Monolever rear wheel vertical alignment

[IanMc]

After a 2 week tour around the UK I have become much more in tune with my 1988 mono.  I've noticed a slight turn in the bike, so if I ride hands off it will fall away to the right.  This is regardless of road camber.  A very slight forward pressure on the left hand end of the handlebars makes it run true.  With some one sitting on the bike and viewed from the front, the rear wheel is obviously out of alignment with the front in the vertical plane.  It's like the swingarm has twisted under load.  Using a straight edge along the bottom of the tires  it's about 25mm out of alignment.  There is no slack in the wheel bearings or swing arm, and I find it hard to believe the swing arm has actually twisted over time.  I can't see any impact damage.  The manual says you can't adjust wheel alignment anyway, but it should be within 4mm - yikes!  It certainly explains the turn.  Anyone with any experience of this and what to do?

[Bob_Roller]

First question, any work done to the bike recently ??

[IanMc]

Sorry  I missed your reply.  Some work on the engine, but nothing around wheels, suspension etc.  I thought I had cracked it when I noticed/thought the swing arm pivot was out of line indicating a collapsed bearing maybe, but it was fine when I had a look.  I tried a rough and ready double check with a spirit level, ie put two Allen keys in the end of the swing arm spindle then packed the centre stand until the swing arm pivot was level (equal distance to Allen keys from the spirit level on the floor). Then set up the level vertically across the rear wheel spindle and measured to the rim.  It seems square within a few millimetres, so I don't think it's the swing arm. However , I tried the same trick with the front wheel which is admittedly more difficult to line up, but that didn't seem that far out either which is confusing given the difference I can see when I visually sight the wheels.  To give you an idea, the edge of the front tire is pretty much in line with the edge of the back tire on the left hand side when viewed from the front.  On the right had side the rear tire sticks out (by the difference of the width of the front and rear tires).  I am assuming now the problem is at the front end.  Any thoughts?

[Barry]

I think the 4mm spec you mentioned may be for maximum permitted wheel track offset.

Not sure about mono bikes but it was normal on the twin shock bikes for the track to be slightly offset by a few mm i.e. the front and rear wheels are not accurately in line.  They even approved of a wider spacer on the rear wheel to alleviate tyre rubbing on the swing arm and that spacer deliberately introduces even more offset which BMW were ok with provided it didn't exceed 4mm.

 Are you sure that you are not mistaking track offset as vertical alignment problem.

[IanMc]

That would make some sense of my measurements not matching what I see looking along the bike.  I can only see the bottom of the wheel so it may be an illusion that it is twisted.  It's quite an offset though!  The spacing of the swing arm in the frame is near enough equal.  The tire is very close to the swing arm now, so I am not sure I could go any further to the right (looking from behind).  I am starting to wonder if this is a red herring - the symptom I am trying to address is a noticeable pull/fall to the right.  If it's not the alignment, what else could it be?

[Barry]

Airheads are not symmetrical bikes. Along with the offset track and for the same reason of facilitating the shaft drive, all airheads have a right hand side weight bias due to the engine, transmission and final drive being mounted off center. The mono bikes must be even more weight biased to the right.

I can't say I've noticed a pull to the right on a twin shock bike because of this weight bias but that might be because of an unconscious correction on my part. I have though heard people describe a right pull before and particularly on mono's. If there is anything amiss such as head bearings, uneven tyre wear or wheel alignment then you can imagine that if an airhead is going to pull to one side then it will be to the right because of the weight bias.

I think all you can do is check everything is in spec and if so it may just be another aspect of airhead character.