Fork dropout woe

ride_whenever
ride_whenever Posts: 13,279
edited October 2008 in Workshop
Rightyho chaps!

My gf has bought a second hand bike, and the dropout on the forks has been bent slightly closed presumably in transit (9mm down to 6mm).

They are custom coloured carbon fibre forks with aluminium dropouts so replacement isn't really an option due to not being able to replicate the colour. As I can see it I have three equally horrific options (listed in descending horror):

1) Try and bend the dropout back, relatively trivial, but as it is aluminium i run a serious risk of snapping/weakening dangerously the dropout.

2) file a flat onto the axle so it slots into the now narrower dropout, slightly more complex but doable, probably the least risk of damage/future death from failure.

3) cut the axle protrusion off entirely and run it with the skewer done up really tight, more difficult and probably fairly dangerous, although IIRC sheldon says something similar can be done with rear wheels on track bike so it can't be that bad.

Opinions

Comments

  • nicklouse
    nicklouse Posts: 50,675
    I know what i would do.

    but lets have a picture to see how bad.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • ride_whenever
    ride_whenever Posts: 13,279
    piccies:

    2906013702_5b3466feac.jpg

    and

    2905163125_fa8d1197eb.jpg

    I'm pretty sure the sides are supposed to be parallel!

    the deformation is approximately 3mm, so the opening is only 6mm wide as opposed to the more traditional 9mm
  • nicklouse
    nicklouse Posts: 50,675
    Ok so there is no visible damage to the join?

    I would be tempted to try opening it a bit but without putting any force on the leg, and then file to get the final bit clear.

    but the first thing i would be doing is checking with the seller about was there any insurance cover in the shipping.

    and then take it to a bike shop for an inspection. Just for a second set of eyes to give it a once over.... as there may be some issue that is not visible.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • ride_whenever
    ride_whenever Posts: 13,279
    I was going to get it serviced (as i get free services anyway :wink: ) this weekend.

    I've just tried offering up the axle to the dropout from the inside and it very nearly fits with only the threads preventing the axle slipping in.

    Thanks for the help
  • W5454
    W5454 Posts: 133
    Bending it could be tricky unless you heat the dropout first which could damage the paintwork as the fork leg got hot.I would file the 6mm end until the gap was 9mm, removing a small amount and checking regularly until the axle fitted.It wouldn't need much filing and there's still plenty of dropout left for the QR to grip.
  • nicklouse
    nicklouse Posts: 50,675
    if it is that close you should be able to squeeze them open enough but take care not to use the legs as a lever. It could be fine but....

    if you put two flat plates in the drop out and then put a wedge between them that should move the bent drop out just enough...
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • robbarker
    robbarker Posts: 1,367
    I'd try squeezing the dropoutin a vice - the metal will have flared out at the bend where its distorted, and squeezing it in a vice might open out the slot just enough. I think Nicks idea of filing the last sliver rather than bending is a good one too.
  • method
    method Posts: 784
    Surely any bending is going to weaken it? I might be being over cautious but I don't think I 'd pick any of the above options. New forks for me, but good luck!
  • Al_38
    Al_38 Posts: 277
    edited October 2008
    No edited thanks to a pinch of salt from synchronicity

    Bending in theory doesn't have to weaken it: it will remove the ductility though so if you continue to plastically deform it then it has a much increased risk of failure.
    If you can heat it up then this should allow the aluminium to recrystallise which will dramatically increse its ductility. Ideally this would be to something like 600C but this would probably wreck the paint (and as pointed out by synchronicity below would likely melt / burn the resin and damage the CF - alloy joint. Do you know if the alloy is heat treated or anything else about it or the composition?. You can however help it to some extent by keeping it hot for a long period of time something like 100C (check if this is safe for CF) for a day or two and this should help it to recrystallise although the change will be much slower. I have no idea what effect these temperatures will have on the CF though. I would advise speaking to a frame builder who uses both CF and alloy and seeing what they would say.

    If you do bend it out and see any stress marks in the alloy then I would be very anxious about using it as aluminium isn't the most ductile of materials in the first place before it has been alloyed. Could you not get a new pair of forks and have them custom sprayed?

