belt drive primary

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Apr 26, 2007
Hi all I have had a belt primary since 06 and was just wondering about tension adjustment. The drive chain spec for chain tension according to manual is 3/4 inch of free play @ the tight spot determined by rotating the rear wheel and taking measurements to find the tight spot and adjust from there. My question is how much free play is normal for a belt drive primary? Does anyone have any numbers for this adjustment so I can check tension of the belt and re-adjust if required. My goal is to adjust my drive chain it needs to be tightened. However I think I need to check tension of the belt first make any adjustments here and then adjust my drive chain to spec. Any help here is welcomed thanks.
Doxford.. 69 750 roadster
I have always been told you need a fair bit of play in them. I was told you should be able to twist the belt nearly 90 degrees in the middle. The important thing is to get the alignment right. You obviously have as its been in a while.
I would probably agree to replace a belt that looks like its getting tooth wear. Your 3/4" sounds about right.
I have had a dry belt primary for 10 years now.
I installed a left side threaded gearbox adjuster same time as belt.
The belt will get tighter as the sprockets and the engine plates heat up and expand, hence some slack setup when cold.

I feel the grab top rung and twist it 90 degrees when cold is erroring on the side of too slack.
This much slack can cause the belt to be bouncing off the bottom of the primary case and causing faster wear on the belt's teeth.
Set it up with maybe 70 degrees slack twist, I know hard to measure, don't sweat it.

When all pinned down, then adjust rear chain.

I have about 5000 miles on my belt, looks just fine, don't know how long they last, have heard as much as 20,000 miles?
my belt has over 23,000 and another one I built has over 25,000 :mrgreen: with no issues.
I too have high mileage beltdrive. A Tony Hayward. Around 30,000mi. I was astounded just how much the belt tightened when hot. That would be when you want to check it. Left side adjusters are pretty much manditory, for alignment. One particular issue for me came up. I got an adjuster that had a lot of play on the bolt. And the bolt had a certain amount of play in the case and in the cradle. It is important to recognize that the rear chain pulls much harder than the primary, so in the battle over the mainshaft, it's going to win. When you adjust the primary you need to realize that the adjusters are there to keep the case from being pulled backward. When you make your preliminary check pull the case backward with you hand, pushing it against the the bolt, and it up against the cradle. Then tighten. If you don't you'll be chasing the adjustment for a long time. Adjust the drive chain last. Also, belts don't last forever, and it doesn't just involve milage. They don't stretch and limp along like a chain. When they go, they're gone.
I snapped my primary chain a couple of weeks ago. I consider myself very lucky that it was the only casualty. The chain could have very easily bound between one of the sprockets and broken the primary case, crankcase, or transmission. Instead, it just piled up between the two sprockets!

The most common failure for belt drives is a shearing of the cogs on the belt. When that happens, it should be the only thing that fails. Even if the belt were to break, it is unlikely to take anything else out, except maybe catch the stator wires. I don't see why a drive belt won't last ten years, regardless of mileage.

Three things lead to the end of a belt's life - age, wear, and chemical contamination. There have been many advances in belt construction over the past thirty years, so much so that many cam belts used in automotive engines are designed to last the service life of the car. I remember the early Ford Escort, circa 1981, where the timing belt could only be tensioned once - if it ever had to come off, it had to be replaced. It also had to be replaced at 60,000 mile intervals, as that was the predicted minimum service life, and failure of the belt could lead to catastrophic engine failure (hemispherical combustion chambers).
There is a lot to be said for a belt drive. Large reduction in weight. No oil to leak out of the primary. I would think there is less rolling resistance with a belt. Replacing a belt is a lot cheaper than replacing a chain.
All those things are true and you can add one more, It changes the ratio and this makes it a bit easier on the transmission. And you no longer have a flywheel for a clutch basket. ala BDM. :lol:
You wouldn't believe how much weight you loose with an alloy clutch basket and a belt. It takes a lot of energy to spin them up.
Bill, you must be living right to have the chain stow itself like that.
For setting up the tension I found the simplest way is to press down on the rear chain to snug all the tolerances then check the primary, belt or chain, before tightening the trans down. If the factory wanted to get away with one adjuster, it should have been on the left. What were they thinking?
I know the ratio is higher on the belt, I'll count cogs today and find out. At least on this one.
the stock primary ratio is 2.19-1 and the standard belt kit from norvil is 2.187-1 with pulley's that count 70-32. he offers overdrive options up to 1.97-1. if I ever have to replace my belt drive I WILL OVERDRIVE it.
When I snapped a primary chain it did the same thing. The only real damage was cutting off the stator wires.
I was when I was a kid. Given the way I abused that primary chain it deserved to snap. I treat this old Norton with kid gloves, no drop the hammer starts, no yanking the bars at a shift. This stuff is forty years old now and 59 years have slowed me too!
and one of the really great reasons to run a belt primary: Dry clutch plates, always ! (and no leak primary!)
I must say, I love my belt primary setup. Even after this. Running it too tight from what I can tell.

belt drive primary
I see different colored primary belts, Im assuming different compositions? The primary belt drive kit I purchased from RGM came with a bluey/grey colored item, Fullauto,s a red (is this a fast model?). Does anyone know or believe theres a better quality one to source? Perhaps the racing fraternity amongst us knows.

Hi foxy,

Well you'll find as many different opinions in the racing fraternity!! I've always used 8mm HTD belts, originally a Norvil item (never again!) and then Maney ones ( far superior!) and in 25 years I've only had one belt was about 5 seasons old and had been well contaminated with petrol and oil over the years!!

I know some people who swear by the RGM ones, but I've seen them fail too! I think a lot depends on the rider. When Gary rode the 930 triple he'd go through about 3 belts a season. Since Cormac's been riding it we've had the same belt on for 3 seasons!!

Catch ya soon!

p.s. Sweden was a disaster, oil tank spilt again!!
I have an RGM belt...It is a polyeurathane type available from Transmission developments UK, I have used their kit on machine designs....Their belt is oil proof unlike the HTD type Norvil uses which tend to delaminate with oil. They also can transmit more torque through the same width of belt...i.e. they are stronger.
The two keys are don't let the belt get tight at any time, i.e. get the tension right, and also get the alignment right. I agree a double adjuster is pretty much essential.
Just a note to Fullauto, and racers....experience with belt drives on high speed spindles at work have resuted in us installing cooling fans on systems running continuously at over 4000 rpm. So I suggest if you ride your bike hard for sustained periods you open up your primary casing to get some cooling in there. I did this on the backplate so visually you cannot see any cutouts.

The rolling resistance from a belt is more than a chain if both are not cutting through an oil bath, because there is friction lost in bending a belt around a sprocket. That is why racing cycles still use a chain drive. I am unsure how much resistance a chain cutting through an oil bath would provide.

The best reasons for a belt are a dry system. The clutch works so much better dry. Also whever you can remove oil from a british bike has got to be a good thing!
Sorry to see that, perhaps that amazing new head is making toooooo much power!! maybe you need a 40mm and ditch the alternator :wink:
I'm of a similar opinion to Seeley on some aspects of Norvil but strangely my BD has been fine though I'm starting to think about a spare belt. Mine has 20k miles on it with some hard use and other than the odd run what you brung drag racing it doesn't get the same stresses as a racing application.
If you had too much tension you would have noticed difficult gearchanging I'd think? Norvil say 33mm total and I run 30mm with a special fixture to set and check. When hot there is a tiny amount of freeplay after the drum and pulley expand. Needless to say I made a big effort to get alignment as good as possible and the primary is vented through large holes in aluminum discs replacing the inspection caps. ... tdrive.jpg
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