Possibly bent rear derailleur

[NewCyclist]

Hi guys,

I dropped my bike accidentally and now the rear derailleur just contacts the wheel spokes when in lowest gear. Other than that everything seems normal. Ive got SRAM Force Axs etap. Can i just use micro adjuster to repair or do i have to do anything else?

Your thoughts would be much appreciated.

Thanks

[NewCyclist]

Also the chain went behind the gears once and contacted frame.

[redsonic]

Most likely a bent derailleur hanger; this is designed as a sacrifice part to prevent damage to the frame (or the derailleur). The micro adjust would do nothing for this. Check out videos online showing the straightening of derailleur hangers. Helps to have a tool, but you could do it by eye with a pair of pliers.
Your bike shop could check it in a few minutes too.

[CmdrBiggles]

Hangers are cheap and easily replaced, but the hanger for the SRAM is specifically for that derailleur — FORCE AXS.
The shop where your bike came from should have spare hangers.

For an initial fix:
Remove battery from derailleur and remove from hanger. Place pieces of hardwood in a vice, with the hanger sandwiched between them and carefully tighten, watching as the hanger straighens. Though this is a common procedure, there is a risk for severely bent hangers to break, and in any case straightening a bent hanger does introduce material fatigue.

If it appears straight to the eye, re-attach to frame, then reassemble derailleur. The Low stop screw should be all the way out to fine tune shifts up to the largest cog, starting with the second largest, using a visual indication. Once maximum shift has been reached (largest cog + largest front chainring) without throw-off or jamming, check for chittering and use the app to microadjust until quiet.

If shifting is erratic across all cogs/front chainring combinations, and not helped by microadjust (which is not a substitute for thorough set-up tuning), replace the hanger and suspect damage to the derailleur itself. I presume you know SRAM Force AXS rear derailleurs are almost $600 as replacements (!) I doubt very much there would be damage to the frame lug where the hanger is attached, but if it is a carbon frame, this area should be scrutinised too.

Check shifting on highest gear too, up and down the freewheel at a steady speed (mounted on a service hoist or whatnot). A fall or crash means the entire local area that took the brunt of the force needs thorough scrutinising, with all gear combinations thoroughly checked for over-runs and throw-offs (particularly the front chainrings if the rear derailleur is compromised).

[NewCyclist]

Thanks so much guys! Unfortunately I dont have vice or other tools to do the repair so I'll need to take it to a bike shop. Is it safe to ride my bike (not to cause further damage to the bike) to a bike shop? There is no unusual noise and everything seems just fine as long as i dont shift to lowest cog.

[antigee]

I'd adjust the low gear stop so you can't accidentally shift so that the bent mech hits spokes

https://www.youtube.com/watch?v=eJTyWKbVyM8

about 1min 40secs in I'd set it with the chain on say 3 cogs down from lowest to be sure ain't go to hit

also check spokes for gouges/damage if any severe gouges I wouldn't ride - you don't want wheel to collapse

PS not a mechanic and I trust you've downloaded and read my standard terms and conditions

[NewCyclist]

Hey guys! It was just a bent rear derailleur. Took it to the bike shop and they were able to straighten it for me.

With hanger acting as a sacrificial part to protect the frame, will the straightened hanger still provide protection (as it will be easier to bend next time the bike falls over)?

[CmdrBiggles]

Hey guys! It was just a bent rear derailleur. Took it to the bike shop and they were able to straighten it for me.

With hanger acting as a sacrificial part to protect the frame, will the straightened hanger still provide protection (as it will be easier to bend next time the bike falls over)?

Short answer, no. You are merely deferring the inevitable failure of a critical component.

Derailleur hangers aluminium or, in some specific cases (finger-pointing to 'weight-weanies'...) carbon, cannot realistically be repeatedly forced back to true shape like the metal hangers (drop-out integral or bolt-on) of 30-40+ years ago — there will be material fatigue introduced by correcting the hanger after the first fall, and that material fatigue increases with each subsequent attempt, additional to the normal operating tension the derailleur imparts on the hanger (which is significant with the Red and Force AXS) . After the first re-jig, if there is another fall, it is sensible to replace the hanger to preserve the shifting and landing accuracy of the derailleur itself, rather than court disaster when it is least expected.

Point to note: it is not going to break the bank replacing a hanger, compared to replacing a SRAM Force AXS rear derailleur!!

[Andy01]

My take is a little different.

Certainly fatigue and plastic deformation (a mechanical engineering strength of materials term) is a thing, so bending and straightening the aluminium hanger will almost certainly have reduced it's strength to some degree.

However, unless the hanger is particularly light duty (thin or soft) I would suggest that it will offer similar protection levels as before - probably "better" protection because it will tend to deform a little easier next time (aluminium, unlike titanium, doesn't work harden). I would also suggest that the likelihood of a catastrophic failure such as it breaking in two during normal use (not a similar drop or crash) after one deformation cycle is probably quite remote. Obviously a CF hanger (never seen one) couldn't be straightened or repaired.

I think the most I have straightened a hanger (admittedly a cheaper MTB hanger) was 4 times, and really didn't seem any different after the 4th time - no obvious difference in the force required to straighten it and no physical signs of damage (ie. hairline cracks). It was on a Norco MTB that I couldn't see a readily available obvious replacement for.

I wouldn't recommend repeatedly straightening the same bent hanger multiple times, but I would surprised if a single deformation event (or even a couple) will noticeably affect it's ability to do it's job.

That said, probably not a bad idea to try to source a replacement so that you have it on hand if it gets bent again.

