Sanjeevan

Oh! crap, and here I thought we got to the bottom of things ...so, what's the answer boys...

RJay

I'm not with you on this one CC. Forget about the fact that these tests were not conducted side by side in the same circumstances like testing methodology, ambient, surface type... Lets assume, none of that is significant and your assertion is correct. If weight isn't a factor in deceleration then surely the inverse must be true.

So how is that the Toureg won't accelerate with the Elise? The most obvious factor beyond gearing, slushbox versus manual, differing levels of drive train loss, different torque curves etc... is that the HP/Weight ratio advantages the Elise. Braking capability can also be measured in terms of HP. Clearly, just as the Toureg would require a more powerful engine if it was to match the Elise's acceleration, in this case it has a far more powerful braking system to be able to equal it. It needs this more powerful system as it has far more mass to slow which requires more HP to do it in an equivalent distance, in the same way that it would require a equivalent HP/Weight ratio (forgetting about all the other variables) to accelerate to match the Elise in 1/4 mile times.

adrial

Thats very interesting data you found CC. I didn't realize Avon published so much tire data, maybe that is a key reason that some FSAE teams chose them over Hoosier/Goodyear.

Anyway, I may have some old (mid 90's) Goodyear data for same sized tire that I will post when I get a chance. It's in new jersey but I will be there this weekend. I expect they will be fairly similar, but we will see.

New data in that tire size is damn near impossible to find from what I've been told because FSAE asked tire manufacturers to keep that data private and force the teams to gather their own data. Again that is just from what I've been told, when that actually happened was before my years...

brucegre

OK, I'm not writing this because I'm dead. The reason I'm dead is because I was sure that my 1300 lb Diasio would stop in a shorter distance than my 2500 lb Cup Car, so I went into corner 8 at Mosport about 250 feet deeper than usual before getting on the brakes. About 100 feet was attributable to the speed difference between the two cars entering the braking zone, but after that I must have died, because the Cup Car has much bigger brakes and really expensive Dunlop rubber and giant cooling ducts and ABS and everything. The Diasio has tiny little rotors with tiny little calipers and the cooling ducts got ripped off a while ago and the only abs are the ones I'm clenching while I drive, so obviously I didn't make it through, and all those laps I got are really only happening up here in Heaven.

CC, I don't usually jump on these ones, but you have to get out of the theory and into the car. I'm an engineer, and the hardest students to teach are engineers because they want a theoretical model they can drive to. The problem is creating an accurate dynamic model that models every variable under all conditions, and then finding a computer to run it on. The inaccuracies of the model are killing it. Just go and drive a heavy car on the track, and then get into a light one and tell me what you think.

Here's something to think of. Why do ground effects cars stop so well from high speed? Obviously, the downforce on the tires is increasing grip (as you've pointed out with some good tire data) which is analagous to a heavy car. But, the downforce is not mass, and so it doesn't increase the force required to stop the car. So, take a car that weighs 5000 lbs, and then take an F1 car that generates a total combination of 5000 lbs weight and downforce at the starting speed, are you telling me that they will stop in the same distance, even with the same brakes and tires?

Bruce

ColorChange

Rjay, I find it ... interesting? ... that you don't find my arguement compelling. And your acceleration analogy is flawed. It is easy to generate a thousand horsepower in braking, very difficult to generate that with a motor.

Thanks Adrial.

Bruce, your implication that I need to get out of the theory implies two things. 1 you are a better engineer than me to correct me, and this is highly unlikely, and 2, I have done something wrong (that I haven't already corrected myself about), and I would love you to demonstrate your engineering accumen and point it out to me.

No, the downforce car will stop WAY quicker than the non downforce car, because it can pull higher decelerating g's (nearly 5). But my guess is you knew that.

kurt M

Nice touch CC! That is a great way to gain influence and persuade people regardless of right or wrong, come across as a fathead. Don't forget the human factor formula. AH=FU. This is an interesting thread with lots of theoretical and some real though provoking information please keep it this way and we all might pick up some good stuff.

brucegre

OK CC, whatever you say. Let me know how the heavy car works out for you.

brucegre

BTW, in case you really are interested, in my downforce example one car weighs 5000 lbs. The other car weighs, say, 1800 lbs and has 3200 lbs of downforce at the start of braking. Both cars have identical brakes and tires. Which car stops faster?

Bruce

Bob Rouleau

Tim,

If the question is: can a heavy car stop in as short a distance as a light one? The answer is yes.

If the question is: can two cars identical in every respect except weight stop in the same distance, then the answer is "it depends". Pete T observes that the lighter car will have more braking ability after repeated use of the brakes sice it has an excess of braking capacity compared to the otherwise identical heavy car. True.

Bruce - in your example, I suspect the Diasio has more downforce than a cup car. I also think the ratio of contact patch to mass favors the Diasio. Is this possible?

Tim I think this was about safety wasn't it? Given a choice of making a mistake and crashing in Bruce' Diasio or a 996TT, I'd pick the TT every time. I have some doubts about the engineering of crumple zones in the Diaso versus the 996. I could be wrong mind you.

One final consideration is the design of the braking system itself. Take a sedan, the MB E 500. The car will stop almost as quickly with 4 people aboard as with just the driver. The brakes are designed to cope with the added mass of passengers and baggage. The additional mass is offset by the additional grip afforded by the extra weight. On the other hand, a car with marginal brakes (a mini-van perhaps) would probably take longer to stop because the brakes themselves are not up to the task. The capacity of the braking system is not a factor in simple models which can lead to some surprises in the real world.

Rgds.

macnewma

RJay, you are certainly correct in that you can't accelerate the Toureag as fast as the Elise because of the weight. The problem in your argument is that the reason weight is a greater factor in acceleration is the fact that neither car has the ability to reach the limits of the coefficient of friction when accelerating.

