beanbag

Hello folks,
I'm trying to fit a Shimano GRX crankset + road BB onto my older full suspension mountain bike (68mm shell, MTB BB and cranks) as part of a conversion to a gravel bike. In terms of Q factor and crank arm to chainstay clearance, that part worked out fine.
What was unexpected was that the 46T chainring barely hits the chainstay due to [reasons]. So I would like to move the drive side over by at least 1 mm, preferably 2mm. To do this, I was thinking of the following:
1) move the drive side BB shell out by 1-2 mm, either by adding a spacer, or using the MTB BB cup (1 mm thinner) and add one 2.5mm spacer (= 1.5mm total) since I already have those laying around.
2) for the non-drive side, use the MTB BB cup to gain back 1mm worth of crank arm spline engagement.
(I would use the road plastic inner cylinder because it is shorter)

One important question is what is the safe range for the crank arm spline engagement? Is it "as long as the locking plate slides back in"? In the "default" installed configuration, it looks like the locking pin is exactly in the middle of the big hole in the splines, and about 2mm of splines in the crankarm are still showing. But the hole is big enough that the crankarm can go +/-2mm in either direction. Is this still considered within the "safe" range?

Any other ideas?
(It's ok to suggest giving up since I can probably still return the cranks, having not actually used them)

Iride01 

Chain line of GRX cranks is 47 mm. Most 11 or so speed road bikes expect a chain line to be about 43.5 mm.

I'm just not sure why you'd want to insist on putting a GRX crank on your road bike unless the chain line, which is determined by your rear cassette position is not what a 11 speed road bike will have.

If you are trying to get a smaller size of chain rings, then there are better options.


edit... coming back to this I realize now that the OP did state it was for an older Mountain Bike. So just take where I said road bike out of the text and substitute older mountain bike. Older mountain bikes had narrower chain lines than what GRX is made for.

70sSanO

A 68mm shell width is odd for a mtb. I think you need to first look at the chainline with the crank too close to the chainstay. If moving it out a mm or two will keep things close to centered then you probably won’t have a chainline problem.

I “think”, not positive, the GRX crank will fit 68/70 BB shells, so you can probably thank those Italians for that extra 1mm to 2mm DS spacer.

John

beanbag

my bike expects chainline of 49mm, but as a 3x, and therefore not a 46T chainring so close

Iride01 

I'd still find a proper size crank instead of using something not intended for the purpose. I guess there is a certain appeal to re-engineering stuff and making square pegs go in round holes, I use to do that myself. But over the last decade or so I find it more easy to get the correct stuff.

And for the old bikes I had that I couldn't get the correct stuff easily or at a cost I was willing to pay, well they are gone... either someone else's issue or in the scrapyard.

Troul 

I'm not familiar with the BB in question, but if you can, (If the NDS crank arm doesn't cause issues) omit the spacers on the NDS & place (stack) those on the DS next to the spacers meant for the DS.
Assuming the tolerance needed is less than 3mm.

beanbag

I used the method mentioned in the first post, and it seems to work out ok. Overall, this resulted in the chainline being moved out 1.5mm, yet the non-drive side only lost 0.5mm worth of engagement over stock.

Troul 

i'm not following the method you used, can you quote the method you used?

wheelreason

I think we called them spacers last century....
"Uncontrolled reentry" made me chuckle today.

beanbag

drive side: mtb cup +2.5mm spacer
non-drive: mtb cup only

Shimano overview:
mtb cranks have longer spindle than road cranks
mtb BB cups are actually 1mm shorter than road BB cups
MTB crank + bottom bracket setup involves spacers
Road cranks and BB uses no spacers

70sSanO

Good job!

John