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Thread: Negative Offset (Dished) Wheels

  1. #1
    Senior Member eastcoastdrifter's Avatar
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    Fitting a set of negative offset or dished wheels that may also be wider than standard requires you keep a number of things in mind. These are discussed below. You must not however, that this information has been described for a rear drive car. And that front-weel drive cars often have amplified problems when fitting varied offset or wider wheels to the driving axles.

    So what to keep in mind? Firstly, since the offset, and size of a wheel will determine the stress that is placed on your cars bearings and wheel studs, the massively dished negative offset wheels that have become quite popular may result in increased wear on your car. Also, with modern cars using larger braking hardware. It will be necessary to specify a precise calliper overhang area before choosing some wheels for your car.

    Additionally, it is important to note that altering your wheel offset of your front wheels will affect the way in which your car behaves on the road. This may make your car more prone to bump steering or torque steer when considering a front drive vehicle.

    So why do manufacturers specify a particular wheel offset for a car? Currently the majority of new cars on the market both rear and front drive designs - feature a positive offset. The reasons for this are many, however of most importance is the fact that manufacturers aim to locate the centreline of the wheel as close to the centreline of the hub bearings as possible. As well as this, a positive offset offers a number of benefits relating to: appearance better handling (particularly in front drives) and for allowing room to fit components such as brakes and CV joints.

    To understand why wheel-to-Bearings alignment is desirable, you have to realise that the bulk of physical forces acting on a wheel will transfer to the hub through an axis that runs straight down the vertical centre of the rim, The best way to minimise the effect of this force being transferred through to your car. Is to line the centre of the wheel up with the centre axis of your cars wheel bearings.

    Now, to move the centreline of the wheel away from the centreline of the hub's bearings will create a situation where the forces from the wheel act with a type of leverage against your cars bearing and wheel studs. For anyone that doesn't already know. Utilising a situation of leverage allows more acting force to be distributed to a smaller portion of space. This means that the force coming from the wheel acts on a smaller portion of the hub materials and therefore increases the risk of causing damage to the hub's components.

    So. To fit some extended dish wheels without massively altering your cars reliability. Or handling. It is important to increase the backspacing area (the area between the mounting face and inside rim of the wheel) by the same amount that you add on to the wheel's lip. For instance if you had a seven inch wide wheel as standard, you could opt for half an inch more dish. With another half inch added to the inside of the rim (space permitting). This however, is not the ultimate answer to fitting wider rims: under cornering loads there will be increased instances of leverage. Thanks to the extra half-inch on either side of the wheel.

    That being said, there is a certain extent to which you can work outside of the optimum centre alignment. Porsche vehicles are one example of cars that commonly have wider wheels fitted to the rear. These fitments quite often alter the centreline-of-wheel to centreline-of-hub alignment. To overcome the problems associated with increasing stress on the bearings and wheel studs. It is possible to buy stronger wheel studs and bearings. This would be a good option for anyone that does alter their centreline alignment from standard by fitting dished rims.

    To Sum everything up, although massively negative-offset. Wide rims will definitely lead to increased wear on your car's wheel mounting components. There are things you can do to counteract this. While for the rear of a car this strength is the only real problem. Changing offsets at the front of the car may see you bump steering off the road, Basically don't Opt for excessive dish up front. Strengthen your components in the rear and you should be able to use some decently dished wheel.
    How many times have you seen kids playing on the Touge at night.

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    Veteran Konakid's Avatar
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    Good find, basically saying that the offset on the front should be kept as close to zero as possible to minimise scrub radius and to prevent excess loads being placed on the studs, bearings and steering.
    RT142 Estate.

    AJPS.

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    cool copy paste man

    most people already knew this but :2thumbs:

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    Senior Member eastcoastdrifter's Avatar
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    Actually I type it all source was from a old Autosalon Mag.
    How many times have you seen kids playing on the Touge at night.

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    autosalon is wak :ph34r:

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    Part of this statement is correct, that is the bit about minimising the scrub radius. Depending on the king pin angle though you may not want a zero off-set rim.
    Steering axis inclination –kingpin inclination as it is called in the attached diagram – refers to the angle of the steering axis when viewed from the front of the vehicle. (So, to be clear: castor is the steering axis angle when viewed from the side; steering axis inclination is the steering axis angle when viewed from the front.) As with castor, what's important is where the imaginary line through the steering pivot points reaches the road.

    If this line touches the road halfway across the width of the tyre contact patch, the steering is said to have zero scrub radius. (Sometimes this is called ‘centrepoint steering'.) If the steering axis inclination line touches the road on the inside of the tyre's centreline (ie closer towards the centreline of the car), the steering is said to have positive scrub radius. If the steering axis touches the road on the outside of the tyre's centreline, the steering has negative scrub radius.

    Steering axis inclination can only be achieved by placing the lower ball-joint further outboard than the upper ball-joint. (This diagram of old Jaguar suspension shows just that.) Therefore, to achieve steering axis inclination and castor, the upper ball joint must be further rearwards and inwards than the lower ball-joint.

    Steering axis inclination is an angle, so is measured in degrees. It is usually not adjustable. However, scrub radius can be altered by changing the wheel offset - and so where the centre of the tyre's contact patch is relative to the steering axis.

    There is plenty of info out there but it is important to get your head around the effects of the big dished rims and S13 Coilovers on the suspension geometery.

    Cheers
    Paul



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