    If you do go for the bend and file option I would advise doing it at a reasonable temperature 80 - 100C and try and bend it out as slowly as possible (This should allow some deformation as creep rather than plastic). Filing down the remaining bit is probably a good idea too.
  • cougie
    cougie Posts: 22,512
    For that to happen - theres been some pretty strong forces involved ? Was it just that dropout thats been damaged ? Good luck !
  • Special K
    Special K Posts: 449
    cougie wrote:
    For that to happen - theres been some pretty strong forces involved ? Was it just that dropout thats been damaged ? Good luck !

    or this is a manufacturing error

    either way, there is no way I would ride that fork.

    forks are too cheap, warranties too good, and importantly life too short to take that sort of risk - it just isn't worth it.

    But then I am the paranoid sort who does eat sushi that has been warmed gently in the sun over the course of an afternoon, but that's another story.
    "There are holes in the sky,
    Where the rain gets in.
    But they're ever so small
    That's why rain is thin. " Spike Milligan
  • How much do you value your face and front teeth?

    Bin the forks and fit some new ones.
  • Al_38 wrote:
    Bending in theory doesn't have to weaken it: it will remove the ductility though so if you continue to plastically deform it then it has a much increased risk of failure.
    If you can heat it up then this should allow the aluminium to recrystallise which will dramatically increse its ductility. Ideally this would be to something like 600C but this would probably wreck the paint, an air quench is probably fine for bringing the temperature down - do you know if the alloy is heat treated or anything else about it or the composition?. You can however help it to some extent by keeping it hot for a long period of time something like 100C for a day or two and this should help it to recrystallise although the change will be much slower. I have no idea what effect these temperatures will have on the CF though. I would advise speaking to a frame builder who uses both CF and alloy and seeing what they would say.

    If you do bend it out and see any stress marks in the alloy then I would be very anxious about using it as aluminium isn't the most ductile of materials in the first place before it has been alloyed. Could you not get a new pair of forks and have them custom sprayed?

    If you do go for the bend and file option I would advise doing it at a reasonable temperature 80 - 100C and try and bend it out as slowly as possible. Filing down the remaining bit is probably a good idea too.

    Sorry but I believe you are out of place to offer this kind of advice.

    Firstly, the most obvious thing that will happen when you heat up the dropouts to 600°C is that the bond between the dropouts & carbon will disintegrate. Epoxy resin cannot withstand that kind of abuse. Not even high-temperature polymers such as PEEK are designed for temps that high.

    There is also more to recrystallisation than what you think. Sure the ductility will increase when you heat aluminium alloy. But then you'll also dissolve the fine precipitates which are dispersed throughout the entire grain structure. They're what give all precipitaion hardened alloys their stregth (think 6061-T6, 7075-T6, etc). At 600°C, the strength will disappear quicker than you can say "Oh boy I think I smell plastic burning".

    Assuming it is 6061-T6 alloy, heat the thing 15 minutes at 425° and you'll completely remove a T6 temper, reducing the strength by 55% to only 124Mpa. Even at 200°C, the strength / hardness is affected. Those alloys are not designed to be heated after their initial heat-treatments. That's one of the reasons why most of those alloys are not weldable... because they need thermal aging treatments around 160° to attain their peak strength... this then drops off if over-aged.

    It's been a long time since I studied all this, but I do know what I'm talking about. If you don't believe me, ask RedDragon, he's doing his Materials degree right now.

    I think nicklouse has the best idea. Don't put any undue force onto the carbon fork leg.
    I wouldn't like to cut the thing off entirely because what support it does provide will be gone altogether. Respect to Sheldon, but don't forget it's also much more dangerous for your front wheel to fall out than your rear one.

    If you say the axle fits without the thread, you might be onto something. Know that the threads don't impart any strength to the axle. In fact it is more than likely that they introduce their own weakness by concentrating stress at the base/root of each thread. But by filing it, I'd be more worried that it won't mate with the dropout properly. You'll have to file it on one side (the front) if you choose that option, otherwise your wheel will be lop-sided if you file the axle all around.

    For me it makes more sense to file the dropout sooner than bend it all the way, especially if it is a high-strength alloy. That way you won't create any more stress in the remaining part of the dropout as you would by bending. Yes you'll lose material, but at least you won't weaken what remains any further than it has alread been. :idea:

    Having said that, I'd be tempted to go with the wedge idea as suggested by nicklouse.

    It goes without saying that I'd also be tempted to throw them in the bin... what caused the damage might have weakened not just the alloy, but the all-important joint with the carbon leg.

    The one thing you shouldn't do it heat it up.

