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Just swapped the stock sram S1250 cranks in my 2015 Stumpjumper for some carbon FSA K-Force Light cranks (BB30 MTB Double) that Wiggle had a big discount on. I have not changed the BB, just left the push fit one that was already installed. The FSA cranks came with ceramic BB30 bearings, but I’ve not fitted them just left the ones that were already fitted in the frame for now.
Problem is that if I do the crank bolt up to anywhere near ‘normal’ (ime) crank bolt tightness, the cranks do not spin freely - there is resistance compared to the old cranks. A push that would spin the old cranks round 5 times, only lets the new ones spin ~2 times. I can address this by loosening the crank bolt off (and still eliminate play by using the preload collar), but this doesn’t seem right to me.
Any ideas? The cranks are smooth, just not free spinning. I’ve not ridden it yet to know if I will notice it while riding, but I suspect I might. Will they loosen up in time? Despite my instincts telling me not, is the relatively ‘loose’ crank bolt okay? Do I need to fit the ceramic FSA bearings for an exact match, or should this not matter? Any help appreciated :-).
Are both BB's the same.? Are they loose bearing races or are they press-fit BB's.?
Both BB30
Press fit
So they are identical apart from one has ceramic bearings inside the race.?
Yes, the FSA comes with ceramic bearings that I could push fit into the frame, but I was hoping to just use the bearings already in the frame.
If you have the tools then swap them over. Sounds to me like the width of bearing or the width of the axle is slightly different. Unusual but not out of the realms of possibility.
Agreed. Was hoping I wouldn't have to do that, as I'll need to purchase a few tools for push fit BB removal/fitting. Guess it will be worth it in the long run.
However...if it is due to the bearing width being slightly different, wouldn't that cause resistance even when the crank bolt is loose?
Problem is that if I do the crank bolt up to anywhere near ‘normal’ (ime) crank bolt tightness
Have you checked against the actual recommend torque to check whether you version of 'normal' is actually right?
Also, as above, check bearing dimensions as there could be a difference. Any spacers/washers that you've missed or added that shouldn't be there?
Just tighten it and ride it, it'll be fine.
Bearings won't be any narrower - they are made to .01mm tolerances.
It's a bike not a surgical instrument!
Have you checked against the actual recommend torque to check whether you version of 'normal' is actually right?Also, as above, check bearing dimensions as there could be a difference. Any spacers/washers that you've missed or added that shouldn't be there?
Torque wrench lent to friend, so to be fair, I am guessing, but it is loose compared to every crank bolt I've ever tightened before. Checked spacers a few times, think I am okay according to the instructions.
I tend to agree with Al that it'll be OK s long as the cranks aren't actually binding, but I can understand your worry if it's clearly different to the old cranks.
Can you tell where the drag is coming from? does it seem like a preload/rubbing/binding or could it be a rubber contact seal somewhere and just a little more drag than the old cranks?
Bearings won't be any narrower
to quote one of your favourites back at you al
where's your evidence? 😉
Is that because you know the parts/spec fo both those cranksets and [i][b]know[/b][/i] that they use the exact same BB bearings or was that a case of the [i][b]shouldn't[/b][/i] be any different?
I wasn't referring to a tolerance difference between same spec parts, but if they are designed to run with a different bearing then there's no harm in checking the current Vs new to check they are (at least labelled) the same size.
BB30 is a standard => bearing dim!s the same.
For every model ever?
I'm being purposefully awkward here, we both know its a standard and that standards should all be the same, but it wouldn't be the first time a particular manufacturere or model was slightly non-standard.
For the 10 seconds it'll take to check, I say check.
Just tighten it and ride it, it'll be fine.
Mate of mine swapped a set of cranks over and put everything together exactly the same as it came off. When the crank bolt was torqued up correctly the cranks were stiff. Less than a month later the drive side bearing was shot. When I pressed a new one in and re-installed the cranks and took a wafer thin spacer off he none drive side arm and it span perfectly. Six months on and it's still running fine.
Do it correctly first time.
Can you tell where the drag is coming from? does it seem like a preload/rubbing/binding or could it be a rubber contact seal somewhere and just a little more drag than the old cranks?
Difficult to say. Drag only develops as the crank bolt is tightened (but it doesn't need to go very tight before it starts). They seem smooth enough with or without the resistance.
Doesn't really seem like binding, more like a tighter contact seal. Difficult to describe I'm afraid.
Amedias - no harm checking indeed.
Neil's - pretty different scenario there.
Neil's - pretty different scenario there.
True that but something is not right somewhere is it.
well if you're 100% sure you've assembled correctly according to instructions, no obvious bits missing/added, bent or damaged, and bearings are all identical then I would go for one of the following options depending on how worried you are
1 > ride it
2 > get your torque wrench back and check, then ride it
3 > fit the new bearings and see if any different
4 > seek professional help locally
My personal suggestion would be 3, followed by 2.
The fact that it's smooth and only gentle resistance suggests that it's either a bit more friction on a contact somewhere (which may loosen up depending on what it is) or that you have over-preloaded them somehow (see point 2).
My guess is its a bit out of tolerance, I presume there's no spacer to remove?
I wouldn't replace the bearings as before - take some very nears or a micrometer to them why not.
It's just a TINY increase in friction (unlike Neil's example) so I think it'll be OK.
My guess is its a bit out of tolerance, I presume there's no spacer to remove?
Spacer removed from drive side of old cranks, and replaced with narrower spacer provided with new cranks, as per the instructions with new cranks. Chainrings are in exactly the same position as before, which suggests to me that the spacing (on the drive side at least)is as it should be. Other spacers were not used (only needed with 68mm width BB, mines a 73mm).
Been there/done that.Sram bb and fsa bb (pf42) have a different shaped bearing seal that matches up with one on the cranks.If you prize it out (carefully) and swap it for the one on the other bb it'll work fine.
Thats interesting Rorschach, I'll take a closer look tonight.
That suggest to me that when tightened the mis-matched bearing seal is causing the extra drag, which matches the symptoms. Thanks.
Thanks all for the suggestions.
Replacing the SRAM bearing seals with the FSA ones did the trick. 30 mile ride this morning and everything worked fine. Internet gold star to Rorschach
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