Recreational & Family - modifying MTB for road use
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I've recently started using my Cannondale M300 commuting bike for road riding (got skinnier tires on it). Problem is that on flat ground I run out of high gears. So I would like to switch out the crankset to a road crank (was thinking of Sora or Tiagra). But the kids at two local shops have suggested that this is not possible, but I get the feeling they just don't know. Plus they want to sell me a new road bike instead.
The bike has 8-speed MTB cassette and chain that I'd like to keep if possible. So, would a road crank fit on Cannondale mountain frame and work with other components? Do I need to stick with a 8-speed crank or would 9-speed work as well? And will there be problems with chainline when going from MTB to road crank?
Any other suggestions/words of caution would be appreciated.
03-18-08, 12:05 PM
Instead of changing the crank, you could change the chainrings (front) and/or cassette for much cheaper.
Better post for Mechanics forum.
03-18-08, 12:25 PM
Your front derailer probably won't handle larger chainrings very well - it is probably designed for a max chainring of 44 to 48 teeth.
A relatively easy/inexpensive fix would be a road cassette in the back (with a new chain, of course). Something like a an 11-23 or 11-25 cassette would give you higher gears for minimum cost.
Masiman, the crankset I have now has the some of the chainrings (don't remember which one) built together with the crank, and thereforeit can't be modified.
Al, Can i switch out just the cassette or do i need an entire new road wheel? Are the road and mountain cassettes the same width (assuming I'll stay with an 8-speed)? Will the the rear derailleur still work? I am not very worried about losing the front derailleur functionality because in the 8 years I have had the bike I never used the smaller chain rings, and even on the back only need the 5 highest gears.
Thanks for your feedback.
03-18-08, 02:34 PM
I assumed your cassette is Shimano or Shimano-compatible (Sram, IRD, etc.) If so, any compatible cassette should work. "Mountain" cassettes have wider range of sprocket sizes but there should be no difference in putting one on a road or mountain freehub.
Check out the late, great Sheldon Brown's articles on the Harris Cyclery website. His discussion of gears, etc. is very informative for those of us who are new to this stuff.
03-18-08, 05:48 PM
Your rear derailleur should work unless you go out of spec on the range, which is unlikely, but something to check.
Sheldon's page on freewheels/cassettes and compatibilities (http://www.sheldonbrown.com/k7.html).
Depending on what gearing you have now you may not be able to do much with the cassette alone. For example, if you have 678mm rims and your high combo is currently a 48x13, and assuming you only do a 12 sprocket you gain %8.33 (23.46mph vs 25.42mph at 80rpm). Not much of a jump but some. If you can get it to a 48x11 you gain 15.4% (27.7mph). This is a nice little gear calculator (http://www.jbarrm.com/cycal/cycal.html).
If you are only using the top 5 gears look for a cassette with a tighter range. You'll have more usable gears that way.
Since your chainrings and crank are one piece I am guessing your bike is not worth alot. If so, I would not spend alot of money trying to upgrade it. Fix a price to modify it, if you can't meet it you should look for something used. It may serve you anyway as you may find that changing the cassette will require a new chain and maybe even a chainring/crank. At that point, new may become more cost effective.
03-18-08, 09:36 PM
I did a little research: the Cannondale M300 came with 42/32/22 chainrings and a 11-30 rear cassette.
So you aren't going to be able to improve the rear cluster top end (a 12-25 would have you use more gears but the 12 would be slower than the 11).
So your choice is to improve the front.
I am shocked at how square taper cranks have disappeared from the market. My favourite supplier only lists a pair that would be appropriate:
(I am working within Masiman's view that the components should be appropriate to the bike - no silk purse for a sow's ear)
A 50T will shift ok (not great but not too bad) with the Avilio derailleur and 50-12 is common set up for road bikes (20% faster than current). Installation requires a crank remover and a cresent wrench (I can't believe I said that - as the man with complete Snap-On tool box)
Who's putting a silk purse on a sow's ear with his daughter's Trail-a-bike....but it will be FAST!
Thank you all! You have been most helpful.
Masiman and Prairie*boy, I do know that there is a real road bike in my future, but as long as my current bike serves me well (which is has done for 8 years), I'll put the extra $$ to other uses.
I appreciate you sharing your experiences.
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