Gear inches - MTB v Road?

itsaghostcar Posts: 59
edited October 2007 in Road beginners
Does the weight of the bike make a lot of difference when it comes to hill climbing and selecting gears ... and is there any difference when comparing a MTB with a road bike?

Reason I ask is that I am currentlly riding a 26" x 1.25 MTB with 50-36 compact on the front and 11-32 cassette. At it's lightest (with puncture repair tools and rack) it probably weighs in at around 13.5kg. Am looking to set up a new 700c x 25 bike for audax / credit card touring type riding. I'm guessing the weight of the new bike would be closer to 9 or 10kg. A compact 50-34 with 12-27 cassette would give me 33 gear inches compared to my current 28. Using a triple would get me down to 29 gear inches, but involve more gear changes and unused gears. I've used the bottom gear a few times - right at the end of Ditchling Beacon and a couple of times during the C2C (with 11kg of luggage).


  • sbullett
    sbullett Posts: 139
    My old MTB had a lowest gear of 25 inches and weighed around 15Kg, my new Giant SCR2.0 triple has a lowest of 32 inches and weighs around 10Kg. I haven't found a hill that I can't get up since changing, that can only be due to the reduced weight and rolling resistance from the new bike cos I certainly haven't got that much fitter!!!
  • Bronzie
    Bronzie Posts: 4,927
    Whilst a difference of 4kg in the weight of the bike will certainly be noticeable, other factors such as heavier tyres and wheels on the mtb and longer chainstays will also make an mtb more sluggish up a climb.

    For anyone not familiar with "gear inches", it's simply the number of teeth on the chainring (front) divided by the number of teeth on the sprocket (rear) times the diameter of the driven wheel in inches. The result is the equivalent size wheel diameter you are pedalling when in that gear (imagine a Penny Farthing with a wheel of this diameter). As an mtb has 26" size wheels, the gearing on an mtb is lower, even if you use the same size front and rear gears as a road bike.

    In terms of gearing, a bottom of 33" is pretty low and will get you up almost anything unladen. It should be sufficient if you are planning just light touring, but for heavier touring (ie tent, stove etc) you'll probably want something even lower. It will depend on the terrain you are planning to cover as well - if you are touring in the Alps, you may want a wider spread of low gears.

    A triple doesn't necessarily give you a lower gear, but it will give you a wider spread - you will have a range of low gears that are similar sizes that can make it easier on a climb to keep the same cadence. It doesn't mean you have to change gear more often, but you will have more choice there should you need it.
  • Thanks. By changing gears more often, I was thinking of how many times I would be going up and down the chainrings and having to adjust the rear accordingly.

    However, after staring at the gear tables for a while, I think it is possible to ride on the moiddle chainring of a 30-42-52 triple for the majority of the time, only going down to the granny or up to the big chainring occassionaly. Between gears 3 and 8 on the rear I can get a range from 48" to 79" which is where I will normally end up. My current setup has me constantly flicking up and down between chainrings as my cruising speed hovers between the two.
  • Bronzie
    Bronzie Posts: 4,927
    I think it is possible to ride on the moiddle chainring of a 30-42-52 triple for the majority of the time
    I've got those chainrings on my winter hack with a 12-23 cassette on the back - very rarely need to change out of middle ring - only use the granny ring when I'm really getting tired on a long ride up a hill or when loaded up a bit.
  • Great - that's sorted the gearing decision out. Now on to the other bits and pieces :D