Cda Adjustment for different positions

[arjunmurali1993]

Hi,

I've recently purchased the Powerpod and I have to say, I've been very impressed with the device and the Isaac software and the features it offers.

I've calibrated the device pretty well and used an O&B ride to dial in the calibration as well. However, I did the O&B ride completely on my hoods and then, yesterday, I went with our group and we did a 2-hour paceline ride with each of us taking a 60-90 sec pull at the front. I saw that the power values were abnormally high and figured that it was due to the fact that I did this ride completely on the drops whereas I had calibrated the Powerpod for the hoods position. (Another rider with a similar height and weight as me, mentioned that his Stages showed a Normalized power of 202W for the 2 hour ride. His FTP is around 15W higher than mine. My FTP is 225W) I know the Cda or Aero factor has to be adjusted in the Profile setting in Isaac, but I'm not sure what value to plug in there. Could someone help me with the values, please?

I would also appreciate it if you could give an approximate range of values for each riding position as well so that I can have a separate profile for each one and set it appropriately before each ride. Thanks!

I'm attaching the Profile, the O&B ride file and the Paceline ride file.

[Velocomp]

When riding in the drops your CdA goes down by about 5-6%--to about .306 in your case.

Also, you're likely riding on racing tires, which have a lower Crr of about 0.004.

I've created a Drops profile with these two changes, attached. Also attached is your 97KM ride file with these factors applied.

You can load the Drops profile into one of your other PP profiles (2,3 or 4). Because you're using the same bike, you'll need to manually set the profile number in Isaac.

So, for example, if your ride today is on the hoods, using Isaac manually select profile 1 before starting your ride. If your ride is in the drops select profile 2 (assuming that is where you place the drops profile).

[arjunmurali1993]

Hi,

Thanks a lot for the prompt help. Really appreciate it

I've set up the Profile you had sent as Profile 2 and yeah, I'll set it using Isaac if I'll be riding on the Drops.

Could you also help me out with the Cda value if I use my TT bars and am in the TT position? We do quite a few ITTs and it would be great to have the ITT profile as my Profile 3.

Thanks and Regards,
Arjun

[arjunmurali1993]

Another thing that I just noticed in the profile that you sent was that the Wind Scaling has been set at 2.889.

I saw in another post on this forum that for the default handlebar mount, this value should be around 1.4. Should I tweak this value?

[Velocomp]

I recommend you use Isaac to select Profile 3 for your TT bike, then use the command "Device/Setup Newton..." to enter the parameters for your TT bike, including the CdA that is correct for your weight, height, and size.

You'll have to do a calibration ride and sensor pairing for your TT configuration. And, if you're using the same sensors for your TT bike, you will need to continue to use Isaac to choose manually between profile 1, 2 and 3.

[arjunmurali1993]

Thanks John. I'll do that when I add the TT bars on my bike.

Another thing that I just noticed in the profile that you sent was that the Wind Scaling has been set at 2.889.

I saw in another post on this forum that for the default handlebar mount, this value should be around 1.4. Should I tweak this value?

Also, should I just leave the Wind Scaling at 2.889?

[Velocomp]

Yes, leave the wind scaling where it is. It has been customized for your bike.

[Sergioperi]

Good day,

I have recently bought a Powerpod. I will calibrate it this week and I will do the out and back ride to check calibration step. But I have a question reading this post: I usually ride in both position (hood and drops) during a normal ride. Could I use a unique profile propertly?

Thanks.
Sergio Sanchez

[Velocomp]

Good day,

I have recently bought a Powerpod. I will calibrate it this week and I will do the out and back ride to check calibration step. But I have a question reading this post: I usually ride in both position (hood and drops) during a normal ride. Could I use a unique profile propertly?

Thanks.
Sergio Sanchez

Set PowerPod to the ride position that you use the most often.

[Mshynson]

There's a HRM that measures chest angle/rider position. Could Isaac/powerpod use that data to determine cda?

[Velocomp]

There's a HRM that measures chest angle/rider position. Could Isaac/powerpod use that data to determine cda?

Possibly; please provide details about the product...

[Mshynson]

I've mentioned this to the owner of the company, he said he'll contact you.

[Velocomp]

We're on it!

[Sergioperi]

Good day,

I have recently bought a Powerpod. I will calibrate it this week and I will do the out and back ride to check calibration step. But I have a question reading this post: I usually ride in both position (hood and drops) during a normal ride. Could I use a unique profile propertly?

Thanks.
Sergio Sanchez

Set PowerPod to the ride position that you use the most often.

Thanks!

[Mshynson]

We're on it!

[pedrok]

Hi,

I've set my PowerPod profile to assume that I'm riding on the hoods with elbows bent. I tend to ride on hilly terrain, and while I'm climbing this is my typical position. However, when I descend (or ride the flats), I'm usually in the drops. Based on other replies here, it sounds like the compromise solution is to pick the profile where I spend the most time, but if you're climbing a lot that means that *half my mileage* will have the wrong profile either way.

