Aerodynamics testing – an excuse to get a new bike!

I’ve been doing a little more aerodynamics testing using a power meter and a local stretch of road. Here’s a link to the the previous set of testing.

This time I’m comparing a new bike with my trusty Planet-X Stealth!

It’s a Chinese open mold, a TT069 (as sold by Carbonzone on eBay). I’ve got mine second hand.

This is an unfair comparison really, as even though the open molds tend to be comparatively cheap, the bike is still worth roughly twice as much as the Stealth.

As last time, the protocol is a simple out and back test, but 1km each way this time, aiming at a steady 40kmph. The setup is nearly the same:

• Planet-X Stealth TT bike or TT069. No bottles or tool-kits (although I did leave a rear light on the Planet-X).
• 50mm Planet-X wheels (old style “pointy” profile) with a home made disc cover on the rear.
• 23m Michelin Power Competition clincher tyres. Latex inner tubes.
• Bell Javelin Helmet
• Tri SPD-L shoes
• Speed-suit
• Shaved legs!

I used the same wheels, tyres and crank-set (where my power meter is) on both bikes.

The Planet-X weights in at 8.9 kg. The TT069 at 9.3 kg.

The tests were:

1. Planet-X base run.
2. Planet-X base run 2 – to see how much variance there was in the results.
3. This is just the average of the first 2 runs, not a run in itself.
4. TT069. Note I haven’t done any tinkering on the aero position. The bars are flat rather than the praying mantis position I found was good on the Planet-X in my first set of aerodynamics testing. They are closer together than the Planet-X allows, and have no width adjustment.
5. Increased tyre pressure on the TT069 – I realised I hadn’t pumped the tyres so they were 70 PSI on the front, 65 on the rear. I increased both to 120 PSI.

Here are the results (the out second – Watts 1/Speed1 – has a mild uphill, and was more into the wind, which was across the route):

	Watts 1	Speed1	Watts 2	Speed2	Av Watts	Av Speed	Change
Base 1	315	39.9	203	40.5	259.0	40.2
Base 2	291	40.2	219	40.1	255.0	40.2	4.0
Average	303	40.1	211	40.3	257.0	40.2
TT069	257	40.1	193	40.2	225.0	40.2	32.0
120 PSI	258	40.3	176	40.3	217.0	40.3	8.0

Strava link.

The weather. I started at 12:30.

So, the difference between the base runs – 4 watts. Pretty close!

The new bike. It’s got all the aero things I could name that the Planet-X doesn’t (internal routed cables, brakes hidden away behind other things, very flat base-bar which is inline with the top tube, so no stem, narrow headset, deeper and better aerofoil shapes on the tubing),  but even so, a 32 watt reduction was more than I expected!

The 8 watts from pumping up the tyres is also interesting.

So, just to make sure, I went out on a 39.3km route I do quite often. As you’d expect, the watts are higher than in the testing (there’s some minor hills and various junctions).

On the Planet-X, with the mantis position, I’d done this recently. 39.0 kmph, 260 watts.

On the TT069, I did this. 41.1 kmph, 264 watts.

That stretches it a little as the weather was a little less windy this time (12:30 on here) vs the previous time (12:30 again on here), but even so that’s still a big jump in speed! 3 minutes difference over the route!

So, there you go. There’s the excuse you’ve always wanted to go and buy a new bike…