Rear wheel offset
- Thread starter Flo
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Flo,
In view of recent comments about Mk3's I can't comment. Seems they have offset built into the swingarm and the wheel centred.
On all others the swingarm tubes are identical side to side with front mount and main cradle having the tubes welded 3/16" to 1/4" offset, putting the whole engine and swingarm assy to the left. Since the front wheel should be centred in the forks and forks centred in the frame that means the rear wheel has to be offset right to compensate and align wheels. On mine the main cradle was 0.25" offset and the front 0.31", twisting the powerplant and wheel assy. It made the bike pull left and had the 'Commando weave'. I corrected that and it now pulls straight.
On why the engine assy is offset (in case you ask) the best reply came from this forum....for tyre/chainguard clearance! Seems crazy but if you think about it , makes sense!! It bugged me for years and no-one had come up with a consistent answer.
So to answer you question the rear wheel offset on non Mk3's should be the same as the cradle/mount offset but the opposite way. Doesn't sound like yours has enough and it's in the wrong direction anyway. If it is closer to the LH S/A tube then it is wrong. You can't have much tyre to chainguard clearance even with std 100 section tyres?
You really need to measure your actual offset and work from there but since it means a total stripdown I don't recommend it. To understand the offset thing have a look at your headsteady, that'll show what's going on since it also has to be offset to fit. It's not a part to take accurate measurements from though.
If you just want to get your wheels inline and you have the front centred then the straight edge trick on the back wheel will show you how good alignment is or not.
In view of recent comments about Mk3's I can't comment. Seems they have offset built into the swingarm and the wheel centred.
On all others the swingarm tubes are identical side to side with front mount and main cradle having the tubes welded 3/16" to 1/4" offset, putting the whole engine and swingarm assy to the left. Since the front wheel should be centred in the forks and forks centred in the frame that means the rear wheel has to be offset right to compensate and align wheels. On mine the main cradle was 0.25" offset and the front 0.31", twisting the powerplant and wheel assy. It made the bike pull left and had the 'Commando weave'. I corrected that and it now pulls straight.
On why the engine assy is offset (in case you ask) the best reply came from this forum....for tyre/chainguard clearance! Seems crazy but if you think about it , makes sense!! It bugged me for years and no-one had come up with a consistent answer.
So to answer you question the rear wheel offset on non Mk3's should be the same as the cradle/mount offset but the opposite way. Doesn't sound like yours has enough and it's in the wrong direction anyway. If it is closer to the LH S/A tube then it is wrong. You can't have much tyre to chainguard clearance even with std 100 section tyres?
You really need to measure your actual offset and work from there but since it means a total stripdown I don't recommend it. To understand the offset thing have a look at your headsteady, that'll show what's going on since it also has to be offset to fit. It's not a part to take accurate measurements from though.
If you just want to get your wheels inline and you have the front centred then the straight edge trick on the back wheel will show you how good alignment is or not.
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- Nov 20, 2004
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As ludwig says, the primary objective is to place both wheels on the centreline of the frame.
Where the rear wheel sits in relation to the swinging arm is of no actual importance provided that the end result of any hub or rim offset puts the wheel on the frame centreline.
As all cradles are supposely offset to the left, if the pre-MkIII swinging arms are in fact totally symmetrical, then the lower ends of the shocks will be pulled over to the left by the same amount as the cradle offset, because the shock lower mountings will not be an equal distance from the frame centre line so will not be in line with the shock upper mountings, the result being that the shocks will either be leaning over, or they will pull the rear end of the cradle and swinging arm assembly over to the right, which would result in the cradle assembly being out of line with the frame?
Where the rear wheel sits in relation to the swinging arm is of no actual importance provided that the end result of any hub or rim offset puts the wheel on the frame centreline.
Keith1069 said:
As all cradles are supposely offset to the left, if the pre-MkIII swinging arms are in fact totally symmetrical, then the lower ends of the shocks will be pulled over to the left by the same amount as the cradle offset, because the shock lower mountings will not be an equal distance from the frame centre line so will not be in line with the shock upper mountings, the result being that the shocks will either be leaning over, or they will pull the rear end of the cradle and swinging arm assembly over to the right, which would result in the cradle assembly being out of line with the frame?
