Rear Disk Runout

[gerrym]

Re "curing" rear disk rotor runout, looking for practical ways of achieving this.

It would seem impossible to perform on car disk machining as the disks are semi-inboard. (wrong side on the strut bodies).

So is the cause of the runout the 3 mounting spots on the outboard drive shaft, or manufacturing tolerances from the disk machining.

If I'm to check the outboard drive shafts, I can't use my dial indicator as there's not a solid surface between the three spots and bumping it on and off will destore the accuracy of the measurement. What would be involved in making a calibrated flat disk (diameter of the 3 spots)?

By the way, I'm shimming the disk inwards about 2mm to gain clearance at full droop between disk and wishbone (adjustable toe-in wishbone from Pat Thomas). I've checked the shim thickness variation and they are less than 0.01mm (say less than 0.5 thou) so problem should not be coming from here.

Total runout on the disk is about 6 thou so clearly something is not right. New disks were purchased from reputable source.


Gerry

[alaric]

Hi. I managed to bend one of the ears on the outboard halfshaft, and that was obvious by the runnout of the disk. I replaced the half shaft and it's fine now. That was caused by my heavy handedness - you live and learn. I wonder if it's possible for wear in the inner and / or outer bearings to cause the shaft to run off centred.

All the best.

Sean.

[bcmc33]

Gerry,

As a matter of pure interest, do the new discs have ground or turned brake pad surfaces?

[weldsol]

Hi Gerry
Are you saying both sides are running out ? if are they out by the same amount ?
If it is only one side you could try swapping the discs to see if the problem travels from one side to the other. If it does then it's probably the disc if it doesn't then it's probably the rotor.
Also have you miked the lug thickness on the discs ? this would only need to vary a couple of thou out to give 6 thou at the disc edge. Even a bit of dirt would give that much.
Re checking rotor lug run out you use a fixed rod (clamped somewhere) and check with feelers on each lug
Paul

[andyelan]

Hi Gerry

If the discs are old, then its possible that they could have distorted through overheating or there could be a build up of rust on the mating surfaces, however I would suggest it's almost certainly due to one of the arms on the output shaft being slightly bent (don't forget any distortion here will be magnified by about 2.5 times at the disc rim). New discs are very unlikly to be wrong as they're quite simple to make and will all be machined at one setting.

You should have no problem clocking up the 3 arms of the output shaft. When using a clock you should only have the very slightest pressure on the item being measured (maybe 5-10 thou max) so the fact the pointer goes on and off the arms won't be a problrm as far as accuracy goes (do use a proper sturdy clock stand though)

If you do find it's the arms of the output shat which are wrong then you're left with either grinding them true or shimming (which is what I did). I wouldn't recommend trying to bend the arms back true as it's unlikely you'll get it accurate enough

If the drive shafts are non failsafe types the grinding the arms true isnt a problem as the original centres will still be in place and can be re-used. This isn't possible with the failsafe type driveshaft though as the centres are covered up so that leaves you shimming. If you decide to shim, then you can either simply use a shim the same thickness as the low arm which chances are will get you pretty close to what your after of you can measure the swash at the disc rim and add shims to bring this in (there's a technique for doing this which I can go through if you wish but I too busy at the moment).

Bottom line is though, whatever the book says, if you get the swash at the disc rim better than 5-6 thou then I don't believe you'll have any problems (I had 130 thou run-out on one of my discs and you sure could feel that).

Hope this helps

Regards
Andy

[andyelan]

Hi Brian

Discs are turned.

It's not normally good practice to grind cast iron as this can leave abrasive particles inbedded in the surface

Regards
Andy

[andyhodg]

Gerry

I had that problem on my rear discs. I was able to clock the mounting "pads" by mounting the drive shaft in my lathe and found about 10 thou' run out. I cured by machining the pads in the said lathe. Remember to machine both sides of the pads as on is a mounting for the disc and the other is the mounting for (in my case) the CV joint.

All the best

Andy Hodgkinson

[bcmc33]

Andy,

My question had some meaning. Up to the mid 70?s all discs were ground on the braking faces, two grinding methods were used, and were the main causes of progressive run-out in service.
It was only when certain CNC machine turning centres became available from the mid 70?s was turning the braking faces a practical option.

