Wheel fitting and its effects

[zygoteer]

This is not a query, just a diary (alright, alright, a LONG verbose diary!) of what I did, that might be of interest to some.


An embedded screw in my tyre meant I had cause to change a wheel for the first time on my 535 (only had this car since last December); and I also wanted to investigate a tin-like cyclic clank that I sometimes got from the front (as expected it turned out to be disc shroud plate that had got distorted by a stone and was catching).


The following tells of the wheel issues I found, and because of this I also checked and corrected all the other wheels too, resulting in a much improved and smoother new-like ride to the car.


When I initially removed the wheels I saw that the black wheel bolt was not fitted in the correct place, so I corrected this on re-fitting.
Not everybody realises that 4 of the wheel bolts are plated and one is blackened, which is a lighter ‘balance bolt’ intended to account for the extra weight of the tyre valve, so should be fitted next to it.
In my case this meant that the wheel balance would now be less than perfect, but as tyres wear they usually need re-balancing at some point anyway.
I have always had my wheels balanced off the car, but with the balance bolt at the correct position.
I have always had to ‘force’ the tyre business to do this though, as it means they have to take extra care when mounting them on their balance machine.
You could also have the wheels balanced whilst still on the car, but I do not recommend this and dislike such machines as they really do wear the shoulder of the tyre excessively during the balancing process.


Before I put things back together I noted some corrosion on the mating/fitting surfaces of the hub and wheel, so naturally I cleaned it off etc., paying particular attention to the ‘frictional mating surface’ and the ‘datum fitting diameter’ on both the alloy wheel and the steel hub.


During this cleaning, it became apparent that the wheel ‘frictional mating surface’ had non-continuous corrosion meaning that the wheel had been over tightened in the past giving rise to the alloy being ‘pulled’ at the bolt positions, so reducing the contact area strength and allowing moisture ingress that starts the corrosion (tyre fitting businesses with air tools, that never get torque calibrated or even setup, have a lot to answer for).
This became self-evident when a steel rule edge was placed on the surface, thus showing where it was not flat.
I corrected this by re-cutting the bolt hole chamfers on the mating surface and re-checking the flatness; fortunately they did not need to be skimmed in a lathe (not many places have a lathe that can take a wheel diameter anyway).
The strength of a wheel mounting is not vested in the shear strength of the bolts, but in the shear friction between the mating surfaces, so it is very important that these surfaces are as flat as possible to spread the load.
It also means that tighter bolts do not equate to a stronger wheel mounting, they only have to be tight enough to ensure full area frictional contact plus a little extra to account for differential expansion of dissimilar materials plus a little extra still to put the bolt into the elastic tension region to avoid them coming loose.
Some people think that correctly torqueing up wheel bolts is a perfectionist attitude and not really worth the bother, but it is especially important with alloy wheels in order to maintain the integrity of the wheel mounting.


The cleaning also revealed that the sizing and profile of the hub and wheel is quite sophisticated as shown in my rough sketch attached (not to scale though), which is actually a slightly better design than that on my similar-age Ferrari, but most other manufacturers simply use plain surfaces/diameters which are then very prone to seizure.
From the sketch, you will note that on the hub, the ‘datum fitting diameter’ is a narrow raised land that is relieved on its inner and outer, to allow dirt to escape during fitting and removal; and it is also similar on the wheel centre bore.


… If you have wheels from another BMW model or maybe after-market, they may require the use of hub-centric rings to make the ‘datum fitting diameter’ fit correctly before the bolts are fitted and tightened, thus ensuring the wheel really is concentric with the hub for ‘true’ running.
It should be remembered that the wheels do not (and should not !) bear the running weight of the car on this ‘datum fitting diameter’. as that is the job of the ‘frictional mating surface’ only (this is why the hub-centric rings can be made of plastic or aluminium – they are actually only a fitting aid, but the exact sizes are crucial of course, and the wheel should be rotated whilst progressively tightening bolts to ensure optimum concentricity).
I would be very suspicious of hub-centric rings that have been mutilated, as this actually means the wheel was very probably never fitted truly concentric.

On cleaning the inner relief of the hub, I found it was packed with ‘grease’, but in my opinion it was far too much, which brings a risk of it migrating to the disc eventually.
On re-assembly I ‘lightly greased’ the diameter to avoid future seizure, using a high-melting point water-proof grease (disks get hot and are subject to weather).


Test drives showed a very smooth ride and solid steering.
Previously, an odd road surface ripple would occasionally give me the famous (?) ‘E34 shimmy’ at 50 to 60 mph, but that has now gone completely, no matter how hard I try to provoke it.
The solid smoothness cruising hands-off at 135 mph (momentarily with hands poised to take back control of course) is now close to that of the F288.


The above doesn’t really cost anything to check and correct, and may well solve a some minor issues for some.

OK ... now I will shut up, but I've probbaly bored you all already - sorry

[TheGeak]

Very interesting write up, brought to mind a few things i'd never thought about

[Gayle]

Thank you for the informative post.

[Zeuk in Oz]

Thanks - and explained beautifully.
Just proves the old saying : "education is the progressive discovery of your own ignorance" !
I had my wheels off last weekend but will take them off again soon to put the black bolts in the right places - I had no idea !

[tim s]

do i need to weigh them to find the odd one???
thanks
tim s.

[zygoteer]

If you have a 'balance bolt' it will be obvious just looking at it ... If you move the bolt then possibly the wheel balance may need doing again if it is too far off true.

[liquidtiger720]

all of mine are bronze?

Thank you for the informative post.

[zygoteer]

It doesn't matter if you don't have the 'balance bolts' ... that only means the (ugly) wheel balance weights are very likely to be bigger.

[DaCan23]

Isnt that black bolt the wheel lock bolt? Where the lock cylinder screws into it and then theres a key for the lock?

If you have a 'balance bolt' it will be obvious just looking at it ... If you move the bolt then possibly the wheel balance may need doing again if it is too far off true.

[zygoteer]

Lock bolts can look similar, but are usually a different hex size and height to allow a reasonably sized locked cover to fit over the top.

The depth, diameter and internal plain profile of the hole would not be sufficient for a cylinder to fit and be retained.