225w on a trainer = ___X___km/hr on the road?

[jdd]

Hello,

My first post here on the cycling forums (long time lurker), I guess my question runs something like the following.

Thru this winter I've been using a stationary bike machine in my school gym. It's an old one--not much info on the readout. But I do 30-40 minutes of the following, 4 minutes of pedaling followed by a 30 second break. Rinse and repeat for seven-ten times. All pedaling is at the same resistance level. The machine does give a watts readout after each 4-minute duration of pedaling. It varies a little, but is usually in the 220s, occasionally into the 230s.

Trying to compare that effort with my riding experience, it seems like about 225w on a machine like this would equate to about 28km/hr (of course trying to think of it as happening on level ground, no wind).

I'm sure there are other variables, e.g., the crank arms on the machine are certainly shorter than the ones on my road bike, and even something like body weight might make a difference here. I'm not looking for a hard & fast rule (such as watts-volts-amps), but if there is some such reliable conversion, then fine. Really tho, I'd just like to see if my above impression rings true with the experience of others.

Looked at from the other side, I've only ever used basic Cat's Eye cycling odometers/speedometers, and so maybe someone using a HRM and other electronics may be aware of their average output in watts for certain riding conditions.

Finally, tho I've looked thru searched the forum some, I haven't found anything connected to this. But if there is something connected to this out there already, I'd appreciate being directed to it--a link to the thread.

tia,

John D.

 

[dhk2]

There is no conversion factor, but there are various "bike calculators" on the web which try to estimate the relationship of power and speed based on "standard" assumptions. Here's a simple one: Bike Calculator that should get you in the ballpark, say within a couple of mph. Note how much difference the position selected makes in the speed. As we know from the road, getting down in the drops, tucking in knees, elbows and head really does make a difference.

Note there are a lot of assumptions used, e.g. cdA (your aero profile), air density, tire rolling resistance, road surface smoothness, drivetrain efficiency, lack of gradient and wind. Other "calculators" can be found online which allow you to input more factors, but unless you have specific data, they may not be any more accurate. The difficulty of getting an accurate estimate of speed vs power explains why so many serious trainers buy expensive powermeters for their bikes.

 

[jdd]

dhk2-- thanks for the quick response!

Yes, that calculator looks like it'll give me what I was looking for, and also let me see what other variables I should be considering.

I'm a little older (58), and by spending some time on the machine in the gym was hoping to hit the ground in better shape than usual when spring weather sets in. This is the first year that I've tried using that, and was just wondering how time on it would compare to some of my usual times/distances.

 

[dhk2]

Also remember that the power reading from the trainer bike isn't exact either. Even if it had an accuracy of +/- 5% when new, it could be off more than that now. Of course, as long as the load is consistent, the number on the display really doesn't matter for assessing your progress.

I'm doing a lot of indoor training now too, using a "Startrac Pro" trainer at the Y. We're actually getting a winter this year here in the southeastern US. In past years, I've actually complained about the great sunny weather not allowing a winter off-season for rest and recovery, but not this year......it's snowing again this morning

 

[jdd]

Yes, margin of error, definitely worth being aware of... The calculator link you provided is giving me rather different numbers than I expected. Apart from the apparently difficulty of figuring this out in the first place (possible error in the calculating), the machine I've been using is quite old, and I doubt that it's ever been calibrated. Probably the only service it's gotten is just to keep it operational. But it's one of two that are designed like a road bike--toe clips, drop bars, saddle--vs a half dozen newer ones that have big platform seats, clipless pedals, and a very upright position.

***

Huntsville, eh? In '74, I spent my last year in the army at the airfield on the arsenal. I wasn't riding then, but can imagine that roads/hills SE around the dam/lake (or even just the back side of Green Mtn) could be wonderful. Some really nice sunsets, and great plate lunches, too.

 

[dhk2]

The calculator seems about right to me, but I'm sure you can find others that will yield different results, or you can play with your own calculations. The formulas for aero drag and rolling resistance aren't difficult; the factors you use for frontal area and drag coefficient are the big "unknowns". All the calculators show the importance of being in the drops and tucked in.
Regardless of the numbers, if you're doing decent work on the trainer you'll be ahead on the road when spring arrives.

Yeah, lots of hills here. I'm looking over to (snowy) Green Mtn now from my kitchen window. The "front side", from the west isn't easy either, starting out with a steady 15%, and averaging 13% for the next mile. I don't ride it too often because of the guardrails right at the edge of the narrow lane and the poor sight lines on the climb up. There is a bit of traffic there now with the subdivisions going in, and I'm paranoid about not being able to move off the road for the dump trucks

 

[Bikemech]

Theres no real answer to this, as there are way too many variables to calculate this... Offcourse, you can use one of the bike calculators, but it will never be the same. Also, if you are interested:

750W=1 horsepower ​