tomato coupe

I think it's a quantum mechanics thing ...

PeteHski

A bit like not being able to measure your last tyre pressure without affecting it.

rsbob 

Ah yes, did it too. Part of the rationale of inflating to the max sidewall pressure was to reduce the likelihood of flat tires, because as we all ‘know’, glass will not imbed itself as readily into a hard surface. that reasoning proved spurious considering the countless flats fixed in my cycling life. (Ah, but what about the flats you never got because of it says, WWBHDD?) The other spurious thought was that the higher the pressure the less the rolling resistance because the contact patch was minimized, especially on my 700/23s.

Now on my tubeless 28s its 68-70lbs and on my tubed 700/25 its 82/85lbs. Certainly is a lot more comfortable and am able to maintain within 1 mph my average speeds when I was in my 40s riding 5000 miles/year - but I do that same amount now too.

I could care less if people use a tire calculator (I use the Silca one) or not. What they want to do or not do, or maintain or whatever, has no bearing on me and my enjoyment. I just like to minimize effort so I can either ride further or faster or both. As has been made painfully clear on BF, that is not many peoples objective, and so be it.

LarrySellerz

The issue is that to truly make an online calculator you need to add stat weights that depend on the changing conditions. For instance, if its horribly rainy or if it is icy, almost every online calculator will overestimate the pressure very badly. Its easier to "misuse" an online calculator than one might initially suspect. When I used to have to ride in the sleet, id take my mtb tires that I normally pump to like 60-80 and take most of the air out, probably run like 20 if that. These numbers are estimates because as has been pointed out in this thread gauges can be deceiving, a ballpark of +-15% of your target is almost always adequate and easily detectible by hand. I'm not taking out air and then rechecking the pressure with a gauge lol, nobody did that.Trying to put that in an equation is difficult.

tomato coupe

I think even the most "misused", poorly written online calculator wouldn't tell you to run MTB tires at 80 psi.

Calsun

Best to check the tire charts as the PSI depends on the weight of the rider and the volume of the tires. A 240 lb rider needs a higher PSI than a 140 lb rider. A 1.95 x 29 clincher tire can run at half the PSI of a 19mm tubular tire with the same rider weight.

My wife and I ride road bikes, road e-bikes, and mountain bikes and so I have 6 different PSI that I use. I made a chart and taped it to my high and low pressure bike hand pumps for fast reference.

PeteHski

Or reduce by 60 psi for wet conditions.

LarrySellerz

Don't get hung up on the numbers, divide by 3 would be better, but my point is that very wet conditions require enormously lower PSI, to the point in which trying to rely on an equation for all conditions is impossible. Using a calculator allows for more user error; any talk of calculators for OP is missing the forrest for the trees because he mentioned he rides in terrible conditions in his first post.

genejockey 

With hard riding tires and excessive rolling resistance.

tomato coupe

3 is still a number.

PeteHski

I don't require enormously lower pressure when riding in very wet conditions. I might drop 5 psi if I know it's going to be very wet before I start, but I've ridden some hard rides with very technical descents in rain storms without an issue at normal pressure. But then my normal pressures are not as high as yours. Dividing my pressure by 3 is just complete garbage IMO. But thanks for the tip.

smd4

Honestly, I wonder if anyone has done a study comparing the rolling resistance of 23 mm tires at 140 psi vs. 38 mm tires at 60 psi?

Chuck M 

One is the loneliest number that you'll ever do
Two can be as bad as one
Three is just something in my wildest dreams

Eric F 

They do? I haven't found that to be the case. Also, enormously lower pressures increase the chance of pinch-flatting, burping (if tubeless), and/or rim damage, and having to fix a flat in the rain is not something I really want to increase the odds of happening. Having to walk because of a busted rim - no thanks. I prefer to keep my tires at the same pressure, and just be a little more cautious when I need to be. All that said, I don't like riding in the rain, and try to avoid it.

genejockey 

Yes, they have, or at least close to it. This has been done repeatedly by a variety of different testers.

Again, you can ride what you like, but I remember you saying you prefer to FEEL fast like you do on 140 psi to actually BEING fast at lower pressures. To me, this means your mind misinterprets the sensations you feel through the bike, so that when you hit a bump or rough pavement, feeling each little bump is interpreted as your rock-hard tires trading comfort for efficiency, when the reality is you're giving up both. Obviously, there's a point in the rolling resistance vs. tire pressure curve where decreasing pressure increases rolling resistance, but the same is true for INCREASING pressure. The curve is a V, not a simple negative correlation.

EDIT: the optimum pressure for lowest rolling resistance varies with how rough the road surface is, which is why 1) drum testing doesn't reveal this, and 2) track riders still ride really high pressures.

LarrySellerz

I was talking about literally sleet and snow/wet ice on the roads, which i was assuming is what OP faces in the PNW winter. I should have clarified that because the psi needed for that and just "very wet" rainy days is dramatically different. My bad

Eric F 

In my understanding, this is accurate. Rolling resistance is only one factor in the story of how efficiently a tire gets down the road. Aerodynamics play a part (in conjunction with the rim). Hysteresis loss plays a part. Vibration and vertical deflection losses play a part. The best tire and pressure choice finds the balance that minimizes all of them as much as possible. When you get outside that balance point, minimizing one factor increases another, and the overall result is diminished.

genejockey 

With the exception of aerodynamics, those are all factors of rolling resistance.

Eric F 

Valid. Mentioning hysteresis separately was probably unnecessarily redundant in my comment. However, vibration and vertical deflection losses due to irregular surfaces don't show up on drum tests (as far as I'm aware). It seems to me that this is an area that tends to be ignored by the "harder is faster" folks, but plays role in picking the right tire and pressure, and is significant in why softer tires may be faster in many real-world conditions.

genejockey 

Right, but that's the difference between "drum" rolling resistance and "real world" rolling resistance.

shelbyfv

FWIW, OP isn't asking about snow, sleet or ice. Says he rides trails when the roads are worse than rainy. For rain, conventional wisdom is to reduce pressure 10%-20% but I don't know that there is actual evidence that it matters.

PeteHski

FWIW Pirelli advise simply reducing by 5 psi for wet conditions. Zipp calculator suggests similar (6 psi for my weight). They don't advise for snow/ice. If I was riding regularly in those conditions I would change to some more appropriate tyres anyway.

indyfabz

It’s a magic number.

shelbyfv

Yes, just trying to redirect from the tangent The Larry went off on after his "enormously" reduce pressure in the wet silliness.

smd4

So that's a "no."