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Bent frame??

Old 02-03-21, 09:03 AM
  #1  
kross57
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Bent frame??

I am working on a 2001 Cannondale R400. This is a phot of the chain alignment. Looks off to me, and I can seem to get the shifting to function properly. Am I dealing with a bent frame, bent derailleur hanger or what?

Any info appreciated.
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Old 02-03-21, 09:22 AM
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The photo doesn't give a clear picture of what's going on, to my eyes, due to some optical distortion but I would check the rear derailleur alignment first. Does the chain angle look that extreme from the top of the chainring to the rear and what does it look like when you have it shifted to the middle cogs?
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Old 02-03-21, 09:32 AM
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It's extremely unlikely that any Cannondale frame would be out of alignment. The other two choices you suggested are both possible, but more information is needed, per the previous post. Was the bike shifting correctly until recently? If so, what changed, and what specific problem are you attempting to address?
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Old 02-03-21, 09:48 AM
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Put the bike upright. Don't worry about the angle to the pulleys. Take another photo w/ the chain in the middle cog.
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Old 02-03-21, 10:08 AM
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Apparently OP not familiar with rule #49
  1. // Keep the rubber side down.It is completely unacceptable to intentionally turn one’s steed upside down for any reason under any circumstances. Besides the risk of scratching the saddle, levers and stem, it is unprofessional and a disgrace to your loyal steed. The risk of the bike falling over is increased, wheel removal/replacement is made more difficult and your bidons will leak. The only reason a bicycle should ever be in an upside down position is during mid-rotation while crashing. This Rule also applies to upside down saddle-mount roof bars.
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Old 02-03-21, 10:30 AM
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Some of us have a fondness for disregarding rules. I turn mine upside down. However there is stuff you can't do when it's upside down. You certainly can't correctly adjust the DR's with it upside down. At least I can't.
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Old 02-03-21, 10:59 AM
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Originally Posted by cxwrench View Post
Put the bike upright. Don't worry about the angle to the pulleys. Take another photo w/ the chain in the middle cog.
Do this.

But if you are adjusting the RD by the book and it still won't shift smoothly across the cassette, it is most like a bent derailleur hanger. Any shop will have the correct tool to straighten it for a few bucks.
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Old 02-03-21, 02:53 PM
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Thanks for all the info.

There is a lot of evidence that this bike took a severe hit. The rear derailleur hanger was bent. I straightened that. The rear derailleur was broken in half (replaced). The chain was twisted and dented (replaced). The front chain rings are uneven, meaning as they go around the distance between chain ring and frame varies. I replaced the crank and rings and it still does that, so either the frame or spindle are bent. Maybe both?

It does shift smoothly across the cassette in the rear. The front shifting is the problem. It is hard to adjust a derailleur when the distance from the derailleur chain guides to the chain varies as the crank spins. This is looking more and more like a lost cause.
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Old 02-03-21, 06:56 PM
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Being that the photo shows the big/big cross over combo no surprise that the chain line looks off, it is by the greatest amount in this combo (usually). With no view from the rear of the rear der and cog set we can't speak to what's going on back there. Although hanger alignment is a frequent need on any bike that has an AL hanger.

Good timing as today i did a deluxe tune on a R500 of about the same era. When I clicked on this thread all I initially saw was the badly corroded/lifting paint in the photo and I at once though "Cannondale"... Andy
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Old 02-03-21, 09:17 PM
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Originally Posted by Andrew R Stewart View Post
Being that the photo shows the big/big cross over combo no surprise that the chain line looks off, it is by the greatest amount in this combo (usually). With no view from the rear of the rear der and cog set we can't speak to what's going on back there. Although hanger alignment is a frequent need on any bike that has an AL hanger.

Good timing as today i did a deluxe tune on a R500 of about the same era. When I clicked on this thread all I initially saw was the badly corroded/lifting paint in the photo and I at once though "Cannondale"... Andy
As I said, The front chain rings move erratically, meaning as they go around the distance between chain ring and frame varies. I replaced the crank and rings and it still does that, so either the frame or spindle are bent. Maybe both?

It does shift smoothly across the cassette in the rear. The front shifting is the problem. It is hard to adjust a front derailleur when the distance from the derailleur chain guides to the chain varies as the crank spins. This is looking more and more like a lost cause. I could replace the spindle but if that doesn't work I have shoveled more sand down a rathole.
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Old 02-04-21, 01:13 AM
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Originally Posted by trailangel View Post
Apparently OP not familiar with rule #49
  1. // Keep the rubber side down.It is completely unacceptable to intentionally turn one’s steed upside down for any reason under any circumstances. Besides the risk of scratching the saddle, levers and stem, it is unprofessional and a disgrace to your loyal steed. The risk of the bike falling over is increased, wheel removal/replacement is made more difficult and your bidons will leak. The only reason a bicycle should ever be in an upside down position is during mid-rotation while crashing. This Rule also applies to upside down saddle-mount roof bars.
I disagree. I come from a horsey background and it is perfectly normal for your steed to enjoy some time upside down - as long as you are not trying to ride it at the same time. Horses always ride better when they regularly spend some time rolling around upside down, and I cannot understand why my bicycle would not benefit from the same.

Of course, if you are worried about scratching your saddle then the answer is obvious - remove it first. Fitting all the important accoutrements before riding, then their removal afterwards followed by a thorough cleaning, should be standard habitual practice that ensures all developing faults are observed before they become serious and adequate hygiene is always maintained.
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Old 02-04-21, 01:43 AM
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Originally Posted by kross57 View Post
The front chain rings are uneven, meaning as they go around the distance between chain ring and frame varies. I replaced the crank and rings and it still does that, so either the frame or spindle are bent. Maybe both?
If the front chain wheels have been replaced and they appear to wobble as they rotate then the problem is not the chain but the spindle. Of course the frame may still be bent, but if the spindle were straight then the chain wheels would not appear to move from side to side no matter what shape the frame is.

