Am I the only one blowing shocks

[maxdlx]

I saw a girl blow a donkey once, but never seen anyone blow a shock. LOL

 

[GotJuice]

I saw a girl blow a donkey once, but never seen anyone blow a shock. LOL

>>>
BWOOOAHHHaHaHaHaHaHaHa..now that's some funny chit right there....LMFAO....!!!!!!!!!!!!!!!
If your not happy with the rebuild dude go some where else don't keep sending it back to the the same sucker after it's blown several times...obviously something needs changed. Go one step bigger on the spring to help eliminatet bottoming worries and take some Motrin if your back hurts ....... do something different!

 

[YammiMoose]

shock

I spoke with Ken at Ohlins yesterday; He took my info and promised to get back to me.

I will keep everyone posted with the results.


-Moose

 

[riverrat]

I had a variable rate spring and a revalve done by hi gear and had to have it rebuild again, been ok for the last 1000 miles though.........

 

[terret725]

The charge pressure definitely varies depending on the type and manufacturer of the shock. They are not all 240-250 psi. The HPG (High Pressure Gas) line for example all run at 300 psi (at least according to the section of the Skidoo HPG rebuild manual I have plus I have been told this by several techs from both Skidoo and Yamaha).]

i didnt say all were 250 just said all the ones i have seen have been

As far as the nitrogen volume in a shock goes, a shock without a reservoir has a very tiny nitrogen cavity. If I use my needle gauge to check the pressure of a non-reservoir shock it drops the pressure by about 20% (300 psi drops to not much over 250 psi). I've tested this and it will continue to drop by about 20% each time I check the pressure. It also makes sense when you compare the volume of my needle valve gauge/filler and the shock's nitrogen volume. A shock with a large remote reservoir would drop much less, but it would still drop.]

if this was true ud automatically lose 20% after u filled the shock. if u fill to 300 psi and then pull the needle out the air in the needle is lost. if u stick the needle in and it goes to 300 psi when u pull it out it lose the same pressure as if u had just filled it. so what ur pretty much saying is that every time u fill a shock not jus check the pressure u lose atleast 20%. i dont see this being possible. i know some pressure is lost but i dont see it being 50 psi or more. ive checked shocks 2 or 3 times and lost maybe 10 psi from the first time i checked to the last.

As far as shocks low on nitrogen pressure being harder to get the shaft to move compared to a shock fully charged - this makes absolutely no sense. The only way it could be harder is if you were actually trying to stroke the shock fast and when rebuilt it had softer valving installed. The key here is you should be trying to find the force required to "just" start it moving, not the force to compress it at speed (compressing it at a significant speed and the valving becomes the dominant force). The steady state force is only a function of shaft diameter and pressure. Shock diameter has no effect on how much force it takes to just start compressing the shock (or more accurately holding it slightly compressed without actually moving the shaft).

i checked 2 different shocks today at work both with the same psi. one a kyb moved with realy any force. the other a walker evan i could barley move with all my body weight. now i understand different shaft sizes and also body size could play a factor but they looked to be close to the same size. now im not sayin ur wrong. to me it just seems like theres more factors that would add to how hard the shock is to get moving then just pressure but i may be wrong.

If you guys are checking your shocks using a gauge you are definitely loosing nitrogen pressure each time you do it. Talk to any shock guru or read any shock rebuild manual and they/it will confirm this is highly not recommended.

ill ask my boss tomorrow. his name is the second name in the polaris race sled manual.

 

[cannondale27]

You would lose pressure each time you checked them.Difference when filling is the pressure is left on as your pulling needle out so none can be lost since its being added as needle is being pulled out.Thing is though biggest job of the Nitrogen is to make the seals seal.Yes it does affect rate but main job is to seal the seals.Even at 150psi which is alot less than Ohlins it should still keep seals sealed.

 

[Alatalo]

You will lose nitrogen pressure every time you ATTACH the gauge to the shock, because a small volume of nitrogen will escape from the nitrogen reservoir to the gauge (thereby reducing the pressure inside the nitrogen reservoir and increasing the pressure inside the gauge). How much you lose will depend on nitrogen reservoir volume and the volume inside the gauge. Big reservoir and small gauge = small pressure loss. Small reservoir and big gauge = big pressure loss.

You will not lose any nitrogen pressure when you REMOVE the gauge from the shock, because - assuming the charge valve is fully functional - zero volume of nitrogen will escape from the nitrogen reservoir (thus the pressure remains the same as set by the gauge).

The main reason for using a pressurized damping system is to avoid cavitation. The pressurizing itself can be accomplished in several ways where mechanical springs is one way and nitrogen charging is another way. The reason for using nitrogen (instead of any other gas) is because nitrogen is "chemically inert" which means it does not react with other materials and it does not explode when coming in contact with oil or grease...
Nitrogen pressure has got very little to do with sealing properties. Ohlins is using as low as 6 bar (87 psi) in TTX shock absorbers without any sealing problems. If anything, you would want to reduce the nitrogen pressure because this reduces the seal friction, but again, this might not be good for avoiding cavitation...

 

[ReX]

if this was true ud automatically lose 20% after u filled the shock. if u fill to 300 psi and then pull the needle out the air in the needle is lost. if u stick the needle in and it goes to 300 psi when u pull it out it lose the same pressure as if u had just filled it. so what ur pretty much saying is that every time u fill a shock not jus check the pressure u lose atleast 20%. i dont see this being possible. i know some pressure is lost but i dont see it being 50 psi or more. ive checked shocks 2 or 3 times and lost maybe 10 psi from the first time i checked to the last.

