Changing the size of the tyre fitted to the car can improve vehicle roadholding
and other handling aspects. Most frequently the new tyres that are selected have
an increased section width (they are wider) and a lower aspect ratio or profile
(they have a lower sidewall height).
But there's a few things that you have to know about when swapping-in bigger
tyres. Firstly, increasing just the width of the tyre while retaining the same profile
will cause an increase in the diameter of the tyre. This would happen if you decided
to change your 205/60 15 tyres for 225/60 15 donuts, for example. Doing this will
give a larger rolling circumference - causing higher vehicle gearing (less engine
rpm per km/h) and also introducing speedo error. Changing tyre circumference may
also affect the operation of the ABS, Traction Control System, Automatic Stability
Control and four wheel drive control. These systems can be particularly badly affected
if the front/rear tyre circumferences vary a lot from each other. So basically,
changing the overall tyre diameter away from standard is not a good approach to
take!
Rolling Diameters
Working out the potential change in rolling diameter that occurs when a tyre
of a different size is fitted requires some calculations. Firstly,
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Aspect Ratio |
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| Sidewall height = |
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* section width |
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100 |
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For example, let's assume that you currently have a 225/50 tyre fitted to a 16
inch rim. The aspect ratio of 50 in the description indicates that the tyre's sidewall
height is 50 per cent of its width.
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50 |
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| Sidewall height = |
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* 225 = 112.5mm |
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100 |
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As there are two segments of sidewall making up the diameter of the tyre, the
tyre walls in this case make up 225mm of the tyre's height. The other portion of
the diameter is made up by the rim, which in this case is 16 inches in diameter.
Multiplying the rim diameter in inches by 25.4 gives its diameter in millimetres.
Rim diameter = 16 x 25.4 = 406.4mm
The two sidewall heights (225) + the rim diameter (406.4) = an overall height
of 631.4, or 631mm.
A replacement tyre must retain an overall diameter within a maximum of 5 per
cent of the standard tyre's diameter - but the smaller the variation from standard,
the better. For example, fitting a 245/50 tyre to the same 16 inch rim gives a calculated
diameter of 651mm, an increase of 3 per cent. Fitting a 245/40 16 tyre gives a
diameter of 602mm, a decrease of nearly 5 per cent. Changing the rim diameter to
17 inches and using a 245 wide tyre requires that the tyre has a profile of 40 if
the overall diameter is to be kept very close to standard.
However, while these calculations are useful for gaining an initial idea of the
tyre diameter changes that may occur with swaps, tyres vary in their actual diameters
on a model-to-model basis. Even those tyres having the same nominal size often have
different actual rolling diameters. The tyre's specification sheet will show the
overall tyre diameter, static loaded radius, rolling circumference, and revolutions
per kilometre. You can use any of these specifications to compare the diameter of
the old and new tyres.
When increasing tyre width, it is very important that the tyre not foul any suspension
components or the bodywork of the car. Wheels deflect laterally a little under cornering
loads, and wheels that are able to be steered (either passively or actively) must
be able to pivot through their extremes of movement without the tyres fouling. The
arcs through which the wheel passes as suspension movements occur may also cause
the tyre to rub. If the outside wall of the tyre touches the inner part of the guard,
enough clearance may be able to be gained by having the guard lip rolled, so that
it protrudes less into the wheel well. Note that the offset of the wheel will also
have a major bearing on the clearances around the tyre.
The tyre clearances should be checked carefully when the tyre/wheel combination
is first fitted. Then, after some initial driving, both the inner and outer
surfaces of the tyre should again be checked for signs of rubbing.
Contact Patches
One of the most common swaps is to change the standard tyres for ones with a
lower profile and wider section. However, a tyre swap like this will change the
shape of the contact patch of the tyre on the road. Changing to the wider tyre will
give a contact patch that is shorter fore/aft, but is wider. The change in width
is easy to understand - a 225 tyre is nominally 20mm wider than a 205 so of course
the contact patch is wider - but what about the contact patch shortening in the
fore/aft direction? This occurs because - despite widespread belief to the contrary
- only the internal air pressure and the weight of the car acting through the wheel
determine a tyre's contact patch area. Nothing else affects contact patch
area- not tyre construction, nor tyre size! As a result, if the contact patch is
wider, the patch must also be shorter, so that the same area is maintained.
The change in the shape of the contact patch resulting from fitting a tyre of
increased width is likely to reduce ride comfort. This is because a tyre with a
shorter contact patch will ride up and over bumps more abruptly than one with a
long contact patch, that can "flow" over the bump will a less rapid distortion.
Of course, a tyre with a longer contact patch (and so one that is narrower) will
also miss bumps that a wider tyre will catch! A tyre with a wide, short contact
patch will also have reduced self-aligning torque (the propensity for the wheel
to wish to track straight). This is obvious in cars with suspension that makes them
prone to tramlining - on the same car, wider tyres will tramline more than narrower
tyres. The widening of the shape of the contact patch will, however, improve turn-in,
as the shorter contact patch takes less time to "re-arrange" itself when
establishing the cornering force. This is because the individual blocks of tread
can only change their direction after they lift off the road - and a shorter contact
patch means that each tread block is in contact with the road for less time.
The wider, shorter contact patch resulting from the fitting of wider tyres gives
cornering gains, but can also result in reduced braking and acceleration performance.
However, the use of stiffer sidewalls and softer tread rubber of high performance
tyres usually results in some improvements to everything!
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