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Can I repair my damaged Carbon fibre chain stay ?

afcbianafcbian Posts: 424
edited February 2010 in MTB workshop & tech
I have a rear chain stay that has been partially worn through by winter riding boots rubbing on it as the crank rotates.
It has worn to a point where the wall thickness is reduced to a paper thin level.
Can this be repaired ?
Also if repairable, I was looking to get the frame powder coated/sprayed. The frame is aluminium and the rear stays are carbon. Is powder coating OK on carbon ?
Any help gratefully received
I ride therefore I am


  • afcbianafcbian Posts: 424
    bigbenj_08 wrote:

    I will try to load some later
    I ride therefore I am
  • Make+model would also be a help. Is it a solid or tubular stay? How do you know it's paper thin?
  • afcbianafcbian Posts: 424
    It's a Giant xtc-1 aluminium/carbon composite
    Aluminium frame with carbon rear stays
    It's a hollow stay as when you put your finger on the centre of the worn mark you can feel the carbon fibres and if you push they flex.
    The soft patch is about 1cm long by 0.5cm high
    I ride therefore I am
  • SloperSloper Posts: 141
    Doesn't the powder coating process involve electrickery and magnetism? It's what attracts the powder to the metal. Carbon conducts but I'm not sure the resin does and would you want to subject your CF to 200 degrees in an oven?

    You can repair carbon fibre with carbon fibre cloth and epoxy resin, but doing a good job would be difficult. Contact Giant?
  • konadawgkonadawg Posts: 447
    Sloper wrote:
    You can repair carbon fibre with carbon fibre cloth and epoxy resin, but doing a good job would be difficult.

    I'll have to second that.

    Nevertheless your chainstay has not collapsed yet so anything you add on to it will add strength even if maybe not quite as much as originally so you may take your chances, after all the only other option is replacement, so may as well get some more mileage out of it.

    If you could possibly inject someting inside the chainstay at the weak point via a hole, that would then set relatively hard, that would probably help a lot. Epoxy (again) is all I can think of. A pack of JB Weld applied via syringe (not through the needle of course) may be viable. You may need to carefully introduce a couple of plugs either side so the JB Weld won't flow away but will build.

    Would not worry about the hole, you'll be building the damaged area afterwards and the wafer thin bit you have now has no strength anyway.

    It would not be the strength of the JB Weld per se that would matter, although it would help a lot, but the fact that the section of damaged chainstay would be forced to retain it's profile rather than flatten and crush under a high stress. (Round tubes generally flatten prior to failing.)

    After the plugging, lay up the area with epoxy (JB should be OK again) and fibre, CF if you can source it or glass fibre. Chamfer the areas if not chamfered enough by the wear (probably).

    Interesting project.
    Giant Reign X1
  • warrerjwarrerj Posts: 665
    Where abouts are you ??

    I've nerver repaired a CF bike frame but do competition CF kayaks on a regular basis and have done all sorts of other CF thigns (e.g. squash raquets etc.)

    If you post some pics I'll be able to give you a fair idea of your best plan of attack. I also have contacts dotted around the UK who may be able to help and or supply materials if you want to have a go your self.
  • afcbianafcbian Posts: 424
    Photo now sorted[email protected]/4318592222/

    The "soft patch" is the lighter grey bit in the middle and having measured it it is approx 2cm long and approx 0.5cm high.
    It is on the horizontal stay, which I believe is in tension rather than compression and I would be more worried if it was in the diagonal stay which would be in compression.

    I am based in Verwood, near Bournemouth BTW

    Thanks for all the advice so far
    I ride therefore I am
  • konadawgkonadawg Posts: 447
    Not too bad. I'd simply lay it up with epoxy and glass fibre or carbon fibre but since this is a repair not quite convinced that there will be a major benefit to using CF, after all you will not be able to lay up in the same manner as per factory or achieve anywhere near the same fibre to epoxy ratio, the works normally wet the layup and then squeeze out every last drop of glue out so it's as light and strong as possible. It's a small repair it has not failed any repair will help and also help with stopping the wear from going deeper (although you will probably protect the stays this time round!)

