Accurate shock pump digital or analogue

I’ve got a digital and a analogue shock pump and both give out totally different readings like 20-30 psi difference –  I have no idea which is correct – they are pretty old and beat up so may answer why

what’s out there these days that’s accurate or at least gives me a indication I’m in the right ball park figure and not 20-30 psi off recommended – I know folk will say it’s irrelevant just use sag but I like to know a starting point is accurate and then adjust rather than just guessing at everything

i ended up with a digital, mainly because a few psi can be quite dramatic on certain setups, however, i only use those figures to record my own settings so i can apply them precisely after servicing etc

TLDR it doesn’t matter what the gauge says as long as it’s repeatable

TLDR it doesn’t matter what the gauge says as long as it’s repeatable

This  Keep the one that’s easiest to use and don’t worry about the actual number that it shows.

How do you know they’re not consistent?

When you unscrew the hose the hiss is the ~200psi leaving the hose / gauge, when you re-connect it the shock fills the hose again.  So depending on the relative size of the shock volume and the pump’s hose / gauge you’ll always get a different answer.  It’s not like tyre pressure where there’s a huge volume relative to the pump so connecting / disconnecting makes no difference.

TINAS as the analogue one has a feature that stops it losing air when disconnecting or attaching

Yep I get all that about it not mattering, but of course it has relevance or what would be the point of having one, may as well have a pump with no readings on, I use them as that’s what they are there for (when accurate~) ? I like to setup as per recommendations on the chart and go from there and if the gauge is 30 psi then I’m starting off wrong, and it’s a complete guessing game and faff to get to desired sag – I just want to be able to attach it and set as per the guides as I find modern day guides are pretty accurate for me PSI wise (when I knew they were both accurate)

Digital pumps resolve better. i.e. will give a number rather than a vague pointer on a (even vaguer) scale, so for repeatability are ‘better’. They’re generally no more accurate though, as other’s have suggested.

Okey doke – I’ll have a look at newer digital ones mines is really old and the display is shoddy too

whats out there digital wise? Any to avoid for any reason? It needs to be shaped to fit a sit post air valve (I know some gauge heads can get be chunky etc)

What TINAS say.

I bet if you pump it up with either, disconnect then re-connect the same pump you’ll get a similar discrepancy.

as the analogue one has a feature that stops it losing air when disconnecting or attaching

It should close the valve on the shock as you unscrew it before it breaks the seal, but when you re-attach it has to open the valve and let some air out into the pump in  order to pressurize the hose and the gauge.  Hence whichever pump you use, if you then try and check it with the other (or even the same one) then it will give a different answer.

yup, fit pump, not value, disconnect pump, refit pump note value, note difference, that’s your loss. Add this next time you inflate to a required value

Add this next time you inflate to a required value

You dont need to add anything.

– Start from nothing/atmosphere. Pump to 200psi. Shock body and pump body are now at 200psi.

– Unscrew. Shock body remains at 200psi (think about that the way the valve springs shut) even though the pump reduces to zero/atmosphere. Nothing is ‘lost’ from the shock at this point.

– Now re-attach. Shock body volume is now increased to include pump body volume.  We havent lost any air at all, but its now residing in a larger volume, so our 200psi becomes 180 lets say.

This is what people confuse with ‘my shock was at 180psi, some must have leaked out whilst I unscrew the pump‘.

It wasnt. It was at 200. Only after you attach them, your shock PLUS pump are now showing at 180.

If you want 200 in your shock, pump it up till it says 200. Done. Theres no need to do any adding or subtracting.

I work as an Application Engineer in for a large manufacturer of Pressure Sensors, gauges, calibrators and controllers for my sins.

The one thing I would note is that “consumer” gauges/sensors be they independent or built into a pump are generally a bit shit, especially once they have been bouncing around in your car/pack/toolbox for months/years, been dropped etc etc

Digital gauges tend to suffer from temperature effects, so if you’re setting your bike up outside in winter at 5C, you might well be getting a different reading for the same pressure than in a warm kitchen or when you’re on holiday somewhere hot.  How much different? Heck knows.

Then you get into non-linearity, hysteresis and repeatability…  :sigh:  I have have never seen an accuracy statement though.

I would hope a digi gauge will be reasonably repeatable for a given pressure and temp, not entirely convinced it is but it’s the best bet.    They tend to quote just the resolution which isn’t the same.

