TD in DC

It would if they had the same braking systems and tires, but they do not. What it does show is that engineering and money can overcome the detrimintal impacts of weight. If you took a stock elise and added 3,000 lbs of lead, it will take a lot longer to stop than the stock elise at 2,000 lbs. Of course, the elise wasn't designed to stop a 5,000 lbs vehicle, but if you retrofitted the Tourag's system to the elise . . . Oh well, you get my point I hope.

Put another way, there is no way I would want to retrofit a Tourag with the brakes and tires of an elise If you want to make a point, you need to compare apples to apples. I was only pointing out that you were comparing apples to oranges . . .

Vince5

I am with you TD. A given braking system (brakes, wheels and set up) is able to dissipate a certain given amount of energy, increase the weight and you increase the energy to dissipate so the brakes/tyres will have to work longer. That's as simple as that if you don't want to venture into tyres CF and no linearuty of CF to weight transfer.

Of course the braking system of a touareg given it's size is much better as dissipating energy than the Lotus's (and that's rather fortunate) so in the end the braking distance of the 2 cars in stock trim is pretty similar. But that's not the point for us track drivers, we just want to lap as fast as possible and there weight is key...

Geo

Very simple (and an excellent example of how you have to look at the whole package). Brakes require heat to work efficiently. I'd bet big money the Toureg brakes would not warm sufficiently to work effectively on the Elise.

ColorChange

Vince: We are ignoring heat dissipation, just talking stopping distance. You're highly likely to be wrong about heat dissipation though. The Touareg's brakes will abosorb much more heat (because so much more heat is generated due to the high mass), but they are very unlikely to dissipate the heat (mostly cooling air) than the Elise as the Elise is intended to need this, the Touareg is not.

Geo: Very possible, but then I would need to see the cF/temp curve of the pad material.

Again, don' t miss the point ... vehicle weight has little to do with the stopping distance of a properly designed vehicle.

Geo

We're only talking brakes here. See above.

This stuff can NEVER be taken in isolation. The real world is far more complex that what we sometimes try to make it. Trying too hard to isolate variables is how major mistakes get made sometimes. And to be fair, it's also how major problems get solved sometimes, but always, a critical analysis is necessary.

TD in DC

Geo,

I was not trying to suggest that it would be a good idea to put Toureg brakes on an elise, or that engineering analysis should be based on anything less than viewing the whole picture. My point was that you need to compare apples to apples, and I did not believe that comparing stopping distances of a stock Toureg to a stock elise was a fair comparison, or proof by any means that "weight doesn't matter." They are two completely different vehicles, and the most you can say is that engineering and money can largely overcome the detrimental effects of weight. Hey, now that I think about it, that statement could be the tagline to the "Story of Porsche's 911" and how performance can improve despite an ever increasing waistline. In any event, I don't think there is as much disagreement here as the point-counter-point my suggest.

TD

Geo

Tim, I assume you are an engineer. Sometimes you have to step away from the formulae and the math and play around in the real world a bit to have a better understanding of the formulae and the math.

The Toureg is a much bigger vehicle and the braking system is designed to dissipate a great deal of heat as a result. The Elise, being much lighter, will not generate as much heat in the braking system (if you somehow adapted the Toureg brakes to it) to work efficiently.

Just ask anyone who has driven with race or even street performance pads that have not been warmed up properly. It can be downright scary.

I'm all for trying to isolate the effect of components, but you can only understand that effect IMHO if you consider it's place in the system as a whole.

Just think things through a little more. My point was not to make a big point, but to point out that you can't just make assumptions.

Geo

1) The point was made that the Toureg brakes on the Elise would stop the Elise quicker. My point was that it almost certainly would not and that is because you have to consider the environment in which the components need to work. It's not as simple as just bolting on bigger brakes.

2) I submit that in the context of Tim's argument, it is indeed a fair comparison, exactly for the reasons you mention. Engineering applied properly can overcome certain disadvantages. If the point is, how quickly can you stop a given vehicle from a given speed, then in the context of this discussion, all engineering means necessary are open to solve the problem. Said another way, Tim was looking at braking distances for a variety of cars and the net net was that weight was not a major factor in the result. That's not to say it doesn't factor into the engineering.

