Lateral Braking Force At Tire Patch

[sbozy25]

Perhaps this is a rather elementary question, but for some reason I am being forced to second guess my self.

I am relatively green to the vehicle suspension industry, and am trying my hardest to learn as I go. I will be attending some seminars in a few weeks that will help, but until then, I am on my own.

Currently I am working on a project that has a dual suspension system at the rear of a vehicle, I am only focusing on the rear, not the front as that will be another company. As I am designing my components and trying to cut some cost out of what was already there, I was asked by my boss to back everything up with FEA's, which is fine by me. However, one of them is giving me fits to understand and he is so busy, I really don't have much of a chance to ask him. He told me to run an FEA for each axle as a result of the braking force at the tire patch on the road. I was told it would be easy to find and get that information, but for some reason I am struggling. Now, I know the vehicle weight, the cg, the traveling speed, all the geometry, even the tire make and model.... Am I mistaken, or is this just a comples dynamics problem that requires me to transfer the weight and speed into a force then free body it back to the point of concern? Or am I missing the point...

I wish I had diagrams to show, but I don't and wouldn't be able to because of the sensitivity of the project. But any help you can give would be appreciated.

 

[BobM3]

I think you mean "longitudinal" force. You need to know how fast the vehicle can stop. Having that info, you can calculate the longitudinal acceleration and plug that into the free body diagram for the car (F=ma). You'll have to estimate the brake force split between front and rear. I'd guess the front brake force is 60%-70% of the total brake force. Others on this site may have better numbers.

 

[GregLocock]

In a truck I think you'd have to run a higher %age at the rear - in fact given the occasional jack knife incident I'd suggest that the brake proportioning is not done especially well, and the rears are occasionally asked for >100% of their capacity. Of course with ABS this will happen as well.

So a safe assumption for strength in braking is that if you use Fx=Fz*mu, and mu is probably no more than 0.7 for a truck tire, 1.2 for a car tire.

BUT braking is not your biggest longitudinal force at the contact patch. Potholes and the like are.



Cheers

Greg Locock

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