Bosch gen 4 crank arm fell off my mondraker crafty r.

[Begazzz]

hi all. Tried a search but nothing concrete.

Pedalling on flat half way through a 25 mile route the race face crank fell off. Luckily the bolt was there as well and found a local house in the sticks with an 8mm key.

My question is I thought these crank arms were meant to need some serious leverage to get off but mine aee easy to get off once the bolt is off. Took a punt on 50nm when I got home and the other side was loose as well. Does anyone thread lock the bolts?

Can anyone share some knowledge?

Tia.

 

[Jag009]

Never had that problem ,but a small amout of purple or blue theadlock should do the job

 

[Mikerb]

There should be no need for threadlock. Check that the bolts are complete with a flat and rounded washer (both are captive ). The Isis splined spindle has a small ramp against which the crank sits and it is the correct torque that forces the end of the crank onto it. The spindle splines should be clean and lightly smeared with grease. Also apply some grease to the bolt threads and flat washer. All of that ensures the correct torque is achieved......and also helps getting the crank off at a later date.

 

[Paulquattro]

Some arms fit better than others
A small amount of loctite on the threads wont hurt , also don't grease the threads of the bolts but some on the flats where the washers and mating surfaces are is fine.

 

[Begazzz]

Ok thanks both. Yes bolts are complete and both your answers make sense. I guess some must be a better fit than others. I’ve opted for grease and observation during use for now and if I get a repeat I’ll perhaps go for thread lock. Either of you suggest a torque setting? I saw a video of the Yorkshire mechanic (YouTube) putting hope ones on at 50nm.

 

[Bndit]

Don't put thread lock, they should stay tight with just a little grease on the threads and 50Nm should be fine, I have Brose with Hope cranks.

 

[Paulquattro]

Don't put thread lock, they should stay tight with just a little grease on the threads and 50Nm should be fine, I have Brose with Hope cranks.

Hope cranks are renowned for being tight i have had several sets
Some cranks are not a perfect fit hence needing help with loctite
Out of 6 bosch motors i have had 3 needed loctite
DONT grease the threads only the mating surface and that should be very little ,
greasing the threads lubs them and lets them undo again if there prone for that in the first place
Where clamping force is needed no grease on the threads.

 

[2WheelsNot4]

Cant think how many threads Ive read saying this bolt or that bolt are chewed and how do they get them off.
All because they were cranked on hard, possibly beyond the set torque, but even if not, the points of the Allen bolt wear and eventually chews.
Thread lock allows you to put the bolt in tight, but not so tight you could be damaging it.
And the nature of nuts,bolts et all on mtb's is vibration loosens things off. So it is standard in all engineering issues to use a bit of threadlock where vibration is an issue.
The stuff was designed for a reason you know.

 

[Begazzz]

Fair points all. Also grease on the threads increases the real torque as it decreases the resistance so I’ll probably take off again. Degrease and a small amount of thread lock.

 

[Bndit]

You guys put locktite to your pedals also? Grease don`t have effect to the torgue. Hope instructs ( and everyone else) to put some grease and there`s grease when you take the original cranks out. So that`s the way. Locktite is good for the smal torgue bolts to keep things safe, put you don`t need it for this. It`s different story with EP8, you should glue the cranks

 

[Paulquattro]

You guys put locktite to your pedals also? Grease don`t have effect to the torgue. Hope instructs ( and everyone else)

1 . Pedals are not falling off for the OP and there also self tightening
2 . Whoever told you grease on threads and mating surfaces doesn't increase torque , You need to stop listening too as there talking out of the arses.

 

[Bndit]

I trust always manufacturers instructions: https://www.hopetech.com/_repository/1/documents/E-BikeCranksInstructionsISISSpline-EN.FR.DE.pdf grease and 50Nm.

 

[Paulquattro]

I trust always manufacturers instructions: https://www.hopetech.com/_repository/1/documents/E-BikeCranksInstructionsISISSpline-EN.FR.DE.pdf grease and 50Nm.

And if you had bothered to read my previous reply to you mentioning Hope cranks specifically being over tight on the splines compared to others you may not have felt the need, to try and correct me with your stupid comment that grease doesn't affect bolt torque,
You are also here giving advice on fitting cranks when you don't own the motor in question have you heard of manufacturers tolerances especially the differences between companies.

