Raod cranks on a Mountain bike

[benkoostra]

I have a Giant Cadex3 from the early 90's that I LOVE. What a great ride.

However, I've been living on Sacramento, CA for a few years now, and have been riding it with slicks on the pavement. If I have any kind of elongated descent, I can spin out the highest gear (44X12). The bike shop guys said I full of it, but that's what happens.
Has anybody put a road crank and rings on a MT bike? I'd like to do it, just to get a slightly bigger gear on the highend, but the mechanic at said LBS said I would have to change out the front derailer, chain, shifter, possibly take out an additional mortgage, etc.
Is it really that big of a deal? I did this in the late 80's and it was no problem at all. It shifted a little roughly, but that's ok.

Anybody have experience with this?

 

[gclark8]

Similar, I set up some ladies MTB bikes for tri use. Use a 11-28 on the rear (7 spin on or 8 cassette) and a 28/38/48 Sugino (square drive) crankset on the front. 650c wheels and 23mm tyres make them go like a rocket. I just use the original 7/8 speed chain and shifters.

The Shimano TX71 crankset (28/38/48) with a CO 51 front deraileur and a 122.5mm BB is a cheaper but heavier option.

I test rode one of these over my 55 min commute on Thursday, it was faster than my Felt!

 

[Retro Grouch]

The issue that you need to resolve, believe it or not, is cable routing.

If you use a road crank with a 53 big ring, your mountain front derailleur won't work very well. The cage has a smaller arc that's designed for smaller chainrings. Most mountain bikes have top tube cable routing and all road front derailleurs are require cable pull from the bottom.

Solutions are to not get too greedy with chainring size - maybe limit yourself to something like a 48, or to use a pulley or something to reverse the shift cable pull.

 

[F1_Fan]

The issue that you need to resolve, believe it or not, is cable routing.

If you use a road crank with a 53 big ring, your mountain front derailleur won't work very well. The cage has a smaller arc that's designed for smaller chainrings. Most mountain bikes have top tube cable routing and all road front derailleurs are require cable pull from the bottom.

Solutions are to not get too greedy with chainring size - maybe limit yourself to something like a 48, or to use a pulley or something to reverse the shift cable pull.

I've been pondering something similar...

Depending on the frame design you may need a wiiiiide bottom bracket axle so that the small ring doesn't hit the chainstay. Looking at my MTB I'm not sure a 42T would go on there. Otherwise, my bike is perfect for that conversion with its bottom pull derailleur.

My big engineering challenge is to do a drop bar swap with quality STi units that work with the brakes and 8-sp XT shifting.

 

[Retro Grouch]

I've been pondering something similar...

Depending on the frame design you may need a wiiiiide bottom bracket axle so that the small ring doesn't hit the chainstay. Looking at my MTB I'm not sure a 42T would go on there. Otherwise, my bike is perfect for that conversion with its bottom pull derailleur.

My big engineering challenge is to do a drop bar swap with quality STi units that work with the brakes and 8-sp XT shifting.

All that it takes is money.

You'll probably need a road stem so the clamp size will fit a road handlebar. Linear pull brakes generally require a Travel Agent to match their cable pull needs to road brake levers. STI's will match your XT derailleur fine on the back. I'm told, however, that you'll need a road front derailleur.

 

[benkoostra]

Thanks for the replies.

I've been thinking about it, and I think a compact, perhaps a "cross" crankset would be best. That way I get a few extra teeth on the big ring, and still use my stock derailleur. Does that sound feasible? Can I run a double ring crank with a triple derailleur? Probably not.
All I want to do is put a !@#$%ing 48T crank on my mountain bike. Sheesh! Things used to be so much simpler.

The last statement implies three possibilities:
1) I've crossed the "event horizon", whereby sufficient age has been accrued to cause such statements to seem reasonable.
2) Things REALLY WERE simpler to use a few years ago.
3) The previous two are both partially true, and I should just buy a road bike.

 

[gclark8]

Thanks for the replies...... All I want to do is put a !@#$%ing 48T crank on my mountain bike.......

Did you not see my reply above?

The Shimano TX71 crankset (28/38/48)
with a CO 51 front deraileur
and a 122.5mm BB is a cheaper but heavier option.

All the above can be done for less than $150 in Aus.

I guess the rear cssettte is a spin on. For less then $40 Aus I bought a 7 speed 11-28 last week, thats faster again!

 

[benkoostra]

Did you not see my reply above? All the above can be done for less than $150 in Aus.

I guess the rear cssettte is a spin on. For less then $40 Aus I bought a 7 speed 11-28 last week, thats faster again!

Thanks George! I guess I missed that while skimming. I'll look into it!

 

[benkoostra]

Problem solved:

http://bike.shimano.com/catalog/cycle/products/component.jsp?PRODUCT%3C%3Eprd_id=845524441771298&FOLDER%3C%3Efolder_id=2534374302039957&ASSORTMENT%3C%3East_id=1408474395181419&bmUID=1143068735707

This puppy comes in a 48T-36T-26T configuration, which is what I'm after, so all I need is a new BB, and I'm set.

 

[breakaway01]

no need to spend money on a whole new crank and BB if all you want to end up with is a 48 x 12 top gear.

just get a 11t cassette as somebody else has already suggested and keep your 44t chainring. 44 x 11 is the same gear ratio as 48 x 12.

 

[DiabloScott]

no need to spend money on a whole new crank and BB if all you want to end up with is a 48 x 12 top gear.

just get a 11t cassette as somebody else has already suggested and keep your 44t chainring. 44 x 11 is the same gear ratio as 48 x 12.

You also ought to be able to find a 48 or 50t chainring to go on your existing crank - that would be the easiest solution by far.

 

[benkoostra]

My big ring is a 42, and it's rivited in place by a robot at Shimano. But thank's for the suggestions.

 

[bobbyl1966]

on my mountain bike i have put 52/42 crank and 7 speeds cassete 13-26. But i want try 53/39 crank and 9 speeds cassete 12-25

 

[RPatterson]

If your Giant is a mountain bike, the 44-tooth large ring is typical of a compact mountain-style triple. Is the granny ring between 20 and 24 teeth? Then it's likely a compact. Before you decide to remedy the problem by simply installing a larger crankset, take a look at your existing setup. Is your front derailleur bolted to a brazed-on boss, or is it clamped to the seat tube? If it's clamped, you can probably keep it if you wish to install an old-school conventional sized triple, 28-38-48.You could then mount the existing front derailleur higher on the seat tube, but you may also need to get some measurements at the shop to purchase an additional, properly sized BB spindle (or cartridge), so that the derailleur's throw is laterally aligned with whichever particular crankset you install. You would then have your higher gear ratio for your high-speed descents. However, this would also affect the low gears adversely. If you were to merely swap out the 44 for a 48, your front derailleur may not perform well at shifting between the middle ring and the big one. But you are likely to get better results by simply going to a rear cog setup that offers an 11-tooth small cog, and you can dispense with the hassle of a new front setup. If you still feel you need a higher gear after this, you may have to consider a different front setup.

 

[oldbobcat]

Be aware of frame clearance when swapping for larger chainrings or road cranks on a mountain bike. Go to large and the inner rings can hit the chainstays. Some work around this by using a bottom bracket with a longer spindle, or shimming the bottom bracket in the frame, but this can mess up the chain line or simply push the rings out of reach of the front derailleur.