OT: Kreitler power curve

[sweet spot]

Just did an FTP test on my Kreitler rollers (2.5 drums) and I'm doubting the results.

There's nothing wrong with the iPro. It has given consistent readings in 'trainer' mode. At 18 mph, for example, the power meter always shows 240 watts.

I'm just wondering whether the power curve, which I assume is given by the manufacturer (Kreitler), is correct. This may be a little off topic, but has anyone independently tested the wattage produced on a set of Kreitler rollers with 2.5 drums?


Thanks,
sweet spot

[travispape]

Unlike trainers, most of the resistance when riding on rollers is due to the rolling resistance of the tires on the drums. Since the amount of rolling resistance depends on the tires, inflation, and weight of the rider, it is not uncommon for the power curve of rollers to have more error than a trainer curve would.

There are a couple of things you can do to minimize the errors. If you have them, use the larger drums and the fan. The fan resistance is not a function of weight, so if it provides more of the resistance, it will reduce the error due to the rolling resistance. Also, using the larger drums will reduce the relative contribution from rolling resistance.

Better yet, if you can borrow someone's DFPM power meter, you can calibrate the rollers yourself for your setup.

Otherwise, you can tweak the power curve you use by adjusting the B coefficient for a better power curve result.

Travis

[rruff]

I think the fact that your computer always shows 240W at 18 mph is because it's reading is tied directly to speed... and nothing else.

If the rollers have no fan or mag, then the resistance will be highly dependent on the rolling resistance coefficient of your tires. This could vary by 100% over the range of best to worst tires, so there is a lot of margin there. What tires are you running, and is the power reading too high or too low?

[sweet spot]

Great responses.

Hadn't thought of the rolling-resistance factor.

I'm using 700-25 IRD tires. I'm 150 lbs. Total weight: 174. And the FTP seemed too high. Went up from 283 (outdoor test) to 310 (today on Kreitler). I've been training smart, and I felt fresh, but I'm going to stay skeptical about this one.

I wish I could set an indoor FTP at 310 and an outdoor FTP of 283, but that would throw off my Performance Management Chart.

Again, thanks for the feedback.

[rruff]

So that probably means that the IRD tires are good as far as Crr goes. You are pretty close on the two numbers actually... maybe just apply a 10% bias to the number you get on the rollers.

[doofus]

I'll second the DFPM recommendation. I've done that for trainers to create a spreadsheet. )Ou could then manually enter the data into WKO+.

[sweet spot]

I've tried, unsuccessfully, to find the answers.

A little help, please.

After I ride the rollers with a DFPM, how do I get the data from the Power Tap to the iBike III software? I've seen the coefficient tab in the 'edit trainer' section. Do I start there? Will the iBike software 'connect' to a Power Tap or do I download it to WKO and then pull it up in iBike? If this is spelled out somewhere, please tell me where to fetch the information.

Thanks.
sweet spot

PS: I double-checked my settings in the 'edit trainer' section, and I think I found the culprit for my artificially high power readings. I had selected the Kreitler rollers with 2.5 drums and hit the 'fly wheel' field. There's just one problem: I'm not using a fly wheel. Duh! Maybe my call sign should be doofus. Today, I did another 20-minute test and ended up with an FTP within six digits of my last one. (Unfortunately, they were six to the down side).

PPS: Although I think that the power curve is now adjusted correctly, I'm still planning to run a DFPM on it.

[travispape]

After I ride the rollers with a DFPM, how do I get the data from the Power Tap to the iBike III software? I've seen the coefficient tab in the 'edit trainer' section. Do I start there?

Yes, that's where you can import the PowerTap data. First, download the PowerTap ride file using Power Agent and export it to a text file.

Then in iBike3 in the trainer coefficients tab, click the "Fit to DFPM Ride File..." button to browse to your DFPM tab. Just click "Force through Origin" checkbox, fill in the manufacturer and model information, and click accept.

Travis

[sweet spot]

Just as I was almost believing my own lie that indoor workouts aren't so bad, the weather here in NC turned 70 and perfect. I went out with five teammates and did 50 miles today, with a trip up Pilot Mountain. Average gradient: 8 percent, with sections at 17 percent. I clocked a personal best, and I have to give credit to the ibike. The whole way up, it flashed messages like, "Come on, you lazy bum, you're not even five beats above threshold!" Seriously, it was my first time on the mountain with my ibike; it helped me pace myself and shave off two minutes.

Of course, the Kreitlers are in exile --- at least for the weekend. When the cold weather comes back, I'll try that DFPM test. Thank you for the instructions.