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Time Trial geometry?(6 posts)

Time Trial geometry?dave_w
Nov 22, 2002 11:39 AM
As an old guy(well 40) looking to get back into some Du/Tri races I have been looking at TT bikes. What do you guys think about the 78 degree bikes? It seems there are many manufacturers using the slacker seat tube angles on their TT bikes now than a few years ago, any particular reasons? Please share your opinions/experiences.

I'm inclined to go with a steeper offering, but maybe because that is what I have thought of as a TT bike for the last several years.
re: Time Trial geometry?Jon Billheimer
Nov 22, 2002 11:47 AM
According to what I read the consistently fastest bike splits at Ironman competitions are recorded on 78 deg bikes. You might also note that pictures of Pro roadies during time trials generally show them riding right up "on the rivet", so to speak, which is their way of creating a steep effective seat tube angle in spite of the stupid UCI rule preventing a seat position more forward than a 5 cm setback from the bottom bracket.

However, in order to determine your own optimum position with respect to power output vs. aerodynamics you'd virtually need to combine a computrainer spinscan analysis with wind tunnel data. Short of that, try riding or training in a variety of positions and guesstimate what seat tube angle feels best. Regardless of the variations in frame geometry among TT/tri bikes you can always adjust your position with the use of reverse seat posts. Cervelo also includes a seat post which can be flipped around, thus accommodating effective seat tube angles from 75 to 78 deg or steeper.
my overstated opinionSteve_0
Nov 22, 2002 12:47 PM
78 degree bikes are great for what they're designed; getting the average person into an aero position (i.e., cking down and going fast by yourself).

unfortunately, theyre not altogether practical or comfortable for everyday riding (groups, crits, etc).

Many people,pros included, can accomplish an aero position just fine on a road bike. others cannot, but still find the comfort of a roadbike allows for a quicker total time (more relaxed on the run) than on a tri bike. And of course, still others improve times with a tri bike. Its very individualized

About half the field at kona uses road geometry. I believe this is because of 1. the practicality of a road bike over a tri bike, and 2. Many people simply prefer the road geometry. Regardless, theyre all amazing athletes.

If you've got a few grand burning a hole in your pocket that couldnt be better spent on charity, vacation, anniversy ring, etc, go for it. Otherwise, a roadbike works fine for many, MANY peolpe

(btw - you could invest in a 0-offset or forward seatpost to help emulate a tri geometry, rather than spending the big bucks just to find out a forward geometry doesnt buy you anything.

good luck.
Time to educate yourselfKerry
Nov 22, 2002 2:27 PM

For adjusting the fit of the bike, there are roughly five starting points:

1. Seat height (top of saddle to center of pedal axle) at 108-110% of inseam.
2. Saddle parallel to ground.
3. Saddle fore/aft adjusted so that a plumb bob from the bony protrusion just below the kneecap passes through the pedal axle when the cranks are horizontal. This is known as KOPS (Knee Over Pedal Spindle)
4. Front hub axle obscured by the handlebars when riding in your "regular" position (drops, hoods, or tops).
5. Top of handlebars 1 to 4.5+ inches below the top of the saddle depending on your flexibility and size.

These are all starting points for "average" proportioned people, and many folks like to move away from these starting points as they learn what makes them more comfortable, powerful, or efficient. You want to get the fit of the frame as close as you can, then do minor adjustments with the stem, seat post, saddle position, etc.
BRAIN FART!!!!!!!Kerry
Nov 22, 2002 2:41 PM
Was posting a reply to a different thread! Sheesh. At any rate, as stated, a 78 STA frame is dedicated to one thing and one thing only - time trialing. They're not much of a road bike, so it's not the best to have them as your only bike.
re: ...consider...Akirasho
Nov 22, 2002 2:21 PM

Personally, I'm 47, run a Cervelo P2K as well as a couple other TT bikes with varied geometries... and kinda like the steep seat, slack head angles of said P2K for flat... to flat to rolling... a slacker seattube and steeper headtube for hillier terrain... Thing is, the steep seattube angle has gone through it's transitions... now, an individual can decide what works best for themselves.

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