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wobble
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op200
Posted 12/13/2004 8:00 PM (#5645)
Subject: wobble


I've got a BMW /2 conversion (/2 frame with an R75 motor ect.) I'm running 15" car wheels & tires and I pull a trailer. The other day I tried rideing it without the trailer and found that it had a real bad low speed wobble. If I can get it up to speed (maybe 25 - 30) it's ok but usually I can't get it out of 1st before the wobble starts. Once it starts it keeps getting worse untill I stop. It has the /2 leading link front end. Pulling the trailer keeps it more stable. I have a steering dampner (replaced it). Anyone have any ideas?

Edited by op200 12/13/2004 8:04 PM
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Bandit Bill
Posted 12/13/2004 11:44 PM (#5650 - in reply to #5645)
Subject: RE: wobble


Interesting that the trailer lessens the tendancy for the rig to wobble, how is the trailer hitch attached? By the rear wheel swing arm, or by the sidecar frame? If by the swingarm, i'd say your swingarm bushings/bearings are shot.. what the trailer is doing in that case is 'preloading' the excess slop out of the swingarm pivot for you, reducing the amount of free play that will pre-dispose the tendancy to wobble.

Any bearing looseness/wear/excess free play will let itself be known with a rig. Check headstock bearings for excessive wear, and correct bearing preload. Since you've got a heavier front car tire front end, check that the wheel bearings haven't gone funny on you - all that extra rotational mass can prematurely wear out wheel bearings. Even unbalanced tires can cause instability in front end handling.

Bear in mind, that rigs do tend to have a wobble range, which steering dampners only really mask the symptoms. not provide the cure to wobbles, dependant on variance from ideal rake/trail of the front end for sidecar use. Any wear in the chassis will amplify this issue to a much greater extent.

my $.02 .. FWIW..

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Hack'n
Posted 12/14/2004 12:54 AM (#5654 - in reply to #5645)
Subject: RE: wobble



Expert

Posts: 4833
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Location: Boise, Idaho
You don't mention a sidecar. If none is present you may very well get an uncontrollable wobble with sidecar rake and trail and a steering damper.
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Posted 12/17/2004 2:15 PM (#5687 - in reply to #5645)
Subject: RE: wobble


I agree with Bill. Check your steering head and swing arm bearing conditon and preload and let us know what you find.

Jim
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op200
Posted 12/17/2004 3:44 PM (#5688 - in reply to #5645)
Subject: wobble


It looks like the front swing arm. With the wheel on the ground if I move the wheel side to side I can see a little movement there. I do have a sidecar. I started to take it apart but it looks like a big job. It's been so long since I put this thing together that I forgot how the singarm works. Maybe I'll head over to the libary adn see if I can find a manule.
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Posted 12/17/2004 5:57 PM (#5689 - in reply to #5645)
Subject: RE: wobble


These are tapered bearings very similar to the rear swingarm but with a bolt as an axle that goes through from the right side to the left.
On the left side there's a female thread in the fork leg and a lock nut, the bolt comes out to the right.
To re-adjust the bearings open the locknut, adjust at the bolt and re-tighten the nut.
It's the best to unmount the frontwheel and shocks to get the right feeling.
If you intend to disassemble the swingarm for cleaning/greasing purposes, then you should have two special tools.
The first is a simple 8mm round steel a bit longer than the width of the fork with a thread M8 at one end.
On the left side of the swingarm-axle there's a short female thread M8.
Here one can screw in this M8-rod to push/hit the axle out.
There are spacers/shims between forklegs and swingarm that will fall down now.
Note where they came from cause they are not necessarily of the same thickness.
At the re-assembly you really should have the above mentioned rod plus the second tool, which is a piece of round steel
with a M16x1,5 thread at the outside and a centric hole diam 8.1.
You screw it in into the left forkleg as a guide for the rod. Then you push the rod which is connected with the axle
through the right leg, the right spacer, the swingarm, the left spacer and this guide in the left swingarm.
This way you have a chance to find the thread on the left with the axle while all the spacers are in place.
Without these two aids it is very hard to find it, the left spacer will fall out repeatetly and you'll very likely
damage the thread at the axle and/or forkleg.
The free play between fork and swingarm should be shimmed to 0.1mm max in order to avoid excessive bend of the fork.

Hope this helps!

Good luck,

Clemens
(wringing 30 year old memories out of his head ;
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op200
Posted 1/2/2005 6:34 PM (#5812 - in reply to #5645)
Subject: wobble


Thanks Clemens, Sorry it took so long to get back but I've been unable to workk on the thing. I was able to make the first tool, don't know how I'thread the second.
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op200
Posted 2/6/2005 6:41 PM (#6219 - in reply to #5645)
Subject: wobble


Well here it is 2 mounths latter and I finlly got it back together. I just got back from the test ride and guess what .... It still wobbles, I put more air in the tires (was 15 lbs. now 50) and it didn't change. It starts at about 15 MPH.
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