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tyre conicity - I want to understand how this is measured

tyre conicity - I want to understand how this is measured

(OP)
I understand some of the theory behind this but I'm struggling to understand how it is measured and calculated.

i am told that you measure the tyres lateral force in both directions of rotation which should give you a + and a -ve value. Then you average these to get the conicity. i.e +350 and -300 would give you 25N positive conicity. ply steer is then the averaged subtraction of these i.e 325N.

For steering pull on a vehicle conicity is deemed the more significant value and you want to minimise the conicity across the front axle. However if the conicity is an averaged figure and not related to the actual magnitude of the lateral force then surely it is only relevant to compare tyres against each other not the effective forces across 2 tyres on an axle.

I think I'm missing something simple hopefully someone will pull it all into perspective.

RE: tyre conicity - I want to understand how this is measured

I'm not very happy with your definitions (I think you have them the wrong way round), but don't have any reference books with me. I'll refer to one as the Lateral force independent of direction of rotation (LFIDR), and the other one as TOO. According to the web http://128.242.141.111/pub/technical/ITW.pdf
CONICITY=LFIDR

If the serial number is fitted outwards and the tyres are not handed (usual production case) then LFIDR cancels across the axle, TOO adds. Therefore when ordering tyres it is important that the LFIDR VARIATION between tyres is minimised, but that the absolute value of TOO should be reduced as far as possible.

However, more generally LFIDR is still a bad thing, as you steer the tyres the two vectors will no longer cancel and you will develop a moment.

Cheers

Greg Locock

RE: tyre conicity - I want to understand how this is measured

(OP)
Thanks Greg, The link is excellent and the very fact you have clarified conicity as the lfidr puts the light bulb on

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