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Adjusted chain - now low speed wobble :-/

Discussion in 'Tech Help' started by Aiji, Jul 6, 2014.

  1. Aiji
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    Aiji Member

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    I'm guessing this is a wheel alignment issue, but after adjusting my chain today I found that during a test ride I got a nasty low speed wobble on deceleration - i.e. just lifting my hands off the bars at about 30kph. Easily controlled once I got my hands back on but a bit disconcerting. I haven't had that before with this bike, so I'm assuming it's something I've done wrong. The alignment looks okay (board along the side of the wheels - looks pretty close). Any thoughts?

    Bike is a 1981 honda CB900F (suspension totally rebuilt by Laurie about 5 years ago and not that many kms since then).

    Cheers
    Jerry
     
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  2. John.R
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    If your swing arm has markings, make sure they're aligned. Also check to see if it could possibly a tight spot in the chain.

    Im sure others will come in with more precise (lasers, pythagoras hypotenuse n shit) methods of getting your alignment A-OK
     
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  3. supamodel
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    supamodel Secret Aaaaaagent Man Staff Member Moderator Supporter

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    Nothing too high tech needed, just a ruler against the rear sprocket to see where it points :). Should point to just inside the chain plate on the side of the sprocket it's against, and you just gotta make sure it's sitting straight on the sprocket, too.

    No more high tech than checking the lines on the swingarm, really.
     
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  4. DonT
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    Swing arm marks are some times out, so good to do a test like Supa mentions to be sure, then see if the marks are right for future.
    I assume chain tension is the only thing you did? Tight spot check looks like a good idea, put it on the centre stand and spin the wheel while pushing up (or down) on the chain to ensure it runs at same slack point. Maybe also check it isn't tight when the suspension is fully compressed?
    If all that looks good I'd check the steering head bearing isn't notchy.
     
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  5. Gosling1
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    Gosling1 Forum Whore of Death Veteran Member Supporter

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    check your axle nut mate - sounds like its still a bit loose.

    As far as the swingarm marks are concerned - on older bikes they are never 100% correct - they can be close, but the only way to ensure proper wheel aligmnent - which is what you need - is to string-line the wheels.

    Doing this properly will ensure your wheels are in alignment - which on a single-track vehicle is absolutely crucial. String-lining can also unearth issues with your front-end which may otherwise go unnoticed (and unsorted).

    Here is a video which shows the technique. Its basically the same as I have been doing for 30+ years - having the wheels aligned properly can make a massive improvement to how a bike handles.



    :cool:
     
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  6. supamodel
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    supamodel Secret Aaaaaagent Man Staff Member Moderator Supporter

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    I shall repeat myself only because I like to :).

    ... if there's a difference between sprocket alignment and wheel alignment, you have other issues... such as a dodgy front end or a bent frame, as Gos mentioned. Assuming you didn't bin it subsequently immediately after adjusting the chain or tweaked the forks, it should be fine to just use a ruler to get everything back in line.
     
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  7. Gosling1
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    Gosling1 Forum Whore of Death Veteran Member Supporter

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    I know what you are saying mate - but how would anyone know if there was a difference between sprocket alignment and wheel alignment by just using a small straight-edge off the sprocket ? - which is only checking the alignment of the chain run.

    You can't just align one wheel on a 2 wheeled vehicle ;) all you can do with a ruler or a laser - is ensure that the chain run is straight, using the sprocket face as the reference point. But if the swingarm is slightly tweaked - the wheel could still be sitting a bit off......and the only way to find this is to string-line using both wheels.

    I have never rated the laser chain-aligner or a short steel ruler as a tool for wheel alignment - because it is only using 1 reference point. You need to use 2 for a proper wheel alignment.....and that needs string - or a pair of really long perfectly straight, straight edges.

    I hope that makes more sense ?

    :)
     
    Last edited: Jul 6, 2014
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  8. supamodel
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    supamodel Secret Aaaaaagent Man Staff Member Moderator Supporter

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    Sure, but in this case you don't really care about wheel alignment, you care about getting the sprocket and chain in line. From which the rear wheel is aligned.

