Workshop tales, trials and disasters.
Maintenance tips, techniques and myths.
Technical discussion, description and outright lies
11 posts • Page 1 of 1
inexplicably, i noticed that it had disappeared when i removed my cranks on the weekend. on this morning's commute, my non-drive crank arm started falling off. i tightened up the pinch bolts real hard - no issues yet (touch wood).
how urgent is it to replace this?
You can ride without it, but should you? Dunno. IIRC, its there to ensure the crank arm is tensioned sufficiently onto the axle, to prevent the whole assembly floating side to side in the BB. It appears to serve no other purpose once thats done. Normally, the two pinch bolts are tight enough that it wont slip out, but I've never checked if it could be levered or knocked out.
MY RIDES: My Velospace Profile
we shall see
Temporarily yes, recommended long term no.
Ive had to do it for 2 weeks whilst in Perth recently on my emergency bike. No matter how tight the bolts are, they somehow loosen (and it only needs one of the two to go). The worst part was thinking the crank arm is going to end up flapping from the bottom of your shoe at the worst time(it got close on several occasions).
Just installed a new one today and the peace of mind is great.
n=8 (2011 road, 2004 road, 2010 track, 2009 foldup, 1990 hybrid, 1992 indoor trainer, 2007 road now a rental, 1970's step through)
Interesting. I also find the bolts keep loosening over time. So I check them fairly regularly, as I do with most bolts on the bike. That's probably why I've never had a problem with two piece Shimano cranks actually coming loose yet. Don't use a torque wrench either as I don't have one that goes low enough. But I'm an experienced enough bolt wrencher to know what bolt stretch feels like and what is too much.
However on your experience I bought a crank arm fixing bolt. Just because it hasn't happened to me doesn't mean it isn't going to happen. Thanks for the advice.
Are you guys using a torque wrench?
As it is 12-14Nm to get the correct tension. I used weights hanging from the end of a fixed length to get the feel for the correct torque and it is quite a bit of force for such a small bolt. Torque specifications are only 25% accurate to get the correct bolt stretch anyway. Now knowing how much force is required, I tighten by feel for bolt stretch.
This reminds me of a time I was replacing the torque-to-yield (TTY) bolts on the head of my 5S Celica engine. I'd got the 10 bolts from two different shops as they were hard to find. I torqued them up and then it required another 1/4 of a turn (or something). When I did the 1/4 of a turn they all felt uneven. So I left them tensioned overnight then repeated the process in the morning and got much more consistent results. That was about 6 years ago and the head gasket is still going well.
I also spoke to a professional torque wrench calibrator who said that steel needs to be worked a bit before it will give you an accurate reading. I have found I can usually see this by tightening a bolt to a set level, then waiting a few minutes. Usually when I come back it can tightened some more.
As a side note, even thought the crank is coming loose, I doubt they would just drop off if the safety tab is working properly. Could be wrong though.
Nobody - i'm not using a TW, but i am using T-bar hex wrench and giving the bolts a fair bit of torque - to the point where i think the bolt head will be stripped if i give it much more (it's only a 5mm bolt head, so it's not meant to be torqued too hard).
what do you mean by the 'safety tab'? i haven't had a close look, but i didn't think the pinch bolts sat in grooves on the BB axle - they seem to just bind the crank arm onto the splines, relying on friction to prevent the crank arm sliding off the splines. as mentioned, that hasn't worked - the crank arm fell off (or would have if i hadn't stopped pedalling and re-tightened it).
If you are concerned about the heads stripping (I've done it while doing the weight trial) then replace them with (stainless) normal socket head cap screw bolts. I replaced the ones I stripped. The ones on there have a tapered head and appear weaker. I'd argue you can't get enough torque with a T-bar as the points of contact with a T-bar is thinner than a normal allen key due to the ball end shape. I use an allen key with a shifter on the end, but a proper socket would work better. This thread is suggesting to me that people aren't doing up their crank pinch bolts tight enough. They have to be retightened after the first 100Km, and I'd argue they need to be regularly checked.
I've broken a safety tab in the past as I didn't know about them initially. You have to loosen the bolts, then flip the plastic tab up (on an angle) out of the crank slot with a screwdriver (or long nose pliers) to get the crank to actually come off.
11 posts • Page 1 of 1
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