I cannot for the life of me figure out how they are measuring what they claim to be measuring.
Normally in wind tunnel testing, the device under test is mounted to a fixture that has a force measuring gauge built in. Simple, obvious.
In this case, though, they’ve got two bikes mounted on rollers. The rollers for the rear bike are just sitting on top of the turntable, so they’re not mounted to the measuring fixture. Ok, so maybe the bike they’re testing has a power meter on it (in fact, I’m pretty sure it does – you kinda see a battery compartment on the ‘rear’ bike’s crank in the video during the close-ups).
So you just measure power needed in the two cases and compare, right? Sure, except that, because the bikes are on rollers (particularly important on the rear bike which is the one whose improvement they are looking to quantify), the bike is constrained by the rollers in the bike’s fore-aft direction (which is the only direction that matters for the power measurement). So what is the power difference they are measuring come from?
If you look at a case where the bike is constrained by being hard-mounted to the measurement fixtures, the power measured by a power meter on the bike is irrelevant, because it’s not providing the force that’s holding the bike in place against the wind (to put it another way – you wouldn’t have to pedal at all – zero power measured by the power meter – and you could do the test, though it wouldn’t necessarily be the most representative of real riding conditions).
While the rollers are probably not as hard a constraint as having the bike hard-mounted to the fixtures, I still don’t see why having the rider pedaling (aside from being able to stay upright on the rollers) should fundamentally matter in the measurement they’re trying to make. Pedaling at “15mph” on the rollers or “25mph” on the rollers will give you different answers even though the wind speed is the same.
And why didn’t they just do the test the same way they did the first drafting test (with the bike mounted to the fixtures and a second rider in different positions, on rollers)? It would have been a lot more straightforward.
It’s been a little while since I took my fluids class, but you can measure the drag of an object by measuring the static and dynamic pressures in the tunnel before and after the test object. In this case, the object would be both riders and the rollers. I think that’s what they are measuring here.
I think what they’re measuring is the change in pressure at the fan relative to an empty chamber (which you could calculate by measuring the total power drawn by the fans) and then calculating drag from that (as Durk mentioned). ie, the baseline is 2 * the solo guy, so 2 riders drafting is 195% of the solo rider, or 162% depending on the configuration. Assuming 2 identical riders.
It’s been a little while since I took my fluids class, but you can measure the drag of an object by measuring the static and dynamic pressures in the tunnel before and after the test object. In this case, the object would be both riders and the rollers. I think that’s what they are measuring here.
Interesting. Seems a lot harder than just putting the rear bike on the normal fixture and measuring drag force directly with the front rider in the different positions.
Hi, happy holidays! Great question. Normally, you’re right in that the stanchions are what’s instrumented so you would need to bolt on whatever you’re measuring. We designed our balance so that the entire turntable surface is “live” (e.g. if I just stood on the balance it would measure the aero forces exerted on me as long as I wasn’t slipping on it). Makes testing different and wild ideas a bit easier.
There were 2 reasons why we used rollers: the functional one: we were able to get a closer overlap between the two bikes without the upstream bike touching the balance table (and thus affecting the force measurements) compared to a trainer and also we wanted the two bikes to be at the same height - 2 identical rollers worked pretty well. The not-so-functional reason: riding rollers in a crosswind is a lot of fun =).
Hi there, the balance table measures a not easy to separate summary of forces. I think this setup is good for relative comparisons of a laboratory closed system with some stable conditions all over, the riders didn’t fall off. All the motions of the test riders are balanced with three forces and only one of them is consummating pedal power to overcome drag.