Studs, Nuts and Spacers

To complete our look into wheels for the RS it’s worth looking at these important but often overlooked components.

Before the arrival of the 996, Porsche road cars used studs with alloy nuts to hold the wheels on. I’m sure a few other manufacturers used alloy nuts – but I can’t recall any (except maybe Lotus). The use of alloy nuts shows the attention to detail in minimising unsprung weight. However for motorsport in the UK the MSA specifies steel wheel nuts. This does mean the degree to which the stud protrudes into or through the nut can be seen – This is a definite advantage when using ‘shim’ type spacers given that the nut should locate on a threaded length of at least 1.5 times the stud diameter. Where the existing studs are too short several longer lengths of stud are available.

While copperslip or similar anti seize compound can be used on the thread, the mating surfaces of nut and wheel should be dry and clean. Nuts should be torqued to 130 Nm (96ftlbs).

Spacers have long been a way to increase the track of a car offering handling benefits. Obviously fitting wider wheels with the right offset is another way of doing this but spaces allow you to work with smaller tolerances (wheels tend to go up in 1/2   sizes).

There are two types of spacer, those which are in effect shims being retained by the standard wheel nuts; and those which use the standard studs and nuts to mount the spacer, which is fitted with its set of studs on which the wheel is fitted.

The first method is obviously simpler and lighter – but does require the studs to be long enough for wheel and spacer. The second method, while heavier allows the fitting of much wider spacers.

Porsche themselves sold spacers for the RS and used them on the cup cars, so that gives some sort of approval  (they were actually fitted as standard to the 928GT).

As mentioned in a previous post  spacers will put additional loading on wheel bearings as they will move the effective tyre contact point further away from the hub bearings. I have not heard of any issues regarding this but it’s worth remembering if fo no other reason than to keep an eye on wear. The second point relates to suspension geometry and handling characteristics. Spacers on the rear axle will not alter the geometry but will obviously alter the track. Increasing the rear track  reduces the weight transfer of the vehicle as a whole (good) altering the front to rear weight transfer balance (decrease oversteer or increase understeer) and marginally reduce the effective spring rate at the wheel.  Spacers on the front will have the same effects as the rear (but vice versa on under/oversteer) as well as altering the steering offset. This steering offset is designed into the suspension geometry to give the steering feel and feedback, altering this will change the feel of the car marginally. The effect will be to add a little more weight to the steering and sensitivity to road input – this should thus be done  cautiously. if you go too far then the RS will probably feel a little too highly strung and nervous.

The most obvious thing to take account of is the change in physical clearance if spacers are fitted. Adrian Palmer in an early 964RS register tech note quotes 15mm and 22mm respectively as the largest spacers front and rear respectively, that can be fitted . His car came from Roock with a combination of 7mm front and 21mm rear.

Remember this is with standard wheels of 55mm offset. With other wheel widths and offsets you will need to be careful, use the offset calculator, if possible talk to those already using your proposed setup.

There is one more factor to consider with none Porsche wheels – the thickness of the wheel centre. This has recently been a matter of personal experience as I’ve just fitted a set of 17” Dymag wheels – fabulously light, but with thicker centres than Porsche wheels which has meant the need to fit longer wheel studs.

One final point on spacers don’t skimp on quality, make sure they are from a reputable company and of high quality and if possible (not on shims of under about 7mm) hub-centric (with a centring lip), this will ensure the wheels are running true.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s