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Rear dropout alignment or...

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Rear dropout alignment or...

Old 03-12-06, 10:05 PM
  #1  
marcusbandito
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Rear dropout alignment or...

So, I recently noticed how close my rear wheel's tire is to touching the left chain stay, right behind the bottom bracket. The tire is almost making contact and there is considerable more distance between the right chain stay and the tire. This can't be normal and this isn't present on my other bikes. What can it be?

Its an older Cannondale 3.0 aluminum frame, maybe 2.8, from 1995. I've used a dropout alignment gauge on the bike and it the dropouts are aligned. My wheels are trued and dished properly, and in fact I have a new wheel set on the bike (the problem was present with the old wheels as well). I've done some limited home-brewed frame alignment checks, and although they everything looks good, I figured an aluminum frame would crack & not bend or flex, so I though this would be a dead end.

What to do? I've considered adjusting (i.e. screwing) with the rear wheel's dish and moving the rim closer to the right chain stay that way, but that would/ could affect the bikes handling & tracking and I don't want that. I've also been considering drilling and tapping a hole in the left dropout and installing a set screw to place the axle where it would center the rim in the chain stays. Or is the frame just had it?

What do you all think? What am I missing? Thanks.
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Old 03-12-06, 10:24 PM
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Sheldon Brown
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Originally Posted by marcusbandito
So, I recently noticed how close my rear wheel's tire is to touching the left chain stay, right behind the bottom bracket. The tire is almost making contact and there is considerable more distance between the right chain stay and the tire. This can't be normal and this isn't present on my other bikes. What can it be?

Its an older Cannondale 3.0 aluminum frame, maybe 2.8, from 1995. I've used a dropout alignment gauge on the bike and it the dropouts are aligned. My wheels are trued and dished properly, and in fact I have a new wheel set on the bike (the problem was present with the old wheels as well). I've done some limited home-brewed frame alignment checks, and although they everything looks good, I figured an aluminum frame would crack & not bend or flex, so I though this would be a dead end.

What to do? I've considered adjusting (i.e. screwing) with the rear wheel's dish and moving the rim closer to the right chain stay that way, but that would/ could affect the bikes handling & tracking and I don't want that. I've also been considering drilling and tapping a hole in the left dropout and installing a set screw to place the axle where it would center the rim in the chain stays. Or is the frame just had it?
The "string test" for frame alignment isn't so reliable for older Cannondales, 'cause they sometimes have bowed tubes...they would sometimes "sag" a bit during heat treatment. Although all of the attachment points tend to be correct, the tubes may curve a bit, making the middle of the seat tube a poor reference point.

I know you said the dishing was correct, but I would advise triple checking it. Install the wheel backwards and see if it's still off the same direction.

You should also check carefully to see if the rear wheel tracks in line with the front.

If you are sure it's the frame, the simplest approach would be to take a file to the front of the left dropout slot. Shouldn't take much.

Sheldon "Alignment" Brown
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|  It is by riding a bicycle that you learn the contours    |
|  of a country best, since you have to sweat up the hills  |
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|  through as you gain by riding a bicycle.                 |
|                             -- Ernest Hemingway, By-Line  |
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Old 03-12-06, 11:10 PM
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Thanks for the info about the frame. I flipped the wheel and checked the dish and have the same results. I'll take your advice and use a file on the drop out. Thanks for the idea!
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