Changing the oil in front struts (79-04 Mustang)

[jw33]

Today I decided to try changing the oil inside of the front struts for something else and added a shrader valve so that the oil can be changing fairly easily to something else if we want to experiment with different viscosities. Yes it is ghetto and it would be better to buy a decent strut, but this is for a lemons/chumpcar. These struts were bought at Autozone in 2007 and there are about ten races on them. I decided I would drill/tap the bottom of the strut since there is room down there when they are mounted on the car and the metal is thick enough to tap/thread.



I used a pair of shrader valves from Napa. They are 1/8 NPT made of brass and cost about $2.50 each.



Drilling the first one made me a little nervous because these are gas charged and I had no idea how much pressure there would be inside. I drilled a pretty small hole first and it wasn't that bad. I then went up to the right size drill bit for the 1/8 NPT tap while using a shop vacuum to catch the shavings. Tapping the hole was mostly straight forward except that you need to use a bottom or finishing tap because you can only go down about a half an inch into the hole. I ended up cutting the tip of my tapered tap off to get the job done.



I packed the tap with grease to catch as much of the metal shavings as possible.



And now drilled, tapped, and ready to drain. I waited to drain them at this point to flush out as much crap from the drilling/tapping.



Next I turn them over and start cycling the strut while catching the oil in a container. It is a pretty vile looking fluid, but it must work alright if they lasted as long as they did. It seems about as thick as cooking oil.





I measured how much fluid came out and tried to put back in the same amount. I used some 15 motorcycle fork oil. You have to put it in pretty slowly, but eventually it will all fit.




Finally I used thread sealant on the shrader valve threads and tightened it down. I charged the strut with about 40psi, but I do not know for sure how much pressure should be inside of there.



Now I have no idea if this will actually be an improvement over the way the struts were. I figure we can play with different oils and see if it actually makes a difference. Just judging by hand in the garage the struts actually feel softer? That doesn't seem possibly because the new oil was definitely thicker that what came out, but I'm pretty sure they are softer now. Everything I have read about this subject suggested that you should start out with the lower number oils first so you don't damage the seals. And now you know....

[ceyko]

heh, would not ever had thought to do that. Wonder if it'll hold up under pressure. Guess so since you said others have done it.

[Blackpony]

If it feels softer maybe there needs to be more air pressure? You could also try nitrogen to fill them???

[racrguy]

The reason they feel softer is because of cavitation in the oil. That shock is ruined now. I'll explain further when I get back to work.

[jw33]

I will be the first to admit that all I know about dampers will fit into a thimble. My basic understanding of how a they work is that a shaft is connected to a disc that has small holes in it that is kept inside a chamber filled with some type of fluid. When the strut is compressed the disc is moved through the liquid and the size/shape/quantity of hole in the disc dictate how much resistance is applied. The same theory is involved when the strut forced back the other way. Since I cannot fool around with the valving I figured I can affect the damper by changing the viscosity of the fluid. As far as why they are charged with pressure I have no real idea. My best guess is that Nitrogen is inert and they keep them under pressure to keep the fluid free of bubbles plus it keeps them from sucking in crap when the temperature changes.

[racrguy]

Sorry I didn't reply sooner.

You have two different forms of shocks, twin tube and mono tube. When you hear the term "gas charged" that doesn't mean there's gas, typically nitrogen because of it's volume stability over a range of temps, in with the oil. Inside a twin tube shock there is a bag of gas that expands and contracts to offset the volume inside the shock that the shaft takes up. Mono tubes have two pistons, one attached to the end of the shaft, another which is free floating that separates the gas and the oil. The two are bros, and they work well together, but they can never meet. The reason that the shock is easier to move now is because not only have you lowered the viscosity of the oil, which lowers the damping ability of the shock on both compression and rebound, but you've also introduced gas, which has a viscosity of near 0 and incorporated that into the oil, which reduces the viscosity of the oil even more.

Shocks and the way they behave is serious business and there's a LOT of r&d money that goes into them. I wish I knew more about them, but even I don't have enough time to study them as much as I want to.

