Any reason NOT to build a TT frame as a road bike?

warrerj

My wife is ikkle! 4'10" ikkle!
She currently rides a Kinesis KR210 L in the smallest size on 700c wheels which is just about ok for club rides, audax, etc, she even did the Welsh 12hr TT on it last year. She's very comfortable on it but really it's too big (down to 50mm stem to get any sort of fit).
She now wants something she can get more aero on and to do that we need to get her on a 650c wheel bike to get the front end low enough and the top tube short enough. Decent 650c bikes are very hard to come by and we were at the point of thinking custom frame was the only way to go. BUT I just spotted a planet X carbon TT frame in XXS which is on 650c wheels and has a top tube of 47cm which is what her bike fit says she should be on. SO... as in the title is there a reason I shouldn't build it up as a road bike for her ?

Also if anybody has a link or info on the gearing required on a 650c to make it equivalent to a standard compact and 11-28 she has on her current bike it would be a massive help.

Cheers

gloomyandy

Sheldon Brown gear calculator should help with the gearing:
http://sheldonbrown.com/gears/

One thing to watch out for with a TT frame is the saddle position. Make sure that you can get the saddle far enough back to have whatever position your wife finds works for her for knees in relation to the bottom bracket. TT bikes often have the seat very far forward (which works in TT position but may not work so well with normal bars), they also often have funky seatposts that you may not be able to change to get any required set back.

Oh also if she wants to use this bike all year round, what about things like mudguards etc. There may not be much clearance on a TT frame...

Giraffoto

warrerj wrote:

. . .Also if anybody has a link or info on the gearing required on a 650c to make it equivalent to a standard compact and 11-28 she has on her current bike it would be a massive help.

Cheers

To get the same distance-covered-per-turn-of-the-pedals:
Measure the diameter of the current wheels - this is a
Measure the diameter of the 650b wheels - this is b

Either get a cassette with sprockets b/a x 11 - b/a x 28
Or get chainrings with a/b x the number of teeth she has now on her chainrings

Jim C

Have seen one of these PX frames built up into a track bike. Worked fine, not sure on fork angle, rakes, etc, but it should be easy enough to get used to its handling. Its not super steep angles from memory. PX will advise im sure

My son has a cheap small Ribble TT frame built up as a road training bike. Rides just like normal to me

Gears- front rings need to be around 5% bigger, so 52 or 53/ 38 instead of 50/ 36

Grill

The biggest issue, my dear cake-eater, is the steepness of the seat angle. 76 degrees will put her quite far forward and in an upright (read: non-TT) position this can cause issues.

Jim C

Without doing an extensive search, a steep seat tube angle might well be a feature of small frames, to try to solve the toe overlap issue. might be less of an issue with 650 wheels, which are ideal on a smaller frame. Or even 26 inch ( I know of 2 mercians built this way for smaller people)
Might be solvable by pushing the seat well back. Layback seat pin not an option, as its an aero special. Definitely worth a call to PX, as they should have come across this before

warrerj

Cheers guys!
All pretty much what I'd thought. I'll double check the seat tube angles but don't think there's much in it. She's quite a way forward on her current bike anyway.

MichaelW

650C bikes are now mainly classed as youth bikes. Almost all women's bikes use 700c even when the frame is too small.
Ideally, a small women's bike needs small wheels, short cranks, low bottom bracket and narrow bars. Good luck.

The steepness of the frame may have something to do with TT aero position but all small bikes use steep seatposts to compensate for using long cranks, to get the rider into a KOPS position (knee over pedal spindle) at 3:00. For the inverse reason, XXL frames use very slack seatpost angles. The correct solution is to for cranks that fit the rider.

warrerj

She's on 165 cranks now but should be on shorter but they too are difficult to come by in a decent spec unless you pay shed loads of ££ :-(

Grill

You can now get 160mm Rotor 3D cranks.

jibberjim

Making a TT bike up in road format in large sizes doesn't work very well. However in small sizes, it works extremely well, and I would suggest just about any 5'2" or less woman with any sort of vaguely normal proportion would do well on such, and yes 650 wheels.

Look at:
http://www.youtube.com/watch?v=J2mgiMST ... ded#t=176s
The P2SL works fine as a road bike (if the position could be yet more aero...)

warrerj

cheers skinny Grill. I'll check them out. Last time I looked I could only find some mega expensive custom frog made cranks with a BB that cost £150 on it's own!!

PS - Guns ??? you've got pea shooters not guns! ;-)

Grill

warrerj wrote:

PS - Guns ??? you've got pea shooters not guns! ;-)

Derringers?

markos1963

Not sure if Cervelo put it into production but they made a 650c wheeled S2 for Emma Pooley a few years ago, granted she was riding for them but you never know.

Wirral_paul

Be careful on the seat tube angle - as pushing the saddle back negates the whole point of having the shorter top tube in the first place.

Another idea for you - how about a custom frame to her ideal measurements?? How about a custom titanium frame - maybe from one of the suppliers out of China to keep the cost down to similar to the PX frame??