Reasonable Goal for CdA?

Triathlon Forum
1

I finished my first Oly on Sunday, and managed 20.7mph, on a 4-lap, ~5.2 mile course. Unfortunately, my power meter crapped out, so I don’t have power data from that day. Based on my RPE, HR, and my resulting run, I estimate that my avg power was ~175 watts. I also have several other files from a closed loop near my house, with similar speeds. Using the calculator on Aeroweenie, I’m estimating (yes, I know its “rough”) that my current CdA is somewhere around 0.280. I ran calculations on all of the data I have where I was in race config, and they spread between 0.278 and 0.282. I’m clearly not 0.350 or 0.190.

I have a road bike setup in a TT type config. Previously posted here:

http://forum.slowtwitch.com/...ost=6421522#p6421522

I was going to embed a race photo, but I can’t shrink them on this computer to meet the 250kb requirement. I will try and update later. In the mean time, race photos are here:

https://www.backprint.com/...tingimage/142662/103#

I’ve got one more Oly race in 3 weeks, so I’m not making any changes until after that. But, what I’m wondering is what a reasonable expectation is for a CdA on this current bike? Since I’m NOT planning on buying a Tri-bike in the next 12 months, I’m trying to set a goal for the fall/winter aero testing…and, am looking for help from the more aero-knowledgeable for what is reasonable. When I get to testing I will use Aerolab in GoldenCheetah, not the rough calculator from Aeroweenie.

2

I’m no help on this.

But all I see are the straps on those shoes!

3

Yeah I know. It’s the first thing I noticed too. They are almost done in anyway and will be replaced for next year.

4

you can find good deals on tri bike frames. your bike is definitely a limiter due to the lack of adjustment possible with front end and the fact that your front end is…suspended. i scored a NP2 frameset for about $600 and sold a classic P3 frameset for $300. so good deals that actually allow you to take advantage of your bike (instead of your bike taking advantage of you) do exist. i mean being under .25 on a bike like that would be a pretty solid accomplishment. your elbows are way, way too wide.

5

For as big as you look in those photos, 20.7 on 175 watts is pretty good.

6

Not sure what the standard is for “big”, but I think those photos make me look bigger than I am. The camera adds 20 lbs right?

I’m 5’ 11", 151 lbs on race day. I wear 30" waste Levi’s, and they are loose. I’m wearing a medium tri-suit, but I think it should have been a small, based on the wrinkles coming out of the swim. I should probably at least try one and see.

7

Yeah I know. It’s the first thing I noticed too. They are almost done in anyway and will be replaced for next year.

Just cut the excess off. Boom…you just reduced your drag!

8

I’m not going to hazard a guess because it would literally be throwing a random dart in a dark room. Priority for you should be to get your arm pads A LOT narrower and probably add more reach as well.

Even if your elbows remain somewhat wide there’s no reason to not have your hands touching or almost touching up front. It’s actually a more relaxed position as it allows your shoulders to be internally rotated. My $0.02, YMMV.

9

Thanks, James. Yes, that’s the longer range plan. I’ll probably spend next year doing just that. I need to let the credit card cool off a bit.

Elbows: yeah. Kinda like the shoes, I noticed how wide my elbows look when I first looked at those photos. I was given the T2+ aero bars as a present. The J2/F19a bracket/pad combo doesn’t let me adjust the pads any narrower. I’m looking at replacing that with the L2/F40TT combo…maybe with pieces parts or with the V4+ barset (replacing the S-bend with ski-bend). These go considerably narrower and about 2cm lower.

10

Thanks. That’s probably another argument for switching to the V4+ carbon vs. the T2 alloy. As far as I know I’ve got the T2 as narrow as it can go.

11

Yea I’d say elbows closer and maybe a bit more drop in the stem.

But I’ve seen guys much worse on much more expensive setups.

12

Yea I’d say elbows closer and maybe a bit more drop in the stem.

Lol. I literally spent months on Ebay looking for a stem with more drop. The problem with the SR900 is the Headshok steerer—its not a standard dimension, its 1.56". So, I have to buy a headshok specific stem—something that isn’t made anymore—so, its all bikewagon/ebay/craigslist, other random old stock. The current stem is 130mm / -20 degrees. Very early in my quest for a TT position on this bike…I saw a 140mm / -35 deg Headshok stem on Ebay. But, at that point I was still riding on the hoods of road-bars with a 90mm / +5deg stem. The 140/-35 stem just looked SUPER aggressive. I couldn’t convince myself that the 140/-35 stem could work…so, I passed on it. Stupid, stupid, stupid.

