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Discussion Starter #1
Breaks and stopping distances...

I noticed that a lot of people, including very respected car journalists, say things like this:

"This car has this massive 17" brakes, and it stops like no other car blah blah"
"When you press break pedal on this car, you can feel massive g-force stopping you etc"

The truth is...
Mini Cooper stops as fast as Pagani Zonda.
(60-0 in 2.6 secs for Mini and 2.7 for Zonda, Autocar)

On modern cars, even Lada breaks have enough power to lock up the wheels as fast as you press the break pedal.
Once your wheels are kept on the locking boundary, there is *ONLY ONE*, and i mean it, factor affecting your break distance - coefficient of friction between tires and road, i.e condition/type of the road and your tires.

So, what distinguishes good and bad breaks?
Two most importaint things here are:
1. Resistance to fading.
2. Feel.
 

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Re: Breaks and stopping distances...

And the relation to the M5 is..? Point?
 

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Re: Breaks and stopping distances...

not true. while the binding power of the calipers is sufficient even on the weakest car nowadays, there are other factors to consider:

1. tire compound/ width/ thread (assuming non-slick type)/ deflection (related to width and height)
2. external temperature
3. tire temperature
4. surface texture
5. surface temperature
6. resitance to fade (less fade allows abs to control pulsations more accurately thus maximizing traction)
7. brake-bias (you brake 90% with the fronts, but the rears help a lot too)
8. weight transfer under braking
9. alignment maximizing tire patch under braking (keep tires perpendicular to road).
10. engine overspinning effort (related to cylinder compression and exhaust path)
11. vehicle weight

in other words- too many things. braking is one of the most complicated issues. i would say- just as complicated as accelleration, if not more...

in other words- a well braking car doesnt mean just bigger and sticker tires. there is much more to it.

alex
few cars
 

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Discussion Starter #4
Re: Breaks and stopping distances...

AJ, have top disagree with you.

1-5: Thats included in coefficient of friction.
6: thats what i said makes good breaks.
10: we are talking about normal breaking only, no engine breaking, sorry, had to mention it, but i thought it was obvious :)
7-11: Neither contact area, nor weight effect breaking in any way. Thats school physics.
 

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Re: Breaks and stopping distances...

Kind of offtopic but the Mini Cooper is one of the worst braking car I have ever test driven :nono:, so the Pagani Zonda must be a nightmare to brake :1:
 

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Discussion Starter #6
Re: Breaks and stopping distances...

ganzbaf said:
Kind of offtopic but the Mini Cooper is one of the worst braking car I have ever test driven :nono:, so the Pagani Zonda must be a nightmare to brake :1:
How is that worst breaking car? :)
 

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Discussion Starter #7
Re: Breaks and stopping distances...

Guys, this topic is not really for discussion here.
Its simple facts about elementary physics which translate in real life with 100% expectable result.
I just wanted to make sure people like ganzbaf dont have this misconseption anymore.

*ALL* morden cars will stop from 100kph in pretty much the same distance.
(I have to note, that this will not be true for extream high speed stops/repetative stops, since fade resistance will play a role as well)

From an independed source (some quotings):
(60-0 mph in seconds)
Aston Martin DB9 2.7
Bentley CGT 2.7
BMW new M5 2.8
Daewoo Kalos 2.9
Ferrari 612 2.7
Ford Focus 2.8
Honda Civic 2.7
Lambo Gallardo 2.9
Range Rover 2.8
Lotus Elise 2.8

I hope you see the point.
 

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Re: Breaks and stopping distances...

Val said:
AJ, have top disagree with you.

1-5: Thats included in coefficient of friction.
Which must be taken into account when discussing the braking performance. A car will obviously take longer to stop on ice than on a race track. Look at your thread title... you mention stopping distances, so AJ's points are valid.
Val said:
6: thats what i said makes good breaks.
10: we are talking about normal breaking only, no engine breaking, sorry, had to mention it, but i thought it was obvious :)
Again, you mentioned stopping distances, and there is typically some engine braking.
Val said:
7-11: Neither contact area, nor weight effect breaking in any way. Thats school physics.
Go back to school. You're telling me that two identical cars, one with 185mm wide tires and one with 255mm tires will stop in the same distance? Or that two identical cars, with one weighing an extra 1000 pounds or so will stop in the same distance? Wow, I guess physics don't work the same in Moscow.
Val said:
Guys, this topic is not really for discussion here.
If it is not really for discussion, why create the thread? After all.. this is a DISCUSSION forum.

