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# CoF in similar tyres with different loads

## CoF in similar tyres with different loads

(OP)
Morning,

I read recently that the coeff. of friction of a tyre to road interface will decrease with increasing mass of the vehicle. If it didn't then any vehicle of any mass running similar tyres would, with all else equal, be able to hold an equal corner speed. Common sense says not to me, my question is why? What assumedly microscopic action takes place at the interface to reduce the value of mu? Or is it macroscopic and am I missing something obvious? Bear in mind i'm not talking about straight line accel or braking distance, just steady state cornering. Disregard any aerodynamic effects too.

thanks,

Kurtis.
Replies continue below

### RE: CoF in similar tyres with different loads

No answers - OK - I'll give this a go with the caveat that I'm not an elastomers engineer and whole books are dedicated to understanding just parts of this.
For constant slip and camber angles, the coefficient of friction does decrease with increasing load.  You are right that there are two general principles at work - macro and micro.
In the macro view, the tire distorts under greater load and alters the tire contact patch shape, pressure and slip distributions.
In the micro view, the coefficient is comprised of two components - adhesion & hysteresis.  The adhesion portion is inversely proportional to the pressure (temperature).  This is also the result of molecular bonding and the strain cycle (bonding, elongating, rupture).  The higher the pressure and temperatures, the easier they are to rupture.
The hysteresis portion is associated with localized tread deformation. A differential area deforms and absorbs a certain amount of energy. This generally absorbs less energy than adhesion.
Kevin

### RE: CoF in similar tyres with different loads

(OP)
Kevin,

good answer. The adhesion portion is I think what I was looking for. I wondered if the contact patch would make a difference and couldn't find a definite answer, so thanks for that too.

At least I know the article was factual.

thanks,

Kurtis.

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