[Closed] my crank fell off today....why?

I'm flummoxed why you lot are going on about the bearing preload nut.

It serves no retention purpose. All the work is done by the pinch bolts.

This happened to me once on a new set of ht2 cranks. I installed the bolts without grease and they weren't cranked as hard as I thought. I had to relctantly conclude user error, even though I'd built loads of bikes, I made a mistake.

The spline in the crank arm was damaged and it never fitted properly tight again and persistently worked itself loose so I had to bin it after that.

You're a big lad ton, you've just dragged it off the end of the axle.

[i]yeah right, cos I have never built a bike up before.[/i]

[i]I have a deore crankset on a tiagra bb. [/i]

😯

It fell off because it was incorrectly fitted using incompatible parts.

You need a Shimano mtb bottom bracket and no spacers. Using a road bottom bracket will mean that there is 2mm less engagement on the splines than there should be. You may get away with it but why risk it? With the wider road bottom bracket and a spacer there was no way it was going to stay attached long term.

You need a Shimano mtb bottom bracket and no spacers

no I don't. I need a shimano mtb 68/73mm bb with 1 spacer at each side.
a shimano tiagra 4700 bb is also 68mm.

I have now fitted a deore bb with 1 space each side.

Did I actually get something right?? 😯

no ideas Dez......my head is fubar now.......... 😆

Doesn't a 68mm bb-shell need one spacer drive-side and two on nds for Shimano mtb stuff?

One on NDS, two on DS with MTB BB on 68mm shell.

Shimano road BB cups are 1mm wider than MTB ones.

So although both are 68mm shell fitting with no spacers your actual space between cranks is now 2mm wider than had you used the right BB.

I'd have thought the plastic hook wouldn't have fitted into the hole in the axle if the cranks wasn't engaged enough (having said that, I've never checked for tolerance).

just rode home and survived the bb/crank apocolypse...... 😆

scotroutes - Member
I'd have thought the plastic hook wouldn't have fitted into the hole in the axle if the cranks wasn't engaged enough (having said that, I've never checked for tolerance).

Having read the instructions, I'd have thought the plastic hook thing was specifically put there for the express purpose of checking the crank was on properly before tightening the pinch bolts. You can't push it in if the crank's not on properly.

One good thing about PF bearings was it did away with all the spacer malarkey!

68mm shell: 1 NDS, 2 DS
73mm shell: 1 DS
E-type front mech or chainguide? Replace one from DS.

Get the right BB and use the right spacers. Done.

Yeh, they're a shit design. Replace them with a SRAM chainset as they don't suffer from this problem and far easier to remove/install.

For sure it's almost like they knew you would have it on and off alot with their shit bearing quality 😉

😀 was hoping someone'd post that

Shimano MTB and Road BB cups are the same size. The only difference between them is the length of the threads on the DS which are usually longer for MTN than for road. Road BBs are also usually for 68mm shells, so have smaller connecting shafts. As Njee says above, just use the spacers appropriately for the shell size.

I have MTB Cranks on road BBs and MTB road cranks on MTB BBs.

The Preload screw is just that, finger tight with the tool (or a pair of opposed butter knives), make sure the plasic pin spacer between the bolts on the NDS crank arm can home into the hole on the axle, then tighten the pinch bolts to the correct torque.

http://www.tobiasfeltus.com/site/blog/the-burning-question-can-i-use-a-shimano-road-bottom-bracket-on-a-mtb/

I'll just leave that here

1mm.

so my tiagra bb with my mtb craks were ok, just as i thought.

the plastic flag with the pin did drop in the hole on the axle when i built the bike. just has it did on every (lots) bike i have ever built.

taking my tiagra bb out and fitting my deore bb with spacers made no differance.

my crank falling off was just one of those unexplained things.

if it happens again i shall install a thing like a old fashioned head doctor. a length of threaded bar with a washer and nut on each end, to old the whole lot in place. sorted.

I’ve played this game (but opposite to Ton) - 105cranks with XT bb. It doesn’t work - the 1mm difference between the width of the outer face of the bearings can’t be sorted properly with the 3spacers irrespective of the frame bb shell width.

I'm confused reading this 😀

I'm confused reading this

not as confused as i am typing/explaining it........ 😆

lol

my crank falling off was just one of those unexplained things

Hahahahahahahah De-nile is not a river in Africa

[quote=philjunior ]Having read the instructions, I'd have thought the plastic hook thing was specifically put there for the express purpose of checking the crank was on properly before tightening the pinch bolts. You can't push it in if the crank's not on properly.
Are you suggesting the OP hadn't fitted them properly?

Are you suggesting the OP hadn't fitted them properly?

I ran XT cranks with a Dura Ace BB and a spacer removed circa 2007 and that BB lasted forever, including a season of XC guiding in Whistler.