Aero position on TT bike cost savings vs. speed

Triathlon Forum
1

I’ve been wondering about this for a while. Let’s say for this example that I’m on my TT bike for the entire test. One position being in Aero and the other non-Aero position. Let’s say the TT (10 miles)is a flat course no wind, take the external variables out of the question.

(Numbers are for easy computation)
Non-Aero position = 275watts @ 30mph
Aero position = 250watts @ 30mph
= 10% gain in efficiency from Aero position

If I used that efficiency I gained and pushed the same 275 watts in the Aero position would that translate into 10% gain in speed? So 33mph?

2

I don’t think so because this assumes that drag grows in linear fashion with speed, which I don’t think it does…although I may be corrected
.

3

No. The force of air drag which you must overcome increases at a greater rate than your speed. ie if it takes 200watts to go 22mph it might take only 20 more watts to go 23. But to jump from 23 to 24 might take 30 watts (not real world numbers)

4

Ok, that makes sense. Are there any numbers out there that would put us in the ball park for this? Guess we’ll have to wait on some serious Aero people for this one.

5

from analyticcycling.com

Edit: 140lb person with a 17 lb bike = ~71Kg

For 30mph at 250 watts:

Speed For These Parameters 13.41 m/s (30mph)
Power 250 watts
Frontal Area 0.288 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 71 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0. decimal

increase to 275 watts gets you:

Speed For These Parameters 13.89 m/s (31.07mph)
Power 275 watts
Frontal Area 0.288 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 71 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0. decimal

6

Sweet @$$ numbers Jaretj! Thanks!

7

You can play with the number yourself at www.analyticcycling.com

On the right in the tool kit click on “static forces on rider” and then power given speed or speed given power

jaretj
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8

I’ve been wondering about this for a while. Let’s say for this example that I’m on my TT bike for the entire test. One position being in Aero and the other non-Aero position. Let’s say the TT (10 miles)is a flat course no wind, take the external variables out of the question.

(Numbers are for easy computation)
Non-Aero position = 275watts @ 30mph
Aero position = 250watts @ 30mph
= 10% gain in efficiency from Aero position

If I used that efficiency I gained and pushed the same 275 watts in the Aero position would that translate into 10% gain in speed? So 33mph?

According to a rough rule of thumb, you should gain ~1sec/km for every 10W it takes to go the same speed…so, that means that in the more aero position you’d be 2.5s/km faster, or ~40s faster over 10 miles (16.1km).

9

from analyticcycling.com

Edit: 140lb person with a 17 lb bike = ~71Kg

For 30mph at 250 watts:

Speed For These Parameters 13.41 m/s (30mph)
Power 250 watts
Frontal Area 0.288 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 71 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0. decimal

increase to 275 watts gets you:

Speed For These Parameters 13.89 m/s (31.07mph)
Power 275 watts
Frontal Area 0.288 m2
Coefficient Wind Drag 0.5 Dimensionless
Air Density 1.226 kg/m3
Weight Rider & Bike 71 kg
Coefficient of Rolling 0.004 Dimensionless
Slope of Hill 0. decimal

That’s a pretty darned low CdA to be assuming (i.e. .288m^2 x 0.5 = .144m^2 of CdA)…I think I’d assume something a bit more realistic.

10

True, but to fit the OP’s (generous) terms of 250 W and 30mph, it’s a necessary (false) assumption.

11

I know but I was using the power and speed he gave me and then just lower the cda untill I got them lined up.

I feel that’s really isn’t enough power to go 30 mph. I’m pretty small and there is no way I could get 30 mph out of 250 watts, it would be well over 350 watts for me, probably over 400. I can’t ride that fast on flats for any length of time to be able to observe my power at 30 mph.

jaretj