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Mk3 rear brake

Submitted by david_henshaw on Fri, 06/04/2012
Forums
Commando

Hi guys,

I have just purchased a 1977 Mk3 electric start commando, unrestored and reasonably original.

The rear disc was rubbing against the side of the caliper, exhaust side.

I took the rear wheel out and the only thing I could find was the bearing lock ring was a touch loose.

I tightened this up with a peg spanner having checked the bearings appeared to be correctly located.

When I put the wheel back in the disc no longer touches the caliper but is extremely close.

The caliper mounting plate appears to be straight and the spacers as detailed in the parts list appear correct.

Wheel is not out of line so the gap at the front of the caliper is pretty much the same as at the rear.

Before I start removing bearings etc and checking everything out in more detail has anyone come across this before?

Many thanks

Submitted by tony_ripley on Sat, 07/04/2012

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Hi,

Are you sure it is the original disk ? I bought a pair of new disks from RGM and they were near 1/8" deeper than the original. They took them back and swapped them for 'real' ones from Andover Norton which sit exactly in the middle of the caliper.

Remember the original disks were hard chrome plated so if yours is not then it has been changed.

If the front disk is perfect then measure the difference in depth of the dish.

If that is not the problem then I can take a few pictures of the rear of mine.

Regards

Tony

Submitted by david_henshaw on Sat, 07/04/2012

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Hi Tony, I have attached a pic so you can see how close it is.

I checked the discs and one is indeedapprox 1/8 deeper than the other. The problem is the deeper one is on the front which is perfectly in the middle of the caliper.

If the back one was deeper it would make the problem worse.

The rear hub has been rebuilt on an 18 inch rim and the only thing I can think is that they also refurbished the bearings and have done something wrong.

One thing I have noticed is that the gap between the wheel and the rear chain sprocket is approx 3/16 inch which appears more than required?

Some photos of both sides would be appreciated.

Many thanks

Richard

Attachments Rear-Disc.jpg

Submitted by ashley_cutler on Sat, 07/04/2012

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Hi, re-check all the spacers are in the correct positions, and the stub axle is fitted correctly. If this fails. check that there are no washers between the caliper and mounting plate, it should be surface to surface contact with any washers under the head of the bolts. Another check is to ensure that the disc is correctly fitted to the hub, If I remember rightly the disc mounting holes are chamfered slightly to enable them to sit down square on the hub.

Submitted by david_henshaw on Sun, 08/04/2012

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The spacers are all in the correct position as best I can see from the illustrtaions in the parts book. The caliper does not have any shims between it and its mounting plate and this plate is straight.

Wheel bearing run smoothly and appear to be correctly installed. At the end of the axle there were two large washers. See photo. I am wondering if one of these should be between the wheel spacer and the brake plate which would have the effect of offsetting the disc enough.

When I undo the axle the swing arm springs out enough to get a spacer in.

Anyone else got a spacer in this position?

Thanks

Attachments DSCF3127.jpg

Submitted by david_henshaw on Sun, 08/04/2012

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Think I now know the issue. The lock ring holding the inner wheel bearing had come loose allowing the bearing to move off its seat. I have repositioned these as per the manual and now the brake disc is nicely central inside the caliper.

The inner part of the cush drive sits slightly proud of the hub so the lock ring appears to tighten up on this and the bearing at the same time.

Is this normal?

Thanks

 


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