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Comparing Compact frames with different seat tubes?....(4 posts)

Comparing Compact frames with different seat tubes?....CaliforniaDreaming
Sep 17, 2003 2:03 PM
How do you compare two compact frames of similar size? Am looking considering a Soloist in 51cm, 73^seat tube, and 530mm top tube. Have riden a 50cm, 76^seat tube, and 520mm top tube Allez that felt a tad bit short. How should I compare these two bikes? What other dimensions are important? If I go with a Soloist, then it will be without a fitting, hence my questions.
seat tube angle effect....C-40
Sep 17, 2003 4:16 PM
According to the geometry chart that I have, the STA of a 50cm Allez is 75, not 76.

Steep seat tube angles increase the effective top tube length, since the saddle much be moved further back to place the rider in the same position. The exact amount is calculated by the formula: saddle height x (cosA-cosB) where A and B are the seat tube angles of the frames being compared. A good average value in 1.2cm per degree.

Add 2.4cm to the TT length of the Allez and you get 54.4cm, compared to the 53cm for the Soloist. If you felt more cramped on the Allez, it was either due to a shorter stem or a failure to move position yourself in your normal relationship to the bottom bracket.

To compare the vertical size of the two frames, compare the head tube length. A head tube that is too short will produce an ugly setup with too many steering tube spacers and/or a high rise stem.
Let me get this straightKerry Irons
Sep 17, 2003 4:20 PM
Your message is not clear (my browser sees a ^ - carrat - rather than a degree symbol?), but I think you are saying one bike has a 73 degree seat tube with a 530 top tube, while the other has a 76 degree seat tube and a 520 top tube. If this is the case, the steeper seat tube will push the head tube forward about 3 cm, giving that bike a top tube that is effectively 20 mm longer than the bike with the 73 degree STA. Your stem would have to be 2 cm different to make these bikes have the same reach - assumes saddle is in the same relationship to the BB. You can't compare "long or short" without considering stem length.

Try these sizing tools and then reach a conclusion.

For adjusting the fit of the bike, there are roughly five starting points:

1. Seat height (top of saddle to center of pedal axle) at 108-110% of inseam.
2. Saddle parallel to ground.
3. Saddle fore/aft adjusted so that a plumb bob from the bony protrusion just below the kneecap passes through the pedal axle when the cranks are horizontal. This is known as KOPS (Knee Over Pedal Spindle)
4. Front hub axle obscured by the handlebars when riding in your "regular" position (drops, hoods, or tops).
5. Top of handlebars 1 to 4.5+ inches below the top of the saddle depending on your flexibility and size.

These are all starting points for "average" proportioned people, and many folks like to move away from these starting points as they learn what makes them more comfortable, powerful, or efficient. You want to get the fit of the frame as close as you can, then do minor adjustments with the stem, seat post, saddle position, etc.

A lot of this is personal comfort, and we all tend to adapt to a given position over time. For example, a given stem length may be right for you, but it may feel long at first. I use the "handle bar obscures the front hub" rule for my fit, but others claim better position (for them) with the hub in front of or behind the bar. I'm 6' tall and ride with 11.5 cm drop from saddle to bar, probably more than most people would like but fine for me. Some are suggesting zero drop from saddle to bars - it's about comfort, efficiency, and aerodynamics. The ERGOBIKE calculator is pretty good, but it is not infallible. I would suggest riding some miles (over 100 total, and over 500 would be better) and see if you adapt to the position. There are no hard and fast rules, just general guidelines, when it comes to these things.

Just as important as your size is your flexibility. If you have a stiff lower back, you may not be able to lean over and stretch out as much. If you are very flexible, you may get away with a longer top tube, with the stem in a lower position. Over time on the bike, too, you may become more limber, or at least become accustomed to being lower and stretched out. So, your first 'real' bike may not be anything like what you will want 5 years from now.

Someone new to road riding is highly unlikely to find their ultimate position on the first go. As they become accustomed to the riding position and get some miles in, sometimes over several seasons, people often find their desired position changing. What was "stretched out" now feels OK, or what was "just right" now feels cramped. With time, if you are working on your position along with all your other riding stuff, seat position tends to rise, handlebars tend to be farther below the saddle, saddles tend to move rearward, and handlebars tend to be farther forward from the saddle. You simply cannot say "this is the right position for someone of your body dimensions" because there are too many variables and things that change with time. Get used to your position, and then oc
don't forget to add Tiemeyer's siteMShaw
Sep 18, 2003 9:51 AM

Very good site for giving sizes.