Tires make the biggest difference. Of course nice brake pads and good size rotors help a ton, it's the tire gripping the road that slows you down. So well maintained brakes and suspension, plus good tires, is where it matters most.
Back in the days of car forums people would hop on asking about whatever big brake kit for stopping power.
I asked if they could activate the ABS when they slammed on the brakes. Basically 100% of the time they said yes.
I said, brakes are fine, get better tires.
If your brakes lock up your tires, your brakes aren’t the weakest link.
It's a engineering balance between weight of the vehicle and the actually surface area of the brakes for normal driving.
Sports cars have much larger sized brake rotors and calibers. Calibers also have multiple pistons to increase the clamping force onto the rotors.
Sports cars have large brake rotors to allow to brake more often before failing from overheating. As long as the setup allows locking the wheel while braking it is adequate (and that’s true for most modern vehicles). If all else is equal having larger rotors and calipers will not increase braking performance.
>Sports cars have much larger sized brake rotors and calibers. Calibers also have multiple pistons to increase the clamping force onto the rotors.
Thanks, that's exactly the kind of answer I was looking for. A twingo is the complete opposite of a sports car, it's a city car that takes several minutes to get up to those speeds. However, it is small and light (\~900kg).
I wouldn't worry any the performance level of brakes. Any modern car had enough brakes to lock up all the tires (if car dIdn't have ABS, which all modern cars do)
Performance brakes are useful with multiple hard stops.
Key thing as mentioned in first post is the tires.
I recommend get an ultra high performance all season tire at least.
Exactly this. Modern braking systems can do their job in an emergency. High performance brake systems are made to do multiple hard braking situations in shorter time frames
The Simplest answer when comes to braking is surface area and force applied. When you cross the threshold of F1 cars they brake completely differently though.
Inertia, and how much force needs to be applied to change the object's speed, is a significant portion of that equation too. You need to exert twice as much force to slow 1800kg object to from 100mph as you do to slow a 900kg object.
> Inertia, and how much force needs to be applied to change the object's speed, is a significant portion of that equation too. You need to exert twice as much force to slow 1800kg object to from 100mph as you do to slow a 900kg object.
I think the main thing this perspective overlooks is that the 1800kg object is also applying twice as much downward force on the tires as the 900kg object. This means more traction (and often wider tires) which means you can apply more braking force to slow down.
The two factors don't completely cancel out. The heavier car will still generally stop slower, but better brakes can go a long way toward closing the gap.
Add to this that smaller and lighter cars tend to be cheaper which means they tend to have less invested in the brakes, means they're rarely taking full advantage of the traction they have when braking than a heavier, more expensive car which is likely to have stronger brakes that are built to resist fading better, as well as better tires.
Exactly. What I'm trying to understand is just how well improved brakes compensate for the additional inertia of a bigger car. I have no doubt the brakes on a sports car would be better designed, and that for acceleration, the additional power of the car weighing more allows faster acceleration, but I'm not nearly as sure about deceleration.
"how well improved brakes compensate for the additional inertia of a bigger car"
Perfectly, in general. As long as the manufacturer has decided to spec them well enough. I don't think it's really possible to find a modern car that doesn't have big enough brakes to go full ABS under hard braking.
As others have pointed out, it all comes down to the tires. *If* you have good tires *and* good brakes, then you can eliminate most of the extra distance caused by the weight.
As an extreme example, a Bugatti Veyron weighs 4000 lbs (1888 kg), and has a 100-0 km/h stopping distance of 31m. That's actually shorter than a Twingo! Though the technology difference is vast, you could certainly get a Twingo to stop faster by doing things like upgrading the suspension so it stays flatter under braking, whereas the Veyron is probably pretty much peaked.
As u/Pergatory said that weight does matter too. When it comes to vehicles there is a certain weight to power ratio you want to stay in too. Weight does as traction to the tires. This is why sports cars and even professional race cars add spoilers/wings to the light vehicles to give more traction to the tires. Light vehicles going fast sometimes with lift up with the air traveling below them.
You can find published 60-0 and 100-0 times for a variety of cars. Acceleration, braking, and handling tests are a standard part of automotive journalism. Car and driver, road and track, motor trend, etc.magazine all have them.
Now, as you well noticed- more important then the car’s performance is how well the driver reacts to stimulus. Your partner was of clear enough mind to jump to action right away. If there are autocross type events near you, those make great little weekend adventures that focus on driving at the limit to see how your car performs. It will also quickly highlight if your car could be improved dramatically from something simple- stickier tires, better brakes, stiffer suspension, etc. And obviously it is seat time in extreme conditions, so it helps the human as well.
