54cm or 56cm
Comments
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Yes, I can see your point if you want to set your saddle position relative to the BB by measuring the distance between the saddle clamp and the top of the head tube. But you can simply set your saddle position by placing the tip of the saddle a certain distance behind the BB. If you do this for two frames with different effective top tube / seat tube angle combinations but the same reach, the distance from the tip of the saddle to the bars will be the same, irrespective of seat tube angle or effective tt length. The saddle doesn't mind whether it is being supported by a steep seat tube and lots of setback, or a shallow seat tube and less setback.
Also, if you are moving between two frames with the same reach and stack but different seat tube angles / effective tt lengths, you can simply set the tip of the saddle the same distance from the bars or the top of the head tube (depending on whether you are comparing frame stack/reach or total stack/reach), and the position of the saddle relative to the BB will be the same.
The value of these figures is that they allow you to compare two frames and work out how different the distance to the bars and the height of the bars will be, assuming you are going to set your saddle at a certain height and a certain distance behind the BB.0 -
Thanks for all your replies. Lots of interesting stuff about fit. Personally, I don't have particularly long legs, so my cross, singlespeed and road bikes are all 5cm and seem fine. I was more interested in what the ride difference would be of having a couple of cm difference in frame size. My first instinct was that shorter would be more twitchy/responsive in the steering and larger would be more stable but less responsive, but I've no idea if that's correct.0
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Depends. First rule, as discussed above, is that actual frame size doesn't necessarily have much relationship to the number on the frame - so a 54 in one brand could be the equivalent of a 56 in another.lukea-d wrote:Thanks for all your replies. Lots of interesting stuff about fit. Personally, I don't have particularly long legs, so my cross, singlespeed and road bikes are all 5cm and seem fine. I was more interested in what the ride difference would be of having a couple of cm difference in frame size. My first instinct was that shorter would be more twitchy/responsive in the steering and larger would be more stable but less responsive, but I've no idea if that's correct.
But let's say that you are genuinely in-between sizes, as determined by the actual frame measurements, and that you can make both sizes fit properly with different stems / spacers etc. On the larger frame you will have a shorter stem, and the wheelbase (distance between front and rear wheels) will probably be longer. Your centre of gravity will be further back in relation to the front wheel. On the smaller frame you will have a shorter wheelbase but a longer stem, and you will be in a more forward position over the front wheel. In general your instinct is probably correct - the shorter wheelbase of the smaller frame and your more aggressive forward position over the front wheel will probably make it feel more nimble, whereas on a larger frame with a shorter stem you can sometimes feel as if you are wrestling with the bike, or as if you are more of a passenger on it, due to the more rearward position. BUT - the shorter stem will to some extent make the steering a little more responsive, so perhaps partially counteracting this, but in a different way... Also, this can be accentuated by the different geometry in the different sizes - smaller frames often have slacker head tube angles, and as the fork rake is usually the same across the board, this can make for a long trail and sluggish steering (although probably not in a 54 size, it's usually only when you get to the "small" and "extra small" sizes that the steering geometry gets weird...).
The best approach is to work out what length stem you would need to use on each of the two sizes to get a good fit, and then pick the frame that gives you a sensible stem length in the 100mm -> 120mm range. If you are having to go down to 90mm on the larger frame you would probably be better off on the smaller frame, but if you are needing 130mm on the smaller frame you might be better off on the larger one (that said, a lot of pros ride small frames with really long stems). You obviously also need to check that you can get as low as you want to on the larger frame, and that the smaller one doesn't necessitate using a ridiculous number of spacers.0 -
Great discussion. Many thanks, everyone, especially neeb.0
