tyre/rims

[cbrown@xxxxxxxxxxxxxxxxxxxxxxxx]

re tyre size/rim widths

the old rule of thumb was that rim to tyre ratio should be 75-85%. This gives
appropriate support to the sidewall. Too wide a tyre on any given rim width
causes the sidewalls to balloon and the tyre to 'wobble' back and forth on the
rim under cornering loads, the bottom line is too wide a tyre reduces steering
accuracy and feel, in effect the slip angles are affected (adversely)

185, on 5.5= 76%, on 6=82%
195, on 5.5=72%, on 6=78%

modern tyre are perhaps more tolerant, and looking at tyre fitting standards,
indicates that a number of sizes are 'acceptable' fittings. The 'design' rim
width for a given tyre is the parameter in question, ie that which provides the
crispest steering/handling.

Overly large tyres also increase steering weight, and significantly affect the
breakaway characteristics. Yes they have more ultimate dry grip (less wet
grip...), but instead of tending to transition into slide mode progressively,
the breakaway point (although higher) when reached, occurs much more suddenly,
ie oversteer becomes much more drastic,  the chassis effectively less catchable. 
Road car, and track cars will accomodate this characteristic differently. 

perhaps those that really know tyre technology can chime in and sort out some 
old chestnuts. Just because it fits, and maybe looks mucho, does not make it
functionally desirable. 


regards 

chris brown

Les, pauline is yours if you want her, we (the rational majority) certainly
don't

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