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When building any Greenpower car the biggest cost is usually the wheels and tyres so it's very important to make the right decisions early on in the design phase. The regulations state that the wheels must be between 300mm and 520mm in overall diameter and have pneumatic tyres; small wheels are easier to fit within a car's bodywork for good aerodynamic performance but high pressure tyres may not be available for them; larger diameter wheels are said to offer lower rolling resistance but will have greater air resistance; clearly there are compromises to be made!
The vast majority of Greenpower cars use either 16” or 20” bicycle wheels, however the sizing system for bicycle wheels is somewhat arcane to say the least! The main sizing system is ERTRO and the most common 20” wheel used in Greenpower is a “406” rim which is commonly found on BMX bikes. 16” rims are usually “349” which is the standard on Brompton folding bikes, but a few teams have used the smaller “305” rim, found on some Dahon folding bikes, to good effect.
In recent years the 349 16” wheel has been the most successful in races, the 305 16” wheel offers the best value for money and can easily be fitted within bodywork, the 406 20” wheel is standard on the Greenpower kit car but is not an obvious choice if building from scratch.
Having decided on a rim size you need a matching hub, the toughest and easiest to use is one designed for the front wheel of a mountain bike, with standard brake disc mounts and designed to fit onto a 20mm axle. British cycle component manufacturer, Hope, produce extremely robust hubs that have been widely used on Greenpower cars.
An alternative to using front MTB hubs is the Sturmey Archer drum brake hub. These make setting up brakes very easy but the standard axle is too small for the cantilever design (supported only at one end) needed for use in a car. However it is possible to replace the standard axle with a larger diameter if you have access to suitable metalworking tools.
In most cases the best way to obtain quality wheels is to go to a good bike shop who will be able to source the components required then build the wheels for what is usually a very reasonable fee, however it's quite normal for a complete wheel and tyre combination to be over £100.
It doesn't matter how good your wheels are if you fit poor quality, under-inflated tyres. All the three wheel sizes suggested have matching high pressure tyres available, “349” and “406” have the very popular Schwalbe Kojaks. The most obvious feature of the Kojak tyres is they're slick which has the advantage of lower surface roughness and so less rolling resistance than a similar tyre with a tread; many winning cars have been shod with Kojaks
Now, you might make you think “What happens if it rains?” The tread on a road car's tyres isn't there to provide more grip, it actually reduces it, only to remove water from between the tyre surface and road surface at high speed and so prevent aquaplaning. On a bicycle or Greenpower car, speeds are rarely more than 30mph, tyre's are narrow and the contact patch is very small so there is no danger of aquaplaning, there will be less grip because water acts as a lubricant but treaded tyres will not give more grip in such conditions.
Tyre pressures are very important when it comes to increasing the efficiency of a Greenpower car. The Schwalbe, Continental, Kenda and Michelin tyres produced for typical Greenpower wheel sizes all have recommended inflation pressures in the range 85psi to 125psi. Whilst there's always a bit of controversy amongst the cycling fraternity about tyre pressures, Schwalbe state categorically that rolling resistance reduces as tyre pressure increases. There are only two reasons to reduce tyres pressure below the maximum printed on the sidewall, comfort and increased grip in the wet. Considering you're building a racing car comfort doesn't come into the equation! If it's raining and you think more grip is needed then reduce to 5-10psi below the maximum, otherwise make sure you have a decent tyre pump and keep them up at the maximum.