[Ze_Zaskar]

Hi all,

Question regarding pressure loss on a Stumpjumper Evo’s DPX2 shock.

First a disclaimer, I know how to set shock pressure, equalizing the two chambers before setting and reading the final pressure.

My previous bike had a different leverage ratio and the RS Deluxe only required 160PSI to achieve 30% sag. It was pretty much set and forget, I took several weeks before I needed to touch up air pressure.

I noticed that I get a lot more pressure drop on the Evo’s DPX2. My baseline pressure for 30% sag is 260PSI. I did the following measurements:
26/06 – 260PSI
02/07 – 240PSI

That’s 20PSI in 6 days, 3,3PSI per day.
Is this normal??

 

[Shartist]

Which shock pump are you using? Also, keep in mind that when you hook up the shock pump you are pressurizing the hose which is at ambient resulting in a lower reading than what the pump was at before hooking it up.

What temperature was the bike/shock at on 6/26 and what temperature was the shock at for the reading taken on 7/2? How different was your sag after taking the reading on 6/26 vs the reading before you hooked up the pump to take the reading on 7/2?

 

[Ze_Zaskar]

Thanks, all relevant questions.

It's a genetic, conventional gauge shock pump from Decathlon. Always served me well and keeps consistent relative readings.

Ambient temperatures are really close between readings. Didn't measure sag, I have trouble considering that reliable enough.

I know when you connect the value there is a shock air loss that pressurises the pump. However, I would expect that to be a handful of PSI, not 20PSI or 8% of initial pressure

 

[Vespasianus]

Thanks, all relevant questions.

It's a genetic, conventional gauge shock pump from Decathlon. Always served me well and keeps consistent relative readings.

Ambient temperatures are really close between readings. Didn't measure sag, I have trouble considering that reliable enough.

I know when you connect the value there is a shock air loss that pressurises the pump. However, I would expect that to be a handful of PSI, not 20PSI or 8% of initial pressure

No it could be easily 20 psi. Also, remember, shocks contain a small amount of air at a high pressure. When there is a leak, it will not loose just a little air.

 

[Shartist]

I don't have the positive and negative air chamber volume values for both your old and new shocks so it's hard for me to run numbers to definitively tell you whether the air loss to the shock hose you are observing is (btw "pressure drop" and "pressure loss" are very different things) possible, but keep in mind p1v1=p2v2 where the first term represents the values prior to shock connection and the second term represents values after shock connection and consider the following:

-P1 of the Fox is greater than that of the RS deluxe, V1 may be smaller (keep in mind when you're hooked up, you're only really tapped into the positive air chamber).

-The greater the P1 value relative to ambient pressure, the higher the absolute value of the difference will be, while the % of total psig lost will be the same with the same pump hose(all else being equal). <edited to clarify>

-Smaller V1 will result in greater in larger P1->P2 change (all else equal).

I don't know the exact volumes for each of these shocks or your shock pump hose. That being said, I ran some rough "back of the envelope numbers" and a change in pressure on this order of magnitude is possible if the OoM of my volume guesses was in the ballpark. For this I assumed a 7 inch shock hose with a 0.18" ID (this is the ID of a SS tubing I could pull off the top of my head) and shock air chamber volumes ranging from 55mL to 60mL. Add in variability in how fast you can get the hose off, gauge reading error, etc. we are well within the realm of possibility. This assumes I guessed shock air chamber volumes in the ballpark. Additionally, I held the volumes of each the same for simplicity's sake knowing full well that they probably aren't.

Suffice to say, the hose theory is a viable possibility based on the gross assumptions I made on some of the metrics.

 

[Ze_Zaskar]

I'll just run a couple of tests.

First I'll set pressure as usual and re-measure right after. That should tell if the difference in pressure is repeatable without the time variable.

Then I want to try to equalise the two chambers. Initially, I'm reading 260psi on an equalised system. When I connect the hose, I measure the pressure of the equalised systems of positive chamber + pump, while the negative chamber is still at the pre-pump pressure. Maybe this explains it a bit

 

[Aglo]

As mentioned before it can easily be 20PSI. When connecting the pump to my DPX2 at around 200PSI I lose around 15PSI. At higher pressures you lose even more.

 

[mlx john]

I also have a dpx2, set at 230psi. When connecting my pump, I consistently lose 15psi.

 

[Schulze]

I lose under 10 psi at 200psi with my pump. My shock loses about 5 psi/month. 2018 DPX2 about time for a service.

 

[Ze_Zaskar]

So, after some investigation, I think my shock is OK.
For convenience, I’ll condense the methodology and the findings together:

1 – Connect shock pump, set pressure to 260psi
2 – Cycle the shock to equalize chambers
3 – Final set pressure to 260psi
4 – Remove shock pump
5 – Re-connect shock pump and read – There it was, 240psi, 20psi loss
6 – Cycle the shock to equalize chambers, without removing pump
7 – read pressure after equalization – 245psi, 5 psi were gained after chamber equalization
8 – Set shock pressure

I repeated the procedure 3 times, always with the same results. I think this explains what I was observing. It seems that, discount the pump effect, the shock lost little to no pressure on my previous weekly readings