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Why is it that some triathlon bikes have a seat post that adjusts straight up and down instead of the more standard axis that aims at the bottom bracket?

This means that if you raise (or lower) the seat on the special triathlon bikes, the torso/leg angle would change more than for a normal bike, right? Doesn't this also mean that the straight-up vertical adjustment of these special triathlon bikes do not fit as wide a range of riders as if the seatpost adjusted in a way that kept the same torso/leg angle along the axis?

This question is not about triathlon bike geometry or efficiency of peddling. Rather it asks about why the displayed vertical seatpost adjustment axis is used instead of a more standard adjustment axis for seatpost.

Here is a tri bike with the vertical seatpost:

enter image description here

Here is a more standard bike:

enter image description here

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I don't have time to give a full answer to this question, but it's because TT and Tri bikes have a steeper seat angle than road bikes. This is a side-effect of aerobars, which are used on TT and Tri bikes. You can get more info from this interview with Dan Empfield, the primary inventor of the Tri-specific bike here. It would be good if you would summarize the article and submit the answer to your own question. –  R. Chung Mar 15 at 19:57
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And keep in mind that the bike is exquisitely sized to the rider, such that only miniscule seat adjustments are apt to be required. –  Daniel R Hicks Mar 15 at 21:13
    
@R.Chung, I will take a look a the link(s) and accept an answer or summarize my own findings. Thanks for the pointers! –  PositiveK Mar 17 at 7:21

3 Answers 3

To crudely simplify things, a triathlon/TT bike position is much the same as a road position, but basically "rotated forward", so your arms rest atop the very-low-set bars. A consequence of this is, the seat ends up further forward.

Position as it rotates forward
(source)

Using Chris Froome's TT position as an example, noting the hip position versus the bottom-bracket position:

Chris Froome TT vs Road position
(source 1 and source 2 - I couldn't find a good side-on picture for the road-position, so it has quite a lot of skewing to align to the TT-position picture)

This "Proper Bike Fit for Triathletes" article has a vastly more detailed explanation (and is the source of the first diagram!)

Doesn't this also mean that the straight-up vertical adjustment of these special triathalon bikes do not fit as wide a range of riders as if the seatpost adjusted in a way that kept the same torso/leg angle along the axis?

As long as the frame is roughly the correct size, the tri bike should be just as adjustable as a road-bike, if not more..

Look at the tri bike image in your question - the saddle rails cover around 3/4 of the length of the saddle, allowing a greater range of movement forward/backwards (compared to the road bike, where the rail often covers less than half of the saddle)

Additionally, the aero-bars are much more adjustable than standard drop-bars (which are mostly limited to stem-length adjustments). Aero-bars can potentially be moved quite far forward or backwards (although at some point you will hinder steering/stability)

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Nice answer. As you no doubt know, that slowtwitch.com article was written by Dan Empfield. –  R. Chung Mar 17 at 13:35
    
There's a difference between your animated gif and the pictures the OP posted. In your gif, it seems like the seat post (and resulting seat position) is actually quite far forward, whereas in the question, it seems like the seat ends up in more or less the same location on both bikes. It seems like in the question, the bike has been designed to optimize for air flow, while still putting the seat in the same place. –  Kibbee Mar 17 at 13:56
    
I suspect that the long rails on the originally posted triathlon bike's saddle is to compensate for the only-vertically-adjustable height. If the adjustment axis was at an angle, then long saddle fore/aft adjustment would not be as necessary. –  PositiveK Mar 17 at 21:41

Triathlon bikes are about one thing, and one thing only. Aerodynamics. Dan Empfield, the creator of the Quintana Roo brand, recognized this early on. Cervelo came along soon after, and their designs basically changed how time trial bikes are viewed, with their breakthrough design of the P3 in 2001 (Company history here.)

This P3 design evolved, and there are a few models such as your pictured Giant that have completely vertical seatposts. Many other bikes don't have this, but do approach vertical, as the frame geometry is completely different than a road bike with the purpose of allowing a rider to ride more forward on the bike, bent over with arms in the aerobars. You will also see much different clothing (esp for road racers) as time trial suits are also designed with aerodynamic considerations.

Time trial fit is very much a science as well as an art form. These include the F.I.S.T. method (Also developed by Dan Empfield), Retul, which are two of the most popular, but there are a couple others as well. They look at such things as upper body angle, hip angle, leg angles at top dead center and bottom dead center (TDC, BDC), forearm to bike angles, etc etc. A very minor change in seat position can have profound effects on aerodynamics, comfort and power.

You will also see some different frames and components in the triathlon world than you will in the road race world, because as always, the UCI (governing body for cycling) has very strict rules for time trial bikes and components, and some of the items used in triathlons are not allowed for road time trial events. For example, Cervelo's latest model, the P5, has two different forks, one which is legal and one which is not.

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Nice. You might want to add something about the 5cm setback rule -- it shows that riders would want to go even farther forward if they were allowed to. –  R. Chung Mar 16 at 15:53
    
@R.Chung - True, but that's more a UCI positioning thing than a frame design detail. There's a few frames (Such as the BH brand) that have a seatpost that has a huge forward to back range, so you could have a UCI legal and a UCI illegal seat position on the same frame/post. –  JohnP Mar 16 at 18:24
    
I view it differently, but I don't have a problem with your answer. Vertical seatposts and steep geometry are a frame response to the desire to get as far forward as possible. The UCI imposed that rule to prevent even more forward frame designs. The vertical seatposts we see today are just a point along a continuum. In a completely aero-focused world frame designs would be even more radical. There's nothing magic about vertical. –  R. Chung Mar 16 at 18:53
    
@R.Chung - I see your point as well. I've heard rumors that the UCI might be allowing the superman position on the track soon, it'll be interesting now that McQuaid is out (Thankfully) to see what changes come around. –  JohnP Mar 17 at 1:06

In addition to aerodynamics, not yet mentioned is the benefit to your running off the bike. By changing the seat angle, you use your muscles differently and can run faster off the bike than a more traditional seat post angle.

As noted in this article, test subjects were about 5 minutes faster in a 40k/10k Brick (stationary bike/treadmill run). There was some time savings in the bike (even indoors on a trainer), but the biggest time savings was the first half of the 10k.

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My interest is in the angle of the adjustment axis. I realize that the resulting geometric angle made by bottom bracket to saddle to handlebars is different for Triathlon / TT bikes. Good point about the overall geometric angle though, thank you! –  PositiveK Mar 17 at 21:35
    
No downvote as it is a legitimate point, but running well off the bike is primarily about fitness. This is still a fairly hot debate among triathletes and exercise scientists that work in cycling/triathlon fields. –  JohnP Mar 18 at 16:04

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