Front Disc Run-out

I bought a new front disc from RGM this year, and mounted it on my restored '74 MK2A Commando.
I was somewhat surprised when the MoT (annual roadworthy) tester discovered that the brake lever was pulsing with the machine on the rolling road.
This is with a disc that has barely travelled 20 miles.
I mentioned this to RGM, and they asked me to return it for examination.
Apparently, mine wasn't the only one, and two were sent back to the manufacturer.
While I was waiting for it to be returned, I bought a dial gauge - something I should have done earlier.
To make sure the hub wasn't at fault, I measured it, and it turned out to be only 0.0005" (half a thou) - video.
Finally, after more than four weeks, the disc was retuned to me, having been tested by the manufacturer (Velocette).
They proclaimed the disc to be good with less than one thou run-out.
I mounted the disc, being careful to tighten the fasteners gradually in a 1-3-5-2-4 pattern and finally to 20ft/lb.
Having mounted the disc, I measured the run-out at four thou - video.
I contacted RGM again, and asked them to run my video clips by Velocette, and comment on the run-out.
Here is their response:

The discs you returned were both measured at less than a thou run out for certain. Obviously these needed to be set up perfectly which is much easier to do in a standards room than on a motorcycle.

There were some useful notes provided by Lockheed about the care needed to mount a disc but also how to settle a disc in to true running by harsh application of the brakes. I believe these were included in the factory workshop manuals.

It is difficult without having the bike but feel certain that some careful attention will soon have this disc running dead true.

Questions:
How can I set up the disc in any other way?
Does a disc require "settling in"?
Will some careful attention have the disc running true?
Am I being fed a load of codswallop?

How accurate was your measuring? A half a thou is quite small. Is it a fair assumption it could be more? It's certainly not uncommon.

When that 0.5 or more is amplified by the diameter of the disc perhaps it gets close to your 4 thou measurement.

As both your hub and disc are not true, I would think that there will be certain positions where it is at its best and where it is st it's worst. So I would try rotating it a bolt hole at a time, measuring it, and finding the sweet spot.

Also, whilst it's off, I'd ask a pro engineering shop to measure it. If they think it's more than RGM say, I'd ask them for a letter. I'd then ask RGM what the max tolerance is, and if the disc is outside that tolerance, I'd then send disc and letter to RGM.

Finally, I'd try and get a pro engineering shop to check, and if possible, re face the hub.

Re “To make sure the hub wasn't at fault, I measured it, and it turned out to be only 0.0005" (half a thou)”

0.0005 At the hub could be 10x that at the disc away from the hub!
Do as fast Eddie suggests and clean both the disc and the face of the hub to ensure there is no dirt between them.

If you are going to demount the disk from the hub, take this opportunity to
thoroughly clean the caliper area around the pistons and the pads.
I know this doesnt make sense but found that it mattered in terms of pulsing.
Red herring maybe, but do it anyway and remove all doubt.
Just my experience.

I'm having exactly the same issue, but haven't returned the disc yet as I don't live in the UK and the postage will cost half the value of it so I'm going to post it direct to Velocette next week as I'll be driving over to London.

I expect the same answer as you have received. I had no issue with the previous disc, but that was a floater from Norvil which I was forced to remove due to the idiotic policy here in Switzerland.

I completely overhauled the caliper, using stainless pistons and Ferodo pads, new seals etc.

Initially RGM told me they have had no previous issues, but all of a sudden they have (I think) 4 or 5 customers complaining. It's got to be a manufacturing issue surely?

I removed the disc and rotated it several times, torqued it correctly and also slightly countersunk the holes on the disc and the hub in case there were burrs etc, but the lever still pulses and the bike doesn't stop smoothly.

I don't have a dial gauge, but using a micrometer I did notice that the disc is thicker towards the outside, and that is also visible when I put my good quality vernier gauge on the disc.

Not sure I'll be using RGM again.

Cheers,

cliffa

An ongoing battle with automotive rotors (NOT just aftermarket) as well. Made by the lowest bidder, in horrid shops with uncontrolled processes and working to loose or unspecified tolerances. :shock:
Hold their feet to the fire. :twisted:

bucksfizz, can I ask you where you bought the dial gauge and stand? Just seen some on EBay quite cheap, but are they any good?


Cheers,

cliffa

The disc and hub were cleaned meticulously.
The calliper has been overhauled with new stainless steel pistons, seals and outer cover.
I tried moving the disc around so that the high point on the hub aligned with the low point on the disc - run-out still an unacceptable 0.0035".
I'm getting a little tired of constantly removing and reinstalling the disc and front wheel.
I'm normally patient and rational, but the remarks from Velocette appear BS (some careful attention will soon have this disc running dead true).
It looks like I'll need to disassemble one more time, and take the disc to an engineering shop to have it measured.

@cliffa - I bought the dial gauge from Amazon, and it appears to be a generic low-cost item available at many outlets. I don't have a control gauge to know whether or not it's accurate.

BTW, I bought a new disc for my Trident, and it works without all this faffing around.

