What's the deal with frame sizing for mountain bikes? There are bikes with horizontal top tubes, and ones with top tubes that slope down towards the saddle. If the sizing is based on the length of the tube that the seat post goes into, then how is the size of a frame with a sloping top tube established, given that the tube of interest is shorter?
Is it derived from the tube length that would result if the top tube was horizontal? IOW, is there a way of comparing the frame size of different frame shapes?
"Wally" <[email hidden]> wrote in message news:[email hidden]...
Quoted message said:
Is it derived from the tube length that would result if the top tube was horizontal? IOW, is there a way of comparing the frame size of different frame shapes?
Generally yes, size is based on a hypothical conventional horizontal top tube. A rough guide to size is measure your inside leg from crotch to floor (longer than trouser measurement) then deduct 12 inches for bottom bracket height and another 3 inches for ballroom. E.g. 33" - 12- 3 = 18". This formula will obviously be skewed by suspension, particularly rear, so as always the thing to do is at least sit on the bike or better yet ride before buying. The main things will be nothing dangerous to your bits if you have to put your foot down on uneven ground and with the standard stem and saddle mid position horizontally you should be comfortable. Again obviously, you will then have fore-and-aft adjustment on the saddle for fine tuning but assuming the manufacturer has fitted the optimum length of stem for the bikes steering geometry you could but wouldn't want to change it for a longer or shorter one. -- Pete http:[email hidden]/P
What's the deal with frame sizing for mountain bikes? There are bikes with horizontal top tubes, and ones with top tubes that slope down towards the saddle. If the sizing is based on the length of the tube that the seat post goes into, then how is the size of a frame with a sloping top tube established, given that the tube of interest is shorter?
This is precisely why inch sizes are no good as an indication of the size of a mountain bike (or, indeed, 'compact road'😉 frame. Different manufacturers use different geometries, so there is no useful or sensible correlation between seat tube length and the size of the frame. There is no alternative to going and sitting on one.
Quoted message said:
Is it derived from the tube length that would result if the top tube was horizontal? IOW, is there a way of comparing the frame size of different frame shapes?
Is it derived from the tube length that would result if the top tube was horizontal? IOW, is there a way of comparing the frame size of different frame shapes?
Quoted message said:
Generally yes, size is based on a hypothical conventional horizontal top tube.
Doesn't seem to apply to Giant - been looking at the frame geometry in detail, and the seat tube is 530mm for the 21" frame, which is just about 21". Extending the seat tube to mimic a horizontal top tube gives something closer to 24". The seat tube extends about 2" beyond the rearward end of the top tube.
Quoted message said:
A rough guide to size is measure your inside leg from crotch to floor (longer than trouser measurement) then deduct 12 inches for bottom bracket height and another 3 inches for ballroom. E.g. 33" - 12- 3 = 18". This formula will obviously be skewed by suspension, particularly rear, so as always the thing to do is at least sit on the bike or better yet ride before buying. ...
I've been thinking again about the 2nd hand Giant I looked at - clearance wasn't quite 3", more like 1.5-2" with feet flat on the road. It felt good when riding it - the whole fit seemed right, no undue leg stretching when the pedals were at the bottom, handlebar height and distance was fine.