    Have also often repaired stuff very successfully such as windsurfers (tricky) and canoes but only using glass fibre do not know whether there are any special issues with carbon fibre, hopefully warrerj can fill in.

    I often use " gel coat filler " by Plastic Padding instead of epoxy. It sets very hard (but with just enough give) wets the matting very well indeed, probably better than epoxy, and can be sanded down to a very nice smooth finish ready for primer/paint.

    The repair becomes VERY strong, I have often repaired windsurfer noses that have been smashed by a falling mast, and often the next incident results in a different failure point, not the repair.
    Giant Reign X1
  • biff55biff55 Posts: 1,404
    afcbian wrote:
    Is powder coating OK on carbon ?

    wont work , has to be metal.
  • warrerjwarrerj Posts: 665
    I'm sure that can be repaired but it may not look pretty.

    Make sure you key the area well and clean with acetone first. I'd fill the worn area with epoxy and Carbon tow (linear carbon fiber not a weave) and pack as tight as possible. I'd over fill the hole and overlap the unaffected areas. Leave to cure then sand back and shape to an even finish. Then I'd wrap the whole stay with a light carbon weave to make it look nice and give a bit of extra strength.

    It'll be fiddly and somehting you get much better at with practice.

    May be worth you taking you frame for a trip to see these guys in Poole. They're who I use for my supplies. As you'll only need a very small amont of material they may have offcuts you could use. They may also be able to give you some local contacts wo may do the repair for you. A top end boat builder or motorsport would prob do it for you.

    Good luck !
  • afcbianafcbian Posts: 424
    Excellent advice. Thanks very much guys
    Just out of interest, based on the photo, providing I protect the area, do you think it is likely to get any worse or fail ?
    I ride therefore I am
  • konadawgkonadawg Posts: 447
    It won't get worse if you don't wear it down again. As far as failing, well that's a tricky one, even undamaged CF frames can fail but that seems to be an unlikely spot unless by impact. As you (?) say I think that the forces are more tension than compression which helps.

    It will be stronger repaired of course. End of the day it has not failed as-is and the options are - don't fix and replace frame now, or fix, ride, and take your chances. I know which one it would be for me.

    If it were to break on the trail it would not be totally unfeasible to strap on a brace of some sort to get you home.

    Good idea re the carbon wrap from warrerj to prettify the repair.
    Giant Reign X1
  • afcbianafcbian Posts: 424
    I agree. I am going to either fix it myself or get someone else to do it for me.
    I have found this on the web ... p-239.html
    Do you think it would do the job ?
    I am a bit out of my depth here, I will be fine with good instructions but it's not an area I have any experience with.
    I ride therefore I am
  • joshtpjoshtp Posts: 4,329
    i would:
    clean and roughen with IPA and sand paper. inject epoxy, then coat a few inches of stay in epoxxy and wrap it very, very tightly in multiple layers of CF cloth, or glass fibre, with epoxy between each layer... then a epoxy top coat, and a few layers of 'copter tape....
    I like bikes and stuff
  • that seem quite far back on the chainstay! How big are your feet! :lol:
  • afcbianafcbian Posts: 424
    It wasn't my feet.
    The bike was my mates, but I bought it off him last week.
    Neither of us knew there was an issue until I gave it a really good clean at the weekend, then I noticed.
    It has been caused by his winter riding boots rubbing as the heal passes the stay.
    I have ordered a repair kit today and will fix it later in the week.
    I will post some photos for all to see. Will make an interesting project.
    I ride therefore I am
  • konadawgkonadawg Posts: 447
    Tips - the matting must be very thorougly wetted, dry matting is worth f-a.

    Get in as many layers as possible. In practical terms if you get more than two I'd be surprised.
    Giant Reign X1
  • afcbianafcbian Posts: 424
    I'll take all the tips and advice I can get.
    I was planning to fill the stay first with something like JB Weld, then fill the gap with the same then effect the repair with the Carbon repair kit ??
    I ride therefore I am
  • konadawgkonadawg Posts: 447
    The "gap" is where the repair needs to be and so is where the matting must be placed.

    Done properly, you would sand off all excess buildup until the stay assumes the same profile as original, no lumps or bumps, i.e. were you to paint the entire stay there would not be any evidence of the repair location.

