Tech Talk FAQ - Wheels and tyres

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Wheel Alignment  FAQ Wheels and Tyres FAQ  
Everything Else... FAQ

Tyres
What have they got to do with suspension systems?
Aspect ratio, what is it and why is it important?
Tyre width, wider is better right?

Wheels
Wheel sizes, when is it relevant?
Wheel offset, who cares?

Tyres - What have they got to do with suspension systems.

The job of a suspension system (any suspension system) is to maximise the grip from the tyres whilst maintaining acceptable ride comfort for the passengers. All the good work we do revolves around making the best of what the tyre has available, regardless of the individual tyre brand, quality or size. Therefore the aim of the suspension is to keep as much of the tyre in contact with the road as possible when turning, braking and even driving in a straight line. It is both the beginning and end of all that happens in suspension and chassis design. Whitelines "Handling Packs" are specifically designed to improve handling through better tyre contact.

Tyre aspect ratio, what is it and why is it important?

Aspect ratio refers to the ratio between the tyres width and its side wall height. That is, a 205/60/15 size tyre has a 60% aspect ratio which means its side wall height is 60% of its width which is 205mm. Note that this is a ratio, not an absolute measurement so a 205/60 has a different side wall height measurement to a 195/60 even though the ratio is the same. The 15 refers to the wheel size in inches.

Key features of a reduced aspect ratio are;

• less side wall flex hence the car will turn-in better with more precision,
• greater carcass rigidity allowing higher effective slip angles.
• Ideal for bigger Selbys Swaybars
• increased effective spring rate at the wheel often requiring reduced physical spring rate
• increased NVH
• ride quality deteriorates
• greater probability of tyre and wheel damage

Tyre width, wider is better right?

Not always, it is a general misconception that wider is 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, such as McPherson at the front and multi-link at the rear on the S15, 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 tyre 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.

It is also quite easy to "over tyre" a car depending on the wheels used. A tyres' width is designed by the manufacturer to work in with the wheel size, suspension geometry and vehicles weight. 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.

This is particularly relevant to standard wheels where overly wide tyres will often handle worse than the original with the undersize wheel creating a balloon that easily walks and distorts though a spongy side wall. Properly matched wider and stickier tyres are a perfect match to a larger Whiteline Swaybars to maximise the extra load transferred to the drive wheels during roll.

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 with the most common symptom being the new tyres rubbing on the guards. 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.

Again, fitting a larger Whiteline Swaybar can minimise body roll to provide a "band aid" solution for tyre rub problems.

Wheel sizes, when is it relevant?

People often start their suspension and handling discussions by quoting their wheel and tyre size. Though they may be very proud of these statistics they are very rarely relevant to what we do. Why? Because a sensible (read legal) upgrade to wheels and tyres should deliver the same rolling circumference as the original wheel tyre combination (plus/minus a small tolerance). That means the outside diameter of the tyre should remain roughly the same. We suspect that people give us these sizes because they expect us to "fill the guards" for them with lowered springs, but as you can see, a correct wheel tyre upgrade WILL NOT change the tyre to guard gap.

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 which is discussed in more detail elsewhere.

One point we would like to make is that when considering wheel sizes, buy your tyre first, then your wheel! By this we mean that for best handling, a good tyre on a smaller wheel will out perform a poor tyre on a big wheel. So, by all means get bigger, better, prettier wheels, but make sure you can afford a decent tyre to go with it IF you're after better handling. If you're after looks, you're probably wasting your time reading this FAQ.

Wheel offset, who cares?

Whiteline do and so should you. This 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.

A lot of Selbys Swaybars have been sold to Hyundai Excel owners that experienced rear tyre rub problems after fitting mags. That's because most of the middle priced wider mags sold have the wrong offset for the car. That is, the increased width has been added to the outside of the wheel causing it to rub on the inner guard. This also radically changes the suspension characteristics resulting in excessive tyre wear, premature wheel bearing failure, heavier steering and poor turn-in.

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. This may not always be possible but try to avoid excessive changes in this ratio.