DaveSSS 

ETRO has a standard for rims and tires. The standard limits tire pressure to 73 psi. This may sound low to those who are behind the times and still run narrow tires with 100 psi pressure or more. It's been proven that wider tires with lower pressure have less rolling resistance. For some reason, michelin tubeless tires list 73 as a minimum and 102 as the maximum, but I used pressures in the 60's for 6,000 miles. Some brands have issued new guidance, saying that tires marked 73psi minimum can safely be used at lower pressures.

Try the tire pressure calculator at zipp.com. With 23mm internal width hookless rims, a 28mm tire can be used by up to a 240 pound rider with a tire pressure of 67 front and 71 rear. I've followed the zipp recommended pressure and it seems to work just fine. I've already descended at over 50mph with tire pressure in the low 50s.

Using a narrower 25mm tire limits rider weight to 180. Using hooked rims actually lowers the maximum rider weight a little more.

GhostRider62

No it has not.

Crr depends more on the road surface than anything. I would agree with your general statement if it only applied to lousy roads.

I ride 25 mm tires at 89 psi and weigh more than 180 pounds. 89 PSI GP5K with latex tubes on my roads is optimal......I get Crr just a touch over 0.004....lower pressure is higher rolling resistance.

DaveSSS 

I've got no way to measure rolling resistance. I'd be interested in knowing how you do that. Zipp has different recommendations for gravel and road, but you don't see different pressures recommended for lowest resistance for racing versus comfort for general riding. Ever tried 28mm tubeless with Zipp's recommended pressure and compared it to your favored setup?

FWIW, Zipp's recommendation for a common tubed setup with 190 pound rider using 17mm internal width hooked rims and 25mm tires is 87 front/92 rear - about the same as you're using.

GhostRider62

I have tried tubeless from 23 to 35 mm.

Your blanket statement was incorrect. There is an optimum pressure and it varies by many factors. On a wooden track, it might be 140 psi. On nice asphalt, it might be 110 psi on 25 mm tires but on lousy roads, it could be 73 psi. On truly awful roads, 40-45 psi on 42 mm tires might be better still. Lower pressure is not always faster and those of us who still run higher pressure and narrower tires are not behind the times.

A 28 mm tire whether tubed or tubeless will be slower than a 25 mm tire at speed (aero) because on most roads, the Crr differential is quite small from 25 to 28 mm but at higher speeds, the aerodynamic losses are far from trivial. OTOH, on bad roads a 30-32mm tire at lowish pressure will likely be faster than either.

Crr can be measured using the Chung method (Google it) solving for CdA and Crr at two different speeds. If you want to do a simple tire or pressure comparison, a low speed rolldown is effective way to roughly compare Crr of two tires or pressures on that road.

icemilkcoffee 

Do go on. Do you have any Crr numbers from different pressures or different tire widths for comparison?

GhostRider62

No thank you. Not sharing my hard earned data. You can do a little digging on your own

What I wrote is obvious and a clear refutation that lower pressure is faster, this is only true up to the point that the suspension effect of the tire is overcome by the size of surface imperfections, then, your tissues start to absorb energy

Here is a simple overview.....

https://silca.cc/blogs/silca/part-4b...-and-impedance

DaveSSS 

I'm old and have no interest in racing. I ride a lot of chip sealed road and I'm really enjoying the great ride I get with 28 and 30mm tubeless tires. My latest setup with the 23mm hookless rims and 30mm tires is the best ride I've had so far. It may not be faster, but I don't think Zipp would recommend pressures that are aimed only at a good ride, with a big rolling resistance penalty. Anyone can choose to add another 5-10 psi if they really think that their rolling resistance is too high.

Dave Mayer

Thank you! Finally some credible quantitative evidence on this subject. Note that the rolling resistance over a wide range of inflation pressures is plus or minus 10 watts, so a distant second-order effect compared to wind resistance.

Not covered in this summary is the aero effects of fatter rims and tires. Narrow rims and tires are obviously faster here due to a lower frontal profile. And narrow rims and tires are obviously lighter, for the same materials and type of construction.

I would like to see the study cover the rolling resistance as a function of riding speed. The only advantage lower-pressure tires have is reducing 'impedance' losses, or the efficiency losses due to the rider bouncing around. My real-world experience of riding gravel (daily) is that narrow (23mm) high-pressure (100psi+) tires perform best on gravel at higher speeds. If you travel fast enough, every surface smooths out.

urbanknight

So if you want to go faster, you just have to ride faster!

GhostRider62

The only "study" I have seen was in German by a number of scientifically minded velomobile enthusiasts who showed

1. that Crr is not a constant, it increases with speed
2. Crr increases quicker for some tires than others
3. Crr is about constant in normal riding speed (16-22 mph for instance)

So, a tire with a Crr of say 0.003 at 20 mph might have a Crr of 0.004 at 30 mph....I am just making those numbers up to illustrate. Your observation about speed and gravel lines up with the common experience with German velomobile riders, they run very high tire pressures with similar view as yours.

It has been a few years, I do not have the link but you could repeat the study. Use the Chung method up to say 40 mph. Time consuming and it also takes wattage