    As previous I would recommend plugging the stay at the damage location, via a suitably opened out hole. In fact I'd open out a slightly larger hole than necessary (and ensure that all edges are finely chamfered at a shallow angle to maximise on contact surface area between the repair section and the chainstay material)

    Then comes the plug (but keep the JB below the inside edge the stay material) and then the repair on top.

    Truth be told I'd prefer a homogenous repair whereby it's all JB or whatever you use/have in the repair kit and all done at the same time.
    Giant Reign X1
  • afcbianafcbian Posts: 424
    I have ordered some JB weld as well as the CF repair kit.
    In order to get the JB weld into the stay properly is it just a case of pushing it into the stay like the old fashoined P38/P40 type filler ? A syringe was mentioned in an earlier post ??
    Sorry for all the questions but, truth be told, I am a little nervous about f***ing it up and making it worse.

    In summary then;
    1. open hole slightly and chamfer edges
    2. Rub stay down for key
    3. gently place resin stops inside stay and fill stay with JB to just below inner surface of stay and leave to set
    4. fill gap with JB and carbon mat and leave to set
    5. Rub down to give smooth appearance to same profile of stay.
    6. Carry out carbon wrpping of stay in accordance with instructions on the repair kit

    Is this about right ?? :?
    I ride therefore I am
  • konadawgkonadawg Posts: 447
    The JB Weld is around the same consistency of Araldite, not filler. (Dont think of using filler instead!) You will need to plug the insides of the chainstay so it won't flow and can build. You will need to be imaginative. Maybe cotton wool? Possibly this is the fiddliest part of the procedure which is why I suggested a wide hole. Yes, a syringe will help with filling through the hole.

    2) No need. You only want the repair where the damage is, you won't want any excess remaining or unnecessarily adhering well outside of the damaged area, as this should all be removed later. Just chamfer the damaged area as gradually as possible, i.e. a gentle taper to zero - the greater the surface area the better the bond. If we can taper 2mm composite windsurfer skins successfully, you should have an easy job with the extra thickness of a chainstay.

    3) + 4) - As previous, my preference would be for a single operation so the JB is homogenous. So once the stay is filled with JB up to the level of the lower (inner) edge of the hole - Wet the damaged area (all of it that is below original stay level/profile) with the JB. Apply a layer of matting, and using a suitable tool (credit card maybe?) press the matting well into the JB until the JB has thoroughly wetted the matting and mostly come through it, so the CF matting is as close to the stay material as possible.Apply a bit more JB over the matting, and apply another layer. Same procedure repeated as necessary until the centre of the repair (the deepest bit) is now definitely proud of the original profile.

    Let it set.

    5) Correct. If after sanding there are gaps that need filling (possible) you may use auto body filler or similar, or more JB, then sand again. Well actually this would be the case if you are painting. If you are wrapping then this may not be necessary.

    6) Don't know what the carbon wrapping is doing in a repair kit, as I understand it the only thing necessary now is a cosmetic layer, be it paint or whatever, if this is what the wrapping is about then fine.
    Giant Reign X1
  • warrerjwarrerj Posts: 665
    Sorry I dissagree !!

    I would not open a hole in the stay unless there is a hole already.

    You don't need to fill the chain stay with anything it won't help the repair strength at all. The strength in a composite repair is in bonded overlapping fibers. You can't key the inside of the chain stay so nothing you pmp into it will bond well to the inside of the stay.

    IF you do already have a hole then you need to fill the stay with something like liquid foam NOT epoxy resign.

    1.Fill the dent (worn aread) with well wetted out CF tow cut into small lengths and build it up bit by bit. Once the whole area is above the original level leave it to cure.
    2. Leave it a few days if you can as it'll make sure it's flly cured. Then sand back to the original level and shape of the stay.
    3. key in the repair and surrounding area then wrap the stay with a light cloth or tissue.
    4. Finish with whaterver finish you like. Paint, lacquer or leave it natural.
    5. Make sure you protect with coppter tape on chain stay protector :wink:
    6. RIDE !! :lol:
  • afcbianafcbian Posts: 424
    Thanks to both of you for your advice.
    It is very much appreciated
    I ride therefore I am
  • warrerjwarrerj Posts: 665
    If you were closer I'd offer to do it for you.

