Compact Double vs MTB Double?

[Scunnered]

If I wanted to replace a compact double chainset with a MTB Double, what differences to I need to be aware of?

[NATURAL ANKLING]

Hi,
BB length probably will need changing, to prevent the small wheel from hitting the frame chain stay.
Front derailleur will probably need changing also.
Front gear changer may also need changing, to a different type.
This will affect maybe the Q factor (pedal spacing lateral)

Give some details of your existing bike parts and bike for a better explanation.

[Scunnered]

Hi, bike is an Arkose 1 with 48/32 FSA chainset and Shimano Sora FD and shifters. I was contemplating changing to a 40/28 MTB chainset.
I had anticipated that I might need to replace the BB as well. Why would I need to change FD and shifters? Q-factor is not an issue, I already have peddle extenders and maybe I could ditch them with a MTB chainset.

[Bonefishblues]

Save yourself faffing (and cash) by looking up Spa super compact chainsets on their site. £60 and should simply slot on (someone will now tell me of my foolishness, I'm certain )

[Scunnered]

The Spa Super Compact is a converted triple with the large ring removed. That's a whole 'nother question!

[Brucey]

An MTB double will usually have a different chainline to a (road) compact double, and not all road mechs will reach out far enough for the wider MTB chainline. If they do reach far enough, indexed front shifting may no longer work perfectly because the shift ratio of the front mech changes through its travel.

cheers

[Scunnered]

Hmm, the difference in chainline is much bigger that I had thought. A Deore M625 has a chainline of 48.8mm, more than even a road triple (45mm), and a road double (Sora) is 43.5mm, so perhaps this was not one of my better ideas. Perhaps a converted triple is the way to go after all...

[tim_f]

I have 'kludged' a Shimano hollowtech external bottom back triple mountain bike chain set work with a Sora triple front mech by moving a BB spacer from the drive side to the non drive side (i have a 68 mm bottom bracket shell.

This enables the front mech to reach across enough. (but it does makes pedals not symmetric.

However it might be different for you

a number of factors which might meant my kludge might not work for you.

1) You have a double not a tripple.
2) you might have a different generation of mech.
3) you might have a 72 mm bb shell
4) you might have a different sear tube diameter - which may inpact from mech position.
5) my bike uses bar end shifters so front mech is not indexed.

With external BB you do not have flexibility to position chain set as you do with square tapper.

[NATURAL ANKLING]

Hi,

Hi, bike is an Arkose 1 with 48/32 FSA chainset and Shimano Sora FD and shifters. I was contemplating changing to a 40/28 MTB chainset.
I had anticipated that I might need to replace the BB as well. Why would I need to change FD and shifters? Q-factor is not an issue, I already have peddle extenders and maybe I could ditch them with a MTB chainset.

Sorry I saw MTB and though it would be 3 speed?
But you did say double

[reohn2]

Save yourself faffing (and cash) by looking up Spa super compact chainsets on their site. £60 and should simply slot on (someone will now tell me of my foolishness, I'm certain )

This is your answer with a 115mm Shimano UN55 BB.
Your existing front mech will probably work with it by moving it down the seatube,if it doesn't a Sora triple front mech will by using the two inner positions with you double STI.

[Scunnered]

Hi,

Hi, bike is an Arkose 1 with 48/32 FSA chainset and Shimano Sora FD and shifters. I was contemplating changing to a 40/28 MTB chainset.
I had anticipated that I might need to replace the BB as well. Why would I need to change FD and shifters? Q-factor is not an issue, I already have peddle extenders and maybe I could ditch them with a MTB chainset.

Sorry I saw MTB and though it would be 3 speed?
But you did say double

Yes I did mean double. I had hoped that staying with a double would be relatively simple, but apparently not.

[tim_f]

Yes I did mean double. I had hoped that staying with a double would be relatively simple, but apparently not.

Not too bad need these steps

1) Remove old chain set and bottom bracket.
2) Buy Spa super compact double chain set and square taper bottom bracket with 115mm length and install
3) move front Mech down a bit.
4) tweak range screws on mech
5) re attach cable (new cable might be a good idea)
6) re index mech

[Scunnered]

Yes I did mean double. I had hoped that staying with a double would be relatively simple, but apparently not.

Not too bad need these steps

1) Remove old chain set and bottom bracket.
2) Buy Spa super compact double chain set and square taper bottom bracket with 115mm length and install
3) ...

Thanks. If I decide to go with a modified triple, I would prefer a hollowtech type BB and so would probably get the Sora triple and ditch the big ring. A 39T (or 40T) ring gives a good range of gears with a 11-28T cassette. I have no need for gears bigger than 100".

[Bonefishblues]

Yes I did mean double. I had hoped that staying with a double would be relatively simple, but apparently not.

Not too bad need these steps

1) Remove old chain set and bottom bracket.
2) Buy Spa super compact double chain set and square taper bottom bracket with 115mm length and install
3) ...

Thanks. If I decide to go with a modified triple, I would prefer a hollowtech type BB and so would probably get the Sora triple and ditch the big ring. A 39T (or 40T) ring gives a good range of gears with a 11-28T cassette. I have no need for gears bigger than 100".

People have been burnt for less

[Samuel D]

Your bicycle’s long-cage, 9-speed, Sora rear derailleur officially supports up to a 34T maximum sprocket size. Would it not be cheaper and arguably better (than messing up the chainline) to replace your 11–28T cassette with one with larger sprockets?

Granted, most relevant cassettes start with the pointless 11T and therefore have larger gaps than they should have and a higher top gear than you’ll ever need. But the larger sprockets at the other end and the middle may allow you to use the FSA chainset as it was designed to be used: in the outer ring most of the time.

You have a nice chainset in the 48T/32T FSA. It would be a shame to dump that.