Digital pressure gauges

[Danneaux]

Hi All!

Today's mail brought a new Schwalbe Airmax Pro digital tire pressure gauge and with it, the hope for more accurate low-pressure measurements.

I currently have four Zéfal Pressograph tire pressure gauges. While they agree to within a pound or so at my usual 85psi/5.86bar, they are off by as much as 6psi/.41bar when measured against each other at what *should* be 29-36psi/2.0-2.5bar. None were spot-on.

Since those gauges were so far off at low pressures, I've been using a certified bourdon tube-type Schraeder tire pressure gauge intended for autocross events. It is accurate at low pressures, but is still an analog gauge, so there is some leeway in accuracy due to parallax when reading the needle against the dial markings. Besides, it is a hassle to use 'cos I have to also fit a gasketed, screw-on presta-to-Schraeder adapter each and every time I want to check pressures on the Nomad.

Plainly, I needed something better and more convenient.

Looking at what was on offer, there were three contenders:
• Schwalbe Airmax Pro: http://www.schwalbetires.com/accessories/helpful_tools/airmax_pro
• SKS Airchecker: http://www.sks-germany.com/?l=en&a=product&r=tools&i=2000&AIRCHECKER
• Topeak SmartGauge D2: http://www.topeak.com/products/Pumps/smartgauge_d2

All three use a CR2032 battery. All can be had for a similar price if you shop carefully (USD$22-$28).

There was nothing "wrong" with the others, but I chose the Schwalbe for several reasons:
• Compact size and light weight. The Schwalbe is small (21x45x97mm at the thickest/widest/longest and 34.7g/1.22oz...something I'd actually take with me on the bike, rather than use just at home. It feels good in the hand.
• No bleed valve and no swiveling head. <-- this would seem a disadvantage at first, but in my experience these features introduce joints that can leak over time. Also, some of the gauges do not re-sample as you bleed the pressure, necessitating a manual re-sampling anyway. I decided to keep it simple and bleed the pressure by hand and then recheck.
• No backlight, conserving battery life.
• It reads in PSI, BAR, kg/cm2, and kPa, making conversions instant for me across the board.
• 0.1psi readout.
• It remembers the last reading on startup, a 1-second push on the button clears it and 2-second push turns it on/off. It gives a confirmation beep when pressure is reached, and turns itself off after a few seconds to conserve the battery. The presta valve gasket in the brass end seals completely and only needs a contact seal to get pressure...no need to wrestle it on/off the valve. It will fit Schraeder or Schwalbe's "classic" valves if the brass adapter is removed.
• Cost was reasonable at USD$22.99 postpaid from an eBay vendor.

If I were to choose a digital gauge for home/shop use, I would likely go with the SKS, as it has a backlit display with large digits and the larger case size wouldn't bother me.

After the gauge arrived today, I put in a lot of miles playing with the Nomad's tire pressures and there have been some interesting results already. For example, I when I have ridden the Nomad fast on rough ground, it induces a bit of a whiplash effect on my neck when hitting bumps. I have been experimenting with decreased front tire pressures with little effect. Imagine my surprise when I found a reduction in *rear* pressure had a much more profound effect, turning the rough ride into a smooth one. There's lots of fun ahead!

Oh! One last tidbit...the Schwalbe gauge is blue, a color they had to license from Park Tools, who has apparently trademarked the use of blue for bike tools! All Schwalbe's advertising and the gauge package note the color has been licensed. I'll bet Schwalbe wishes they'd thought to trademark the color first! D'you suppose Pedro's "owns" the right to produce yellow bike tools? If not now, then soon....

Best,

Dan.

[JimK]

Did I miss it Dan, or maybe you haven't told us: down in the 30 - 40 psi zone, how accurate does the Schwalbe gauge seem to be? Very nice that you have a more accurate reference to compare it to!

[Danneaux]

Ah, Jim...I think my mind was typing faster than my fingers and I forgot to be specific on this; thanks for the reminder!

