rruff wrote:
Rob84 wrote:
... I was initially worried about relying on high- and low- speed runs for precision work because CRR and Cd vary with speed...
...Ultimately it’s actually not that hard to validate a delta CRR method. You can do runs with the same tire at different pressures, where you have a small but roughly known CRR change (say, based on BRR’s data at 60-80-100 psi). If the deltas you measure lines up roughly with what was determined expect from 3rd party CRR tests at different pressures, then I’ll consider it validated.
Crr increases quite a lot with speed on the roads where I tested... with the caveat that it could have been a mysterious "something else". Temperature effects Crr too.
I won’t provide a watt savings, or actual CRR value. I’m looking for a % difference in CRR between CS2 and Record. If someone wants to figure out watts saved for Records, they’d have to baseline their watts for the CS2 off of all the data that’s out there, and apply the delta.
When I compare my results to Aerocoach’s and BRR’s results, I’ll convert their wattage results into a % compared with their baseline CS2 measurements (at 80psi for BRR).
So it’s true, my tests will be at 80 psi, smooth roads, low speeds, and a temperature dependent on the test day. Race conditions will definitely be at much higher speeds, and the other variables may or may not match. This would introduce error if the variables changed at different rates between the 2 tires, as conditions changed.
For example: hypothetically, the Record could have 10% lower CRR at 10 mph, but actually be 5% slower at 22 mph
Or one tire could do more to minimize vibration losses than the other on rough roads, in a way that doesn’t correlate with CRR.
Or is one tire could be faster at 50F, but the other tire is faster at 90F…. Or at least less slow.
These are all possible. I would think these impacts are minimal but I can’t prove this. I definitely welcome any insight here!
The most prominent & risky difference with race conditions, I think, is the slow speed of this test, around 10 mph.
For CRR vs speed we do have some data… When BRR did their test of CRR vs speed they found an a average increase of 12% CRR from 9mph to 22.5. There was a range of 6.6% to 14.5% between the 6 tires tested. But the article stated that all variation here was within the margin of error, so they could actually all have varied by the same %.
If there is in fact a dramatic difference in how much CRR increases from 9 to 22 mph between Records and CS2, say 5% vs 15%, then this would definitely throw off my results. Based on the BRR data, I wouldn’t expect these to be actually off by more than a few % points, if at all. But it’s a good point, and I’ll make sure this + the temp differences a stated assumption & potential source of error.
Overall I don’t think these assumptions are much (if any) worse than the assumptions made for various lab testing. I don’t know from any test results that GP5000s are slower than CS2s at a tire temp of 105F, or at 40F… but I think we all still assume the CS2s are still faster at those temps. Same goes for smooth vs rough roads, I understand that supple tires manage impedance losses better, but it the CS2 and Records are both in the “extremely supple” category, and nobody (I’m aware of) has publicized big differences in impedance losses between otherwise similar tires.
rruff wrote:
BRR's data for Crr vs pressure would only be valid for the apparatus he was using. This would certainly not be valid on a road where vibration losses are a factor.
What you really want to do is compare the overall resistance of one tire vs another, at race speed and expected race conditions. If you are testing without airspeed measurement, then very low wind is your best bet, although you will miss aero differences. I suspect on most wheels the VFR will be aerodynamically better at yaw, because it is smaller.
I hear you. I’m really trying! But I don’t know what I can make more race-specific, given the constraints I’ve got on speed required to amplify the CRR signal. Any specific ideas are most welcome, this whole thread had been a huge help for me to refine my process and consider new variables.
I’ve outlined above why I think this test will still lead to worthwhile results in spite of the differences against race day. But like any test, the results are imperfect representations of the actual use case, and while I think the assumptions are reasonable, they are far from perfect and will make any “close call” results suspect.
Yes the assumptions are too vague to generate specific error bars, so it will be up to every reader to make conclusions about precision against the real world. But in this world of aero & CRR, let’s be real, let’s be real, this is ALWAYS the case. We always have to assume to extrapolate. We don’t race on drums and in tunnels :-).
What I can do is to be sure to include these assumptions when I ultimately discuss results.
Ultimately this test isn’t meant to be better than the other public test results on these tires, just another data point to help people make decisions.
It’s worth repeating, the delta CRR between CS2 and NEW Records was something like 20% in Aerocoach’s tests, while BRR found them almost equal. That’s a massive difference. I think even my test has a good chance of falling clearly on one side or the other.
rruff wrote:
Question is: What is the precision of your testing relative to the small difference in Crr you expect? Doing a bunch of test runs where you are trying to keep *all* variables the same (or compensate for the changes), will show you if you have a reasonable chance of picking the winning tire with any certainty. No tire swapping is necessary to do that.
My initial day of runs, with one high speed tun for CdA, and about a dozen roll downs with CS2s and Records, came up with .00300/.00300/.00301 for the CS2s. Two were consecutive, one was almost an hour later, after a wheel swap. The Records weren’t as consistent. Could have been wind coming into play. So precision is TBD. There are some bugs to work out but also reasons to be optimistic about the precision.
Good point about testing on different days! But I’ll tell you, it’s brutally hard for me to find test days/times with no wind, minimal sun, and a few hours of free time. If I also needed to temperature match test days, I’d be lucky to get 2 tests in a year!
What I really need is a second identical set of 38mm Sunrise wheels. At $320 on Amazon it’s actually tempting… I wonder what the resale value is for these things?
If anyone would want to take a lightly used set of those off my hands, send a PM :-)