Proper TT bike Stem Length-All you TT experts



bgoetz

Active Member
Nov 25, 2010
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I just picked up my first TT bike, a Cervelo P3SL that a friend of mine previously had. I have not spent any time with it yet, but I already know that the bars will need to be moved closer to me, as my friend was a couple of inches taller. The bike is a 55cm, I ride a 56cm road bike, but it is a TT bike and I actually think a 54cm would have been optimal. The top tube length on the Cervelo is a bit longer than my road bikes, but with the seatpost not being offset the measurement from the head tube to the center of the seat clamp is pretty much the same. But just from having ridden my road bike with aero bars I know that I feel real stretched out.

The bike has a 110mm stem on it, but my thought is I am going to be better suited with a 90mm stem. My only question is, instead of getting a new stem why couldn't I just adjust the aero bars so they are closer to me?? I am assuming that this is not the optimal option though or everyone would just use a 100mm stem and make adjustments with the aero bars??
 

daveryanwyoming

Well-Known Member
Oct 3, 2006
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Yes, if you have aero bars that allow the elbow pads to be slid back behind the base bar you can do some or all of your cockpit length adjustment with the pads. if you push the pads way back you might not like the handling when you're down in the aero bars steering with your elbows but moving the pads back a cm or two shouldn't dramatically alter the steering response.

In terms of TT fit and where the pads should be, there are different approaches and one of the tricks is figuring out what approach leads to your best combination of sustainable power, aerodynamics and to some extent comfort if you ride longer time trials. From a big picture perspective the two approaches are:

- Conventional short and low TT positioning where your elbow pads are far enough back that your upper arms are pretty much at a right angle to your back so that you're resting structurally on the elbow pads and supporting your upper body weight largely with the bones in your upper arms. This is pretty standard for triathlon fitting as it's pretty comfortable for long races in the aero bars. How low you go with the pads depends on your flexibility, how far forward you've set your seat to help maintain an open hip angle and the limit of comfort and or being able to see up the road in the aero bar position. Lower isn't always more aerodynamic but for a lot of riders with good hip flexibility, good pelvic girdle forward rotation, fairly straight backs a low position down to the point where your shoulders are within a couple of inches of your hip joints (or more precisely the origin of your lats is level with the outside edge of your shoulders) is usually pretty aero. Much lower than that and you start showing less chest to the wind but exposing more of your upper back and risk both a lot of power drop as well as a lot of neck soreness trying to see up the road without additional aero benefits. In this position the pads have to be pretty far back and the overall cockpit is pretty short so given your TT frame is actually longer than your road frame (is the top tube still longer when riding in a steep seat tube angle, saddle forward position) you might need both a shorter stem and pads slid behind the base bar.

- High and long 'Faux Superman' or 'Deep Diver' position where your base bars and aero bars are higher but your pads are further forward and your upper arms are stretched forward and not square to the angle of your back. In this position the seat angle is generally similar to your road bike seat position, the bars are higher but you're stretched out longer but within the legal UCI limits if that matters to you (i.e. you do national level events at least here in the states or you race in a country that adheres to UCI regularions). Your torso and hip angles shouldn't really be much different than the other position but you get there with higher bars and more forward stretch coupled with a slacker effective seat tube angle. This position can work for shorter to medium length time trials but might get real old for five plus hours of racing during something like the Ironman. If you go this way you will be somewhat stretched out and the pads will be further forward. It also has the advantage of leaving your seat angle unchanged or at least closer to your road bike position so there's less adaptation between the two positions which is nice for someone who mostly trains on the road bike. Whether this position is sufficiently comfortable and or is at least as aerodynamic depends on you and your flexibility. Personally I'm far more comfortable in a high and long position, my power drop off relative to my road bike is much less (as in within a percent or two relative to the 5 to 7 percent drop off I see early season in a short and low position before I've adapted to it) and surprisingly both aero field testing and race results tell me I'm more aero in the high and long position but this obviously doesn't apply to everyone.

Here's some screen shots taken from fitting videos I took a while back comparing my best high and low position and even though the high and long position(top photo) looks a lot less aero, it's actually faster for me even at the same power and I tend to get a few more watts there as well.



Anyway, yes you can slide your pads back if you need to. The biggest reason folks don't do that all the time is that many aero bar designs do not allow you to slide the pads back so it's often not an option unless you use certain brands of clip on bars.

-Dave
 

bgoetz

Active Member
Nov 25, 2010
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38
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Wow, thanks Dave! Great information. With that I think it would only make sense to wait until I have spent some time with the bike to buy a new stem. I would bet that with the way the steerer tube has been cut, I may need to get a more aggressive rise than the typical 7 deg to get into the "Faux Superman". Maybe I can get ahold of an assortment of stems, or borrow the local shops adjustable stem fitment adapter. Looks like I have got lots of playing to do!!
 

daveryanwyoming

Well-Known Member
Oct 3, 2006
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If you're going to explore both low and high positions you should consider picking up an adjustable stem. The most versatile are the double jointed Look Ergostems as you can configure them for a wide range of both height and length settings but they're very expensive. But even a tilt adjustment only stem can be really useful to help you dial in your best TT position. I'd recommend something like this: http://www.performancebike.com/bikes/Product_10052_10551_1083476_-1___

They're cheap but more importantly very easy to adjust while still on the bike. Others like the Ritchey adjustable stem forces you to take apart the headset clamp every time you want to make an angle adjustment which really sucks when doing field testing and comparing different candidate positions.

One other tip, get a notebook and make careful measurements of things like seat height, seat setback behind the bottom bracket, aero bar pad height and reach to the ends of the bars. It can save you an awful lot of chasing your tail when you get a fairly good position, try something else out and then want to go back to what you had or at least make sure you're not just trying the same thing over and over again. It can take a bit of time to figure out what position helps you both from a power generation and aero standpoint and you won't likely get there with your first attempt.

A professional fitting can be a great idea if the fitter is very experienced with TT fits but even then it may or may not be your most aero position unless you go all out and have your fitting done in combination with a wind tunnel session but that's a pricey process. I had a couple of pro TT bike fits over the years and they helped with things like comfort and power generation but it took field testing to show me that those positions weren't my fastest options.

-Dave