The amount of pressure you loose when checking totally depends on the gauge and the shock's nitrogen volume, but you will always loose pressure when checking it and in many shock/gauge combinations you will loose lots of pressure..

You actually only loose pressure when the gauge is inserted. This is because the gauge is filled with nitrogen from the shock. When you pull the needle out you don't loose any more from the shock and whatever the gauge was reading is how much pressure remains in the shock.

The problem is every time you insert the gauge the shock looses pressure if you don't top it back up.

i checked 2 different shocks today at work both with the same psi. one a kyb moved with really any force. the other a walker evan i could barley move with all my body weight. now i understand different shaft sizes and also body size could play a factor but they looked to be close to the same size. now im not sayin ur wrong. to me it just seems like theres more factors that would add to how hard the shock is to get moving then just pressure but i may be wrong.

As I said before (possibly not very clearly though), to check pressure you are trying to find out how much force it takes just move it a tiny bit and hold it there. If you are actually stroking the shock at any speed you are primarily measuring the damping force, not the pressure induced force.

Take those same shocks and lean on them enough to get the shafts to compress a tiny bit and hold them there. If the pressure is the same and the shafts are the same diameter the force needed to keep the shocks compressed a little bit will also be the same - regardless of the manufacturer, shock valving and shock body diameter.

The pressure created force will vary with the square of the shaft diameter so a larger diameter shaft will increase the force by quite a bit.

 

[ReX]

You would lose pressure each time you checked them.Difference when filling is the pressure is left on as your pulling needle out so none can be lost since its being added as needle is being pulled out.Thing is though biggest job of the Nitrogen is to make the seals seal.Yes it does affect rate but main job is to seal the seals.Even at 150psi which is alot less than Ohlins it should still keep seals sealed.

While many seal designs are such that pressure on one side helps to seal them, Alatalo is correct in that the primary reason shocks are pressurized is to avoid cavitation.

Cavitation is where the shock oil or contaminants contained within the oil suddenly flash into a gaseous state. If a shock doesn't have enough nitrogen pressure, it won't provide as much damping force at higher compressive loads because there will be a pocket of gas created on the oil side (and the floating piston will move down temporarily under shock compression).

The higher the pressure, the less likely cavitation will occur.

In reality, once the nitrogen pressure gets too low, since the floating piston starts to rapidly move up and down as the oil cavitates, what nitrogen there is quickly makes it way past the floating piston's seal and mixes with the oil. On top of that the constant cavitation quickly breaks down the oil. You end up with lots of nitrogen in the broken down oil. At that point the shock doesn't work very well at all since the nitrogen filled oil has lost its viscosity and the nitrogen in the oil makes the oil compressible.

This is the failure mechanism I've seen in almost every case with the shocks I've rebuilt (nitrogen on the oil side with black, sludgy oil).

 

[cannondale27]

Yes forgot about cavitation.For sure Nitrogen pressure prevents it.Especially if shock is mounted shaft upwards.It wierd how there are many shocks out there that work fine with no bladder or floating piston/bag and still pressurized.Other arent pressurized at all.They may be cavitating but doesnt seem to matter.Remove pressure from a Ohlins or KYB and seals will leak very quickly.KYB instantly because of seal design in most.

 

[Alatalo]

While many seal designs are such that pressure on one side helps to seal them, Alatalo is correct in that the primary reason shocks are pressurized is to avoid cavitation.

Cavitation is where the shock oil or contaminants contained within the oil suddenly flash into a gaseous state. If a shock doesn't have enough nitrogen pressure, it won't provide as much damping force at higher compressive loads because there will be a pocket of gas created on the oil side (and the floating piston will move down temporarily under shock compression).

The higher the pressure, the less likely cavitation will occur.

In reality, once the nitrogen pressure gets too low, since the floating piston starts to rapidly move up and down as the oil cavitates, what nitrogen there is quickly makes it way past the floating piston's seal and mixes with the oil. On top of that the constant cavitation quickly breaks down the oil. You end up with lots of nitrogen in the broken down oil. At that point the shock doesn't work very well at all since the nitrogen filled oil has lost its viscosity and the nitrogen in the oil makes the oil compressible.

This is the failure mechanism I've seen in almost every case with the shocks I've rebuilt (nitrogen on the oil side with black, sludgy oil).

When discussing cavitation, let us not forget the most impotant reason why you want to avoid cavitation - sudden loss of damping. Cavitation normally occurs during a compression stroke because of insufficient operating pressure caused by either too low nitrogen pressure or incorrect damping distribution between main compression valve and additional compression valve. Besides from the risk of completely collapsing during this particular compression stroke, the shock absorber can also lose quite a lot of damping during couple of following strokes (in both directions, compression as well as rebound).

Regarding failure mechanisms, I still insist that the main problem with the Apex GT shock is the leaking shaft seal. This shock, at least the american version, has got a compression damping spec where the damping distribution is very biased towards the adjustable EC valve. Because of this the operating pressure inside the shock absorber will never go below the level of the nitrogen pressure. Assuming that the nitrogen pressure is set to spec (or not affected by the leaking shaft seal...), this shock should be on the safe side against cavitation.

 

[cannondale27]

Some guys have had good luck running 300psi seal doesnt leak and seems to help ride.Maybe Pioneer valved for lower psi and has seal problems because of it.Ohlins does size thier seal/shaft fits with low friction in mind.