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Help with Cannondale Aero frame sizing(7 posts)

Help with Cannondale Aero frame sizingQubeley
Oct 14, 2002 8:18 AM
Hi, I am considering getting a Cannondale Aero frame from GVH, but am unsure about the sizing.
My road bike has seat-tube 50c-c, 52.5 top-tube. That seems to be a good geometry for me. The Aero frame has the same measurement, however, the top-tube is much lower. And I also thought you need a slightly smaller frame for time trial bike. Does anybody have experience with the Cannondale time trial/tri bikes?
BTW: wheels size is 700c.
re: Help with Cannondale Aero frame sizingdivve
Oct 14, 2002 8:37 AM
An Aero frame with a 52.5 top tube similar to your road bike is often too large. You'll need something like a 50cm top tube. The tube is indeed lower due to aerodynamic reasons. The extra drop in height however will somewhat be negated depending on the bars you use. The shorter length for the top tube is necessary due to the extra reach of the bars and the for more forward seating position on the bike.
re: Help with Cannondale Aero frame sizingSpunout
Oct 15, 2002 4:00 AM
But if you adopt a forward position, all of the top-tube length must be required. Too short and you are looking at too much weight over the front wheel, and it is a bad-enough handling proposition at best.
re: Help with Cannondale Aero frame sizingdivve
Oct 15, 2002 9:12 AM
I see now that I should have explained it differently. The top tube is shorter so you can rest your elbows more comfortably on the aero bars. Also, the more rearward position of the bottom bracket in comparison to a road bike will decrease some of the weight on the front wheel. Your weight isn't as far forward as one might think.
re: Help with Cannondale Aero frame sizingdivve
Oct 15, 2002 9:25 AM
Scrap that last bit...there's actually more weight forward...you can easily feel it.
re: Help with Cannondale Aero frame sizingSpunout
Oct 16, 2002 8:01 AM
So, if your weight IS more forward, are you mis-fit for the TT bike? Just wondering.

We've all seen too-extreme aero positions sapping comfort and power. On my road bike, I can ride in the drops, flat forearms, flat back. How much more drop can one need?
re: Help with Cannondale Aero frame sizingdivve
Oct 16, 2002 9:08 AM
...it's very difficult to answer these questions without writing a whole article:) So many factors are dependent of each other that it's hard to make generalizations. I'm sure you're aware that there's no such thing as a correct fit. It's one big (personal) compromise.

To get back to your initial question, no, the fit on a tri bike isn't a "mis-fit" it's a different fit. Although your weight is more forward it's not to the extreme that you'd get when converting a road bike to a tri bike. Key in the geometry of a tri bike is the more rearward placed BB.

Let me illustrate by an example. To achieve the desired "tri" seat to pedal position on a road bike you'd have to move the saddle forward by a considerable amount. What happens next is that now your reach is too far forward. So, to compensate you stick on a longer stem. Result, your weight is way over the front wheel and you have a bad handling proposition at best.
Now for the tri bike, the seat angle is typically 5 or so degrees steeper and the BB is placed more rearward. The combination of these two factors will facilitate a correct seat to pedal position without moving the rider too far forward. Together with a slightly shorter top tube the elbows and subsequently the weight of a rider will be placed in a much more optimal position in comparison to a rigged up road bike.

>>>>We've all seen too-extreme aero positions sapping comfort and power. On my road bike, I can ride in the drops, flat forearms, flat back. How much more drop can one need?<<<<

Normally the stack height of a tri-bar is considerably higher in comparison to a road bike set-up. The lower top tube and shorter head tube just bring things back into place. Generally speaking, a tri position is considerably more "open" than a road bike seating position. Amongst a few things it will make pedaling on long flat straights more efficient, increase breathing, and it will ease the acclimatization of the body when transitioning from cycling to running.

BTW, In my opinion one shouldn't even attempt buying a tri bike over mail-order unless you know exactly what your body requires. If you think an ill fitting road bike is bad, an ill fitting tri bike will be 10 times worse...