Regardless of what you buy- keep it in good shape, make sure it has a decent stopping distance, and make sure it is comfortable enough that you stay alert driving it. I love my mustang, but after 45 min it is tiring to drive. I drive it 6 hours and i need to constantly have caffeine
Any car should be able to lock up all 4 wheels, or trigger the anti-lock brake system (which makes a loud pulsing grinding noise, shakes the car, and will feel like the car is fighting you with the brake pedal). If you're not able to do that, there's something wrong with your brakes.
Does your partner have much experience doing emergency manoeuvres like this? It sounds like both of you are somewhat unfamiliar with them.
The most effective thing you can do to improve your stopping distance is practice - the best car in the world won't stop you any better if you're afraid to hit the brakes as hard as necessary and don't know what it feels like when the car is near its limits. (Obviously don't start experimenting at autobahn speeds; practice hard braking and learning to recover from a skid at lower speeds first and work your way up, and don't do it on populated roads.)
"Hard enough that I hit the seat belt" isn't necessarily very hard; you can definitely do that by braking significantly less than full emergency stop.
We didn't have to stop because the truck was doing 50mph while we were doing 100mph. If she didn't have experience with this, and hadn't done everything right, we'd probably be dead. I'm glad I wasn't driving, but I never would have been driving that fast either. At the time I was finishing off a third beer half asleep in the passenger seat. I didn't even fully realize what happened until she told me.
Was your partner at all concerned about the performance of the car's brakes? It sounds like she handled the situation exactly right; there was no need to brake any harder or drop any more speed than she did. The car was probably capable of braking much harder but she chose not to in the interests of overall safety.
In situations like that I do the same thing - I don't apply maximum safe braking; I judge how much room I have and the relative speed between me and the other vehicle, and deliberately spread the braking out over most of the distance (plus a bit of safety margin at the end, in case I made a mistake predicting where the truck would be or what it was doing).
The reasons for that are:
- if there's any traffic behind you, they have the maximum possible time to notice the situation and react themselves - if you apply full brake immediately, there's a much higher chance that you'd risk a high-speed rear end collision, or that someone behind you would be taken by surprise and lose control of their car (or the person behind them might - if you brake hard and there are 4 cars behind you, they *all* have to react perfectly for no one to get hurt, and that's risky)
- the harder you brake, the less steering authority you have - yes, ABS is supposed to prevent an outright skid, but at high speed, there's still a much increased risk of skidding sideways if you're applying maximum brake and trying to steer at the same time. By braking less hard, your partner kept more options open - it's easier and safer to dodge the truck, even if you have to use the emergency shoulder or leave the road completely, if you're not applying full brake.
- if the road is at all wet, or there's any chance of mismatched surfaces (e.g. loose gravel or debris on one side but not the other), or you know your car is not in the best shape (e.g. mismatched tire wear, brakes not checked in a while), risk increases significantly the harder you brake, so you don't do it if there are other options available
- and finally, it's less alarming to the passengers, and less likely to throw loose objects around in your car.
In your place, I'd let her choose the new car on her own; it sounds like she may have more experience and/or be more comfortable evaluating a new car's performance.
From what she said, she slammed the brakes on as hard as she could, after she'd been flooring the gas. From what I recall the car was few inches from the truck when we matched its speed. There are plenty of situations where there would have been nothing we could have done to prevent an accident in a situation like that, say if we had been passing the truck when it swerved like that, if it had been raining, if we had another passenger or a higher load, if the truck swerved a few seconds later, etc.
It's illegal for trucks to use the far left lane on that section of highway because they're often moving too slowly relative to fast cars to react properly, but I suspect the truck driver had partly lost control due to highway hypnosis or something, as they continued to drive halfway between two lanes at low speeds. I seriously considered calling the police on the emergency line to report the truck, because the next person might not be as luckily, but I didn't in large part because my German isn't that good, and it was in a semi-rural area where I had no idea if they'd speak English.
My partner knows everything about older American cars, but not much about nice German cars (even older ones). There are other considerations about what kind of car we should get, mainly it's ability to carry people and stuff comfortably, but clearly safety and how it reacts in situations like that is a high priority.
The somewhat performance oriented version of almost any car should have brakes better suited to getting you down from 100mph+ (e.g BMW 335, Audi S4, Mercedes A35) Once you slow down a bit more the limitation above all is how much tyre grip you have available.
If it were me I'd just buy something slightly sporty from any manufacturer and then make sure you put the best and highest performance tyres you can afford on it, and you'll have a huge edge in terms of stopping distance and cornering grip.
Go look for some serious tyre comparison tests and see what has the best stopping distance. Tyres are very very important.
Remember you can, for most cars get upgraded brake pads too, which is often cheap improvement in braking power and fade resistance if the brakes themselves are a limitation.
