Slowman wrote:
i don't disagree with the anything you wrote above. i just know that when i have broached the subject of rolldown testing, regardless of the protocol, i've gotten a toilet flushed on my head for the very valid reasons you post above.
Yes...rolldown tests are a subset of the field testing protocols (most field testing procedures are really just evaluating the same equations of motion for a bike+rider system, just from different "angles"...at least they should be.) The main advantage of roll-down methodology is that a power meter isn't required, and thus power meter accuracy and precision doesn't come into play. But, to do it correctly, one WILL need accurate speed recording (GPS speed isn't going to cut it, and multiple magnets for a magnetic speed sensor are an advantage...although the same is true for other field test methods). Then, of course, tire rollout measurement will be critical as well as part of that speed measurement (again, same as other field test methods). If one want's to attempt to "pry" CdA and Crr, then that will require having entry speeds for the start of the roll-down segment vary as well.
All that is saying...if one proposes to do roll-down testing like some manufacturers have (i.e. hand-timing through a segment after a roll-down), then one is rightly getting a "toilet flushed on their head", because doing it in that manner doesn't allow one the sensitivity to reliably state that one setup is different from another...no matter how many runs are performed, nor what statistics are applied. I've seen claims made from that type of testing as "there's no difference", when in reality it's just that the method isn't capable of detecting the differences that are there. This is one of the reasons a "Tom Compton Challenge" type test is a good first start for any method.
I say that if one is going to do field testing and one DOES have access to a good power meter (and the late, great PT hubs are ideal for the purpose), then it's far better and more time efficient to do one of Virtual Elevation variations, such as "Half-pipe" out-and-backs, Loops with varying speed (i.e. Shen method), or even the classic "regression" methods of out and back runs at varying speeds on a flat course (and again, they're all just looking at the same math in different directions).
Slowman wrote:
as to the treadmill, again you're right. healthy skepticism is in order. but it's not a running treadmill. it's not a woodway. the segments are not designed - or necessarily designed - with compliance in mind. so, i would just hesitate to make any assumptions on material density, sag, compliance, damping. that established, i would also not make any assumptions that the entire system isn't subject to vibration beyond what's in the belt or track. for example, i wouldn't mount a camera or photosensor on the frame of the unit to measure vibration. i'd mount it on the ground.
You are right that I don't have detailed knowledge of the treadmill structure...but, knowing what I know about polymer materials that are apparently being used, and being able to visually SEE the planks moving/deflecting in the low speed video (even across the "flat" plank sections) I'm pretty confident in saying that someone is going to have to SHOW me that aside from the surface roughness and vibration amplitude, that the material behavior of that surface and structure is consistent with a paved road. The fact that the data shown looks more like what one would expect on a compliant surface rather than a rigid paved road points to the idea that there's some of that behavior influencing the results. That's just my engineering opinion though...I welcome information that shows it's not so.
Slowman wrote:
all that established, i like the rolling road for the same reasons you like drum testing: ease of use, precision, repeatability. i'm not the scientist you guys are, but my instinct tells me if you could (say) put an accelerometer on the bike frame, ride it on the rolling road, and then find the road surface that most closely reproduces vibration of a similar amplitude, maybe you could make a statement about the road surface the rolling road (without any features installed) mimics.
The "rolling road" could be a great tool, with the caveat being that it still needs to be shown that it reflects road properties (especially for smooth pavement applications). One big advantage it has is that it side-steps the inability of a lot of folks to be able to wrap their heads around roller testing being a good proxy for on-road performance. Another is that for rougher conditions, it brings in the damping of the rider body into the system....so, just like with rough roller testing using a load support with representative damping could do, the testing can actually demonstrate the breakpoint pressures of the systems, like one sees in field testing. That could be cool.
However, even if the vibration amplitude and frequency content at the rider is the same for the rolling road and representative pavement, if ADDITIONAL damping in the rolling surface is introduced, then the results are NOT going to reflect the pavement performance. More energy is going to have to be input to the system to get the PSDs to match in that case, because additional losses are occurring in the rolling road. In fact the results will end up more like what has long been measured for MTB rolling resistance, where no matter the tire, the lower the pressure the faster it rolls...and then pressure selection just becomes "what's the lowest pressure you can run without pinch flatting for the tire you've chosen"...which is EXACTLY what their power vs. pressure plots from the rolling road look like in this wheel introduction. It's another clue that there's something amiss at that interface that doesn't reflect actual pavement. But, like I said, that's just my semi-educated engineering speculation.
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