1974 MKIIA 850 shakes when i hit 70

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krullbeast

krullbeast

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Hi everyone. I'm looking for advice as to why it feels like my back tire starts shaking when i start to hit 70. I have tried figuring it out to no avail. everything is A ok up until i reach about 65 then if i start to accelerate it feels like the back tire starts to wobble until i pull in the clutch slow down a bit and let it out. it feels very dangerous and i've never had that happen before with my Norton. im running 32 psi in the back tire and i have one rim lock on it. the chain is good and the tightness seems fine to me. i measured everything when i was putting the tire back on, it all looks even. Has anyone else had this issue and figured it out? is it my swing arm bushings going bad? isolastics? all spokes front and back are in good shape. could it be my rim lock throwing my bike off that much? it never used to..i'm so confused and i would appreciate any advice you might have. Thanks for listening!
Lee
 
First thing I would look at would be if the wheel is balanced. lots of things to look at after that, like shocks, swing arm bushings, or even
if the wheel sprocket is correctly in line with the drive sprocket and front wheel.
 
thanks for the reply! it's not balanced great. it already has a lot of weights and i feel like the rim lock is keeping me from statically balancing it. i put the valve in front of me on an axel and it definitely moves. would this cause a high speed wobble? it never did it before until recently. i always use the center stand and the dunlop k81 tire has a lot of life in it. how do i check my swing arm bushings? how do i know if my wheel sproket is in line with the drive sproket?
 
With the bike on the center stand, grab the wheel at the rear, bid way up and wiggle side to side.

Any clunking or play?

Do the same thing from the very bottom.

If there is, grab the swingarm by the speedo and wiggle it side to side, and then also try to rotate it .

If yes on the wheel and no on the swingarm play, check axle tightness and then wheel bearings.

If yes on both then is likely swingarm bushes, but still check axle and wheel.
 
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Is your tyre ok. I had a problem like that years ago. 70 mph on a sweeper and almost a tank slapper. Tried everything first then was advised to see what happened with only one hand on the bars. Wobbled, so front tyre was removed and new one fitted. Problem solved. There was no visible problem with the original tyre. It ran true all the way when spinning it up.
 
I think play in the swing arm comes out as rocking motion more than a wheel wobble. I am with Lazyeye6, wheel balance, tyre itself then bearings. All easy to check and replace. Also on a road bike (my personal opinion, others will disagree) I don't think a rim lock is strictly necessary. It does make balancing the tyre harder. If you ride in the gravel with low pressures fair enough.
 
Agree re balancing as a likely suspect. I have never had rim locks on my current Commando - purchased in '06 - and I removed them from my original Commando back in the day. As noted, they complicate balancing and, IMO/experience, add nothing useful for street riding.
 
gortnipper said:
With the bike on the center stand, grab the wheel at the rear, bid way up and wiggle side to side.

Any clunking or play?

Do the same thing from the very bottom.

If there is, grab the swingarm by the speedo and wiggle it side to side, and then also try to rotate it .

If yes on the wheel and no on the swingarm play, check axle tightness and then wheel bearings.

If yes on both then is likely swingarm bushes, but still check axle and wheel.
Click to expand...
Do the above checking
This will also show if your rear axle has cracked,if you haven't already checked for that
Cheers
 
Last edited:
Also you might check around the headsteady for loose bolts and failed rubber mounts.
 
Front wheel balance & tire condition (and age) will play into that, rear wheel not so much.
 
thank you for all of the advice! after going over it again today, i found that my center stand was loose on one side, so i tightened it up. then i checked where my rear shocks attach to my swing arm, one side was a little loose, so i tightened that up.
lazyeye6 said:
First thing I would look at would be if the wheel is balanced. lots of things to look at after that, like shocks, swing arm bushings, or even
if the wheel sprocket is correctly in line with the drive sprocket and front wheel.
Click to expand...
thanks for the advice! my left shock was loose where it bolts to my swingarm. i'm gonna try it out again!
 
gortnipper said:
With the bike on the center stand, grab the wheel at the rear, bid way up and wiggle side to side.

Any clunking or play?

Do the same thing from the very bottom.

If there is, grab the swingarm by the speedo and wiggle it side to side, and then also try to rotate it .

If yes on the wheel and no on the swingarm play, check axle tightness and then wheel bearings.

If yes on both then is likely swingarm bushes, but still check axle and wheel.
Click to expand...
thank you for the advice! i felt no play when checking my tire. no wobbles on top or on bottom, it moved the whole bike so i'm hoping everything is good there. i grabbed the swingarm and felt no play there either. axle is good and tight. my left shock was a little loose where it bolts to the swingarm, i'm hoping that that could be it, if not, i think i'm going to remove my rim lock and get it balanced properly.
thanks again for the help!
 
Steves said:
I think play in the swing arm comes out as rocking motion more than a wheel wobble. I am with Lazyeye6, wheel balance, tyre itself then bearings. All easy to check and replace. Also on a road bike (my personal opinion, others will disagree) I don't think a rim lock is strictly necessary. It does make balancing the tyre harder. If you ride in the gravel with low pressures fair enough.
Click to expand...
thanks for the reply! if it still does it after tightening my shock, i'm going to remove the rim lock and get it balanced properly. i appreciate the help
 
baz said:
Do the above checking
This will also show if your rear axle has cracked,if you haven't already checked for that
Cheers
Click to expand...
the axle doesn' t
kerinorton said:
Is your tyre ok. I had a problem like that years ago. 70 mph on a sweeper and almost a tank slapper. Tried everything first then was advised to see what happened with only one hand on the bars. Wobbled, so front tyre was removed and new one fitted. Problem solved. There was no visible problem with the original tyre. It ran true all the way when spinning it up.
Click to expand...
it's pretty new and has no signs of weirdness...so if all else fails, i'll be buying a new one and trying it out i guess hahah. thanks!
 
could my missing speedo spacer be the culprit? i just realized it. and i probably have the axle tightened too much then too. i'm going to back out the axle a bit and wait for that spacer..ugh
 
Improperly adjusted rear brakes can also cause a vibration or even poorly installed , aligned shoes ... these old beauties can be finicky , much like a few very good looking people .... would not ride at speed until totally sorted .....
 
krullbeast said:
could my missing speedo spacer be the culprit? i just realized it. and i probably have the axle tightened too much then too. i'm going to back out the axle a bit and wait for that spacer..ugh
Click to expand...
You may need to get a new speedo drive aswell

Is it still working?
 
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