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I'm looking to buy my first MTB and I had an online calculator tell me to get a 17.5". My measurements are height: 177cm and leg 76cm.

Thing is, I found a bike that I REALLY want, but it's the last one and it's a 15.5" hence the good price. If I don't get this particular one I'll have to get something significantly lower spec as this is massively discounted.

I don't intend on doing really long rides ever. Maybe 2 hours max.

1.5" difference doesn't seem that much.. Will this be ok or am I about to waste allot of money?

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Better a bike a hair on the small side vs one that's too big, but either is less than ideal. –  Daniel R Hicks Mar 1 '13 at 23:24
    
A reputable bike shop will be able to fit you to a bike sized properly for you. –  Van Mar 4 '13 at 5:18
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3 Answers 3

up vote 3 down vote accepted

You can use a bike that's "too small" or "too big" without too many problems, so long as you set it up properly for your own biomechanics.

Try using this fit calculator instead: http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO

Compare the bike you really want with the results of the calculator. It will give three separate options, and provide you with a rough idea of whether the bike is likely to be a reasonable fit for you.

1.5" doesn't sound like a lot, but in the world of bike fit it can make a massive difference. Ideally you want to work around things that don't fit by changing components, but you need to know first if changing the components will help.

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Yeah, you could probably make up 1.5 inches by sliding the seat back and getting a slightly longer stem. –  Kibbee Mar 2 '13 at 15:59
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The most critical thing is the "reach" -- how far forward and up/down the handlebars are from the seat, relative to your torso and arm length and, shall we say, avoirdupois. Too short a reach and the torso must curl up, compressing the lungs and causing strain, and the ability to assume an aerodynamic crouch (if you would wish to do so) is reduced.

On the other hand, too long a reach places a strain on the arms and back and significantly increases the pressure on the hands.

Moving the seat back to increase "reach" makes the angle between torso and legs more acute. Up to a point this moves one closer to the "sweet spot" of maximum power from the legs, but this can go too far, plus the problem with an over-ample gut compressing the lungs is increased. Moving the seat forward generally moves one away from the "sweet spot" and also increases the weight on the hands and arms.

Moving the handlebar forward/back to adjust "reach" upsets steering stability/agility.

A too-small bike may (in rare cases) not allow the seat to be raised high enough to attain full leg extension, or (more often) not allow the handlebar to be high enough for comfort. A too-large bike may not allow the seat to go low enough, forcing the cyclist to slide side-to-side as he pedals.

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Ride it, see if it's comfortable. That's the only way to know for sure.

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Also, be aware of the top five length, this is important and not add easy to adjust as the seat height. –  alex Mar 2 '13 at 6:12
    
top tube* & as* –  alex Mar 3 '13 at 4:19
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