Is 0.30 MM enough to cause a pulsing brake at low speed?

Hi everyone,

just come to the end of an expensive restoration on my combat commando. Everything works just fine except the front brake. On the first test ride i noticed that the front brake was pulsing heavily - the abrasion marks on the disc confirmed this, so i purchased a new disc and pads.

Once fitted went for another test ride and to my horror the pulsing was still there albeit not so extreme as before. Removed the front wheel and got it on my truing stand and corrected the spoke adjustment - the rim is still the original dunlop so i was satisfied at having got it true to within a millimeter or two which i.m.o shouldn't have any appreciable effect on the disc.

i checked and cleaned the mating surfaces between the hub and the disc which were fine. What i did notice about the hub, was that it has a vertical runout of 1 mm on the disc side (manufacturing tolerance??).

Anyway, with the disc back on - tightening the nuts (tight but not too tight - have no compatible torque wrench) in a star pattern so as not to distort the hub- i measured the runout of the disc with my gauge which is around 0.30 mm - i cannot imagine that this small amount causes such noticeable pulsing at low speed (approaching junctions etc.). The pulsing can be felt at the lever but much more so through the braking which feels almost as if i was braking and then letting the lever go and then braking again , if you see what i mean.

The wheel bearings don't appear to excessively worn with little to no appreciable play.

One side of me says "Oh sod it! Just buy an upgrade disc set from norvil", but that would stretch the finances to much especially when i take into account, that the build is well over budget and that i have already bought a new disc and pads. Other than that problems that i can't fathom play on my mind and really annoy me so i would really appreciate any help / tipps that you guys (oops, and girls!) may be able to offer!

Thanks

Tim

When everything is fully assembled, is the opening in the caliper centered on the rotor?

Hi Panda_good,

thanks for your help!

Well, when i reassembled it i had to force back each brake pad, as the old disc was probably thinner and they were new (thicker) pads. I certainly had no difficulty getting the disc back in.

At the moment the wheel is out again so i will put it back in paying attention to the alignment and post again.

By the way,

The wear from the 100 KM ride seems to be even on both sides of the new disc.

.3MM is, roughly, .012", brake pulsation becomes noticeable just north of .003".

So, either the new disk is warped or the hub is (could be a bit of both). The hub is relatively easy to measure, just remove the disk, put the hub back in the forks and use a dial gauge to see what the mating surface looks like. If it is warped I'm relatively sure that it will stand a discrete skimming, you may have to shim your caliper; good idea to check in any event; best if it is centered on the disk. If the hub is good then the disk is warped , return it or get it Blanchard ground (all three surfaces parallel).

If the hub is warped no amount of other brake kits will solve your problem.

I have just replaced the back wheel and checked the alignment of the opening in the caliper with the disc - the alignment does not appear to be correct, being approx 3 mm on the inner side and 1 - 1.5 mm on the outer side. Could this really be a contributing factor? Wouldn't the caliper pistons just compensate for that?

I have checked the new disc by placing it face down on a mirror and trying to get a feeler under it without it lifting - it doesn't appear to be warped. What amount of tolerance is acceptable on the mating surface of the hub? - i Will check that now and repost.

timsha said:

IWhat amount of tolerance is acceptable on the mating surface of the hub? - i Will check that now and repost.

Click to expand...

If the rotor is 10.7" in diameter and you want less that 0.003" runout at the rim of the disk the hub mounting surface of the rotor should have less than ~0.0008" runout.

Disclaimer: I don't have a disk braked Commando so I don't know the exact diameter of the disk mounting on the hub ( guessed 3" bolt circle). Regardless, the runout at the hub has to be less than 0.003" X the ratio of the radius of the bolt circle at the hub to the radius of the disk ( the number should be smaller than 0.003). If you find the 'high' and 'low' spots on the rotor are diametrically opposite, then you need to use the diameter of the rotor and hub.

The same valve should apply to the hub's mounting surface too.

Greg

timsha said:

.... approx 3 mm on the inner side and 1 - 1.5 mm on the outer side. Could this really be a contributing factor?

Click to expand...

Probably not, I was wondering if it was way over to one side.

Hi Tim,

Out of interest where did you buy your new disc from ?

Cheers,

cliffa.

Hi,

it was the hub, which has 0.25mm run out. That accounts for the slightly higher run out (0.3mm) on the disc. The good news is i know where the problem lies - the bad news is more costs! The spokes are also not good enough to put on a shiny new hub and if i have nice shiny ones on the front i will need them on the back too!! Oh well, its only money!! Luckily i am Switzerland where the prices for commandos are considerably higher - even higher than in the uk so i shouldn't make a loss should i choose to sell it!

@cliffa

i purchased the disc here in switzerland....its not an andover-norton or norvil disc but a cheaper pattern part (95 Pounds) from i don't know where. The dealer said he has sold loads of them over the years with only one reclamation. Will be interesting to see if its any good or not.

Thanks for your help everyone!

Hi Tim,
that's interesting. I got my disc from RGM and I can feel pulsing at the lever. They assured me they have never had a returned disc, and they (of course ) have sold hundreds. When I called them about the issue they were out of stock, but asked me to call them back after a bit more time as it may settle down. And as I don't have a dial gauge I could not really argue. My bike doesn't have the original studs but stainless bolts, so i took the disc off and lightly countersunk the holes on the disc & hub as i wondered if the hub has "pulled" around the bolt holes. It did seem to be better afterwards, but is still discernible. £95 is pretty cheap for here! Can you give me the details please?


Cheers,

cliffa.

cliffa said:

I got my disc from RGM and I can feel pulsing at the lever. They assured me they have never had a returned disc, and they (of course ) have sold hundreds.

Click to expand...

My MoT tester said that my new disc from RGM was pulsing at the lever.
I wrote an email to RGM, and this is their response:

Hello Peter, yes there seems to be a problem, got one faulty report a couple of weeks ago, and one back today and now yours, so there seems to be a problem sorry about that we will have to look into it. We have sold them for a few years now without problems until now. If I could ask you please to return the offending item we exchange it immediately. Again apologies for the inconvenience. best regards, Roger

I think you were told a fib.

Sounds like runout on the hub is the issue.
I agree with Roadscholar: " If it is warped I'm relatively sure that it will stand a discrete skimming"
If you have access to a large lathe, get it skimmed with the rim and tire on it.
Otherwise, dismantle the wheel and get the hub skimmed.
Skim the hub mounted on its axle and bearings. Hopefully your axle is straight, or that introduces another issue.
BTW, a bent axle won't create the issue on the bike, but will create an issue on the lathe. (the axle is not rotating on the bike, but is on the lathe.)

Stephen Hill

Hi bucksfizz,

Sincere thanks for responding,

I guess i was the the first complainant just before Easter. At least they were honest enough to concede that there is a problem (to you), however I am extremely disappointed that they didn't get back to me when they realized there is an issue, as they have my contact details. I mean (worst case) we are talking about life or death stuff here.

Sending and email to them now...


Cheers,

cliffa

I had extreme pulsation with just 4 thou runout on a Madass disc. He sent me a new disc and that resolved the problem.

Glen

I have had similar problems. Make sure that after you mount a new disk and you scrub off the paint by use
you then make sure that the paint doesnt bung up around the caliper and piston. Yes hard to believe this
can happen but it did to me or at least was what made me clean the area completely and problem solved.

Hello Tim
Check your disc for parallel thickness all the way around. Its not unknown for new discs not to be parallel. A few tenths out will cause pulsing.
Original worn discs will exhibit this problem as the chrome wears off unevenly.