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I am 182cm (5' 11'') tall and am looking into buying a new frame for my mountain bike.

Right now i have a 19'' frame and i found a good deal on a 17'' frame.

The guy from the shop told me that it shouldn't be a problem for me to get a 17'' frame since you can have a longer stem and seat post so its fits alright.

Smaller frame means lighter but what are other advantages and disadvantages ? Would you choose a smaller frame ?

Update The difference top length (seat tube to handlebar stem) from my 19'' to this 17'' is 1cm.

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Solo, you really need to compare effective top tube length to truly understand the comparison. Make sense? –  GuyZee Jun 1 '11 at 21:03
    
Instead of faffing about with a ruler, can't the shop guy give you a test ride? –  cmannett85 Jan 10 '12 at 18:25
    
A friend recently got a small-ish bike because it was a good deal. Was very sorry after that. Though a too-small frame would be nice for downhill, I suppose. –  Vorac Jan 29 at 16:06
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7 Answers

up vote 9 down vote accepted
  1. With a smaller frame, you will need an appropriately longer handlebar stem in order to retain the cockpit length. This will affect handling of the bike: it will feel more "lively", which can be good in some cases (tight turns, difficult terrain) or bad (less stable) - in most cases.
  2. A smaller frame will be marginally less reliable, and more prone to break, due to bigger stresses in its structure; longer stem and seat post mean bigger levers to exert force on the frame. But this should not be a significant effect.

As a rule of thumb, a 17" frame is an inch or two too small for your size. I would not recommend taking the smaller one without actually riding it for a couple of hours and making sure it's comfortable for you.

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Agreed. The 17" is definitely too small, aside form a few special purpose bikes, or an unusually proportioned rider, based on your height. –  zenbike Jun 20 '11 at 13:04
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+1 For the stem comment. Increasing the stem length has the same effect as steepening the head angle, which is usually a bad thing. –  cmannett85 Jan 10 '12 at 18:23
    
Actually a smaller frame made with the same techniques and materials is more sturdy than the same design in a bigger frame, due to the shorter leverage of the internal forces in the frame. –  Jahaziel Jan 12 '12 at 13:18
    
@Jahaziel The internal forces in the frame will be smaller, but the external forces exerted by the necessarily longer stem and seatpost will more than make up for it, because proportionally more leverage will be added externally than removed internally. –  ttarchala Jan 22 '12 at 21:27
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I'm 6ft and just brought a medium Lapierre (43) and so far am finding it fine. I was riding a large Giant Trance before this and often felt I was over reaching when going down hill (also had upper back pain often). The only thing I've noticed so far is needing to adjust my riding style to ensure I'm not too far forward and getting the weight over the bars.

Might condider a dropper post with a slight lay back if I can find one.

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There is some good information in some of the answers here, but, in reality, more information is needed to give a better answer. (Inseam, reach, type of riding you do...just to name a few key factors.)

I'm sure you've already made a choice by now, but just because you had a 19" frame doesn't mean that it was correct or incorrect for you. Certainly different size frames, with different stem combinations, tire sizes, and steering & frame angles will affect the handling of the bike.

Most likely there's going to be nothing "wrong" with choosing either size (given most mainstream manufacturers fit guides and your height), but they will ride differently.

In most (possibly all) types of mountain bike riding, if you need the seat moved to one end or the other of the rails, or if you feel you need a stem longer than 100mm, you probably need a different size frame.

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Ultimately there is a simple calculation which is.

Your height x the length of your beast / divided by 26.5 which would suggest your best with a 19" frame.

Trust that helps out.

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Generally a slightly smaller frame than recommended is a good idea. I am 6 ft and I ride a 56 cm road bike though 57 - 58 cm is recommended by most manufacturers for my height. The advantages are lighter, more aerodynamic, more compact, more 'on top of things.' I have made adjustments by putting the seat as far back as possible, raising the seat post and substituting the stem for a -17 degreee 140cm.

However if you are small and my g/f is 5 foot 2 inches and she rides a 13" mtb then the problem is that with such a small frame I think you will lose speed, she'd be better with a 14 - 14.5" frame imo.

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Both Neil & Ttarchala are spot on. The one additional comment or perspective that should be considered is with a smaller cockpit your body may not be stretched out enough and for me the impact of mountain biking on a bike with too tight a cockpit wreaks havoc on my BACK!

Think of your spine as a coil...is there more flex when it pushed together or stretched out...that flex softens the ride and allows for more movement which ultimately improves your riding, is less taxing on the body therefore less fatiguing.

Also, too long a stem tends to put too much weight forward which increases the likelihood of issues with your upper body and increasing propensity to endo :(

Be careful..probably not a viable option!

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I have a small bike and am constantly worried about the endo on technical terrain. –  Mranz Oct 25 '11 at 20:59
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A smaller frame also means a smaller reach--that's the distance from the saddle to the handlebars, and if you have short arms and/or torso height then a smaller frame may be a good idea. If not, you may find this frame too small.

This can be mitigated by sliding the saddle backwards in the rails a bit, but this can account for perhaps an inch at most. Changing the stem size can also help. If you want a more upright position on the bike, a smaller frame could help somewhat.

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You can also get what's called a layback seatpost in order to make the reach a little longer. You won't be quite aligned with the pedals in the same way as before. I've never used one and can't attest as to how comfortable they are. –  Kibbee Jun 2 '11 at 14:28
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