I’ve done some searching but can’t quite come up with what I am looking for. So for sake of this question, say I ride a 54cm c-c bike.
If I buy a 54 c-c bike with a 74* seattube angle, versus a 54 c-c with a 78* seatube angle, what does that steep seat tube angle do? I have heard people say that 1* equals 1cm. So does that mean that if your STA is 78* versus 74*, there is a 4cm difference in top tube? ALso, how does this affect compact sizing?
It’s not quite clicking with me.
Thanks.
Generally, I think that when bike manufacturers list top tube lengths, they list them from the center of the seat tube to the center of the head tube. So, whether the bike is a 74 or 78 degree seat angle, if it has a 54cm top tube, it has a 54cm top tube.
If you are asking what I think you are asking…and 1 degree = 1 cm…then if you fit comfortably on a 74 degree bike with a 54cm top tube, you will want your 78 degree bike to have a top tube that is about 4cm shorter to work well with the aerobars.
I think that Cervelo lists ‘reach’ as one of their measurements, and I believe that Dan has posted about using reach as opposed to top tube length. But having manufacturers do that would probably take a lot more ‘work’ than they are willing to do since top tube length is so easy.
Compact sizing can get complicated as some manufacturers list the actual top tube length along the top tube, which is sloped. Other list the virtual top tub length, which I think is an imaginary horizontal line drawn from the head tube to a point on the seatpost where the top tube would intersect if there were one.
Confused?
you will want your 78 degree bike to have a top tube that is about 4cm shorter to work well with the aerobars. and as an option if one were to have purchased an incorrect size, you could obtain a stem that is 4cm shorter in length. Is that also correct?
I think that you could just use a stem that is 4cm shorter, if that is the difference that you think you need. But, then you get into the issues of if my road bike has a 110mm stem and a 54cm top tube then my tri bike is going to have a 70mm stem and a 54cm top tube. Some folks will tell you that it’s not going to handle well. Personally, I like my bikes to fit such that they have between a 100 and 120mm stem on them.
http://www.slowtwitch.com/mainheadings/techctr/techctr.html
Here’s Dan’s tech center. It has several articles on bike fit. He doesn’t explicitly say that 1 degree = 1 cm, but he does mention that his road bike has around a 59cm top tube (and we can assume a 74ish degree seat tube) and that his tri bike has a 55.5cm or so top tube (and he rides 80ish degrees). So, if his stem lenth is about the same, then 1 degree = 0.75cm or so.
As much forum activity as there has been on fit, etc. I’m suprised no one else has replied just yet.
No, that explains it a bit better. So, here’s my issue. I am 6’ with a 31" inseam(pubic bone height). I ride a 56 cm Fuji Team Pro with a 100 stem and pretty short reach bars. I have always thought that I was a little stretched out, so I am going from a Ritchey setback post to a 0 setback, whenever it arrives. I figure I “should” be riding a 55. So, going 1 size lower, as I have been told to fit a tri bike, puts me on a 53cm tt. I have ridden a friends 52 Cannondale Ironman and with a few adjustments, found it to be comfortable. I have been looking at compacts for no particular reason and am flustered with what I am coming up with. To get the top tube I want, I would have to ride a Small frame. No problem, except I am 6’ and I’m not sure how that would work. Using the inseam x .65 calculation, that would put me on a 51 cm seat tube, well into the small range. Riding a 55cm traditional frame would also fit into the compact small frame sizing. But again, at 6’ tall on a small? That seems way off.
I have been looking at the Bp Stealth’s and Giant TCR’s. The BP has a steep seat tube angle where the Giant is slack. I own one Giant meduim, with a top tube of 55.5 and find it too large to run aerobars on. I am going to go down and try a small Giant today, but I just don’t see how that would even come close to fitting correctly.