Sounds like the manufacturer has a new machinist who doesn't know how to machine discs. I bet he has machined one side then removed it from the chuck, turned it around, remounted it then machined the other side. Guaranteed to make the lever pulse.
I'll put money on that your disc is not the same thickness all the way around.
When I machine discs I machine both sides at the same setting.
Regards Peter

So, I was getting ready to wrap the disc to post it, and I thought I'll try and check the overall height in various places. We have a polished granite worktop in our kitchen, and I put a good steel rule on it and could see absolutely no light underneath it - so pretty straight (Swiss engineering at it's best ) I put the disc on the worktop - flange down, started measuring and noticed a slight wobble, turned it over and the bloody mounting face is cone shaped !!...







Surely conclusive proof !

bucksfizz, you may want to do the same.

Thanks for the info on the dial gauge

Peter, can i send you my disc to put right? :wink:

Just going to send these pics to RGM.

Cheers,

cliffa

3.5 thou run out is stuff all. Should not be a problem. If you guys are not careful, the only way the suppliers will guarantee no run out is to supply the disc mounted to a hub [ at what cot ]. Then you will have one with no run out until you mount your wheel on it, and tighten the spokes unevenly to get the rim lined up. Then you will have run out.

Use the bloody bike. They are never perfect anyway. Your front tyre will probably give you more problems than your disc.

As has been said here, 0.5 thou run out at the hub will multiply out 10 fold at the rim of the disc. You cant win.


Dereck

I had four thou runout and the braking was awful, almost made the bike unrideable the pulse was so strong, especially pulling up to a stop. Thought I might end up on the tank a couple of times.

New rotor smoothened things out.

Glen

To clffa
Yes I can sort your disc out. I'm,in Switzerland at the moment, returning to the uk later this morning. Pm me later today or tomorrow.
Regards
Peter

dobba99 said:

I'll put money on that your disc is not the same thickness all the way around.

Click to expand...

You're spot on.
I have a top quality Moore & Wright micrometer, and the minimum and maximum are 0.256" and 0.258" - that's a 2 thou difference.

cliffa said:

So, I was getting ready to wrap the disc to post it, and I thought I'll try and check the overall height in various places. We have a polished granite worktop in our kitchen, and I put a good steel rule on it and could see absolutely no light underneath it - so pretty straight (Swiss engineering at it's best ) I put the disc on the worktop - flange down, started measuring and noticed a slight wobble, turned it over and the bloody mounting face is cone shaped !!...







Surely conclusive proof !

bucksfizz, you may want to do the same.

Thanks for the info on the dial gauge

Peter, can i send you my disc to put right? :wink:

Just going to send these pics to RGM.

Cheers,

cliffa

Click to expand...

Good work finding the sophomore machining error. Isn't it amusing (sic) how these parts salespeople are QUICK to A) assume you have no idea what you're talking about
and
B) attempt to discredit your methods/skills

Sickening really. Selling junk and swearing it's the real article.

"The discs you returned were both measured at less than a thou run out for certain. Obviously these needed to be set up perfectly which is much easier to do in a standards room than on a motorcycle.

There were some useful notes provided by Lockheed about the care needed to mount a disc but also how to settle a disc in to true running by harsh application of the brakes. I believe these were included in the factory workshop manuals.

It is difficult without having the bike but feel certain that some careful attention will soon have this disc running dead true."

Four weeks to test and return a disc is atrocious service. It might run true on a lathe but when bolted to a hub with a dish like that it could end up anywhere. I would be asking for a refund rather than persevering with a poor product.. Velocette also supply fork stanchions and they have little to do with the original Velocette company. was it cheaper than the AN equivalent product?

Re; “You're spot on.
I have a top quality Moore & Wright micrometer, and the minimum and maximum are 0.256" and 0.258" - that's a 2 thou difference.”

As someone who has worked in a toolroom, that is a big runout I worked to a tenth of that.

Looking at your photo, the surface under your straight edge appears to have been turned. Looks like the turner forgot to lock the carriage on his lathe, Schoolboy error. What this does show is a distinct lack of quality control. Just bang them out and flog them.

Hi all, a quick update. I sent the pics to Roger (Mr. RGM) and he agreed to give me a full refund.

After i posted the pics last night I did some more checking, and basically the whole disc looking from the outside, is convex. I have renamed it, it's now called "The wobbly fruit bowl".

Not shown here is me holding a 7" thou feeler gauge under the straight edge on the mounting flange.

















gripper, FYI: @£80 + vat these are just over half the price of the AN discs. I think I'm in the wrong business. I can buy new disc for my MGB for £12.30 !!!!!!


Cheers,

cliffa

I was told once, that if it had racing or motorcycle in the description, you paid a premium for it. Cars are a different story. They are built like brick shit houses in most cases. 4 thou run out on and MGB would probably not be noticeable either.

I measured my commando front disc the other night. As far as I know it is original and has only 1 thou run out. I slightly worn front tyre [ Malaysian TT100 ] causes more wobbles at slow speed than the brakes do

Good to hear Roger is looking after you. He cant be expected to put a micrometer on everything he sells [ until a problem is evident excepted ] . I have been happy with all the stuff I have bought of him so far and he has been very helpful when I haven't been sure of which item I needed.

When I was in the motor trade we often machined the car discs on the cars. So long as the wheel bearings on the car were ok, you ended up with a perfect job. No run out or no variation in thickness. Cars like Mercedes though, you just threw on a new disc as by the time you saw them, they were totally worn out.
Dereck