850MKII Rear sprocket graft.

Brewer · Mar 15, 2009

Hi All.
I would like to know if there is a modification for putting a conventional rear sprocket on to the rear hub of a 850 MKII.
That would replace the original norton rear spocket / brake drum single unit setup!
I am curious that perhaps someone out there has been successful in this operation???

Brewer

Jeandr · Mar 15, 2009

Not impossible, but difficult. You will have to machine the old teeth off and find a sprocket that would fit. The main problems would be:

1- the new sprocket may be bigger (more teeth) because the original 42 tooth sprocket hugs the drum very closely.

2- using a bigger rear sprocket means replacing the gearbox sprocket to maintain the same overall gear ratio.

3- welding the new one on may distort the drum.

That being said, I would do it if my bike needed a new rear sprocket.

Jean

Ratso88 · Mar 15, 2009

I haven't checked into it in a while, but . . . Sprocket Specialists used to do an "over lay" set up. You sent them your MKIII sprocket and they did the rest. I've never seen one. If anyone could do it these guys can.

Probably for less than a new Norton sproket.

Link . . . http://www.sprocketspecialists.com

Shoot 'em a e-mail.

Sorry dude, I mis read. You're looking for a MKII. Give 'em a call any way.

Brewer · Mar 15, 2009

Thanks Jean,
I do understand that there may be problems involved.
If the counter shaft sprocket was reduced .( Currently I have a 21 teeth on).
Then the rear sprocket could be increased in teeth numbers to retain the same gearing. I thought that this would allow for enough diameter to fabricate a slip on and welded sprocket carrier perhaps???

Then there may be a problem with the hubs material "weldability" and also.... the distortion factor of the rear brake drum hub after welding.

I was just wondering if anyone has had any success on this type of modification/procedure.
It does seem a waste to change the whole unit when only the sprocket is worn and not the rear brake drum hub!

I also think that perhaps you may get better wear and durabillity from the drive train, with a rear sprocket made of better material than the original Norton unit?

Brewer.

Ron L · Mar 16, 2009

Check out the conversion by Fred Eaton at Old Britts.

http://www.oldbritts.com/ob_start.html

Go to "New Products", then "Mk3 Rear Sprocket carrier with Removable Sprockets"

It's not cheap, but it works.

Jeandr · Mar 16, 2009

ludwig said:
Why not go all the way and exchange the complete wheel for something off a midweight enduro bike ?
The original Norton hub is needlessly complicated , extremely heavy , has a lousy brake and is expensive to change sprockets .
A complete switch gives you : an 18inch aloy rim , disk brake , choice of alu sprockets , a real cush drive , one piece spindle , 20 + lb ( unsprung !) weight saving , etc ..Only problem is to graft the speedo drive .

Agreed, even a drum brake from an older Japanese bike would be better, they all have very good cush drives too. There are a number of electronic speedos which can be driven from either the front or the rear wheel, even some that fit right in the original bezels.

Jean

Dr_Hiller · Mar 16, 2009

I've got the Old Britts alloy carrier, and it's a huge improvement over the integrated setup. The cush setup is very nice compared to the previous. Changing ratios without pulling the clutch is also useful. (I've got a 38 tooth "highway" gear and a 42 tooth "town" gear that require nothing other than a chain adjustment to swap.

However, it's a MkIII piece. Going the MkIII conversion route is very expensive and I wouldn't do it again unless I had a complete donor setup to work from. Add up all the new bits and you'll see what I mean.

I think Ludwig has the most viable suggestion. I'd be curious to know which hubs are a good match.

Sawsall · Mar 16, 2009

your thinking is wrong on the gear ration... going larger in the back and/or smaller in the front lowers the gear, like down shifting... nice for take off, wheelies, but bad for the highway...

bigger front, and/or smaller in the rear raises gear, like upshifting. great hyway, sucks taking off...

if you want to keep it the same, increase both, or decrease both... as a guide, 3 to 1... 3 teath rear to 1 tooth front.

JimC · Mar 16, 2009

Sawsall said:
your thinking is wrong on the gear ration... going larger in the back and/or smaller in the front lowers the gear, like down shifting... nice for take off, wheelies, but bad for the highway...

bigger front, and/or smaller in the rear raises gear, like upshifting. great hyway, sucks taking off...

if you want to keep it the same, increase both, or decrease both... as a guide, 3 to 1... 3 teath rear to 1 tooth front.