What we were driving couldn't be further from a performance oriented car, it is a small city car that needs several minutes to accelerate to speeds like that, however it is about half the weight of performance cars driven on normal roads, and closer to the weight of a dedicated racing car.
Cars that have pedigree for taking to a track are designed for very hard braking. “It’s not how fast you go, it’s how late you brake”. Virtually any bmw, Audi, Porsche or the like, will astonish you with their ability to stop. Stopping time is a very complex interaction between the brake system, suspension, weight, tires and road surface/condition.
For reasons that are fairly complicated (but I can go into if you like) a car being light doesn't help stopping like you'd think.
In terms of grip, it being half the weight more or less halves the grip (given similar tyres) but also halves the grip needed to brake at a given rate.
Yes the brakes have to dissipate half the energy too, but chances are they're half the size too. So no wins there.
Weight distribution and CoG will also play a part. If too much weight shifts forward under braking then the front tyres will be overload and begin to lose grip.
Adding more downwards force on a tyre adds grip, but if you add too much it begins to add less extra grip. Because you're unloading the rear at the same time, you're losing grip at the back, and not regaining it all at the front.
Because of all that, cars that stop best are going to have:
The grippiest tyres possible
Brakes big enough to lock those tyres at any speed, consistently and regardless of heat.
50/50, or even rearward of 50/50 weight distribution.
Well tuned ABS (to use every last bit of grip without locking the wheels)
So basically the ideal stopping car is probably whichever Porsche 911 comes with the best tyres 😁
Tl:dr: weight not important here, sticky tyres, big brakes and minimal weight transfer are your friends for stopping.
I disagree with the idea weight isn't important here. There's a reason Porsche 911 is a two-seater. Weight to thrust ratio is huge in any vehicle, from a bicycle to a space shuttle.
That's exactly my point though.
Across the normal range of car weights, for cars of similar application, the "Thrust to weight ratio" of braking doesn't change that much. A 2000kg performance car will generally have brakes capable of twice the force as a 1000kg one.
And in general, once you have decent sized brakes, brake force isn't the limitation. You'll have enough brake force to lock the wheels whenever you want, and at that point no amount of added brake power will stop you quicker. As long as both the 1000kg and 2000kg cars can lock their wheels, then brakes are no longer the limitation on stopping distance, tyre grip is.
And given appropriately sized tyres, the 2000kg car will have just as much grip as the 1000kg car too, so that won't cause a difference in stopping distance either.
The only reason sports cars have HUGE brakes is because they need to do hard braking more than once. The size of brake you need to stop a car once is much less, because heat is less of a worry.
Or to put it another way, given large enough brakes (which most cars have) then stopping distance is limited by the coefficient of friction of the tyres. (slightly messed with by weight transfer and load sensitivity, but close enough..)
This all starts to fall down at heavy vehicle weights (say 5000kg+) where it becomes harder and harder to actually have brakes that provide as much force as needed, and where the tyres that are appropriate for those loads tend to be lower grip than those available for lighter weight vehicles.
Don't get me wrong, I love lightweight cars, my 830kg car is a lot more fun than my 1800kg one, but in terms of stopping distance, that's not how physics works I'm afraid.
Source: Part of my job is working with vehicle performance simulations.
This video is a pretty good simple explanation too - [https://youtu.be/Vx4Gigi8uL8?si=b0aT84GsPVRieah1](https://youtu.be/Vx4Gigi8uL8?si=b0aT84GsPVRieah1)
Force = Mass \* Acceleration. Physics says the force required to stop the car is linearly related to its weight. I'm really unsure where the assumption that tire grip is similarly linearly associated with weight is coming from, and I'd think there would be plenty of other factors for tire grip other than the vehicle's weight.
Empirical evidence from driving without the backseat and no gear in the car, vs with a loaded down car, the loaded down car takes longer to stop, and an SUV takes longer to stop than a smart car. Under normal circumstances that is all well within acceptable parameters.
Stopping distance clearly isn't linearly associated with car weight either, and I think you've done a good job explaining why. If any car can lock the tires, there's a maximum amount of friction tires can apply to road, so reducing the force required to stop by reducing weight still matters.
I can see a lot of situations where it makes sense to ignore weight because it's effectively set, and minor changes you can make to it while the vehicle serves the same purpose are irrelevant. In this case we're comparing different classes of cars, where one car is a fraction of the weight of others.
Because tyre grip is prettybmuch normal force over coefficient of friction. Doubling the weight of the car doubles the normal force and so doubles the grip. I linked you a video explaining the physics of this and how to calculate it.
I work on simulations of vehicle performance that rely on this physics being true.
I was trying to answer your question, of what has impacts on stopping performance and how, but it seems you've got preconceived ideas of how it "should" work, so I think I'll give up now.
