Wheel offsets and clearances
We often get asked about wheels and whether the bodywork will need modifying, whether they affect gearing and what arch extension width is needed. So here are some points you need to know.
It is commonly believed that fitting larger diameter wheels gives less clearance to the wheel arch. This is not true.
The difference in wheel diameter is not reflected in the outside diameter of the tyre. Or, at least, the difference is negligible.
Here are some examples of tyre diameters:
145/80x10 - 486mm
165/70x10 - 486mm
145/70x12 - 508mm
165/60x12 - 502mm
175/50x13 - 506mm
As you see, there is only 22mm between the largest and smallest tyre, or 4%.
Therefore, as far as gearing goes, if you had been travelling at 60mph with 145/80x10 tyres, with 145/70x12's fitted and at the same engine revs, the car would be travelling at 62.4mph. Not exactly an earth shattering difference! And certainly not too much to worry about. In any case, your speedo is likely to be far less accurate than this anyway!
This is because the second part of the tyre definition is the 'profile'. This is a percentage of the width. So a lower profile on a larger wheel could maintain the same basic rolling diameter.
The tyre diameters quoted are taken from an on-line calculator and actual sizes can vary slightly with different manufacturers.
So, why does the bodywork need modifying sometimes?
Widths and offsets
What we should be talking about here is tyre width and wheel offset.
The wheel offset is the distance from the hub face to the centre line of the wheel. Your tyre is then distributed equally about this centre line.
There are positive offsets which mean the centre line of the wheel is inwards from the hub face.
And negative offsets which mean the centre line of the wheel is outboard of the hub face.
Therefore, a +20 offset would mean the centre line of the wheel is 20mm inboard of the hub face.
So, to get a rough (and there are reasons why it is rough!) idea of how much tyre will be protruding from the bodywork, take the offset and half the tyre width to calculate the distance from the hub face to the outside of the tyre....
Using the offset example above of +20 and a 165 tyre, the calculation is:
165 ÷ 2 - 20 = 62.5mm
Remember that a positive offset means the centre line of the wheel is inboard of the hub face so it needs to be deducted from the half a tyre width. So, in this example, the distance from the hub face to the outside edge of the tyre is 62.5mm.
Now you can measure the distance from the hub face to the ouside of the bodywork on the car. The difference is what width arch you need...... in principal!
However, the width of the wheel affects the way tyre sits on the wheel rim. A wider rim will 'stretch' the tyre making the wheel wider than the tyre whereas a narrower rim will leave the tyre wider than the rim. So the above is only a rough guide.
One other thing on this subject is that fitting wheel spacers adds negative offset (moving the centre line of the wheel outwards), so this also increases the risk of clearance issues. So, if you start with a +30 offset wheel and add 20mm spacers, you end up with a +10mm offset (wheel centre line is 10mm inwards of the hub face) . And starting with a +10 offset wheel and adding 20mm spacers, you end up with a -10 offset (wheel centre line is 10mm outwards of the hub face).
With the above in mind, you should be recognising that the tyre diameter is not what causes clearance issues. It is, in fact, the tyre width and offset that creates the clearance issues.
When you turn the steering, the front wheels don't rotate about their centre. They turn in an arc. As you turn the steering to the left, the right hand wheel moves in an arc forwards in the arch. The further out the wheel is set, the larger the arc it forms and the further forward it moves. This is what causes the clearance issues. The outside shoulder of the tyre fouls the wing and front panel in the area behind the headlight when the offset and width is too great.
In practice, we have found that a 5x12 wheel with an offset of +30 (centre line 30mm inboard of the hub face) fitted with 165/60x12 tyre is as far as you can go before fouling may occur. Even with this wheel and tyre combination, it is still possible to get fouling on some cars.
Other factors in tyre clearance
Lowered cars (either using Hi-Lo's or because the rubber cones have collapsed) have far greater clearance issues than cars with sensible ride height.
Bent tie rods will pull the wheels forward in the arch increasing the risk of fouling.
General factory tolerances.
Let's face it, the Mini was never the best build quality and the later cars were far worse than earlier models. So the wheel position relative to the body can vary considerably from car to car.