I would like to try the Lowes Motor Speedway TT series and at the same time do a new bike build (a TT bike), So I thought I would ask here. BTW, these questions will not be about components, just frame sizing.

I have used and seen most of the online fit calculators for road bikes, but I have never seen one for TT bikes. I am not even sure where to start. Is the frame sizing comparable? Do I use the same top tube measurement that I feel comfortable with on my road bike? I see that seat tube angles range from 75 to 78 degress and probably more. What are the advantages/disadvantages when your at either extreme of the range? I have seen and read about what you should look like for riding position, and I think I am flexible enough for an old fat guy to get there, just need some guidence.

Currently I ride a 56/57cm frame with a TT of 55-56cm and a stem length of 110-100mm. I use either 172.5 or 175mm cranks. If anyone needs more info just ask.

Thanks in advance.

Hey, hows it going? I'm new to the forum and thought I'd reply to your post. Before I answer your question I'll say that I'm 6'0 tall (to give you a reference for size). When I went to buy a TT bike I was fitted for a 54 cm frame which is smaller than I'm used to riding. I was shocked but the guy fitting me said that TT bikes are meant to be smaller than your road bike. Once we got everything adjusted correctly it worked out and I have no problem riding 70 or 80 miles at a time. On the subject of the seat tube angle, I would strongly recomend you look at any cervelo TT bike. One of the nice features is the reversable seat tube, allowing you to have any seat tube angle between 74 and 79 degrees. I recently bought the Cervelo Dual and along with the variable seat tube geometry it's a blast to ride. Hope this helped.

Joe

In regards to TT frame sizing.

All my road bikes are in the 56 - 57cm frame size.

Even my bike set up for TT's (IM's) was until recently on a 56cm road frame. Just with the seat as far forward on the rails and the aerobars (Vision Tech integrated) as low as possible. This worked adequately without impacting the handling characteristics of the bike too greatly.

Limitations of the road bike frame is the top tube length and length of the steerer tube. So even with the seat forward, it is difficult to get the upper torso as close to horizontal as possible.

The benefit of the steeper seat tube and shorter steerer tube is to allow the hip angle to stay as 'open' as possible and get lower at the front (imagine rotating your position up and forward around the bottom bracket).

To put this all into perspective, I have just recently bought a 52cm P3 frameset. Although the frame is considerably smaller than my roadbikes, the fit is amazing (due to the extreme downsizing the front end is a little twitchy) which I am willing to manage for the aero benefit. I was very sceptical about such a significant downsize, but a triathlete friend of mine who is nearly the identical size let me ride his 52cm P3 and I was sold on this sizing immediately!

In regards to crank length, go longer rather than shorter. The additional leverage of the longer cranks should allow for greater wattage.

Check what rules the TT series you are looking at race under, as if UCI the saddle nose must be 5cm behind the bottom bracket. Steeper seat tube angles will most probably put you well forward of that - so a stumpy nose TT saddle may be required to get around that ruling or a more moderate seat position.

Try the slowtwitch.com forum (taking most with a grain of salt like any forum) plus also check out some of the tech fit articles at https://bikesportmichigan.com/wheels/wheels.shtml

New Slowtwitch article on bike fit. https://www.slowtwitch.com/mainheadin...bikefit06.html

Thanks guys, I have started looking over the Slowtwitch site. It's a lot of info to digest.