So far, the gauge is a match for the certified bourdon-type analog gauge I have, though the resolution (to 0.1psi) is far finer on the Schwalbe than on the analog gauge. In other words, what looks like 25psi on the analog gauge can read 24.8-25.2psi on the Schwalbe. It also seems to be pretty much spot-on in the 30-40psi range. I'd be happy with 0.5psi accuracy. My Zéfal gauges are optimized for accuracy in the 85-105psi range and are terrible at lower pressures. So far, the Schwalbe seems to be good at the lower ranges. I haven't tried it on the rando bike or tandem yet; both those bikes' tires run at 85psi.

[EDIT: Jim, my certified bourdon analog gauge tops out at 40psi, so I can't check the Schwalbe at higher pressures. It is close to what the Zéfals are, but I have no idea how close to accurate the Schwalbe is at those higher pressures 'cos I have no standard to compare it to].

Without sending it in for separate testing, the best I can say is, "It's pretty much a match", but far more convenient than what I had been using; no adapters to switch in and out, so I can make changes much more quickly.

Speaking of changes, I experimented with pressures a lot today, and was astounded at how well the bike still rolled and how comfortable it was with F/R pressures of 25/26psi or 1.72/1.79bar. The sidewalls weren't bulging, and rollout was good.  I really want to do some extended rollout tests at different pressures to see what feels optimum when the bike is largely unladen. Of course, a touring load will require increased pressures.

Best,

Dan.

[Andybg]

If only ford had thought to trademark black for cars.

Has the world gone mad?

Andy

[Andre Jute]

I use the same pressure gauge, Dan, except that instead of Schwalbe Airmax, mine is branded BBB. I've had it for donkey's years and carry it in my ultralightweight toolkit on the bike to use if I should top up the wheels at a service station.  I don't care so much about accuracy but I do care about consistency, and this gauge is perfectly consistent. (It's also pretty accurate.) Good choice!


http://bbbcycling.com/accessories/pumps/BMP-90

http://www.youtube.com/watch?v=A5LFDZlPRGk

Andre Jute

PS I looked at the Airmax but with a green bike (with gold coach lines) I was simply not having a bright blue gauge. So I bought the BBB one instead. I like BBB, a Dutch bike parts, accessories and clothes brand.

[Matt2matt2002]

Following this one with great interest.

By changing the pressures are you increasing or decreasing tire wear?
Is this a measurable factor or indeed important?

Matt

[Andybg]

My gut feeling would be that there will definetly be an effect on tyre wear with varying pressures. The simplest maths would say with reducing pressure you are reducing tyre wear but doing so over a larger contact area but unfortunately having done some mathermatical modeling of tyre behaviour there turns out to be nothing simple about it.

I would say the priority has to be comfort and handling with tyre life being the "to be seen" part of the equation.

Andy

[Danneaux]

By changing the pressures are you increasing or decreasing tire wear?

Hi Matt!

You ask an excellent question I can't answer except to say, "It depends". Pressure that is excessively high or low for a given load, tire section, and width, can increase tire wear.

My primary interest in changing tire pressures is to find the pressure that is most comfortable at a given load rating without unduly increasing rolling resistance.

I can say from some experience that increasing pressure means there is less rubber in contact with the road surface, and wear at the contact patch does seem to increase as a result of the reduced surface area. I've had some tires I ran at the upper end of their pressure range that started with a domed profile and ended up essentially flat in the center. On the other hand, if the tire is a bit low on pressure, more of the tread is in contact with the road surface, so the rate of wear should be reduced as a product of increased surface area. Of course, there are diminishing returns; beyond a certain point, rolling resistance will increase as a result of casing deformation and sidewalls can crack if pressure is too low for a given load; cracking is definitely "wear", and riding a too-low tire is no fun.

Is this a measurable factor or indeed important?

