Belt Drive Tension / 1974 850 Commando

nomadwarmachine · Oct 23, 2010

I have a 1974 Commando Roadster with a chronic vibration problem that I have not been able to resolve despite numerous isolastic adjustments. The next step will be to rip apart the isolastics and replace the rubber, but before I do that, I wanted to ask if it is possible that my belt primary could be too tight and, if so, whether this could be the cause of my incessant vibrations. Frankly, it seems unlikely, as the vibration occurs even when the bike is not moving.

I have been told that the belt should be loose enough such that I could take the belt and twist it 90 degrees when cold. The belt is nowhere close to that loose, seeming to be more like the 3/8" tension recommended for the chain primary. What should my tension be? Do others agree with the 90 degrees or does this seem too loose?

One other question: the right-side top gearbox bolt appears to be a single piece with the adjuster. Is this correct, and if so, how do I adjust the primary tension. I would think I need to loosen the top and lower gearbox mounting bolts and then move it with the adjuster, but I do not see how to loosed the top bolt. It's a 1974 model year.

Thanks in advance!
-Keith

highdesert · Oct 23, 2010

sent you PM

Fullauto · Oct 24, 2010

I ran my Hayward belt drive too tight and lost the teeth. The belt should be quite loose as when it gets heat into it the slack runs away quickly. If you're not sure, pull the inspection cap after a good run and feel the tension. It should have a small amount of play.

pete.v · Oct 24, 2010

The belt is WAAAAAY too tight. "YIKES"! You need at least 1" to 1.5" up and down movement and/or 90 degree twist at least if not a little more.

I run a RGM unit and I adjusted mine to the point where kicksterting would jump a tooth and then bring it in a little. I am serious. If you find it jumps a tooth under load, bring it in a little more but not much. You can run with the cover off easy enough to checkit out, although the open air will keep it cooler/looser longer, hence the swiss cheese covers.

These thing run that loose. It is somewhat amazing how much they tighten up when running. The clutch hub really swells.

Hobot · Oct 24, 2010

I've seasoned experience to share on Commando belt drive. Its too hard to tell tension by the up/dn slack method. Cold belt should be fairly easy to twist 90' in bottom run, about no matter the length of the run nor width of the belt. When hot should still be able to slide/work on/off the belt by finger and blunt probes.
I spent deep detailed time with the builder of the outrageous blown dragster at Barber's. There are at least six belt drives on it of various widths 1/2" to 6". He and I both agreed to the Nth degree that round profile HTD teeth set for 90' twist when cold works the best.

As well seasoned primary and drive train destroyer I have possible bad news to convey, it only takes one deer in helmet to ruin a few seasons of spinal cord and Command health and same with over tensioned when hot primary. Better check your various shafts for candy cane bends/twists for the long run. Once set gear box can be firmly fixed - not welded as may need to remove gb for service but not to change belts.

May compare your set to feel twist to 90' tension -pulley center measures- with this chart, that neither me nor custom drag builder bother to use anymore.

zotz · Oct 26, 2010

pvisseriii said:
If you find it jumps a tooth under load, bring it in a little more but not much.

I have my belt adjusted as loose as possible within the slots of my engine plates. It does not slip when I kick start it but you can hear the bottom run of the belt slapping the primary case a bit at idle when it's cold. I have not noticed large amounts of expansion in the clutch hub after driving although I haven't put in a lot of distance on a hot day yet. What I was curious about though was whether it might be slipping a bit under hard acceleration in 4th gear. There seems to be a bit of a juddering sound but I thought maybe it was the slack run of the belt slapping the primary case. I know a slipping toothed belt on an industrial drive is quite noisy and obvious but I'm not sure what to expect from the one on my Commando.

pete.v · Oct 26, 2010

I don't know how loose you have it but if it is slapping the primary, maybe a it is a bit too loose. How is the vibration now?

Keith1069 · Oct 26, 2010

Just put a 30mm dia socket or something between the top run and dowel boss. Should be a tight fit for nominal slack in belt.
Zotz, BTW I used to work for a company in Cambridge...Conestoga Blvd, until they went bust last October. Always enjoyed my visits over there.

zotz · Oct 26, 2010

There never has been a vibration issue, just a minor sort of HMMMM, whats that? Sort of hard to explain.

Keith1069 said:
BTW I used to work for a company in Cambridge...Conestoga Blvd, until they went bust last October. Always enjoyed my visits over there.

Yeah, there's been a lot of companies gone bust around here lately. I almost lost my job as well.

Hobot · Oct 26, 2010

Belts can take a lot of abuse on their flats and teeth sides, but they are rather frail if the sides become frayed, can come undone and wrap around and into front pulley and alternator. I'd tighten up slack so no slapping bottom case, but still might touch the upper case boss for a bit of rubber dust evidence.

BTW there is a conversion kit for rear wheel belt drive on Brit Iron bikes.

Belts don't change length, not sure what expands thermally to account for the increased tension, which can be dramatic to traumatic.

concours · Aug 2, 2013

I don't choose to run a belt drive on my Commando, but read all the mail here. I've worked on a lot of them in industrial settings, thought maybe some techno could spill over. The tension issue seems to come up over and over. Has anyone used any proper measuring devices? Any notes published? Pressure/deflection gage is hard to get in the cover, no doubt. I thought this might be of interest, although pricey for a shadetree, some are not intimidated by pricey things for the bike.
http://www.gates.com/brochure.cfm?broch ... on_id=3539 sells for $600

I'm impressed by the repeatability, ease of use. I'm using it here setting the belt tension on the spindle drive belts of a horizontal milling machine.
http://s1040.photobucket.com/user/gsx80 ... 3.mp4.html

[video]<embed width="600" height="361" type="application/x-shockwave-flash" allowFullscreen="true" allowNetworking="all" wmode="transparent" src="http://static.photobucket.com/player.swf" flashvars="file=http%3A%2F%2Fvid1040.photobucket.com%2Falbums%2Fb404%2Fgsx800ho%2FMachine%2520Tool%2FIMG_1237_zpsef8b9f73.mp4">[/video]

comnoz · Aug 2, 2013

concours said:
I don't choose to run a belt drive on my Commando, but read all the mail here. I've worked on a lot of them in industrial settings, thought maybe some techno could spill over. The tension issue seems to come up over and over. Has anyone used any proper measuring devices? Any notes published? Pressure/deflection gage is hard to get in the cover, no doubt. I thought this might be of interest, although pricey for a shadetree, some are not intimidated by pricey things for the bike.
http://www.gates.com/brochure.cfm?broch ... on_id=3539 sells for $600

I'm impressed by the repeatability, ease of use. I'm using it here setting the belt tension on the spindle drive belts of a horizontal milling machine.
http://s1040.photobucket.com/user/gsx80 ... 3.mp4.html

Any belt tension gauge I have seen, including the one your using only works for a belt that is run under tension. The toothed belt on a Norton can not run under tension so deflection is the only way of measuring it. Jim

Belt Drive Tension / 1974 850 Commando

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