ColorChange

TD Hooray!!!! That is the point. Cupcar, yes, the stopping distance does go up, but you need to add ... but not by much! (the whole point of this thread)

One other intersting point, when you load the front tires in braking, the cf goes down, but the rear tires are unweighted and the cf goes up in the back. This further reduces the differences you would (intuitively) expect in stopping distance.

Vince5

Fine I can live with that

PTskater944

Please explain this. You say cars stop fastest when they are sliding 10-20%. Technically, a car can not stop at all if it is still sliding. It has to stop sliding and regain traction, or else it will never stop. I don't understand what you mean when you say cars accelerate faster under 50% slip, because first of all, I've never seen amount of slip expressed as a percent before, and second, it doesn't seem logical. If the wheels are slipping 50%, wouldn't that mean that only 50% of the cars power is getting to the ground. How will that make you accelerate faster than a car getting 100% of its power to the ground (0 slip)?

And yes, I do have more to learn. I've been looking at these forums for about a year, and my post count is only at 29. I try to only post when I know what I'm talking about, or when I'm asking an important question.

Cupcar

Great question! I wondered the same thing.

Here is my thought on it: Torque is what accelerates or decelerates the chassis. This torque is applied on a lever that originates at the axel center and extends to where the tire touches the ground.

The amount of torque that can be applied is related to the coefficient of friction at the ground, the higher the coefficient the higher the torque that can be applied in either acceleration of deceleration.

Looking at the graph in my prior post note that I circled a point on the highest coefficient of friction which occurs at around 8% relative slip. This relative slip heats the tire and increases the coefficient of friction. Energy is lost but the torque transmission is improved and acceleration or deceleration greater.

ColorChange

PT, cool, I'll try to help. Cupcar, I think you'll find this explanation helpful.

There are two different terms used to describe tire function, slip angle (for turning - lat g), and slip ratio for accelerating and decelerating (long g). For slip angle, the tire acheives its maximum cornering force at about a 6-8% slip angle of the tire relative to the direction of travel of the car. If you look at the actual race tire data I posted, it shows how the cornering force changes as a fuctnion of slip angle. Street tires often have a wider range of optimal slip, may 6-10% or een 12%. That is why you can normally be fast on streets throwing the car around and kicking the back end out quite a bit. On r-compounds, you have to be quite a bit more tidy.

There are similar curves for slip ratio (in this case, how fast the tire moves relative to the speed of the car). In this case, the optimal braking force is usually in the 10-20% slip ratio range before a big drop off, while with acceleration, the slip ratio may not drop a lot until 50%.

You just have to be sure you keep slip ratio and slip angle separate.

M758

Ok guys this is getting a bit out of hand here.

So lets sum up.

Firstly all agree that lighter cars do stop faster (lets not get into how much right now).

The reason for it is non linear cF propers of tires.

Now CC is arguing this non linear aspect results in minor change in distance. Others are arguing it is a major impact.


So what is it? Well it all depends on the tire. The graphs from the 25 year old books are accurate in concept. They probably don't represent modern tires. However that basic behaviour is there. So the impact of a change vehicle mass can only be determined by tire testing to determine the real cF curve for that tire over it proper operating temp, pressure and load range.

Lets also rememeber that this is a purly academic discussion. Reason is that well all know that on a track weight is bad. That is plainly obvious just like weight is bad for braking on a track. At the track you never make just ONE stop.

macnewma

You mean this isn't a drag racing forum? Where am I?

Geo

Why?

I think it does illustrate Tim's point. What I am getting from this is Tim's point is that you can compensate for the heavier vehicle and get it to stop just as short as a lighter one.

You and many others want to restrict this discussion to the single variable of changing only the weight (mass actually). While that is an interesting discussion unto itself, the discussion of braking distances of vehicles of different mass leaving all variables open is also an interesting one.

In the case of Tim's pondering buying a car of one mass or another, the discussion came up of braking distances. For his purposes he has found that mass does not seem to affect the braking distances given the ability to choose different braking components.

Geo

Ok, Tim has explained it some. I'll go on to say that this is one of the reasons you cannot simply use Cf to describe the behavoir of a modern tire. Text book physics of Cf of static vs sliding simply do not apply to a modern tire. It's more complex than that and there was a very complex discussion of this fact in this forum a while back.

TD in DC

George,

I think that you have just hit upon the reason why there seems to be unnecessary disagreement. You and others are linking this discussion directly to CC's question about car safety. I and others apparently thought that this new thread was started to separate out the weight (mass) question from Tim's car search, otherwise it could have continued there. Here was the original post:


Both questions are interesting, but if you are trying to answer different questions, you are bound to disagree a little over the answers. You know what they say about arguing on the Internet . . .

On that point, why did I feel like I was in the special olympics when Tim said "Hooray TD" to me?

TD

ColorChange

TD, I think that's a slam on me but it's too funny to get mad about.

TD in DC

No slam . . . more a hopefully humorous comment on the strange phenomenon of Internet communications. When you wrote that to me, the following very, very UN-PC picture came to mind . . .

Cupcar

This is getting funny.

The title of this thread is "How does weight impact car braking". One would think that this meant what it said.

Weight is a force=mass x acceleration of gravity. Weight is what the tire contact patch "sees" and I thought this was the discussion point.

Obviously, the discussion centered around whether or not two cars of differing mass could be designed to stop in the same distance from a given speed given unlimited braking design variables. I missed that point somewhere along the multiple pages of the discussion.

All this thread proved is the last point made above: "Internet discussions have their limits".....

ColorChange

CupCar "How does weight impact car braking" ...the answer is "not much" - (with normally engineered cars, and within reasonable weight limits).

M758

Tim,
Don't forget that this discussion was started off by producing the physics equations showing that weight played ZERO role in braking. Knowing that not to be the case I got want to prove that to be an incorrect use of physics. It has clearly shown to be the case that car mass plaing zero role in braking is flatout wrong. We have all agreed to that. Your calcautions seem to show that while mass does impact braking that precent of that impact is small.

I think this discussion is over with the salient point being answered in academic fashion as appropriate for an academic topic.