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Chainset - can do or labotomy?

BokdrolBokdrol Posts: 15
edited August 2012 in MTB workshop & tech
Dear Techies
I am doing a lot more road work on my Carrera Vengeance MTB than usual, the benefit of country lanes, and I am having to modify it for this. Slicks etc.

Main issue is that the current chain\crank set (Shimano Dual SiS index, 42-32-22) is not making road cruising easy. I frequently spin out on the 42 and need to upgrade to more teeth. Problem is I haven’t a clue what to upgrade to.

Do I go for a 48-38-28 or some other combo?
What will be great for road , but also when I hack it in the forest and hills?
Any brand you recommend that doesn't alert the bank man?
Can I only change the 42 or is the unit integrated?
Any video’s on how to do modification?
Will I need a longer chain?
Front derailleur must be moved higher?

Crikey, help please?
Thanks in advance
Bok

Posts

  • supersonicsupersonic Posts: 82,708 Lives Here
    You must be riding fast, a 42/11 combo at 120rpm is about 35mph! Your only option (assuming the rear cassette does have an 11t small cog) is the get the 48/38/28 chainset. You might get away with the existing mech, but specific ones are curved different, and you'd need a longer chain to be on the safe side.
  • BokdrolBokdrol Posts: 15
    Cheers for your thoughts SuperS.

    Is this a job for me or the shop? How do I determine the correct length of a longer chain?
  • supersonicsupersonic Posts: 82,708 Lives Here
    There is a guide in our FAQ on chain length.

    What is important is that the chainset matches your bottom bracket for fitting, and length.
  • YeehaaMcgeeYeehaaMcgee Posts: 5,740
    Er, I doubt that you're spinning out, frankly. What kind of pedalling RPM are you achieving?
  • BokdrolBokdrol Posts: 15
    Too fast ;) for my legs to keep up. No idea about rpm.

    I would appreciate more advice on brand and technical issues from the boffins.
  • YeehaaMcgeeYeehaaMcgee Posts: 5,740
    Well, here's one technical issue, like Sonic already said, you're looking at around 35mph from a 42t chainring and an 11t sprocket, when pedalling at 120rpm - that's two whole crankset revolutions per second, which is hardly beyond attainable.
    25 mph only needs a cadence of 85 rpm, which begs the question, how fast exactly do you want to cruise?
    Even roadies don't ride everywhere at 20+mph the whole time.
  • The RookieThe Rookie Posts: 27,802
    I'm running a 46T on the front of my commuter and I'm only using 8th and 9th on steep downhills (40mph on GPS), I spin at 120+rpm, I know this from the machine at the gym when I'm away on buisness, I suspect you need to work on your leg speed.
    Current steed - Whyte T129, 2013 frame, mongrel Revelations, Giant dropper, Stans S1 wheelset. 12, Magura Trail Sport brakes, 1x11. 12.8Kg
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