New KYB Rear Struts - Too Soft?

[Hardway]

I just received a pair of rear KYB strut inserts, part# 361001.  I know they have to be compressed and allowed to rebound a few times to "activate" them.  As I did this the pressure needed to push the piston down did increase some but after 10 - 15 cycles, it still does not feel stiff enough.  I know these are considered as close to a factory replacement that is available but I was hoping for a little more pressure.  What has been your take on these out of the box?

[NVZEE]

Basing your evaluation on driving the car or how the new and old shocks "feel" when you compress them? If the old ones needed replacement, there definitely should be a difference.

[Zed Head]

3 hours ago, Hardway said:

I just received a pair of rear KYB strut inserts, part# 361001.  I know they have to be compressed and allowed to rebound a few times to "activate" them.  As I did this the pressure needed to push the piston down did increase some but after 10 - 15 cycles, it still does not feel stiff enough.  I know these are considered as close to a factory replacement that is available but I was hoping for a little more pressure.  What has been your take on these out of the box?

They're gas-filled and the gas is pressurized so they tend to be difficult to press down if they're new and everything is in order.  If the gas has leaked out the shaft will stay down.  But the gas isn't meant to hold a load, it just keeps the seal tight and stops foaming, I think.  Not clear what you mean by "stiff".  Shocks work on rate of motion.

I know that mine, when new, would hurt the palm of my hand as I pressed the shaft down.  The gas pressure is fairly high, that plus the damping took some effort.  And the shaft moves right back up once you let go.

[Namerow]

Trying to hand-test a shock absorber tells you very little about its on-car performance.  All you're really detecting is the resistance of the gas charge inside the chamber.  While the effort required to compress the gas charge may seem like a big deal to you, it's fairly insignificant in comparison with the hydraulic resistance that the shock generates when it's compressed by the wheel running over a ridge in the road at 20 or 30 or 40 mph (or when it's extended by the force of the suspension spring when the wheel tries to slam down into a pothole).

Although I haven't experienced driving a Z equipped with KYB's for many years, my recollection is that they're valved pretty soft.  Not as soft as the OE set-up, mind you, but way softer than a performance shock like a Tokico. 

Unfortunately, it's difficult to compare shock absorbers numerically... something you can do with a spring (or, at least with a non-progressive spring), where the pounds-per-inch rating pretty much tells the whole story.  You need a shock dyno to generate performance data for a shock absorber.  The shock manufacturers use these for both development and QA/QC purposes, but they rarely supply any performance specs with their products.  It's just possible that someone out there (probably a racer) has dyno-tested some of the shocks available for the Z and can post their measurements.  Anybody?

[Diseazd]

I just installed new Tokicos in my 73 47,000 original mile Z. When you pushed down on each corner, the car was soft and bouncy. I figured they must be the original blown shocks. When I removed the struts, I was surprised that they were loaded with KYB's .......just my personal opinion that the Tokicos are a better shock for our Z cars.....you'll live with your decision for the life of your car....Tokicos ain't that much more.

[87mj]

I just put them on my '71 and I am very happy with them.  They feel factory to me.

[Hardway]

Thank you everyone.  You are right, testing them by hand is not an accurate test, I just hoped they would have more resistance.  I had the rear suspension apart today and compared the KYB's to what I had installed before, some Ebay inserts that actually have the same part number as the KYB's.  They pretty much felt the same and rebounded just as quickly as the KYB's so I left them in for now.

[Captain Obvious]

Those KYBs are your typical low pressure twin tube design. Don't get too hung up on the amount of gas pressure or the rebound speed. As Zed Head alluded to earlier the gas inside those shocks isn't intended to have anything directly to do with the damping force at all. It's just in there to reduce the tendency of the hydraulic oil to foam under repeated quickly occurring cycles.

In other words... Even if the gas all leaked out and there was no rebound at all, the dampers would still dampen the same for a couple cycles. They just wouldn't do it for as many cycles in a row because the oil might foam.

[Burl]

Koni shocks rock!

 

[gundee]

On 4/21/2017 at 9:34 AM, Hardway said:

I just received a pair of rear KYB strut inserts, part# 361001.  I know they have to be compressed and allowed to rebound a few times to "activate" them.  As I did this the pressure needed to push the piston down did increase some but after 10 - 15 cycles, it still does not feel stiff enough.  I know these are considered as close to a factory replacement that is available but I was hoping for a little more pressure.  What has been your take on these out of the box?

 

I never knew you were supposed to do that first. (Activate)  I just put them in straight out of the box. Maybe that is why I don't like them with my new springs.

[Hardway]

Yeah, I have always done this when installing just a new shock or strut.  What springs are you running Lumens?