ngoldrich

I am not a chassis dynamics engineer by any stretch of the imagination, but from a simplistic physics approach I would have guessed the following is true -

(1) If you use the same car for both tests to insure the same suspension, tire compund, etc...
(2) The car weighs 2000 lbs
(3) Run the car to say 100MPH and threshold brake. Measure the stopping distance
(4) Add 500 lbs as evenly as possible to the car
(5) Run the car to 100MPH and threshold brake. Measure the stopping distance.
(6) Since the momentum and interia will be higher with more mass, I would think that the additional energy would require a higher coefficient of friction on the contact patch to stop in the same distance.

Is that incorrect, maybe I am missing a key component to the idea..

Thx,
Norm

ColorChange

The formula I have shown for theoretical stopping distance does take weight into account as well as several other factors including braking efficiency, coefficient of road adhesion, rolling efficiency, frontal area of vehicle, drag coefficient, density of air, mass factor, and aerodynamic resistance. But I really don’t want to go there so instead, let’s look at actual tire data!



http://www.avonracing.com/tech/dload.asp

Here is the actual data taken off of one of their charts.

1.87/150 = 0.0125

3.01/250 = 0.0120

4.04/350 = 0.0115

So, the cf changes by 8% while adding 440 lbs to the tire (that’s like adding over 1,000 lbs to the whole car). Yes, the cf for a decent tire does change with load, but only a little like I said, not a lot like chart that Adrial showed. Maybe that data was from some lousy street tire.

Now for more real world data, and this is a devastating argument to those who don’t want to do the math.

A Volkswagon Touareg weighs over 5,000 lbs but stops in 122 ft. from 60 mph. (Motor Trend)

A Lotus Elise weighs under 2,000 lbs but stops in 114 ft. (also Motor Trend)

How is this possible if weight is important? 8 feet longer stopping distance for OVER 3,000 LBS of high center of gravity mass? Amazing isn’t it. As I have just indisputably shown, weight is not a significant factor in stopping a properly set up car.

Now, if you want to play games and put tiny tires on the Touareg and the Elise, it will take way longer to stop the Touareg (assuming they don’t explode form the load), but this is really not a proper/realistic comparison. Yes, adding the 1,000 lbs to one of our cars will make it stop longer, but not by a lot. The original point of this thread was my false assumption that the Elise would stop quicker and therefore be safer … and that was wrong. As just shown with the SUV/Elise example the mass of the vehicle is not a significant factor in stopping distance!


Thank you,


Thank you very much …


Thank you.




CC has left the building.

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ColorChange

Chris C: I am so glad I am able to repay the light you turned on in my head when you said "rotate the car under trail braking". Duh, but I hadn't thought of it yet.

Norm, the 500 lbs would probably cause the car to stop something like 5 or 10 feet further. Not too much.

ngoldrich

CC: Thanks for the info...

So if the car was 1000 lbs heavier, I would only obliterate one car in front of me if I tried stopping at the same point at the same speed :-)

Norm

TD in DC

I would buy this argument if both vehicles had the same braking systems and tires. All the makers of the performance SUVs know that they need to make those things stop on a dime, so they all have monster brakes. I bet that if you put the exact same brakes on the elise (and tires, since that is another crucial variable), the ligher elise would have an even shorter stopping distance. Another way you could do this is to take two Tourags, one in stock form and the other with everthing stripped out of it except what you needed to reach test speed and hit the brakes. I put my money on the lighter Tourag, and weight does matter. Comparing the stopping distances of radically different vehicles is not a very scientific way to determine the effect of weight on stopping distances. In the real world, weight matters. Hell, I stop 20 feet shorter before lunch than I do in the afternoon after I have eaten Summit Point's 100 MPH Chili for lunch.

Geo

Yes, if you couldn't figure out how to adjust your braking zone.

kurt M

CC, are you perhaps missing the possible idea that car builders are designing for the same approximate stopping point? The light car could be built with a shorter stopping point but this would take away from other aspects of the car. “All tire, no motor” That is to say after all is said and done and all the compromises are added in cars are all a bit the same in function.

Metaphysical mumbo jumbo time. Why do cars have 4 wheels? Because even if 3 or 5 wheels might have some pluses they would also have negative aspects that outweigh the positive for normal use. Cars are built to a human design constant for the most part. The heavy car has bigger brakes and tire contact patch to compensate for the added weight. Take the added weight out and leave the contact patch and brakes the same or even reduce the brakes some but leave the rubber the same. Yes, there will be less grip from normal force but there will be much less stored energy to ablate as well. Tires can be overloaded and the actual tire material will sheer away regardless of down or normal force induced grip. Think of it as regardless of how hard you grip the rope you skin rips away when it is pulled.


