# Tire/Shock Pump Gauge Accuracy



## Too_Fast_46 (Apr 16, 2011)

I have recently purchased a RockShox shock pump, then an Avenir tire pump (while just cheapo tire pump it still makes a point.) I have compared both of these to two digital tire guages which I consider to be the 'correct' pressure to compare all other gauges. 

What I have found is the RockShox pump measures 10psi low and the Avenir pump 8psi low. Has anyone else had a similar experience or is it just me?


----------



## the-one1 (Aug 2, 2008)

Use the gauge as guide and not rely on it for accuracy. And use the same pump on the same fork/shock, that way you know its consistent.


----------



## Too_Fast_46 (Apr 16, 2011)

I understand what you mean, which is why I'm still happy with the purchases. But it makes me wonder because in this shootout Review of Shock Pump Shootout - Competitive Cyclist it shows similar results. And with the max 50psi in my fork, it would be easy to over inflate.


----------



## sasquatch35 (Jun 8, 2010)

Whenever you remove a shock pump from the valve, you usually lose 5 to 10 psi because most thread onto the valve rather than clamp over it. It's always a good idea to pump an extra 5 or 10 psi into the suspension to compensate. If you're checking the pressure with your gauge after inflating it with the shock pump, you will most likely read less than you thought you really put in there.


----------



## BShow (Jun 15, 2006)

I have trouble believing that any of these guages are accurate. They should be consistent, however. With that in mind, I'm firmly in the camp that they're good to have and use so that you're consistently running the pressures that you want or need - even though you might not know exactly what that pressure is. Running 10+/- psi more or less than the recommended settings certainly won't hurt anything. In fact, you may like the ride better. Use the guages to figure out what works best for you and once you found the spot, keep it consistent by using the same guage.


----------



## Wildeyes (Jun 14, 2007)

Your shock pump should NOT leak air out when taking it off. The air you hear leaving is from the tube and the lever or contraption you switch before you take off the shock pump should keep the air in the fork and let the air out of the pump tube so it comes off easy.


----------



## 4slomo (Jul 4, 2006)

This is the correct interpretation. I'll state it a different way: The fork/shock air valve shuts before the pump head disengages from the valve stem. The air you hear escaping when the pump head comes off is from the air pressure remaining in the pump tube, and not from the fork/shock.

When you connect a pump tube to an air fork/shock, high air pressure in the fork/shock equalizes with low pressure in the pump. The resulting pressure drops, and you have to pump to get it back up to get to the pressure you want.

When you connect an air gauge, it has a much smaller air volume than a shock pump does, and the equalized pressure therefore doesn't drop as much.



Wildeyes said:


> Your shock pump should NOT leak air out when taking it off. The air you hear leaving is from the tube and the lever or contraption you switch before you take off the shock pump should keep the air in the fork and let the air out of the pump tube so it comes off easy.


----------



## gdlals (Mar 3, 2008)

Gauge accuracy is really not important. Gauge repeatability is what is very important. If a gauge consistently measures 10psi below what you consider to be the correct reading, this is OK because you can adjust scales.

if however everytime you use a gauge in exactly the same conditions you get a different result then it is a crap gauge.


----------



## customfab (Jun 8, 2008)

sasquatch35 said:


> Whenever you remove a shock pump from the valve, you usually lose 5 to 10 psi because most thread onto the valve rather than clamp over it. It's always a good idea to pump an extra 5 or 10 psi into the suspension to compensate. If you're checking the pressure with your gauge after inflating it with the shock pump, you will most likely read less than you thought you really put in there.


The air you hear hissing out comes from the pressure that is built up in the hose and gauge not from the fork. The reason it will read a little lower when you hook it back up is that you just expanded the volume.

OP use the same pump or at least the same brand all the time and your golden. Don't sweat the small stuff.


----------



## axelbaker (Jun 17, 2011)

What you might want to do is find a more appropriately sized gauge. As has been said, the number isn't as important as consistency. But an appropriate size will assist in being consistent. (Think trying to measure 1 cup in a 5 gallon bucket vs a 2 cup measuring cup)

I run my fork at about 45 psi which on a 300 psi +/-10% accuracy gauge is essentially impossible to measure, consistent or not. The gauge needs to matched to the purpose, for the fork I use a 0-60psi +/- 10% gauge, for the rear shock a 200psi. The 300psi gauges on most bike shock pumps is just dumb. You can go to a motorcycle shop and get a pump with a more appropriate gauge (they also use the same Chinese factory for their pumps as the bicycle industry), or just change the gauge out. It's not that hard to take them apart and pull off the gauge, a decent hardware store or industrial supply (McMasterCarr, Granger, etc... ) has hundreds of gauges to choose from and depending on what you want to spend you can get them to about 1% accurate.

One of these days I keep meanting to put a T-fitting on my pump and put both gauges on one pump.

Lastly don't worry about that "50psi max" on your fork. The pressure goes way over that while you are riding, I would be 100psi internally at 1/2 way down the stroke, and 200 at 3/4. That's just the max useful pressure while it's sitting still. It would just be harsh and uncomfortable if you put more than 50 psi in it.


----------

