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56cm road bike = 54cm TT bike? sizing question

3K views 2 replies 2 participants last post by  mprevost 
#1 ·
Hi,
I ride a 54cm Specialized which has roughly a 56cm top tube (they apparently measure their bikes differently). I keep reading that TT bikes have a shorter top tube. Does this mean I would likely want to go one size smaller on a TT bike? I don't do triathlons so the 76 degree seat tube angle is not a benefit to me... I am wondering if going with a 54cm TT bike (since I normally ride a 56) and using a recessed seat post (like you find on compact geometry frames) makes sense? I assume this would put me back closer to the more efficient 72 degree seat angle?
Thanks!
 
#2 ·
TT Bikes

mwilcko2 said:
Hi,
I ride a 54cm Specialized which has roughly a 56cm top tube (they apparently measure their bikes differently). I keep reading that TT bikes have a shorter top tube. Does this mean I would likely want to go one size smaller on a TT bike? I don't do triathlons so the 76 degree seat tube angle is not a benefit to me... I am wondering if going with a 54cm TT bike (since I normally ride a 56) and using a recessed seat post (like you find on compact geometry frames) makes sense? I assume this would put me back closer to the more efficient 72 degree seat angle?
Thanks!
72 degrees is not more efficient on a TT bike. It is less efficient. Your hip angle gets too small and it zaps your power. Pros ride shallow because UCI will not allow steep seat tube angles. Many "cheat" the angle a bit by sitting on the tip of their saddle. THe steep seat tube angle on Tri bikes opens up the hip angle to approximate the same hip angle you would have while riding a road bike. You can get much lower in the front while preserving power with a steep seat tube angle. However, it may not be USCF legal. Others here will know the USCF TT rules. If it were me, I would get as steep as USCF would allow. But...don't take my word for it, go over to slowtwitch.com and read the tri bike fit articles. Also, check out the cervelo web site for some more info.

Mike
 
#3 ·
TT BIke fit

I should have mentioned that I ride a 55 c road bike (top tube) and a 53.5 tri bike (78 degree seat tube angle). The steep geometry lets me get much lower in the front than I could on my road bike and do so comfortably. I have ridden 120 miles in this position on my Tri bike. If you keep the same seat tube angle, your top tube measurement would not change much. If you go steeper (and I think you should, if for no other reason than comfort), you would probably need to go with a shorter top tube.

Mike
 
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