Fast forward a couple months, I bought the TT bars, SRAM shifters/brake levers/RD, installed the forward seat post and the PR 2.0. I flipped the stem over, used Dan’s static fit article to figure out a position starting point…and REALLY, REALLY, REALLY wished I’d bought that -35deg stem. I sat on ebay, and googled daily for months. But, the only thing I kept finding was the one Ebay Auction that I’d passed up…just sitting there taunting me.

The only realistic way (unless I get lucky on ebay) that I’m going to move the cockpit is with an undermount setup. That would drop the pads 20-30mm. Interestingly, the undermount versions seem to allow the extensions to go a little closer together, and the pads a little narrower compared to the over-mount versions (and a LOT narrower than the setup I have currently). It seems the J2 bracket that comes with the T2+ alloy bars, is just about the widest and least adjustable clip-on bracket made.

I’m still shopping and comparing, but the Zip Vuka clip (under-mount) looks better for my config that the PD V4. It looks like it has a slightly lower stack, and allows a narrower mounting bracket compared to the L2 bracket used in the V4. My bar clamp on my stem must be wider than a non-C-dale clamp, because my current bars are tight to the clamp, but my elbows are still “WAY wide”, and even the extensions are being called “too wide”.

But I’ve seen guys much worse on much more expensive setups.

175 watts / 21mph isn’t anything to write home about, obviously. But yeah, I passed a 2 P5’s, a P5x, a Felt IA, a P4, and a P3 on Sunday. I’m happy with my $$/mph quotient at the moment.

With careful ebay hunting, I’m at about $600 in upgrades. Yes, the front wheel is Chinese…and accounts for almost half of the upgrade expenditures. I’ve tried to buy things that I could transfer over to a new frame. I’ve saved all the road-bits that I took off the road-bike in the conversion—so I can put it back into road config once I have a TT frame.

13

I finished my first Oly on Sunday, and managed 20.7mph, on a 4-lap, ~5.2 mile course. Unfortunately, my power meter crapped out, so I don’t have power data from that day. Based on my RPE, HR, and my resulting run, I estimate that my avg power was ~175 watts. I also have several other files from a closed loop near my house, with similar speeds. Using the calculator on Aeroweenie, I’m estimating (yes, I know its “rough”) that my current CdA is somewhere around 0.280. I ran calculations on all of the data I have where I was in race config, and they spread between 0.278 and 0.282. I’m clearly not 0.350 or 0.190.

Then none of the above numbers mean anything.

With full nutrition I’d say that a competitive tri CdA should be <.220m^2. Of course if you don’t have a reliable method to calculate CdA then it doesn’t matter.

14

Ah I see. There are headshok adapters on eBay. Would these let you run other fork?

15

I finished my first Oly on Sunday, and managed 20.7mph, on a 4-lap, ~5.2 mile course. Unfortunately, my power meter crapped out, so I don’t have power data from that day. Based on my RPE, HR, and my resulting run, I estimate that my avg power was ~175 watts. I also have several other files from a closed loop near my house, with similar speeds. Using the calculator on Aeroweenie, I’m estimating (yes, I know its “rough”) that my current CdA is somewhere around 0.280. I ran calculations on all of the data I have where I was in race config, and they spread between 0.278 and 0.282. I’m clearly not 0.350 or 0.190.

I have a road bike setup in a TT type config. Previously posted here:

http://forum.slowtwitch.com/...ost=6421522#p6421522

I was going to embed a race photo, but I can’t shrink them on this computer to meet the 250kb requirement. I will try and update later. In the mean time, race photos are here:

https://www.backprint.com/...tingimage/142662/103#

I’ve got one more Oly race in 3 weeks, so I’m not making any changes until after that. But, what I’m wondering is what a reasonable expectation is for a CdA on this current bike? Since I’m NOT planning on buying a Tri-bike in the next 12 months, I’m trying to set a goal for the fall/winter aero testing…and, am looking for help from the more aero-knowledgeable for what is reasonable. When I get to testing I will use Aerolab in GoldenCheetah, not the rough calculator from Aeroweenie.

Tom - have you used the Aero Analyzer on Best Bike Split? That may be a good place to start.

16

Then none of the above numbers mean anything.

I hesitated to mention the race data, because I figured someone would make this comment. That’s fine.

But as I mentioned in the OP, I do have powertap/speed data from OTHER efforts in race-configuration on a similar course (closed loop, 6.9mi course, 400 ft elevation gain/loss per loop, and similar grades). All of those data points suggest that my current CdA falls in the 0.280 (+/- 0.002-ish) range. I’m fully aware that this is just a ballpark and could be off either way by some amount. However, the data wasn’t meant to be DEFINITIVE, rather just illustrative. I only ran it through aeroweenie to get an idea to the 0.05 precision level (0.4, 0.35, 0.3, etc).