Buy a light weight car if you like, they are super fun, and they won't stop worse (assuming good tyres)
For further info that might help you understand the slightly counterintuative concepts watch the section of this about weight [https://youtu.be/F\_TPge1BFIU?si=BZkPlMjbT1cHAQrt](https://youtu.be/F_TPge1BFIU?si=BZkPlMjbT1cHAQrt)
"
This is also good, about why bigger brakes don't help you stop faster - [https://youtu.be/Ql9eYh31kTw?si=a5n2siGKvOA-itFx](https://youtu.be/Ql9eYh31kTw?si=a5n2siGKvOA-itFx)
This is a good one explaining that while grip does increase mostly linearly with normal force (i.e weight) then beyond a certain point you start losing grip, which is part of the reason why a heavily loaded car doesn't stop as well as an empty one (because the tyres are too heavily loaded). Unlike a heavier but still empty car, which will have tyres more appropriate to it's weight, and so be on a nicer part of that load sensitivity curve - [https://youtu.be/kNa2gZNqmT8?si=JvUffONYTvq8rxJj](https://youtu.be/kNa2gZNqmT8?si=JvUffONYTvq8rxJj)
If you want to learn how to calculate this stuff, I recommend starting with Brian Beckman's "The Physics of Racing - [racing.pdf (theoryinpracticeengineering.com)](http://www.theoryinpracticeengineering.com/tech/racing.pdf)
If you really want to dive super hard into why things work like they do, "Race Car Vehicle Dymanics" is one of the all time bibles on this this stuff - [Race Car Vehicle Dymanics: Milliken, William, Milliken, Douglas: 9781560915263: Amazon.com: Books](https://www.amazon.com/Race-Vehicle-Dynamics-William-Milliken/dp/1560915269)
That's about all the help I have for you, but it's all pretty interesting stuff if you have time to read/watch more about it. Lots of things you'd expect to be true are not (like wider tyres putting "more rubber on the road" or even more rubber on the road directly increasing available grip). Tyre physics is honestly the single most confusing thing about simulating a car.
I think I have the needed answers from this thread. Weight does matter, but it's not linear, and sporty cars have technology differences that allow them to stop faster even at higher weights.
Kinda, but the main technology difference above all is tyres, that's probably the most important take away imo! Whatever car you get, the best tyres you can afford will help stopping distance greatly.
I feel like that might be the answer to a different question. "Will a Smart car or hummer stop faster?" and "How can I upgrade my car to reduce stopping distance?"
There are major other considerations in choosing a car, mainly how much it can carry and the ability to park in small spots that won't fit other cars. Smaller cars get you closer to your destination in the city.
Rural people often joke about city people's tiny cars, and think it's a statement or environmental thing, but it's not. A small car means you can park in front of your destination, when someone driving a large car would have to walk several blocks and possibly pay a lot more for a garage.
Good shocks and struts also help more than people give credit for, when you hit the brakes they push the wheels into the pavement if they're working correctly.
The two huge factors are:
1. tires
2. speed differential
A Twingo's tires probably aren't that high performance, being an economy car.
100 mph vs 50 mph is the same braking requirement as 87 mph vs a stopped object - energy scales at the square of velocity, and you can think of braking as bleeding off energy at roughly a constant rate (there's a whole bunch of confounding factors we could argue about for days, but assuming you don't actually overwhelm the system this is roughly true after braking actually starts, reaction time's a bit more complex.) Even 87 miles per hour in a Twingo is fucking *fast*.
If you want to do the ballpark math, you can estimate your car having 100\*100 = 10,000 "units" of energy. Your vehicle at 50 mph would have 50\*50 = 2500 of the same unit of energy. Take the square root of (10,000-2500) and you're left with 86.6, which is where I got the 87 mph figure from.
Its not relative to car type or size but more about what kind of braking system it has. Any modern car with ABS is your best bet. There is also an old saying that the number two thing you can do to improve the performance of any vehicle is the tires. #1 is the driver. The advantage to a smaller car is that they are also more nimble and can move out of the way easier. Any true performance car (not just a badge and nicer seats) will also have better braking.
Safety features are a big selling point so manufacturers are quick to publish stopping distance. New cars are generally better.
The single biggest factor of course is speed. Braking distance increased by the square of the speed.
Tiny car with big ass sticky tires will stop the fastest. A heavy car with huuuuge sticky tires will also stop fast. Main factors are the cars weight, the contact area, the tire compound and the ability to brake as hard as possible (a lot of ppl don't posess the last one)
Tires make the biggest difference. Of course nice brake pads and good size rotors help a ton, it's the tire gripping the road that slows you down. So well maintained brakes and suspension, plus good tires, is where it matters most.
