Turbo trainer design?

On Thu, 22 Jan 2004 21:21:35 +0000, Zog The Undeniable <[email hidden]>

Quoted message said:

There probably isn't an answer to this, but here goes. I have a traditional Tacx Speedbraker turbo
trainer - one of the kind where the resistance is provided by a fan, and it's only used in the
garage because it is *very* loud. It's simple but it's good for getting fit quickly and for
maintaining condition at this dark and wet time of year. The power required increases with the cube
of the speed, just like air resistance when riding.

Now using a given gear combination and pedalling cadence I can work out the speed of the back
wheel. For instance, at my anaerobic threshold (HR of 170-ish) I can just about turn 52 x 17 on
a slick 26" tyre, which at 100rpm equates to about 23mph. (Yeah, yeah, I'm not exactly at race
pace yet.)

However, I don't know how the size of the fan is determined. The turbo "feels" a bit easier than
riding on a flat road, where I'm not sure I could actually maintain this speed without drafting
another rider.

Do turbo manufacturers generally specify fan size on the assumption that the rider wants to feel
the reduced resistance that would be given by riding "on the drops" or on tri-bars?

Check this out: geocities.comtrainerpowerOpen ↗

This guy used his Power Tap wheel to plot power needed for various speeds on different trainers.
No doubt there are undeclared variables (Tire pressure? Roller pressure?), but it gives you a
rough idea.

You can see that many trainers appear to follow the cube shape curve of real life, some with
generally greater resistance, some with generally less.

With the exception of the old Vetta squirrel cage trainer, the fan units are generally pretty good.

dianne_1234 said:

Check this out: geocities.comtrainerpowerOpen ↗

This guy used his Power Tap wheel to plot power needed for various speeds on different trainers.
No doubt there are undeclared variables (Tire pressure? Roller pressure?), but it gives you a
rough idea.

You can see that many trainers appear to follow the cube shape curve of real life, some with
generally greater resistance, some with generally less.

Thanks - that's an interesting graph. Maybe it's the rough roads round here that make it hard work!

Zog The Undeniable said:

Now using a given gear combination and pedalling cadence I can work out the speed of the back
wheel. For instance, at my anaerobic threshold (HR of 170-ish) I can just about turn 52 x 17 on
a slick 26" tyre, which at 100rpm equates to about 23mph. (Yeah, yeah, I'm not exactly at race
pace yet.)

Quoted message said:

Do turbo manufacturers generally specify fan size on the assumption that the rider wants to feel
the reduced resistance that would be given by riding "on the drops" or on tri-bars?

You're giving them way too much credit!

The manufacturers try to get a somewhat realistic correlation between "speed" and power, but it
rarely matches actual road riding.

I have an original "TurboTrainer" from about 20 years ago. Riding at a given speed on that was
*much* easier than on the road. I added a second "Racermate" fan which attaches to the seat
post (the fan contacts the top of the wheel). With both sets of fans providing resistance, the
effort required to ride at about 17-21 mph seems realistic (or maybe even a little harder than
on the road).

So it depends on your particular trainer. I guess you could use a HRM to calibrate the trainer to
equivalent road speed (e.g., 20 mph on the trainer equals 18 mph on the road, etc.) But if you're
training by heart rate, you don't really have to worry about "speed."

Art Harris