I assume you are referring to Brewer rather than David. Yes?

Brewer · Mar 16, 2009

Ok! Thanks guys.
The enduro back wheel does sound like a resonable option, however I did not want to sway away from the norton originality too much.

Though, I am very surprised that no one has performed a modification to put interchangable rear sprockets on a 850 MkII back wheel! (well to my knowledge anyway) There may even be a market for a clever invention to do so.

I will let you guys know how I go.

Thank you all for your info and advice.

Brewer.

lcrken · Mar 17, 2009

I did this mod on my race bike back in the '70s. I turned the teeth off the drum, brazed on a ring to bolt sprockets to, trued the braking surface of the drum in a lathe, and made up a set of alloy sprockets from blanks. The only real drawback is that the smallest sprocket you can fit is larger than 42T. I don't recall what the smallest size was. I'd have to pull it out of my storage container to check. I have a couple pictures of the conversion on Photobucket at

The stub axle looks different becaus I drilled it out to allow me to use a one-piece axle.

Ken

RennieK · Mar 17, 2009

Nice job Ken and it's great to have you on this forum! Welcome!

splatt · Mar 17, 2009

I had thought about this,but to get the gearing back you would have to use a bigger gear box sprocket, increasing the load on the gear box so probably defeating the whole purpose.Changing the complete back wheel would probably give you alot more gearing options.

Jeandr · Mar 17, 2009

??

Why would putting a bigger gearbox sprocket change the load on the gearbox? I assume the same overall gear ratio, for example, the gearbox sprocket is 21 teeth and the back is 42 for a 2:1 ratio. Going up to a 46 tooth back sprocket and using a 23 teeth gearbox sprocket gives the same 2:1 ratio, the loading on the gearbox should be the same shoudn't it? I think the only change would occur on the chain, there would be less force exerted on the chain, but with a longer "lever" the same amount of torque ould be transmitted to the rear wheel.

Jean

splatt · Mar 17, 2009

The faster the gearbox spins the less load on all of its internal components,you will notice they only really started to shit themselves when the factory upped the gear box sprocket. I was told they used to run hillman imp side chair motors through the gear box reliably, :wink: :wink: but they spun the gear box over a 1/3 faster.Like I said , I had thought about this but obviosly not hard enough :idea:

fastback · Mar 21, 2009

I'll admit I don't know much about this topic, but....
Bruce Chesse- of carb sleeving fame - showed me a sprocket installed on a bike, that he just turned the width down to fit a modern chain.
Can it be that simple? Perhaps I misunderstood?

Phil

Dr_Hiller · Mar 21, 2009

Splatt:

It's true there was more load when the rear sprocket stayed the same, but the torque equation changes when the rear sprocket is altered.

In short, gear inches is gear inches - same ratio = same torque. e.g. 19x42 = 20x45 = 21x48. Each of those combinations put the same stress on the gearbox because they are mechanically equal.

Fastback, yes, its that simple. To run a 520 chain you can just turn the stock sprocket(s) down.

splatt · Mar 21, 2009

splatt said:
.Like I said , I had thought about this but obviosly not hard enough :idea:

swooshdave · Apr 16, 2009

lcrken said:
I did this mod on my race bike back in the '70s. I turned the teeth off the drum, brazed on a ring to bolt sprockets to, trued the braking surface of the drum in a lathe, and made up a set of alloy sprockets from blanks. The only real drawback is that the smallest sprocket you can fit is larger than 42T. I don't recall what the smallest size was. I'd have to pull it out of my storage container to check. I have a couple pictures of the conversion on Photobucket at

http://tinyurl.com/2q6jy7

The stub axle looks different becaus I drilled it out to allow me to use a one-piece axle.

Ken

I counted 45 on one sprocket.

850MKII Rear sprocket graft.

Brewer

Jeandr

Ratso88

Brewer

Ron L

Jeandr

Dr_Hiller

Sawsall

JimC

Brewer

lcrken

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RennieK

splatt

Jeandr

splatt

fastback

Dr_Hiller

splatt

swooshdave

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