Thinking of building a tt bike on the cheap or getting a used tt bike. I have a non tt aluminum frame that would probably work. Just wondering what A tt frame can do that a standard frame cannot.

it is a comfort thing......faster . But if you cannot ride it does not matter...

um ok.

Thats true but its really about the different geometry giving you a more aero position on the bike. This means head down, bum up and minimal frontal area. The seat tube will be steeper (79 v 76 degrees) and if its a modern TT frame, the rear wheel will be jammed up under the seatpost too. Google Cervelo P3 and look at the images to see what I mean. You can put aero bars on a regular roadie and get pretty aero but you may feel quite stretched out or struggle to get the saddle forward enough on the seat post so that you can actually ride leaning on the aerobars. Really, if you want to build a TT bike, you need a TT frame.

Does this help? Post more questions if not.

well I see that I can use a custom or bent seat post to get forward enough.

IMHO and from my limited experience I find the most notable benefit of a TT type frame to be the postion of the rider over the cranks. I'm just the typical "bike as a lifestyle" type of cyclist so there's a damn good chance that I cannot exploit all of the benefits of a TT frame but the bike I built for Triathlons is a late 80's steel framed Nishiki with a modified seat post to bring my body over the cranks more. When the weather and conditions are good I can do one of my training routes of 21 miles in ~52 minutes (about 23.7mph average) on the Nishiki. I tried the same course under similar conditions several times on a Bianchi D2 Crono TriCarbon and shaved off only a minute at best (about 24.7mph average). Not a very exacting or scientific approach but it did help put things into perspective for me... a 48 year old amateur "athlete" who is more a utilitarian/commuting cyclist than a competitive cyclist. YMMV

There's one more extremely important point to mention...

The steeper angle and different geometry works your leg muscles differently. Switching to TT frames from road bike geometry won't just help your bike time, it will noticeably drop your runtime.

I said tt not tri lol. Not a triathlete. . . yet.

Could you say a tt frame is safer handling than a converted road frame?

What is a common size tt frame for a 6 footer with medium flexibility.

Seems to be pretty much the same thing concerning frame design.

1. a person's height is not something you should base a frame size on. At the very least inseam, torso length and arm reach are to be considered.
2. what do you mean by "safer handling"? A road frame usually is a compromise between stiffness and ride compliance... a frame that efficiently transfers a rider's effort to the wheels yet is compliant enough to ride smoothly and not beat up the rider after many miles. A TT frame is designed mainly for speed.

It's possible to set up a road frame with the same rider ergos as a TT frame, the big difference will be in frame handling characteristics.

what the advantage of a tt frame?

You look cool riding it.

safety in regards to balance and steering, not ride quality.

Sorry but the bit in bold is still too vague. Some people would consider a relatively sluggish handling bike safe whereas other's feel that a bike with a quick steering response is safer. The shorter wheelbase, steeper steering tube angles and reduced rake and trail of a TT bike will make it quicker handling ("twitchier" to some) than a road bike but it's up to the user to decide if it's safer.

Either way whether the bike has a road frame or a TT frame safety is compromised once the rider drops onto the aero bar pads.

TT bikes have more trail and slacker head-tube ("steering tube") angles than road-bikes. Cervelo's P-series have 72.5 degrees, while road machines usually sport 73deg, or even 73.5deg. Focus' Izalco Chrono sports 72deg, Trek's Speed Concept 72.5deg again.

A TT/Tri bike needs more relaxed angles and sluggish steering to counteract the fact that you're basically putting your torso over the fork while simultaneously putting yourself in a position that makes it harder to control. Your centre of gravity is far forward, which makes any movement more significant.

The steep part of a Tri bike is the seattube, which is where (traditional) TT and Tri geometries differ: A TT bike needs to provide as aerodynamic and efficient a position as possible - while a Tri bike has to do all and save some running muscles. By placing the rider further forward, some muscle-groups work less - it might make your bike-split slower, but it saves some strength for the run.

I stand corrected on the head tube angle. My experience has been that TT/Tri bikes felt twitchy compared to road bikes so I falsely assumed it was due to steeper head tubes. I'll need to see what the ratio is of wheelbase to trail.

**Update**
Apparently fork rake is also slightly greater on a few of the bikes I checked (Cervelo, Kestrel, Quintana Roo, Jamis) so trail will be reduced and the handling should be "more predictable; slower" than a road bike. I find this odd since my experiences/impressions have been to the contrary.

I apologize for my incorrect assumptions.

Yeah, I'm pretty it's the handlebars that give that feeling. TT handlebars are forward (talking about bullhorn bars...not just the aero bars) so it puts your arms in a more aero position at expense of comfortable steering. At least that's my experience.

Rake and wheelbase affect how well a bike balances itself while riding. To my knowledge, that's why the TT bikes are different in those measurements. To help make up for the different steering.

Yeah, I'm pretty sure it's the handlebars that give that feeling. TT handlebars are forward (talking about bullhorn bars as well...not just the aero bars/pads) so they put your arms in a more aero position at expense of steering dexterity. At least that's my experience.

Rake and wheelbase affect how well a bike balances itself while riding. To my knowledge, that's why the TT bikes are different in those measurements. To help make up for the different steering position.

oh snap I didn't realize there were two different fits. so does that mean there are two different frame styles ? or do both fits share one frame.? is that why jamis has a two spots to attach the seat to the seatpost?

The difference is not huge and can usually be "cured" with a seatpost - many offer their bikes with two interchangeable posts: A ~73deg "traditional" and a ~78deg "Tri".

Both markets are not yet large enough to truly separate, but the regulations for triathlons are different from the UCI rules governing TTs. For example, some baseboards for the aerobars have shapes that would be TT-illegal but work in a Tri - or the famous Zipp 2001 and Trek Y-Foil. Quintana Roo, I believe, produced a limited-edition bike that would not be UCI-legal.