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Old 02-26-09, 08:42 PM   #1
PMK
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Co-Motion aluminum frame question

We now ride an aluminum Co-Motion Roadster with disc brakes.

Tonight while on a ride, we were stopped at a light. Just being goofy I was holding the rear brake and noticing how much front to rear rolling movement was happening with the brake locked. After crossing the road, the wife asked to stop for a moment. With her off the bike, I had her hold the rear brake locked and I rolled the rear wheel with my hand in a for and aft movement. The torque load from the brake was flexing the drop out / frame tubes enough to feel this rolling motion.

My first thoughts were the brake caliper mount is loose though it's not.

So anyone else with an aluminum frame and discs seeing this or similar flex.

Any thoughts? My initial one is to run a strut or brace from the forward caliper bolt to the v brake post helping to stabilize it.

Just asking.

PK
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Old 02-27-09, 05:50 AM   #2
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To add a touch more information, this left side drop out is intricately manufactured and joins the tubes well.

I believe the majority of this flex under load is from where the drop out was lightened and there is a blended radius between the axle mount and brake mount. Almost as if this had not been "notched" there would be a lot more strength through the cross section.

I'll attempt some photos tonight.

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Old 02-27-09, 08:28 AM   #3
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Originally Posted by PMK View Post
Any thoughts?
Yes, what did your dealer or the folks at Co-Motion have to say about your observations?
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Old 02-27-09, 11:56 AM   #4
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Hi PMK, I have an aluminium Cannondale and the rear is rock solid. No move at all at the drop or tube level.

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Old 02-27-09, 12:30 PM   #5
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It sounds like you have the play or movement traced to the aluminum frame.
My first thought when reading your tread was that you were observing wind-up from the wheel spokes. I have seen disk wheels with wind-up that behave similar to your experience. I wonder if the play is a bit of both ?
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Old 02-27-09, 02:04 PM   #6
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Something's not adding up here. As you can see in the pic below the stays, drop out, and mount for a disc brake are pretty robust on a Co-Motion aluminum frame. Something is wrong if you can cause significant flex in the stay rotating the wheel with one hand.

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Old 02-27-09, 04:51 PM   #7
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OK guys, here's a quick update, contacted Co-Motion today. They assured me they don't have failures or concerns about this. And if we were the original owners it would be covered if it failed. These frames and the company so far for us is top notch.

As for the flex, the photo Merlin posted is an identical drop out to ours. While the drop out is robust,(no pun intended), when you add the disc brake caliper adapter, and the caliper itself installed in a 203mm disc application, it adds a lot of height to that brake mount.

I will take some time and see if I can pin down the exact flex point or see if it is cumulative of several points.

PK
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Last edited by PMK; 02-28-09 at 04:16 AM. Reason: Added brake installation photo
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