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Rear shock twist / rotate


MrJacques
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Is it normal for a rear shock to be able to twist / rotate a little bit from side to side? I'm able to move it about 3mm in either direction. No real lateral movement, just clockwise / anticlockwise. I phoned a friend who works at a bike shop and he says it's fine, they have it happening on lots of bikes. It's a DT Swiss xm180 on a bmc fourstroke.

 

And while we're getting technical, is this a good shock? I've only seen a few reviews on it and it seems like its not too bad. There's a Fox RP23 advertised in the classifieds, would it be worth it to rather put that on the bike? I'm also thinking long term for serving costs etc.

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Is it normal for a rear shock to be able to twist / rotate a little bit from side to side? I'm able to move it about 3mm in either direction. No real lateral movement, just clockwise / anticlockwise. I phoned a friend who works at a bike shop and he says it's fine, they have it happening on lots of bikes. It's a DT Swiss xm180 on a bmc fourstroke.

 

And while we're getting technical, is this a good shock? I've only seen a few reviews on it and it seems like its not too bad. There's a Fox RP23 advertised in the classifieds, would it be worth it to rather put that on the bike? I'm also thinking long term for serving costs etc.

 

DT Swiss shocks are mounted on conical bushings that allow a bit of rotating play as you describe.

 

Other shocks are mounted on straight bushings and don't allow such play. IIRC come Manitous are also mounted like that. It is neither here nor there and doesn't make or break the shock.

 

Is it a good shock? The carbon models are seriously crap. The fine thread strips right out of the air cannister and, they tend to leak along the head-to-toe seam in the canister. Certain materials are just not appropriate for certain applications. The alu ones seems fine. I have no idea what spares cost though. That's another indicator if you want to own one or not.

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Cool, thanks for the reply. I was curious and took the shock off and noticed the conical bearings you mentioned. Good to know that it's supposed to be like that. This is an alu one, so I hope it will be fine. I'll ask my lbs to find out how much the service costs are for the dt swiss and fox.

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Just to expand on Johan's post above:

 

The "spherical plane bearings" which DT use in their rear shocks is a super idea. it essentially prevents the shock from taking excessive side loads as a result of the frame/ swingarm flex or any misalignment in the frame/ swingarm. In this aspect the spherical bearings allow the shock to "self-align" preventing premature wear on the seals and bushings within the shock. Why don't other MFG's use it? Two reasons:

1.) Price: those buggers are expensive and when selling OEM every cent counts.

2.) Structural support: the same thing that makes the spherical plane bearings great in the shock make the shock ill suited to many suspension designs. Unfortunately many frames on the market actually rely on the rear shock to provide additional support for the swingarm, and with a DT style shock their design is plagued with flex.

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Makes much sense, especially point #2, thanks! It seems like bmc's frame and swingarm design has enough structural support to use the 'rotating' shock and still be stiff.

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Just to expand on Johan's post above:

 

The "spherical plane bearings" which DT use in their rear shocks is a super idea. it essentially prevents the shock from taking excessive side loads as a result of the frame/ swingarm flex or any misalignment in the frame/ swingarm. In this aspect the spherical bearings allow the shock to "self-align" preventing premature wear on the seals and bushings within the shock. Why don't other MFG's use it? Two reasons:

1.) Price: those buggers are expensive and when selling OEM every cent counts.

2.) Structural support: the same thing that makes the spherical plane bearings great in the shock make the shock ill suited to many suspension designs. Unfortunately many frames on the market actually rely on the rear shock to provide additional support for the swingarm, and with a DT style shock their design is plagued with flex.

 

Thanks for the clarification. I didn't have a DT shock in my workshop to remind me exactly why they rotate, but I did have a spare set of busings for them. Looking closely at the bushings I can now see that it isn't their conical shape but rather the spherical plain bushing that does the rotation.

 

If you examine the wear on standard shock bushings you'll see that they tend to go conical. This is from the side loads described by Rush Sports above. They are designed to only rotate around the bolt but in reality us strong okes make the frames flex and we wear our bushings to smithereens.

 

Have a look at the picture below and see how they all differ.

 

post-1761-0-35813300-1319443502.jpg

 

On the left we have a pair of standard shock bushings. They allow the shock to move in only one plane.

 

In the centre we have the spherical plain bushing. My photo's angle is from straight above but picture the centre sphere moving ball-joint style in any direction. On the right is the bushing insert from a DT shock. These would fit inside the spherical bushing. These in the picture won't fit each other since the speherical bushing is from a Specialized shock and the black bushing from a DT shock that's much smaller.

Edited by Johan Bornman
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