Second hand TT bikes

[bianchi928]

Hi,

I am looking for a Tri / TT bike and my budget dictates second hand.

If my road bike top tube length is 56cm, is that a reasonable guide for sizing of a Tri/TT bike.

Many Thanks

[Travis_R01]

Hi,
Yes a 56cm top tube is often the best way of measuring a frame size for best fit. Best place to find second hand TT bikes is online, such sites like Facebook marketplace, eBay and Gumtree are good places to look, and the trick is to look regularly.
cheers.

[MattyK]

Short answer, no.

Get a (TT position) bike fit, get your Pad X/Y numbers, and get someone (ideally the fitter) to help translate that into a frame size.

Second point is that every frame maker uses a different frame sizing system, often with no relation to the classic seat tube length measurement.

Third, you typically want a longer frame in a TT position for stability/reducing weight on the front end, and typically a steeper seat post angle. This often messes up the top tube length number.

That said, it's probaby a good starting point to say that if you're a "large" frame in one model/brand, then "Large" would be fairly close in other brands.

[Derny Driver]

TT bikes are often marginally smaller than road bikes.
The seat tube angle is steeper so position is more forward. And seat tube shorter.
By putting more seatpost above the frame it allows the rider to get a lower front end for aerodynamics.
Aero bars /cockpit are postioned further forward than normal flat bars or drop bars so a shorter top tube allows you to position you arms without being too stretched.
Ideally though, as MattyK said, you dont want to have heaps of weight in front of the head tube, or too long a handlebar stem. So if your road bike was say 56 square, you'd go 55 seat tube and 56 top tube for TT. The forward seat position and steeper seat tube angle will move your whole body forward anyway. A shorter top tube is not required.
But of course, TT bikes are not custom built so you really need to try them for fit.

[warthog1]

They still need to comply with the nose of the saddle being 5cm behind the bottom bracket though.
Stupid rule that it is for TTs.
You need a specific saddle with a short nose to get the hips forward.
As Derny says moving forward opens the hip angle and allows a lower frontal position for better aero whilst still developing power.
Rode a 56cm effective top tube in tt and around a 57 in roadie.
Edit: (just looked up the chart and it states it was a 57 in the rear seat hole and 54 in the front which I used. Had a shortened slr t1 so I sort of fail to see the relevance of the 2 positions a bit. I run the seat at 5-5.5cm behind bb on the roadie too, but it aint shortened)
I ordered my p2c new and had to swap it for another size larger as the bike shop had me on one that was too small.
Max seat tube extension and needed a longer stem.
Yeah it would pay to try it out before buying.

[MattyK]

Top tube is not a number to pay attention to. If you want to compare TT/Tri frames, use the Stack and Reach numbers.

[g-boaf]

When I got a Cervelo P5, we worked out the medium frame was okay. I ride also a medium frame on a Giant TCR Advanced SL1 road bike.

Garmin-Sharp people recommended that and the guy who did my bike fit did his work and we came up to the same result and were able to set up the bike with the right fit. It was comfortable enough and a pretty good position on the bike as well.

If you can find a bike fitter with an adjustable stationary bike setup, you might be able to replicate the TT bike you are interested in to see if it will work.

[twizzle]

Another vote for stack/reach. My "go to" for fit calculation was always Competitive Cyclist, but they don't have TT.

Grab a tape measure and try this one instead.

https://www.jensonusa.com/bike-fit-calculator