FTP test - Indoor trainer speed to power conversion?

[Deleted User]

Hey all,

I don't have a fancy power meter type gadget, so I guess I'll have to have resort to other methods to determine my FTP. Instead, I have an indoor trainer (CycleOps Fluid2) and a relatively simple Cateye on my bike (rear wheel sensor) so I can track my average speed. Would this be an acceptable way to do a 2x20' test of my FTP?

Of course, we want some sort of number to come out of this, and it seems that I can somewhat reliably estimate my power output using empirical data. Here's a link that describes a reasonably good conversion between speed and wattage on that trainer. Note that the numbers work out remarkably well, seeing that the trainer was calibrated by the manufacturer such that 25mph corresponds to 400W.

I'm guessing there will be some systematic error to using this method. My FTP measured using an indoor trainer will probably be slightly lower than if I did the test in outside conditions due to things like wind and temperature, but it's the best I can think of.

 

Peter Greagg

Hi Ansom — you don't say whether (or not) you are going to buy a power meter? If not now, you can use heart rate to judge your intensity. See here http://members.endurancenation.us/Resources/Wiki/tabid/108/Default.aspx?topic=test+our+fitness

Cheers
Peter

kurshing

Have to use HR and/RPE if not power.

Speed, on a trainer, can be a decent metric, but from ride to ride, simply hanging the tension on the rear wheel changes the speed. However, once you are 'in' a ride, you can peg a speed to hang out at for those FTP intervals if you want to - I don't see a problem with that; as long as you see what HR or RPE correctly corresponds.

GMAN 19030

I wouldn't concern yourself with a speed number for the indoor trainer. It's a hocus pocus number anyway. Do the HR test and follow the EN protocol to figure out your zones.

jburbatt

Plus eventually it'd be pointless to try and SWAG your FTP this way because you won't be riding outside on your trainer during training or a race. If you aren't going to go the PM route, start learning to train by HR. The earlier you start the better you will be at reading yourself, as learning about HR and RPE and what you can do for a race is highly individual.

William Jenks

What's worse is that you have no way to figure out "85%" or whatever because the resistance/velocity curve isn't linear on virtually any trainer. So, once you have your "FTP Speed", you can't really use it to accurately judge the intensity of other workouts.

That said, you COULD use it to judge long term progress. The trouble is that the error bar on the measurement is almost certainly larger than the change from test to test, unless you're just starting out and are going to make very big gains.

If you are going to try to do this, you have to be very careful about (at least) two things: the tension of the flywheel/roller/whatever onto your tire, and the tire pressure. Either of those being measurably different will give yo a different power/velocity curve.

astokes30

@Anson...interesting link. I clicked on it and it took me to the article that discussed a bit about the Kurt Kinetic Power Computer ($80) Good for power reading only on the trainer. I have a KK fluid trainer. Has anyone heard of this computer? Would it be worth the money to have some sort of a power # on the trainer only (hope to go to power next year). At this point I have been going to heart rate...curious...

[Deleted User]

Thanks for all the tips everyone! I do agree that using HR/RPE would be a better metric in the long term. I also forgot to mention that I pretty much keep the same bike hooked up to the trainer at all times and use it to log most of my bike miles, so I figured that the consistency of the setup might be useful in determining some sort of power metric.

@William: My intent was to have table in front of me of the speed vs. power numbers (or commit the relevant numbers to memory beforehand for a workout) so I know exactly what numbers I should be hitting. Crude, I'll admit . And yes, I do realize now that the error bars will be substantially larger than any small gains I might make. For example, the difference in wattage (according to the derived formula for the trainer's power curve) between 18.0 mph and 18.3 mph is about 6W, which is around the same size as the error I'd expect using this method.

Al Truscott

Following up on what others have said, two main points:

• The value of a metric (be it power, or HR) is two-fold: to guide training intensities, and to guide race execution. There is no way to transfer the data you get from the speed:power on the trainer to either situation.

• Without a power meter, HR is a perfectly adequate method to achieve the above - many ENers use it successfully, including me up until about a year ago.

This speed:power assumption may be interesting from a curiosity perspective, but don't use it for trainng or racing purposes.