Hard to measure without a remote-sensing pyrometer to detect temperature changes as a function of casing deformation and serving as a realtime proxy for "wear" under controlled conditions, along with tread-depth measurements correlated with load, pressure, road, surface, speed, etc. Some tire makers do test development and production samples in just this way, but there are so many variables and the equipment is so expensive, it is impractical for consumers.

Is a correlation between wear and pressure important? It can be at the extremes, but if a user follows recommendations for load (weight) and tire section width, wear won't be excessive and reasonable tire life can result.

Andre!   I was perfectly happy with my new gauge until I saw your stealthy black one!   What a pity mine doesn't match the Nomad!   Ah, me...and not a BBB dealer in sight here in the States. Good to know they function the same. I'll keep telling myself that as I pine for a black one. <sigh>  

 

All the best,

Dan.

[Matt2matt2002]

Thanks folks
I thought I this might a difficult one to answer subjectively.

Is a correlation between wear and pressure important? It can be at the extremes, but if a user follows recommendations for load (weight) and tire section width, wear won't be excessive and reasonable tire life can result.

Excuse the pun, but where will I find the recommendations for load and tire pressures?
On my Marathons there appear to be a min and max range.
Nowt about owt else.

Matt

[Danneaux]

where will I find the recommendations for load and tire pressures?

Hi Matt!
'Bout everything you could hope for on pressure recommendations is in this thread, along with links to Andy Blance's and Schwalbe's recommendations: http://www.thorncycles.co.uk/forums/index.php?topic=3798.0

The main thing is to not over-inflate fat tires on (relatively) skinny rims, else the rims can split (AndyB's warning, referenced above). Sschwalbe say don't ride the tires with too little air or you'll crack the sidewalls. In-between...that's the back-and-forth in the above link.

For more, go to these Forum archive searches I've customized for you: http://www.thorncycles.co.uk/forums/index.php?action=search2 ...and... http://www.thorncycles.co.uk/forums/index.php?action=search2

Best,

Dan.

[Danneaux]

Hi All!

Updating this thread...

Last week, Performance sent me a USD$10 coupon to use in their store as I'd like (apparently this perk is available to all registered customers who live near one of their retail outlets), so I dropped by to see what they had. After perusing the aisles, I noticed they had an "open box" example of the SKS AirChecker digital tire pressure gauge heavily discounted from USD$24.95 down to USD$15. It was complete except for the cardboard and plastic blister packaging and included a fresh battery and all paperwork. Only USD$5 after my discount -- Score!

Checking it against my SKS digital gauge, I see they differ from each other by a consistent 0.2psi across their range. They are pretty evenly split on either side of my certified Bourdon gauge, so close enough. As noted earlier, the SKS is slightly larger but also has a lot more features for the price/packaging and is really the business for shop use and is only a little bigger for touring than the Schwalbe version. The SKS...
• Comes with a little drawstring case to keep the gauge's LCD viewport from getting scratched and I use the ties to store it hanging from the handle of my SKS Rennkompressor pump (thanks again to Andre for the pump recommendation).
• The head swivels 90° each way to place the gauge in an easy-read position. One side is Presta, the other Schraeder; the Schwalbe has a single universal port that is nicely angled.
• Unlike the Schwalbe which stores records but must be cleared before each subsequent use, the SKS will read dynamically so long as it remains pressed to the valve. This is particularly handy because...
• The SKS has an air-bleed valve that works a treat; just push to release a tiny bit of air, and the gauge will recalculate as soon as you let go and pressure stabilizes. You can adjust pressure downward to the nearest 0.1psi.
• The SKS also uses a standard CR2032 battery, but the display has an orange backlight, a real help in dim light -- they even optimized the viewing angle. Of course, it also reads in PSI or BAR.
• Like the Schwalbe, the SKS also shuts down after a short delay to preserve the battery.