LAB, I think it was you that told us the MK3 tubes were not symmetrical. The 750 and 850 arms I've seen were all symmetrical. Since the lower fixings are wider spaced than the top they splay out at the bottom anyway but the RH is more vertical on mine than the right, measured with the bike level (9" spirit level on the springs). Not totally accurate but there is a definite difference.
maylar
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L.A.B. said:
True, but there's no gurantee that they will be, even from the factory. And correcting factory tolerances isn't in the capabilities of most owners. A secondary objective is to get both wheels in line, which can be achieved by careful alignment of rear wheel to front (string method, etc).
As I mentioned, the front wheel that NVT supplied with my MKII was off to one side by almost 1/8". I rode it that way for 30+ years and never knew why the damn thing wanted to lean left when I took my hands off the bars.
My front wheel is out 0.08", I shall be taking it to Norman White in the winter to pull it straight.
I asked about the back wheel because if that was out, it would be going with it. I think I will see if it is in line with the main top frame tube, but not yet. I refuse to dismantle it until the winter.
I asked about the back wheel because if that was out, it would be going with it. I think I will see if it is in line with the main top frame tube, but not yet. I refuse to dismantle it until the winter.
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Keith1069 said:
Yes, my own findings were based on measurements taken from a spare unused MkIII swinging arm.
But I just can't believe the factory would have overlooked or dismissed the fact that the cradle offset would have affected the shock mountings and not corrected it before the MkIII model?
And I had also read this on the NOC website NOC-L notes which gives reference to a 1973 850 model where the owner had measured offset in the swinging arm?:
Quote:
_________________________________________________
"Commando wheel offset - the definitive answer
I have the definitive answer as to what is offset, how much, and which way!
I set up a 1973 850 on the frame table and verified it was straight. I then put together a dummy engine and installed it with new isolastics, new washers, Hemmings adjusters, and a Norvil head steady. I put both adjusters on the left side, checked the swinging arm in my fixture and then installed it in the frame. I took all the play out of the adjusters and started measuring:-
The frame is symmetrical
The engine/gearbox cradle is offset 1/8" to the left.
The swinging arm is offset 1/8" to the right so the axle pads end up centred in the frame
I then installed the rear wheel:-
The spoke flanges are offset 1/8" to the left as mounted in the arm, so the rim is laced off 1/8" to the right to put the tyre in the centre
The centre of the rim is 3.3/8" from a straight edge laid across the brake drum (not the backing plate
With the stock Dunlop rim, if you sight across the rim on the brake drum side you will see about 1/16" of hub when it's in the right place. In my opinion, front and rear adjusters should be on the same side. Otherwise as things wear and you take up the slop you angle the engine and gearbox in the frame. Since the swinging arm mounts to the cradle, a little one way in the front and a bit the other way in the back and the rear wheel is out of line considerably. The chain will still line up but the wheels will not." __________________________________________________
I read that which totally confused me. I don't think anyone will ever agree until we have the 'gospel' on this from someone like Bob Trigg (the main Commando designer in 68). For example my current bike's cradle and front mount tube 'stickout' difference measured 0.255" and 0.310" respectively before correction(outside of sideplate to tube face). Just measured my old 850 headsteady. The head mounting holes are 5/32" offset left to box section centreline. Also the rear aircleaner plate to battery tray holes are drilled offset by a similar amount (never measured it though). So 1/8", 3/16", 1/4", !!! who really knows? They all seem to vary a lot.
Next time I have the S/A off I'll remeasure it (std 750).
When I set up the bike last it was with a bare frame, cradle, S/A, wheel and bare cases. Using a plumb bob from centre front frame tubes (midway between tabs) and a line stretched to the S/A pivot centre I needed to pull the rim 1/4" to the right to get it on line. There is no guarantee it was totally accurate but it at least matched the cradle/front mount offset.
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