Load Sensitivity Effects On Adhesion Of Pneumatic Tire

[JonnyK]

I was wondering if anyone could help me to understand the behavior of the automotive pneumatic tire. In particular I am interested about the load sensitivity. I have read various sources on the subject but have yet to find a satisfactory answer.

I understand that the two primary means by which ‘traction’ is generated are adhesion and deformation:

*Source:

http://www.insideracingtechnology.com/tirebkexerpt1.htm

Adhesion is the more predominant mechanism on dry smooth (i.e paved) surfaces. Deformation on the other hand is critical for traction on rough and/or wet surfaces.

What I don’t understand is why increasing the load on tire when adhesion is the predominant traction mechanism results in a reduction of the effective coefficient of friction (eCf). It seems logical to me that increasing the load on the tire should increase the contact patch to some extent and also increase the load per unit area on the contact patch. Both of these seem desirable for increasing the eCf of the tire. So why then does increasing the load per unit area on the contact patch reduce the eCf when adhesion is the predominant mechanism for grip?

TIA

 

[crysta1c1ear]

I'm no expert, I just read the same article as you.

Taking the picture on the right, if you double the load, it would increase the area in contact. But it wouldn't double it. So you wouldn't have twice the area for the adhesive effects. So you wouldn't have twice the advhesive force, and that would mean the coefficient of friction had dropped.

I assume the explanation is of that nature.
Your assumptions of increasing area etc seem right to me, just that the areas don't increase as much as the force that caused them. So friction would go up, but coefficient of friction would drop.

(This is a layman's view of things, not a rubber engineer's view of things. Sorry I cannot resize the image, but this doesn't seem to be an HTML based BB interface.)