The lack of electronics in mechanical gauges means that they have a different set of issues but with analogue gauges where your eyes are relative to gauge makes a huge difference.

TLDR:  I use a digi gauge on both tyre and suspension pumps.  I know they are a bit shit, it probably doesn’t really matter but use them as a rough reference only.

You dont need to add anything.

– Start from nothing/atmosphere. Pump to 200psi. Shock body and pump body are now at 200psi.

– Unscrew. Shock body remains at 200psi (think about that the way the valve springs shut) even though the pump reduces to zero/atmosphere. Nothing is ‘lost’ from the shock at this point.

– Now re-attach. Shock body volume at 200psi is now increased to include pump body volume.  We havent lost any air at all, tbut its now residing in a larger volume, so our 200psi becomes 180 lets say.

This is what people confuse with ‘my shock was at 180psi, some must have leaked out whilst I unscrew the pump‘.

It wasnt. It was at 200. Only after you attach them, your shock PLUS pump are now showing at 180.

If you want 200 in your shock, pump it up till it says 200. Done. Theres no need to do any adding or subtracting.

Very much this, Schrader valves will be sealed before you lose the seal between pump and valve.  That hiss you hear is pressure equalizing between pump and atmosphere.

– Start from nothing/atmosphere. Pump to 200psi. Shock body and pump body are now at 200psi.

– Unscrew. Shock body remains at 200psi (think about that the way the valve springs shut) even though the pump reduces to zero/atmosphere. Nothing is ‘lost’ from the shock at this point.

– Now re-attach. Shock body volume at 200psi is now increased to include pump body volume.  We havent lost any air at all, tbut its now residing in a larger volume, so our 200psi becomes 180 lets  say

Most shock pumps have a two step attachment – step one fits the hose to the shock, step 2 then opens the schrader valve. So, as long as you know roughly what pressure you expect in your shock (say 150psi)

– do step 1 of attachment so hose is connected but schrader is shut

– pump until guage reads approx what you expect (ie 150 psi)

– do step 2 of pump attachment process so schrader is now open and pump is reading the pressure in the combined shock+pump system. If the shock pressure was still where you expected it to be then the guage won’t move.

I suspect the pressure gauges fitted to shock pumps are pretty low quality but they’re a standard and easy to buy part.  You can buy replacements from less than £10 on eBay. I’ve got one pump that does fork pressure (middle of scale is about 80psi) and one that does high pressures now. They’re accurate (checked to shockwiz and each other) whereas the old ones on there were no longer. And have a readable scale.

cheaper than a new pump. I believe fluid filled gushes are more resistant to drops and vibration etc but more expensive.

thepurist
Full Member

Most shock pumps have a two step attachment

Yep, though it’s not always obvious. Tbh if it doesn’t, is it really a shock pump? Except those occasional tiny “emergency trail fix” ones.

But the bike industry being what it is, people keep selling it as a benefit rather than a basic so quite reasonably lots of people fall for it.

I have the Topeak Pocket Shox DXG – analog, and is repeatable.

I believe some forks/shocks are very sensitive to 1-2psi but I’m sure I can easily read this to within 2psi – with good eyes, good light, and a shock that isn’t in an awkward position. There are markers every 5psi so:

needle over the marker (equidistant from the neighbouring markers) = e.g. 100psi

needle just higher than the marker (marker just visible) = 101-102psi

needle halfway to the next marker (equidistant between two markers) = 102-103psi

TLDR it doesn’t matter what the gauge says as long as it’s repeatable

I bought the small lezene pump. I was very sad when it’s readings were massively out compared to my old analogue pump.

It’s out compares to the Mrp ribbon, fox and rock shock recommendations for my weight.

However it is small, repeatable and it has lasted for ages- 6 years.

https://www.merlincycles.com/lezyne-digital-shock-drive-97153.html?utm_source=PHG&clickref=1011lA5wdsga&app_clickref=1011lA5wdsga&source=PHG&utm_medium=Affiliates&utm_campaign=genieshopping

I removed the analogue gauge from my cheap fox pump and fitted a digital one from eBay to it and it’s miles easier to use.

I can only assume it’s accurate, but I measure my fork sag and use whatever the pump says as my base pressure.