TT frame sizing

[JohnO]

I've decided to take the plunge and build up a TT bike. All I really need is a frameset and handlebars/shifters, my parts bin has enough to do the rest.

Is sizing on a TT frame the same as on a road frame? In other words, since I ride a 60-62cm road frame, is that also the size I should be looking for in a TT frame? None of the LBS's in my area seem to have the slightest clue.

 

[Eden]

I'd always head that TT frames should be a little smaller than your road frame so I asked the same question to a shop that specializes in Tri bikes. They said just a little smaller - like a top tube that is only several cm's shorter than your road bike. The only prob for me is that I already ride a teeny tiny road bike so finding something smaller is a challenge to say the least. You do have to watch out -some TT frames are "sized" based on their actual size and others are based on the size of road frame that the rider would normally use so you really have to look at the actual geometry to get an idea of how it should fit. From what I understand you want more drop and less length on a TT bike than on a road bike.

 

[RapDaddyo]

First, a lot of TT frames are offered in fewer size options. For example, the Trek TT frames are offered in S/M/L only, whereas the Cervelo P3 is offered in 6 sizes. Second, if you take a typical road frame and get in the TT position your thighs will bounce against your torso at the top of the upstroke. To create the right angle, you have to move forward, positioning yourself more vertically over the cranks. That's why TT and triathlon frames often have a steeper seat tube angle, to create the right angle between the torso and the thighs at the top of the stroke. This position pushes you more forward on the frame, reducing the required top tube length. Third, when you get your elbows in position for the aero bars, the head tube can't be too high or you won't be able to get low enough. So, you typically end up with a smaller frame than your road frame with a shorter top tube and a higher seat post. There ought to be a TT frame sizing methodology, but I have not seen one. Let me know if you find one.