I’ve read the Platypus thread, and as many of the other related/similar threads and external resources as I have been able to find. I’ve been an R&D engineer for 25 years in gaseous-fluid-dynamics related fields, and I’m aware of the need for rigor (baselining, repeatability, etc), and precision required in this type of testing when it comes down to “brass-tacks”.

Also as mentioned in the OP, once I get through my last race of the season, I’m going to spend some time with GoldenCheetah/Aerolab to get some precision data. I have a very good location to go and do that testing, again around the corner from my house: sheltered with trees on both sides, nearly zero traffic, etc. Once I’m past the point where every workout matters (mentally, if not physiologically), I’ll go “waste” a day, several, or weeks/months and start learning what it takes to make reliable/repeatable test runs across a variety of conditions, and then collect a baseline, then start making some changes and see what effects they have.

With full nutrition I’d say that a competitive tri CdA should be <.220m^2. Of course if you don’t have a reliable method to calculate CdA then it doesn’t matter.

Thanks, but that doesn’t answer the question that I asked. There have been lots of threads here and elsewhere reporting results of CdA testing/optimization on TRI and TT bikes indicating that 0.22-0.20 (and in a very few cases, even sub-0.20…maybe even you are one of those, I don’t recall) is achievable. But, I don’t have a TRI-bike; I have a ROAD-bike—and its an old one at that with external cabling, large round tubes, road geometry, etc. I haven’t found any threads that give an indication of how those factors raise the optimal floor from the 0.20-0.22 range on a tri-bike.

So, my question is, “Given those limitations, what is a realistic expectation for CdA, for me, on THIS bike?”

I’m really just looking to have a ball-park goal to shoot for. For other people who have tried to optimize a road-bike for TT/tri…how good were they able to do? 0.26? 0.24? 0.22? In the end, I’m only going to “get what I get”.

  • James suggested that 0.25 would be a solid effort for my config.
  • I’m okay with an answer like GreenPlease gave—“can’t do it…would just be a wild-ass-guess / making shit up.”
17

Thanks, but that doesn’t answer the question that I asked. There have been lots of threads here and elsewhere reporting results of CdA testing/optimization on TRI and TT bikes indicating that 0.22-0.20 (and in a very few cases, even sub-0.20…maybe even you are one of those, I don’t recall) is achievable. But, I don’t have a TRI-bike; I have a ROAD-bike—and its an old one at that with external cabling, large round tubes, road geometry, etc. I haven’t found any threads that give an indication of how those factors raise the optimal floor from the 0.20-0.22 range on a tri-bike.

So, my question is, “Given those limitations, what is a realistic expectation for CdA, for me, on THIS bike?”

I’m really just looking to have a ball-park goal to shoot for. For other people who have tried to optimize a road-bike for TT/tri…how good were they able to do? 0.26? 0.24? 0.22? In the end, I’m only going to “get what I get”.

  • James suggested that 0.25 would be a solid effort for my config.
  • I’m okay with an answer like GreenPlease gave—“can’t do it…would just be a wild-ass-guess / making shit up.”

~.220m^2 is plenty achievable (as long as you’re not really big or tall). The frame isn’t the issue as long as you can get the position (you’ll likely need to ditch the bouncy fork).

18

Tom - have you used the Aero Analyzer on Best Bike Split? That may be a good place to start.

No, I haven’t. I’m aware of it…I do get the BBS emails all the time from training peaks. My first plan was to use the AeroLab tools in GoldenCheetah simply because more info seems to be publically available about it vs. BBS—All the public posts from Robert Chung, Andy F, Tom A and the like.

That said…I actually was going to load my race data into BBS. I was quite annoyed when the powertap stopped working for that reason (that and the fact that I was going to have to pace on RPE/HR alone). I was really wanting to play some of these what-if games with that data. Alas…

19

(you’ll likely need to ditch the bouncy fork).

Well…that’s an interesting thought. As near as I can tell that would drop the head tube by about 2cm. Its hard to tell graphically, and real drawings are hard to come by. It might be closer to 3cm.

I’m aware there are bearing reducer kits to convert to a standard 1 1/8 steerer tube. the final drop would also depend on the exact configuration of the reducer kit. There seem to be several options, depending on exactly which Headshok configuration I have…each have different stack effects.

Dropping the head tube 2-3cm would steepen the head tube angle by between 1.2-1.7 degrees…starting from a stock angle of 73deg. This would also reduce the trail. That, of course, is all going the wrong way from a handling perspective. I’ll have to do more math later and see what aftermarket fork geometries are available…

…and read up more on steering geometry effects of head tube angle and offset. That’s not something I’ve ever spent a lot of time on, but I wouldn’t want to create a dangerous handling situation.

But, its certainly an interesting thought to ponder. thanks.

20

If you end up going new fork route: I have a version of the cervelo chord fork. 10" steerer tube. Sell it for what I paid $60.