i know that as elevation increases, air resistance should also decrease, thus for a given spec and power combination one should have a higher CdA (estimated) at elevation. i’ve been searching but cannot seem to find the posts that described this relationship.
i happen to be interested in ~1500m (1300m to be more precise), but since i live and train at 2,000, often race at 1300, and occasionally race at sea level i’m curious about the relationship.
in a recent TT, based on my speed and power i was at a CdA of about 0.22 (full aero, deep front, disc)…had i been racing at sea level. not sure how dramatically different that would be for where i actually raced (1300m). i’m slightly under 6’ tall and was testing an extreme position over 40km. i’m going to evaluate whether it is better for me to give up a little aerodynamics in order to gain a little power.
thanks for any input! i did search and could not find the references i’d seen in the past.
The important input is that the air resistance is directly related to the air density and air density decreases at altitude. There are a number of online calculators if you google around that you can use to determine “rho”, which is a primary input to tools like GoldenCheetah’s Aerolab, or other VE spreadsheets. The key inputs you want are typically altitude, temperature and dew point.
Hope that is enough to get you started. Let me know if you need more pointers.
thanks. i found this interactive model that allows one to enter their weight (rider & bike), frontal area, drag coefficient, drivetrain loss, grade, Crr and Rho, then allows you to see the relationship between power and velocity.
based on an estimate of Rho, 0% grade (course was very slightly rolling), out-and-back with some wind but nothing major, 3% drivetrain loss, i was seeing an estimate of CdA at 0.23 based on the speed i traveled.
my speed includes getting up to speed at the start, a turnaround, and an acceleration for the last few minutes of a 40k, but other wise was pretty steady.
i know it’s just an estimate, but it gives me a good starting point.
i’m thinking that at a CdA of ~0.23, i could probably afford to move my position up a little to see if i gain back any power. of course, determining if the power gain vs aero redux is a good trade-off will depend. my current position is, i believe, limiting my power production. i’m as low as i could possibly go and found it limited me to upper tempo for the duration of the hour.
You may be interested to know that Steve Gribble also coded the rho estimator into goldencheetah, so you can use it with aerolab. it is under the tools menu.
“…i’m thinking that at a CdA of ~0.23, i could probably afford to move my position up a little to see if i gain back any power. of course, determining if the power gain vs aero redux is a good trade-off will depend. my current position is, i believe, limiting my power production…”
Yeah, finding the best balance that gives you the best speed or watts/CdA for a full hour can be tricky but it sounds like you’re probably giving away a bit too much power at the moment. One of the tricky parts is we do tend to adapt to the lower position if we spend enough training time in that position so it’s hard to know if the position itself is too aggressive or if you just haven’t spent enough hard training time in that position.
FWIW, when I lived and trained in Jackson I’d do complete loops up the highway and back around Antelope flats as big single lap Chung courses both for the training value of 40+ minutes in the aero bars and to compare candidate positions from a power, CdA and ultimately a speed over distance standpoint. If you start laps right at the Kelly junction you can stay in the aero bars all the way around just by easing up enough for the turn onto Antelope Flats and again for the turn to the South at the Shadow Mountain Road. Pick calm mornings and do an entire lap in the bars from a known position, stop, swap some spacers or adjust an adjustable stem, do another lap in the next candidate position and so on. I’d usually only get a couple of runs per morning but it’s great SST training and as long as it’s not too windy and you stay in your aero position with minimal to no braking you can quickly see what positions are more aerodynamic and which are more comfortable for that sub maximal power production. My only issue was windy days or rogue Bison that you have to keep an eye on…
i routinely bike up and around kelly as part of longer training loops (though not on the TT bike), and the bison jams in the summer are not all that infrequent.
they’ve now paved a path north from town to the GTNP entrance (not in moose–but the first parking lot), and the segment that extends from that lot up to moose is all but done. a cyclist friend was injured quite severely (and permanently) last summer heading south between GTNP & the airport. i ride to kelly all the time, and i’ll do whatever i can to avoid riding between the airport & the park. something about the motorists on that stretch of road scares me - and i’d feel even less visible in the aero bars.
anyway, you raise some good points and i have determined i’m going to do some experimentation in a slightly higher position. unfortunately, i was racing this weekend (low position) and have races next weekend, so i think i may make the change and race on it. i know that’s generally taboo, and i’d rather do the multiple trials in various positions over time, but ill have to save that for later. i’ll lose direct ability to compare before and after, but i’ll see quite clearly my ability to sustain particular power levels. i’m fairly confident the aero loss and anticipated power gain will be a net reduction in time. i’ll likely know right away.
ps i’ve never NOT had a day with some significant wind on that loop, but admittedly my schedule has me out there later in the day vs early morning.
Bummer to hear about yet another accident on the park highway. I’ve always figured the Tetons are a major distraction to drivers and can’t count the number of times I’ve seen drivers taking photos or videos out their driver side window AS THEY DRIVE! But I’ve always figured riding Northbound was riskier as drivers look to the West so it’s really disturbing to hear about a rider getting nailed while riding Southbound. Huge bummer.
FWIW, while it’s not great for looped Chung testing, the Northern end of Henry’s road is a great place for roll down style tests (which can be analyzed in GC Aerolab as Chung VE data even if you don’t make laps of it, just use the same start and end points and ideally very similar starting speeds to keep comparisons simple) and it’s both much safer and is more sheltered from the wind. It may not tell you what power you can sustain in a given position but it can give you an aerodynamic comparison.
Bummer to hear about yet another accident on the park highway. I’ve always figured the Tetons are a major distraction to drivers and can’t count the number of times I’ve seen drivers taking photos or videos out their driver side window AS THEY DRIVE! But I’ve always figured riding Northbound was riskier as drivers look to the West so it’s really disturbing to hear about a rider getting nailed while riding Southbound. Huge bummer.
FWIW, while it’s not great for looped Chung testing, the Northern end of Henry’s road is a great place for roll down style tests (which can be analyzed in GC Aerolab as Chung VE data even if you don’t make laps of it, just use the same start and end points and ideally very similar starting speeds to keep comparisons simple) and it’s both much safer and is more sheltered from the wind. It may not tell you what power you can sustain in a given position but it can give you an aerodynamic comparison.
-Dave
generally speaking, i encounter so few cars in my typical 50-mile loop, but i admittedly avoid that stretch. during the peak tourist seasons i recognize that people are here, expressly, to GAPE at those mountains, so i expect they’re watching that and not the road, and i ride accordingly.
i feel like southbound is riskier because the wind tends to blow a southbound rider into the road. in this particular incident, it was a commercial delivery driver who just plowed into the rider–no excuses. just a bad mistake.
henry’s in hoback? hadn’t thought about that.
i’m also going to take some pictures from the front of the lower and higher positions while wearing aero gear and do a quick photoshop maneuver to compare frontal profiles (counting pixels).
i know that as elevation increases, air resistance should also decrease, thus for a given spec and power combination one should have a higher CdA (estimated) at elevation. i’ve been searching but cannot seem to find the posts that described this relationship.
I made a spreadsheet awhile back that should do what you want. It includes a power vs altitude adjustment as well.