And what exactly is the purpose of your last posting? A bunch of random stopping distances from a wide range of different types of cars? What does that "prove"?
 

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Re: Breaks and stopping distances...

Val said:
7-11: Neither contact area, nor weight effect breaking in any way. Thats school physics.
Well then you need to go back to school hiha

The grip on the road a tire has is NOT linear with the force applied to the tire. If this wre not true then there would be no point in running wide tires on a sports car. Friction is only one part of tire grip and its not the biggest. You forget about mechanical keying and adhesion.

If you really want to learn how tires work I would suggest:
The racing and high performance tire

One of the reasons my lotus stops better than the M3 is not because it has better brakes. It doesn't really. Its because the car has a rear weight bias and under braking that give the rear tires a chance to contribute more without overworking the fronts as much.
 

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Discussion Starter #10
Re: Breaks and stopping distances...

vndkshn, concerning coefficient of friction, maybe you should read my post once more?? That was the whole point, and i was just saying AJ was putting it as something i havent mentioned.

I hope Enigma cleared that school stuff up for you...

"And what exactly is the purpose of your last posting? A bunch of random stopping distances from a wide range of different types of cars? What does that "prove"?"

Err... Well, if you look more closely, all cars stop in pretty much same time.
So, that proves that all cars stop equally well.


Enigma, I am not sure the bias affects anything.
We have constant amount of weight, say 1 tonn.
If all weight, say, goes on front wheels, then they get 1 tonn.
Frictional force is 1x then.
However, if half weight goes on rear wheels, then frictional force is 0.5x for front wheels and 0.5x for rear. Its still 1x in total.

? :)

(Its the same argument which explains why weight doesnt affect braking distance. And since all these equations are linear, that means that it will apply for a variable weight as well)

This is also consistent with the data. Bentley stops just as fast as Lotus.
 

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Re: Breaks and stopping distances...

Val said:
Enigma, I am not sure the bias affects anything.
We have constant amount of weight, say 1 tonn.
If all weight, say, goes on front wheels, then they get 1 tonn.
Frictional force is 1x then.
However, if half weight goes on rear wheels, then frictional force is 0.5x for front wheels and 0.5x for rear. Its still 1x in total.

? :)
I guess you missed the part when grip is more than just friction and the other components are NON LINEAR with force. Ever seen a rock stick to a tire? Explain with your formula how that works ;)

Just because you learned a new physics formula yesterday don't think it tells you the whole story.

I'll attach a REALISTIC graph of tire grip vs weight applied.


From this article: Click, Read, Learn
 

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Re: Breaks and stopping distances...

friction is proportional to normal force (and hence contact area is irrelevant) in a coulomb friction model, but a coulomb friction model can only accurately be applied to very rigid surfaces such as two metallic objects. One only has tp look at drag racing or any racing for that matter to observe this.
 

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Re: Breaks and stopping distances...

digger said:
friction is proportional to normal force (and hence contact area is irrelevant) in a coulomb friction model, but a coulomb friction model can only accurately be applied to very rigid surfaces such as two metallic objects. One only has tp look at drag racing or any racing for that matter to observe this.
I think what you're saying here is tyres are sticky...

Bottom line here, lighter cars stop in shorter distances, generally speaking. Bigger tyres help a lot, proportionately, but yes, most modern cars have the ability to achieve a lock-up in a single stop.

My racer stops a helluva lot quicker than any road car I've driven (except maybe my old Exige), simply because it's light and the tyres are reasonably grippy. Except last weekend the bias was too much to the rear, so that messed things up a touch at Oulton Park...
 

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Re: Breaks and stopping distances...

Val said:
vndkshn, concerning coefficient of friction, maybe you should read my post once more?? That was the whole point, and i was just saying AJ was putting it as something i havent mentioned.
I did read your post multiple times, it still made no sense. AJ mentioned other aspects of braking that you were not taking into account that has impact on stopping distances... and defining "good brakes".

Val said:
I hope Enigma cleared that school stuff up for you...
It seems that he is working on clearing it up for you.....
Val said:
"And what exactly is the purpose of your last posting? A bunch of random stopping distances from a wide range of different types of cars? What does that "prove"?"

