Originally Posted by GhostRider62

Low of 0.0042 to high of 0.006, depending on road surface and temperatures. I generally get around 0.0045 on decent (no holes) but worn pavement. Wattage is a matter of plugging those values into calculator like Gribble. It seems that suspension losses on real roads (worn and bumpy chip seal) give me about double the Crr as the steel drum measurements that one reads online. Rolldown tests in windless conditions are easiest to rank tires but if you want Crr, Chung VE testing isn't too hard. I did tire pressure optimization measuring Crr but that was 25mm tires and for me on my roads, it was 90 psi. On the 32 mm tires, I measured Crr but did not optimize pressure that way because I knew something was wrong. I used rolldown tests to figure that out. As long as the temperatures are stable, the road surface represents the roads, and there is no wind and you do sufficient replicates, I find this method faster. IIRC, the wattage at my speed was under 10 watts but more than several watts.....around 6-8 watts. I calculated how much extra sleep it would give me on long brevets. I know the bearings were full contact RS with too much grease, but I really suspect they were not mounted properly in the factory. When cornering, there was weird sounds from them. When rotating freely, they would stop rotating pretty quickly and they would not rotate when placing the valve stem at 1 o'clock. I know this is not quite satisfactory answer to your question but good tire testing to get Crr takes a lot of time, I was mostly trying to determine a problem's solution. I went from the equivalent of a biketire rolling resistance of an old Schwalbe One to what you would expect from GP 5000 S TR tire. On my rolldown hill in benign summer conditions, I know where my tubulars roll out, where my Rene Herse tires roll out, where 25mm GP5000 tires roll and at the tippy top, I know where my Vittoria Speeds roll. The order of rollout is consistent with drum testing you can read from online. When my GP5000 S TR rolled out worse than old Scwalbe tires, I knew the approx. magnitude of the problem. Once good bearings were installed, rolldowns were much better. Varying tire pressure from 50-60 psi had marginal impact on distance.

Each rider's roads will be different and their impedance (fat) will be different, so, my pressures may be lower than what works for others as I tend to find a little lower works for me.

Edit: 25mm at 100 psi on my paved roads, Crr goes from 0.0042 to 0.005x.....I run 85-90 psi as a range.

Originally Posted by Mtracer

Thanks for the information. I may try some test of my own. Just for the fun of playing with some numbers.

FYI In the Silca article on rolling resistance, they mention that tires with higher rolling resistance have a larger penalty running both below and above the optimum. So, another advantage to using better performing tires is they are also less sensitive to pressure deviations from the optimum.

Originally Posted by Mtracer

I wish they would put numbers on this as well. But really, for any calculations I have run, the differences are pretty small. 1-2 PSI. Given the variables and the fact that most of us aren't using calibrated tire pressure gauges, I'm not sure how meaningful this is. Though maybe with setups much different than mine, lighter rider, narrow tires, there are larger differences between these generalized speeds.

Originally Posted by GhostRider62

I always suspected the impedance inflection point on those graphs came from a skinny Cat 1 racer and I also suspect those of us with some fat have a higher "impedance" on lousy roads, which would support lower tire pressure

Originally Posted by chaadster

So, I was looking at this fact and trying to reconcile it with what I understood based on the charts of their test results and I can’t get it to make sense.

According to their blog post, if I read right, their tests were based on 190lbs system weight, and 25.8mm (actual) GP4k performance tires. Plugging that into the calculator with new pavement selected , 26mm tire, TT bike, and CAT1/2/3 racing, the result was 93.5psi, but their charts show breakpoint out at 112psi or so.

What am I missing??

Originally Posted by Sy Reene

...

Other great (:-)) questions I've asked, relate to inflated tire width and pressure recommendations. If 2 different tires inflate to eg. 26mm width, but one inflates to 22mm height and the other to 25mm height -- how do we compensate accordingly on tire inflation calculator?

Originally Posted by Mtracer

I think the calculation strongly relates to tire cross sectional area. Silca is probably assuming some common aspect ratio of height to width. But of course rim width is going to affect this. Could simply be that the width and any reasonable tire cross section aspect ration, is close enough. I.E., don't try to compensate for height.

Originally Posted by Sy Reene

I emailed and asked Silca just this a number of years ago. The answer if I recollect is that there is variance between their testing conditions (as reflected in the chart) as relates to road surface, and how they felt most people would interpret and select their own road surface. IOW, I think they were saying that their "new asphalt" was super great and bound to be better than the rest of our road asphalt.

Originally Posted by chaadster

But the calculator recommendations don’t appear to align with any of their test result breakpoints, not for any surface setting. Not for new asphalt, 2 year old, coarse intermediate, nor for machine roughened.

Hopefully they have a more satisfying answMtracer .

Originally Posted by Mtracer

I don't think the characteristics of the rider or bike (other than tires), have any effect on the losses on rough roads. The bump expends energy lifting your mass, and when you come back down, I don't there is a mechanism to convert this back into forward motion.

When Silca uses the word "impedance", I think they a referring to the mechanical characteristics of the road due to its texture.

Originally Posted by Mtracer

But, I'm willing to give up a watt or two if the ride is going to be significantly more comfortable, as was this recent ride.

Originally Posted by chaadster

But the calculator recommendations don’t appear to align with any of their test result breakpoints ...