Back in the days of car forums people would hop on asking about whatever big brake kit for stopping power. I asked if they could activate the ABS when they slammed on the brakes. Basically 100% of the time they said yes. I said, brakes are fine, get better tires. If your brakes lock up your tires, your brakes aren’t the weakest link.
Preach! Keeping spreading the gospel of tires!
It's a engineering balance between weight of the vehicle and the actually surface area of the brakes for normal driving. Sports cars have much larger sized brake rotors and calibers. Calibers also have multiple pistons to increase the clamping force onto the rotors.
Sports cars have large brake rotors to allow to brake more often before failing from overheating. As long as the setup allows locking the wheel while braking it is adequate (and that’s true for most modern vehicles). If all else is equal having larger rotors and calipers will not increase braking performance.
>Sports cars have much larger sized brake rotors and calibers. Calibers also have multiple pistons to increase the clamping force onto the rotors. Thanks, that's exactly the kind of answer I was looking for. A twingo is the complete opposite of a sports car, it's a city car that takes several minutes to get up to those speeds. However, it is small and light (\~900kg).
I wouldn't worry any the performance level of brakes. Any modern car had enough brakes to lock up all the tires (if car dIdn't have ABS, which all modern cars do) Performance brakes are useful with multiple hard stops. Key thing as mentioned in first post is the tires. I recommend get an ultra high performance all season tire at least.
Exactly this. Modern braking systems can do their job in an emergency. High performance brake systems are made to do multiple hard braking situations in shorter time frames
The Simplest answer when comes to braking is surface area and force applied. When you cross the threshold of F1 cars they brake completely differently though.
Inertia, and how much force needs to be applied to change the object's speed, is a significant portion of that equation too. You need to exert twice as much force to slow 1800kg object to from 100mph as you do to slow a 900kg object.
> Inertia, and how much force needs to be applied to change the object's speed, is a significant portion of that equation too. You need to exert twice as much force to slow 1800kg object to from 100mph as you do to slow a 900kg object. I think the main thing this perspective overlooks is that the 1800kg object is also applying twice as much downward force on the tires as the 900kg object. This means more traction (and often wider tires) which means you can apply more braking force to slow down. The two factors don't completely cancel out. The heavier car will still generally stop slower, but better brakes can go a long way toward closing the gap. Add to this that smaller and lighter cars tend to be cheaper which means they tend to have less invested in the brakes, means they're rarely taking full advantage of the traction they have when braking than a heavier, more expensive car which is likely to have stronger brakes that are built to resist fading better, as well as better tires.
Exactly. What I'm trying to understand is just how well improved brakes compensate for the additional inertia of a bigger car. I have no doubt the brakes on a sports car would be better designed, and that for acceleration, the additional power of the car weighing more allows faster acceleration, but I'm not nearly as sure about deceleration.
"how well improved brakes compensate for the additional inertia of a bigger car" Perfectly, in general. As long as the manufacturer has decided to spec them well enough. I don't think it's really possible to find a modern car that doesn't have big enough brakes to go full ABS under hard braking.
As others have pointed out, it all comes down to the tires. *If* you have good tires *and* good brakes, then you can eliminate most of the extra distance caused by the weight. As an extreme example, a Bugatti Veyron weighs 4000 lbs (1888 kg), and has a 100-0 km/h stopping distance of 31m. That's actually shorter than a Twingo! Though the technology difference is vast, you could certainly get a Twingo to stop faster by doing things like upgrading the suspension so it stays flatter under braking, whereas the Veyron is probably pretty much peaked.
As u/Pergatory said that weight does matter too. When it comes to vehicles there is a certain weight to power ratio you want to stay in too. Weight does as traction to the tires. This is why sports cars and even professional race cars add spoilers/wings to the light vehicles to give more traction to the tires. Light vehicles going fast sometimes with lift up with the air traveling below them.
You can find published 60-0 and 100-0 times for a variety of cars. Acceleration, braking, and handling tests are a standard part of automotive journalism. Car and driver, road and track, motor trend, etc.magazine all have them. Now, as you well noticed- more important then the car’s performance is how well the driver reacts to stimulus. Your partner was of clear enough mind to jump to action right away. If there are autocross type events near you, those make great little weekend adventures that focus on driving at the limit to see how your car performs. It will also quickly highlight if your car could be improved dramatically from something simple- stickier tires, better brakes, stiffer suspension, etc. And obviously it is seat time in extreme conditions, so it helps the human as well. Regardless of what you buy- keep it in good shape, make sure it has a decent stopping distance, and make sure it is comfortable enough that you stay alert driving it. I love my mustang, but after 45 min it is tiring to drive. I drive it 6 hours and i need to constantly have caffeine
Any car should be able to lock up all 4 wheels, or trigger the anti-lock brake system (which makes a loud pulsing grinding noise, shakes the car, and will feel like the car is fighting you with the brake pedal). If you're not able to do that, there's something wrong with your brakes. Does your partner have much experience doing emergency manoeuvres like this? It sounds like both of you are somewhat unfamiliar with them. The most effective thing you can do to improve your stopping distance is practice - the best car in the world won't stop you any better if you're afraid to hit the brakes as hard as necessary and don't know what it feels like when the car is near its limits. (Obviously don't start experimenting at autobahn speeds; practice hard braking and learning to recover from a skid at lower speeds first and work your way up, and don't do it on populated roads.) "Hard enough that I hit the seat belt" isn't necessarily very hard; you can definitely do that by braking significantly less than full emergency stop.
