valleysuper

do most gravel bikes come with 68mm bb shells?

am trying to convert old mtb into gravel bike but i have a 73mm shell.

cant seem to find an appropriate solution to run any crank with 50/34 teeth. Mtb cranks do go up to 50 and road bike spindles cant go in a 73mm shell.
Right now im thinking of putting the same deore crank back on as a double, by removing small ring and then adjusting my road fd to shift it, but that still leaves me with 42/32 for rings.

Ive looked at specs on a few gravel bikes online, but they dont specify the bb shell width usually.

Thanks for any help

Tim_Iowa

Most so-called "gravel bikes" are based on cyclocross frames, which are based on road frames, so they use a 68 mm wide BB shell.

You have a MTB with a wider shell. If you try to put a road compact 50/34 crankset on your frame, it wouldn't have the correct offset to line up with your cassette in back.

Your second idea (using the existing crank and BB) is a much easier path. Research the crankset to find its Bolt Circle Diameter, and then you can buy exactly the rings you want. 48/32 sounds like a better set-up for climbing rolling gravel hills than 50/34 anyway.

You may also run into issues with fitting a road FD to the MTB frame; the road FD is designed for less offset due to the narrower road BB shell width, so a road FD may not reach out far enough.

What shifters are you trying to use? Some are compatible with MTB FDs, especially vintage MTB FDs.

valleysuper

Thanks Tim.

Trying to run Dura ace 2x9 spd shifters pretty sure they are 7600's which from what i have read should shift the Deore LX RD as indexing was the same, but i had not looked into the FD. maybe keep the Deore LX front deraileur and set up the limit screws to negate the removed granny ring?

seemed from reading most road and mtb shimano were relatively compatible as long as you stayed pre 10 spd for everything??

i did look into diff rings the 94 bcd 5 bolt options are few and not above 42 teeth, when i looked quickly last night

justin1138

square taper bottom bracket and an applicable double crankset?

I've used a handful of 'non-sanctioned' front derailleur combos and have had pretty good luck. It's worth a shot.

And yes, your rear shifting set up should work fine. My cross bike is set up with 10 spd road shifters and an 8 spd mountain derailleur. All works great.

valleysuper

Yes it is square taper now Justin. So I think I’m gonna leave it be and put cranks back on as a double. Use the deore FD as it’s top pull and see what happens...
The dura ace FD I was hoping To reuse is bottom pull.
Learning a bunch doing this.

Tim_Iowa

Shifting:
DA 7700 is the 9-speed group, and yes that right shifter will index a 9-speed Shimano MTB rear derailleur perfectly.

HOWEVER
the left shifter will not index with a MTB front derailleur. Shimano has used different cable pull ratios for road and MTB front derailleurs ever since they started doing index shifting.

The Jtek Shiftmate 7 is designed to solve this problem (Road shifter -> MTB FD). Seems like it's only available from the UK at the moment... And, it says it's meant for road triple -> MTB triple. I can't attest that it would also work for road double -> MTB double.


Or, you could try a Shimano road triple FD and see if it has enough reach for the chainline of the MTB crankset. The Claris FD-2403 would be a cheap experiment.


I see a lot of bar-end shifters on drop-bar-converted MTBs because of these headaches. 1x is another popular option; ditch the FD completely.

Gearing:
I did manage to find a 46t chain ring in 94 mm BCD, but that seems to be the largest. 46 x 11 gives a decent top gear.

A 110/74 BCD triple (widespread on 80s/90s MTBs) could easily fit 48t and 50t rings. My 88 Schwinn has a Deore XT FC-M730 in 48/38/26 (Biopace, natch).

valleysuper

i was wondering why alot of them were this way, im finding out this is not so easy to do, let alone have it look good when youre done..
I admire the work that goes into some of the bikes i see made up, but they at times deserve the "Frankenbike" moniker.

Anyway, my final solution is 1 I was hoping to not go to, as noone ever has spare campy parts. But im gonna run my Campy chorus 10 spd levers and keep the Deore Front and rear Deraileurs. Im gonna drop the granny ring off the triple crank grab a 46 tooth big ring and throw that back on. throw on a 11-34 9 spd cassette. I will be able to sweep the original triple fd over enough with the Chorus lever, once limit screws are set properly

In the end it will look neat and clean with cables run under bar wrap, and keep all cables running as intended for the frame using the original top pull deraileur.

thanks for all the help If you see anything wrong or concerning with above set up, please let me know
Everybody likes pics so

Original. And prepped for paint and rebuild.

Tim_Iowa

That's a decent solution, actually.
Campagnolo front Ergopower shifters aren't really indexed, so you can probably get them to work with the MTB FD in double mode.
And you can index a 9-speed Shimano cassette with a 10-speed Campy shifter if you use the "Hubbub" cable routing. https://artispin.wordpress.com/hubbu...ergo-solution/

I've always like the old "triple triangle" GTs. I have an '89 Avalanche that I built as a kickaround town cruiser bike for my son (cruiser bars and 2" slicks). It has a rear U-brake (derp).

valleysuper

That’s a good link Tim.

Here’s another if it helps anyone.
Helped me a bunch with pulls and ratios.

https://blog.artscyclery.com/science-...compatibility/