Simple fix: Would it be possible to add a feature that assumes drops if I'm over XX mph? Basically, if I'm under 17mph I'm on the hoods. If I'm above that I'm in the drops. I'd assume this goes for a lot of riders, although the cutover speed may vary.

Thanks!

[Velocomp]

We love this kind of thinking: it's innovative and practical.

Here are some questions to consider:

1) if your speed increases above "x", is it because you're on a downhill? If so, how steep is the downhill?

2) How does PP "detect" that your increased speed is a result of more a aero ride position, as opposed to just hammering on the pedals?

3) Is "increased speed = changed riding position", a general statement, or is it just something that applies to only a smaller set of riders?

We are tremendous advocates of this kind of practical thinking: it combines physics with real-life behavior.

In summary: what, specifically, is your proposed "algorithm"? And how general is it?

We are all ears!

[pedrok]

Great questions.

1) If my speed increases above "x", it's not necessarily because I'm on a downhill. Certainly if I'm on a downhill -- pretty much any grade -- I'll go to the flats for both aerodynamics and for handing. But in general I tell myself not to sit on the hoods if I'm over, say, 17-18mph. My opinion (which may not be well informed) is that somewhere around that speed the cost of riding on the hoods becomes too high to justify the added comfort (or, for climbing, the better position). So for me it's more based on speed. (Also, if I were in a paceline where my aerodynamics don't matter as much and my speed is 25+, I wouldn't be on the hoods -- much more safe to ride in the drops and with hands on the brakes).

3) Per the above, I'm not sure if increased speed = changed riding position for everyone. But I would think there's definitely a correlation? I don't think many guys going 25+mph on the flats are on the hoods (if they are that strong they probably are also experienced enough to be thinking about their aerodynamics). I'd also guess that few people doing 10mph or less are riding in their drops. But to be safe, you could just make this an expert-level customization in Isaac.
One edge case is serious wind: if I'm facing a 30mph headwind I might actually ride in the drops at 10mph! But perhaps this can be part of the cutover speed definition: bike speed + headwind (e.g. with a cutover of 18mph, I'll be in the drops if I'm riding 18mph with no wind, or 16mph with a 2mph headwind).

2) I'm not sure how PP detects that increased speed is related to changing aero position as opposed to hammering more. I think the safest change would be a setting in Isaac where you can specify the "cutover" MPH between two postures, if that's how you ride. If that's not how you ride, or you think it's a lot more variable, then you can pick a single posture.

That said, if you wanted the magical auto-detect function of "change of posture" then you have a killer feature! It not only "solves" this but you could do some amazing post-ride analysis. This is beyond my pay grade, but could you do some analysis of *immediate* changes to bike speed compared to *immediate* changes to wind speed? I'm imagining what happens if I'm riding and I unfurl a parachute behind me. If there's not any change in external wind (i.e. no gusts), my bike speed will immediately decrease. No matter how quick I am to react, it will take hundreds of milliseconds, probably seconds. After a second or two you can't really tell what happens -- I'm just riding more slowly and you don't have any other data. But the change might be a clue? The question is what else could have caused that deceleration. Brakes are most likely. I'd guess that for changing aero positions though the accelerometer data should be different for brakes (sudden; likely other speed changes soon) and change of position (less sudden than brakes, and likely a PREDICTABLE change in speed).
More fundamentally, I can't think of what other sensors would help in this case. A little chip that put on your helmet that measures distance to the PowerPod (via signal strength I guess)? Or some other sensors that rule out the other possibilities? Happy to continue pondering this, it's a fun problem to think about...

[BBQcyclist]

I agree. Interesting questions indeed!

Additionally though, let's not forget - for many (or most) cyclists there will likely eventually be a tradeoff between (short vs long-term) maximum aero position / speed / power / stability ... vs. maximum capacity to breath efficiently / recruit cycling-specific muscles efficiently vs. comfort (e.g. hoods vs tops vs even standing pedaling...) when riding on flat roads, up or down hills, or against strong winds.

This guy is a cyclist and a mathematician, and has some interesting youtube videos using PMs (albeit possibly slightly technical, but generally easy to follow).
Checkout his video on the subject of pacing efficiently on climbs / descents using physics/science and a PM.

He covers some of the notions re. differences in power when riding hoods vs drops vs standing-pedaling - starting around minute 13 - until around the 17 minute mark.
Enjoy (particularly if you're a geek for this kind of stuff)!
https://youtu.be/HlglnUNnAnY

[pedrok]

Thanks, that was a great video.

Ironically, I did face 20mph winds today and found myself in the drops at 10mph (bike speed). So any feature ought to incorporate a cutover that includes bike and wind speed.

As a workaround for now, I'll change my default position to the drops. I believe this compromise minimizes the errors: if I'm climbing, assuming the most aero position will cause only a small under-reporting error, but if I'm hammering on the flats or a descent the readings should be more accurate. To whit, I saw abnormally high power in a relatively mild descent (with a nice tailwind), which I'm attributing to the PP assuming I was on the hoods and not in the drops.