Experimenting with Fixing KDSS Lean

I have a 2024 4Runner with KDSS, I picked up a set of the TRD Pro Fox Shocks and, as you might have guessed, I’ve encountered the dreaded KDSS lean. The Fox shocks should only provide about a 1” lift in the front and no lift in the rear, the TRD Pro uses the same rear springs as the SR5 and TRD models.

There are several opinions about the causes and solutions for the lean. I’ve referenced the diagram of the hydraulic connections in the KDSS to try my hand at solving the issue. Here are my findings so far:

KDSS Basics:



KDSS is a dynamic sway bar system that uses two hydraulic pistons on the left side, one on the front and one on the rear, to function in two modes: One that ‘unlocks’ the stabilizer bars to remove tension and one that ‘locks’ them to realize full tension. When KDSS is not active (the normal state) there are ‘accumulators’, which are gas-charged springs, which let the volume of the hydraulic system change when force is applied; this helps decouple the pistons for more natural movement. When KDSS is ‘activated’ it locks solenoid valves that cut off the path to the accumulators, restricting the volume of hydraulic fluid shared by the pistons and forcing them to push on each other directly.

The system has a ‘high side’ with fluid in channels above the pistons (not shaded in the diagram), and a ’low side’ with fluid in channels below the pistons (shaded in the diagram). The amount of fluid on the high and low side should, theoretically, force the pistons into an equilibrium position. Later I found this might not be true, take note.

The pistons are sensitive to each other’s movement and move ‘in-phase’ or ‘out-of-phase’. In-phase means the pistons are moving in the same direction, up or down, at the same time. Out-of-phase means the pistons are moving in opposite directions at the same time. There’s a KDSS video on Youtube from Toyota that shows this animation in great detail.

During in-phase motion, such as during cornering when the axles move together, the pistons are moving in the same direction and are stopped by hydraulic force; neither piston is strong enough to push the other up or down. This means the stabilizer bars are able to realize full tension and keep the vehicle's body level with the axles. Note that the front KDSS sway bar connects to the axles at an odd angle on the LCA’s, meaning it doesn’t exert maximum force, which is why it’s extra beefy.

During out-of-phase motion, such as moving over uneven terrain when axles are moving in different directions, the pistons are not fighting the hydraulic force and push each other up or down. The stabilizer bars are moving somewhat freely here and don’t realize as much tension, allowing the axles to flex farther than normal.

IMPORTANT: KDSS has two sets of valves, the solenoid valves and the shutter valves. Solenoid valves are normally open and are closed when KDSS is activated, see the “Accumulator Solenoid Valve Switching Timing” diagrams above. The shutter valves are bolted into the KDSS accumulator housing and are opened manually (on the unit under the body on the left side). There is a common misconception that doing something to the solenoid valves (e.g. Dr KDSS disconnect, removing the KDSS control unit fuse, etc) will help calibrate the pistons but it’s the shutter valves that help adjust the high/low fluid volume.

VERY IMPORTANT: Only back out the shutter valves 1-2.5 turns, I used about 2 turns. If you go any further then you risk letting high pressure fluid out and letting air into the system, then you would need to bleed and pressurize the system.

My Experience:
I installed the Fox shocks with little drama, I was able to remove the front KDSS sway bar and drop the LCA’s to make it easier. As recommended by other lift instructions I opened the KDSS shutter valves, this made sense to me because after the lift the piston for the front sway bar would have moved and high/low side hydraulic volume should be adjusted. When I was done with the install, and closed the shutter valves, I had about a 1” drop on the right side:
(These measurements are from the ground, so look for relative differences):
LF 35 7/8 RF 34 1/2
LR 38 RR 36 5/8

Looking at the front stabilizer bar it was definitely not even (and the rear bar to a lesser degree), as if the piston was pushing down on the bar and acting like a lever, with the fixed stabilizer link as a fulcrum, to compress the front right shock.

Experiment #1: Compress the KDSS pistons.
There’s a video where someone demonstrates fixing the KDSS lean by opening the shutter valves, rolling over a block of wood, and then closing the valves. The idea seems to be opening the KDSS high/low sides, compressing the KDSS piston(s) to move fluid from the high to the low side of the system, and then closing the valves with the new high/low volume.

I did something similar but rolled my left rear wheel up on a ramp, compressing the piston. However when I closed the valves and rolled off it was like nothing changed.

LF 35 3/4 RF 34 1/2
LR 37 7/8 RR 36 3/4

Taking this idea further I used a floor jack on the right side to roll the car and dramatically compress both the front and rear pistons. However again, after closing the valves and removing the floor jack it’s like nothing happened, there was still about 1” lean to the right.

LF 35 11/16 RF 34 5/8
LR 37 7/8 RR 36 11/16

Experiment #2: Disconnecting KDSS
To sanity check my Fox shocks I decided to disconnect the front and rear stabilizer bars so KDSS would have no effect on the suspension. I first removed the front, then the rear, then backed out of the driveway and returned. This resulted in an almost perfect alignment, so KDSS is definitely the culprit.

