UK Karting - Notice Board (General Karting) : Re: tyre pressure altering gearing?

Notice Board (General Karting)

Re: tyre pressure altering gearing?
Posted by 'SimonS' on 26 Jun 2009 @ 11:23

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SimonSJoined: December 2004
Total Posts: 4
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"obviously the tyre would have a much bigger rolling diameter"

It isn't that obvious after all.

I've just measured the circumference of a (worn) cold rear Le Cont.(11 inch diameter)

At 10psi it was 853 mm
At 20 psi it was 858 mm
At 30 psi it was 860 mm
The answers with another tyre might be completely different, it isn't unknown for 2 new tyres to differ by more than 1 cm circumference at the same pressure.

Worn tyres will differ by tread depth (1mm less tread will be 3.2 mm circumference) which could be the equivalent of 10psi tyre pressure.

The gearing difference of course will be the difference between the two circumferences, approximately 7:853 between 10psi and 30psi

7:853 is a change of 0.0082

A change of ratio from 20/70 to 20/72 is 0.0079

So, on an 11 inch tyre increasing the rear wheel circumference by 7mm (from 10 to 30psi) is almost the same as 2 teeth on the rear sprocket.

http://www.russellkarting.com/settings.htm suggests that a gear change is called for with 12-15 mm of change in circumference

What the change in pressure does to the tyre performance is another question.

Message Thread:

tyre pressure altering gearing? by 'JCC' (26 Jun 2009 @ 8:08)

Re: tyre pressure altering gearing? by 'SimonS' << You are here!

Re: tyre pressure altering gearing? by 'Hi99' (26 Jun 2009 @ 11:56)

Re: tyre pressure altering gearing? by 'SimonS' (26 Jun 2009 @ 20:28)

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