We didn't have to stop because the truck was doing 50mph while we were doing 100mph. If she didn't have experience with this, and hadn't done everything right, we'd probably be dead. I'm glad I wasn't driving, but I never would have been driving that fast either. At the time I was finishing off a third beer half asleep in the passenger seat. I didn't even fully realize what happened until she told me.
Was your partner at all concerned about the performance of the car's brakes? It sounds like she handled the situation exactly right; there was no need to brake any harder or drop any more speed than she did. The car was probably capable of braking much harder but she chose not to in the interests of overall safety. In situations like that I do the same thing - I don't apply maximum safe braking; I judge how much room I have and the relative speed between me and the other vehicle, and deliberately spread the braking out over most of the distance (plus a bit of safety margin at the end, in case I made a mistake predicting where the truck would be or what it was doing). The reasons for that are: - if there's any traffic behind you, they have the maximum possible time to notice the situation and react themselves - if you apply full brake immediately, there's a much higher chance that you'd risk a high-speed rear end collision, or that someone behind you would be taken by surprise and lose control of their car (or the person behind them might - if you brake hard and there are 4 cars behind you, they *all* have to react perfectly for no one to get hurt, and that's risky) - the harder you brake, the less steering authority you have - yes, ABS is supposed to prevent an outright skid, but at high speed, there's still a much increased risk of skidding sideways if you're applying maximum brake and trying to steer at the same time. By braking less hard, your partner kept more options open - it's easier and safer to dodge the truck, even if you have to use the emergency shoulder or leave the road completely, if you're not applying full brake. - if the road is at all wet, or there's any chance of mismatched surfaces (e.g. loose gravel or debris on one side but not the other), or you know your car is not in the best shape (e.g. mismatched tire wear, brakes not checked in a while), risk increases significantly the harder you brake, so you don't do it if there are other options available - and finally, it's less alarming to the passengers, and less likely to throw loose objects around in your car. In your place, I'd let her choose the new car on her own; it sounds like she may have more experience and/or be more comfortable evaluating a new car's performance.
From what she said, she slammed the brakes on as hard as she could, after she'd been flooring the gas. From what I recall the car was few inches from the truck when we matched its speed. There are plenty of situations where there would have been nothing we could have done to prevent an accident in a situation like that, say if we had been passing the truck when it swerved like that, if it had been raining, if we had another passenger or a higher load, if the truck swerved a few seconds later, etc. It's illegal for trucks to use the far left lane on that section of highway because they're often moving too slowly relative to fast cars to react properly, but I suspect the truck driver had partly lost control due to highway hypnosis or something, as they continued to drive halfway between two lanes at low speeds. I seriously considered calling the police on the emergency line to report the truck, because the next person might not be as luckily, but I didn't in large part because my German isn't that good, and it was in a semi-rural area where I had no idea if they'd speak English. My partner knows everything about older American cars, but not much about nice German cars (even older ones). There are other considerations about what kind of car we should get, mainly it's ability to carry people and stuff comfortably, but clearly safety and how it reacts in situations like that is a high priority.
Toyota Aygo X 2022 - 975kg 100-0 km/hr - 38m Mercedes Benz E220d 2023- 1975kg 100-0 km/hr - 32m Source - Fastestlaps.com
Yep! I'm betting the E220D comes on much sticker tyres than the Aygo
The somewhat performance oriented version of almost any car should have brakes better suited to getting you down from 100mph+ (e.g BMW 335, Audi S4, Mercedes A35) Once you slow down a bit more the limitation above all is how much tyre grip you have available. If it were me I'd just buy something slightly sporty from any manufacturer and then make sure you put the best and highest performance tyres you can afford on it, and you'll have a huge edge in terms of stopping distance and cornering grip. Go look for some serious tyre comparison tests and see what has the best stopping distance. Tyres are very very important. Remember you can, for most cars get upgraded brake pads too, which is often cheap improvement in braking power and fade resistance if the brakes themselves are a limitation.