There's no doubt the SKS would be my choice for shop use, but the Schwalbe will remain in my underseat bag for its smaller size and  more rounded profile. If I could only get one, I think it would be the SKS. However, for take-along use I'd likely go with the Schwalbe for longterm reliability and battery life as noted in my first post above.

Both digital gauges are much more accurate and easy to use than my collection of analog Zéfal Pressographs (partly 'cos the digital display is more precise) and are right up there with my certified Bourdon gauges for accuracy.

Best,

Dan. (...who is declaring himself "air" to the Gauge Empire)

[julk]

+1 for the SKS Airchecker, it is excellent.

I treated myself to one recently, full price though!
Julian.

[Andre Jute]

Congrulations on your fancy gauges, gentlemen.

Dan, did you check the accuracy of the Rennkompressor analogue gauge? I know there's a possibility of parallax error, and the divisions are awfully small, but it never seems more than 10% out by comparison to my digital gauge, which is good enough for us mortal cyclists.

As an aside: on checking my tyres today, I found both the front and the rear down to 1.27 bar.  I rode the bike for the first time in weeks yesterday, though only down to town, and noticed no ill effects. In fact, today, on properly inflated tires (2.03 bar), the slippery roads were more wriggly than the low-inflation tyres yesterday. Makes you wonder what the snakebite limit of balloons on proper-width rims actually is.

[Danneaux]

Dan, did you check the accuracy of the Rennkompressor analogue gauge?

Well, no, Andre, I haven't.

The tick marks on my Renkompressor gauge are so coarse as to be useless for fine measurement, so I pump to the next increment above what I need, then bleed down and re-sample with my digital gauge till I get the pressure spot-on, allowing for the blowoff puff when I remove the gauge.

For my rollout trials and such I want consistency and approximate just won't do; it has to be spot-on to 0.1psi if possible and certainly to 0.2psi as a minimum. I really needed both these digital gauges.

Does your Rennkompressor have the same indices? I'll check next airing, minimize parallax, and see how close the measurements really are. I'll report back here when I do.

All the best,

Dan. (...who thinks "approximate" is close enough for riding but not for testing and any gauge beats a looksee or thumb)

[Andre Jute]

The tick marks on my Renkompressor gauge are so coarse as to be useless for fine measurement, so I pump to the next increment above what I need, then bleed down and re-sample with my digital gauge till I get the pressure spot-on, allowing for the blowoff puff when I remove the gauge.

Yes, that's what I do too. The Rennkompressor gauge is also pretty small and a long way away between your feet. All the same, "a spot over 2 bar plus a couple of strokes to make certain" is pretty consistently 2.35 bar on my BBB digital gauge (exactly the same as your Schwalbe one but in a dignified black) which is precisely what is required to reduce to my desired 2.03 bar with one finger on the Presta and the other on the digital gauge reset button for the few moments until the gauge reset resets, at which point the double-check measurement reads 2.03 bar. This is so repeatable that if I'm in a tearing hurry, with the pedalpals already warming up, I don't bother to check.

Since someone is bound to ask, 2.03 bar is scientifically calculated (I sucked the thumb on my left hand while focusing on the rotary log scale of my old flying watch, as illustrated at http://coolmainpress.com/andrejutewatches.html#Navihawk -- don't try it at home, you'll go cross-eyed and give yourself a migraine) and empirically proven (okay, I did it over and over and the result was the same every time) to permit me a soft enough ride at the beginning of the month, and not to require inflation again for at least a month, at which point the pressure will be about 1.65 - 1.7 bar and the ride is really cushy. Engineering with all the required precision, and not a gnatbite of a decimal more!

PS Of course, what matters in a gauge for everyday riding or even racing isn't that the precise measurement be exactly spot on, as long as the error is not so large as to interfere with operation of the component being measured,  what matters is that the reading, including the error, be consistent every time. That's what makes a reliable gauge. That's what makes the top racing mechanics in the world trust the Rennkompressor; it is what they mean when they speak of its "precision".