When deciding on what new bike to get, my decision was between two things: lighter climbing bike, or aero road bike? Because I usually don't have any difficultly staying with the peloton throughout hilly races, I figured that having the aero advantage at the end of races when speeds are 30+ MPH would benefit me more, and lead to greater chance at better results. That's what the research I could find told me, at least.
But, how could I be sure? Well, before I sold my previous road bike (a CAAD10 with traditional frame shape) I did aero tests to see how it would stack up against the bike I landed on - the Fuji Transonic SL SRAM Red build (full review of that to come). In order to give something to compare the results against I also tested my CAAD9 CX bike, and my Cervelo P2 TT bike.
How could a regular guy like me do an aero test? I puzzled over it. I figured if all the bikes started at the same place on the top of a hill, and then I simply let go of the brakes and didn't pedal, the more aero bikes would pick up the most speed on the descent, as well as hold that speed when the road went back up. So, the more aero the bike, the further up the hill it should be able to go. The bike that goes the furthest up the hill is the most aero and the fastest. How much further up the hill it goes than the other bikes is how much faster it is.
A road close to my house (Lanier Boulevard) has a long stretch of road with no stop signs, has a ~.5 miles descent and then an uphill. That's where I did the test.
On all the bikes my hands were on the hoods, and I tried to hold myself up at the same, relaxed position. The one exception was an additional test I did on the Cervelo when I got into the aerobars to see how much of an advantage that gives. I recorded the distances on my Garmin, uploaded to Strava, and took screenshots of where each bike stopped to compare. On all the bikes I did two tests and I compared the furthest run of each bike.
The results were both expected and a little surprising:
1. The Cervelo was fastest, Transonic second, CAAD10 third, CAAD9 CX last.
2. There was a little uptick about halfway down the hill and the CAAD9 CX wasn't able to keep enough speed to crest it - that's why its map results aren't pictured. It's really not fast on the road compared to a road bike.
3. The Transonic was just about as fast as the Cervelo. That was surprising to me.
Here are the results of the Cervelo - you can see that being in the aerobars gives a clear advantage. The width of a house lot - around 45 feet.
Here are the results between the CAAD10, Cervelo P2 and the Transonic SL. The Transonic went about 20ft further than the CAAD10, and the Cervelo went about 5-8ft further than the Transonic.
The Transonic actually went a few feet further than the slower of the Cervelo results.
FACTORS TO CONSIDER
There are a number of factors that could have made this test inaccurate:
1. Wind direction shifts, and small differences in position on the bike.
2. The Transonic comes with deep dish wheels, and it's possible that greatly contributed to the overall aero speed of the bike.
3. All things were not equal - to be truly testing just the frames they would all have the same wheels and weight the same.
4. The Transonic was tested with 2 bottle cages installed, and 1 water bottle - the other bikes did not have bottles or cages. So it was actually at a disadvantage.
I'm comfortable with the conclusions of the test: that aero is substantially faster than traditional road frame shaping. 20 feet faster over .5 miles along this sort of course. I'd say that translates to about half a pedal turn. Over the course of a road race those are some BIG savings.
I'm comfortable with this conclusion because of the consistency of results as well as clear distances between bikes that are consistent with the design intentions. I think this will especially be the case at race speeds; but I think this test proves this is the case at lower speeds as well.
Also interesting that aerobars give an advantage of about double an aero frame over a traditionally shaped frame. I always knew it was the case, just nice to know by how much.