Why are large slip angles faster on low grip surfaces?

[adambrouillard]

My theory is that on low grip such as ice/gravel the longitudinal grip of a tire that is spinning is significantly higher than the lateral grip so using more longitudinal grip makes cornering faster and why 4 wheel drive is so much faster off road. I'm curious what the typical optimal slip ratio is.

Thoughts?

[TEAM78]

thats a massive question and very complex.
You are correct that longitudinal tyre to road interaction forces are greater than lateral in most cases. As for slip ratios, this is generally speed related, at low speeds its easiest to go of a speed difference then at say 60-80mph you look at an approx 6-8% slip difference, this number varies a lot depending on the interaction between tyre and road. I have little experience of running vehicles on ice so id be guessing there are are other factors coming into play.
As for large slip angles, serious consideration has to be given to tyre deformation, generally from plots I have seen i have seen nothing to indicate that on low mu value interactions (not surfaces as mu is dependent upon the interaction between tyre and road) large slip angles are benificial. i think its to do with F=ma and maybe playing on newtons law of action and reaction, note I say think that means im having a guess.remember your aiming to have the greatest resultant force which links straight back into a

[zac510]

Are you mixing vehicle slip angle with tyre slip angle?

[TEAM78]

Good point, based on Adam discussing longitudinal and lateral grip, im assuming tyre slip angles not vehicle is being discuused here.
i wonder when gravel surfaces will come into this discussion,

[tristancliffe]

On a fixed surface you can also take into account the need to maintain speed, so finding the optimum amount of force with the minimum amount of drag is good. On a loose surface you are still worried about generating high forces, but you're not so worried about drag as the surface moves. Indeed, as the gravel (heh heh) or whatever builds up in front of the tyre(s) it can add to the turning forces...

It's a lot more complicated than that I'm sure, but how much understanding do you want?

[phoenix]

Maybe I have got hold of the wrong end of the stick, but my understanding is that a given tyre will develop a higher slip angle for the same cornering forces on a slippery surface and a lower slip angle on a grippier surface.

In other words, if you have the same slip angle on a low grip surface as you had on a gripper surface, the cornering forces must be lower.

I would be pleased if someone would correct me on this.

[TEAM78]

Originally Posted by phoenix

Maybe I have got hold of the wrong end of the stick, but my understanding is that a given tyre will develop a higher slip angle for the same cornering forces on a slippery surface and a lower slip angle on a grippier surface.

In other words, if you have the same slip angle on a low grip surface as you had on a gripper surface, the cornering forces must be lower.

I would be pleased if someone would correct me on this.

Thats correct except what confused me is you didnt mention the reducion in vertical load on the tyre, Without a reducion in vertical load (caused by a lower lat G) then the statement isnt true.

[zac510]

Originally Posted by TEAM78

Good point, based on Adam discussing longitudinal and lateral grip, im assuming tyre slip angles not vehicle is being discuused here.
i wonder when gravel surfaces will come into this discussion,

Sorry my post wasn't directed at you but it would be good if the OP would clarify it

[phoenix]

Originally Posted by TEAM78

Thats correct except what confused me is you didnt mention the reducion in vertical load on the tyre, Without a reducion in vertical load (caused by a lower lat G) then the statement isnt true.

I still need help!

By cornering forces, I meant the combination of lateral G and vertical loading - these increase with each other don't they?

So, if you have a certain slip angle on a slippery surface, the lateral G and vertical loading must both be lower than for the same slip angle on the grippier surface. To put it another way, if the you increase the lateral G and vertical loading on a slippery surface, the slip angle will be greater than for the same loads on a grippier surface.

Is that correct?

[Big Dave]

I just find that if you can see tyre smoke from in the car (a real mini) to much angle!!!

[ubrben]

http://dspace.lib.cranfield.ac.uk:80...D%20Thesis.pdf

[Big Dave]

wow thats heavy reading that is!!!

[phoenix]

Originally Posted by Big Dave

wow thats heavy reading that is!!!

It is indeed - but the answer to the question at the beginning of this thread is within chapter 4!