bent aluminum seatstay; fixable?
08-08-21, 08:20 AM
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bent aluminum seatstay; fixable?
So yesterday's group ride was cut short by a minor incident that probably would count as "Comedy Of Errors" if it weren't for the fact that a couple people sustained some minor injuries. In addition to some gnarly looking bone bruises on my knee and shin, my biggest personal takeaway was a bent drive-side seatstay. See pics:
Frame is aluminum (Cannondale CAADX). My vague recollection of Metallurgy For Cyclists tells my that aluminum has a finite fatigue cycle limit (or something?) that suggests that bending this seatstay back into straightness might compromise its effectiveness? Da hell do I know? So I'm asking:
- Can this be bent back to straightness?
- Should this be bent back to straightness?
- Presuming the answer to those first two questions is "yes" is this a task that most Local Bike Shops can perform?
- Given that I'm already predisposed to not having extreme faith in any of my Local Bike Shops, what other questions should I be asking them to give me confidence that they understand the scope of the problem and the correct solution?
e.g., I'm pretty sure I want them to confirm the dropouts are square, right? Anything else obvious -- or not so obvious -- like that?
Any insight into this problem would be greatly appreciated. Thanks,
Frame is aluminum (Cannondale CAADX). My vague recollection of Metallurgy For Cyclists tells my that aluminum has a finite fatigue cycle limit (or something?) that suggests that bending this seatstay back into straightness might compromise its effectiveness? Da hell do I know? So I'm asking:
- Can this be bent back to straightness?
- Should this be bent back to straightness?
- Presuming the answer to those first two questions is "yes" is this a task that most Local Bike Shops can perform?
- Given that I'm already predisposed to not having extreme faith in any of my Local Bike Shops, what other questions should I be asking them to give me confidence that they understand the scope of the problem and the correct solution?
e.g., I'm pretty sure I want them to confirm the dropouts are square, right? Anything else obvious -- or not so obvious -- like that?
Any insight into this problem would be greatly appreciated. Thanks,
08-08-21, 10:10 AM
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Best advice is to find a frame builder or experienced shop to provide their opinion after hands-on evaluation.
My opinion is that this has "repair" potential, but would never be a frame to trust in critical moments. From the pictures, the bend seems to have allowed the seat stay to remain cylindrical, rather than collapsing or creasing the tube wall. That would give me hope that a gentle and careful repair could be effectuated.
EDIT:
You should also be concerned with damage at the seat cluster, dropout, and bottom bracket. It isn't possible to change the length of the side of a triangle without also changing the angles that the sides meet.
My opinion is that this has "repair" potential, but would never be a frame to trust in critical moments. From the pictures, the bend seems to have allowed the seat stay to remain cylindrical, rather than collapsing or creasing the tube wall. That would give me hope that a gentle and careful repair could be effectuated.
EDIT:
You should also be concerned with damage at the seat cluster, dropout, and bottom bracket. It isn't possible to change the length of the side of a triangle without also changing the angles that the sides meet.
Last edited by Unca_Sam; 08-08-21 at 10:16 AM.
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08-08-21, 10:30 AM
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If a repair is intended it definitely needs to be done by a reputable professional frame builder. In that it is an aluminum frame I would strongly consider frame retirement. Looks like all the other components survived!
But you know this... Ha
If there are any still around... Looks like your going to get another frame and switch out all the components for a new bicycle... Fun Fun Fun...
But you know this... Ha
If there are any still around... Looks like your going to get another frame and switch out all the components for a new bicycle... Fun Fun Fun...
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08-09-21, 07:50 AM
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You got it sort of half right -
a fatigue limit is the amount of stress that, if a material never sees stress above this limit*, it can theoretically never experience fatigue failure. NB that these stresses are always in the 'elastic deformation' range - minor deflections
Aluminum is known for not having a fatigue limit - any amount of stress, no matter how small, is likely to eventually lead to a fatigue failure.
It kinda sounds dire, but that is only a property of the bare material. Through design and engineering and construction, a part (in this case a frame) can be made to last billions of stress cycles. If you were to assume one pedal stroke = one stress cycle, and assume you move 2 meters per pedal stroke (700c wheels with a roughly one-to-one ratio, say 24-24), you get 2 billion meters of riding, or 2 million kms.
