Tire data/tire behaviour

[HenningO]

I came across something today that I need some help understanding, I was looking at how different amounts of camber affects the longitudinal force a tire can produce during several slip ratios but with a constant normal force.

What I've seen on the 13" hoosiers and avons (haven't looked at any other ones yet). Is that the tire seems to be more sensitive for camber (ie. lower longitudinal force at same slip ratio) when the slip ratio is positive (ie. accelerating). Why is that? Is it due to the carcass design of the tires? Any thoughts?

[HenningO]

I came across something today that I need some help understanding, I was looking at how different amounts of camber affects the longitudinal force a tire can produce during several slip ratios but with a constant normal force.

What I've seen on the 13" hoosiers and avons (haven't looked at any other ones yet). Is that the tire seems to be more sensitive for camber (ie. lower longitudinal force at same slip ratio) when the slip ratio is positive (ie. accelerating). Why is that? Is it due to the carcass design of the tires? Any thoughts?

[ben]

The brush model of tyre grip predicts a slight increase in lateral potential with combined braking than combined acceleration. This might be a similar situation.

Ben

[HenningO]

Ok, I'm using the Milliken model. So this would just be a phenomena related to the model and nothing that could be observed in real world?

[ben]

The brush model is the simplest physical model of how force and moment characteristics result from a given friction level and carcass stiffness.

The Milliken ND tyre and Pacejka formulae aren't really models in them same sense - they're curve fits. I'm not sure about the Milliken approach (Mr Kasprzak is the man to ask) but I'm fairly sure Pacejka includes this effect in its combined slip model.

Ben

[VinceL]

Quote:
"So this would just be a phenomena related to the model and nothing that could be observed in real world?"

If you want to check that you could always make plots of the raw data included on the DVD. It takes some time at first to make a small Matlab program that takes in the data then outputs the plots you want. But once you do it you can use it over and over again to make many interesting plots to compare to the models.

As for your question about the MRA model I don't know the answer, but I will echo Ben's comment Dr. Kasprzak would be the best person to ask.

[Christopher Catto]

I would argue that it is related to the amount of available grip the tyre gives at different longitudinal slip ratios.

You will find in RCVD (page 488 or thereabouts??) plots of normalized braking and accelerating forces. The plots are quite different, the one under braking being a much smoother curve. So say at braking SR = -0.4 the available grip is quite different from acceleration SR = 0.4. But these are plots from real world tyres with their asymmetrical construction.

If you think of the problem with the "principle of superposition" (which I dare say is wrong!) maybe this will point out at why grip in corners may be better under braking rather than acceleration. In theory though the cases should be the same if the tyre construction is symmetrical, but some asymmetries probably are present simply due to the loading case and I think the brush model may show these up.

[HenningO]

Left RCVD back at the office so I'll look into it tomorrow. I should also point out that I'm looking at zero slipangle.

I'm waiting for Dr. Kasparzak to get back to me.

I'll take a look at the raw data and see if it's worth the effort.

Christopher, there are some differences between maximum avaliable longitudinal force between acceleration and braking. At zero camber they are less then 1%. Something I would say is within the error margin of the test. But at 3 degrees camber, it's almost 10%. Too much to ignore IMO