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Burman Clutch Drag

Vic Youel

Well Known and Active Website User
Non-VOC Member
I thought I would report progress with the Burman clutch on my Comet rebuild. It worked ok before I dismantled it and although I carefully replaced all the plates in the correct order it suffered badly from drag. The 5 springs were all of unequal length so I replaced them with new ones of 1 ¾ inch length and tried to appropriately adjust the springs for a “clean lift.”

Firstly I was curious as to the correct spring adjustment so researched the subject of Burman boxes in various books:

1. Nicholson (2nd edition 1945), a Canadian book, very comprehensive and an excellent reference says “spring nuts should be tightened 1/32 inches past the stud ends”
2. Matchless book by FW Neill (1949) states “screws should be unscrewed 4 complete turns from screwed home position” ….it does not explain what the “screwed home” position is….. does it mean the point when the spring is totally compressed?
3. AJS book by FW Neill (1952) is a similar book to the Matchless one and extremely comprehensive on Burman boxes but makes exactly the same ambiguous statement.
4. Ariel book by CW Waller (1952) was again very comprehensive on “things Burman” with excellent illustrations and drawings. It states “all clutch springs should be screwed down only just sufficient to allow the end coil of each spring to be visible when sighted across the face of the centre of the outer spring plate”
5. Pitmans book on the Panther by WC Haycraft (1953) states “note that the five adjuster nuts for adjusting spring tension are correctly adjusted when their heads are just flush with the outside of the clutch spring plates.”

Therefore I endeavoured to achieve a balance of the various recommendations without success. Drag and crunch persisted.

There were no problems with plates getting caught in grooves etc so, although the plain plates seem very flat there was slight distortion in all of them. I consulted the experts on the next section night and there was general agreement that this was the cause of the problem. Vintech only offer one clutch plate of undefined thickness on the web site whereas VOCSC offer three specified thickness so I ordered from the latter of the 0.062 variety as those wer what was fitted. I also consulted various Ariel and AMC web sites but could not be sure their respective (cheaper) plates were a correct fit. The VOCS plates arrived the very next day and needed a little “dressing” to fit to the drum but the clutch is now devoid of drag. So it’s off for an MOT today and to test for slip.

I opted for the Ariel screw adjustment which coincides with the limit one can use a large blade screwdriver before the slot hits the end of the stud. The clutch spins absolutely true and is crunch free.

So for the next generation doing their PhD in old bikes I hope this helps their research! Anyway I think it is fun to share such info and I’m still surprised how little plate distortion caused such annoying drag.

Vic:)
 

samueljohn

Active Website User
VOC Member
Vic,
Try removing one plain plate, or using thinner plain plates overall. I had success going from .06 to .03 plates, which are available. This made the action and quite pleasant, and adjustment easier.
Good Luck.
John R
 

Vic Youel

Well Known and Active Website User
Non-VOC Member
Clutch inserts

Trevor, the basket has a new one from the VOCSC, one that looks faily similar and three which @I think are asbestos with bronze wire interwoven. It works fine now..... no slipping and the MOT passed with 65 miles on the clock now.

I promise I did not exceed 40 mph in third uphill!

Vic :)
 

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