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Tire Gauges are Never Accurate

Old 11-29-15, 03:27 PM
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kenshireen
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Tire Gauges are Never Accurate

I have a blackburn 3 pump and wanted to see how accurate it was so I purchased a spin doctor digital tire guage.
The only problem is that whenever I pop the guage on the tire it loses some pressure (presta).

So what is the purpose of a tire guage if you always lose pressure when you use it...
How do you get an accurate reading.
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Old 11-29-15, 03:48 PM
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you always lose some air using a pump or garage . air pressure is garage in a rank . like 80-100 psi . if you within the ballpark you are good .
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Old 11-29-15, 03:53 PM
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You have rediscovered the Heisenberg Uncertainty Principle. The very act of measuring something, changes it. Therefore, you can never know its original state. But how closely do you need to know air pressure?
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Old 11-29-15, 04:14 PM
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there isn't a purpose. precision is more important than accuracy and a tire gauge separate from the pump is good for neither
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Old 11-29-15, 04:51 PM
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Originally Posted by zeeway View Post
You have rediscovered the Heisenberg Uncertainty Principle. The very act of measuring something, changes it. Therefore, you can never know its original state. But how closely do you need to know air pressure?
Actually the OP has discovered the Observer Effect. In Physics, this is the effect that says subsequent measurements can't be accurate, as the system is affected/thrown out of static equilibrium by the measurement instrument. Or something like that. It has been 35 years since my last college physics class!
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Old 11-29-15, 04:58 PM
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Originally Posted by kenshireen View Post
I have a blackburn 3 pump and wanted to see how accurate it was so I purchased a spin doctor digital tire guage.
The only problem is that whenever I pop the guage on the tire it loses some pressure (presta).

So what is the purpose of a tire guage if you always lose pressure when you use it...
How do you get an accurate reading.
Also, why are you so sure the Spin Doctor gauge is any more accurate than the dial gauge on your pump? Neither are precision gauges.
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Old 11-29-15, 05:07 PM
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You need to calibrate your digital gauge. (No not that "digital" gauge you picked up in a bike store. The gauge you were born with. Index finger and thumb. Flesh and blood digits.) Pump up to your best ride. Squeeze the tire. Do this every time you ride. After a while you will know what proper inflation feels like for your tires under your weight.

Best part? Works when you don' have your tools. Works with other's pumps that may well not be calibrated as your pump is.

I rode 25 years on sew-ups and raced without a gauge.

Ben
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Old 11-29-15, 05:38 PM
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Car (especially truck/SUV) tires make a very handy crude calibration tool. The volume of the tire is very large compared to the volume lost in a gauge/pump so you can use a tire as a reference pressure to tell you how close the pressures are. This will tell you if one gauge reads high or low compared to another gauge. What it won't tell you is what the absolute pressure is.

My experience is that most gauges are +- 5psi (which is a stupid huge range). Ideally, you need a properly calibrated gauge to compare against. Alternatively, if you always use the same pump, the absolutely calibration doesn't matter, since the offset will be constant.
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Old 11-29-15, 05:47 PM
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Yes. A large reservoir's pressure will change little with the small amount of air released when changing gauges. A compressed air tank works well for this. The Meiser Accu-Gage is a decent relatively inexpensive mechanical gauge that is calibrated to a specified accuracy of ± 2% from 30% to 60% of scale and ± 3% below 30% and above 60%. Few if any bicycle gauges/pumps have any accuracy spec.
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Old 11-29-15, 06:18 PM
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My Accu-Gauge is: a- accurate. They don't mess around. B- it's repeatable. I lose a fraction of a PSI to measurement air loss in a measurement. C- if I ever get so nuts about this (or facts come to my attention) that I think this is important, I'll over-inflate by a couple pounds and use the button to drain the tire to what I want.

There's a phrase that is much more common in engineering than in physics, "for all practical purposes." By that standard a good pressure gauge is good enough.
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Old 11-29-15, 06:20 PM
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Accuracy is less important than repeatability IMO. Once you find a (indicated) pressure that rides the way you like it you just need to be able to get back to it. How it stacks up to a calibrated instrument is immaterial as long as you can get the same pressure consistently. You don't need you tire pressure traceable to national standards, just what works for you.
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Old 11-29-15, 06:31 PM
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Try an experiment, and post the results here.

Fill your tire, then apply and remove the digital gauge 10 times. How close are the readings? Is each reading slightly lower due to air loss, or are they about the same? I wonder how repeatable these gauges are.

And that will be good practice toward applying and removing the gauge quickly and correctly.

