Head Up vs. Head Down

I few years ago I heard John Cobb say that when he tested aero helmets in the wind tunnel it didn’t matter if the rider’s head was up or down. That always struck me as strange. When looking down the long tail of an aero helmet sticks up into the wind and would seem to add a fair amount of drag thus slowing the rider. A couple of weeks ago I got the opportunity to test helmets (and other gear) in the wind tunnel in Ft. Collins, Colorado with the guys from PowerTri, an online triathlon store with a retail outlet shop in Lehi, Utah. And now I understand what John was saying and why he was right.

The first picture here shows a common head position for athletes when racing in a time trial or triathlon. The second picture is the head-down position I mentioned above. I see this a lot at races. It seems riders want to frequently look at their gears or are continually resting their necks. Notice the helmet tail pointing up nearly vertically. What we found out was that the key here is not the tail of the helmet, which is small, but the position of the head, which is relatively large.

In picture 1 most of the rider’s head sticks up above the shoulders. This increases drag. In picture 2 the head is completely below the shoulders. This decreases drag. In fact, drag with the head down – even with the helmet tail sticking up – is significantly less than with the head in the common position of picture 1. The rider is more aerodynamic.

The PowerTri people checked the drag characteristics of several helmets that day both in the common and head-down positions. On average for all of the helmets there was a 1.4% decrease in drag when the head was down compared with the common position. That amounts to about a 51-second time saving in a 1-hour race.

Notice the third picture here. This is what might be called the “optimal” position. I wrote about this position last February following my trip to the A2 wind tunnel in Mooresville, North Carolina.  You can also see pictures of the common an optimal positions in that post. The optimal position, compared with the average of all of the head-up positions, decreased drag by 1.5%. That’s a time savings of about 56 seconds in a 1-hour race – right in the same time range as head down.

I would certainly not recommend racing in the head-down position of picture 2 due to the inability to see where you’re going. It’s just not safe. A friend of mine once ran into the back of vehicle riding in this position in 1987 and has been in a wheelchair ever since. While it’s not safe for extended periods, an occasional glance down is apparently not going to slow you down, and, in fact, might even make you slightly faster. But, again, be cautious with taking your eyes off of the road ahead even for a few seconds. It’s quite risky.

On the other hand, tucking your head down between and lower than your shoulders may even be faster than looking down. And you can still see where you are going but it requires rolling your eyeballs up to the tops of their sockets as if peering over the tops of your sunglasses. This sometimes referred to as the “turtle” position. The challenge here is holding this position for long races. If you’re unable to do that the times when you can get the greatest gain from such a position is when you are going the fastest as on a downhill section of road or riding into a strong head wind. Tucking your head down like a turtle at these times will reduce drag when it is potentially the greatest.

Comments

[Eric D.]

There is only one problem here and perhaps it's the camera angle but I don't think so. The rider in the above photos appears has anything but a good aero position. I have to think someone with a more aggressive aero position may see some different results. No?

[Nick]

Joe, I had a similar experience after visiting the wind tunnel last year. Lowering the head, or sticking your neck out and 'lowering' the ears and still keeping the tail of the helmet down, coupled with narrowing the pads took two and a half minutes off my 40km time, simple but effective. We also raised the stem by 20mm which increased power and made it easier to see and lower the head. Seating the helmet on the back of the head makes it easier to see too. Drag down, power up, massive savings, safer position. 45W saved by spending 2hrs in the wind tunnel. In total 4:50 off 40km time, 9min off 50M TT. Race proven!

[Joe Friel]

Eric D--The rider does have a rather high shoulder position. He does some ironman distance races which may be the reason for this. But all of the tests were relative to the bike set up you see here so his position was not a variable.

[Susan]

Love that you call it the turtle -- it's the term I use to remind myself to tuck my head between my shoulders, assuming it would be faster. Good to know it really is. Wonder if turtles really do that....

[Trevor]

Joe,

First off I'd like to say thanks for your blog and the information you provide in it. I own the training bible and ordered your new book yesterday. I'm sure I will enjoy. The aerodynamic question I have is did you happen to test different water bottle options (i.e. - Front (handle bar) mounted v/s an aero version on the seat tube)?

[Joe Friel]

Trevor--Several different types of drinking systems common for triathlons were tested. The testing was paid for by www.PowerTri.com so they own the data. I was given permission to comment on head position. They will publish the remaining in due time. Keep checking in with their website.

[Bert]

When drag is lowered by 1.5%, then 1.5% power is saved at that speed. Power (phys) needed to drive a bicycle increases as the third power (math) of speed. This means that for small variations the speed increases by a percentage approximately 1/3 of the percentage extra power (phys) you can spend. In this case this means a speed increase of 0.5%. This rule is illustrated by Nick's findings above: assuming that his 45W power savings are about 21% of his total power, his speed should increase by about 7 percent, which corresponds quite closely to his improvement of his 40km time: something like 4:15 on a presumed 60:00, although this increase may be a bit too large to apply this simple rule of a percentual speed increase being equal to 1/3 of the percentual power (phys) increase.

[Michael Paul]

Do you have wind tunnel data, actual numbers, on the new helmets compared to the old? I have an old Troxel (the plastic one) with the dimples--I also still have an old Bell Stratos that I pulled out of moth balls in the mid 90's and beat all my best times. How do these two compare? What about riding with a normal helmet, like a Giro Ionos? Trying to decide if it's worth my $$ to get a helmet newer than my 12 year old Troxel.

[Joe Friel]

Michael P--power Tri has the data. Start by searching their site (http://www.powertri.com/). At one time they posted the data there. If that search doesn't produce it then write to them directly. Good luck.

[Michael Paul]

Thanks Joe! I love messing around with numbers. I used to be so delicate with the aerodynamics, back when I was pretty darn good at time trials.