Article on The Internet Mini Encyclopædia
IMPORTANT: We do not warrant the effect of any changes to the tyre pressures profile size etc etc on your mini AWAY from those specified by the manufacturer, these were the results of extensive testing when your car was developed. ONLY change the tyre characteristics of your vehicle if you are ENTIRELY confident they will not have a retrograde effect and such changes ARE AT YOUR OWN RISK.
"What do tyre codes mean?"
Tyres are described as x/yRd (Eg: 165/70R10), where:
x is the width of the tread in mm. Usually accepted as the width of the tyre, but the tread does tend to go down the sides slightly
y is the wall height as a percentage of x, or the distance from the rim to where the tread starts
d is the wheel or inside diameter in INCHES
R means Radial construction.
Often a letter preceding this letter specifies the 'speed rating':
So 165/70R10 has 165mm of tread width, and a 70% (of 165mm
= 115.5mm) wall height, and fits on a 10" wheel.
This means that 165/70's have a slightly larger diameter/circumference than 145/70's. Hence your speedo will read low if fitted 10x6 alloy wheels in place of standard 10x4.5 steel wheels.
Low profile tyres have a smaller 'y' figure (eg: 175/50R13)
Normal road tyres have a 70 or 80 profile. Expensive sporty ones have 60 or 50 profile.
Here are some wheel sizes and typical tyres:
|145/70R12 to 165/70R12
Different widths of tyre can fit different wheel widths.
You just get tyre 'overhang' on wider tyres.
Often a letter follows the wheel size. Eg: 10x6J. This is the rim type. Common ones are B & J. You don't normally need to worry about these letters. It's for the tyre fitter.
And tyre pressures?
Some people find 30psi front and 25psi back to be best. Others prefer 28 all round.
Do not go above 30 on the back end or you will spin off,
unless perhaps you're carrying heavier loads than normal. Do not run less than
25 on the front or the steering will be affected.
Power Distribution and Cornering Forces
OK 2 things to think about, power distribution and cornering (side) forces. Any tyre will have fixed compound adhesion properties per cm squared of contact area, break away occurs when shear force exceeds the forces pressing down on the tyre, I guess a total torque of x can be transmitted to the road more easier with a larger contact area (and a lower shear force per cm2, same torque transmitted over a larger area).
BUT, cornering (side) forces are more likely to be capable of spinning you off if there is less pressure per cm2 from the weight of the car. SO, F1 cars have big fat tyre TO GET THE POWER DOWN AND NEED THE EXTRA DOWNFORCE TO STOP TYRE BREAKAWAY ON ACCELERATION TOO.
BUT ALSO, S**T loads of ground effect stuff to increase
the down force to keep it glued on to the road... to increase the force per
cm2 of the vehicle weight to act against the side force on cornering IF wide
tyres helped cornering better AND to transmit power F1 cars wouldn't need all
the wings and things..eh?
So there must be a compromise between tyre width to get more power down and excessive width causing lack of grip on corners...maybe we need to start fitting aerofoils to our Coopers with 175s?
You can't remove the contribution of correct suspension alignments on handling, narrow profile tyres reduce flex of the sidewall, lower the car (and its C of G) and alloys reduce unsprung weights to further improve things...take these out of the equation and the use of wider wheels and tyres of the same side height (ie change from 155/70/12 to say 180/60/12s "fictitious") would only be an advantage if you were suffering from wheel spin on acceleration, the force per cm2 from the weight of the car would be reduced so theoretically the car would spin out easier?
Wider tyres change the track and important suspension angles to effect the handling in ways NOTHING to do with the tyres...fit "nutty wide boots" on their own expecting a radical improvement from the tyres and you will be disappointed, if these new tyres have a lower rolling diameter and are fitted to alloy wheels the improvement in handling might be from the added stability from the lowered Centre of Gravity and unsprung weight NOT from the fact you got 10cm wider strips of rubber!
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Updated 16 Dec 2002
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