Belt drive question

I have an older RGM belt drive, and my belt needs to be replaced. I was running an 890, and I had a spare, but it was a 920. So I tried to replace the 890 with the 920 and found it was too big. Even if I pull the top bolt of my gearbox all the way back in the slot, there's hardly any tension on belt - the bottom sags and touches the inner primary cover. The 890 fit fine, so I'm a little confused as to why I had a 920. What size belts do other people run? Does anyone run a 920? Maybe it's meant to run with a bigger drive sprocket?

seventyfour said:

I have an older RGM belt drive, and my belt needs to be replaced. I was running an 890, and I had a spare, but it was a 920. So I tried to replace the 890 with the 920 and found it was too big. Even if I pull the top bolt of my gearbox all the way back in the slot, there's hardly any tension on belt - the bottom sags and touches the inner primary cover. The 890 fit fine, so I'm a little confused as to why I had a 920. What size belts do other people run? Does anyone run a 920? Maybe it's meant to run with a bigger drive sprocket?

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We're going to be as confused as you as to where you got a 920. :mrgreen:

seventyfour said:

I have an older RGM belt drive, and my belt needs to be replaced. I was running an 890, and I had a spare, but it was a 920. So I tried to replace the 890 with the 920 and found it was too big. Even if I pull the top bolt of my gearbox all the way back in the slot, there's hardly any tension on belt - the bottom sags and touches the inner primary cover. The 890 fit fine, so I'm a little confused as to why I had a 920. What size belts do other people run? Does anyone run a 920? Maybe it's meant to run with a bigger drive sprocket?

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Belts are available in all different sizes for different sprocket combinations or different models of bike. The correct size should be readily available from RGM to replace yours. Jim

Like Jim said, different lengths for differnt combinations of pulley sizes and different chassis. If you're talking about the HTD style belts, I've used 880, 920, and 960 belts. I believe the 880 was for a Norman White drive, which uses a smaller AJS clutch hub, the 920 for a BNR setup on my Commando PR, and the 960 for Steve Maney's Seeley style frame with his belt drive setup.

Ken

890 is the size.
i got a 920 locally thinking i could slide the gearbox back and fit it. No go.
890 is the size.

Peel had Dreers version of Haywood drive. Came with 920T count but found 880 would fit and ran it a while fine but didn't like how forward that left gear box.

Mine is a 920 (Gates) which I broke recently and Norvil replaced with a Goodyear. Pulleys are 35 and 72 which puts the adjustment about middle of the cradle slot.

Mine is a 920 (Gates) which I broke recently

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Very curious on your mode of belt breakage as ain't no way pure Norton power can pull a 30 mm belt apart, maybe could pull a tooth off on drag race clutch dump but usually the gear teeth go first if not just spinning tire in smoke lubed relef. Only way I lost a belt was from edges rubbing front slide plates to completely unravel belt and store it all around the front pulley and alternator spaces. The last 920 belt I was running on Peel was ventilated by rotor nut going through the teeth leaving a few holes, biggest a pencil could be stuck through. It held up fine a few 1000 miles more and would still be using it but switched to 40 mm for future power. Sleeve shaft bushes got worn on hours of WOT low gear flings [before * how I learned they don't get lubed when engaged] so clutch wobbled which made belt run off center so it rubbed on front pulley plates. Belt edges fraying are its weakness that matters to avoid. I ventalated primary later, just for fun, and found grit wearing edges so put foam filter behind the forced air intake at front of case.

Very curious on your mode of belt breakage

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The belt teeth rounded off. I had a ride a while back where the nut on the gearbox adjuster decided to go its own way, and for the rest of the ride home, the belt would slip every time I accelerated much. I think that ride must have started the wearing, and after that I would have to tighten the belt a little after a few rides because it would start to slip under acceleration. I finally took off the belt and the teeth aren't square any more, they're looking more like a semicircle.

Thanks for the replies confirming the 890/920 issue. I thought maybe the 920 was also supposed to fit when the gearbox was pulled back, so it's good to know that's not the case and there's nothing wrong with my geometry. I'll put the 920 on the for sale forum in case anyone uses that size. It's an AT10/920/32.

Very curious on your mode of belt breakage

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Hobot, I guess that was directed at '74' but if you look at the Gates site they have a breakdown on belt failures, most common, reasons etc. Mine was tooth rotation or something similar. Still not really sure how it happened except a loose belt can contribute over time and mine is run loose until clutch drum etc heats up.

Hey we is all family here with crazy making children with teething issues. I've studied and interviewed a number of drag racers on their belt tensions to find that unless belt so loose ya can run a finger under it they don't jump teeth. Informative feedback to quell my worries about belt teeth as shows something else wrong first before belt reveals it dramatically.

My holey belt also had most the teeth profiles nicked out by running over the nut but didn't delay Ms Peel power hits one bit during a period she was over rev'd in low gears to remove sneers from modern sports riders.

i found a very informative webpage to explain belt failure at this website

http://www.industrialsupplyco.com/june-2010.html

very informative and good pictures and discriptions for diagnosing belt failures

hobot said:

Mine is a 920 (Gates) which I broke recently

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Very curious on your mode of belt breakage as ain't no way pure Norton power can pull a 30 mm belt apart,

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really?
when racing at Monthlery (Paris France, track similar to original Brooklands) and lining up for the flying kilometer , we stupidly thought a really tightened belt was the way to go ,alas for us we did not made the kilometer , stripped nearly all the teeth of the belt AND ripped it apart Belt is still hanging in the workshop as proof what wrong tension can do to a belt
ps this was a 40mm i seem to remember

really?

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Yes really - your too tight belt example once heat stretched then coming apart is only proof that belts don't stretch and 30 mm width is no match to resist the thermal forces in fairly massive components, not a good example of engine power over whelming it. Belts do have an rpm teeth engagement limit but think its beyond range a Norton can spin the primary. Yet the most powerful liter size Nortons have reported failure of 40 mm. Boosted Peel 920 will start with 40 mm, if that breaks then back to the oily chain gang and loss of some bragging rights on lightness.