Help Picking the Right Size TT Frame

[RyanH]

Hi all,

I got a Retul pre fit for a TT bike but I'm not sure which size to get based on the numbers. It looks like a BMC tm01 Medium-Long (515mm stack and 418mm of reach) would be about right except for the seatpost being relatively low (57cm center to top with my total length being 72.5cm). Am I translating the numbers correctly or should I be looking at small frames? It seems stack is really low on small frames and I'd be running a lot of spacers.

Of most interest is a Felt Da1 in medium (416 and 497 for reach/stack) but again seat tube is 54cm center to top.

Is that going to look wonky? Or should I look for a small and run spacers? Fit specs are attached.

[dudemanppl]

SMALL OR BUST DUDE

[Kayrehn]

Read up lots on Slowtwitch, they have many articles discussing fit issues and also comparisons of different brands' geometry.

I'm curious - why did you not ask the bike fitter when you went for the session? I told my bike fitter I wanted to get this tt bike, he made some measurements and he advised me on the size to get (I'm between sizes, so the smaller one of course [emoji1]), then I went back to him after the build to get it sorted out.

[RyanH]

He's on a plane to the London 6 day so I figured I'd post here to see what others said while I wait for his feedback.

[Rudi]

Maybe I am reading it wrong, but the reach measurement of 450? Both frames you mention seem to be 3cm shorter than that.

On the long and low classic Cervelo P3 you'd need a 58 to get that sort of reach - 445mm, but that would bust the stack - 535

Are you really tall? - edit - tall in the body?

[Rudi]

Again, just checking I have read it right. If the frame stack from the fit is 509 and the frame reach is 450 - a quick check of the Slowtwitch Stack and Reach sheet shows nothing along those lines.

http://www.slowtwitch.com/stackreach.html

You'd be looking generally at the largest sizes to get that reach, and the smaller to medium sizes for the stack.
I have a feeling that the reach is wrong on the fit - that guess is coming from your 726 saddle height.
(please tell me I haven't messed this up)

[wingguy]

You're working on incomplete information. So you have one theoretical frame stack and reach that they think would work... but what is the spacer stack, stem length, pad setback and pad stack on the base bar to get that frame to the right point?

Without all of that information you're kinda asking for guess work. Unless people work backwards from the pure pad stack and reach figures, but that's a reasonable amount of work.

[RyanH]

The Retul is dynamic, so it's frame stack and reach doesn't necessarily need to be translated to a real bike. I suppose that the primary important number is the Arm Pad Reach (471mm) /Stack (599mm) and making sure that the combination of the frame and the bars/stem/extension/pads can accommodate that. It seems there's a lot of flexibility in hitting your fit on the TT bikes. I guess my primary concern was that if I went with a medium, on a lot of frames the amount of seat post exposed would be low. On a road bike, that'd look odd, but does it on a TT bike?

[Rudi]

For the Felt DA1, you'd need a stem and bar combo that gives you roughly 10cm of stack over the frame to get to the arm pad stack. A 100mm 6degree stem will give you about 4cm of that so 6cm for the bars sounds plausible.

The arm pad reach from the BB suggests 471mm to the back of the pad. Assuming the pads sit directly above the bar clamp, then the 100-120mm stem would probably get the middle of the pad in the right area.

I'm not a fitter so as mentioned above, a view from your guy is probably best. I have in the past bought the wrong size of TT frame because I didn't understand stack and reach and only paid attention to one of them. Ended up on a frame that was way too small, hence my interest in trying to help.

[nathanong87]

SMALL OR BUST DUDE

*XS or bust