Disclaimer

This article does not pertain to all types of lacing patterns and rim styles. It is specific to wire wheels with CENTERED spoke drillings in the rims, such as those found on pre-war MGs. It does not directly relate to other types of wire wheels where there are multiple rows of spoke holes in the rim.

Spoke lengths and dishing

While true that spoke length affects wheel dish, it is not solely responsible for it. The length of the spokes will set the range of allowable dish. Spoke tension will specifically set it within that range. The amount of required dish must be known before ordering spokes, in addition to hub flange size (measured at spoke holes) and rim diameter which is measured at the rear (tire side) of the spoke holes in the rim.

This is all important because a correct spoke length at in incorrect tension will not give the desired dishing, nor will proper tension with incorrect spoke lengths. Also keep in mind that spoke tension is not equal between sets of spokes. Inner spokes are shorter and at a more direct angle to the rim compared to outer spokes. This means they must run at higher tension and will offer less lateral support than their outer counterparts. This is why dishing is important not only to prevent fender rubbing, but for optimal wheel strength.

If a wheel is dished too far, the shorter set of spokes will offer little to no lateral strength and the outer spokes will be put under additional strain making up for the loss of support during cornering. Additionally, a wheel with too much dish will be more prone to breaking spokes not only because of the aforementioned problem but also because the inner spokes will be very tight and will fatigue more rapidly. As a wheel is dished more, the difference in tension between inner and outer spokes grows exponentially. Therefore lowering overall spoke tension to help prevent broken inner spokes will cause the wheel to come out of true more easily as the outer spokes become very loose.

Speaking in terms of strength, a wheel's optimal dish would be as far out as possible without the risk of rubbing a tire on the fender lip. The reason for this is that the closer a rim is to being centered between the inner and outer spoke holes of the hub, the stronger the wheel will be and the better it will hold true.

On a centered wheel, lateral forces are shared equally between inner and outer spokes, reducing the maximum stress placed upon any one set. Thinking about it geometrically, an Isosceles triangle is far better in this situation than Scalene!

(There may be a slight variation in this formula based on the size difference between the inner and outer hub flanges which I have not accounted for)

Other considerations

The other aspect of this issue is the vehicle's suspension and steering geometry. An improperly dished wire wheel can place added strain on critical safety components such as king pins and wheel bearings. Too great a positive rim offset will multiply the forces transmitted through the wheel into the king pin area, risking component failure and increased wear of bushings, bearings and pivot points.

It has been suggested, and rightly I predict, that the optimum position for the rim of some cars including the MG TC is at or near the point on the ground directly below the center line of the king pin axis. Varying from the offset specified by the factory will not only risk increased wear or component failure as pointed out above, but will affect steering performance and response.

With these considerations in mind, it may be far more important to achieve proper positioning of the rim in relation to the car's suspension and steering systems than to attain the best offset for the wheel itself.