Front disc runout? - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 12 (permalink) Old 10-19-2019, 11:46 AM Thread Starter
Supersport 600
Main Motorcycle: 1977 T140V(J)
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Front disc runout?

Hi all,

I have just fitted a pair of DP Brakes sintered pads to my T140. I had a "spare" 250mm dia disc which I got from an autojumble some time ago, so I took off the original chrome 10" disc, almost no chrome left on now, and fitted the new 250mm disc.
Went out on bike with the view of bedding the new pads in. Did not take many gentle applications from 30mph to feel the extra bite coming in from the new pads.
So, tried some gentle stops from around 60mph. As I applied the brake, this started a very noticeable handlebar shake that got less as the speed dropped. This happened every time I applied the brake. Tried some low speed stops to rest with gentle braking and can definitely feel the brake lever pulsing.
So, I am sure that the higher speed shake and also the low speed pulsing is caused by the "new" disc having excessive run out.
Does this fit in with the experience of others on here?
I can always re-fit the original 10" disc and if the problem goes away, would confirm the disc is the issue.
As an aside, I had Ferodo Platinum pads before. The DP sintered pads in the 30 miles I did today, appear to have much better bite and stopping than the Ferodos.

All the best,

Andy
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post #2 of 12 (permalink) Old 10-20-2019, 05:16 AM
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Hi Andy, I have a lot of experience with disc brakes.

Most likely your "new" disc is out of parallel. Meaning you have thin & thicker areas on disc. Sort of like a pot hole in a road. Every time the low & high spot transitions it moves the pad back & you feel it in lever & usually the brake will judder shaking bike.

You mention chrome is almost off.... Any difference in friction surface of disc such as a chrome patch or rust patch that is uneven can cause jerking & often pulsation as the pad is pushed back over the uneven surface.

A good disc will measure nearly perfect. Maybe vary .0001-.0002". Quality discs are ground or turned very accurately.

Generally you start feeling something at about .0005-.0006". .001" will be noticeable by most users. .004-.003" will give a very good pulsation & jerkiness during braking. Sometimes if not too bad hard braking my calm down, were normal medium braking will be quite noticeable.

To measure parallelism, use a marking pin & put against disc holding pin very steady. Rotate wheel so it leaves ink line on disc such you can get your micrometer next to line. Then measure next to line in about 8-10 places as you rotate wheel & record readings. Subtract thinnest from largest readings. That is the out of parallel. The disc will normally wear tapered. This is often not a real problem, but if you don't mark a line you can miss read taper for out of parallel.

Runout is the combination of out of parallel & "wobble".

Wobble of disc with a 2 piston caliper like we have is fairly forgiving. Generally the max wobble is about .004". However I've seen many with twice that & the brake feels normal. The caliper pistons can move in/out very quickly to help compensate for wobble.

Wobble can be caused by the disc itself not machined true, being bent, or the hub flange disc is bolted to not true. Often a combination of all this. Generally you use a dial indicator to test for wobble often called runout, but dial indicator will not accurately measure for parallel.

Loose/worn wheel bearings can give a "false" wobble reading also. Generally wobble from loose wheel bearings will not be felt as pulsation by may cause too much pad clearance & give excessive lever travel.

Please measure your disc for parallel & report what you find.

My strong hunch is skimming the disc will correct the problem. Luckily the disc is close enough to automotive size, many brake or machine shops have equipment that it will fit on for skimming.

I don't see specs on what the minimum thickness is after skimming. If discs are too thin & pads worn thin, the pistons come out too far to be safe.
Don
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post #3 of 12 (permalink) Old 10-20-2019, 09:03 AM
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What Don says is true....people called it warped discs, but a lot of times it's the variations in disc thickness caused initally by poor machining or later by brake pad deposits or gauling...On my T140 when it had the stock front disc, the runout was a few thousands and it felt ok

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post #4 of 12 (permalink) Old 10-20-2019, 03:06 PM Thread Starter
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Hi,

Many thanks to Don for his informative and detailed response.

Unfortunately, I was up early today and replaced my original disc on the bike, before reading Don's reply. However, due to family life and yet more rain, I have not ridden the bike to see if the issue changes with the disc swap.

With the "new" disc on the bench, I used a standard micrometer to measure the disc thickness at 6 points around the disc. I measured around 3/8" in from the disc rim. Smallest was 0.212", largest 0.215". Went around the six points several times and readings were consistent.

So, plan is to ride the bike again at the earliest opportunity. If the issue is still there with the original disc, I am minded to deplace the DP sintered pads with the Ferodo Platinum pads and again see if the issue goes away. May be the issue is caused by the different disc/pad interface with sintered and standard pads.

Regards,

Andy
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post #5 of 12 (permalink) Old 10-20-2019, 03:51 PM
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Your friendly MOT place might let you use their brake checker so you could see what's happening?
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post #6 of 12 (permalink) Old 10-20-2019, 04:14 PM
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Hi Andy, Indeed different pads can make the problem worse or better, even though pads is not the root problem.

At Mercedes a few years back we had a rash of discs going out of parallel for no obvious reason. Factory determined pad composition was causing discs to wear uneven like that. They issued new pad material. New pads did not cure already bad discs, but it stopped the problem reoccurring with new discs.

I don't understand how pad material can cause disc to wear out of parallel. However it can. I don't know if sintered is worse or better regarding pulsation. Also some cars like earlier Honda & Mazda the disc go out of parallel for no good reason. On my Mazda truck I used various brands of pads & discs. All went out of parallel & started pulsating bad. I don't understand what causes these would wear that way either.

Looking forward to seeing how the old disc does with the new pads. At .003" that disc defiantly needs skimming or new.

One thing about disc compared to drum brakes is much easier to get discs skimmed than a brake drum turned on motorcycle.
Don
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post #7 of 12 (permalink) Old 10-20-2019, 05:26 PM Thread Starter
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Don,

Just had a thought and been out to the garage to measure the old disc, that I re-fitted to the bike earlier.

Got 0.205" at every point I measured round the disc, so certainly much better than the "new" disc.

Perhaps that is why the "new" disc was for sale at the autojumble!

hopefully, a road test will throw some light.

All the best,

Andy
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post #8 of 12 (permalink) Old 10-20-2019, 06:44 PM
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My original '76 disk got out of wack in the first year of its life. It wasn't delivered in that condition so the heat of use took its toll. I took it down to the local truck machine shop and they ground it straight again. The chrome was gone but it worked well after that and it did not go bad again. Sure did rust easily though.
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post #9 of 12 (permalink) Old 10-21-2019, 06:49 AM Thread Starter
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Hi,
Another thought open for discussion. Could this be a fork twist issue that is caused by the extra bite of the sintered pads?
My bike has the standard 1977 steel brace/mudguard support between the fork legs (97-7004).
Hopefully a short road test may provide some answers.

Andy
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post #10 of 12 (permalink) Old 10-21-2019, 09:46 AM
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