    Unless something has happened to the front end after adjusting the chain, the front end hasn't changed alignment (otherwise it would have wobbled before doing the chain adjustment). So why bother with more work than you have to?

    That's where I approach it from :).
     
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  9. Ron50
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    Sounds like a worn front tyre to me and just coincidence that you've noticed it after adjusting the chain. My shafty tells me it's time for a new tyre by behaving like this - no chain/alignment issues involved. New tyre stops the wobbles.
     
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  10. Gosling1
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    :ridgy - I can see where you are coming from.

    The reason I rate string-lining so much more than just putting a straight edge against the sprocket, making sure its in line with the chain - and then assuming this means the rear wheel is aligned - is due to prior experience with bent swingarms. Not bent that much that the eye could see it - but bent enough so that the wheels remained out of alignment even when the chain run was straight. The only way to spot this was to do the job properly.......by stringlining both wheels. The difference this makes, is worth every minute of the extra work you put into it......

    The other thing that stringlining can show, is how much the swingarm marks are out. And they will be ;) ....Once you know how much the marks are out - you can take this into account when setting the chain tension :ridgy

    This may not be Jerry's issue anyway - sounded to me much more like the rear axle nut was just a bit loose - this will cause the rear wheel to change position in the swingarm when accelerating/braking (or rolling off the throttle) and contribute to slight wobbles as he has experienced......

    I have just always worked from the approach that wheel have to be aligned in reference to each other - not just assume that the rear wheel is in alignment with the front, simply because the chain run is straight. The chain run could be straight with a swingarm that may be out of whack - which means the rear wheel is out of whack in reference to the front wheel - and this can cause just the same sort of probs....

    :cool:
     
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  11. nige69
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    Prior to re-tensioning the chain, when was the last time you rode rode no hands?

    What's the condition of your front tyre like? Scalloping on the front tyre will cause head-shake/speed wobbles
     
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  12. Aiji
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    Aiji Member

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    Thanks for all the responses guys :)
    I have re-torqued the axle nut to the manual settings (listed as 58-70 foot/lbs - I used 65) and dealt with tyre pressures - the front was a bit low 27psi instead of the usual 32. Seems to have helped a bit but the issue is still there. Next lot of daylight I'll have another go using the string technique - thanks for that Gosling :) - I had tried a rough alignment using two 2m straight square dowels of jarrah, but the string method looks more accurate.

    I have no doubt there may be some steering head bearing issues which I think might be my next port of call if it's not alignment. This does only happen at low speed - I haven't had any issues up to 100kph, and it's only on that last bit of deceleration

    The antique is going in for new cam chains in a couple of weeks, so I'm not taking it out much before then, and I might get the steering head bearings looked at at the same time. After that yep tyres will be the next thing.

    No I didn't bin it, and I suspect the axle tension and tyre pressure both contributed. But I reckon it will come down to making sure the front and rear wheels align with each other - I know that the swing arm marks are likely to be out - especially that vintage.

    So couple more things for me to try and I'll post up how it goes

    Cheers
    Jerry
     
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  13. Gosling1
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    look forward to hearing how it goes mate :)
     
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  14. suspensionsmith
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    It may also be just a characteristic of the bike owing to its age.....by that I mean its old style steering geometry and heavy but flexable forks triples brakes frame etc...lots of older bikes have this characteristic of shaking/wobbling their head with the hands off the bars on deceleration...its usually associated with the natural frequency of the structures involved coming into tune with the frequency of the forces being put into them and they thus build in intensity....putting your hands on the bars acts as a damper and takes the energy out of the wobble....modern bikes have stiffer but lighter structures and their natural frequencys usually lie outside of the frequency of the forces they are exposed to.
     
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  15. Aiji
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    Aiji Member

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    Thanks Laurie - that makes sense it terms of how it feels - I'll still double check the alignment and get it as good as it can be, then see how we go :)
     
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