You crack on and do what you want and i will go on my experience of 40yrs and counting of being a mechanical engineer
Have a great day.

 

[Bndit]

Yeah but ISIS is standard, so I assume same principals generally would apply to all manufacturers? Or not?
Ps. Don't want to start fight here, just a different opinion...

 

[Paulquattro]

Yeah but ISIS is standard, so I assume same principals generally would apply to all manufacturers? Or not?
Ps. Don't want to start fight here, just a different opinion...

Fair enough on fighting

ISIS is standard but that's where i said about manufacturing tolerance
No two items are ever the same especially across different manufacturers
The Hope cranks for example on the bosch motor are slightly too tight where as on the brose they maybe different again
Even batches are different you would be shocked what i see sometimes and from top quality manufacturers.

PS. i do stand on my advice dont grease the threads with a bolt prone to coming loose as the grease will stay there and do its job allowing the thread to work loose , where as on the flats the grease will get dispersed when torqued up allowing the two metals to contact thus creating a friction to prevent loosening , and as for loctite thats its job no matter what size the bolt or nut to prevent loosening.

 

[Mikerb]

Yes this is an ISIS crank and the Race Face cranks are non self extracting. I have the same cranks on both my Bosch cx gen4 bikes which of course uses the ISIS crank. I have never used threadlock and always apply a smear of grease on the splines and the (female) bolt thread. Even after removing the bolt it still needs an extractor to pull the cranks off the splines. Note that the ISIS crank is strictly speaking not splined but rather it is castellated and it provides 100% surface contact with the crank. Without some grease the 2 faces will bind preventing an accurate torque on the bolt.
With reference to the bolt it is the application of the correct torque that prevents it loosening. That torque figure comes from a fastener guide book for engineers and is designed to be that which creates the greatest friction between the thread pitch faces but below the sheer point for the bolt. Again a small smear of grease prevents friction giving a false torque reading.
In this case I can only assume the crank was installed incorrectly either initially or removed subsequent to that initial build and not re installed correctly.
Was the crank that fell off drive side or non drive side?

 

[Begazzz]

Yes this is an ISIS crank and the Race Face cranks are non self extracting. I have the same cranks on both my Bosch cx gen4 bikes which of course uses the ISIS crank. I have never used threadlock and always apply a smear of grease on the splines and the (female) bolt thread. Even after removing the bolt it still needs an extractor to pull the cranks off the splines. Note that the ISIS crank is strictly speaking not splined but rather it is castellated and it provides 100% surface contact with the crank. Without some grease the 2 faces will bind preventing an accurate torque on the bolt.
With reference to the bolt it is the application of the correct torque that prevents it loosening. That torque figure comes from a fastener guide book for engineers and is designed to be that which creates the greatest friction between the thread pitch faces but below the sheer point for the bolt. Again a small smear of grease prevents friction giving a false torque reading.
In this case I can only assume the crank was installed incorrectly either initially or removed subsequent to that initial build and not re installed correctly.
Was the crank that fell off drive side or non drive side?

It’s was drive side. But the other was loose after checking. Highly likely just not tightened at the point of assy.

 

[Mikerb]

I always check over a new bike.........and I do all my own maintenance!! A common assembly mistake is to fit and tighten, planning to change tool to a torque wrench later..........then forgetting!! I also see folk using a torque wrench incorrectly. The correct torque is of course when the torque wrench "clicks".....but that click should be during a continuous movement of the bolt, not just after re-positioning the ratchet to get a better leverage point. If that happens, loosen the bolt a little and then re apply leverage on the torque wrench.

 

[Hardtail]

Grease your threads people! unless you want to have 40% less torque than required.

https://www.engineeringtoolbox.com/torque-lubrication-effects-d_1693.htm

I grew up building modified, often force inducted motors, they often had dry or lubricated torque settings.

I have always believed Loctite works as a lubricant, Kevin seems to agree.
Ask Kevin: If I Lubricate Threads, or Use a Threadlocker, Should I Reduce the Tightening Torque?

Old but gold:
Tech Tuesday - Thread locker Basics - Pinkbike

Torque wrenches are cheap and accessible now but not many work on reverse threads, including the one I bought for the Bosch Lockring...