Hi
A little off topic, perhaps...I’m building a salt racer, the base is a 1948 M21 which came with a very sorry six spring clutch, the gearbox is an external clutch lever type like WD M20’s. I will be running a dry primary, so the six spring isn’t desirable for that reason, never mind it’s bad reputation in general, albeit resolvable with patience! So my choice would seem to be finding a single spring clutch, or a four spring. One option is that use my road M20’s clutch, a single spring, and ‘upgrade’ my road M20 to a four spring, I’m not keen on that as I don’t like breaking one thing to fix another...
Anyway I’ve looked around and I’m confused about which four spring is suitable, obviously with some work for the centre to match the gearbox. Some places offer a new four spring with adapter which appears to be suitable but it’s got the cush drive. I guess that doesn’t matter, but I won’t need it. I also then have the ditch the plates and pop in some dry ones.
I know that I need to do some additional work to fit a sealed bearing for the basket on the centre to be able to run with a dry primary. I also don’t want to opt for a belt drive conversion, because that’s higher loss than chain and rules out gearing adjustments via engine sprocket change and primary chain length. So my questions are:
1) can I use the non-cush four spring?
2) is there any benefit in terms of machining work between starting with a single spring or either of the four spring clutches?
Cheers
Richard
email (option): dickie.bobbie@hotmail.co.uk
Since you are building a racer I’d advise you to look at race options for Triump Bonneville. BSA and Triumph all have the same taper on the main shaft as far as I know (someone please chip in if I’m wrong!) and there should be a fair amount of alternatives for you. Most racers run a dry clutch and a belt.
Biggest problem may be that the Triumph unit engine, which is the most preferred to race, uses a duplex chain and thus won’t work with the original primary engine sprocket. These also have the cush-drive inside clutch.
A clutch for a pre-unit Triump will probably serve you better, but you will have to do some research.
Good luck!
/Simon
email (option): SimonofSweden
Kidderminster Motorcycles do a 4 spring clutch to fit the M20 for £350 all in or the just the adapter for about £70.
If you are keeping the shock absorber on the drive spocket you don't need to (some would say shouldn't) use a clutch with a shock absorber.
Regarding the clutch centre use a Triumph pre unit clutch from an early model 650...These don't have the shock absorber center. Not only isn't it necessary if you are retaining the engine shock absorber but the 'shock absorber' center is considerably more expensive...
The 650s have more plates so the clutch is wider...For that reason make sure the clutch springs, clutch spring cups and studs, sprocket and centre are all compatible if you are buying from different sources....Aluminium sprockets, friction plates and pressure plates are available if you want to save some weight and reduce inertia. All pre unit Triumphs run a single row primary chain.
As noted in a previous post the clutch adapter is now made in a modified form to fit the earlier gearboxes so no machining work is required to fit it all...The taper angles of the various gearbox types are the same but the diameters are not. That is why a modified adapter is required..
There are also two types of clutch spring for those clutches....The shorter ones are heavier than the longer ones and make for a heavy clutch operation. I'd at least start with the softer springs and see how that runs...
You don't really need to fit a bearing to the clutch to replace the loose rollers, just grease the clutch rollers/tracks with some Lithium grease....They only operate when the clutch is held in and that condition is probably best kept to a miniumum if you are running a dry clutch to avoid it overheating..It's better to select neutral than 'hang' on the clutch. The rest of the time the whole assembly is rotating and the rollers aren't doing anything much...
Standard plates may be OK to run dry...I've seen plenty of competition bikes with open cases/no cases running dry Triumph clutches...I guess it's a case of 'suck it and see' with that and worry about it if you encounter problems!...Much of what you do will fall under the heading of 'try it and see if it works'...It's often the case with any aspect of a non standard competition machine...Ian
email (option): ian@wright52.plus.com
Didn’t know an adapter was needed to fit a Triumph clutch! But always found it to be to good to be true that they all were interchangeable.
You may want to also look at the Norton Commando clutch, sometimes used on Bonneville racers. But probably more of a hassle to fit.
Anyway I have a three spring with built in shock absorber on my 69 (ish) mildly tuned 750 Bonneville and I find it to be real sweet. I can easily operate it with two fingers (mind you my daytime job is behind a desk so weak hands). No drag and no slip. Properly adjusted it is almost on par with a modern clutch i’d say. The adjustment process is however a somewhat different story... :D
/simon
email (option): SimonofSweden
Hi all
Many thanks for the input, this fills in the gaps in my knowledge beautifully, I now know what to look for!
Thanks
Richard
email (option): dickie.bobbie@hotmail.co.uk