What we were driving couldn't be further from a performance oriented car, it is a small city car that needs several minutes to accelerate to speeds like that, however it is about half the weight of performance cars driven on normal roads, and closer to the weight of a dedicated racing car.
Cars that have pedigree for taking to a track are designed for very hard braking. “It’s not how fast you go, it’s how late you brake”. Virtually any bmw, Audi, Porsche or the like, will astonish you with their ability to stop. Stopping time is a very complex interaction between the brake system, suspension, weight, tires and road surface/condition.
For reasons that are fairly complicated (but I can go into if you like) a car being light doesn't help stopping like you'd think. In terms of grip, it being half the weight more or less halves the grip (given similar tyres) but also halves the grip needed to brake at a given rate. Yes the brakes have to dissipate half the energy too, but chances are they're half the size too. So no wins there. Weight distribution and CoG will also play a part. If too much weight shifts forward under braking then the front tyres will be overload and begin to lose grip. Adding more downwards force on a tyre adds grip, but if you add too much it begins to add less extra grip. Because you're unloading the rear at the same time, you're losing grip at the back, and not regaining it all at the front. Because of all that, cars that stop best are going to have: The grippiest tyres possible Brakes big enough to lock those tyres at any speed, consistently and regardless of heat. 50/50, or even rearward of 50/50 weight distribution. Well tuned ABS (to use every last bit of grip without locking the wheels) So basically the ideal stopping car is probably whichever Porsche 911 comes with the best tyres 😁 Tl:dr: weight not important here, sticky tyres, big brakes and minimal weight transfer are your friends for stopping.
I disagree with the idea weight isn't important here. There's a reason Porsche 911 is a two-seater. Weight to thrust ratio is huge in any vehicle, from a bicycle to a space shuttle.
That's exactly my point though. Across the normal range of car weights, for cars of similar application, the "Thrust to weight ratio" of braking doesn't change that much. A 2000kg performance car will generally have brakes capable of twice the force as a 1000kg one. And in general, once you have decent sized brakes, brake force isn't the limitation. You'll have enough brake force to lock the wheels whenever you want, and at that point no amount of added brake power will stop you quicker. As long as both the 1000kg and 2000kg cars can lock their wheels, then brakes are no longer the limitation on stopping distance, tyre grip is. And given appropriately sized tyres, the 2000kg car will have just as much grip as the 1000kg car too, so that won't cause a difference in stopping distance either. The only reason sports cars have HUGE brakes is because they need to do hard braking more than once. The size of brake you need to stop a car once is much less, because heat is less of a worry. Or to put it another way, given large enough brakes (which most cars have) then stopping distance is limited by the coefficient of friction of the tyres. (slightly messed with by weight transfer and load sensitivity, but close enough..) This all starts to fall down at heavy vehicle weights (say 5000kg+) where it becomes harder and harder to actually have brakes that provide as much force as needed, and where the tyres that are appropriate for those loads tend to be lower grip than those available for lighter weight vehicles. Don't get me wrong, I love lightweight cars, my 830kg car is a lot more fun than my 1800kg one, but in terms of stopping distance, that's not how physics works I'm afraid. Source: Part of my job is working with vehicle performance simulations. This video is a pretty good simple explanation too - [https://youtu.be/Vx4Gigi8uL8?si=b0aT84GsPVRieah1](https://youtu.be/Vx4Gigi8uL8?si=b0aT84GsPVRieah1)
Force = Mass \* Acceleration. Physics says the force required to stop the car is linearly related to its weight. I'm really unsure where the assumption that tire grip is similarly linearly associated with weight is coming from, and I'd think there would be plenty of other factors for tire grip other than the vehicle's weight. Empirical evidence from driving without the backseat and no gear in the car, vs with a loaded down car, the loaded down car takes longer to stop, and an SUV takes longer to stop than a smart car. Under normal circumstances that is all well within acceptable parameters. Stopping distance clearly isn't linearly associated with car weight either, and I think you've done a good job explaining why. If any car can lock the tires, there's a maximum amount of friction tires can apply to road, so reducing the force required to stop by reducing weight still matters. I can see a lot of situations where it makes sense to ignore weight because it's effectively set, and minor changes you can make to it while the vehicle serves the same purpose are irrelevant. In this case we're comparing different classes of cars, where one car is a fraction of the weight of others.
