How to size a TT bike?

JesseD

Been starting to think about building up a cheap TT bike to ride in local 5 and 10 mile TTs but am a bit clueless about how to size them?

My road bike which fits me perfectly is a 2017 Kuota Kougar in size large with a 56.5cm top tube, a 16cm head-tube (with a 10mm top cap and a 6mm spacers on under the stem) and a 110mm stem, however I do not know how to translate that to a TT bike for size? Should I be looking at bikes with a much shorter top tube like a 53.5cm one? Is there a rule of thumb or formula people use to get the right size?

I want to buy bits second hand (or a cheap complete second hand bike) so don’t want to go to my LBS ask loads of questions and waste their time then buy a bike elsewhere, I have tried google but have not found anything that answers my questions?

I know it’s a bit hit and miss as none of you know my flexibility, measurements etc, but if anyone can offer advice on the size range I should be looking at, it would be very much appreciated.

Grill

Honestly? You can't. At least not first time out. Best bet is to go with something similar in size to your road bike with super adjustable bars so that you can easily adjust pad stack and reach. The only reason to go with a smaller frame is in order to get a lower stack as opposed to less reach (be careful with this as though 3cm is on hold for this season, we have no idea what's going to happen next year).

JesseD

Thanks Grill, was considering something like a second hand PX Stealth in a medium which according to their website will suit someone between 5ft 10” to 6ft, I am 5ft 11”, so looking at the geometry chart it says it has a 53cm top tube which is 3.5cm shorter than my roadbike. The large has a 55cm top tube but according to their website it says it should be for 6ft upwards.

Do I go for something like a medium and then get a very adjustable front end and alter reach with a stem? Was thinking of a base bar with a set of extensions that have the risers under the extensions and pads, bit like Wiggo used to ride as this will give me the option of raising the front to get comfortable now, then I can take out spacers as I get more flexible?

No idea what the 3cm refers to, is this a rule of some sort introduced by a governing body?

meesterbond

As a single data point, I've got a road bike that fits with a 545mm effective top tube and a TT bike with a 510-550mm top tube (depending on where you have the saddle presumably)... Stem on the TT bike is a little shorter but I do have a pretty adjustable cockpit (ADR Ultimate). It's taken quite a bit of fiddling to get to a good fit and I'm a macro-adjuster (if that's what you call someone who isn't massively sensitive to bike fit.

If you have a look on the Time Trialling forum you'll see a nice concise 40 page thread about the pros and cons of the 3cm rule... basically you are supposed to have your elbows no more than 3cm from the axis of the steerer tube (or something like that). Meant to stop people adopting a superman position but proved a little more contensious than CTT thought.

Once I've got round to building my Trinity up, I'll have a Dolan Scala with ADR Ultimate bars to off load - I'm 5'10 so should fit you just fine...

Grill

JesseD wrote:

Thanks Grill, was considering something like a second hand PX Stealth in a medium which according to their website will suit someone between 5ft 10” to 6ft, I am 5ft 11”, so looking at the geometry chart it says it has a 53cm top tube which is 3.5cm shorter than my roadbike. The large has a 55cm top tube but according to their website it says it should be for 6ft upwards.

Do I go for something like a medium and then get a very adjustable front end and alter reach with a stem? Was thinking of a base bar with a set of extensions that have the risers under the extensions and pads, bit like Wiggo used to ride as this will give me the option of raising the front to get comfortable now, then I can take out spacers as I get more flexible?

No idea what the 3cm refers to, is this a rule of some sort introduced by a governing body?

PX Stealth and Exocet 2 are good places to start as they have very low stack. With that in mind you can't really go to big on them. I'm the same size as you and had an Exocet 2 in medium which was good, but would have needed an XL when the whole 3cm debacle was in full force. Nice thing is that because they're quite cheap, you won't lose your ass if you need to change to something larger/different down the line.

Grill

meesterbond wrote:

If you have a look on the Time Trialling forum you'll see a nice concise 40 page thread about the pros and cons of the 3cm rule... basically you are supposed to have your elbows no more than 3cm from the axis of the steerer tube (or something like that). Meant to stop people adopting a superman position but proved a little more contensious than CTT thought.

More like 140...

JesseD

Thanks for coming back to me guys, I am comfortable with road bikes but TT is a whole new world and a whole new set of kit to spend my money on. Whilst I accumulate parts to build a cheap TT bike, I think a skin-suit, TT helmet and overshoes are the way to go and I will just compete on my road bike until its built, don't want to fit tri bars to the road bike as I don't want to scratch the bars on it.

meesterbond wrote:

Once I've got round to building my Trinity up, I'll have a Dolan Scala with ADR Ultimate bars to off load - I'm 5'10 so should fit you just fine...

Those ADR bars are the ones I was looking at, I quite like the fact you can play with the height of the bars and pads from the base bar, so as I get more flexible I can lower the pads and bars gradually.

I will keep a look out in the classified!