Originally Posted by asgelle

The plots of test data are from one test series with one rider, bike, set of wheels, etc (note there are no error bars, standard deviation estimates or confidence intervals). The calculator is based on a curve fit to the results from hundreds of tests over a wide range of conditions. The fact that one test doesn't fall exactly on the curve fit should not be at all surprising.

Originally Posted by asgelle

The plots of test data are from one test series with one rider, bike, set of wheels, etc (note there are no error bars, standard deviation estimates or confidence intervals). The calculator is based on a curve fit to the results from hundreds of tests over a wide range of conditions. The fact that one test doesn't fall exactly on the curve fit should not be at all surprising.

Originally Posted by GhostRider62

I am 100% sure you are wrong. I have seen Uni studies to the contrary.

Originally Posted by Kapusta

Burping just some pressure without going flat does not violate any laws of physics, as evidenced by the fact that many people have actually experienced such a thing. Myself included.

The tire bead gets pushed in away from the rim long enough for some air to get out, but then snaps back in place.

This is different than the bead coming off the rim altogether.

Originally Posted by Peruano

You will find out when you hit your first significant chuckhole or crack in the pavement. Pinch flats create abnormal low tire pressure.
You may be over thinking things, but simply stated, if the pressure is too low, you will damage tire walls, and the tubes.

Originally Posted by Carbonfiberboy

My experience is completely different. I'm a geezer with geezer ideas. I still run 23mm tires and like them fine. I'm always the first descender down, sometimes by minutes and it's not because of my cornering skills. My BMI is about 24.5, 5'6", and because of having short legs and only about 8cm drop, my position in not particularly aero.

I used to run my 23mm at 140 lbs. I rode with a more compact rider than I who could never understand why he couldn't coast by me. I told him to buy the same tires I was running and pump them to 140. He did so and the next time we went out, he went right by me. 15 years later, on Conti 5000 I run 80/100. They seem rougher at that PSI than my old tires were at a higher pressure. But whatever, I still coast past everyone in sight, never been passed except by tandems.

I hear all the above words, but don't understand them. I have examined Silca's test methodology. It seemed to me that they didn't test on the same smooth asphalt I ride. Sure, on chipseal it's a little rougher and slower, but I ride a comfy carbon bike so no big deal and other bikes still don't pass me on descents. Maybe it's because I don't use my brakes much, but that idea seems a little too weird. Why not do 40 or so?

My experience is more like the steel roller data.

Originally Posted by Mtracer

I certainly could be wrong. Just an educated guess. Do you have any references to the studies you mentioned. Specific ones would be great, but I understand you may not have these handy. But anything that might help me find them. Just curious. I'm guessing these were not bike specific, but perhaps related to automobiles. Which would still apply.

Originally Posted by Mtracer

I don't think the characteristics of the rider or bike (other than tires), have any effect on the losses on rough roads. The bump expends energy lifting your mass, and when you come back down, I don't there is a mechanism to convert this back into forward motion.

When Silca uses the word "impedance", I think they a referring to the mechanical characteristics of the road due to its texture.

Originally Posted by Carbonfiberboy

My experience is completely different. I'm a geezer with geezer ideas. I still run 23mm tires and like them fine.

Originally Posted by chaadster

I honestly don’t think you’re missing anything. A lot of what gets tossed around as axiom, specifically stuff like “try lower pressure,” is oversimplified and, and devoid of the context within which the core idea was born. Guys like you know the history; it wasn’t 15 years ago that rims had narrow 15mm internal widths, the fast kids’ go-to tires were 20-23mm wide, and their pressures were +120psi.

That’s the context, right? Skinny rims, skinny tires, high pressure. The new models (championed by guys like Poertner at Zipp) were internally wider rims and lower pressures, while a concurrent thread of thought was the supple casing thing. Those merged into the new paradigm: more volume and lower pressures.

What has happened is that the shorthand, “wider and lower,” has been stripped down to just “lower pressure” because the focus on internal rim width race got nullified by the wide adoption of disc brakes which removed the limitiations not just of rim brake caliper clearance, but of frame max tire width capacity. So today, “try lower pressure” has become the mantra because people forgot, or never even knew, the original context. Forgotten history; shocker…

The pressure numbers being thrown around in this thread and elsewhere more generally should shock you; I mean, I’m shocked that folks could possibly consider a 30mm tire at 55psi is optimized for lowest rolling resistance on typically decent American roadways. In fairness to the OP, they never claimed as much and in fact acknowledge there was probably some wattage cost, but the net effect is the same as has been happening for years: oversimplification leading to a singular focus on lower pressure without understanding of the mechanisms and context.

Lower pressure is not key; the magic happens when it’s the right mix of rim width, tire width, tire casing and tire pressure. I get that it’s a mouthful, and even worse, that it’s hard-to-impossible to know how to formulate the right mix, because it depends on a lot of shifting variables that just aren’t clearly discernable.

Originally Posted by esasjl

The jargon is often a barrier. I think there are two factors, energy loss in deforming the tyre system (hysteresis) and energy loss in deforming the bike (probably small) and the rider (probably large) sometimes called suspension losses.
https://www.renehersecycles.com/the-...re-calculator/ and the Silca explanation are largely comparable but with different jargon.