* 'if a material never sees stress above this limit' is a big 'if'. Designers and engineers will make assumptions about the type of loads a bike will experience, and try to design and construct a frame that can take these loads and no part of the frame will see stress above the fatigue limit, or in the case of aluminum or other materials without a FL at stresses so low that some high number of cycles (billions?) are expected. The problem is that the use the bike sees may or may not exceed the assumptions made during design. Also, fatigue is only one way a material can fail. Material can fail from a single extraordinary load, or it can corrode, or it can be constructed in such a way that the strength of the material is compromised. As a buddy of mine says, it's far more likely that your bike will die from 'misadventure' long before fatigue failure.
About the dent
First, I cannot see the pictures. But in general, if an aluminum frame is damaged but still ridable, it's almost always preferable to leave it as-is because every successive bend the metal sees weakens it significantly. The discussion of 'fatigue limits' is not relevant here because bending the metal to a point where it stays is 'plastic deformation' - beyond the elastic deformation range where fatigue limits apply. A single bend might reduce the strength by 25%(just guessing at numbers for the sake of discussion), but bending it back might reduce it another 40% or 50% or more (again, guessing), or it might crack as soon as you try the repair.
As long as the wheel still sits straight in the frame, leave it. If the wheel does not sit straight, then there is no reason not to try a repair as the bike is garbage if it can't be fixed anyway. But either way, understand that the strength of the frame is already compromised and treat it as such, and also know that any repair attempts will weaken it further.
This same logic applies to steel frames but they do generally have the ability to be straightened out a couple times before the strength is so compromised that they shouldn't be ridden.
a fatigue limit is the amount of stress that, if a material never sees stress above this limit*, it can theoretically never experience fatigue failure. NB that these stresses are always in the 'elastic deformation' range - minor deflections
Aluminum is known for not having a fatigue limit - any amount of stress, no matter how small, is likely to eventually lead to a fatigue failure.
It kinda sounds dire, but that is only a property of the bare material. Through design and engineering and construction, a part (in this case a frame) can be made to last billions of stress cycles. If you were to assume one pedal stroke = one stress cycle, and assume you move 2 meters per pedal stroke (700c wheels with a roughly one-to-one ratio, say 24-24), you get 2 billion meters of riding, or 2 million kms.
* 'if a material never sees stress above this limit' is a big 'if'. Designers and engineers will make assumptions about the type of loads a bike will experience, and try to design and construct a frame that can take these loads and no part of the frame will see stress above the fatigue limit, or in the case of aluminum or other materials without a FL at stresses so low that some high number of cycles (billions?) are expected. The problem is that the use the bike sees may or may not exceed the assumptions made during design. Also, fatigue is only one way a material can fail. Material can fail from a single extraordinary load, or it can corrode, or it can be constructed in such a way that the strength of the material is compromised. As a buddy of mine says, it's far more likely that your bike will die from 'misadventure' long before fatigue failure.
About the dent
First, I cannot see the pictures. But in general, if an aluminum frame is damaged but still ridable, it's almost always preferable to leave it as-is because every successive bend the metal sees weakens it significantly. The discussion of 'fatigue limits' is not relevant here because bending the metal to a point where it stays is 'plastic deformation' - beyond the elastic deformation range where fatigue limits apply. A single bend might reduce the strength by 25%(just guessing at numbers for the sake of discussion), but bending it back might reduce it another 40% or 50% or more (again, guessing), or it might crack as soon as you try the repair.
As long as the wheel still sits straight in the frame, leave it. If the wheel does not sit straight, then there is no reason not to try a repair as the bike is garbage if it can't be fixed anyway. But either way, understand that the strength of the frame is already compromised and treat it as such, and also know that any repair attempts will weaken it further.
This same logic applies to steel frames but they do generally have the ability to be straightened out a couple times before the strength is so compromised that they shouldn't be ridden.
08-10-21, 07:09 AM
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Agreed with above, the chain stay, has been stressed / stretched , i.e. its become longer, so if it is made straight again, it could well be longer, & therefore could push the rear wheel out of alignment.
Just my thought.
Just my thought.
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08-10-21, 02:34 PM
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I agree with ClydeClydeson . This has nothing to do with fatigue (which happens below the plastic deformation limit). It's bent = you have gone beyond the plastic deformation limit of the material. At that point, and in particular with heat-treated aluminum, you have significantly lowered the resistance to further deformation; and if you were to bend it back, the probability of fracture is very high, but at the minimum the resistance would drop much further. Rear wheel alignment would be the least of your problems. You have two options: ride it as-is, assuming the rear wheel is not horribly off; or replace the frame.
