You can do both, you know. That's allowed.

Almost always, the rank ordering of tires on rollers is the same as on the road. Sometimes two tires will swap ranking, but usually that's when the tires are pretty close to begin with. An exception can occur when the impedance break point on one tire comes a little earlier than the other. If you're in the neighborhood of that threshold, you can also observe a ranking swap.

Almost always, if you see a discussion of impedance breakpoint they're talking about real world field testing on roads as opposed to on a roller.

Thanks for that - yeah, that does makes logical sense. Is there a very *approximate* ballpark range in terms of closeness where the rankings can change, in your experience? I wouldnt expect a tire to make up a 7-8W gap, but are we talking fractions of a watt,1-2 watts or some other number? To be clear - i realize this is not going to be a scientific calculation, just a guesstimate borne of experience.

Wouldn't this be dependent on the road surface and sprung weight? As shown in the article linked, rolling resistance/friction increases with surface roughness. If friction is a function of the area of the contact patch, as well as the per-watt flex of the sidewalls, then on a perfectly flat surface, the best tire would be completely stiff. As surface roughness increases, the tire would be spending less and less time in contact with the ground, due to bouncing, leading to a pedaling inefficiency.

For a straight line path then, the ideal (efficiency-wise) tire would then be a combination of the minimum surface area for "good" traction with the minimum amount of deformation necessary to maintain that surface area.

It's conceivable then to me, that the Turbo Cotton tires would outperform the GP5k in certain surface roughness conditions.

That depends on how careful you're being in testing. If you're being really careful, you can reliably discern a difference in Crr *on the road* of maybe around 5% or maybe a bit less (I prefer to work in Crr rather that watts since watts depends on your speed and total weight). So if you're measuring a tire that has a Crr on the road of around .004, that means you can reliably discern a difference of around .0002, or a bit less.

So, one thing we noticed in addition to the rankings not changing much between roller and road testing: Real roads tend to be a little less perfect than even the kind of diamond plate Jarno uses for his big drum, plus when you pedal you're moving around and leaning the tire this way and that. So Crr measured in field tests tends to be maybe 1.5x higher in absolute terms than what Jarno measures on his drum. That is, the relative rankings and the relative differentials tend to be very close between rollers and road, but the absolute raw Crr values vary by maybe 1.5 (and this will depend on the condition of the pavement).

Here's the practical bottom line: if I absolutely positively need to know--like when I'm working with someone making a record attempt--I'll test CdA and Crr on the surface being raced on. However, for most purposes, I just use the roller tests, multiply by maybe 1.5, and use that. That usually gets me close (but, of course, I check).

Btw, RChung - not to impose on your time too much, but can i get the benefit of your expertise on this thread of Slowtwitch:
https://forum.slowtwitch.com/forum/S...=unread#unread

Ta!

Sorry, just saw this, and just saw the ST thread. I think you got the answers there. There was a study in J. Sports Sci in 2018 or 2019 by Thomas Maier et al. on rim width and rolling resistance in a downhill MTB context. The conclusion was that tire pressure mattered more than rim width for Crr. For CdA, I think matching the tire to the rim so that the tire/rim combo is slippery is right. However, as far as total drag (i.e., aero + rolling drag), there's a point where good aero performance and good rolling performance are in opposition (the best examples are 19mm pizza cutters that had good aero performance because of width but terrible RR; and on the other end, Bontrager R4 Aerowing tires that also had good aero performance but they got there with stiffer sidewalls, so the RR component was also bad enough to negate the aero advantage in most instances).

To bounce off this......the point of opposition is a blend of overall weight, road surface roughness, and anticipated airspeed for the "event".

You can shift the point of opposition one way or another simply by being a faster or slower rider. You can shift the point of opposition by being a heavier or lighter rider.

The point of opposition isn't the same for a 160lb rider going 27mph in a TT as it is for a 220lb rider going 20 or 21mph in a full Ironman. The point for the 160lb rider shifts to "narrower". The point for the 220lb rider shifts "wider".

The point for a faster rider shifts towards narrower (less yaw angle, more grams aero drag going faster). The point shifts to wider/CRR for slower riders (more yaw angle, less grams aero drag).

I think I've got that right in my head.

Yes, I think you've got that right, too: the exact balance point matters, and the things you list are how they're put together, but it also depends on what the road surface is like. Wooden velodrome vs. regular asphalt vs. rough asphalt vs. gravel race.

Dumb question about all these claims about grip..

Unless you're racing crits and laying your bike down a couple times a month, how do people claim to say (x) tire has better grip than (y) tire? Is it just blind faith in the tire product hierarchy and marketing? One post early in this thread mentioned that they could tell the tire had more grip by the second turn. Do you all really take your tires to the edge of sliding out that regularly to find the breaking point?

short answer. Yes.

In training, no. In races... yeah.

It's why I won't race Michelon Power Comps or Shwalbe Pro Ones. On the former I lost it completely and crashed. On the latter I had to drop out of the race when it started raining, and I couldn't take the turns anymore.

You don't have to have that extreme of an experience to feel when the tire is not holding, though.

Funny that, I've just recently ditched clincher GP5000s with latex tubes precisely because they're a horrible pain to get off and get on my wheels when I do get a flat (which is, admittedly, exceedingly rare and mostly when it happens it's due to the latex tube somehow getting pinched, while I can put on Corsa Speed TL tires on with my hands. We'll see about puncture resistance, but it's a TL tire, should be OK. First 100+ k ride on them, they seemed very nice, although I didn't push the traction too far because it started snowing.

First installed Corsa Speed TLs on my wife's TT bike, flatted once due to hitting a gash in the road with a sharp corner but have been trouble-free otherwise.

I have been intrigued by this brand too....

But this is printed on the product page of their own website for their cotton road tires:https://wolfpack-tires.com/product/5522/?lang=en

ATTENTION!

Our Cotton cannot be complained!

Ongoing checks for tread separation are absolutely necessary to continue to drive safe with this Wolfpack tyre!
Unfortunately, there have been problems with tread detachment with this product for some time now, which is why we have removed the article from our range for the time being. However, as there is still a high demand, we are now offering all cotton tyres at a lower price so that our customers can continue to purchase them and at the same time overlook any problems that may arise!

Look at the 4000s for dry rot if they've been hanging around a long time. The better the tire quality, in my experience, the more dry rot potentially becomes an issue.

I got and mounted a new GP5000s right before the lockdown (due to a flat and went from 23 to 25 at the same time) and spent the next 9 or so months riding almost exclusively on the trainer (if I'd known that was coming, I would have bought a cheaper trainer tire). When I mounted it, I briefly fretted about the mismatched tires, but it didn't matter on the trainer. The first long ride I took outside, the casing on the GP5000 got all cut to hell. Don't know if it was heat from riding rollers (I ride rollers with a forkstand as a trainer and rollers shouldn't heat up much) or what. There were maybe 1000-1500 miles ridden inside and a couple dozen outside, so I was pretty bummed. Lots of cuts in the rubber and it was torn/flapping in parts. Nothing got through the kevlar belt, so I saved it to potentially put back on as a trainer tire sometime in the future if I don't have a race coming up and want to see if it's still usable.

Since there weren't any shops open and I had a Zwift race that night, I put the 23mm GP4000sII on the rear and rode it while I ordered new GP5000s (upsized again from 23 in the front to 25 and to 28 in the rear - should just fit). I'm still riding the GP4000 on the trainer (a fancy tire I have is cheaper than a trainer tire I don't) and have the matching GP5000s to put on when I finally go back outside. No need to put extra wear on them now. Never would have thought the GP5000s would tear that easily.