Given the questions a lot of people have with discrepancies of indoor/outdoor power values, trainer speed/power curves etc
Are there a lot of people here with power meters who also own the popular indoor trainers ?
Can any volunteer to do a basic calibration for others who don't have power meters ?
Eg, for a given trainer just give the wattage for a range of speeds eg between 15,16,17-30mph etc Some kind of wheel tension calibration would also be required, eg tire pressure and spin down time from a known speed eg 20mph.
Given the questions a lot of people have with discrepancies of indoor/outdoor power values, trainer speed/power curves etc
Are there a lot of people here with power meters who also own the popular indoor trainers ?
Can any volunteer to do a basic calibration for others who don't have power meters ?
Eg, for a given trainer just give the wattage for a range of speeds eg between 15,16,17-30mph etc Some kind of wheel tension calibration would also be required, eg tire pressure and spin down time from a known speed eg 20mph.
The numbers will not be very useful. First, there is power/speed drift over the course of an interval. Second, the power/speed relationship is a function of the specific tire and not just tension and tire pressure. There's a thread around here somewhere where I did what you're talking about. I changed tires in the middle of the calibration data collection and the new tire was significantly different from the old tire at the same speed. Three, there is likely a slight difference from trainer to trainer (even with all other factors constant).
Given the questions a lot of people have with discrepancies of indoor/outdoor power values, trainer speed/power curves etc
Are there a lot of people here with power meters who also own the popular indoor trainers ?
Can any volunteer to do a basic calibration for others who don't have power meters ?
Eg, for a given trainer just give the wattage for a range of speeds eg between 15,16,17-30mph etc Some kind of wheel tension calibration would also be required, eg tire pressure and spin down time from a known speed eg 20mph.
Forget that with the Flow, unless we try either a) with full blown pressure applied against the tire or b) with as little resistance as possible (bah! that's not even an option because of the tire slip).
There's an issue in trying to apply as much pressure as possible. To do that you need to bring the screw really near its limit. It can come off anytime during the workout and it takes about 5 min (at least for me) get it to work again.
By this mere parameter (pressure against the tire), I can bring my Flow to actually either underestimate or overestimate power (compared to powertap). 280w can either be 280 or 220 depending on that.
Quite frankly I'm not dismayed by the fact I don't have an on-the-bike power measuring device. My indoor trainer that measures watts is satisfactory for what I need it for - proper training to increase fitness. From what I've seen on-line, indoor trainers overstate or understate wattage results compared to on-the-bike products. I know there are many in this forum that look down their noses with regard to the merits of indoor trainers as power measuring devices, but that is their issue - not mine.
Based on this on-line forum, I have found individual comparisons, even between different users with the same on-the-bike power meter, are of little value due to the myriad of variables affecting power results. Basically, my belief is that the only reasonably valid comparison is one's current workout result with his/her past workout result keeping all other variables constant. This is the only comparison that concerns me now...
It's all relative...but not all of it's relevant...
Second, the power/speed relationship is a function of the specific tire and not just tension and tire pressure. There's a thread around here somewhere where I did what you're talking about. I changed tires in the middle of the calibration data collection and the new tire was significantly different from the old tire at the same speed.
Didn't check it, but I know the resistance was the same because of the way resistance is set on that trainer. It's a stepped lever adjustment and you can leave it set between rides.
Didn't check it, but I know the resistance was the same because of the way resistance is set on that trainer. It's a stepped lever adjustment and you can leave it set between rides.
Actually, your description makes it reasonably clear that the resistance wasn't the same.
Actually, your description makes it reasonably clear that the resistance wasn't the same.
Well, the only variable under my control that was different was the tire. So, if the resistance changes from ride to ride without changing the resistance setting, then any power/speed numbers would be completely worthless.
You're saying the rolldown time isn't under your control?
I'm saying that on this particular trainer that I can't make fine adjustments in resistance. I haven't changed resistance in a year. I always pump my tire to a constant pressure, so it should be fairly consistent from ride to ride. But, I don't care much because I have a PT so I really could care less whether it is exactly consistent from ride to ride (because I manage power with my PT). I did the testing as an accommodation to one of the forum posters who was interested in the power/speed relationship on that specific trainer.
If you can't adjust the rolldown to calibrate for different tires, that'd be a pretty serious design flaw. What trainer is this?
It's the Tacx Swing, the one the OP of the other thread has (which is what prompted me to do the testinig described by the OP of this thread). It has adjustable resistance, just not a way of making fine adjustments to resistance. I got it primarily for pre-race warmups and it meets my needs just fine. It has a bit of drift as it heats up, but this is common if not universal. It's not nearly as bad as some other models according to reports. It's not too noisy and one resistance setting gives me the full range of power for everything but L7s. I think trainer requirements are different if one doesn't have a power meter and is thus dependent on a rollout test together with fine adjustments to resistance to attain repeatability. There are lots of trainers out there. If I did not have a power meter, I would not buy this trainer. In fact, even with a PM, I would not buy (or recommend) this trainer again. I would buy (and recommend) the KK Road Machine (or Pro) because I think it is a better trainer.
is rolldown time a truly good indicator of resistance setting ?
No, but we're talking about rolldown in order to calibrate between tests. For that type of comparison, it's essential. For a trainer, there are only two sources of drag: from the trainer's resistance unit or from the bike. All of the bike drag forces get transmitted through tire-to-roller contact so if you can control for that then you know everything else is from the trainer.
Hmmm. I don't have a Swing but I do have a Flow. The Swing doesn't have a black screw knob under the red press-on lever to adjust the tire-to-roller pressure?
Hmmm. I don't have a Swing but I do have a Flow. The Swing doesn't have a black screw knob under the red press-on lever to adjust the tire-to-roller pressure?
You're right, there is a screw knob there. I have never adjusted it. In fact, I haven't changed resistance in over a year. Actually, I thought about taping it in place, then decided, "Why bother?" I don't use speed to manage power. All I care about is that the thing isn't too noisy, resistance is reasonably consistent from ride to ride (so that I can ride at a given intensity in the same gear) and that it doesn't drift too much during the course of a 20-30 minute interval. It meets those criteria, so it's fine for my needs (even though I wouldn't recommend it and wouldn't buy it if I were buying a non-ergo trainer again).
"The Kinetic PC has a rear wheel sensor that picks up speed off the rear wheel and then converts speed to power using the formula provided by Tom. The end result is an accurate estimation (+/- 3% of a PowerTap reading) using some very simple and inexpensive hardware. "
"The Kinetic Road Machine power formula is accurate for all riders because the variables experience on an outdoor ride are controlled indoors. We are so confident in the accuracy of the Kinetic Trainer; we encourage you to try this test. Complete a 20 minute time trial on the trainer using a rear mount bike computer or Kinetic PC (Power Computer) Complete a similar ride on a flat course with a neutral wind. The distance covered will be within a range of + /- 3%. "
RChung said:
Hmmm. Where on that chart does it say the PT derived KK formula work so well?
You're right, there is a screw knob there. I have never adjusted it.
The screw knob is used to adjust the tire-to-roller contact in order to calibrate the rolldown. If you've never adjusted it, I'm not surprised that the observed power-speed curves differed when you switched tires -- in fact, I'd've been surprised if they hadn't.
