[Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynamics

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Itogliano
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[Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynamics

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I was reading an article on racecar-engineering.com about tire dynamics (full article https://www.racecar-engineering.com/tec ... -dynamics/).

The last paragraph under the section discussing CoF says, "Imagine the contact patch as a matrix of discrete elements. A wider tyre reduces the contact pressure at each element for a given vehicle weight which increases the CoF." Wouldn't the contact patch have to increase in order for the contact pressure to reduce for each element? A wider tire (all else being equal) would change the shape of the contact patch, but the overall area would remain unchanged. Am I looking at this correctly and that statement is incorrect, or is there something I am missing?

The other line of reasoning I come up with, assuming the statement is true, is that a wider contact patch would mean each discrete element in the patch is displaced a less amount (i.e. the tire stays "rounder") which certainly has other benefits, but I don't see how that would lead to an increased CoF.

Thanks in advance

Greg Locock
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Re: [Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynami

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Yes it does smack of 'lies we tell children'. If you have a wide tire running at a given pressure and vertical load, and make it slightly narrower and keep the sidewall height and stiffness the same and make the rim narrower so we don't flex the sidewalls (see how quickly the ifs and buts pile up in real engineering) then a black box regression/physics based model says the lateral dry grip peak drops at that load, partly because the narrower tire has a lower T&RA rated load, as shown in the article, mu drops with load, and indeed load/T&RA load. The contact patch is longer, or at least linear range pneumatic trail increases. The vertical stiffness drops as well.

If you run the two tires at the same proportion of T&RA load then the difference in mu virtually disappears.

gruntguru
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Re: [Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynami

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No doubt the article is assuming the wider tyre is run at a lower pressure.
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Greg Locock
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Re: [Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynami

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Which would also usually get a compound change as well, which directly affects mu.

So as articles go, rather a lot of smoke and mirrors and don't look behind the curtain.

gruntguru
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Re: [Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynami

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Yeah, I long assumed the grip benefit from wider tyres was entirely about the ability to soften the compound courtesy of reduced shear stress but the fact that mu increases with reducing contact pressure is new to me.
je suis charlie

johnny comelately
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Re: [Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynami

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From elsewhere in the forum
http://mccabism.blogspot.com/2019/06/fl ... ffect.html
highly informative and well worth the read imho.
Extract:
'High-performance' racing tyres, of course, are something of a world of their own, and tend to use carbon-black as a filler in high concentrations because it increases hysteresis (i.e., viscous dissipation) and grip. One can find statements in the academic literature such as the following:

"For a typical rubber compound, roughly half of the energy dissipation during cyclic deformation can be ascribed to the agglomerated filler, the rest coming from [rubber polymer] chain ends and internal friction [of polymer network chains]," (Ulmer, Hergenrother and Lawson, 1988, 'Hysteresis Contributions in Carbon Black-filled rubbers containing conventional and tin end-modified polymers').

Given the higher concentration of filler in a racing tyre, one might expect more than half of the energy dissipation, and therefore the frictional grip, to come from the agglomerated filler.

And now comes the interesting bit. Filled rubber compounds suffer from the 'Payne effect'. This is typically defined by the variation in both the storage modulus, and the loss modulus (or tan-delta) of the tyre when it is subjected to a strain-sweep under cyclic loading conditions. (The storage modulus is related to the elasticity or stiffness of the material, and the loss modulus is related to the viscous dissipation).

Greg Locock
Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: [Vehicle/Tire Dynamics] Please help me understand this concept from a racecar-engineering.com article on tire dynami

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Mu increases at low psi (partly) due to cogging. This is the mechanical interlocking/microwelding between the rubber and the road surface. It doesn't need pressure to start happening.So if you plot lateral force vs vertical force the lateral force starts at non zero. If you then divide Fy by Fz this gives a falling characteristic to the mu curve.