terrymorse 

Hey, testing your hypothesis and finding out it's wrong is also some super power learning. "Getting something wrong" may be the best kind of learning.

Yeah, measured width. I got that bit of learning recently, too. GP5k 25s now measure about 27.5mm on my new rims, and they're precariously close to rubbing on the chain stays. Also, the Silca calculators recommends substantially lower pressure for 27.5mm than 25mm.

dedhed

Who would or even have the means to calibrate their home tire pressure gauges?

PeteHski

The Silca calculator must derive rim width off the measured tyre width as there Is no input for rim width. The SRAM calculator uses rim width and nominal tyre width. But they seem to end up with the same result for my tyre and rim combo.

genejockey 

I should go through my stable and check tire widths. I'm running 25s on everything except the Canyon which runs 28s, but I have some old (i.e. 80s) rims that are pretty narrow, as well as some later ones that are a bit wider.

genejockey 

Alternatively, actual tire width is the relevant variable, and others are using rim width and nominal tire width to calculate actual tire width.

prj71

Yeah. Sure. OK.

One of the big problems with that calculator is that the surface condition is fixed for calculating the tire pressure. Out of the Selections available and if I'm riding my road bike, I encounter all of the conditions listed except gravel, cobblestone and track.

If I'm riding my gravel bike I encounter every condition in the list except track.

So then which one is supposed to be correct?

It's not that difficult to figure out tire pressure and a calculator isn't required to do so, unless for some reason you don't have an ounce of common sense. The calculator is bogus.

pdlamb

Sorry, I got hung up on the 0.4 mph less than average speed. What kind of variation do you normally see in speeds -- what's your standard deviation, to use statistics? While I don't know anything about your fitness or route, that kind of deviation from the mean is in the noise for me on a 60 mile ride. I'll see more than that if the wind is out of the north, or south, or 5 mph above some average value I don't track; or 5 degrees warmer, or 10% more humid, etc.

terrymorse 

Pick the condition where you will spend the most time (minimize total energy), or pick the condition where you will be struggling the hardest (minimize hardest effort), or pick the condition where you expect the race will be won or lost (maximize chance of a win).

It's called "optimization". Pretty simple.

prj71

It doesn't work that way where I live. I can go on a 30 mile ride from my house an encounter new pavement, worn pavement/some cracks and poor pavement/chipseal. And in that ride I'd be spending 1/3 of my time in each condition. I've experimented and found a pressure that feels comfortable for all the surface conditions I encounter and it's about ~20psi higher than what is recommended on that calculator.

Goofy thing. Seems too many folks get caught up in analysis paralysis. Just go ride.

genejockey 

Per Strava, I have done that route 31 times, with an average speed of 15.8, and only two rides slower than this one, and I have ridden in similar conditions. In addition, those 31 rides were on at least 5 different bikes, and the bike I was riding is reliably the fastest of them. I'm comfortable with the conclusion that it was roughly 0.4 mph slower than I would expect. But here's a histogram of the average speeds on that route, with the average speed for the ride in question highlighted in red:

PeteHski

Yeah of course you know better and your higher pressure is bound to be faster because you have tested it far more thoroughly. You are super smart and the Silca guys are just silly.

My local roads also have many different surfaces and with the knowledge that lower pressures are less of a compromise on smoother roads compared to higher pressures on rougher roads, I generally bias toward the optimum pressure for the significant rougher roads I encounter.

If you can figure out your optimum tyre pressure for lowest rolling resistance all by yourself and it’s completely different to what Siilca suggest then good for you.

PeteHski

There is no analysis paralysis for me. It took me all of about 10 seconds to get a suggested pressure from the Silca calculator. I rode it, liked it and never looked back.

Calsun

I use a chart that uses the riders weight and the size/volume of the tire. The volume determines the PSI needed to support a given amount of weight. If the chart recommends 38 PSI for a tire I do not care if is 100% or even 95% accurate. I rode for decades and used the force needed to compress the sides of the tire to decide whether to add air or not. More complicated with mountain bike tires where there are sidewall and tread and terrain considerations in the mix.

icemilkcoffee 

Do you guys have some magical pump heads where you can pull the head off without losing a bunch of pressure? Because every time I try to pull off my lever action pump head it loses probably >5 psi just from that. Rendering any kind of precise pressure experiments moot.

PeteHski

It sounds like you lose pressure, but it is usually just air releasing from the pump, not the tyre. You only lose pressure when you attach the pump, which obviously doesn’t matter.

prj71

The other problem is that gauges are rarely correct. 3 different gauges will have three different readings. I have 3 on pump gauges and 2 standalone gauges. Every damn one of them reads something different plus or minus 2 lbs.

But accuracy isn't important with a gauge. Repeatability is. Once you settle on using one gauge then you know your magic number.

icemilkcoffee 

No, from the time I flip the lever, to the time I actually manage to wrestle the head off the stem, it continually loses pressure.
Previously when I was using the push-on head with my Silca pump, the pull off pressure loss was less. But the push-on head had other annoyances. Nowadays I use an air compressor with the lever-flip head. It works well with different stem thicknesses, but the trade-off is pressure loss getting it on or off.

DangerousDanR

Prestacycle claims that their digital gauge is +- 2% accuracy with .5 PSI precision. While I don't know which sensor they are using, I have been working on a farm machine product that uses a sensor that gives better accuracy than that and costs less than $3.00 Add another $.50 for the display and $1.00 for a low end microprocessor along with the other parts needed and you could have a pretty good gauge that cost under $10 to make.

PeteHski

Ah ok. I don’t have that issue with either of my track pumps (Specialized and Topeak). Both have QR levers. My standard Silca Tattico head releases ok too, but the BT version I briefly tried and returned had the problem you describe.

PeteHski

+\- 2 psi would be fine for me. I’m not that sensitive to tyre pressure. I set my road tyres to 65 psi and if it was 2 psi out in either direction I wouldn’t notice.

I do have several gauges and they agree within a couple of psi, so I know I’m not miles out.

ofajen

It’s easy for me to consistently set tire pressure using a Meiser Accu-Gauge. I’m now on my second one in 45 years.

The new line has a bleeder valve so you can slightly overinflate with the pump and then bleed the pressure to the intended setting with the gauge. They have dial gauge models with ranges of 15, 30, 60 and 160 psi.

My gauge has a range of 100 psi, which has apparently been discontinued. I can read it to 1/2 psi easily.

The 30 psi gauge could be read to 1/10 psi. I can’t find a spec for the accuracy they claim but they are very consistent, which is what matters if you use the same gauge. And you don’t lose much air taking it off the valve.

Otto

KerryIrons

So we (and you?) should automatically assume that whatever you "normally use on [your] road bike" is correct and any other suggestion is wrong?

ScottCommutes

Has somebody seriously not yet invented a tire pressure measuring valve stem for bikes? Cars have this. It could broadcast to a phone or whatever and we could all continuously monitor our tire pressures,

As with hydraulic brakes, the tech comes to cars first and then eventually finds its way to bikes.

Sy Reene

Of course this due to calculated volume, however afaik, nobody takes into account tire height above the rim. Some tire brands measure ~2mm different from one another.

PeteHski

Of course they have. Quite expensive, but been around for a while.