Err... Well, if you look more closely, all cars stop in pretty much same time.
So, that proves that all cars stop equally well.
Yes, it proves that their initial stopping distance was nearly equal. But, that has nothing to do with fade, or fade resistance, or feel, and without looking at the larger picture (including the list that AJ mentions) it really doesn't prove anything. Your right, most cars stop in pretty much the same time/distance up to a point. What defines good brakes is not a single stop from 100kph. Try those same cars from 200, or 250, and with repeated stops back to back... there is when you start to learn something. That list of figures tells you nothing about how the brakes deal with heat, or how good or bad they felt during that stop... the testers likely just stood on the brakes and hung on. What does that prove? Exactly nothing.

It takes more than a 100kph-0 stop to define what good brakes are.
 

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Discussion Starter #15
Re: Breaks and stopping distances...

vndkshn, i completely agree :) thast what i was saying.

However, 100-0 figures do show interesting information, and single stop 200-0 wont be that much different.

Enigma, why does Range Rover stops as fast as Elise then?
 

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Re: Breaks and stopping distances...

Val said:
However, 100-0 figures do show interesting information, and single stop 200-0 wont be that much different.

Enigma, why does Range Rover stops as fast as Elise then?
You might want to rethink your thought that 200-0 wouldn't be that much different. Use your Range Rover vs. Elise example above... The math doesn't come to mind right away (hey, its Friday!) but there is alot more energy at 200 than at 100. I suspect you will see the Elise stop MUCH quicker than the Range Rover from 200.
 

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Re: Breaks and stopping distances...

Val said:
Enigma, why does Range Rover stops as fast as Elise then?
I have seen a few SUVs that have posted unexpectedly good braking performance from lower speeds. The best guess is the high profile, soft tires tend to flatten out under extra load in the front giving them an increased contact patch under braking. Its interesting that optimizing a tires for lateral acceleration is very diffrent than logintudinal. For braking (or a drag race style start) the SUV tires often work better with their longer contact patches than the low profile sports tires on most high performance cars. There is a reason that the drag race guys put big, tall, soft sidewall tires on the back of their cars and then only put 6psi or so in them.

The other issue here is 100-0 measures more than just the brakes. There is usually also a engage/disengage time along with drivers reaction time that add a lot of noise to these numbers. 200-0 is a much better measure because you have 4x the braking to do compared to the noise elements.

One other thing to note. On the "extreme" cars like the zonda sometime the brakes are not warm enough to be working 100% on the first stop. I saw this with the other brake pads on the M3. They need a little heat before they start working.

Summary: For a single 60-0 stop weight ballance and tires are the primary factors that determine the result.
 

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Re: Breaks and stopping distances...

Good point on the soft sidewalls of the SUVs possibly helping out. I think you could take that a step further and say that it is most likely easier to cause an initial lockup of the brakes on cars with short sidewalls as the shock is transmitted directly to the contact patch, where the taller profile tires absorb some of that initial shock. Again, looking at the drag racing world where those soft sidewalls wrap around, lowering the shock to the contact patch.
 

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Discussion Starter #19 (Edited)
Ok, ive got 160-0 times ;) (same source - Autocar, reaction time EXCLUDED)

Audi TT 3.2 DSG: 100-0mph: 4.44secs

Lotus Elise 111R: 100-0mph: 4.09secs

Lamborghini Murcielago: 100-0mph: 4.41secs

Pagini Zonda C12S: 100-0mph: 4.05secs

Peugeot 206 GTi 180: 100-0mph: 4.39secs

Alfa Romeo 147 GTA: 100-0mph: 4.39secs

BMW M3: 100-0mph: 4.61secs


Ok, looks like heavier car dont do as well as lighter ones.
But its 0.5sec difference (between two extreames!! :)). It will shave off a few meteres at most.
And there is NO WAY you be able to feel this difference!
 

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Val said:
Ok, ive got 160-0 times ;) (same source - Autocar, reaction time EXCLUDED)

Audi TT 3.2 DSG: 100-0mph: 4.44secs

Lotus Elise 111R: 100-0mph: 4.09secs

Lamborghini Murcielago: 100-0mph: 4.41secs

Pagini Zonda C12S: 100-0mph: 4.05secs

Peugeot 206 GTi 180: 100-0mph: 4.39secs

Alfa Romeo 147 GTA: 100-0mph: 4.39secs

BMW M3: 100-0mph: 4.61secs


Ok, looks like heavier car indeed dont do as well as lighter ones.
But its 0.5sec difference (between two extreames!! :)). It will shave off a few meteres at most.
And there is NO WAY you be able to feel this difference!
 
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