Clarification on weight transfer

[mirial]

Please would it be possible for someone to clarify this one.

In general, vehicle books often state that when cornering, an increase in weight transfer from the unloaded side to the loaded side of the car will result in less grip (average) and an increase in slip angle.

Ignoring tyre deformation, how could this be? Assuming the car has some roll, surely it's the centrifugal force which is acting perpendicular to the car, and starting to take effect through the roll centre, and not the actual vertical loads being added and subtracted from the outside and inside tyres.

This could be further supported by the fact that typically with racing cars, if you increase ride height, you can decrease the slip angle by increasing the vertical load onto the outside tyres, resulting in more tyre rubber interlocking with the tarmac, assuming the roll stiffness has not been altered. The car would be more likely to break loose when maximum weight transfer exerts itself.

I think the best example of this I saw was with the Lancia Delta S4s when on tarmac, which used lots of ride height. This probably had to be done as the cars were very light (900 kg), used very wide tyres, and ran on narrow roads, so this could afford to be done. A setup for a road course would dictate that the car be much lower.

 

[NormPeterson]

Tire grip is somewhat less than a linear function of normal load. Double the vertical load while holding the other stuff constant, get less than double the lateral grip.

Tire deformation does matter.

There are at least three components of lateral load transfer that each cause variation in the tire normal loadings.

BTW, load transfer is the proper term. Weight transfer implies that part of the sprung mass is moving about (laterally, in this case). "Weight transfer" would be valid for contained fluids, unrestrained passengers, and loose cargo, but not much else.


Norm

 

[mirial]

Hi Norm,

Thank you for your response.

I really appreciate that this is a hugely complicated subject, so therefore I'll need to try and be as specific as I can.

Please would it be possible for you to clarify how you term tyre grip from slip angle? I think my comments I made about the increase in ride height/decrease in slip angle are valid to some degree, which is partly based on the observed examples of Delta S4 setups that ran with huge ground clearance on flat tarmac. This totally contradicts everyone's belief that a car should be as low as possible!



Thank you, Michael

 

[NormPeterson]

Near as I can define it, slip angle is a measure of the tire distortion required to develop some given lateral force (aka "grip") when there is a specific normal load. Plotted on a 2-D chart, it'll be a whole family of curves (that are representing a 3-D surface). There is a similar effect for longitudinal grip where the slip is defined as a percent.

I'm not at all familiar with the Delta S4, though it sounds like a tarmac rallye car where things like road crown and gutter profiles and required suspension bump travel following an airborne episode probably had some input to the design (in order to finish first, first you have to finish). If that's the case, then I'd have to say that it works as well through the corners as it does in spite of its overall arrangement rather than because of it.


Norm

 

[mirial]

Your assessment of the Delta S4 makes sense.

Many thanks for your help.

Kind Regards,

Michael

 

[GregLocock]

Typically if you double the vertical load on a tire the maximum lateral load it can generate will only increase by say 70% (it varies a lot for various reasons, 50-90% would be the typical range). In that case the inside wheel would be unloaded, and so the maximum cornering force on that axle has dropped from 200 arbitrary units to 170, that is, a 15% drop.

Raising the cg of a car is always a bad thing to do, so they must have done it to avoid something worse. On public roads it is essential to have fairly soft (less than 3 Hz say) suspensions, otherwise you just won't get grip on typical road profiles (potholes, kerbs, patches etc). So that sets a minimum ride height.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[mirial]

Hi Greg,

Thank you for your comments.

You stated that in your example that there would be a 15% drop in cornering force available; I take it one of these reasons is because that the rubber of the tyre would not compress in a linear fashion when loeaded?

Is it in any way possible to have tyres that are underworked, even when fully loaded? If a tyre creates grip which is biased towards mechanical friction, it might be possible to have tyres that are too wide, where the contact patch (load over area) is not creating enough mechanical interlocking in the road surface?

If you take a look at old racing cars from say 30 or 40 years ago, you will notice that the front tyres were very much narrower than the rears, typically because the weight distribution was about 65% rear 35% front. If you were able to measure the depth of tyre interlocking under all loads, it would probably be similar for all four tyres, even though they are of very different widths.