Spacers for GXP BB?

me-109

I've replaced an FSA Vero Compact on square taper BB with a SRAM Apex onthe GXP BB it came with. All installed ok and only had to tweak lower limit screw of front mech to avoid chain rub on 1st gear. However, chain seems to want to climb off the outside of the outer ring when in top gear. Have reset front mech cable and need to turn outer limit screw to set outer plate against chain otherwise the chain looks like it is trying to curl itself off the teeth.

Measured chainline of new setup as 43mm to inner ring, 49mm to outer ring. FSA webbie states Vero chainline is 43.5mm. Could it be that the chainline is too severe in top gear and needs the chainset spacing out? GXP BB does not come with shims like Hollowtechs seem to. Installation instructions make no mention of them. If such things exist, or I can use Shimano ones, I presume I fit one or two behind the DS shell to space the crank out?

nicklouse

GXP road and HollowTec road do not use any spacers.

or to add to that if used with road cranks.

if you used an MTB crankset you would need to use the correct spacer config.

chain line is measured differently by different makers. and i do not know how FSA measure.

a Sram 49.5MM is the same as a Shimano 50MM

me-109

I figured that they shouldn't use any, but since I've got a problem I didn't have before, this seems a logical approach to me. Can you offer any alternatives?

nicklouse

set your mech again.

and add spacers to the drive side will do nothing other than potentially crush the bearings.

me-109

At the moment I'm having to restrict the movement of the mech in order to use it like a chain device and stop the chain peeling off the outer ring. If I set the mech correctly the chain throws within a couple of revolutions. In 9th gear or below it runs fine, which is why I think the chain approach angle, hence c/set chainline needs moving out.

I did wonder whether the addition of spacers would lead to added side loading of the bearings.

nicklouse

sounds like you mech is set too far out.

and yes there is no room for any spacers. and adding them to the drive side would not move it out. to move it out you need to remove from the Non drive side.

is it a triple or a double?

ride_whenever

how worn is the chain? Sounds more like the chain pitch is longer than the chainring pitch and so it climbs off.

amaferanga

If the chainrings are the same size (so the mech is at the right height) then I'd have a look at the angulation of the front mech. It may have been wrong all along, but you were just lucky that it worked with your old chainset.

But as above, new chain first if the old one has done a lot of miles.

me-109

Chainset is compact 34/50, same as the 'old' chainset.

Chain has only done 600 miles since a 10sp conversion. Old chainset has only done another 600 miles so there is no significant disparity in wear. There are no signs of damage to the chain or stiff links.

The problem does not relate to the front mech. If the front mech was not there it would still happen. It is not pushing the chain off with the inner plate; the inner plate is free from the chain.

I also noticed that when on the inner ring and outer sprockets on the cassette the chain catches the inside of the outer ring more than it did with the FSA chainset. This reinforces my opinion that the new chainset is sitting closer to the centreline of the bike. If 1-2mm shimming is not possible through the design then it is just unsuitable for this frame.

nicklouse

Me-109 wrote:

I also noticed that when on the inner ring and outer sprockets on the cassette the chain catches the inside of the outer ring more than it did with the FSA chainset. This reinforces my opinion that the new chainset is sitting closer to the centreline of the bike. If 1-2mm shimming is not possible through the design then it is just unsuitable for this frame.

yes this happens on most set ups and is not a normal sue of the gears.

and there is no way to move the crankset "out" unless you want to remove material from the frame.


this is the same with most External BBed cranks sets. Shimano ones can be tweeked a bit.

I would still just get it set up and ride.

gloomyandy

since you can't adjust the chainline from the front is there any chance of adjusting it at the rear?

amaferanga

I'm not sure what the problem is here, but it sure isn't chainline.

mpdouglas

Have you definitely got the drive side oil seal fitted to the GXP bottom bracket cup. Without that, the chainrings will slide further into the BB by 1mm or so. But as said above, I struggle to believe that 1mm would be enough to cause the chain to try and ride off the outside of the big ring. The chainring isn't bent or not seated on the spider properly is it?