At some point I want to put a compact crankset on my Tri bike. I’ve seen some pretty decent deals of Ultegra compacts, but I see that bulge around the from the star nearly to the teeth of the big ring and wonder what that does for the aerodynamics. I suppose I could see it making things worse, better or no effect. Could it act like a toroidal zipp wheel and straighten out the flow of air off of the chain? Could it just cause more turbulence and increase the frontal area? Or, does it have no net effect?
I don’t think either the ultegra or da were designed with aero in mind.
eyeball wind tunnel says the FSA gossmaser looks significantly better than both:
If only FSA would fire up a 50T/110 version of their lenticular TT ring… the extra thickness seems like it’d mate with the sculpted arms on hte newest cranks better than a typical ‘flat’ ring
I’m pretty competitive. Won my AG at IMAZ at my first IM. Ranked 15th in my AG in USAT rankings despite limited time for training this last year.
Seconds matter to me.
seriously though, Damon Rinard has hinted that bigger chainrings are MORE aero not less, and you are strong, so why not go get a 53/54 chainring, run an 11x28 if you need an easy gear =)
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Thanks, Jack.
That’s interesting about the larger chain rings. I wouldn’t have guessed, but hard to argue with Damon Rinard. Dan brought up using compacts on tri-bikes a few years ago and my road bike came with one (50-34), so I was looking that way.
I find myself in the 14-19 gears most of the time in races and I hate gaps in those gears. At IMAZ I borrowed a 13-23 (9 speed) so that I didn’t have any gaps in that range, because I seem to have a fairly narrow optimal cadence (85-93) for longer races. I suppose that a 55-39 (which I own) would accomplish essentially the same with a 12-23, 12-25 or 12-28 rear, depending on the elevation profile. A lot cheaper, too.
obviously you have no choice but to go with super record 11 speed!
(joking!)
Thanks, Jack.
That’s interesting about the larger chain rings. I wouldn’t have guessed, but hard to argue with Damon Rinard. Dan brought up using compacts on tri-bikes a few years ago and my road bike came with one (50-34), so I was looking that way.
I find myself in the 14-19 gears most of the time in races and I hate gaps in those gears. At IMAZ I borrowed a 13-23 (9 speed) so that I didn’t have any gaps in that range, because I seem to have a fairly narrow optimal cadence (85-93) for longer races. I suppose that a 55-39 (which I own) would accomplish essentially the same with a 12-23, 12-25 or 12-28 rear, depending on the elevation profile. A lot cheaper, too.
At some point I want to put a compact crankset on my Tri bike. I’ve seen some pretty decent deals of Ultegra compacts, but I see that bulge around the from the star nearly to the teeth of the big ring and wonder what that does for the aerodynamics. I suppose I could see it making things worse, better or no effect. Could it act like a toroidal zipp wheel and straighten out the flow of air off of the chain? Could it just cause more turbulence and increase the frontal area? Or, does it have no net effect?
Anyone know if this has been tested?
What a great question!
Disclaimer: I have never aero tested cranksets and have no experience with this specific problem. I also have no insight into directly answering this question.
Keep in mind that the “time-average of the flow CdA” is not the same as the “CdA of the time-average of the flow”. I’m going to guess that the only way to get to the bottom of this is to perform some kind of aero field test.
This is a fully transient, moving boundary, multi-body (feet, ankle, tubing, crankset, wheels, …) flow problem, making it difficult to perform in the tunnel, and probably impossible to properly model with CFD. While I don’t doubt that people who use these tools have developed a lot of insight into the general flow in the crank region, I’m pretty sure no one’s quantified it.
I’ll put this on my list of things to try, I promise. Thanks for asking it.
We had some threads on this topic many years ago and IIRC, the consensus from the aero gurus was that there is** very little aero gains to be had from “aero cranks”.** Any measurable aerodynamic gains from cranks would stem from lowering your Q-angle (becoming more narrow), for example, the Walser crankset.
obviously you have no choice but to go with super record 11 speed!
(joking!)
Thanks, Jack.
That’s interesting about the larger chain rings. I wouldn’t have guessed, but hard to argue with Damon Rinard. Dan brought up using compacts on tri-bikes a few years ago and my road bike came with one (50-34), so I was looking that way.
I find myself in the 14-19 gears most of the time in races and I hate gaps in those gears. At IMAZ I borrowed a 13-23 (9 speed) so that I didn’t have any gaps in that range, because I seem to have a fairly narrow optimal cadence (85-93) for longer races. I suppose that a 55-39 (which I own) would accomplish essentially the same with a 12-23, 12-25 or 12-28 rear, depending on the elevation profile. A lot cheaper, too.
I WISH.
I HAVE upgraded to 10 sp as of a 3 years ago (on my 8 yo Cervelo P3 Al), but I’ll have to wait until Campy gets eleven speed down to something below Athena before my wallet (and my wife) will allow.
Now that I’m fully awake, vs 5:40 this morning, I realize that I already use at 12-XX for racing. I guess I’d better just race flatter courses.
I must admit, I haven’t yet found a PM that I can afford. That is a bigger priority. I’m hoping one of the ANT+ pedal based PM will be in my price range.
At some point I want to put a compact crankset on my Tri bike. I’ve seen some pretty decent deals of Ultegra compacts, but I see that bulge around the from the star nearly to the teeth of the big ring and wonder what that does for the aerodynamics. I suppose I could see it making things worse, better or no effect. Could it act like a toroidal zipp wheel and straighten out the flow of air off of the chain? Could it just cause more turbulence and increase the frontal area? Or, does it have no net effect?
Anyone know if this has been tested?
What a great question!
Disclaimer: I have never aero tested cranksets and have no experience with this specific problem. I also have no insight into directly answering this question.
Keep in mind that the “time-average of the flow CdA” is not the same as the “CdA of the time-average of the flow”. I’m going to guess that the only way to get to the bottom of this is to perform some kind of aero field test.
This is a fully transient, moving boundary, multi-body (feet, ankle, tubing, crankset, wheels, …) flow problem, making it difficult to perform in the tunnel, and probably impossible to properly model with CFD. While I don’t doubt that people who use these tools have developed a lot of insight into the general flow in the crank region, I’m pretty sure no one’s quantified it.
I’ll put this on my list of things to try, I promise. Thanks for asking it.
Since the leading edge of the chain is hanging out in the wind, for most of the pedal stroke, I wondered if it had an effect like side-pull brake. Otherwise, why would QR and now Cervelo have assymmetric downtubes? But, you’re probably right. There is just too much going on out there.
Small point of clairification:
Other than the frame structure broadening in general near the bottom bracket (to earn the power transfer benefits of BBright),
Cervelo frames have aerodynamically symmetrical down tubes.