Braking is a function of weight if neither car has adequate brakes to reach the threshold of the tires.

If you look at the fact that brakes stop the wheels (or nearly stop) and the tires stop the car by locking up or nearly locking up and generating friction. That specific coefficient of friction is the determining factor in how long it takes to stop (assuming your brakes have the power to reach threshold). When accelerating, except for the initial distance when the tires may slip, you are not accelerating fast enough to allow coefficient of friction effect acceleration.

The question is very simple here, with brakes that are adequate to reach threshold braking, what effect does additional weight have on a single run stopping distance. If we lose sight of this, by talking about multiple stops or if a braking system is not adequate, we won't answer the question.

Everyone agrees, that if you stop 100 times from 150 mph, the lighter car will win because the heavier car will fade due to heat (unless their braking system is adequately beefed up to handle the heat...yada...yada...yada). That is not related to this thread or question.

Bruce, I can't address the effect that downforce has because I know very little about it. Maybe it shoots holes in the theory. I think it would be interesting if someone could explain that in greater depth as it might help with the discussion.

Remember, every vehicle built (hopefully), regardless of it being a 400lb race bike, a 1900lb Elise or a 8000lb F350, the brake system, at the very least should be able to reach the threshold limit of the tires' traction. In other words, brakes are not the limiting factor for a single stop...unless too much weight is added.

I think the interesting point to be gathered from this discussion is that weight does affect the coefficient of friction between the tire and road...BUT to what extent does it affect the coefficient of friction???

CC, maybe your comparison of the Toureag and Elise is insiginificant because from 60 mph, the difference is not great enough to show. Maybe from 100mph we would begin to show.

BTW, lets keep this civil guys.

macnewma

By the way, this wasn't the safety/Elise thread started by CC. This was an offshoot of that. CC originally made the assumption that a lighter car will stop much quicker than a heavier car. It was then questioned. The discussion was led down a tangent and M758 and myself started this thread to discuss this very narrow topic.

I suggested we discuss this very narrow topic because I thought it may have been a common fallacy to believe a lighter car stops faster than a heavier car (in a bit of a vacuum given some contraints).

Vince5

Some contradictions there.
Can't agree with the 1st proposition in the case of 2 cars in every point identical except for the weight the heavier car will take longer to stop even with adequate brakes. It's all a matter of energy and tyre grip.

Total tyre grip is a fonction of the coef of friction of the tyre (depends of the make and of a lot of parameters like temp, pressure etc...) and load on the tyre. Max grip is reached at some point where the load is optimal given the tyres coef of friction, at that point if the load increases the tyres start to loose grip. In a turn you slide, when braking you lock your wheels or the ABS kicks in.

In 2 identical cars with only a weight difference and with ABS and adequate braking system it means the heavier car will load it's tyres up to the point where they loose grip and ABS kicks in faster than the lighter car. The heavier car will then have to spend more time at a lower braking threshold than the other one to dissipate more energy. Braking distance will be longer.

It's really the same mechanics as in turns and why lighter cars can corner faster than heavy cars.

macnewma

I think this is an interesting article and excerpt that I just came across:

This is appears to argue that in essence, a tire or set of tires has a specific, will always provide the same amount of grip regardless of the weight it supports. Unfortunately, I am just a layman trying to understand all of this. What do you guys think of this concept?

What do you make of this part:

The maximum acceleration a tire can take is , a constant, independent of the mass of the car! While the maximum force a tire can take depends very much on the current vertical load or weight on the tire, the acceleration of that tire does not depend on the current weight. If a tire can take one before sliding, it can take it on a lightweight car as well as on a heavy car, and it can take it under load as well as when lightly loaded.

ColorChange

Bob: I’m in pretty good agreement with your post, except you tended to not strongly support the primary point here … that a heavier car does not stop slower, or if you prefer, weight has very little affect on stopping a properly designed vehicle. Two exact same vehicles and strapping in 200 lbs will have almost no effect on stopping distance. At 1,000 it would probably have a small effect, maybe 5 or 10 feet. Obviously, in track situations, you want as little mass as possible because, and not just for brake thermal management, but that is not the point of this thread.

Max, good post but you missed a couple of big things. 1. The down force does nothing to change the FACTS I have presented (as opposed to the theory you called it). Read on for more explanation. 2. The data I showed shows exactly how the cf changes with weight. Look at it again, and 8% drop with a 440 lb weight increase. Also, if weight was important, the cars could not possibly stop in about the same time. You are absolutely correct, that if we stop from a high enough speed, one car will fade before the other, but that depends upon the level of engineering of the braking systems, and is beyond the point of this thread.

Vince, you are wrong if I read your post correctly. Identical cars with one weighing 250 lbs more than another, will stop in the same time (give or take a foot or two). The ABS system keeps the slip ratio near its optimum (10-15% slip) so who get’s there first is insignificant.

Max, your second post is largely correct and says that mass is unimportant in the traction limit of a tire. Now, if you really look at the cf of a tire as a function of load, it actually increases more than linear for a while, has a nearly linear range for a long time, and then as you continue to add load it gets more and more non-linear. Most cars and tires are sized to operate in the roughly linear range, thus the point of this thread:

Weight is insignificant in stopping distance (within limits and boundaries of course).

Larry Herman

Not that I really want to get imbroiled in this, but since everyone is espousing "real world examples" I was wondering about the following: In F1, the cars make so much power (over 750 hp) that 50 pounds or so really shouldn't make too much of a difference, and yet as the fuel load lessens towards the last few laps of a stint, the cars get noticeably faster. Sometimes it is more than 2 seconds per lap. Some of that has to be due to better braking and handling as the car gets lighter.