Looking to buy a TT bike, but can't seem to get my head around the fit. Having never ridden af TT bike, I have a hard time figuring out how long the top tube should be. I'm 178 cm tall, and guess I'm 50/50 in torso and leg length. I usually ride an Orbea Orca size 54 - where the top tube is 55 cm with an 11 cm stem. I'm thinking that the top tube has to be shorter, but any comments to fit is greatly appreciated.

I know the best thing would be to go to a pro fitter and ask all the questions, but it's also a good idea to have some knowledge yourself about the same

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FWIW my tt bikes top tube is 4 centimeters shorter than my road bike and both with a 100mm stem. Best fitting tt bike I have ever owned.

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It's fairly common to run shorter stems on TT bikes, as the actual reach of the bars equate to something alot longer. Not apples for apples ofcourse comparing reach of TT bars and drop bars, but you get what I'm trying to say. About top tube length, look at actual reach. 55cm top tube with 73 degree seat tube is not the same as 55cm top tube with 76 degree seat tube. There is ofcourse also the matter of actual vs effective seat tube angle.

Go to a fitter so you can actually try different geometries.

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The Shivenge

stack and reach, stack and reach, stack and reach, stack and reach. Buy the frame based on stack and reach not TT length. Look for similar reach as your road frame and the narrow down the frame based on stack and desired crank arm length.

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+1

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If you have an adjustable stem and bars, TT bikes are really easy to fit. You will likely change your position away if you try to optimize, so just get it close. I'd get a frame that is big enough to avoid toe overlap though... that gets annoying.

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formerly rruff...

Ok, can you explain that a bit more? If I look at my Orcas reach 379 mm, and compare that to a Planet X Exocet frame, it doesn't go lower than 401 mm? Or should I be looking at the reach of 379 mm + stem length of 110 mm which equals to 489 mm, than I should run the Planet X with a 90 mm stem which would equal 491 mm?

The stack I can't seem to figure out at all. 559 mm for the Orca, and the Planet X in small has a stack of 480,8 mm.

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Take your time to read through these. Stack and reach is discussed very thoroughly.

http://forum.slowtwitch.com/Slowtwitch_ ... e_P3106733

http://forum.slowtwitch.com/Slowtwitch_ ... _P3108442/

http://forum.slowtwitch.com/Slowtwitch_ ... _P3110570/

Do you want the bike for triathalons or Time trials? from what I've read the "ideal" fit is completely different

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I don't really understand the reason behind this (even after reading those links). If I have a frame with a similar stack and reach, but with a much slacker ST I would be stretched too much.

I'd think TT length is also a factor for fit. Now I'm sure I miss something obvious, so where am I mistaken? Or is the consensus that you fix this with the saddle seatback (effectively adjusting the ST angle).

TT's only, and the general consensus on that seems to be a more compact position compared to triathletes.

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Only if you are silly enough to let the saddle fore-aft position be dictated by the frame, rather than what is best for you.

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formerly rruff...

I assume that you're looking for a TT bike to gain speed (it could be just to look cool, then ignore the rest of the post)... The bike itself is not magical, it won't make any real difference in speed. What does make a difference is the position of the rider that the geometry allows you to achieve; the rider accounts for the vast majority of the drag. This is why it's so important with a proper bike fit for a TT bike (and even more so with a tri bike, but a mute point for you). Get a bike fit first and then buy the best possible bike (i.e. geometry) for YOU.

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I don't know what "compact" means... but this is a good place to start. Put the saddle where it needs to be (nose 5cm behind BB if UCI), adjust the reach so that your upper arms are at least perpendicular to your upper body and your elbows clear your knees (or a little more... there is a UCI limit on that also), and adjust the pad height so your chest area is parallel to the ground.

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formerly rruff...

It's certainly a good idea to get a fit... but a fit for a noob is going to be a crude guess on what would be optimal. An adjustable bar and stem (or an assortment of stems) are essential for someone just starting. It greatly improves your position options, and allows experimentation.

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formerly rruff...