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greggow

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Bozman

Use the effective top tube measurement as your size guide.

smoggysteve

Effective top tube length is a good starting point, but if it has a sloping geometry it can still be too big in other areas. You can only ever get a good idea of a bikes ride position by actually sitting on it and riding it.

ben@31

I had a similar problem with the last bike I bought ( a Specialized Allez).

Because of my height (176cm / 5 foot 9 in) I was at the top of one frame size ( 54cm ) and the bottom of the another size ( 56cm )

The 54cm or 56cm measurement did not relate to the height of the seat tube, in fact the seat tube was nowhere near. But it's actually the top tube length.

Specialized complicated things even further by using a curved top tube. So they used an imaginary straight horizontal line from the seat post to the stem called an effective top tube length, rather than measuring the actual top tube.

I ended up going with the 56cm and later regretted it. It sometimes feels too long and I've noticed I shuffle about on the saddle trying to get comfortable, subconsciously shuffling forwards.

For my next bike I'm going to have a proper bike fit before I buy anything. Some bike fits can even compare different make and models of frame from a database to tell you what frames would suit you best. The bike fit would then set up the stem, bars, brake levers and saddle... up, down, fore, aft positions and angle. It might be expensive getting a bike fit but it's not as expensive as getting the wrong bike, being uncomfortable and low confidence when thinking about riding a big route.

I've heard of guys suffer injuries because of badly fitting bikes. I.E, knee pain caused by the crank length and riding position.

gozzy

greggow wrote:

Hi Im looking to get a road bike i viewed a bmc which had a 57 size sticker on it when i measured the seat tube it was 52 cm I sat astride it and it didnt feel too big not a lot of clearance but not uncomfortable but the top tube was 57 cm im small 173 cm tall with in leg of 76cm checked the evans and bmc site which seems to show I need a 53 cm and the bike is too big ! I suppose the real test would be a nice long ride ?? thnx

You will definitely need a smaller frame. I ride bikes with a 57cm top tube length and I'm 190cm tall. You'll do yourself an injury trying to ride something as big as that 57 BMC.

diamonddog

My inseam is the same as yours but I am 165cm and I ride a Xs/50 Canyon and a Sm Pro Carbon the seat tubes measure 46.5 & 44 so seat tube size is really no guide, as others have mentioned you need to look at the ett length. The ett's on mine are 525 & 530 respectively just to give you a bit of a guide.

I would guesstimate you need something between 535 and 550 ett depending on the type of riding you do (51 or 54 frame size on a BMC Teammachine) but you really need to sit on a few and get some good bike fitting advice.