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Wheel offsets

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The answer to this is probably blatantly obvious, but I'll ask it anyway if only to confirm my ignorance!

I have a '73 750 with a disc front brake and the standard drum rear. The wheels were original equipment when dispatched as far as I know, so all was standard when I took the machine over.

I noticed that the front rim does not run centrally to the forks in standard configuration and that's always rankled with me.

I have fitted a narrower caliper on the front and so I have been able to move the rim over when I rebuilt the wheel (The spokes will hit the standard caliper if you do this, hence the offset.).

It now runs centrally to the forks as I believe it should. However I built the rear wheel exactly as it was originally assuming that it should have always been on the centreline of the bike.

The question is; did they build the rear wheels 5mm off the centreline originally to track properly with the offset front? I'm beginning to think I ought to have nudged it 5mm to the right as I have done the front.

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The wheels run centre to the centre line of the bike. Don't know what part of the forks you're referring to but, if you project the steering head down, the wheel centre should lie on that line. Or, if you prefer, the wheel should sit centre on the fork stanchions. This does assume that everything is straight, which is rarely the case on a 50 year-old Commando.

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I'm sorry to disappoint you Stan, but the standard Commando disc brake wheel does not run on the centre line. It's offset 5mm away from the disc so the standard caliper will clear the spokes.

Hence why I have gone to the trouble to move it to the centreline, ie equidistant between the legs and plumb to the steering stem. The front I understand, my question was about the rear. If someone can confirm that they were built on the centreline then I need do nothing, but if Norton had them over the same as the front, then I need to move mine to the centreline.

If the rear is in fact on the centreline it would explain how folk get away with swapping the fork legs and putting the disc on the left hand side. This is in fact moving the rim 10mm to the right, or 5mm to the right of the centreline instead of the left.

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My front rim is on the centre line and it's never been rebuilt and is a Nov 73 build MK2a, it may have a racing lockheed caliper now but I never moved the rim when I fitted it after removing the original caliper. The caliper side spokes are nearly vertical to get the rim centralised and still give room for the caliper, very hard to build correctly and the factory instructions included standing on the rim to move it over before final tightening of nipples.

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When building the wheel, the rim does not sit centrally relative to the spoke FLANGES on the hub. In fact , it sits NEARER the disc side, because the hub has a large projection to mount the disc on, so the flanges are offset to the non-caliper side, relative to the fork centreline. So the standard caliper is NOT  the reason for the offset. ( I have just been to the garage to measure/observe it.) In fact, if spokes were more evenly angled relative to flanges, the spokes would have a bigger clearance from caliper.

    The wheel on mine has been relaced to be centred in the forks once all assembled with axle spacers, and wheel pulled to right when axle tightened( caliper on right).

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All the disc brake wheels I have seen have been offset away from the disc. The manual even tells you to fit the brake pipe bracket between the fork and the mudguard bracket to push the mudguard over to the left, so I think that kind of confirms the issue, but no matter, thanks for telling me lots more about the front wheel, it's actually the rear I want to know about.

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If Gary's front wheel rim is set centrally between the forks, and the rear wheel is in line with it, all is well.

However, if the front rim was previously not central with respect to the forks I suggest that this would have created potentially serious handling problems (with or without the rear being in line with it, although "without" would probably be more problematic still).

Consider that if the rim is set so as not to be central in the forks then it will also be offset with respect to the axis of the steering head — which in turn means that the centre point of the tyre contact patch will be laterally offset with respect to where the steering head axis meets the road.

While I haven't attempted to think through the consequences of this they would seem to involve, at the very least, differential castor action between left and right turns, and possibly an inbuilt tendency to steer to one side.

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Think we have to let him get on with it; he's adamant that his front wheel was offset in the forks.

We've all spent enough time staring at our bikes, checking out other bikes in pub car parks, swapping wheels in and out that surely a wheel cocked 5mm off to one side would have attracted attention enough to get a ruler out. Half a century ... someone would have thought it was odd. Can you imagine the fun the Domi crew would have had taking the piss out of our Commandos : they only go round left-handers you know.

Oh, the original, rust-and-all wheel from my '74 Mk2A Interpol is centre to within 1mm (maybe closer but I can't be bothered).

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Both wheels should be on the frame centreline as others have pointed out. I placed a card template inside the stanchions to match a centre mark on the rim as in the attached pic. This was a custom fitment but the same principle as standard. The rear, again a custom fitment with a different swingarm had the rim centred between the shock mounts. The fact Gary had to fit a thinner calliper to get his wheel central on a stock bike show there is something badly wrong.

Don,t use the engine as a guide either, it is offset around 5mm to the left of centre.

The front setup is critical the rear not so much. We made a pilgrimage to see the late great Paul Smart,s  race Trident at his dealership in 1988. Interesting bloke, friendly, unpretentious but very knowledgable. He mentioned on one occasion after testing or a race, they checked the wheel alignment to find the rear was around 3/8" out but he never noticed. Given his skill level probably compensated intuitively.

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... but taking it from an angle makes it look as though the rim is in fact not central!

As central as can be! Wheel doesn,t have the custom spacers fitted and the short covers were removed for final fitting once everything was in place. Pic was to to show the concept. How do you check yours?

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Setting the front rim centrally in the forks  and  lining the rear to the front  or equally between the rear suspension units  has always worked well for me. It has also been the case that offsetting the wheels to one side has reduced   handling faults in the case of a badly balanced machine. Before making any changes I would do some hands off tests on a level road as you could make things worse if you move rims away from heavy side.

