Belt Drive tightness

Happy new year everyone.
Got an 850 Commando with a belt drive.
My question is how tight the belt need to be ? any slack at all?
Thank you

Norvil or other?

Search (upper right google search) is your friend...

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zebian2 said:

Happy new year everyone.
Got an 850 Commando with a belt drive.
My question is how tight the belt need to be ? any slack at all?
Thank you

Click to expand...

Yes, lots of slack. Too tight will cause failure when it gets hot.

Some say 3/4 inch flop with two fingers.

Some say you should be able to twist the belt 90 degrees with two fingers.

Some say just tight enough to keep the belt from jumping teeth when kickstarting cold.

Any of these will get you there.

With everything at operating temperature you should still have slight slack -no tension. Jim

Thank you.

It's the Norvil primary belt drive.

Norvil instructions state 33mm total and that depends on pressure but I have a Norvil setup last 8-9 years and run a medium tight 30mm. Like Jim says, lots of slack as that clutch drum does expand loads....

All belt drives of any size and run should all be fairly easy to twist almost 90's when full thermal tensioned which seem too loose cold as then able to work belt on off w/o loosing gearbox optimal setting. Not speculating as quizzed world class cycle dragster with couple feet wide tire and belt drive everything, oil, case vacuum, fuel and power train - who I double checked twsiting all its belts to be firm believer going by feel only. Belt should stay on w/o front pulley side plates on and will tend to flap enough to leave some insignificant rub dust collections. Only takes one over tight event to be remembered forever in main shaft shape. The industrial standard way is a formula-chart with pulley size and belt lenght to settle on idea center/center distance/tension but they don't expand as much as Nortons so can't use that unless measuring when too hot to handle.

I have run a Norvil 30mm belt primary for the past 15 years

THE Concern of setting the belt too tight - is that when the primary is fully hot as hell and the belt sprockets have fully expanded so that all possible slack is taken out of belt - that the belt will be just so tight that is pulls the sprockets towards each other which then puts possible bending pressure on the mainshafts and bearings

so the right amount of belt slack with a fully hot primary is.....just a tiny bit of slack

and you get there by setting the belt while the primary is cold, in my opinion from measuring, with the belt slack at about 28mm or some 65 degrees or so of twist in the upper middle belt when cold

so why then do the instructions say to set it a full 90 degrees of twist? - to be extra and unnecessarily sure that a buyer does not error and set it too tight

so what is the negative in setting the belt too loose (full 90 degrees of cold twist IMO) ? - because it slaps the bottom of the primary which fatigues the belt itself and can also less easy for the belt teeth to align with the sprocket grooves on every revolution

don't overdue the looseness factor, there is such a thing as too loose, its no more difficult to get it right than to get it wrong

Well ever the fellow with the odd problem: my puzzle is that the top of the belt has worn against the stator fixing stud towers. After reading all the warnings here about too tight I went with the rather loose approach. So I pulled it a bit tighter and then checked when it was fully warmed up and
found the belt rather tight. So loosened a bit and still it was eating off the top of the belt.
Riding season now over I have given thought to easing the aft sides of the towers ever so carefully as there isnt much meat there. And I shall
haul back once again on the gearbox just a wee bit.
Clearly it is too loose if Im losing belt material off the top.
BNR belt and clutch. 850 Mk2.

Do not confuse two separate issues, proper tension and proper belt slap clearance vs tension strain on shafts. Relieve the stuff in the way as over built for the loads on it and two more backing it up. Many have triplex chain tracts in them too.

I run it the same slapness / looseness /slop /play as you feel is right. Mostly I go by the FORCE and interstellar decisions. 21,000 Kms. now , no stretch or sloppage wear or worrys to report. No mainshaft warps or sleevegear bush issues. To tell you the truth I fired her up with the primary cover off and observed. Intuition and perfect alignments did it. Primary cover back on , no ventings. :wink:

How about you set it where it was before you took it apart? It ran a long time that way
Doug

One thing I needed to mention, that the main primary cover is vented.

Someone needs to put a IR belt-tire temp sensor in vented and unvented primary to see if the most famous belt manufacturer's, vendors and installers are as thermally un-informed as those making crop circle primary cover decorators seem to be by constantly suggesting their cover holes are anything but cool art deco, I love to see too.