Refilling shock

[Greg Davis]

I had to disassemble one of my Fox nitrogen pressurized shocks because of a bent shaft. I lost a little oil, but saved the majority of the fluid. How precise does the amount of oil in the shock have to be? Also I am unsure about re-filling the body. Is there any proportion of distribution of the fluid on either side of the valving when assembling, or does this equalize through use? When using new oil what type is recommended? Any help with these questions would be appreciated.

[problemchild]

If it is a front, take apart the other one to find the exact level, put them both back together using that level ... or a bit more, as you loose a little as you assemble it. You can buy a qt of oil from a Penske dealer or general race vender. That will be enough to put fresh oil in both.
You probably need a hands-on coach or access to a Penske manual if you are doing a rear. It is not hard ... it is just alot easier if someone shows you. Being a little shy or a little extra on the oil level is less important if you are not matching up to an existing shock ..... which is why most are serviced in pairs. You want the quality of the parts and oil (and assembly) to be matched.

[hardingfv32-1]

Use a fork oil from a bike shop to save a little. I use 5W Honda Pro Fork Oil. Change oil every chance you get or about every 8-10 races. The oil does get dirty.

The oil level is set at .5" below the top of the body with the shaft all the way in.

You do not need much nitrogen pressure. I use about 75 lb. Do not use air, as I assume these shocks do not have bladders.

Brian

[Greg Davis]

Thanks for the input Greg and Brian. These shocks do have remote bladders. Can they be pressurized with air? I have a nitrogen tank, but if they can utilize air it would save hauling around the nitrogen bottle.

[problemchild]

They are not supposed to leak down .... so you charge them with nitrogen and they should be good. Resealing the fittings for hoses, etc should help. If you are starting at less than 100 psi .... and need to recharge them daily, I would think their performance is probably suffering during use as pressure is inconsistant. Many of us get by with shocks that lose some pressure by topping them up every morning @ the track but that is certainly not an ideal.
Before I had a nitrogen bottle, I would get my oval racer neighbour to charge them with dry air. Many oval racers put a dryer inline from their compressor. I have used regular air before but the pressure would vary 50 psi from the morning cold to afternoon sun, shade, etc. Certainly was better than no pressure. You do what you have to. Many would never notice the difference.

[hardingfv32-1]

Your shocks are an emulsion type where the pressurizing gas is in contact with the oil. Air will degrade the quality of the oil. I can not state what the long term effect of this will be. I would not imagine it is the end of the world. I have not noticed any lose of pressure over say 6 months for a shock in good condition. It takes a very good driver to spot a low pressure shock.

Brian

[brian]

Penske shocks are not emulsification type and the gas chamber is separate from the oil. In that case, the oil chamber must be completely full. I fill the shock with the gas diaphram completely compressed, no gas pressure, then force the excess oil out when installing the shock top. Refill the gas after the shock is assembled. Any gap or voids in the oil chamber will cause air bubbles in the piston and ruin the dampening.

[Mystique Racing]

Compressed air works fine. I have been using it for a couple seasons and was told it would be fine by my shock guy.

[brian]

Only problem with compressed air is that it will be more sensitive to temperature changes. I've seen the small black front Penske's change 15 psi just from heat of the day. You can use compressed air but will have to check pressures more often. Condensate in compressed air can cause corrosion and rust as well.

[Speedsport]

Does the air pressure really have an effect on the shock? Has anybody actually confirmed a difference on a dyno? I take my air bottle and re-charge my front shocks about 3 times a year. As long as my airtank has more then 100 lbs in it, I never cared if it was 100 or 120.

[SR Racing]

Does the air pressure really have an effect on the shock? Has anybody actually confirmed a difference on a dyno? I take my air bottle and re-charge my front shocks about 3 times a year. As long as my airtank has more then 100 lbs in it, I never cared if it was 100 or 120.

And you are correct. It has little effect. It does change the efffective spring rate a very little bit. But basically anything above 80 lbs will keep it from foaming, You could see a very minor shock dyno change, but not enough to worry about if you are within a broad range.

[smsazzy]

Does the air pressure really have an effect on the shock? Has anybody actually confirmed a difference on a dyno? I take my air bottle and re-charge my front shocks about 3 times a year. As long as my airtank has more then 100 lbs in it, I never cared if it was 100 or 120.

Whatever you set them at for the runoffs seemed to work well. I would stick with that.

[Mystique Racing]

I have found through my testing that the shock air pressure makes a difference. Not huge but noticeable. I run pressures that are radically different then everyone else and have found it to be a benefit.

[problemchild]

I used to have guages built right into the shock fitting. As I said above, the pressure swing from cool morning shade to afternoon sun was as much as 50 psi with compressed air. It was less than 10 psi with Nitrogen. The varience may be minimal in the summer, but an early Runoffs session at MO or RA would be quite different than the mid-afternoon session on a warm day. Your tires are also experiencing that pressure shift BTW.