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I seem to be having similar problems regarding where the rear wheel should sit . Early stages of full rebuild and trial fitting the wheels i have had rebuilt to same position as the old rims . The wheelbuilder said that the old wheels had been rebuilt before , with a mixture of incorrrectly bent spokes of differing gauges , wheel is 13mm offset to main centre tube ( ie rim would need moving 6.5mm to center it ) The info i was searching for was whether the wheels should be in center or not . Not got forks or front wheel on yet

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That "looks" well off.   How do the sprockets line up ?   If you spin the wheel with the chain on  , the way the chain "finds" a position on the sprockets will be revealing.

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Definitely offset to the left but if you have the correct axle spacers fitted the sprocket should be in the right position relative to the frame centreline. In your case the rim needs adjusting 6.5mm to the right. The tab on your seat hoop looks to be central so if you,re lucky it will all line up by measurement and visually once it,s reset. 

Nice restoration, I only dream of having that much space to work on my bikes under cover. Old wooden shed for me.

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Yes sprockets look to be spot on in line althought not tried chain yet . There seems to be quite a space between back of drum and wheel ( picture included ) is this normal ? all felt fine and positive when tightening up . Asking as it was a 2 person job to get wheel on - the new cush rubbers want to squeese the wheel back out so needs holding whilst somone else gets bolt and spacer in . Back to zinc plating today , couple of bits required before i can get front end on and definate conclusions on wheels

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It seems logical that the wheels should be in line with the frame and steering geometry, surely?

I’m currently in mid rebuild of a 72 Commando and have had the wheels rebuilt with the aim of getting both rims centrally lined up with the frame. For the rear wheel this means that the rim is off centre in the swinging arm since this is not symmetrical with the frame. I wonder if Lawrence’s has been centred in the swinging arm.

Luckily my wheel builder was patient with me and adjusted the front rim twice for me to get it in the centre of the forks. This is with the standard Lockheed caliper and the proper thicker gauge spokes on the brake side and the clearance between the very upright spokes and the rear of the caliper is acceptable.

 

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Looking great steve , we are at about same stage by look of it . So are we saying wheels should be centered to frame , or offset 5mm or 1/4" to left as bike is heavier on left due to engine offset and clutch n sprockets , my vote would be central to frame and steering .  Anyone got an opinion on my pic of space between brake drum and wheel , would save me a few hours looking for a problem that doesnt exist , thanks

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Centered to the frame otherwise the front wheel would not be centered to the yokes.

Why this gets into the ether is odd but BSA Unit Singles get the same stories which are just as wrong. Its a function of the engines starting off at low capacities and growing, then the wheels get bigger and wider, chains get wider and the engineers have to get creative to get it all to fit, engines end up going sideways.

Two rules to guide you, wheels on centre line and sprockets/chain in line.  

What does not help is the disc wheel having the disc side spokes vertical and so the non disc side need to be under tensioned to keep the correct alignment. Even then the rim moves to the non disc side as the vertical spokes have no sideways pull to speak of.

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Yes, Laurence, remarkable how close our builds are!

I think wheels must be central to frame. Steering would surely be off if front wheel pivot point was off to one side from steering pivot? Also, both front and rear mudguards are centrally mounted. Hard to believe that wheels would intentionally be offset in mudguards.

I too had a swear inducing job fitting the rear wheel with new rubbers.

My gap looks similar to yours.  I too was initially concerned about this, but I think that as long as all axle components are metal to metal when tightened then all should be well.

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Quote John Holmes. "What does not help is the disc wheel having the disc side spokes vertical and so the non disc side need to be under tensioned to keep the correct alignment. Even then the rim moves to the non disc side as the vertical spokes have no sideways pull to speak of."

Well that might explain why mine and so many others I have seen have their rims offset away from the disc, although it seems odd that they would move over in an even way?

I will keep an eye out for any migratory habits with the new rim.

I will also perform some drop tests on the rear wheel to see where it is although the chain alignment when the wheels are in line will tell me more about whether I can accept it or not.

Sorry that so many people don't believe my claim that Commando disc front wheels are not in the middle of the forks. It will be interesting to see how many we can find when the show season gets going again this year. A simple finger test between the fork leg and rim/tyre will be enough to establish any difference. I haven't seen one in the middle yet, apart from the one I built that way.

It's a shame that they didn't make some slightly wider yokes when they added the disc to the Commando. It might have helped with the wildly asymmetric spoke angles and tiny clearance between disc and fork leg. The caliper is enormous too, especially considering the dinky little pads it uses. But discs on motorbikes were quite new then and Norton were not the only ones to make a clumsy job of it. Shame they persisted with the same stanchion centres when they redesigned the rake angle on the later 850!

The Triumph design of the same era is so superior, even allowing for fitment of twin discs with the same hub. I think the hubs interchange front and rear too?

I've yet to ride a Commando to see if it's as good as they say, but I suspect I will be chopping it in for another Triumph.. :-)

 

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Gary,

My Mk3 front end is yet to be reassembled to the bike, but I can put the assembled fork legs in the yokes and then fit an original, not rebuilt front wheel and see what respective slider to rim measurements are.

Andy

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Front wheel now trial fitted , seems to be about 2 mm closer to disc side fork leg . Regarding Gary's last post about rim migrating away from disc I don't know if it's worth getting it moved over by 1mm

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As long as you make sure the spoke ends inside the rim are not sitting proud of the nipple and smoothed down with no sharp edges/burrs then you can adjust the rim later after a few miles if it has not settled over to the non disc side.

 

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