Can replacing the oil in a damper really improve performance?

On old British cars with lever action shocks they're refillable with a plug on the side. Tried this several years back, seat of the pants no difference.

On old British cars with lever action shocks they're refillable with a plug on the side. Tried this several years back, seat of the pants no difference.

In reply to The Staff of Motorsport Marketing :

Where are you getting these legends? Who are these People?!?!? Lol. 

Yes I can... on my motorcycle.

purplepeopleeater said:

On old British cars with lever action shocks they're refillable with a plug on the side. Tried this several years back, seat of the pants no difference.

I think the shocks have to work before changing the oil in them will do much of anything. 

But does increasing the damping actually increase performance of the car?  I'd guess on a lowered/stiffer sprung car yes, but that is not always the case.

In reply to dcamp2 :

Most street cars are significantly under damped for performance use, in favor of better apparent ride quality. So while that may not always be the case, it probably will be most of the time for people looking into such things.  Already well damped or over damped cars don't generally make people want to stiffen them up as much.

for a sealed damper it seems like the ticket would be to drill a hole in the side and tap the hole for a plug to fit.

cheap dampers plus good shock oil = good dampers?

Awesome I'm a so gonna try this!...

Wait where is it safe to drill the hole exactly?(as in is there stuff inside that won't work after I hit it with the drill bit? ) Could I drill two holes and make it adjustable with a bleeder screw and an external set of lines?

I have drilled a hole in the side of a strut, welded a nut to it, reassembled the strut, filled it with motorcycle shock oil, and plugged it with a bolt and crush washer. It worked for the most part. Ended up blowing the seals out of them. 

I refilled the wet struts on my '85 RX-7 with ATF.

Worked great, until the winter.  Then the front suspension would compress with bumps and not rebound.

Common mod on the Swift/Metros

If there is a viable way to do this to sealed struts count me in.  Id love to experiment.  I can easy TIG any fitting to the body to make this work

I'd also be careful if wanting to do this on pressurized shocks.

In reply to purplepeopleeater :

I used motorcycle fork oil on my MG level arm shocks. Went to 50 weight and found a serious increase in reaction. 

I changed the oil in my TR3 lever arm shocks to 30wt and changed the valve to an AH 3000 valve and found a significant increase in damping.

The limitation is that it will increase both compression and rebound by the same amount.  Most (good) single adjustable shocks only increase rebound for roll control.  Increasing compression tends to make a bumpy ride.  When increasing spring rates for performance use, the compression should be reduced and rebound increased.  That's because the spring and shock work together during the compression stroke, but work against each other on rebound.   Many of the "YO JDM" super coilovers adjust both compression and rebound with one knob.  That may be OK in Japan where the roads are glass-smooth, but it doesn't work in the old US of All those speed bumps and potholes.

Now, if you have some worn out shocks that won't go "ka-blam!" when you open them up, you can add thicker oil and get back some of the damping lost to wear.

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.

I also have done this with TR6 rear lever shocks.  Going from the stock rear springs (~350 lb/in) to 470 lb/in springs required more damping.  The valving was adjusted per JK Jackson's recommendations, but going to 30 wt. oil really dialed them in.  No more bouncy bouncy.

If you're drilling/tapping the pressurization in the shocks could be seen a good thing.  It keeps the metal shavings from entering the shock body by putting them into your face. 

Just go back to friction shocks.   Could be adjustable.

iceracer said:

Just go back to friction shocks.   Could be adjustable.

At least friction shocks have the desirable digressive "valving" built in by nature of physics. Getting separate damping curves in compression vs rebound might be a little more difficult though.

I' going to play around with some of this on my challenge car to help with my budget

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. 

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.