Totally agreed. Even according to this chart which is supposed to espouse a low-pressure principle, nobody over like 180lb can ever possibly ride 20mm tires, because who's going to be riding on pressures over 110? My fat ass on 20mm would need something like 250psi according to the chart, and that's just not true.

When I started this thread 12 years ago, my impression at the time was that this chart was an artifact of 'ancient history', i.e. already 10 or 20 years, and accepted lore among the BF community

I put no more faith in sidewall minimum pressures, than in expiration dates on canned food. Go with what is comfortable.

Porque no los dos? If your tire is only dropping 15% while just cruising along, there's no way you'll be able to get a pinch flat unless you ram straight into a curb without braking (or to be fair, I guess you could also slam into a big sharp pothole)

The original article the chart came from (not the Rene Herse article, it cites Bicycle Quarterly) goes into this. I recommend you try the exercise of getting your bike through a doorway or next to a wall, so you can get on it in a riding posture with the front and then the rear wheel on a bathroom scale. I think you'd be surprised how much more weight the rear wheel takes

In an industry devoid of actual science, it seems odd to single out one guy.

I didn't single him out -- I merely responded to a thread where several posts cited his questionable results. There are others in the industry that make questionable claims, but they weren't mentioned in this thread.

I was playing around with the calculators, and I notice that both RH and Silca roughly agree when it comes to heavier riders, but diverge drastically with lighter riders
for 28mm tires:
total weigh= 200lb
RH: soft:=65psi; hard=82psi
Silca: front=77psi; rear=79psi

total weight = 140lb
RH: soft=46psi; hard=58psi
Silca: front=73psi; rear=75psi

I think Silca is wrong for the lighter riders

all those numbers, what width is it for?

So no "scientifically" derived tire pressue calculators are mentioned in this thread? :roflmao2:

28mm.

RH feels pretty scientific. They measured the rolling resistance, found two dips, and gave a calculator that reports where those two dips are for your values:
https://cimg9.ibsrv.net/gimg/bikefor...f5778c73aa.jpg
They came up with a theory about why there are two dips:
https://cimg5.ibsrv.net/gimg/bikefor...e48cecea99.jpg
But that's kind of irrelevant. We know from the rolling resistance testing that they are there already anyway.

Your posts sometimes make no sense at all.

Perhaps the poster above, who understood, could explain it for you?

When the indication is a range (or ranges for tubes and tubeless), I'm tempted to take it into account. I got 3 pinches in a year following these "comfortable" tire pressures from tables and calculators, even when they offer options for the terrain and conditions, in addition to the weight and tire width...

I guess your roads are really well maintained. I'm using my main bike on all kind of roads, worst is tracks or gravel with some stones when you gain some speed...now I'm checking the pressure much more often, not only with my fingers, and never use these lowest pressures. For many people it might be possible to separate those 2 activities, pure road vs gravel (or MTB), not in my case. So I have 2 wheelsets with different tires depending on the worst conditions expected, but even in this case, there will be a good chunk of asphalt on the way, too.

I'm fine with that 40-60% weight repartition, my issue is with the pressure values indicated in the table if I follow the process and the table, the value for the front tire will be so low that I'm certain to get a pinch as soon as I'm not on a well maintained road. I know there is much more weight on the rear wheel, but when the terrain is very rough, I'm often out of the saddle and the front wheel takes more shocks than the rear (most the pinches were on the front). Might be different if you only do roads, where you usually remain seated at higher speeds...

That's interesting, so every tire might have different "sweet spot" pressures depending on its casing, thread, etc...

That said the graph makes the difference in performance look huge, but it's less than 10%, also do you know what the 140-150 watts values represent? That tire has a rolling resistance around 15 watts on bicyclerollingresistance dot com...

Note, rolling resistance is not what's measured/displayed in "the chart" -- that measures the relationship between load, psi, and tire width combinations that exhibit 15% tire drop. That 15% itself is just a rule of thumb.

ok I pythonned it and arrived at a better understanding of the data pattern in the chart.

These are the slopes and intercepts of the lines in the chart. (Note that the intercept of 25mm is an outlier, you can actually see that in the original)

https://cimg2.ibsrv.net/gimg/bikefor...2cae4050aa.png

The problem is that this data pattern does not extrapolate much past the 37 shown in the chart, at about 40 things start to turn around -- lines get steeper again, with lower intercepts. Here's how it looks out to 50:

https://cimg7.ibsrv.net/gimg/bikefor...cdaae73bdf.png
Note carefully the sequence: it behaves like the original chart up to 35mm, then 40 is purple on the bottom, 45 is brown above that, and 50 is pink way up near green (30)

So my new question is, what is the physics of how the ideal gas law PV=NRT interacts with sidewall rubber (different for every tire!) to yield a load-->pressure at 15% drop? Is it actually linear? What happens when widths get large? Presumably air pressure becomes less significant than sidewall stiffness.

HT @mkane, I thought this is an appropriate pic when discussing Tire Drop

ICYMI, that's "internet slang"! :D

You have to hope their Samurai swords don't get caught in the spokes...

I think only the champion gets a sword.

I think those are Sumos, not Samurais. Very different athletic skillset (and ideal body type). Good thing the Sumos get to wear kimonos when they're not competing in the ring!