Thanks to Marcag and others we have come to know that proper tire inflation for the conditions can really cost you a lot of watts or save you a lot of watts
Update I found a few pumps and another pressure gauge now even more confused
Schwinn Air Center Plus Pump
Pumped to 90
Air checker 2.0 shows 70
Topeak SmartGauge D2 56
Joe blow Pro
Pumped to 80
air checker 2.0 shows 91
Topeak SmartGauge D2 shows 77
Control tower Pump
Pumped to 80
Air checker shows 68
Topeak SmartGauge D2 59
I have a race in two days and I’d like to optimize tire pressure but now I am more confused than I was before. I got the tire pressure gauges .
Add to this that the pressure gauge doesn’t fit in the rear disc so how do you really know what the pressure is there?
Now you know why a Silca SuperPista Ultimate retails for $500, and why I was asking questions about the gauge being used for the tire pressure test.
There are good inexpensive tire pressure gauges out there, but you have no way of knowing which ones they are if you can’t test them with a known good gauge - which doesn’t come cheap. When you find one, you should wrap it in foam and baby the hell out of it.
It’s incredibly likely that the answer to your confounding results is that all your gauges are innacurate/imprecise/both.
That skepticism is entirely warranted. It’s worth checking out the stuff that John Karrasch has been doing with gravel tire rolling resistance. In particular, he has been talking/posting about the process of putting together a pressure gauge fit for purpose.
Personally, I have several pressure gauges - including one I put together that is very similar to what John wound up building for himself - and a Silca Ultimate. I benchmark my other pumps against the known good gauges, and know what the offsets look like at given pressures. For local races, I use my Silca and inflate before I leave the house. I replaced the gauge a while back, and the new one is both accurate and precise, given the visual constraints of the tiny dial markings and my crap eyesight.
For travel, I typically use a Fumpa that is good to within a couple of PSI for my road tires, reads high for gravel tire inflation pressures, and isn’t reliable at all for CX so I use an old Craftsman inflator that is somehow still dead on between 15-25 psi.
I have a couple of small gauges that I use for work/travel when I might not have access to my own pump. These were also baselined against the big expensive gauge.
Every once in a while I hook everything up and re-test, especially if anything gets dropped or tossed about. Or I just feel like I’m getting wonky numbers from something.
In short, it’s kind of a giant PITA to do this stuff “right”. And it would be great if it only impacted performance, and safety margins weren’t insanely close to error margins. But here we are.
FWIW, I was bored, so I just tested all of these in the 60-70 PSI zone, and they seemed to roughly agree within a few PSI:
Silca SuperPista Digital Floor Pump
Fanttik X9 Ace Mini Bike Tire Pump
CYCPLUS AS2 Ultra Pump
Topeak D2 SmartGauge
Topeak SmartGauge D2X
Accu-Gage
The only thing that fits in my disc wheel is the Fanttik X9 Ace Mini Bike Tire Pump hose/adapter. Or the Silca disc wheel adapter (or similar), so anything else would need to be connected to that.
it’s probably worth reiterating that if you’re seeing a “few” psi variance at 60-70 psi, you’re butting right up against the etrto safety margin of 10% if that degree of (im)precision holds through to the max inflation range.
Use a pump or gauge on your rear disc that fits in the wheel opening, and one that you have checked and determined its known and repeatable error.
So, say you want 85 true psi on the disc wheel, but you will be using a pump or gauge that reads 10 psi higher than actual. Well, use that pump or gauge and fill your disc wheel tire to “95 psi”.
Now you should have 85 true psi on your disc wheel.
Just a question, what was the methodology of those tests?
Did you blow up, then use the air checker, then the Topeak each time?
As you will lose pressure each time you connect as you depress the valve that allows the air to balance between the tyre and the measuring tool, then when you disconnect the valve closes sealing the air in the tyre, and then the air in the tool just escapes to atmosphere.
That said, 12psi would need a lot of volume* in the Topeak to account for that, so it’s possibly a contributary factor not the full story.