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08-11-21, 08:51 AM
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Agree with 'best do nothing' approach but with the proviso that the dent has not affected the wheel angle in the frame.
A google on checking frame alignment will give some simple tests to assess alignment issues that just an eyeball
of whether the wheel sits straight in the frame will tell you. In the event the tube cracks at some point what would
be the effect? probably the wheel would shift into the frame and you would slow down rapidly, so riding at <25 mph
or so and wattage outputs under 250 would be prudent. That is if you were sprinting and it cracked you would crash,
but just tooling along you would stop and maybe fall over.
A google on checking frame alignment will give some simple tests to assess alignment issues that just an eyeball
of whether the wheel sits straight in the frame will tell you. In the event the tube cracks at some point what would
be the effect? probably the wheel would shift into the frame and you would slow down rapidly, so riding at <25 mph
or so and wattage outputs under 250 would be prudent. That is if you were sprinting and it cracked you would crash,
but just tooling along you would stop and maybe fall over.
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08-11-21, 06:38 PM
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Klein Bicycles told me once the only way to correctly bend aluminum is by heat treating it. If you can find a frame builder who can heat it up and bend back, THEN re-paint, go for it. I would think a replacement frame is in order though.
08-12-21, 06:56 AM
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Sounds like the consensus is to not attempt a repair. So, two options:
1) Ride it as-is. What could go wrong? Even if the seatstay fails, you still have another seatstay and two chainstays to hold the rear triangle together. Will it be catastrophic? I dunno. We do know that the frame's alignment is off.
2) Retire the frame (or the whole bike) to trainer use exclusively. If it fails on your trainer, you're probably not gonna lose any teeth or end up with a concussion.
I would take option 2, but then, I really like my teeth.
1) Ride it as-is. What could go wrong? Even if the seatstay fails, you still have another seatstay and two chainstays to hold the rear triangle together. Will it be catastrophic? I dunno. We do know that the frame's alignment is off.
2) Retire the frame (or the whole bike) to trainer use exclusively. If it fails on your trainer, you're probably not gonna lose any teeth or end up with a concussion.
I would take option 2, but then, I really like my teeth.
08-12-21, 07:40 AM
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Trainer bike if you still want to use it. When I got hit on my steel bike, the rear triangle got knocked out of alignment. I kept the frame, but if I ever bend it back, it'll only be as a trainer bike. Unless I get an actual frame builder to do it for me and tell me it's safe. And that's a steel bike that's just knocked out of alignment. Actual bend on an aluminum bike, I'd only ever ride it on the trainer, and still check it every ride.
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08-12-21, 02:41 PM
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don't think so. it bent and is stressed at the bend point, but it is not longer in length, the bend would have caused it to be shorter from attachment point to to attachment point (not much, but in alignment not much can be lot) effectively pulling the rear out of alignment
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08-12-21, 03:11 PM
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Personally, if I was to use it, I'd straighten it, and verify overall alignment.
Maybe not do dent repair, but get it straight and aligned.
Doing so will help reduce stress on adjoining parts which may or may not be experiencing a permanent bend. It would also reduce every bump sending shocks through that single point bend.
I think the wheel may be out of alignment in the third photo.
Maybe not do dent repair, but get it straight and aligned.
Doing so will help reduce stress on adjoining parts which may or may not be experiencing a permanent bend. It would also reduce every bump sending shocks through that single point bend.
I think the wheel may be out of alignment in the third photo.
08-12-21, 05:54 PM
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Seat stays are about the last tube to break on a frame. I've never seen one anyway, other than the welds from the seat tube or dropouts. I'd get a couple blocks of wood and some C-clamps, and have at it. If it does ever break, and I bet it won't, you'll probably catch it because it'll start creaking.
08-12-21, 08:06 PM
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Pretty much no one will touch this because it's compromised and also a normal, well equipped bike shop probably can't sort this out--it's pretty common to have tooling to correct the spacing of the dropouts relative to the centerline of the frame and also to align the dropouts, but pretty much only a frame builder would have tooling to align dropouts at effective different distances from the bottom bracket. For all practical intents, its a gonner. It is not a valuable enough frame to be worth the only real correct fix (alignment, re heat treating, and repainting, if not tube replacement).
Cannondale might offer a discount on a crash replacement frame, or you can build it back up on whatever. Whoever caused you to crash technically is liable, though you may or may not want to pursue this for your local group ride.
Please don't pursue trying to make this frame rideable.
