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Question needed on sizing(2 posts)

Question needed on sizingchanhoward
May 6, 2003 8:15 PM
Hi,
When I am doing sizing/fitting for the size of the road bike I should get, these are the numbers I got, effective top-tube of 56cm(560mm) & stem length of 100-110mm.
I was planning of getting a Specialized Allez bike and I am thinking which of the 2 sizes to get, either: 54cm or 56cm.
The geometry for 54 & 56cm bikes as stated in their website are: 54cm bike (top tube length:549mm(54.9cm), stem length:110mm(11cm)) & for 56cm bike(top tube length;563mm, stem length:120mm(12cm))
Please advise , give your opinion.
Thanks
Regards
Howard C
advice...C-40
May 7, 2003 5:17 AM
Trying to size a bike only by TT length is a mistake. Stems are easily changed to produce the same total reach on either of these frames. The 54cm frame will require a 15mm longer stem to produce the same total reach as the 56cm frame.

You must also consider the vertical dimensions of the bike. The standover clearance should be no less than 2-3cm and no more than 5cm to hard crotch contact, in bare feet. The head tube length is also important. The smaller frame has a 2cm shorter head tube, requiring either more steering tube spacers or a high rise stem angle to get the bars up to the same height as the larger frame.

You should either get a professional fitting or take careful measurements on a bike that you have ridden and fits properly and use those measurements as a guide.

Never rely only on body measurements alone to estimate TT and stem length. The accuracy of such measurements is rarely good enough to be of much value. Saddle position has a big impact on total reach to the bars. Body measurements don't take into account the saddle position that you may need for maximum pedaling efficiency.