Now, having said that, I personally do think weight will influence stopping distances, but not as much as we might think sometimes. I think the example of the pick-up truck empty and loaded down is not a fair one in the context of Tim's evaluation of cars. If I can reengineer the braking system of the loaded truck to account for the greater weight, I'll be it could be make to stop in pretty near the same distance, and from what I've followed of this discussion (despite the tangents it has taken to discuss certain points in isolation), that is the crux of the matter.

I apologize for getting so long winded.

macnewma

I think part of the problem is stopping distance (assuming a single stop, not worried about fade) is not a factor of the braking components ability but of the coefficient of friction between the tires and the ground. That is assuming the brakes themselves are up to the task of applying enough force and dissipate enough heat to overcome enough of the wheels inertia.

Brakes stop wheels. Tires stop cars.

Brakes will affect stopping distance if they can no longer slow the wheel to the threshold of locking the wheel. At that point, which is usually a factor of heat dissipation, you are no longer extracting the ultimate coefficient of friction from the tires. The tires are no longer the limiting factor.

Now weight has a huge impact on the energy required for the brakes to slow the wheels. That is what Chris is talking about. That is why a road bike can have tiny little brakes yet still lock the wheels (2 brakes for about 150lbs). That is why 911s over the years have received progressively larger brakes as the car has gained weight. Brake sizing also has to take into account repeated use but that is a whole different discussion.

The question at hand is not if the Elise or 911 has a better braking system than the Toureag. The question at hand is if you take two identical cars with identical tires and add weight, does the heavier car have a longer stopping distance. That is the question that CC has answered with his formula above.

From what I have gathered, additional weight does increase stopping distances. The catch is that it increases them very minimally (although to some this may be a huge difference). It is ultimately a factor of the tires and the coefficient of friction does not change greatly with additional weight.

I think the disconnect here is that we are arguing to two separate points. The initial question was what I stated above. It doesn't matter if you have brembos or if you have drum brakes from a 1965 F100 truck. If they can lock em up, the tires are the limiting factor.

At the same time, the quality of the braking system is still exceptionally important but for reasons beyond single stop stopping distance. We all know that we would rather use the brakes on a 911 than a Toureag. But not because we would stop faster.

TD in DC

Max,

I agree completely. With respect to CC's initial question, weight should be a consideration in picking a track car, but a lighter car will not necessarily be a safer car (far more variable need to be taken into account). The whole package needs to be taken into consideration.

TD

Vince5

I was not talking about heat dissipation, just the amount of energy created by the car in motion being higher the heavier the car is. I was not refering to cooling this is another issue.

With the same breaking system (wheels, brakes, geometry) a heavier car will on paper require a longer braking distance at max braking force given the KE that has to be dissipated. that's the way you should look at it from a track driver perspective. To us in the track driving forum the comp between the Elise and the Touareg is not entirely relevant IMHO.

macnewma

TD, agreed. It is very difficult to determine if one car is safer than another car. The number of factors that go into safety are phenomenal. It really is testament to the research that manufacturers like Volvo and Mercedes did for the vehicles that moved safety forward.

One thing that I think is interesting is the enormous effect weight has on cornering versus the minimal effect weight has on braking distance. I think discussions such as this are good to have because it is very easy to assume false conclusions based on "common sense" physics.

Sanjeevan

Now, if there is any mathematical models available to quantify "safety", would we be crashing cars to test them,...the models are imprecise and the subject (the human body) is too sensitive...The only conclusions we can draw from this is,
Lighter cars do not necessarily stop quicker....

kurt M

If you lower the CF will the lighter car have any advantage?

This is an interesting question but it is not the one that CC needs to ask. CC wants to increase his safety factor and merely looking at braking distance is not the way to go. As was said you need to look at the car being used not just one of many factors. I agree that a factory built late bodied race car is best. I will give you this, a street TT is not the safest ride from an energy storage point of view. On the other hand a 912 with low hp but also archaic crumple zone design or many other modern improvements is not ether. Just slower...

trumperZ06

Guys, have we "lost sight" of the application?

No one runs ONE CORNER at 60 mph. Slams on the brakes to stop, and then goes into the pits !!!

IMO... on the track...

The lighter car will out-brake the heavier car over a 30 minute session... all else being equal !!!