A watt is a watt is a watt? Trainer vs. Road Power.

joule

Now I've only had the PT SL for a week and I know what everyone will say, but I've done a few sessions both on the road and on the trainer, holding a steady heart rate at my tested LT over a 10+ minute interval and consistently showed lower power numbers on the trainer. Possible reasons? If it's a motivation issue, then I've syched my heart out?!

frenchyge

What's everyone gonna say? Something crazy like "higher body temps indoors because of less airflow produce higher HR's," or "daily HR variations or adrenaline could explain what you're seeing" or "HR is cadence dependent, so make sure your cadence is the same," or "why the heck are you still worrying about HR, anyway?"

You know us too well.

joule

LOL! Well after a year of watching my heart rate monitor I guess it's a hard habit to break. It just seems so much easier to hit my power at threshold on the road compared to doing the same on the trainer. I could see having to adjust a training session depending on which I'm doing. Right now however it's not much of an issue since I've been focusing in on 5 sec power on the road to build up strength and work on technique. On the trainer it's strength exercises, mostly big gear work that won't slip my wheel.

frenchyge

I posted a while back, and got some confirmation that it does take a little while to get used to riding on a trainer because of the slightly different feel. http://www.cyclingforums.com/t27984...d-exertion.html
Are you just recently starting back indoors?



I almost mentioned in my previous list that wheel slip can cause a slight drop in trainer power. I notice that when I get out of the saddle or hammer the downstroke and start hearing the chirp-chirp-chirp, that my overall power drops ~5%.

Weisse Luft

inertia. Look at the sampling rate of the wheelspeed...since it is far easier to accelerate the wheel in the trainer, the velocity variations of the wheel are much greater than when on the road, propelling your combined mass. The sampling rate of the wheelspeed being only once per revolution cannot capture these accelerations.

It matters not one bit that you turn the resistance up, resistance is damping, not mass. And the piddling flywheel effect of all trainers does not match the road.

The same thing also affects the variations between gears since increasing mechanical advantage can significantly increase the acceleration rate in a manner that will not be captured by the roadspeed indicator.

Increase the sampling rate of the wheelspeed to 6 times per revolution and the differences will decrease but alas, this is not possible on any system.

frenchyge

You sure about that? At 20mph in a 53x19 gear @ 87 rpm cadence, the rear wheel is spinning 2.8 times per crank rpm. That's 1.4 turns of the rear wheel during each downstroke alone and certainly seems like enough sampling to average out the brief accelerations during each push.

Not to mention that he's measuring everything using his Powertap hub rather than a wheel magnet.....

Weisse Luft

The problem is regardless of how the averaging is taken, it does not represent the actual integration of the power function, leading to variances between road and trainer.

The variations are from sampling rate, not from measurement technique. This is true in all models. Some have claimed the variation in the Polar unit is from the shift in the chain position relative to the sensor. How so? The Polar sensor measures vibrational FREQUENCY, not AMPLITUDE. The FFT is not at all hampered by amplitude...the transform converts to the time domain...its DIGITAL for all purposes. There isn't enough data logging in any unit we would want to carry to capture the exact power log so we must be satiated with averaging.

rmur17

have you studied this in detail? What is the magnitude of the error you mention? How does it vary with wheelspeed? Any data??

Wrt. to the SRM for example which measures power at the crank, are you saying it's affected in the same way?

rmur

Spunout

I'm with you there. It is either inside motivation, the cold weather, or my fitness taper towards my off-season. I cannot get the same wattages inside as I could outside, even in the same week. Less than 10% drop on a FT effort, more on sprint efforts as the bike is locked in (Fluid 2 Trainer).

About the conversation above, I believe a trainer is more difficult because there is not any inertia with the Fluid 2. You really have to pedal around the clock. Stop pedalling outdoors, you keep gonig. Stop pedalling on a Fluid 2, you're stopped in 2 seconds. This is my excuse for dropping 20 watts FT for indoor sessions.

frenchyge

That's true on both the road and trainer, so I don't see how it explains a difference between the two. The same averaging interval is used in both instances.

Yes, the rear wheel accelerates quicker on the trainer, so you'll have a cyclical velocity profile which could be seen with a higher sampling rate. On the road the interia of the bike and rider provides greater resistance against the acceleration, so the velocity is more steady, but you have a cyclical torque profile reflecting the downstrokes and deadspots of the pedalling motion against the higher inertia. Since the power measurement is a product of torque x velocity, how does this create a gross difference in power over time on a trainer vs. the road, and what does it have to do with the sampling rate?

Let's forget about the Polar unit for now. I agree that if parts are wiggling around then there's certainly room for error there. I'm interested in any data you have that shows a gross power error on the trainer caused by sampling rate. In contrast to wheel speed, which is measured once per revolution, I'm pretty sure the Powertap and SRM power "readings" (ie, the display updates every second) are taken from an integration of the power function over that 1 second time interval. I don't believe they are 'snapshots' of power taken each second.

frenchyge

It's a valid excuse. The trainer requires different muscle action and timing, so there's no reason to expect that you'd be able to perform equally well under different circumstances without an adjustment period.

jhuskey

Wow, you guys are far more detailed about trainer vs road than I.
I ride on the road when possible and know that I will not perform as well in cold weather as in warm.
The mag trainer that I use is basically consistent and the only comparison I use is to try and out do my last session on it by calories burned, or on a bad day at least match my best.
It works for me and I don't stay up nights worrying about the comparisons.

__________________
Whenever I can't get excited about riding I just fantasize about someone else's bike.

frenchyge

If all of a sudden you weren't able to burn nearly as many calories as you had just a few days ago, wouldn't you be curious about the reasons for your suddenly impaired performance? That's really all that's being discussed here.

During race training, unexplained performance drops can really hurt a person's motivation, so it's helpful to understand why the observed results are suddenly different from what was expected.

jhuskey

Actually he was asking why the difference between road and trainer and, as I read into it, no drop in either one individually just a difference between the two which may mean nothing at all.
I have a good track record myself, over a long period of time, so I know when I am having a bad day.
If he was referring to impaired performance I obviously misread the text, but my point is that I never compare two different types of training I observe them both separately.
There is no substitute for road training, but an indoor trainer is a lot better than sitting on your butt all day ,by far.

__________________
Whenever I can't get excited about riding I just fantasize about someone else's bike.