Cannondale might offer a discount on a crash replacement frame, or you can build it back up on whatever. Whoever caused you to crash technically is liable, though you may or may not want to pursue this for your local group ride.
Please don't pursue trying to make this frame rideable.
08-13-21, 07:21 AM
#16
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btw, my personal feeling re: liability is that group cycling is a potentially dangerous and expensive activity, **** Happens, and anybody/everybody who willingly gets involved in this activity accepts that risk. It would never even occur to me to pursue compensation from a rider who inadvertently caused damage to my equipment, and I've even declined the offer the one or two times someone was humble and generous enough to make it.
If I'd been hit by a car I would have a very different attitude, but this was a Perfect Storm involving only a few fellow cyclists. No need to be litigious.
Last edited by cb400bill; 09-08-21 at 01:17 PM. Reason: please don't change the spelling of words to bypass the forum censor
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08-13-21, 08:01 AM
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don't think so. it bent and is stressed at the bend point, but it is not longer in length, the bend would have caused it to be shorter from attachment point to to attachment point (not much, but in alignment not much can be lot) effectively pulling the rear out of alignment
08-13-21, 08:12 AM
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btw, my personal feeling re: liability is that group cycling is a potentially dangerous and expensive activity, **** Happens, and anybody/everybody who willingly gets involved in this activity accepts that risk. It would never even occur to me to pursue compensation from a rider who inadvertently caused damage to my equipment, and I've even declined the offer the one or two times someone was humble and generous enough to make it.
If I'd been hit by a car I would have a very different attitude, but this was a Perfect Storm involving only a few fellow cyclists. No need to be litigious.
If I'd been hit by a car I would have a very different attitude, but this was a Perfect Storm involving only a few fellow cyclists. No need to be litigious.
Last edited by cb400bill; 09-08-21 at 01:18 PM.
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09-07-21, 02:10 PM
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Yes, Cannondale does have a Crash Replacement Policy and since the consensus here as well as everywhere else I've asked is mostly "don't repair, replace" that's the path I have decided to pursue. Now if only the Cannondale dealer I bought the bike from would return my calls...
Amazingly, after bringing my bike to the local Cannondale dealer and listening to his sob story about how crappy parts availability has been industry-wide and how I shouldn't get my hopes up about a replacement frame showing up within the next 6-12 months...the replacement frame showed up Friday, less than a week after he'd placed the order!
I dropped the damaged bike off at the shop this afternoon; I'm going to have them transfer all the components over to the new frame.
Hopefully the only follow-up to this thread will be Happy Me with my reincarnated CAADX! Though tbh the way life's been going this past year or so it won't surprise me if the shop calls me later this week with some kind of excuse for why this frame swap didn't go as planned.
Last edited by Bob Ross; 09-07-21 at 02:14 PM.
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09-08-21, 12:00 PM
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Yes, Cannondale does have a Crash Replacement Policy and since the consensus here as well as everywhere else I've asked is mostly "don't repair, replace" that's the path I have decided to pursue. Now if only the Cannondale dealer I bought the bike from would return my calls...
btw, my personal feeling re: liability is that group cycling is a potentially dangerous and expensive activity, **** Happens, and anybody/everybody who willingly gets involved in this activity accepts that risk. It would never even occur to me to pursue compensation from a rider who inadvertently caused damage to my equipment, and I've even declined the offer the one or two times someone was humble and generous enough to make it.
If I'd been hit by a car I would have a very different attitude, but this was a Perfect Storm involving only a few fellow cyclists. No need to be litigious.
btw, my personal feeling re: liability is that group cycling is a potentially dangerous and expensive activity, **** Happens, and anybody/everybody who willingly gets involved in this activity accepts that risk. It would never even occur to me to pursue compensation from a rider who inadvertently caused damage to my equipment, and I've even declined the offer the one or two times someone was humble and generous enough to make it.
If I'd been hit by a car I would have a very different attitude, but this was a Perfect Storm involving only a few fellow cyclists. No need to be litigious.
For me, if the replacement price is not good enough I might just scrap the frame and pick something else. It sounds good taking a step up in frame technology that is compatible with my existing components.
Last edited by cb400bill; 09-08-21 at 01:19 PM.
09-08-21, 01:42 PM
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09-08-21, 02:08 PM
#23
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Oh, absolutely: Cannondale isn't giving me a new frame, they're selling me one at some sort of pro-rated price that's ostensibly cheaper than if I just tried to buy a new frame outright. They're charging $600 for a new frame, fork, and headset.