(When you pull off a floor pump, that air escaping is mostly (or almost all) from the hose, not the tire. )

~~~~~~
Using thumb pressure:
I can tell a difference in the ride between 80 psi and 90 psi. I'm not sure I could feel the difference with my thumb. I have ridden with thumb test if I was in a hurry, and it showed I had enough air to avoid pinch flats.

Last edited by rm -rf; 11-29-15 at 06:36 PM.
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Old 11-29-15, 06:31 PM
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My pump says my rear wheel is 100, and my front's 90. Close enough.
If they're really 103 and 92, it doesn't matter.
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Old 11-29-15, 06:42 PM
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Originally Posted by dsbrantjr View Post
Accuracy is less important than repeatability IMO. Once you find a (indicated) pressure that rides the way you like it you just need to be able to get back to it. How it stacks up to a calibrated instrument is immaterial as long as you can get the same pressure consistently. You don't need you tire pressure traceable to national standards, just what works for you.
Bingo! Andy.
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Old 11-29-15, 07:04 PM
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Ex-racer (cars, not bikes), I'm a bit of an accuracy freak when it comes to tire pressures. I use a pro quality inflator and gauges for for my truck/car tires to this day. For bicycles, I don't use stand alone gauges for pressure checks. Use a shop quality inflator attached to an up to pressure air compressor...and presta valves. +/- a few psi on a nominally 95psi bike tire doesn't actually matter that much to me, I just want to know what it actually is. At least this one's a benign fetish...unlike some of my others.
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Old 11-29-15, 07:17 PM
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Charlie Chan said "Man with one watch knows the time. Man with two watches, never sure." The same is true of bicycle tire air pressure gauges.
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Old 11-29-15, 07:49 PM
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Originally Posted by ltxi View Post
Ex-racer (cars, not bikes), I'm a bit of an accuracy freak when it comes to tire pressures. I use a pro quality inflator and gauges for for my truck/car tires to this day. For bicycles, I don't use stand alone gauges for pressure checks. Use a shop quality inflator attached to an up to pressure air compressor...and presta valves. +/- a few psi on a nominally 95psi bike tire doesn't actually matter that much to me, I just want to know what it actually is. At least this one's a benign fetish...unlike some of my others.
To add to this, the gauges are not that expensive. Decent aircraft tire guages are about a hundred bucks.
This really isn't that big of a deal for someone who owns high end car, maybe a nice motorcycle or a high end bicycle or two.

I'm a bit retentive about pressure, inspecting tires for cuts, replacing when they show signs of wear, etc. Considering that I regularly go 45 mph on the bike with contact patches the size of quarters, I wouldn't call an accurate and precise gauge a fetish.
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Old 11-29-15, 08:02 PM
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From what I have seen, the gauges on offer for bicycle use are utter junk; complete rip-offs.

However, I would certainly find good use for an accurate one if I had it: I still have the nagging suspicion that I had to prematurely retire an otherwise fine pair of pretty expensive studded tires because of excess pressure leading to center row studs wearing through the casing and causing chronic flats.

The problem, with just going by 'what feels right', or settling for a gauge which, though it might help me in repeatedly inflating to the same pressure cannot actually tell me what PSI is in the tire and whether or not I am exceeding the manufacturer's rated maximum, is that it leaves me in the dark as whether the problem was with the tires or with their user.

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Old 11-29-15, 10:14 PM
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People are overly concerned about tire pressure accuracy or precision. Since there's no to calculate ideal tire pressure for any bike, tire, rider o road condition, the numbers themselves are meaningless, except as a reference. So find the number that corresponds to your personal preference, then use the gauge to consistent fill to that number. It might say 95psi, but actually be 85psi, bit it doesn't matter because an "indicate" 95psi is right for you whatever it really is.
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Old 11-29-15, 10:17 PM
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I believe at least the tire gauges for fat bikes are accurate to within a PSI or so. Granted they are only used up to about 6 PSI or so...
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Old 11-30-15, 06:41 AM
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Old 11-30-15, 07:51 AM
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I totally disagree with those who say there is no need for an accurate gauge.

Manufacturers put recommended pressures on the sidewall of the tire and it makes no sense to buy $50 tires and run them too low or high, outside of manufacturers spec because your gauge is off.

Given the relatively small contact patch and high speeds on a modern bicycle, getting a tire within manufacturer's specified range is not being "overly concerned." Having an accurate gauge, correct pressure, inspecting for cuts, replacing tires when worn, etc., are all small prices to pay when compared to the risk.
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Old 11-30-15, 07:53 AM
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Originally Posted by TimothyH View Post
I'm a bit retentive about pressure, inspecting tires for cuts, replacing when they show signs of wear, etc. Considering that I regularly go 45 mph on the bike with contact patches the size of quarters, I wouldn't call an accurate and precise gauge a fetish.
A "fetish" for inspecting tires for cuts and replacing them when worn is a very useful trait. Agonizing over the exact pressure reading and insisting on knowing it to within ±1 psi is not.
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Old 11-30-15, 08:29 AM
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Originally Posted by TimothyH View Post

Manufacturers put recommended pressures on the sidewall of the tire and it makes no sense to buy $50 tires and run them too low or high, outside of manufacturers spec because your gauge is off.

Given the relatively small contact patch .....
Stop and think for a moment. The size of the contact patch is a function of tire pressure and load. Since load on a bicycle varies so greatly among riders, there's no way that a manufacturer spec. can meaningfully relate to contact patch size.

Those who prefer to paint by numbers can blindly follow a meaningless spec (except as it relates to maximum pressure), but the correct pressure is something that each rider has to discover himself by finding the best balance between road resistance, traction and comfort. Once that's been found experimentally, a tire gauge can be used to record that pressure, and duplicate it for future reference. That ability to match the optimum pressure doesn't depend on accuracy, but on repeatability.
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Old 11-30-15, 09:55 AM
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I generally go with "what works" on my bikes with one notable exception. I recently purchased 45Nrth studded tires that can be run anywhere from 35-75 PSI. After e-mailing them, to get maximum grip on REALLY bad days I should run it between 35-40 PSI, but NO LOWER. To achieve this I use a gauge with a release valve on it. (The ones that hold the pressure until you push a button.) Using this button, you can release the pressure in your tire to a desired PSI, then the only air you lose is when you take the gauge off of the valve, which is often much less than the air you lose when you put the gauge ON. I'm not sure if the PSI is 100% accurate, but at least it's precise.
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