H: Hubs, Wheels and Tyres Brake drum skimming

Bill Thomas

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VOC Member
I set up some cams for a friend some time ago,
Did it by eye as always !, Then looked at the marks and they were right !,
Nearly fell off me stool :D.
 

Magnetoman

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When set up in the lathe we simulate how things are with the brake applied. This is not necessarily exactly the same as when the whole assembly is installed and working on the bike, particularly with the brake plate.
The problem with your line of reasoning is illustrated by the following diagram where for clarity I've exaggerated the combined effects of new linings that aren't quite the same thickness as each other, shoes manufactured with pivot points that are slightly different, and pivot pins in the brake plate that weren't made by the factory perfectly concentric with the axis of rotation.

Brake_issues.jpg


Although the result of any or all of the above would result in the brake plate not being concentric with the brake drum, shown by the offset 'x's, it can be seen that the shoes still would make 100% contact if bedded using the emery paper approach. Unfortunately, this would make for a terrible brake when installed on the bike because at that point the two axes shown in the diagram would be forced to be the same. In contrast, none of these production tolerance effects would matter if the shoes were arced to size on a lathe.
 

Magnetoman

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it seems to me that the emery cloth/paper solution is at least one way for us to try improving our brakes, all be it that it will be a slow process.
A slow process indeed. If half the circumference of an 8"-dia. × 2" brake drum needed to have just 0.001" removed from it to make the drum round, that's 0.025 cu.in. of material. That may not sound like much, but it's the same amount of material as in a ⅛" length of ½"-diameter rod. Imagine how long it would take to remove that much material from a ½" steel rod by rubbing it back and forth on a piece of emery paper. That's how long it would take to make the drum round by manually removing "only" 0.001" from half of it with emery paper. Compare that with how long it would take to remove the same amount of metal in a lathe.

This is why motorcyclists who work on their own bikes, but who don't have a lathe, should do their best to become best of friends with someone who does.

I've been lucky (or prescient, or foolish, or ...) in obsessively accumulating tools, machinery and machining skills specifically for my motorcycle interests throughout my totally-unrelated career. As a result, this has reduced the number of times I have to rely on "Argentine engineering" to keep my bikes running. JB Weld, sheet metal screws, duct tape, etc. can work sufficiently well in some situations but, although emery paper might improve brakes in some cases, it only will accelerate the onset of arthritis and elbow-replacement surgery in others.
 

Monkeypants

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Non-VOC Member
I spent about an hour to go from a severely pulsating brake to a smooth very strong brake.
An hour isn't much on an old motorcycle.
It takes quite awhile to set things up in the lathe by the time both procedures are done.
I doubt that I could have completed the work with the lathe in one hour.
Then when you consider removing and replacing the tire as required for the lathe...the emery method was much faster on that drum.
The analogy of rubbing emery back and forth on a rod doesn't fit.
With the brake linings you have mechanical advantage and a large abrasive area.

Glen
 

Martyn Goodwin

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Non-VOC Member
Pic of homemade drum arcing tool - found on the net
 

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davidd

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It was 2017 where we covered this:


Many old drums can be skimmed to remove the inner and outer ridges. Sanding to finish.

DSCN4152.jpg


David
 

Martyn Goodwin

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Non-VOC Member
It was 2017 where we covered this:


Many old drums can be skimmed to remove the inner and outer ridges. Sanding to finish.

View attachment 42005

David
what's old is new again!
 

fogrider

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VOC Member
The photo shown at 47 above reminded me of the way I checked then trued my touring twin drums : Before I fitted the rim and spokes, I put the assembled hub, drums and spoke flanges in the lathe and skimmed the whole assembly true, running on its' own bearings which were set to nil play. A slight slackening of the drum bolts let me fit the spokes, nip back up and Bob's your uncle as the bolts are a zero fit.
Regards all, Terry.
 
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