Extended Track Width

June 1, 2007 at 7:18 am 1 comment

One of the limitations placed on modifying a vehicle’s suspension, is restricting the increase in track width to one inch (25.4 mm, so some States say 26 mm., some 25). They would, wouldn’t they! Can’t even agree on how to round out metric conversions.

So I’d better define track width first. It’s the measured distance from the centreline of the crown of one tyre, across the vehicle to the centreline of the other on the opposite side.
So why do they bother? As part of the program to discourage such modifications, State legislation prohibits tyres extending past the width of the body-work (though you can buy mudguard flares). Even with those, you might still get knocked back because of the fall-back position, which is to also legislate for track width limitations.

There is a sound reason for the limitation. The wheel is supported by two bearings on the axle. The inner bearing is load bearing (only), and is cylindrical in shape, whilst the outer bearing, which is tapered, is a thrust bearing. When the car is loaded up by either extra load, or cornering, the stresses on the wheel rise, and are transferred to the bearings. The outer bearing is designed to cope with sidethrust, the inner bearing with load carrying.

When the wheel track is widened, generally by fitting wider wheels and tyres, more load is transferred to the outer bearing, as the centre of the load is moved outboard. Wheel manufacturers can compensate for this by moving the centre disc of the wheel (the nave plate) further outboard, so that the extra width is carried inboard as well as outboard. These might be known as “deep dish wheels”.  It is possible to find wheels with the same hole spacing, which will fit on your hubs, but the offset may not be suitable for your car. Incidentally, the wheel is located on its “Bore Hole” the centre hole, and the wheel nuts are located on studs at a specified “Pitch Circle Diameter” (P.C.D.), which is what you find in wheel makers’ catalogues.

The life of the outer bearing is important. Should it collapse, then the car’s steering can be greatly affected, and it can lead to loss of steering control. Designers of front wheel drive cars are well aware of the risks, the front wheel drive is already “busy” steering and driving, so they applaud these limitations. A note of caution- if your FWD car’s drive joints screech when you corner hard, please get them replaced. If they collapse, you might end up in “big trouble”. How much trouble depends on the design of the front end.

Entry filed under: Sports & Racing, Tyre Sizing. Tags: , , , , .

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1 Comment Add your own

  • 1. elinorloewen  |  January 8, 2009 at 7:25 pm

    Interesting Read! Very detailed blog.
    Thanks for sharing


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