Wheels n Tyres

by Fuggles
7 years ago

Tyre width

Wider is not always better.  The object is to have as much tyre contact with the road as possible, but there is an optimum limit to this.  On cars with anything other then a live axle suspension, a wheel will have already have some negative static camber in it’s standard form. This means that the weight of the vehicle is already not evenly spread over the width of the tyre, and is only further shifted towards one edge (usually inside) of the tyre as the suspension moves through its arc due to built-in camber curves. Wider tyres have other detrimental effects such as increased aquaplaning and friction (resistance) as well as increased steering feedback, which may be good but up to a point.

Increasing the width of the tyre will not necessarily increase the contact patch as the weight of the car is the same. The shape of the contact patch will however change under these circumstances, hopefully to the benefit of lateral (cornering) grip but only if they are fitted to the correct rim size.

Tyre width can also be a problem when the extra width grows out from the wheel hub face. This is one of the least understood but most common problems encountered when upgrading wheels and tyres. Wheel offset is the real issue here but the wrong offset coupled with excessive width in the wrong direction becomes a nightmare on all fronts.  Fitting a swaybar can minimise body roll to provide a “band aid” solution for tyre rub problems.

Wheel size

The most relevant aspects of upgraded wheels and tyres to suspension design are wheel offset, aspect ratio and tyre width. Of these three, the one that gives people the most problem is wheel offset.

Wheel offset is the least known and understood concept in wheel sizing and selection. It refers to the distance from the wheel mounting face to the outside of the wheel and is critical when purchasing wider wheels. It can also be viewed as a ratio between the inner and outer distances.

Ideally, any increased width should be proportionately split between the inner and outer mount face dimensions keeping the offset in approximately the same position relative to the wheels overall width. Doing otherwise will change the “track” or distance between the wheels which is optimised by the designer for best performance.  Not always possible however!