    Forgot to mention one of THE most important things. Make sure you do the repair and let it cure in a warm (ish) place. Min.10 deg C better 15+ deg C and dry. SO no doing it the garden shed :wink: Any colder or if it's damp you'll have problems gettign it to cure and/or it'll be weak.
  • afcbianafcbian Posts: 424
    Thanks for the offer.
    When you say well wetted, you mean with JB for example, yes ?
    Also when building up the dent with wetted carbon tow, you mean starting with small bits of tow and getting larger, like an inverted triangle ? Eventually overlapping the edge of the dent onto the good portion of the stay ?
    Also you say key in the repair and surrounding area and cover with a cloth ? is this just to keep clean pending the final coating of whatever ?
    I ride therefore I am
  • konadawgkonadawg Posts: 447
    Suggested the JB plug as a means of ensuring the stay retained it's cylindrical form, as a backup. It is true that it is not, strictly speaking, part of the repair, but due to the fact that the repair will not in itself be as strong as the original anything that would keep the stay cylindrical would prevent it from collapsing. On the basis that a cylindrical form is strong but fails when it first flattens then kinks. In an ideal scenario one could even consider fitting a tubular metal insert at the repair location but this is not really possible unless one is prepared to saw the chainstay straight through, maybe we'll leave that for another, more seriously broken bike :wink:

    Well, opinions and all of that, that's what I'd do in this case in consideration of the particular circumstances, factually speaking I have never "plugged" any windsurfer board before because they are plugged already (full of EPS)

    The EPS as we all know is far from strong and a windsufer's skin is what, 2-3mm max in most places including the top coat and snaps easier than a crumbly biscuit but when combined result in a board that can take very considerable stresses.

    Not sure you can inject anything similar to EPS at home and normal builders' expanding foam does not have the same density/properties so I would use whatever I could obtain and use, and also the stresses per unit area on a chainstay are likely to be far, far larger than on a windsurfer so even if one could fill with EPS it may not be dense enough.

    Oh well, whatever!
    Giant Reign X1
  • warrerjwarrerj Posts: 665
    Well wetted out means make sure the cloth is saturated with epoxy. Don't just paint it over the top. It won't take as much epoxy as you think. I mix mine in the bottom of old yohurt pots (only about 1/2" deep depending on the size of the repair).

    Yes sort of inverted Tgiangle. Shortest bits in the bottom of the dent and building up to longer bit that overlap the area around the dent.

    By key the area I mean give it a good sanding with coarse sand paper. Then give it a good clean with Acetone.

    Once you've done phase 1 sand back a start again and wrap the stay in a woven cloth to finish the job off.
  • FFS make sure you wear gloves/eyewear as appropriate. Don't want any Todd from neighbours incidents.

    Also - It's probably about time someone said something along the lines of "Do this at your own risk, that should really be a bin job, if it fails and stabs you, don't blame us..."
  • afcbianafcbian Posts: 424
    warrerj wrote:
    Well wetted out means make sure the cloth is saturated with epoxy. Don't just paint it over the top. It won't take as much epoxy as you think. I mix mine in the bottom of old yohurt pots (only about 1/2" deep depending on the size of the repair).

    Yes sort of inverted Tgiangle. Shortest bits in the bottom of the dent and building up to longer bit that overlap the area around the dent.

    By key the area I mean give it a good sanding with coarse sand paper. Then give it a good clean with Acetone.

    Once you've done phase 1 sand back a start again and wrap the stay in a woven cloth to finish the job off.

    Found a really helpful Boatbuilding company whose MD is heavily into MTB riding local to me and he talked me through the whole process (for free) and his advice was almost exactly the same as warrerj.
    I am waiting for my CF repair kit to arrive and will then get going.
    Thanks for all the advice from everyone specifically warrerj and konadawg.
    I now feel I have the knowledge to do a good job (AT MY OWN RISK :roll: )
    I will keep you informed :wink:
    I ride therefore I am
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