This is a great thread but you have not used an apples to apples example yet. Come on back inside and find one. No sneaking off before you clean up after yourself.

Geo

Actually, I'd bet you my next paycheck that the stopping distances would be longer.

Too much braking system for such a small car. The Toureg brakes wouldn't be efficient on the Elise.

TD in DC

I am not saying it would be a good idea, I am just saying that it doesn't make sense to reach conclusions about one variable in an equation by comparing disparate vehicles unless you have kept all of the other relevant variables constant. Also, I am assuming that all of the brakes in question have properly functioning ABS, so, the stopping distance of the elise might not be any shorter with the Tourag brakes and tires (IF the elise's stock brakes and tires were already as efficient as possible such that stronger brakes would not result in shorter stopping distances), but I cannot see how under any circumstances the stopping distance would be longer with the Tourag's bigger tires and brakes (unless the Tourags tires were less efficient than the elise's stock tires).

Premier Motorsp

Color,

If you are going to say 'significantly shorter stopping distance' then perhaps you are correct. And given that you'd initially asserted that the whole reason for making a car change WAS to significantly decrease stopping distance, then I guess what you have said, "Making a car change based on mass decreasing the stopping distance is foolish." is correct.

Indeed going back it looks like your assertions are correct. In this case the effect of load sensitivity non-linearity is not too important. However, it is still correct to say the lighter vehicle always stops in a shorter distance, everything else being equal.

BUUUUUUUT..... In the case of race cars, light cars (open wheel generally) always have softer compound tires, so they should stop faster based on this alone. Light cars also always have a better contact patch size to weight ratio.

My suggestion for a safe car would be a 993 Cup. Based on:

You can't beat an FIA certified cage and seating setup.

The car has properly designed crush zones and was not originally constructed to be incredibly light. (Reasonably thick sheet metal)

At 5 feet 11 inches you will fit in the car without being really close to the cage.

When properly set up the car is easy to handle and forgiving.

The car is fast enough that you will not be bored with it immediately.

The car rewards your style of 'correct' driving.

993 Cups are just plain fun.

I have walked away from two HUGE accidents in 993s.

996 Cup would be better yet except they are SO much faster.


Geza,

You are probably right. I am too lazy to look it up. Lucky for me, the addition of the 1/2 doesn't change the nature of the argument.

Chris Cervelli
Premier Motorsports

ColorChange

TD: No. Weight does NOT matter if you properly size the tires and related components as my example indisputably shows. If you add (or remove) 1,000 lbs to an existing car, stopping distance will go up and down respectively, but not by nearly as much as you would think.

Kurt: No way, engineers try to stop the car as quickly as possible, and the tire determines most of that. I am running from nothing. How about you show ANY data or fact that disputes what I have shown.

Geo
You are tire grip limited, not caliper/brake tourque limited. If you put the Touareg's brakes on the Elise, it would stop at exactly the same distance. It wouldn't stop 1" quicker! (assume the ABS works correctly, the brake pad cf doesn't change a the pads heat up, etc. ...). Now, if you are saying the ABS wouldn't match ... then you would be correct. But bigger pads, more pistons, etc. would do almost nothing.

Let's please not loose sight of the important point, vehicle weight is a minor factor in stopping distance in a properly set up car.

TD in DC

CC

My point to you is that comparing the stopping distances of an elise and a Tourag does nothing to prove the point you are trying to make. Also, the assertion that putting the Tourag's tires and brakes on an Elise would not result in a shorter stopping distance is based on the assumption that the elise's stock brakes are as efficient as they possibly can be. If the elise's stock tires and brakes are as efficient as possible, then I agree, it would make no difference (although I don't agree with Geo that it would be longer unless the Tourag's tires would be worse than the Elise's stock tires). If the elise's stock tires and brakes could be improved at all, then the brakes and tires of the Tourag probably would stop the elise more quickly.

ColorChange

Chris C: OK, we are in agreement. The lighter car will stop ever so slightly quicker only because it takes benefit of the very small non-linearity of cF as a function of Normal force (weight), or maybe a slight cg change. In the real world, pulling 200 lbs out of your car is usually a huge project, and you would be lucky to shorten your stopping distance by over 1 ft. It's there, it does work, but it's not significant. That's my point.

TD in DC

I agree with this.

ColorChange

TD, if showing a > 5,000 lb vehicle stops in the same distance as a < 2,000 lb vehicle doesn't show that weight isn't important in stopping distance, I don't know what does.