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Crack Carbon FD post

Posted: Sat Nov 01, 2008 11:38 am
by Low Racer
Hi guys,
I have sad news. I cracked the carbon FD post on my M5.

Image

Overzealous tightening of the screw by probably more than 1/4 turn. Really p...s off with myself. I only noticed it when the boom would creaks on heavy load. Initially I thought it was the boom bolts not been tightened enough. Good job no damage done there with me tightening the bolts more than required.

The FD started to play up and would need constant adjustment. That's when I noticed the cracked in the carbon :oops: .

I have cut a metal piece and inserted it between the FD clamp and the crack. Seem to hold ok. I contacted M5 and a new boom is 450 euro but they will sell it to me for 250 euro. That is very good of M5 to offer me 200 euro discount. My other alternative is to change the crank system to Schlumpf High Speed Drive altogether. This will negate the use of FD. Since the post is not a high stress area, I think it should be alright with the slight crack. What do you think guys?

Chong

Posted: Sat Nov 01, 2008 11:40 am
by Kalgrm
Have you got a bigger picture? I can't make out the crack in that photo (make it 640 pixels across).

Cheers,
Graeme

Posted: Sat Nov 01, 2008 11:48 am
by Low Racer
Hi Graeme,
That's a stock picture of the boom from M5 website. I haven't taken a picture of the crack boom yet. Will download it here later.

Regards

Chong

Edited

Here is the picture.
Image[/img]

Posted: Sat Nov 01, 2008 12:16 pm
by John Lewis
Chong,
If the problem is only the deraileur post it's probably easily repairable. Might I suggest you go find a gliding club and ask for help. They are sure to have experts available to help. If they can repair damaged carbon fibre gliders they can surely do a deraileur post.

You might try at Camden if you're in NSW.

John Lewis

Posted: Sat Nov 01, 2008 1:39 pm
by Kalgrm
Yeah, I think you should be able to repair that crack. It might even be as simple as putting some epoxy resin on it.

Some of the guys I ride with don't use a front derailleur at all. They just move the chain over by hand (or foot) when they know they are about to go hill climbing. Is that an option for you in the short term until you can get a repair done? Do you spend nearly all your time in one ring? It might prevent the crack from propagating.

Cheers,
Graeme

Posted: Sat Nov 01, 2008 3:27 pm
by just4tehhalibut
Yep, repair is the best and easiest option. Also not likely to cost $700.

To retro-fit a Schlumpf would still require a bit of machining to install, it needs one side of the BB to be chamfered (and they sell a special tool for this). And this introduces more weight and friction to the drive system, hardly dazzling for a carbon bike.
The speed Schlumpf is more useful for small-wheeled bikes or for nuts like me who like to combine it with a really large chainring and bomb down steep hills pedalling. But then I broke mine through excessive use, didn't I? Twit.

BTW if you have any kayak makers in the neighbourhood they might be able to do the repair.

Posted: Sat Nov 01, 2008 8:03 pm
by bradwoodbr
Easy fixed by sanding down and resin on a patch on the outside. You could cut off the top and do the same on the inside as well as adding a reinforcing spacer out of balsa wood. That will stop future flex and cracks around the patch. Then glue/resin the top back on.

Cheers

Posted: Sat Nov 01, 2008 9:06 pm
by Low Racer
Kalgrm wrote:Yeah, I think you should be able to repair that crack. It might even be as simple as putting some epoxy resin on it.

Some of the guys I ride with don't use a front derailleur at all. They just move the chain over by hand (or foot) when they know they are about to go hill climbing. Is that an option for you in the short term until you can get a repair done? Do you spend nearly all your time in one ring? It might prevent the crack from propagating.

Cheers,
Graeme


Hi Graeme,
I use the 56 teeth majority of the time but do need the granny ring for long ascend. If I changed out the 10 speed 11 -26 rear cluster to 11-32, might get away by using just the 56. From time to time, I have to unclip and use my foot to derail the chain to the granny (the FD doesn't work so well with the boom pushed in to the max). Without the FD, I have to stop and dismount to move it back to the largest chain ring. This will be very inconvenient.

I have posted on BROL too and someone came up with suggestion of drilling a small hole on top of the post and fill the space with epoxy resin. Once set should have the strength to prevent further propagation of the crack. The plate seems to be doing its job well and I am going to leave it just as it is for the moment.

Thanks for everyone suggestion.

Regards

Chong

Posted: Sat Nov 01, 2008 9:40 pm
by Kalgrm
Yeah, I saw the post on BROL. They had a few good ideas. In fact, they had enough ideas that I reckon you can let your breath out now. It looks like it's going to be salvageable without too much pain.

Cheers,
Graeme

Finally Fixed

Posted: Tue Nov 04, 2008 8:22 pm
by Low Racer
Hi guys,
The boom starts to creak again. Took the FD off and guess what, the metal plate was slowly eating into the carbon.

Went to Bunning to check out what I could get to patch the crack. Came home with a tube of steel reinforced epoxy putty. I went ahead to saw off the top and kneaded the putty into the column. I was able to shape the top to resemble the original post and the colour almost matched too. The putty hardened within minutes and is rock solid now. Reinstalled the FD and went for a test ride. Pretty happy with the result as no more creaking.

Thanks again guys for your input.

Regards

Chong

Posted: Tue Nov 04, 2008 8:58 pm
by rog on a bike
Hi Chong

Coffs airport has a fair few aircraft based there. I would imagine there's a L.A.M.E. operating there some where.
If you have future troubles take it to him if he can't fix it he will put you in touch with some one who can.

Cheers Rog