    That looks suspiciously like an older mango-yellow coloured Kestrel EMS fork.smiley_shiftyeyes.gif
  • ride_whenever
    ride_whenever Posts: 13,279
    Cheers for the advice guys, it is the forks on the bike in this thread
  • Well they don't appear to be Kestrel forks... :oops: :lol:
  • Al_38
    Al_38 Posts: 277
    edited October 2008
    Granted I did put in the reply that I wan't sure what the temperature would do to the CF or the resin that encases it. Personally I wouldn't ride it, as you don't know how the dropout was bent in the first place and what damage there could be to the joint anyway.

    (I do a bit more about materials than written above, but as you clearly pointed out there is a lot more to it than a simple do this do that. I'd also expect that dislocations, recrystallisation, precipitation hardening and the TTT / CCT -diags etc that you would need to actually consider to sort this properly are beyond the knowldge of most of the readers of this forum) However I would still suggest that opening it up slowly (if that option is chosen) would be a better idea than trying to do it in a few minutes

    Surely someone like argos http://www.argoscycles.com could spray up a new set of forks to suit... They have done some fairly complicated paint jobs based on their gallery
  • Al_38 wrote:
    I'd also expect that dislocations, recrystallisation, precipitation hardening and the TTT / CCT -diags etc that you would need to actually consider to sort this properly are beyond the knowldge of most of the readers of this forum.

    Agreed. They're also beyond the comprehension of most engineers. :wink: *tongue in cheek*
  • Al_38
    Al_38 Posts: 277
    Hmmm, had more of a think about this when I was out on a ride this afternoon:
    6061 is pretty common as an aluminium alloy for bikes, but it depends on if it was heat-treated or not. If it is T6 then it will fail somewhere round about 8% strain i.e. where the length at any point has beeen increased by 8%, treated to a lesser degree or not at all increases this.
    I suspect it might well break if it was bent back fully, but if it didn't then a fair proportion of it (the outer and inner edges) will be fully plastic. Bearing in mind that the centre is also going to be fairly work hardened by all of this if will be a lot more brittle than before. As it will be subject to vibration in use you would be playing a game of statistics before you hit one bump too many and it does fail.

    Given that it failing in use would be fairly unpleasant, I personally wouldn't use it again even if you didn't do anything to the dropout and modified the hub to fit, the risk is greater than before. Would it be possible to have the bent dropout removed and a new one bonded in place? This would also make sure that the joint between dropout and fork is good too.
  • mididoctors
    mididoctors Posts: 18,093
    replace the fork

    peace of mind
    "If I was a 38 year old man, I definitely wouldn't be riding a bright yellow bike with Hello Kitty disc wheels, put it that way. What we're witnessing here is the world's most high profile mid-life crisis" Afx237vi Mon Jul 20, 2009 2:43 pm
  • Al_38 wrote:
    Bearing in mind that the centre is also going to be fairly work hardened by all of this if will be a lot more brittle than before.

    At the centre lies the neutral axis. Effectively, the front edge will have been put into tension, while the inner edge where the axle goes will be put into compression. There is a gradient of forces between the two. Result: theoretically there is a line in the middle that doesn't experience any stress.
  • Al_38
    Al_38 Posts: 277
    Yes, however the neutral axis relies on a couple of assumptions: I'm pretty sure it is only a valid theory for elastic deformations, if it goes plastic (which it has then you will get a hinge). However depending on how / if it was bent back out then it could quite conceivable not be bent back with the same neutral axis as the damage occured with.

    What I really meant was that the inside and outside edges will have been significantly plastically deformed. The more central regions will maybe not have reached the plastic modulus of the section and so will not be as badly affected. However they will have still have experienced some work hardening (with presumably a linear gradient to the neutral axis, if it exists). This would to an increase in the dislocation density and so to the size and number of flaws. Then it comes down to Millar's (I think) empirical law to determine the life cycle and how much was left, also the fast fracture statistics would need to be considered

    What I am unclear about is what exactly happens when you form a plastic hinge in something with a very low slenderness ratio. Also I am assuming a rigid - perfectly plastic model rather than the more realistic but also much more comlicated elastic - perfectly plastic one. In future I shall make sure to be especially pedantic :P

    My conclusion is that I personally wouldn't ride the fork as it is or with out the drop-out being replaced. If for nothing else at least you would have piece of mind and wouldn't wonder what will happen next time you hit a bump