Road cranks on a MTB frame?

Lowell Walmsley said:

I have a mid-90's StumpJumper that I ride only on the road and I'd like to
increase the gearing a little. I also have an Ultegra 3 speed crank from a
9 speed group (splined bb). The crank I have on it now is a LX MTB crank
with a 46 tooth ring. The cassette now is a 13 - 26.

Can anybody tell me if it would work to put that crank on that bike? What
would I need for a bottom bracket? Would this make a noticeable difference?
Would this be a dumb thing to do?

Thanks,
Lowell

Check to make sure the rings will clear the chainstay, might run into
problems with the front derailleur also. BB depends on the chainstays
and desired chain line.

-nate

Lowell Walmsley said:

I have a mid-90's StumpJumper that I ride only on the road and I'd like to
increase the gearing a little. I also have an Ultegra 3 speed crank from a
9 speed group (splined bb). The crank I have on it now is a LX MTB crank
with a 46 tooth ring. The cassette now is a 13 - 26.

Can anybody tell me if it would work to put that crank on that bike? What
would I need for a bottom bracket? Would this make a noticeable difference?
Would this be a dumb thing to do?

Thanks,
Lowell

Dear Lowell,

Assuming a 52-tooth Ultegra triple chainring, yes, it will make a
noticeable difference.

The 13% increase in front chainring size will lower your cadence at
the same speed.

If your MTB has a 26x1.5 rear tire witha 2045mm circumference, then
your cadence will drop from about 74 to about 66 rpm at 20 mph.

Here's another way to look at it. How fast do you have to pedal in
familiar gears to go 20 mph? Here's the rpm table:

20mph 20mph
rear 46 52
gear front front
rpm rpm
12 68.4 60.5
13 74.1 65.6
14 79.8 70.6
15 85.5 75.7
16 91.2 80.7

Switching from a 46 to a 52 gives you a bigger jump than dropping a
tooth on your rear sprocket--an extra gear, plus a bit more, so to
speak.

Cheers,

Carl Fogel

"Lowell Walmsley" <[email hidden]> a écrit:

Quoted message said:

I have a mid-90's StumpJumper that I ride only on the road and I'd like to
increase the gearing a little. I also have an Ultegra 3 speed crank from a
9 speed group (splined bb). The crank I have on it now is a LX MTB crank
with a 46 tooth ring. The cassette now is a 13 - 26.

Quoted message said:

Can anybody tell me if it would work to put that crank on that bike? What
would I need for a bottom bracket? Would this make a noticeable
difference? Would this be a dumb thing to do?

Specialized have used 73mm bottom bracket shells in the past (my
mid-nineties Rockhopper has one). If you have one, this may be a
complicating factor. Shimano made two versions of the Hollowtech spline: one
for road and XTR, and another for the lesser mtb groups. The road models
were not made in 73mm width, and the longest XTR axle (116mm) is shorter
than the road triple (118.5mm). The effect of installing a 68mm unit in a
73mm shell is to shift the whole assembly rightward 2.5mm with respect to
the centre line of the bike. That could be advantageous from the point of
view of chainring clearance, but you may run into trouble with the flanged
left-hand cup of the bottom bracket.

If this were my bike, I'd start by changing the cassette for something with
an 11 or 12 on the bottom. If your cranks have one of the common bolt
circles, a big chainring with 48 or 50 teeth would be another option.

James Thomson

Lowell Walmsley said:

I have a mid-90's StumpJumper that I ride only on the road and I'd like to
increase the gearing a little. I also have an Ultegra 3 speed crank from a
9 speed group (splined bb). The crank I have on it now is a LX MTB crank
with a 46 tooth ring. The cassette now is a 13 - 26.

Can anybody tell me if it would work to put that crank on that bike? What
would I need for a bottom bracket? Would this make a noticeable difference?
Would this be a dumb thing to do?

Thanks,
Lowell

'May' be able to use the standard 118mm BB for triple w/o the small
ring hitting the right stay...may not tho as these frames are made for
large tires, hence wide stays.

You can also out bigger rings onto the LX crank if its a 110/74 BCD
type.

Lowell Walmsley says...

Quoted message said:

I have a mid-90's StumpJumper that I ride only on the road and I'd like to
increase the gearing a little. I also have an Ultegra 3 speed crank from a
9 speed group (splined bb). The crank I have on it now is a LX MTB crank
with a 46 tooth ring. The cassette now is a 13 - 26.

Can anybody tell me if it would work to put that crank on that bike? What
would I need for a bottom bracket? Would this make a noticeable difference?
Would this be a dumb thing to do?

Thanks,
Lowell

The combination you already own is unlikely to work without hitting the
chainstays. Check your BB shell width. The Shimano road BB's I have
seen specify 68mm shell width. The longest spindle Shimano offers is
118.5mm, which is designed for road triples. You will almost certainly
need something longer to clear MTB chainstays. You could go to an ISIS
crankset and choose a BB long enough to work, but that would cost more
than $100. No matter what you put together, you will need to shift it.
I doubt that your MTB shifter and derailleur will shift any road
crankset gracefully. I had a MTB shifter/derailleur setup with a
compact road crankset and while it did shift, it had too many problems.

You could get a bigger chainring or a smaller cassette, but best of all
would be to spend nothing and admit that you already have all the gears
you need. I know it may seem like you need a big gear at the bottom of
that hill, but trying to keep up with your rear wheel accomplishes very
little. Coast a few seconds longer and your 46/13 will be back in the
game.

Thanks everybody for the advice and info. It sounds like buying a BB to see
if it fits wouldn't be a very good bet. Maybe a smaller cassette would make
the most sense.

Lowell

"Barnard Frederick" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Lowell Walmsley says...

Quoted message said:

I have a mid-90's StumpJumper that I ride only on the road and I'd like
to
increase the gearing a little. I also have an Ultegra 3 speed crank from
a
9 speed group (splined bb). The crank I have on it now is a LX MTB crank
with a 46 tooth ring. The cassette now is a 13 - 26.

Can anybody tell me if it would work to put that crank on that bike?
What
would I need for a bottom bracket? Would this make a noticeable
difference?
Would this be a dumb thing to do?

Thanks,
Lowell

The combination you already own is unlikely to work without hitting the
chainstays. Check your BB shell width. The Shimano road BB's I have
seen specify 68mm shell width. The longest spindle Shimano offers is
118.5mm, which is designed for road triples. You will almost certainly
need something longer to clear MTB chainstays. You could go to an ISIS
crankset and choose a BB long enough to work, but that would cost more
than $100. No matter what you put together, you will need to shift it.
I doubt that your MTB shifter and derailleur will shift any road
crankset gracefully. I had a MTB shifter/derailleur setup with a
compact road crankset and while it did shift, it had too many problems.

You could get a bigger chainring or a smaller cassette, but best of all
would be to spend nothing and admit that you already have all the gears
you need. I know it may seem like you need a big gear at the bottom of
that hill, but trying to keep up with your rear wheel accomplishes very
little. Coast a few seconds longer and your 46/13 will be back in the
game.