Out of curiosity, what did the bike shop charge to straighten it ? Reason I ask Is that it is fairly easy to build your own straightening tool for under $20 and it takes about 10-15 minutes to do the job.

https://www.youtube.com/watch?v=sWdO4dnu18g

The hardest part is finding the M10 x 1 (from memory) bolt & nuts. A "standard" M10 bolt is a 1.5 thread, so this is finer and generally only available from specialty bolt shops - about $4 for the bolt & 2 nuts, about $10 for square tube from Bunnings and you are away. I built one and have used it several times (I already had the 300mm stainless steel ruler) and it works a treat.

[OnTrackZeD]

Out of curiosity, what did the bike shop charge to straighten it ? Reason I ask Is that it is fairly easy to build your own straightening tool for under $20 and it takes about 10-15 minutes to do the job.

https://www.youtube.com/watch?v=sWdO4dnu18g

The hardest part is finding the M10 x 1 (from memory) bolt & nuts. A "standard" M10 bolt is a 1.5 thread, so this is finer and generally only available from specialty bolt shops - about $4 for the bolt & 2 nuts, about $10 for square tube from Bunnings and you are away. I built one and have used it several times (I already had the 300mm stainless steel ruler) and it works a treat.

A rear QR axial is M10x1.0 and has a few nuts aswell.

[CmdrBiggles]

Obviously a CF hanger (never seen one) couldn't be straightened or repaired.

Schmolk CF parts; listed on their website until around a month ago, with hangers for Campagnolo and DuraAce.
The hanger for SRAM Red and Force AXS rear derailleurs is angled slightly inward — this is why straightening them after a fall has more than a modicum of guesswork as a fraction in-or-out will affect axial movement and landing (requiring more tweaks of microadjust).

[NewCyclist]

My take is a little different.

Certainly fatigue and plastic deformation (a mechanical engineering strength of materials term) is a thing, so bending and straightening the aluminium hanger will almost certainly have reduced it's strength to some degree.

However, unless the hanger is particularly light duty (thin or soft) I would suggest that it will offer similar protection levels as before - probably "better" protection because it will tend to deform a little easier next time (aluminium, unlike titanium, doesn't work harden). I would also suggest that the likelihood of a catastrophic failure such as it breaking in two during normal use (not a similar drop or crash) after one deformation cycle is probably quite remote. Obviously a CF hanger (never seen one) couldn't be straightened or repaired.

I think the most I have straightened a hanger (admittedly a cheaper MTB hanger) was 4 times, and really didn't seem any different after the 4th time - no obvious difference in the force required to straighten it and no physical signs of damage (ie. hairline cracks). It was on a Norco MTB that I couldn't see a readily available obvious replacement for.

I wouldn't recommend repeatedly straightening the same bent hanger multiple times, but I would surprised if a single deformation event (or even a couple) will noticeably affect it's ability to do it's job.

That said, probably not a bad idea to try to source a replacement so that you have it on hand if it gets bent again.

Out of curiosity, what did the bike shop charge to straighten it ? Reason I ask Is that it is fairly easy to build your own straightening tool for under $20 and it takes about 10-15 minutes to do the job.

https://www.youtube.com/watch?v=sWdO4dnu18g

The hardest part is finding the M10 x 1 (from memory) bolt & nuts. A "standard" M10 bolt is a 1.5 thread, so this is finer and generally only available from specialty bolt shops - about $4 for the bolt & 2 nuts, about $10 for square tube from Bunnings and you are away. I built one and have used it several times (I already had the 300mm stainless steel ruler) and it works a treat.

Appologies for the late reply mate. Bike shop charged 35 bux. They also lubed the chain, did an overall check of the bike and put some extra gluey substance into the tubeless tires.

[NewCyclist]

Gents, I may have not been clear with my question. I wasn't asking about the ability of the hanger to maintain shifting and landing accuracy after straightening it post fall. I was referring to the ability of hanger to still provide the level of protection to the carbon frame after being straightened (hanger will take impact first rather than the carbon frame).

For example, the hanger that's been straightened multiple times may have lost a lot of strength, so next time the bike falls, the hanger bends more significantly (or even snaps) hence the frame impacts onto concrete.

Your thoughts would be much appreciated. I appologize if i misunderstood your replies.

[Duck!]

Weakening of the hanger through cycles of bending & straightening does not have any real bearing on risk of damage to the frame from subsequent falls. The major concern is that weakening the hanger can make it more prone to breaking as a result of the forces imposed by shifting gears and the movement of the derailleur, to the point that it may break from nothing more than the force of a gear shift. A weakened hanger can break at any time, chucking the derailleur into the rear wheel (probably taking out a few spokes in the process), which will swing the derailleur around and whack it into the seatstay, often causing significant (but not irreparable, although not cheap) damage.

The whole point of a sacrificial hanger is that it will bend or break in an impact, absorbing a lot of the impact energy and thus saving the more expensive bits from major damage.

[NewCyclist]

Weakening of the hanger through cycles of bending & straightening does not have any real bearing on risk of damage to the frame from subsequent falls. The major concern is that weakening the hanger can make it more prone to breaking as a result of the forces imposed by shifting gears and the movement of the derailleur, to the point that it may break from nothing more than the force of a gear shift. A weakened hanger can break at any time, chucking the derailleur into the rear wheel (probably taking out a few spokes in the process), which will swing the derailleur around and whack it into the seatstay, often causing significant (but not irreparable, although not cheap) damage.

The whole point of a sacrificial hanger is that it will bend or break in an impact, absorbing a lot of the impact energy and thus saving the more expensive bits from major damage.

Thanks so much for clarifying!