*I say volume here as if the volume of the system was large that would allow a larger measured pressure drop.
Good questions. It sounds like a methodology issue or really bad presta valves. I have two Airchecker 2.0’s and a Smartgauge D2 along with a handful of analog gauges and wheel sensors. It’s rare I will lose even 0.5 psi checking pressure. Going Airchecker to Smartgauge and losing 10-15 psi is unusual.
I did a fairly obsessive comparison a couple of years ago to trouble shoot the (crappy) Zipp pressure sensors and found excellent agreement between most of the hand gauges and my two pumps near worthless. More recently, the Cycplus inflators I have used are incredibly precise.
The Outrider TPS sensors are GREAT for disc wheels(as long as your rims are wide enough)
I got tired of guessing what the pressure was and after installing them in my race wheels it makes setting up in transition much easier. Well worth the cost
here is an interesting experiment I tried. I tied two pumps together by using an old presta vale with the core removed. I then attached the two pumps together via the presta tube. There was no leakage via liquid bubbles ie where the two attached when you put a soapy solution on the connections. I then pumped using one pump and compared the two gauges for reading and they were very very close. This provided evidence they were reading the same. I had a 3rd pump which was not close, it was out maybe 3-5 psi low. I do not use that pump much and mainly for my fat bike where pressure is less important to me. one of the pumps is a Silca Ultimate which I consider to be the “truth”. The hard thing about pumping a tire and then measuring is there are many chances of error:
1- air loss when removing the air chuck
2- temperature change (compression heats the air then it cools)
3- air loss to get a pressure gauge reading
not to mention just how accurate the gauge is. It is really hard to check a hand held air pressure test gauge. They typically don’t have a way to fit them securely during the reading so you can not use the pump to gauge attachment method. The best option IMHO is to test the pump gauge against some pressure gauge you know is true. Then just use that pump and its gauge to be your truth going forward. If you think you want to check tire pressure check gauges, I think you need to follow a careful protocol which is pump, wait, read. Then pump again, wait the same time and read again. If you get the same reading twice for the same pressure gauge reading set by the pump at least you know that you have a certain level of precision in your reading, if not then your test is not good, if the same gauge can’t get the same reading on repeat tests then it is likely you not the gauge that is in error. Testing is hard, precision is harder, accuracy is unknown unless you have a defined standard for comparison. Atmospheric pressure also plays a role depending on the type of gauge.
I have hesitated to join this discussion but I suggest one of your error sources isn’t and another is <1% so minimal.
1 When an air chuck is released there is no “air loss” from the tyre ie through the valve. Even if air in the pump is at greater pressure, at the instant of release the valve immediately shuts. The air release one hears is the air escaping from the pump and its cable where there is one.
2 The temperature of the air inside a tyre as it’s being pumped may increase by a few degrees above ambient but the impact on its pressure is minimal. An increase of 3oC (5oF) will have an effect of 1% on pressure (so half a psi in a tyre inflated to 50psi. Experimentation would be needed to quantify the average temperature of (all) the air inside the tyre after inflation.
a 7 deg C increase in pressure (assuming that the change in volume was approximately constant or zero) when the temperature returned to 20 deg C would be a change in pressure from 70 at 27 deg C to a pressure of 68.4 at 20 deg C using the ideal gas law and converting temperature to deg K as required. This is a 2.3 % change in pressure but would be noticeable on many gauges.
Also Silca had a discussion at one point about air loss when removing the chuck. It depends on the chuck, some actually depress the valve so those do leak some air, the others like theirs does not depress the valve and use pressure differential to get the air into the tire. those do not leak air from the tire. Errors are cumulative. Most people do not ride at a pressure of 50 psi, more likely in the 70 psi plus range…. unless you are riding really big tires.
So I grant they may be small but when added up the can count.
So as I said it depends and you at least know if you are precise if you get the same value 2x using the same technique/ protocol.
