Can replacing the oil in a damper really improve performance?

In reply to freetors :

In SCCA Formula 500 (850 lbs with driver) the cost restraint for expensive coil over shocks is the use of round rubber pucks for compression in both directions; these are tunable with different durometer strengths.  Add rocker arms for different multiplications and torque the rocker arm pivot bolt for different friction dampening and you have a good suspension.  These low budget formula cars ($35K new and $15K used) run comparable lap times with the more expensive Formula F cars and fit the bill perfectly for grassroots motorsports.  And there are two drive trains - snowmobile and motorcycle.

JimMurphy said:

In reply to freetors :

In SCCA Formula 500 (850 lbs with driver) the cost restraint for expensive coil over shocks is the use of round rubber pucks for compression in both directions; these are tunable with different durometer strengths.  Add rocker arms for different multiplications and torque the rocker arm pivot bolt for different friction dampening and you have a good suspension.  These low budget formula cars ($35K new and $15K used) run comparable lap times with the more expensive Formula F cars and fit the bill perfectly for grassroots motorsports.  And there are two drive trains - snowmobile and motorcycle.

Yeah those f-mods are pretty cool!

Did this back in the early 80's on a Capri that was being preped for autocross on a very tight budget. Used motorscycle fork oil. learned this after a friend who raced in Showroom Stock road racing replaced the oil in his struts with something way thicker and blew out the seals.

The stock struts could be taken apart and put back together without any holes drilled, etc.

In reply to Jere :

External lines and an adjustor will not do anything except give you LESS damping force, as there are already holes in the strut piston. 

Driven5 said:

djsilver said:

When increasing spring rates for performance use, the compression should be reduced...

You seem to have misunderstood the explanation used to describe the general belief that there should typically be some amount more rebound than compression damping...But that is still, and always has been, based on needing to increase both proportionally to the spring rate in order to maintain the same relative damping ratios for each.

A shock naturally has more rebound stiffness than compression  stiffness, as there is more area on the blind end of the strut. More area =less pressure at equal force levels (spring rates). 

In reply to tamago :

so in theory I just need a super heavy oil and it should work... Too many "ifs" to make the idea plausible I suppose

In reply to Daeldalus :

Most are so thin they could not be tapped and seal. It would be tricky to find a thick enough area. Perhaps the cap that's screwed off would have a large enough area. The only other option would be a bung and you can't have it where the spring rides and then you're into welding.

rslifkin said:

In reply to freetors :

I dunno if I'd call digressive valving desirable in a lot of applications.  It works on very smooth surfaces, but on pretty much anything else, it's bad IMO due to blowing through the travel way too easily on big bumps. 

I read a long time ago a writeup on using digressive SPRING RATES on a desert racer.  The setup is, there is a low rate tender spring that is installed with a large amount of preload against a secondary spring seat that the high rate spring sits on, and is allowed to move only in one direction.  The high rate spring sets the ride height, and a little below normal ride height the low rate spring comes into play.  So to make nice round numbers, say the low rate spring is 100lb-in and the static load on the suspension is 500lb, the low rate spring would be installed with 5.5 inches of preload.  (I hope I am describing what is happening well enough!  And my source was very MUM on actual numbers so don't take these as gospel)  Of course there are bumpstops in play as well.

The effect was, the car was kinda harsh until you got up over a certain speed, and then it got limo-smooth over bumps.  But it still had the body control characteristics of a stiffer suspension.

In reply to iceracer :

They usually are, but now you've added stiction to the equation. They won't allow the suspension to return to a neutral position. Imagine driving a car with constantly changing corner weights.

Did this exact thing to a first gen rabbit in the mid 80's. Measured old fluid and refilled with same amount of ATF. It made a noticeable improvement, and never had a seal leak problem.