[jyro]

anyone know what pressure the bladder in a shock is charged to?

to change the oil and make it gas charged you would have to put the oil back in under enough pressure to compress the nitrogen charged bladder. About any hyd. unit could do that. The device used to charge the bladder in a hyd. accumulator or mx bike shock can also be used to connect to the Schrader and put the oil in under pressure because it has a valve to open and close the Schrader valve to prevent pressure loss when you disconnect it. I bet the charge is less that 100 lbs.

[racrguy]

Bladder? Mono tube shock? I've never seen a bladder in a mono tube, only twin tube, and never have I seen a twin tube that you could charge yourself.

However, the pressure that you set the gas at varies depending on how you want the shock to feel, I'd get with the shock manufacturer to get recommendations. I can explain what variances in shock pressure does on a sprint car, but not anything else.

[jyro]

you could be right

Bladder? Mono tube shock? I've never seen a bladder in a mono tube, only twin tube, and never have I seen a twin tube that you could charge yourself.

However, the pressure that you set the gas at varies depending on how you want the shock to feel, I'd get with the shock manufacturer to get recommendations. I can explain what variances in shock pressure does on a sprint car, but not anything else.

I've never cut one apart. I just figured the gas was separate from the oil to prevent cavitation.

[racrguy]

It is, but if it's got a bladder I've never seen one you can adjust the gas pressure on. Adjustible pressure is typically reserved for mono tube shocks and are CHOO CHOO!

[jw33]

I cut open another strut and this is what it looks like inside. I came across more info on the gas bag, but this one does not have one???









And by the way, the oil that is inside of these rivals gear oil for skankness.

[racrguy]

Then it's not a gas charged shock, they just fill it with oil and allow the pressure to build inside the shock as it's compressed.

[jyro]

Ok, I was suggesting adjusting the oil recharge pressure to compress the bladder

It is, but if it's got a bladder I've never seen one you can adjust the gas pressure on. Adjustible pressure is typically reserved for mono tube shocks and are CHOO CHOO!

Unless the bladder inside a oil charged shock actually ruptures when you remove the oil, you should be able to recharge the oil under compression to still utilize the bladder pressure. Just a theory. I've never tried it.

[racrguy]

What? No. If what you're saying is what I think you're saying, you're doing it all wrong. The only reason the gas bag would be there is to compress as the shock shaft takes up volume inside the oil column, thereby reducing rod pressure. On street cars if you notice an increase in rod pressure you're driving too hard, in a Lemons car if you can feel an increase in rod pressure you're racing the wrong class. An increase in rod pressure acts just like more spring rate, and typically makes the car feel "on top" of the track, in my experience.

[jw33]

This weekend we had a chance to compare these struts back to back with a brand new pair of Monroe 801803 ($32ea from rockauto). On Sunday we took the car to Eagles Canyon Raceway where one of our teammates is a member. The weather was perfect and the car had a fresh set of tires. We sent him out first to do 12-15 laps with the modified struts on the front of the car. The car appeared to be very stiff looking from where we were watching from and did not appear to nose over at all under braking. This guy hasn't driven this car in it's current configuration, but after the getting a better feel of the brakes he came back to the paddock so we could switch out the struts. That was the only change that we made to the car before we sent him back out again with the brand new Monroes. After another 12-15 laps he came back in and immediately said the car felt much much better. Lap times were basically the same as far as we could measure and really couldn't be used to for anything because he was not very familiar with the car on the first outing. As far as we are concerned we will be leaving the Monroes on the car unless we decide to bite the bullet and get a real performance strut/spring.

Below is a video we made using a tiny RC camera mounted under the hood facing out to the passenger side spindle area. No data, just a cool 2min video of the struts doing their job on the track. The suspension moves around a lot!

7yr old Autozone struts with an oil change.


Monroe 801803



This is the camera we used.

http://www.hobbyking.com/hobbyking/store/__17200__HD_Wing_Camera_1280x720p_30fps_5MP_CMOS.html