Aero advice - praying mantis position

I’m trying to fit myself on my bike in the praying mantis position (for triahlon) and I have a doubt. My frontal area looks like this:
Which is faster (aerodynamically):

  1. filling the gap between my arms with a water bottle.
  2. leaving the gap between my arms open; you can see the seat tube trough it, so maybe air can go through it undisturbed? I would put my bottle somewhere else, maybe I could use a soft flask which I would put in my jersey pocket, or maybe a bottle behind a seat.

I think I should leave the gap between and below the arm pads and the frame open, as I read on some other threads/groups. I’m not sure however if this is the same with the gap between the arms.
Any recomendations/ideas? Thanks :slight_smile:

I think you’re at a level of fine tuning where it’s pretty difficult to tell either way without going to a wind tunnel. Between the arms and behind the seat are both good options for hydration aerodynamically (the big no no is having round bottles on the frame). So if it was me I wouldn’t worry about it, and would instead ask how important hydration is for your event and which option is more likely to see you drinking enough?

Personally I don’t like reaching around behind the seat for a bottle from the TT position unless I’m on a smooth straight road with good line of sight and space around me, which is rarely the case on a technical or crowded course. And sitting up to drink is a way bigger aero penalty than bottle positioning. So if it’s an event where hydration is important (longer TTs and any triathlon) I always go for hydration between the arms. If that costs me a couple of watts, it’s still a lower penalty than not drinking enough. If it’s a shorter TT where I can get away with little or no drinking, or a TTT where I can sit up for a long drink while at the back of the line, then I’ll go with behind the seat.


I agree with you, I might just end up putting a BTA bottle. I’m preparing for a 1/2 IM relay, so drinking will be vital. I thought that maybe there is a generic answer to my problem, saying something like “in the praying mantis position, if your arms are not tight close together, you should put a bottle between them” or “… you should leave the gap open if it is at least 5cm” :wink:
I will try to do some aero testing if I manage to get some track time for me and my bike :slight_smile:

It might not be bad to leave that spot open, but it’s also probably not bad to fill that space with a bottle. And if you need a bottle anyway, there probably isn’t a better place for it.

In my limited experience trying to set up a bottle there, though, I’ll note that the Speedfil A2 seems better suited for aerobars that are angled up. The Profile Design HC Aero has its bullet tip pointed up instead of forward, the Xlab Torpedo has a similar problem (and I’m not sure where the straw ends up internally, whether it gives you everything in the bottle if it’s tilted up), and the TorHans AeroZ looks like the straw will be in a useless spot if you tilt it up.

If you go with a regular bottle that won’t be an issue, but I don’t like that because regular bottles can get ejected out. The “fix” there is a really grippy cage, which then frustrates me for frequent sipping. YMMV, of course.

Practice turning. I found with my arms up that steep I wanted to leave the bars for too many corners & have flattened them again. Still upward, but just a more gentle 10% or so. Maybe a longer stem or something would have helped…

I will practise turning for sure. Seems like the bottle will have to go there… thanks.
Another question. Should I go narrower with my hands? Because I can. Or maybe untill it is within the width of my hips it is ok?
I mean… does it make sense to try to go narrower? For sure wider is mlre stable.

My personal experience is that you can gradually go narrower and get used to it. I wouldn’t start super narrow, although if you want to speed up the process add some easy indoor sessions on a pair of rollers. You also have to have finger room. If I have mine fully in it also restricts elevation of the aerobars because of how the aerobar holders and the stem interact.

I had one other idea: what if I made a small fairing that will cover the aerobars from the front. Say in roundy shape, like a half of a plastic 2l bottle. I know it would probably look silly, but I guess it could be beneficial. What do you think? PS: I don’t think it is banned in TRI, is it? :slight_smile:
@matthew.weigel I actually purchased the F2 model. We’ll see how it fits.

I think some small nonfunctional fairings are okay, but I can’t remember the details. If I were adding a semi-custom fairing like that, though, I would probably focus first on finding a good protocol for aero field testing to try to establish that it actually provides a benefit. I don’t think I’ve seen the F2 in the wild, I’d be interested in hearing how it goes for you.

Having spent time in a tunnel and in front of a green screen going narrower is a crap shoot. Hands width, whether narrow or wide (relative to you) affects shoulders which in turn affect head position.

As far as isolating narrower hand position the wind tunnel guys said no people are alike in how the stream of air flows around wider or narrower positions.

IMO this is super high hanging fruit. Comfort is king. If comfortable up front you can sort of melt into position. Shoulders relax and narrow. Head can get a touch lower and out of the wind. Both cause frontal area to lower=a very good thing.

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