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Geometry questions from the unenlightened (long)...(3 posts)

Geometry questions from the unenlightened (long)...MVN
Sep 1, 2003 2:55 PM
I'm curious as to the terms "steep" and "laid back" , referring to seat tube angle. Example, my GT has a STA of 73.5, and my 5200 has one of 74. I read on one website (http://www.billbostoncycles.com/seat_tube_angle.htm)that, "In an effort to produce smaller bikes with shorter top tubes, some manufacturers have increased the seat tube angle. While on paper, this does provide a shorter top tube, in reality, the saddle will need to be moved rearward to place the rider in the correct position, thus nullifying any top tube reduction". The reason I mention this is because my frame size is 54, but the top tube on the GT is 55.5 and the TT on the 5200 is 54.6. I'm having a devil of a time getting my saddle fore/aft set correctly. I injured my knee right after getting the Trek b/c I had the seat height set the same as my GT. Well, that was wrong, way too low and the saddle was too far forward. I'm also using a different (but more comfortable) saddle than what's on the GT. My seat height is a lot higher than it's ever been, but it's comfortable so far. The reason I raised it is b/c it flexes in the center more than my old saddle did and the higher it is, the easier it is on my knee. I figured from reading other's posts that my fore/aft on the Trek would be approximately 6mm back from where it is on the GT due to the difference in STA. That has proved to be still too far forward. I'm having trouble finding the most efficient and comfortable position for the saddle. So my question is this: What is a "steep" STA and what is a "laid-back" STA? Is 74 steep or laid back? With regard to the top tube difference, does it mean that the saddle will have to be further back on the Trek to achieve the correct position? Sorry if some of this is unclear but I'm getting a little frustrated with the time it's taking to get the saddle set correctly. I can give more fit and size info if needed. Any assistance and all replies greatly appreciated.

Thanks
Mark
Depends a bit on frame sizeKerry Irons
Sep 1, 2003 5:25 PM
Smaller frames tend to have steeper angles. A "typical" road frame might have a 74 STA in the smallest size ranging to 72.5 in the largest. Your 74 for a 54 cm frame is pretty much "normal" for a road frame. That same 74 in a large frame would be considered steep, while a 72.5-73 in your size would be considered more laid-back. Moving your saddle fore/aft to get the right position relative to the BB is standard practice, and a frame is too steep or too slack only when you can't get the position with saddle adjustment. It's sounds more like your knee problem was due to a too-low saddle than frame angle issues.
Nominal sizes.Al1943
Sep 1, 2003 5:58 PM
I'm not familiar with GT. A 55.5 TT sounds awfully long for a 54cm nominal size. How is the GT measured, c-c, c-t?
The 54cm Trek is measured center bottom bracket to top of seat tube extension. The Trek top tube is lower on the seat tube than most bikes. The head tube is shorter and has a relatively steep angle. It sounds to me as if these bikes are really not the same "size". It's not so simple to compare sizes from one make to another.
~Al