As yours are turned it is unlikely to be a disc that is the problem - so that means the wheel spindle legs remain the prime suspects.

[andyelan]

Hi There

I take your point Brian, I was simply assuming that anyone fitting replacement discs today would be using modern CNCd items. Not necessarily always the case I agree.

Regards
Andy

[freddy22112211]

Me too. One of the 'ears' were low. Schimmed, problem gone. Many years ago, still good.
Final measurement on the disc, not the ears.
Gordon

[toomspj]

I bought some of Pat Thomas' outboard shafts - the discs don't sit properly on these and you need to grind away a bit of the lugs on the id of the disc to get them to sit flush. Of course it may be something quite different.

paul

[mikealdren]

My memory agrees with Brian, I'm sure that disks used to be ground and the initial surface finish affected brake bedding and performance.

Gerry , this isn?t really for you because I know your engineering pedigree but ?
I would start by checking the disks themselves. Disks are probably much better now that when the cars were new. Nevertheless, I would start by measuring the disk thickness at a number of points to check that they are even, it?s quick and easy and you?d expect them to be perfect having been turned/ground but??..

Make sure the mounting faces are very clean and free from rust/contamination and be careful with tightening torque settings.

With regard to run out, there are two possibilities, the disk and the mounting. Install the disk and check for runout, if you have a problem, refit the disk one bolt hole round and see if the runout changes. If it does, the problem is with probably with the disk, if it doesn?t, it?s probably with the mounting ? you may have problems with both!

Mike

[alexblack13]

Hello Guys, Hiya Gerry....

My exp' with the runout problem was this.

As I was going the Spyder driveshaft route I was able to check and machine out the problem. Some one posted above the fact that failsafe ( ) shafts hd a spigot welded over the center drilled end of the outboard shafts. True. But I was able to machine this off.

You must be very carefull when mounting the shafts in the machine. I gripped the bearing 'land' and left enough protruding to allow 'clocking' the shaft true using a DTI before I cut anything. When I had it where I wanted it. +/- NIL.. or say 0005'' runout max, I machined off the stub for the failsafe. Gently... you don't want to move it whilst machining. Gently 'faced off' the end of the driveshaft then put a center drilling in for a rotating center. Then I removed the shaft and remounted it gripping the outer bearing land. Not too tight as you don't want to mark this area. Brought in the rotating center and clocked the shaft again using the inner bearing 'land' You must get this bang on. +/- nil if poss for best results. I then put the dti on the three leg faces and found an error of just on 5 thou. This would have resulted in a very wobbly disc! A complete waste of the new, (still in their boxes) discs!!

I then machined off the error, again VERY VERY gently. Clocked them at less than 0005'' runout. (Whilst still in the Lathe)

Did this with both the outer driveshafts. and got less than 002'' runout on the discs. 'Miking' the discs on the cardinal points showed less than 001'' variation of thickness. I blamed this for the runout I was left with. I made do with the 002'' runout as this was half the 'books' max figure.

Got to say I have been rewarded for the effort with great brakes and a super firm pedal. Well chuffed.

I am happy to assist / loan the use of my workshop for anyone needing to do this process. Or you could send me your outer driveshafts. I will true them for you for free.... Nil... Nought...

Very worth the effort. I once had a set machined by a local shop and they made a pigs ear of them. They will remain nameless.

Alex B....

[gerrym]

Alex, could I trouble you for a sketch of your setup on the lathe. It looks as though the ears on the outboard shaft are not running true. However, don't want to make things worse by poor setup in the lathe.

Thanks

Gerry

[alexblack13]

I will have a look and see if I have a pic or two Gerry. Not sure If I took any of the parts in the lathe but I will check.

I came back into Aberdeen today from Shetland. Where are you in Aberdeen area. You are welcome to bring your shafts down some weekend if you want.

For your own use .. You need a good rotating center and tailstock. And a dti with mag base. A good steady one. Get the shaft running true and machine it. Remember its the inside faces you need to true up.

PM me and I will give you my tel number.

Alex