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tire lateral loading

tire lateral loading

(OP)
This is the first question I have posted on this forum. Not sure if I'm qualified to be here. Background: BS in computer science. I started off as a mechanical engineering major but switched to CS somewhat short of my junior year. So I have enough knowledge to be dangerous. I also spent about 30 years in an aerospace engineering environment (so I claim to know how to talk to real engineers).

Right now I'm interested in the lateral loading of tires during cornering. If you draw a FBD of a simple axle under lateral load, the lateral force contributed by each tire is statically indeterminate, dependent on tire sidewall lateral stiffness. I'm willing to assume this stiffness does not change very much with variations in vertical load due to weight transfer (bad assumption?), and thus, up until the limit of adhesion of the inside tire (<< less than the ultimate cornering load), both inboard and outboard tires contribute roughly the same lateral force.

Is this correct? It seems somewhat counter-intuitive.

P.S. I'm trying to determine the worst-case moment seen by wheel bearings given varying wheel offsets.

RE: tire lateral loading

For a first approximation, you are likely not far wrong.

If you want better than a first approximation (probably not needed for what you are trying to do?) the problem is that every tire size and profile and brand will be subtly different and if the vehicle owner changes (or neglects ...) the tire pressure, it changes again, because the lateral stiffness is dependent to at least some degree on the tire pressure. And all it takes for all your sophisticated calculations to be thrown out the window is for the car to hit a bump while rounding a corner ... or slide sideways into a kerb. Too many variables for what you are trying to do.

I'd just go with your first approximation and see where it takes you.

RE: tire lateral loading

tire lateral compliance when defined as rate of change of pneumatic scrub radius with Fy is a complex field (it actually changes sign at low Fz), but I doubt that stead y state cornering is the most stressful load a wheel bearing sees, more likely kerb strikes are the killer.

Cheers

Greg Locock


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RE: tire lateral loading

I calculate the potential traction available at each tire and assume the amount of resistance that each tire is generating to counter the centrifugal load is proportional. This seems to work great in my simulations, up to the point where the inside tire reaches saturation.

RE: tire lateral loading

It depends, too, on the function of the vehicle. I design a sports type car for 100% lateral load transfer to the outer wheel (we've all seen race cars lift their inside wheels, front or rear). Street cars don't see this lateral weight transfer (unless your teenage son is driving).

You have to add some dynamics to the loads also. This is very indeterminent - 3g's, 6g's?

As mentioned, tires are manufactured to provide certain cornering forces, noted previously as being defined by running slip angles, and these vary all over the map and are not necessarily proportional to tire vertical loading. Firestone will only share their racing tire dynamics with those who pay large for their tires. But in the end, you can figgur tire cornering forces at, say 1.0+ g's plus dynamic forces.

Increasing tire offset of course increases the bending moment on the bearings which may counter or increase the moment due to cornering forces, depending on the direction of the offset.

Finally, bearings are usually designed for a certain service life under a realistic service loads (depending on if it is a granny car or a Mario Andretti car) and not necessarily for ultimate loads - usually (and I accept challenges to this) a suspension component will fail before a bearing ruptures from overloading. Bearings generally fail by wearing out, prematurely or otherwise.

And yes, you are qualified to ask all your questions here.

Bob

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