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Bike Fit(8 posts)

Bike Fitoak1
May 16, 2003 6:18 AM
I used the Competitive Cyclist fit calculator and it has me at Seat Tube range C-C = 54.5 Seat Tube range c-t = 56.2
so what size frame should I go with if I am purchasing a TREK a 54 or 56. does anyone know how Trek measures the frame C-C or C-T.
forget fit charts.Steve_0
May 16, 2003 7:23 AM
get the largest frame you can straddle with at least an inch standover height.
Sorry, can't buy that advicejtolleson
May 16, 2003 9:47 AM
Ignore TT length? Maybe if you are proportioned exactly like a mannequin (or whatever they use to design stock bike sizes). Your advice is particularly problematic for the leggy rider who is shorter torsoed (like many women). Too long a tt and getting a 6 cm stem is NOT a good solution.

Fit charts have their weaknesses, but your advice sounds like the guy who does the standover test before selling a bike at Wallyworld.
trek measures center to top.colker
May 16, 2003 7:47 AM
you should get a 56 and make sure you set your bars high enough to ride in a comfortable position.
more bad advice....C-40
May 16, 2003 8:05 AM
Many folks just can't seem to interpret a drawing or a geometry chart.

Trek measures the length of the seat tube which is not c-c or c-t measurement, since the seat tube is extended above the top tube.

A 58cm Trek would be the closest to a 56cm c-t frame.

The standover height is often the best way to judge the vertical frame size. A typical 56cm c-t frame would have a standover height of slightly over 80cm. The 58cm Trek has a standover height of 80.7cm and the 56cm has a standover height of 79.0cm. The standover height should be 3-5cm less than your cycling inseam (measured in bare feet to hard crotch contact). Which side of this range you select will impact the amount of steering tube spacers and stem angle required, more than the stem length. The effective TT length of the 58cm Trek is only 4mm longer than the 56cm, taking into account the difference in seat tube angle.
Right about vertical, with a horizontal wrinkle.djg
May 19, 2003 7:16 AM
C-40 is correct that, measured from the center of the bottom bracket to the top of the top tube (the usual c-t measurement) a 58 cm OCLV will measure about 56 cm. I had a nominal "56" Trek 5200 for a while that measured about 54 cm on c-t criteria. Was it "really" a 54? Well, judging by the seat tube, perhaps, but it had a 56 cm c-c top tube, which is unusual for a 54 cm bike, but fairly common for a 56.

What it's called doesn't matter nearly so much as whether it fits. And that's substantially a matter of reach to the bars (both horizontal and vertical), once you've got your saddle in the right place. If you don't know what you need, I'd consider looking for a shop with some reliable fit expertise.
I'd be careful . . .RJF
May 16, 2003 9:19 AM
buying based solely on standover clearance. If I bought a bike that had just an inch of clearance, it would probably have a TT about 3cm too long for me.

Fit is a combination of a lot of factors, not the least of which is your own specific body dimensions. I'd suggest that if in doubt, find a local shop with a fit kit to at least get you in the ballpark. It's probably going to be your best bet.

Either that, or convince C-40 and his slide-rule to come over to your house and hook you up. As a math-challenged individual, I'm always amazed at his grasp of the cosine. ;)
sure...C-40
May 16, 2003 9:54 AM
You never want to by a frame solely on standover height, but with stock frames, the TT length generally changes at half the rate of the frame size. It's also rare to find variations of much more than 1cm in the TT length of frames that are the same vertical size, regardless of the brand.

The vertical fit (mainly head tube length)is just as important as the horizontal fit (determined by TT length & STA). If you have unusual proportions that a stock frame won't accomodate, then a custom frame is probably in order. Changing brands won't get you a 3cm shorter top tube than another brand.