Shock fluid replacement for stiffer damping

[presslab]

I had previously added some grease into the bladder of my rear air shocks to stiffen up the spring rate. This worked fine enough but it didn't really address my complaints of too-soft damping, particularly the rebound damping. The shocks were only a few years old, so they were hardly worn out.

 

 

The OEM air shocks are a twin tube design made by Tokico. Because of this twin tube design I was more confident I could replace the fluid without drilling into something important.

 

I drilled a hole in the bottom of the shock - upon pressure release it was pretty strong and on the second one I broke the drill bit. I then tapped the hole for 1/8" NPT - the steel is pretty thick here and seemed fine to tap. I paid special attention to not get metal chips inside the shock by keeping some fluid in the shock to flush it out and also putting some grease on the drill bit and the tap.

 

I then drained the original fluid, just about 6 oz. To determine the viscosity of the original fluid I set up an orifice with a reservoir; I used a stopwatch to time how long it took the fluid to drain out. The original fluid took 9.6 seconds to pass through the little orifice I had set up. I had on hand some shock fluid. The Silkolene RSF 5wt (26.7 cSt@40C) took 11.1 seconds and RSF 10wt (47.36 cSt@40C) took 18.5 seconds.

 

I bought Schrader valves from the hardware store and installed them into the tapped holes.

 

http://www.acehardwareoutlet.com/productDetails.aspx?SKU=40899

 

I used a large syringe to inject 6 oz of RSF 10wt back into the shock, and topped it off with 130 PSI of dry SCUBA air. I know some people insist on nitrogen but that's hogwash; as long as the air is very dry it is good. To bleed the shock, I compressed the shock while upside down, held it compressed, turned it right side up and extended it. I did this about 20 times for each shock to get all the air out.

 

 

It's much, much better now! The damping is now firm, a very noticeable difference. Total cost was about $25. I'll tackle the fronts too some day, but they aren't near as soft as the rears used to be.

 

[ATTACH]6046[/ATTACH][ATTACH]6047[/ATTACH][ATTACH]6048[/ATTACH]

[subiemech85]

USRM candidate!! wow

[DaveT]

Excelent.

[presslab]

After a week of driving on the modified shocks, I am quite happy with it. It's not too firm at all, I've done some very bumpy back roads and it's still pretty smooth. Perfect for a daily driver. The corner turn-in is much more stable, and control is much better especially at speeds over a buck. For someone who doesn't have too many bumpy roads the 15wt would work well, but with the roads we have here I wouldn't want it any stiffer.

 

Something I forgot to mention was how the shocks seemed to have two distinctly different damping modes. I noticed this while bleeding the shocks. When the shock is mostly extended, it is much stiffer in rebound and compression damping. As the shock is compressed, about half way in the travel it becomes softer. There must be some sort of bypass circuit. It makes perfect sense, as you would want two different types of damping depending on if the ride height was set to 'high' or 'normal.'

 

I believe the XT4T shocks have stiffer valving in them; my car is an 88 Wagon with stock rear shocks. I am considering what struts to install on the front. The XT4T shocks are about $40 more each; for that price difference it would be more cost effective to use Wagon shocks and change the fluid. I would also gain the ability to tune the damping.

[nipper]

The only question is how it would do in zero degree weather, but excellant otherwise.

 

 

 

nipper

[presslab]

The only question is how it would do in zero degree weather, but excellant otherwise.

 

 

 

nipper

 

It would be at least as good as any other shock. The synthetic shock fluid I used by Silkolene has an excellent viscosity index of 303; it's one of the best in terms of temperature stability.

 

http://www.peterverdonedesigns.com/lowspeed.htm

[presslab]

Ahha, I wasn't just imagining it. In section 4-1 of the Subaru XT service manual:

 

[ATTACH]6052[/ATTACH]

 

Yet another reason to keep the air suspension: Position sensitive damping!

[presslab]

Got around to doing the fronts. I drilled the hole just above where the knuckle mounts, the metal was thick here as well. They had the same original fluid viscosity and volume as the rear. I replaced it with 6 oz of 10 wt Silkolene RSF, just as I did the rear.

[presslab]

Just an update, this has been working great. I eventually decided to go with RSF 15wt in the rear instead of 10wt as it was a bit soft when loaded up with mountain bikes on the hitch rack, etc.

 

It doesn't get really cold here, maybe 30 F, but the damping has been just fine from 30F-100F.

[idosubaru]

dang dude - that's nuts.

[Txakura]

dang dude - that's nuts.

 

takes OCD to all new heights - but by posting it, everyone can benefit from his hard work - proves it's possible, and shows the way to recreate the results - well done

 

not sure why the disdain for nitrogen, been used in aircraft struts for over 60 years

[presslab]

takes OCD to all new heights - but by posting it, everyone can benefit from his hard work - proves it's possible, and shows the way to recreate the results - well done

 

not sure why the disdain for nitrogen, been used in aircraft struts for over 60 years

 

Thanks, I guess.

 

Don't get me wrong, there is nothing wrong with nitrogen. My disdain is for the people who insist that nitrogen is required. If nitrogen is on hand use it, by all means.

[Txakura]

Thanks, I guess.

 

Don't get me wrong, there is nothing wrong with nitrogen. My disdain is for the people who insist that nitrogen is required. If nitrogen is on hand use it, by all means.

 

wasn't meant as a serious slam, just a tease, it took some dedication to research all that - a person has to be a little obsessive to pioneer things that other people take for granted

 

does this mean regular struts/shocks can be re-tooled?

 

that could open a ton of options for the horrible roads over here

[idosubaru]

did installation go like this:

1. install valve. 2. fill oil 3. compressed air

 

or like this:

1. fill oil 2. install valve 3. compressed air

 

you would recommend 10wt or 15wt oil depending on usage/climate then?

 

i think i'd want a little stiffer too - 15wt if it could handle the winter temps here which is when i often drive the XT6.

[presslab]

did installation go like this:

1. install valve. 2. fill oil 3. compressed air

 

or like this:

1. fill oil 2. install valve 3. compressed air

 

Either will work. While it is a bit easier to get the oil in without the valve in place, with the schrader insert removed it's not that hard to get the oil in there with a syringe. Be sure to clean the oil off the threads on the shock with solvent so the sealant will hold.

 

you would recommend 10wt or 15wt oil depending on usage/climate then?

 

i think i'd want a little stiffer too - 15wt if it could handle the winter temps here which is when i often drive the XT6.

 

The 10wt front, 15wt rear has been just fine, even with my 205/50-15 tires. We have some of the worst roads in the USA here. For street only use I would go with 15wt front 20wt rear but I occasionally go off-road. If you regularly go over 100 MPH I'd recommend this heavier viscosity as my current setup is a bit soft for this.