Oh bugger

Discussion in 'Bicycle Mechanics and Repairs' started by Jacomus-rides-Gen, 4 Mar 2008.

  1. Jacomus-rides-Gen

    Jacomus-rides-Gen New Member

    Guildford / London
    I have just knocked Gen off the repair stand :sad::sad::sad::sad::tongue::cry::biggrin:::smile:

    Her landing was cushioned by the mech hanger, which unsurprisingly bent. Using a bit of brute force I bent it back, but now it has telltale flaked paint.

    The aluminium under the paint doesn't appear cracked, but I am aware that it is compromised now. My question is - how much load does the mech hanger have to take?

    Even at full power is there much load on the rear derailleur? It seems to me that the top run of the chain, thus the chainrings and cassette are heavily loaded, but the derailleur just keeps the bottom chain run from flapping around.

    I don't have the money to buy a new hanger this month, and am wondering if it will suddenly and disasterously snap 50km into a club run.
  2. jiggerypokery

    jiggerypokery Über Member

    You should be ok JRG, I was looking at this last night when I was putting a new chain on X and from what I can tell the rear mech is providing tension between the jockey wheels and the big rings (techy I know ;)) when you change up and down the gears thus moving the rear mech. You are pulling the mech obviously and the force is distributed through the mech to the hanger - the over all surface area of the joint between the hanger and the frame dissipates this force in to the frame, this force distribution is also helped by the joints and springs of the rear mech soaking it up some what. Assuming that you really haven't cracked the frame you should be ok. Having said that you are right that you have compromised the material integrity of the aluminium but to what extent you'll never know.

    At full power ther should be very little load, other than the sprung tension it gives itself, on the rear mech as it is providing tension and guidence and if you follow through the line of the chain you will see that the most tension is at the load points where the chain sits on the teeth of the rear and front rings and at every pin along the top of the chain. As the chain leaves the front rings right the way through the jockey wheels of the rear mech there will be very little tension.

    Not sure about shifting under load though so would ease up and then shift.
  3. fossyant

    fossyant Ride It Like You Stole It!

    South Manchester
    Don't forget alloy doesn't like being bent back - it gets stressed (ahem) - you might need a new hanger at some point - wouldn't panic too much..

    Steel - not a problem - did this to one of my bikes many years ago in a crash.
  4. I bent the hanger on my commuter recently and bent it back too, it was fine for the couple of weeks (approx 100mls) whilst I waited for the new one to come into the LBS albeit I was a couple of gears down.
  5. llllllll

    llllllll New Member

    Bent my 'hanger back into shape with a vice years ago. Done several thousand miles on it since, no problems.
  6. ColinJ

    ColinJ Hillfinder General

    On my first cycling holiday in Spain, I met a Scottish guy whose Bianchi'd just had its hanger bent by clumsy airport baggage handlers. He was pretty pissed off about that (as one would be) but he just bent it back into position and came out with my group that first day. About 30 miles into our ride there came an agonised scream from the back of the bunch as we started to climb a steepish little hill. Bent-Bianchi owner had just changed gear and stood up and his rear mech had gone into his wheel. Result: one snapped off hanger, a mangled mech, several broken spokes, and a snapped chain xx(! It might have been because he'd bent the hanger over too far causing it to go into the spokes when he changed to bottom gear, or perhaps the hanger had snapped first and everything else happened as a consequence of that?

    Between us we had enough tools and knowledge to improvise repairs which left him with a single-speed bike to ride back to the hotel. What a crappy start to a cycling holiday - he wasn't a happy chappy! However, his luck was about to change big-stylee...

    (The) Robert Millar arrived at our hotel with a fleet of top racing bikes to test (Bianchi, Pinarello, Colnago etc. etc.), and also a selection of top race wheels to switch between the bikes. He was doing comparative reviews of the bikes and wheels for one of the big mags, I think it was ProCycling, and he was willing to let us borrow the wheels and bikes in return for us writing mini-reviews to help him. Best of all, our sad Bianchi owner just happened to be RM's size so all those great bikes were the perfect size for him. He got to ride a different fantastic new bike every day for a week :biggrin:!

    Unfortunately, the bikes were all far too small for me... :smile:
  7. barq

    barq Senior Member

    Birmingham, UK
    I think the tricky part is getting the hanger bent back so all the gear shifts work - if you've overcome that then well done. Presumably you bent the hanger to one side (rather than forward or back) so I'd be picking my gear ratios carefully to keep a straight-as-possible chainline.

    What Colin says about the devastation caused by rear mechs going into wheels is true though. I did it a year or so back when one of my MTBs clipped a fallen tree and it pushed the rear mech through my wheel - it elongated the rear triangle of the frame, destroyed an XT mech, and ripped out four spokes. Nasty. So if you have one of those plastic spoke protectors to fit behind the cassette then now is a good time to use it as a cautionary measure, because if your hanger does break you know where the rear mech is headed don't you!
  8. domtyler

    domtyler Über Member

    Great story! :biggrin:
  9. OP

    Jacomus-rides-Gen New Member

    Guildford / London
    Thats reassured me a bit, thanks all.

    Fossy - the hanger is Aluminium, so is a write off... just as soon as I next get paid!

    It didn't bend too much, just enough to make the derailleur just skim the spokes on 23t.

    So I bent it back to verticle, tuned my gears and everything runs perfectly, apart from slightly imprecise downshifting. But I always tune my gears to shift up very fast, and require more input on the way down so it feels almost totally normal.

    I have checked it again after riding in London tonight - and I didn't exactly hold back, even got a few heavy handed upshifts in there :biggrin: and it seems exactly like before.

    I must remember to back off properly before each shift, even if accelerating hard out of TLs.
  10. bonj2

    bonj2 Guest

  11. NickM

    NickM Veteran

    And severely bruised knackers?
  12. NickM

    NickM Veteran

    ...which works very nicely on steel frames!
  13. ColinJ

    ColinJ Hillfinder General

    That might explain the agonised scream! I had that problem once when my gears slipped on a steep climb... ;)
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