Top gear Ratio with Belt Drive Setup

[thunderbolt]

I am fitting a Norvil belt drive to my '74 Mk2 850cc Roadster with 33/69 on engine sprocket and clutch basket. This gives me a primary ratio of 2.09 compared to the triplex chain ratio (original) of 2.19. Using a 20 tooth final drive (gearbox) sprocket that gives me an overall ratio of 4.38 which is the same as the quoted ratio for the '73 and '74 850 Roadsters.

My question is does using a belt drive as opposed to a chain primary drive bring into play the need to go up or down a size on the gearbox sprocket? I.E. Is there extra give in the belt that would require me to come down to 4.18 ratio, etc.

Any thoughts appreciated thanks.

P.S. My Mk2 had the 20 tooth gearbox sprocket on it when i received it with the standard primary drive.

Thanks Don

 

[Nortoniggy]

Over the years of production Norton sold the bikes with various size gearbox sprockets. Usually in the range of 19 to 21 tooth until the Mk 3 came along. There is no right or wrong sprocket and no need to maintain the original overall ratio. It depends what you want and how you use your bike. There is no 'give' in the belt, just the change in ratio.

I have a belt drive and mine is overall under-geared a little as I mainly ride on country roads and rarely go on motorways [freeways]. You may want yours different.

Ian

 

[Onder]

If you choose to go to a higher gear keep in mind that first gear may be a tad high too. So if you want to greenlane and go up on the dual carriage as well then a set of lower first gears might be worth it. However Im not in the know for the stock box and what is available gearset wise.

 

[Fast Eddie]

Modern vehicles spoil us, I was driving in my car recently (on private closed roads of course) and at 90mph it’s doing 2500rpm.

No wonder my Commando or T140 feel a tad frantic by comparison !

 

[SteveA]

Modern vehicles spoil us, I was driving in my car recently (on private closed roads of course) and at 90mph it’s doing 2500rpm.

No wonder my Commando or T140 feel a tad frantic by comparison !

Eddie.......No, you were exceeding the public road speed limits! Most of the roads I drive these days are limited at 80km or under.....thank god for a race bike or two!

Biggest problem with my camping car and box trailer is keeping the speed below limits going down hills!

To the OP, belt drives (intentionally) change the primary ratio to spin the box faster and reduce some torque loading, reduce your final drive sprocket size to suit you riding needs. 1 tooth at a time!

For belt stretch to change the ratio it would beed to be an infinite stretch! Belts don't stretch. Set the belt tension once, if nothing moves, job done, even if you change the belt. Chains stretch.

Some of us think belt drives are great for racing, mainly because a well designed one can reduce weight and inertia and adjust the primary ratio, but are not really needed on a road bike!

 

[Onder]

Belts dont leak oil. They dont need ongoing adjustment. They are light.
So they are nice on a road bike.

 

[thunderbolt]

Thanks guys for the replies.
Appreciated.
While I'm at it, how tight or loose should the belt be run? This is my first time with a belt drive.

Thanks Don

 

[Peavey Jeltz]

Try twisting the belt 45 degrees from flat at the mid point with your fingers. Jeltz doesn't do this for a living and 'is advise is worth 'alf what you paid for it. Haven't shredded a belt. Yet...

 

[swooshdave]

Thanks guys for the replies.
Appreciated.
While I'm at it, how tight or loose should the belt be run? This is my first time with a belt drive.

Thanks Don

Way looser than you think prudent. But it works. Check the belt again when it’s hot and you’ll see how much it snugs up.

 

[Fast Eddie]

According to this excellent table from Norvil 33/69 requires a pulley centre to centre measurement of 232mm.

They also state 30mm to 33mm of total up and down movement.

I ran a Norvil kit on my 850 as it was fitted by a PO and it was perfect.

Ignore the highlighted one, that’s what I was running.

 

[Brooking 850]

Thunderbolt, I run a couple of belt drives, one in a race bike and one in a Roadster.
I run my belts quite loose, probably closer to 70 or 80 degrees off horizontal when twisted between the front pulley and the clutch chain wheel.

The 1st gear is quite tall in the 5 speed race bike which really suits, and I run a 21 tooth g/box sprocket and a modified rear brake drum so I can change the rear sprockets the smallest being a 44 tooth and up from there.
The Roadster has a 21 tooth g/box sprocket on a standard AMC 4 speed and a standard 42 tooth rear sprocket.
Once again with the 30 mm Maney belt drive the 1st gear is a little tall, although this converts into 3850 (ish) rpm at 70 mph where as it would be doing 4100 or 4200 rpm with the standard primary gearing
Hope this of some help
Regards Mike

 

[thunderbolt]

Eddie, my ratio is 33/69 but centre to centre is 239.6, but take your point on the other items you mentioned.

Cheers Don

 

[thunderbolt]

Thunderbolt, I run a couple of belt drives, one in a race bike and one in a Roadster.
I run my belts quite loose, probably closer to 70 or 80 degrees off horizontal when twisted between the front pulley and the clutch chain wheel.

The 1st gear is quite tall in the 5 speed race bike which really suits, and I run a 21 tooth g/box sprocket and a modified rear brake drum so I can change the rear sprockets the smallest being a 44 tooth and up from there.
The Roadster has a 21 tooth g/box sprocket on a standard AMC 4 speed and a standard 42 tooth rear sprocket.
Once again with the 30 mm Maney belt drive the 1st gear is a little tall, although this converts into 3850 (ish) rpm at 70 mph where as it would be doing 4100 or 4200 rpm with the standard primary gearing
Hope this of some help
Regards Mike

Thanks Mike, appreciate your comments.

That's the thing to change a gearbox sprocket all the primary and clutch has to come off. That's where my Velo is a breeze - between 15 to 30 mins and a sprocket change is done.

Thanks Don

 

[Fast Eddie]

Eddie, my ratio is 33/69 but centre to centre is 239.6, but take your point on the other items you mentioned.

Cheers Don

Oops, I overlooked the fact there are two 33/69 combos !

At least you know what the centres should be. When I fitted mine, the specified c to c measurement was cock on I thought, that’s how it’s run and it’s been faultless.

 

[thunderbolt]

Oops, I overlooked the fact there are two 33/69 combos !

At least you know what the centres should be. When I fitted mine, the specified c to c measurement was cock on I thought, that’s how it’s run and it’s been faultless.

That's right Eddie, the c to c was spit on with my bike as well. Now I have fitted the 2nd gearbox adjuster on the drive side I am ready to go ahead and fit up all of the primary (inner & outer cases) as required.

I am planning to run the 20T final drive sprocket, which with the 2.09 primary ratio gives me the exact same ratios for all gears that the '73 -' 74 850 Roadster originally had from the factory.

I must say though that I have been tempted to go with a 21T gearbox sprocket which would reduce the overall ratio (mine currently 4.38:1) to 4.18:1. But concerned that 1st gear may be a bit high making getting away off the mark a bit difficult.

Cheers Don

 

[SteveA]

Eddie, my ratio is 33/69 but centre to centre is 239.6, but take your point on the other items you mentioned.

Cheers Don

My 35/65 Norman White belt drive uses a 916 belt, Ii would say the tension is set tighter than shown above.

Norman told me that he likes to get the centres as close as possible for best efficiency....it drove him mad making the belt set up for my Rickman with an Atlas style vertical motor! He wanted me to make new plates (and exhausts) to get the centres closer and I would not budge on it!