http://forum.slowtwitch.com/...0crr%20plot;#1782584

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I'm very "old school" and I can assure you this was only the fashion among the ignorant and misinformed. I think a lot of people think that just because you can run 140+psi in tubulars on the track, you can do it on the road too. It's NEVER been a good idea.

Thanks,
Can you spill the beans yet, or is the article still forth coming?

Styrrell

Do you mean by that what pressure I found for me to be the "best" with my particular tires/tubes, my weight, and my road? ;-)

The test I did was with relatively large steps between pressures (1 bar = 14.5 psi). For this particular setup (see the caveats above once again) the low "resistance to forward motion" was at 8 bar. Going up to 9 bar was far more harmful to speed than going down to 7 bar.

YMMV, objects may appear larger than actual, etc., etc.....:-)

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Can you give an idea about how much the change was between 8 and 9 bar? Would over inflating by a bar cost 10 sec, a minute, or 5 minutes in a 40 k TT? Your recent P3C / P2 test piqued my interest. In the off season I'd like to do some similar testing. I'm thinking tire inflation may be a good test, mainly because the result is cheap to implement.

Styrrell

Sure...going from 8 to 9 bar increased the "apparent Crr" by over 30% in my test (Again...MY tires, MY weight, MY road). Whereas going from 8 down to 7 bar only increased the "apparent Crr" by ~5%.

That 30% increase by my reckoning could account for anywhere from 30s to 1 minute over 40K.

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Tom,

If you were running a Zipp 404 front, 808 rear on a Trek TTX, clinchers with Pro Race 2's, 20 mm front, 23 mm rear, weighed 155 lbs., Air Comp latex tubes, course was on asphalt, very very smooth, what would the recommended tire pressure be? And any critique of the above choices? And where do I send a paypal payment!

I would follow the recommendations printed on the Michelin tires package :-)

http://two-wheels.michelin.com/...04111118&lang=EN

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7.5/110 psi it will be. Although I'm curious going back to the OP question about "real world" performance, if Michelin recommends certain PSI based on a factpr of "safety", "comfort" and "durability" which might be different in a race than at other times. Obviously, most would choose to be "less safe", "less comfortable" and "less durable" in a race...

"Correct air pressure affects safety, comfort , efficiency and durability , and especially overall riding enjoyment. Tyre pressure should be checked before every ride."

 
Yeah...but they didn't say it was a "one way street", right? ;-)

In other words, those things can be affected by having too much pressure just as easily as not enough, right?

I look at it this way: The reason pneumatic tires are better than solid tires is because they allow a form of suspension between the bike and the road surface. On any reasonable surface, running solid tires would be slower than running pneumatic tires. So, why would you pump up your tires so hard that they start mimicking a solid tire? It defeats the whole purpose of them being pneumatic...

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I understand the principle, but then how do streetluge folks (think X-games...basically laying on a skateboard) reach speeds over 65 miles per hour? I can't prove they'd not be faster on pneumatic tires, and polyurethane wheels aren't exactly solid but if there was a 30% increase in rolling resistance from 8-9 bar you'd think these skateboards wouldn't even roll.

That's simple: low aerodynamic drag.

Just think how fast they'd be if someone could make reliable itsy-bitsy pneumatic tires ;-) Actually, I wonder if they're already reaching terminal velocity for those slopes....

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It's well established among the mtb riders that lower pressure tires are faster over broken terrain because hitting obstacles doesn't exert as much force back through the bike hurting forward momentum.
What seems intuitive is that for a given tire/surface/load combination that there is an inflation pressure that minimizes rolling resistance. What's not at all clear to me is where that pressure is. I know you have a number that you arrived at using a methodology (which I respect), but has that experiment been repeated? I only ask because repeatability is a hallmark of a well designed experiment.

It would be great to see a formula that would take in to account weight, tire properties, pressure, some factor representing the surface and give us a pressure. Could probably be more easily done for a given tire.

I'm very "old school" and I can assure you this was only the fashion among the ignorant and misinformed. I think a lot of people think that just because you can run 140+psi in tubulars on the track, you can do it on the road too. It's NEVER been a good idea.

Old school or not, I still see many, many triathletes in races and in transition zones pumping their tires up to ridiculously high pressures. All I can say is these people must have Iron-crotches! :)

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Steve Fleck @stevefleck | Blog

So, why would you pump up your tires so hard that they start mimicking a solid tire? It defeats the whole purpose of them being pneumatic...

Less contact patch = less friction = faster rolling.

As far as I am concerned, you can run YOUR tubulars at 115psi. When the air pressure in mine drop down to 110-115 psi, it feels like I am in mud. When I pump them up higher, I can coast for longer. Can you logically explain that?

Oh, and BTW, at the State 40K TT last weekend, the fastest 3 pros were using 140+ in their tires. I know because I was there, was parked right next to them and personally know the guy that won in 2007. He came in 3rd fastest overall this year.

AB

interesting chart, but there are few problems.

for one, it is not exactly clear if the chart also incorporates the weight of the bike.
there is also no consideration for different road surfaces.

and then, the curve on the graph is apparently optimized for comfort, durability, and efficiency.

but what would the actual curve look like if one was only interested in efficiency?

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Where would you want to swim ?

Based on your tests here and given that on the AFM chart tires are tested at 120psi... that is over inflated for some of the tires on the list. How can we assume the order of that list does not change at correct inflation for load?

On smooth rollers?


Why would you assume otherwise?

By me? No. But that's just because of lack of motivation. To be honest, testing a lot of this stuff can be a pain sometimes (getting up early, making sure conditions are favorable, etc.) and sometimes it's just plain downright boring ;-)

However, it may interest you to know that the tires used in this test are the EXACT tires Al used in his "bumpy roller" testing. When plotted against that curve, up until 9 bar the data follows the same curve (with an appropriate scaling factor, of course).

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Yes that does interest me, thanks.
Forgive me for being relatively new here, but what/where is Al's 'bumpy roller test'?

I think I'll actually take the time to do rolldown tests with various wheelsets and inflation pressures.

Well...not exactly. First of all, rolling resistance isn't caused by friction between the contact patch and the road. Second of all, your simplified assumption may hold on perfectly smooth surfaces...but a typical road surface is a whole 'nuther kettle o' fish :-)



Let me guess...you're running Tufos, right? ;-)

I learned a long time ago (and keep re-learning it...see my "Something borrowed..." thread") that just because something feels fast, that doesn't necessarily mean that it is fast. Call it the "going 80 mph in a CJ5 vs. going 80 mph in a Mercedes AMG" effect :-)



Great. I personally know a guy who was on the podium of the Masters Nats TT that did it on slow tires too. That doesn't mean he couldn't have been even faster, or perhaps a step up further on the podium if he'd been just a smidgen "smarter"...

There's an old saying that applies here...something like "you don't always know if pros are successful because of, or in spite of, the things they do..." ;-)

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IMO, it would look exactly like what you see.

I think you might be trying to be to "precise" with what the chart is telling you...you need to use a range of +/-5 psi or so to "fine tune" things for your particular application and preferences. The chart is a good starting point.

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http://www.biketechreview.com/tires/AFM_tire_crr.htm


Your response reminded me that my results where pretty much in line with a rolldown test that was published in the last couple of years by Bicycle Quarterly. I'm not really enthused about pointing this out since IMO their methodology leaves a LOT to be desired, but in general they apparently found that excessively high pressures were actually slower as well. I'm sure you can find stuff on the 'net about this testing...but I warn you to make sure you fully understand the potential ramifications of their test procedures...

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