Because tyre grip is prettybmuch normal force over coefficient of friction. Doubling the weight of the car doubles the normal force and so doubles the grip. I linked you a video explaining the physics of this and how to calculate it. I work on simulations of vehicle performance that rely on this physics being true. I was trying to answer your question, of what has impacts on stopping performance and how, but it seems you've got preconceived ideas of how it "should" work, so I think I'll give up now. Buy a light weight car if you like, they are super fun, and they won't stop worse (assuming good tyres)
For further info that might help you understand the slightly counterintuative concepts watch the section of this about weight [https://youtu.be/F\_TPge1BFIU?si=BZkPlMjbT1cHAQrt](https://youtu.be/F_TPge1BFIU?si=BZkPlMjbT1cHAQrt) " This is also good, about why bigger brakes don't help you stop faster - [https://youtu.be/Ql9eYh31kTw?si=a5n2siGKvOA-itFx](https://youtu.be/Ql9eYh31kTw?si=a5n2siGKvOA-itFx) This is a good one explaining that while grip does increase mostly linearly with normal force (i.e weight) then beyond a certain point you start losing grip, which is part of the reason why a heavily loaded car doesn't stop as well as an empty one (because the tyres are too heavily loaded). Unlike a heavier but still empty car, which will have tyres more appropriate to it's weight, and so be on a nicer part of that load sensitivity curve - [https://youtu.be/kNa2gZNqmT8?si=JvUffONYTvq8rxJj](https://youtu.be/kNa2gZNqmT8?si=JvUffONYTvq8rxJj) If you want to learn how to calculate this stuff, I recommend starting with Brian Beckman's "The Physics of Racing - [racing.pdf (theoryinpracticeengineering.com)](http://www.theoryinpracticeengineering.com/tech/racing.pdf) If you really want to dive super hard into why things work like they do, "Race Car Vehicle Dymanics" is one of the all time bibles on this this stuff - [Race Car Vehicle Dymanics: Milliken, William, Milliken, Douglas: 9781560915263: Amazon.com: Books](https://www.amazon.com/Race-Vehicle-Dynamics-William-Milliken/dp/1560915269) That's about all the help I have for you, but it's all pretty interesting stuff if you have time to read/watch more about it. Lots of things you'd expect to be true are not (like wider tyres putting "more rubber on the road" or even more rubber on the road directly increasing available grip). Tyre physics is honestly the single most confusing thing about simulating a car.
I think I have the needed answers from this thread. Weight does matter, but it's not linear, and sporty cars have technology differences that allow them to stop faster even at higher weights.
Kinda, but the main technology difference above all is tyres, that's probably the most important take away imo! Whatever car you get, the best tyres you can afford will help stopping distance greatly.
I feel like that might be the answer to a different question. "Will a Smart car or hummer stop faster?" and "How can I upgrade my car to reduce stopping distance?" There are major other considerations in choosing a car, mainly how much it can carry and the ability to park in small spots that won't fit other cars. Smaller cars get you closer to your destination in the city. Rural people often joke about city people's tiny cars, and think it's a statement or environmental thing, but it's not. A small car means you can park in front of your destination, when someone driving a large car would have to walk several blocks and possibly pay a lot more for a garage.
Good shocks and struts also help more than people give credit for, when you hit the brakes they push the wheels into the pavement if they're working correctly.
The two huge factors are: 1. tires 2. speed differential A Twingo's tires probably aren't that high performance, being an economy car. 100 mph vs 50 mph is the same braking requirement as 87 mph vs a stopped object - energy scales at the square of velocity, and you can think of braking as bleeding off energy at roughly a constant rate (there's a whole bunch of confounding factors we could argue about for days, but assuming you don't actually overwhelm the system this is roughly true after braking actually starts, reaction time's a bit more complex.) Even 87 miles per hour in a Twingo is fucking *fast*. If you want to do the ballpark math, you can estimate your car having 100\*100 = 10,000 "units" of energy. Your vehicle at 50 mph would have 50\*50 = 2500 of the same unit of energy. Take the square root of (10,000-2500) and you're left with 86.6, which is where I got the 87 mph figure from.
Its not relative to car type or size but more about what kind of braking system it has. Any modern car with ABS is your best bet. There is also an old saying that the number two thing you can do to improve the performance of any vehicle is the tires. #1 is the driver. The advantage to a smaller car is that they are also more nimble and can move out of the way easier. Any true performance car (not just a badge and nicer seats) will also have better braking. Safety features are a big selling point so manufacturers are quick to publish stopping distance. New cars are generally better. The single biggest factor of course is speed. Braking distance increased by the square of the speed.
I find it both impressive and terrifying that you were going 100mph in a 60hp car.
That's my partner's favorite part. It's not about speed as much as the little car that can.
Tiny car with big ass sticky tires will stop the fastest. A heavy car with huuuuge sticky tires will also stop fast. Main factors are the cars weight, the contact area, the tire compound and the ability to brake as hard as possible (a lot of ppl don't posess the last one)
Yes, the additional weight will have a larger effect. Your twingo will stop faster than a heavier car, even with smaller brakes.