With your bike leaning against something you can tape or tie a piece of string to one spoke at the back, and run the string around the tire, all the way to the front wheel, around that tire and all the way to the back again, avoiding all fenders and stuff on the way. With the front wheel in the straight ahead position the string should just touch the tires at every point it passes. If it does and the the tires are centred between the forks and the rear frame members + it rides neutrally on the straights and feels equally comfortable in turns to the left and right then the frame is not far out. If you want to use your frame seriously then you should consider finding someone who can check frames professionally because an unknown accident damaged frame is probably not a good choice if used under extreme load.
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Old 02-04-21, 04:24 AM
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Originally Posted by Geepig View Post
If the front chain wheels have been replaced and they appear to wobble as they rotate then the problem is not the chain but the spindle. Of course the frame may still be bent, but if the spindle were straight then the chain wheels would not appear to move from side to side no matter what shape the frame is.

.
Yeah, after some thought I came to this conclusion. It's great to have someone more experienced say the same. I don't believe the frame is actually bent. The main problem was getting the front derailleur to function properly, but as I said, with the wobble in the chainrings, that was impossible. Thanks!
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Old 02-04-21, 07:51 AM
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Sorry if this is obvious but getting laid down on the drive side and doing damage can certainly bend the chainrings? Are they wobbling, the distance from the seat tube to the ring varying exactly the same as they rotate around the bottom bracket? It is not uncommon to need to bend the chainrings back straight to get your derailleur to function properly, Bicycle Research even sells a tool for this.
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Old 02-04-21, 07:53 AM
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Originally Posted by easyupbug View Post
Sorry if this is obvious but getting laid down on the drive side and doing damage can certainly bend the chainrings? Are they wobbling, the distance from the seat tube to the ring varying exactly the same as they rotate around the bottom bracket? It is not uncommon to need to bend the chainrings back straight to get your derailleur to function properly, Bicycle Research even sells a tool for this.
I tried changing the chain rings and the crank and the wobble is still there. So, the only thing left is the spindle. Correct?
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Old 02-04-21, 08:24 AM
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If you change the crankset (crank) and the chainrings wobble exactly as they had it is a bottom bracket most likely, I think I would pull the crank arm and turn it 180º and install, then if the distance from the arm to the seat tube is the same in the same spots it is the crank (rings or spider) and if it moved the bottom bracket or the frame.

Last edited by easyupbug; 02-04-21 at 09:12 AM.
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Old 02-04-21, 08:30 AM
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It’s a bit of work but not difficult so it might be necessary to disassemble the bottom bracket assembly to rule-out or rule-in a bent spindle. Roll the bare spindle across a flat surface and it should be pretty easy to notice if it’s bent. Not exactly sure but I’m thinking that more than a fair amount of force would be required to bend a spindle though it’s possible. That amount of force would likely result in collateral damage to other more vulnerable components and be pretty obvious .
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Old 02-04-21, 11:49 AM
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sovende- The BB looks to be a cartridge type, likely a Shimano UN 52 by the light grey LH side "cup's" edge. So the ability to remove the spindle and roll it on a table top is not going to happen. Although for a loose ball design this method is a good idea (just like rolling hub axles too). But you don't need to remove the axle to see a bent spindle. As easyupbug said remounting the RH arm at the 4 90* points and careful measurement will work. If the bend (and that's a big if IMO) is large enough mere spinning the spindle and watching for the end to orbit/wobble works.

For those who do have a loose ball BB and recently cleaned and greased it if there's 1 too many balls in a side (the typical 11 balls count would mistakenly be 12) the odd ball rides high in the "ring of balls" and this high point will orbit about the spindle at half the spindle's rotation. This will result in a chain ring that looks to be bent but the location of the bent portion is constantly moving about at a different (1/2) rate of the crank's rotation. When this, one too many balls, id done in a wheel the rim looks to be out of true, you align that spot with spoke tension and give the wheel a spin only to find out that you still have a wobble but now in a different spot (and actually you have two wobbles, one due to the 10th ball not fitting in the race fully and the other due to your trying to use spokes to fix a hub problem).

It's that last sentence that should be re read. Knowing where the problem actually is prevents mistaken "corrective" work being done to other areas. Andy
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Old 02-06-21, 10:31 AM
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Do the string test for a bent frame. Remove the rear wheel, run a string from the left drop out up to and around the head tube and back to the right drop out. Then measure the distance from the string to the seat tube on both sides. The distance should be the same. Then check the alignment of the rear drop out where the RD mounts.

Last edited by rydabent; 02-09-21 at 10:30 AM.
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Old 02-08-21, 06:42 AM
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It turned out that the crank was not sitting straight on the spindle. Once I corrected that, the front derailleur started functioning much better. At this point I still have a slight chain rub in 24th gear (largest cog in front, smallest in rear), but that's it. Many thanks to all!
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Old 02-08-21, 06:48 AM
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Originally Posted by kross57 View Post
It turned out that the crank was not sitting straight on the spindle. Once I corrected that, the front derailleur started functioning much better. At this point I still have a slight chain rub in 24th gear (largest cog in front, smallest in rear), but that's it. Many thanks to all!
Excellent news!

This week I discovered that with one of my frames the wheel does not sit centered between the chain stays, and I suspect I will need to do some heavy work with blocks of timber and a lever bar...
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