Belt Drive tension

What is the procedure for the correct tension of a belt primary drive system?
Is it measured like a standard primary chain, with a certain amount of play in the top run? If so, what should this be?
Or is it measured by twisting the belt on the top run a certain number of degrees? Again if so, how many?
My installation is a Maney belt drive system on a Norton Commando, but I'm sure it is the same procedure for all belt drive systems.
Thanks in advance.

Keep the tension loose. On a cold bike i.e. not run. You should have a easy sliding fit at the clutch basket. The belt slides back and forth with ease . Twist the belt easily to 90 degrees at the center spot. These baskets are aluminum and the expansion rate is far more pronounce which causes the belt to become too tight. Watch for the belt track. It needs to stay centered at the pulley. Don't cheap out by using just one lollipop adjuster.

IIRC you need 30mm of up and down movement with that type of belt.

The issue of belt alignment is extremely important and you should spend whatever time is necessary to get it dead on. Adjusting the chain tension is hit or miss anyway due to slop in the adjuster, slop in the transmission mounting bolt holes etc including the possibility that someone left out the washer between the trans and the left side of the cradle and cranked it untill everything is tight. It always takes a series of "ride it and check, ride it and check". Repeat until the chain tension is where you want it. With a belt alignment becomes critical. You could end up with a snaggle of wires and a potential of real destruction.
Using a left side adjuster is the best idea. I have even gone to cutting a bit of aluminum to just fit into the slot which will positively prevent the transmission from shifting while under load. Now, if you are trying to actually cock the transmission to get the belt to run true, it takes very little movement of the trans to send the belt drifting to either side. What I am getting at is to check the alignment and tension regularly for the first few and short rides.

+1 on the 90 degrees twist test at the mid point (upper or lower run)

One important thing you need to do when you adjust a belt.

Move the transmission back until the belt is completely tight. Then snug up the mounting bolt just enough to hold the trans in position firmly but not so tight the adjuster can not cause it to slip.

Then use the adjuster to push the trans forward until you have the correct amount of play. Turning the belt 90 degrees easily with tow fingers usually works. Then tighten the trans mount bolts.

If you use the adjusters to push the trans back until the tension is correct then the rear chain pull will eventually pull the trans back just a few thousandths more and take up the play around the mount bolt. Then your belt will be too tight and it will fail the first time you get it good and hot.

I ran my first belt too tight & it failed .
I do it as mentioned above & have had no problems since.

Fast Eddie said:

IIRC you need 30mm of up and down movement with that type of belt.

Click to expand...

Which is why it's important to check with the manufacturer, the 30mm play specified by Steve does not allow for a 90 degree twist mid-run.

OP, when you get the Maney kit installed, do as comnoz suggests and pull the tranny back to tighten the belt, then progressively move the tranny forward (loosening belt tension) until you hit 30mm total play (measure with a caliper opened to 30mm)

I don’t run a belt drive on my Commando, but I have repaired & set them up (industrial cog belt drives) for a decades.

All these tension values are incomplete.

Belt deflection specification MUST include the amount of force applied to accomplish that deflection. One persons piano fingers vs another gorilla’s giant catchers mitt hand is a huge variable.

https://www.royalsupply.com/store/p...MIubKQ_pq95gIVRz0MCh2qkgA6EAQYAiABEgI2tPD_BwE

Like “20mm deflection with 2.5kg force”
would be a proper EXPRESSION of a tension specification. (Example)

Otherwise, like:
“Don’t go too fast”
or: “air up the tires”

It’s wildly subjective with only half the info.

JMWO

Edited to clarify.

concours said:

I don’t a belt drive on my Commando, but I have repaired & set them up (industrial cog belt drives) for a decades.

All these tension values are incomplete.

Belt deflection specification MUST include the amount of force applied to accomplish that deflection. One persons piano fingers vs another gorilla’s giant catchers mitt hand is a huge variable.

https://www.royalsupply.com/store/p...MIubKQ_pq95gIVRz0MCh2qkgA6EAQYAiABEgI2tPD_BwE

Like “20mm deflection with 2.5kg force”
would be a proper specification.

Otherwise, like:
“Don’t go too fast”
or: “air up the tires”

It’s wildly subjective with only half the info.

JMWO

Click to expand...

I would agree. But the needed tension also depends on how much play is in your sleeve bushing.
I don't think a tension measuring device would do much good.

I just start out loose and get the alignment right. If it's too loose you will feel it bind when you try to kickstart the engine or jump teeth under acceleration when cold. Then you know you need to snug it up slightly, equally with both adjusters.
But don't tighten the belt with the adjuster.
Instead loosen the top bolts. Loosen the bottom bolt just enough to slip.
Move both adjusters about 1/2 turn and then pull the trans back by stepping on the chain.

I understand the loose fit for the clutch/ mainshaft thing.
So, perhaps “50mm deflection, with 1kg. force applied” (I use a fish scale & bent wire hook for the light stuff) for example, or wherever the successful, long living set-up measures out at.

If a person has a lot of mechanical experience, np, instincts.

But, for shade trees, some spec would clear up a lot.

concours said:

I understand the loose fit for the clutch/ mainshaft thing.
So, perhaps “50mm deflection, with 1kg. force applied” (I use a fish scale & bent wire hook for the light stuff) for example, or wherever the successful, long living set-up measures out at.

If a person has a lot of mechanical experience, np, instincts.

But, for shade trees, some spec would clear up a lot.

Click to expand...

I agree, but due to variables, I don't know of anything that always works -except just tight enough to keep it from jumping teeth.

I have never seen a belt damaged because it was too loose -but I have seen a lot of them fail because they were too tight.

That’s why I prefer the up and down free play figure to the twist method. There really isn’t much variation in my opinion when looking for up and down movement. You’d have to be a bit of a He-man to stretch the belt this way with fingertips. It’s quite obvious (to most) when you’ve reached the end of the free play amount of movements.

However, the ‘twist through 90 degrees’ method is massively subjective to the amount of force applied, I find it very difficult to ‘feel’ with this method.

As Comnoz has said, slightly too loose is better than too tight, so if in doubt, always Go on the loose side of correct.

Fast Eddie said:

That’s why I prefer the up and down free play figure to the twist method. There really isn’t much variation in my opinion when looking for up and down movement. You’d have to be a bit of a He-man to stretch the belt this way with fingertips. It’s quite obvious (to most) when you’ve reached the end of the free play amount of movements.

However, the ‘twist through 90 degrees’ method is massively subjective to the amount of force applied, I find it very difficult to ‘feel’ with this method.

As Comnox has said, slightly too loose is better than too tight, so if in doubt, always Go on the loose side of correct.

Click to expand...

Yeah, and that is how I do it on my bike. I put a load on the rear drive belt. With that held tight I check the primary belt and look for about 1/2 inch of slop with light finger pressure.
But I arrived at that by learning where it skipped teeth and went from there.

Perhaps this only applies to the BNR setup because that is what I have. My problem seems to be that the belt, when set where is seems to be correct, will rub on the mounting bosses for the stator. I have relieved them as much as I dare and still I will get some belt contact probably when the engine
is reverse loaded. This is not only on the Norton but on my Interceptor as well just not as bad. It would seem that a needle bearing in the place where
the slipper used to go would be a good idea. When cold you get a lot of slop.