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Sportster Motorcycle Suspension, Frame, Forks, Handlebars, Fuel Tank, Oil Tank, Fenders Discuss Sportster Motorcycle Suspension, Frame, Forks, Handlebars, Fuel Tank, Oil Tank, Fenders problems, advice, and/or how tos.

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  #21  
Old 12th May 2009
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Cosmo, I suspect the groove was worn in by movement in the vertical axis, and has nothing to do with lateral play of the swing arm. Since the hole in the backing plate is larger than the pivot shaft, the backing plate is not supposed to touch the pivot shaft. However, this may be one of the things that happens over time as the rubbermounts get weak and move around a bit more.

Back to basics. The rubbermounts do not prevent lateral play of the swing arm and motor. The pivot shaft connects the swing arm to the rear motor/transmission mount. There should not be any lateral play there, between swing arm and rear motor mount. The 3 stabilizer links connect the engine to the frame. Again there should not be any lateral play there.

If any of these metal to metal connections are loose, only then will the rubbermounts have to stabilize the lateral play, but they are not supposed to be doing that, so the actual problem is play in either the pivot shaft or stabilizer links.

The stabilizer links do allow vertical movement of the motor and front of the swing arm in response to vibration. The rubbermount also allow for vertical movement to absorb vibration, this is also normal. This is how the motor's vibration is isolated from the frame. You can see this when the motor is idling. The motor is moving, the swing arm pivot bolts are moving, but by the time the vibration gets to the rear wheel, it is NOT changing the contact patch of the rear tire on the ground.

Well, not much anyway. An overtightened drive belt will cause the engine to be jerked backwards as the rear wheel goes over bumps in the road. This is not normal. If you take off the shocks and loosen the belt so it does not bind the rear suspension, the engine and swing arm will not jerk backwards.

I thought of replacing the rubbermounts with a harder material. But then you may get into frame cracking problems. And I am not sure they would help because there are so many other things to be fixed related to the suspension before the rubbermounts become the last and only problem. Once again, with all the things I had to fix, rubbermounts were not one of them. The XR 1200 has rubbermounts, and there are plenty of pictures of racers dragging threir knees on them.
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Last edited by XLXR; 12th May 2009 at 18:29..
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  #22  
Old 12th May 2009
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Quote:
Originally Posted by XLXR View Post
Cosmo, I suspect the groove was worn in by movement in the vertical axis, and has nothing to do with lateral play of the swing arm. Since the hole in the backing plate is larger than the pivot shaft, the backing plate is not supposed to touch the pivot shaft. However, this may be one of the things that happens over time as the rubbermounts get weak and move around a bit more.
XLXR, true. The wear was on the bottom of the pivot shaft as it sits in the bike so it would have been from vertical movement.
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  #23  
Old 12th May 2009
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Seems like the perfect solution would be to replace the rubber with urethane like they do for the handle bar dampers.
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  #24  
Old 12th May 2009
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Quote:
Seems like the perfect solution would be to replace the rubber with urethane like they do for the handle bar dampers.
My guess is that would only transmit more vibration to the frame and rider and risk frame cracking. It will not reduce lateral play of the swingarm and will not improve handling because the rubbermounts DO NOT CONTROL LATERAL PLAY OF THE SWING ARM.
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  #25  
Old 12th May 2009
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Originally Posted by XLXR View Post
My guess is that would only transmit more vibration to the frame and rider and risk frame cracking. It will not reduce lateral play of the swingarm and will not improve handling because the rubbermounts DO NOT CONTROL LATERAL PLAY OF THE SWING ARM.
When you say lateral play, you're talking about side to side?

If so, what do the stabilizers do?
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  #26  
Old 12th May 2009
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If the rear wheel contact patch moves forward and back while riding, the effect on handling will be barely noticable, at street legal speeds. Normal swing arm up and down movement in response to bumps in the road moves the rear tire contact patch forward and back as the wheel swings on the arc set by the swing arm. This is normal. The rubbermounts and stabilizer links do allow this front to rear movement.

However, if the rear wheel contact patch moves side to side, or if the tire tilts side to side, or the tire is allowed to twist like a steering wheel of a car, handling will immediately be effected for the worse and the rider will lose control very quickly. You cannot have the tire moving side to side. The rear tire must stay in line with the front to rear axis and centerline of the bike. Lateral play or side to side play, is when the rear tire turns away from the centerline. In effect, this becomes rear steer, and that does not work on a motorcycle

There is metal to metal contact, with no lateral or side to side play between the rear tire and axle, swing arm, pivot shaft, rear engine/transmission mount, and 3 stabilizer links to the frame. There should be no lateral or side to side play in any of these connections. These metal to metal connections are much stronger and have much less flex than the rubbermounts. These metal to metal connections are keeping everything in line, not the rubbermounts.

If you ever have to ride home on a crash damaged bike that allows side to side play, this will become very obvious.
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  #27  
Old 12th May 2009
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On my Road King the stabilizers play an important role in wheel alignment. Is that the case with Sportsters?
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Old 12th May 2009
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I doubt that I ever do it, but I'd like to make up a jig, that I could bolt down to the concrete, that would attach solidly to the frame, with the rear wheel off of the ground. Then see just how much movement, and in what directions you could get out of the rear wheel. Yes, I'll just have to do that.
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  #29  
Old 12th May 2009
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Very interesting post to this Ironhead rider. Is this system something like Norton used on the Commando? I seem to remember that system tied the engine and swingarm together, too.
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  #30  
Old 13th May 2009
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Originally Posted by cootertwo View Post
I doubt that I ever do it, but I'd like to make up a jig, that I could bolt down to the concrete, that would attach solidly to the frame, with the rear wheel off of the ground. Then see just how much movement, and in what directions you could get out of the rear wheel. Yes, I'll just have to do that.

Cool....do it.....I'd be very interested!
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