Cracked magura durin

Discuss light weight issues concerning mountain bikes & parts.

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halington
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Joined: Sun May 13, 2007 10:57 am
Location: UK

by halington

Hi guys, just thought I'd warn you about a place to look out for cracks on the magura durin fork, seemingly other folk have had this happen too. It's the top caliper mount...
Image

I don't think the bolt was done up too tight, but can't be sure as i don't have a torque wrench.

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Benno
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Location: Fort St John
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by Benno

I think with post mount you are also not supposed to run the washer between the bolt head and the caliper. This means more of the bolt goes into the fork. I say this because I've stripped an R7 post mount (luckily they warrantied it for me). Though this looks like a separate problem all together. I will check mine and keep an eye on it since that would lead to injury and/or death :shock:

by Weenie


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Cheers!
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by Cheers!

halington wrote:I don't think the bolt was done up too tight, but can't be sure as i don't have a torque wrench.


I bolt that is too lose and not properly torqued is just as bad a bolt that has been over torqued.

With all the exotic alloys being used, delicate carbon fibre I don't know why people still go without a torque wrench. All these materials used in aerospace applications are carefully torqued, all the threads are cleaned with solvents, the proper thread prep (antiseize, bra-cote grease, loctite, etc) are all perfectly applied to each thread and then the bolts are torque in stages with a QC technician verifying for the click and each torque wrench is checked before use.

6061, 7075, magenisum and titanium alloys used in bikes are the same as the ones in aerospace. Just that in aerospace a huge stack of paper work follows each peice. How you assemble a bike should be as much care as putting together a satellite.

Bikorexia
Posts: 104
Joined: Sun Jan 06, 2008 5:03 pm

by Bikorexia

I remember how I waited for torque wrench to complete bike 1 month. Ignorants at service practically laughed bu there's really no way around dealing with lightest fibre avaible, scandium, magnesium. F.e. my stem really works degreased with friction paste at 2,5/3,5 Nm tested well ;-). Sometimes 0,5Nm makes tangible difrence - I know that for sure :-(

pedalon
Posts: 10
Joined: Fri May 16, 2008 1:56 pm

by pedalon

tbh i would be very surprised if it was due to over tightening, the threads in the posts are not the strongest, i would have thought you would strip the thread before anything else. However like 'cheers' said, if it was too loose and was being knocked each time the brake was applied it may start a crack. Also like someone else mentioned if the bolt is not in deep enough then this could cause the crack. But hey, i am not a metallurgist !, i did my post mounts tight, but not mad tight, i would have thought if they are that critical there would be a sticker on them saying so and making sure users don't over or under tighten. My guess is they were not tight enough, but who knows......... i will check mine tongiht !

Wictor
Posts: 138
Joined: Mon Nov 13, 2006 12:14 pm
Location: Wageningûh, Netherlands

by Wictor

Cheers! wrote:With all the exotic alloys being used, delicate carbon fibre I don't know why people still go without a torque wrench.


<bold>'Cause there's little point.</bold>

As you correctly point out:

Cheers! wrote:All these materials used in aerospace applications are carefully torqued, all the threads are cleaned with solvents, the proper thread prep (antiseize, bra-cote grease, loctite, etc) are all perfectly applied to each thread and then the bolts are torque in stages with a QC technician verifying for the click and each torque wrench is checked before use.


I have never seen bike part torque specs coming with a specification for which anti-seize type or grease they should be used with (if any), and this has a significant effect on thread friction and hence how tightening torque relates to clamping force on a part. A good example of this effect is cleaning and regreasing the seatpost clamp bolt when the thing stops clamping. :wink:

Add to this my suspicion that some manufacturers simply pull torque figures out of their ... . I've fitted parts with calibrated wrenches only to see them slip under reasonable loads, and I've seen people slaughter lightweight components with the same wrench.

by Weenie


pedalon
Posts: 10
Joined: Fri May 16, 2008 1:56 pm

by pedalon

I agree, like i mentioned if the torque was so critical it would be at least written on the post itself (my scott spark had torque values written on clamps, pivots etc), i am pretty sure they allow for ham fisted people like us when they design these things or they would take measures/advertise the fact that we could break them.

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