Shock pressures

steve-oh · Post by **steve-oh** » Tue Mar 08, 2016 5:29 pm

Finally got to drive it pretty hard this evening, I started out with 150 psi all the way around, been driving great on the street , no sway, little stiff got i n the dirt and it was extremely stiff, dump all the nitrogen out suspension really loosened up like it should, high speed still felt stable(60-70) any problems with no charge in the shocks, ....I'm in Florida running mostly dirt roads with wash board, back trails with 2-3 ft woops , no 3-4 hr desert racer mostly Street riding, but it gets drove hard when in the dirt, can check out the build thread in rides, non broncos

johncharlesb · Post by **johncharlesb** » Tue Mar 08, 2016 5:46 pm

As I understand it, no pressure in a reservoir shock will cause it to cavitate with high shaft speed.

When I order new schrader valves the tech told me that low pressure will cause seal leaks.

Relevant race-dezert threads:
http://www.race-dezert.com/forum/thread ... rging.991/

http://www.race-dezert.com/forum/thread ... psi.15457/

steve-oh · Post by **steve-oh** » Tue Mar 08, 2016 6:00 pm

That's the two things I was thinking would happen, I think I'll start with 50 and see how that affects things

steve-oh · Post by **steve-oh** » Tue Mar 08, 2016 6:37 pm

Ok after reading that, if I understand correctly, revalving the shocks is what I'm looking at rather than removing pressure, but little pressure as possible is what I'd be looking for

mobil1syn · Post by **mobil1syn** » Wed Mar 09, 2016 8:40 am

we have found 200 psi is what works well. the pressure is set with shock full extended

lowering the pressure as mentioned above can lead to all kinds of badness in the shock. seals destroyed, in rare instances the ifp will turn in the reservoir and wedge its self in there.

steve-oh · Post by **steve-oh** » Wed Mar 09, 2016 9:03 am

After more reading I think a revalve is in order and going with a flutter stack on the compression side

Tchajagos · Post by **Tchajagos** » Wed Mar 09, 2016 10:42 am

The pressure in the shock is not for stiffness it's to keep it from cavitation. Essentially when you compress the shock so rapidly the piston pushes the oil up and forms a vacuum underneath. At zero pressure (a vacuum) liquid instantly boils into a gas causing the oil to foam up. So not only does the shock now have a gap of no oil to use to slow it on its way back down but now the oil that is there is partially vapor and flows a lot faster through the valves plus vapor can compress. Faster flow through the valve and being able to compress the fluid/gas equals less dampening and you get shock fade. The pressure try's to push the fluid together to keep it from cavitation. At zero pressure there is really 14.7psi (at sea level) keeping that Piston from drawing a vacuum. Think of how easy it is to block the tip of a syringe and draw a vacuum inside. Now imagine a 200lb weight on the handle of the syringe. A bit harder to draw a vacuum.

You are correct that you need to revalve. This should be the only thing that gets changed to give a softer or more firm ride.

steve-oh · Post by **steve-oh** » Wed Mar 09, 2016 5:47 pm

I spoke with JD today at Foa, and he said I need to drop down to a 200 upper, he thinks it's the spring rates are to stiff for my liking, but with no pressure in the shocks , I know that's not right, the suspension is a lot more soft to my liking so I was figuring a revalve, I have access to some 175 uppers that I'll try but I am skeptical about the spring change considering what I've seen on some other solid axle trucks and I figured my 250/350 would be to light to begin with

hollandermotorsports · Thu Mar 10, 2016 8:38 am

Couple questions Are you using a cross over ring? If so you need to be looking at the bottom spring not top. If not you need to otherwise you are relying on coil bind as your spring spring stop which will cause you to have a catastrophic failure eventually. I run 250psi in my shocks on all of our off-road rigs. 200 upper is what I run on my bronco but I only use it as a tender spring. My lower is a 450 I believe. Meaning its to filter small chatter and stuff and allow my springs to stay together through droop travel that is all it is good for. My base spring is what filters out big bumps. If you are not using a cross over than you you need to figure out what your combined spring rate is don't just throw upper spring at it thinking it will fix the problem because it could make it way worse. Springs are literally just to hold the vehicle at ride height the internal valving does the work. 250/350 sounds light so you may just be feeling you compression stack or cycling through your travel until coil bind. How much preload do you have in it to hold the vehicle up at ride height? How much shaft is showing? Can you post a pick of the set up?

You need to look at your compression and rebound stack both will affect the harshness of shocks. As well as spring rate. I do not know JD from FOA but he should have indicated this as well.

Tchajagos · Post by **Tchajagos** » Thu Mar 10, 2016 11:27 am

Don't dorget he converted to solid axle so the spring rate will be a lot less than TTB

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:29 pm

Yes I have a cross over ring, but I have it set extremely high to make sure it wasn't hitting that, I have a 175/350 on it now and it still is to firm, I would think that would be way to light for a f 150 with a straight axle,as far as coil bind ,since I swapped to the lighter spring there are a little closer than I'd like but I don't think that's it, even when driving in the street and you hit a spot where the road has a little sharp bump ,say repaved a spot in the road, the hits are very sharp and abrupt, driving down a dirt road say 20-25 with washboard or small little roots or bumps 2-3 inches in height seems to stiff like it's not soaking up the hits

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:31 pm

Reason I ask about shock pressure because when dropped out it was a lot softer ride and you didn't feel everything in the road

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:35 pm

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:36 pm

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:38 pm

And the bumps do need to be shortened a little , but I remover then just to make sure it was hitting them and it wasnt

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:38 pm

Edit* removed them

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:40 pm

http://f-o-a.com/wp-content/uploads/Com ... alving.jpg

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 2:42 pm

I believe there where med or med firm on the valving,....from what I've read it seems I need to do a revalve and I probably need a flutter stack o. The compression side am I thinking the right way or is it something else

mobil1syn · Post by **mobil1syn** » Fri Mar 11, 2016 3:17 pm

that spring split is a little large. Generally I try to keep them within 100lbs on a solid axle.

dump the bump pressure to 50psi, destroke them, go with extra light on rebound and light/med on compression.

steve-oh · Post by **steve-oh** » Fri Mar 11, 2016 5:58 pm

I had 250/350 first

Shock pressures

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Re: Shock pressures