[Speedsport]

I fail to see how shock pressure can make any difference at all. The air is there only to keep pressure on the oil to prevent foaming. Whatever the air pressure is set at, the oil on both sides of the shock piston is at the same pressure. If it's the same on both sides of the piston, there is no differential to affect the shock. The only possible result of higher air pressure that I can think of is more pressure against the seals, perhaps creating more seal drag, but I find it hard to believe anyone could tell that difference on the track.

Unless someone has dyno data to support shock pressures make a difference, or someone can point out what I'm missing, I still consider it a myth.

[Mystique Racing]

The pressure charge is contained in it's own chamber. At one end of this chamber there is a floating piston that separates the air chamber from the oil chamber. When the shock shaft is forced inward or you might say in compression, the pressure in the air chamber offers a slight resistance to the inward shaft force. This is why the air pressure is said to add spring rate to the suspension, which it does. Additionally, this same pressure has the opposite effect on the outward movement of the shaft, AKA rebound, by speeding it up slightly. So, the amount of pressure absolutely has an effect on the performance of the shock. Is it a big deal? well I think so, but others may not.

Speed secret #456, It only takes about 20 lbs of pressure to prevent cavitation on a 7500 and about 40 on a 8300. I know because I have done it on the dyno. Every additional pound of pressure effects the performance of the shock negatively.

[Speedsport]

OK, that makes sense. I forgot to consider the added volume of the shock shaft. But the differences are still small - a 50 psi change on a shock with a .5" shaft would only change the rate by about 10 lbs if I'm thinking clearly this morning, and it should be linear regardless of the starting pressure. I guess that's one more thing I'll have to start paying attention to.

[brian]

I think Scott's on the right path. The effect on handling will be greatly dependent on the capacities envolved. I can really tell the difference in pressure on the small black Penske's. Not so much on the rear shock with a 40+" line running up to the cockpit. On my old Penske 8100 with the canister only a few inches away, I could tell the difference in 250psi, the recommended pressure, and 150psi the pressure I usually ran. I have used pressures to regulate hardness when there wasn't time for a shim change. Dropping pressures for a rain race is an easy fix at pregrid. Shocks like the Penske blacks need at least 45psi to make sure the retaining rings stay in place.

[hardingfv32-1]

I would say the pressure issue is all about stickion, the drag from the seals. It would be most noticeable at the nose of the shock curve. This would be where the shock travel changes direction. The more stickion the deader the shock feels in this area. Probably more of a problem with a suspension with low shock movement than with the high movement FV.

Brian

[jpetillo]

Michael has the numbers right - 2 lbs of spring force for every 10 PSI for those 1/2 diameter shock rods. I find it interesting why a long rear shock line would make such a difference since it's filled with oil - drag of the oil, maybe? I felt as Brian H. does, that pressure mainly affected stiction. John

[hardingfv32-1]

I believe the added force from the shock pressure affects the static ride height only, not the spring rate. It is a constant that is added to what ever the spring is doing. You could use it to change your ride height, but not to change the spring/wheel rate of the corner you are tuning. So, lowering the pressure is not going to help in the rain.

Brian

[Mystique Racing]

Brian, I have yet to see a FV with high shock movement. Most FV's have a rear motion ratio of less then .4 and the front isn't any better. After re- reading your post I think you mean hi shock travel.

[kps]

I believe there is a pressure difference as you must subtract the area occupied by the piston shaft from the piston diameter on one side. The interesting thing is that this pressure difference remains constant for any piston position subject to the time needed for flow of fluid. By changing the pressure you can add or subtract spring rate. John

[hardingfv32-1]

Yes, your are absolutely correct that the motion ratio on most Vees is bad and that the shock motion is low. I guess we need to worry about stickion then.

I would still say the the shock does not add spring rate to the system. As stated "The interesting thing is that this pressure difference remains constant for any piston position". I would say the pressure needs to change when the shock piston changes position if we are to have a spring rate change. That is why I say we have a one time change or constant ride height adjustment added to the spring's action when discussing shock gas pressures. I do not know how you would describe this numerically in relation to the actual spring rate.

I'm assuming that the volume change caused by the shock shaft entering the main shock body does NOT significantly change the internal shock gas pressure.

Brian

[Speedsport]

Just to throw some more numbers into this discussion to see if anything makes sense:

I don't have a shock in front of me, but If I assume an air chamber behind the air / oil piston of 1" diameter by 1" long, a shock shaft diameter of .5", and air behaving as an ideal gas, and based on the fact the regardless of how much the shaft is compressed into the shock the quantity of air remains the same (mass, moles, whatever), then if we started at 80 psi and moved the shock 1" the end pressure would be 105psi, a difference of 25 lbs and about 4 lbs of additional force on the shaft.

But if we started at 100 psi and moved the shock 1", the pressure would be 133 lbs, a difference of 33 lbs and about 6.5 lbs additioanl force on the shaft.

So yes, changing the pressure has an effect on the spring rate of the shock, but it's so small I don't think it matters other then the initial static addition.