Wheels & Tyres

[Bananaman]

March 2003 - WHEELS & TYRES

It doesn't matter what you do to the engine or the suspension, the end result has to be transferred to the road and this will be done via the wheels and tyres. A constant source of debate, conjecture and argument, the importance of the wheel-tyre combination should never be overlooked. Whilst wheel choice may be restricted by budget, tyre choice is critical. Although consumable, every ounce of grip, traction and horsepower goes through the tyres so it is a false economy to buy anything other than the best tyre for the conditions in which it will be used.

WHEELS

SIZE - Caterhams have a choice of four different diameter wheel rims; 13", 14", 15" and 16". The standard 14" road wheel offers the best road-going compromise between ground clearance, ride and road holding. The 16" diameter wheel has aesthetic qualities at the expense of ride and handling. With the introduction of the R300 model came the new 15" ten-spoke wheel and CR500 tyre.

This Caterham exclusive design, has an additional heat treatment strengthening process during manufacture that has enabled us to make the wheel lighter than all but the 13" magnesium wheel, supplied as standard on the R500. This gives it handling comparable to a 13" wheel, combined with the aesthetic benefits of a large diameter wheel and low profile tyre. Nevertheless, because the Seven is so light, large diameter brake discs are not required. This means that 13" wheels can be used, giving the associated benefits of low rotational inertia (see in 'construction' for an explanation), so in general the best wheel size for the Seven is 13". It is no coincidence that the Superlight R500, and all Caterham race-cars run on 13" wheels. With a choice of rims, the 13" wheel offers the best unsprung weight, tyre choice and tyre wall aspect, although adjustable spring platforms may need to be fitted to ensure sufficient ground clearance. We strongly recommend the use of 13" wheels to achieve the best on-track performance from the car. To this end, all the following information assumes the use of this size wheel.

CONSTRUCTION - Although more attractive than steel wheels, the original purpose of alloy wheels was to reduce unsprung weight. Additionally, light wheels reduce rotational inertia - ie. it takes a lot of effort to spin a large diameter, heavy object and equally as much to stop it spinning again. For example, spinning a roulette wheel is much easier than spinning the roundabout at the park laden with kids, and a roulette wheel won't break your arm if you try to stop it. So if your wheels are lighter, the car can accelerate more quickly and stop more quickly. The choice of 13" rim is basically between the standard, traditional, multi-spoke Aluminium alloy (a smaller version of the 14" wheel - exclusively engineered for Caterham with additional machining for a significant weight saving) in 6" and 7" rim widths, and the R500 Magnesium wheel - Magnesium being 40% lighter than aluminium - in 6.5" and 8.5" rim widths. In both cases, the wider (rear only) rims, allow a wider tyre to be fitted. The Magnesium wheels are only suitable for cars with a Watts linkage and underslung (if fitted) rear anti-roll bar.

PERFORMANCE TYRES - ORIGINAL EQUIPMENT & RECOMMENDED

There is an overwhelming choice of tyres from a variety of manufacturers. However, we have developed a working partnership with Avon Racing Tyres over the last decade that has resulted in a range of tyres specifically engineered for the Seven. Performance tyres are typically much softer than a 'normal' road tyre, so tyre life may seem low in comparison. However, it should be remembered that the rigours of UV light from the sun and oxygen in the atmosphere deteriorate rubber and turn tyres hard over time, drastically reducing their wet and dry performance.

AVON CR500 - The ultimate tyre and the only road legal tyre developed specifically for the Seven, this is standard fitment to the Superlight range and the control tyre in the SuperGrad and Roadsport B race series. It has been designed with a very square profile and flat contact patch ensuring the maximum rubber on the road, coupled to light weight - some 25% lighter than a Yokohama A032R. Available in three sizes, the CR500 gives superb performance in the wet and even greater performance in the dry. We recommend the CR500 as the tyre of choice for all Caterham owners. Tyre life is about 6k miles.

Sizes available (rim suitability - see below) - 175/55x13 (a,c), 205/55x13 (b,d), 195/45x15 (e)

AVON ACB10 - In its 6" form, this is the current Roadsport A control tyre. Available in 6", 7" and 8" widths, the ACB10 offers phenomenal dry grip - unsurpassed this side of a slick. However, its cross-ply construction has a tendency to tram line and is less predictable than radials. Wet weather performance is poor. Replacing radial tyres with ACB10's will require different de dion ears to set the correct rear camber for this type of tyre. Tyre life is about 2k.

Sizes available (rim suitability - see below) - 6x20x13 (a,c), 7x21x13 (b,c), 8x22x13 (d)

AVON CATERHAM RACE SLICK - Significantly re-engineering for the 2003 season, the Avon Caterham Slick is specifically designed for Caterham racing and features a Kevlar reinforced carcass making it exceptionally light at just 4.5kg (more than 40% lighter than a Yokohama A032R). Slick tyres dramatically increase dry weather grip. To get the most from these slicks, camber needs to be adjusted at the front and rear (which necessitates a change of de dion ear) to suit their cross-ply construction. Cars using slick tyres must be fitted with a dry sump.

Sizes available (rim suitability - see below) 7.0x20x13 ©, 8.2x22x13 (d)

AVON RACE WET - To accompany the slicks, Superlight race cars have an optional (radial) moulded tread wet weather tyre that fits on the standard alloy rim. The wet tyre is exceptionally soft and offers superb on-track wet performance. Despite its appearance, it is not road legal. Furthermore, it is suitable for wet conditions only, overheating rapidly if used in dry or drying conditions.

Sizes available (rim suitability - see below) 160/530x13 (a), 180/565x13 (b)

PERFORMANCE TYRES - STOCKED ALTERNATIVES

In addition to the tyres that are used, or have been used, on our model range, there are a number of select popular performance tyres that are still supplied by Caterham Cars Parts and Aftersales departments.

YOKOHAMA A021R - Formally the control tyre for the Roadsport race series, the '21' is still popular thanks to its good road behaviour. The simple directional tread pattern and soft compound offer good performance in wet and dry conditions. Tyre life is usually about 5k miles with some track use.

Sizes available (rim suitability - see below) - 185/60x13 (a,c)

YOKOHAMA A032R - A popular tyre in club racing, the '32' has a distinctive large diamond shaped central tread block created by the overlapping strakes running full width across the tyre. This 'supersoft' compound directional tyre is ideal for dry road and track use. Tyre life is usually about 5k miles.

Sizes available (rim suitability see below) - 185/60x13 (a,c), 205/60x13 (b,d)

RIM SUITABILITY

When choosing your tyres from the list above, it is important to consider the rim you are putting them on. The popular, but incorrect, view is to put on the widest tyre that a rim will take. In reality this may be detrimental to the tyre performance. Fitting a wide tyre to a narrow rim allows the rim to move around within the confines of the tyre walls, deforming the tyre, reducing grip and resulting in poor handling. For ultimate control, a (non-race only) tyre should stay rigid, this is the advantage of low profile tyres where the side wall has little movement. To enhance this, a wider rim should be used where possible, 'stretching' the tyre out, which reduces tyre wall movement. Refer to the key below for choosing rims to suit the above tyres.

A - Standard alloy 6"

B - Standard alloy 7"

C - Magnesium 6.5"

D - Magnesium 8.5"

E - 10 Spoke alloy

NEXT MONTH - SUSPENSION…

If you need any advice on tuning your car, please email Aftersales@caterham.uk.com or call 01322 625804