After removing the front stabilizer, rear stabilizer, then settling.

LF 35 9/16 RF 34 3/4
LR 37 11/16 RR 36 15/16

LF 35 ½ RF 34 7/8
LR 37 ½ RR 37 1/8

LF 35 ¼ RF 35 1/8
LR 37 ¼ RR 37 1/4

Experiment #3: Attempt neutral position for stabilizer
With the stabilizer bars reconnected I attempted to use floor jacks to lift the KDSS pistons so they aligned with the fixed links (a ‘neutral’ position). However, even with the shutter valves opened I noticed the pistons didn’t freely compress, lifting the piston also partially lifted the car. Noticing that I couldn’t force the pistons into a neutral position, without compromising the level height, I gave up on this experiment.

Experiment #4: Sending it
Feeling frustrated I decided to send the pistons all the way home. With valves opened, and the pistons disconnected from the stabilizer bars, I used two floor jack to fully compress both pistons (KDSS be damned). INCREDIBLY, after closing the valves and removing the floor jacks BOTH pistons slowly and fully extended back, as if there was no change in the high side fluid volume at all. This was very surprising as I expected the pistons to only extend a small amount, not extend back to their full length.

Reattaching the stabilizer bars I was right back where I started:

LF 35 3/4 RF 34 9/16
LR 37 5/8 RR 36 5/8

Conclusions thus far:
I assumed that the pistons themselves wouldn’t exert much force, however watching the pistons fully extend - after removing so much volume on the high side and disconnecting them from the stabilizer bar - I’m guessing there’s more physics at play. That or I made some simple mistakes like not closing the valves properly.

At this point I’m looking for some feedback from the community. My biggest questions are:

1. Why can’t I calibrate the pistons to a certain height, why do they always fully extend?
2. Why are the pistons pushing with such great force? Shouldn’t they simply rest on the stabilizer bars when the shutter valves are open?

Other notable “solutions” from the community:

• Adding spacers to springs:
  • Because the right springs are being compressed by the stabilizer bars, adding a spacer on the right will lift the body and could help correct the corner heights. However I think the stabilizer bar itself would still be ‘crooked’.
• Dr KDSS spacers
  • The KDSS piston is hanging low, adding a spacer to the fixed link to match the length of the KDSS piston should even them out but also changes the distance from the sway bar to the skid plate. When the suspension articulates, and the KDSS piston extends further, it might impact the skid plate.
• Swapping springs
  • Because the KDSS piston and stabilizer bar are acting as a lever and compressing the passenger spring, having a stronger spring on the right side could help as it will compress less than a weaker spring. This could affect other driving dynamics so maybe not a great solution.

 

Keep in mind that the trd pro Fox shocks have a different spring perches left to right to combat driver’s side lean in non kdss trucks. Depending on how you installed, your front shocks may be pushing you in the wrong direction. Kdss trucks tend to be flat or lean passenger side so they need correction on the opposite side as pros.

 

When I took the stabilizer bars off, meaning KDSS was no longer a factor, the truck evened out perfectly with the Fox suspension. See ‘experiment #2’, does that describe what you’re saying?

Are you also saying that the stock TRD OR springs, without KDSS attached, would induce driver side lean expecting to compensate for KDSS’s passenger side lean? I’m don’t think that’s true, I’d expect the truck to sit level, but I could find out.

 

Without the swaybars in place they will tend to lean driver side because of fuel and other additional weight on that side. Check the spring perches for your front coilovers and see if they are uneven left to right. You may find the driver side perch is set higher than passenger.

 

I don’t think that would be the case. These are the OEM TRD Pro Fox shocks, so I don’t believe they have adjustable perch’s, and I’d expect they’re already designed to compensate for the weight of the gas tank.

When sitting in a flat surface I’d expect stabilizer bars on non-KDSS trucks to not be under tension, i.e. on a flat surface with no stabilizer bars the OEM suspension should sit mostly flat (like mine did with KDSS disconnected).

 

They are not adjustable. Oem Fox shocks come offset from the factory. If you look at product images online you can see it clearly. Easiest would be just to look at them on your truck and confirm. My point is that if they are offset you will be chasing your tail trying to correct it.

People with lean on kdss trucks tend to offset front shocks in the opposite direction to compensate for the fact that they passenger side lean.

edited to add a link - you can see it in these pictures: https://www.4runners.com/threads/2020-4runner-trd-pro-fox-oem-shocks.18886/

 

You’re saying, from the factory, Toyota switches the R and L front coil overs? I’m pretty sure the SR5 and TRD have the exact same coil overs, this seems unlikely.

I don’t know why you’re saying that the Fox shocks being offset is a problem, why would you expect them to be even? This is a factory set and I’ve installed them exactly where they’re meant to go. The car sits perfectly level without KDSS so what am I chasing?

 

See the image link I sent above. Non pro trucks are even perches left to right and tend to lean left due to weight distribution. Kdss perches are even left to right and tend to lean right due to kdss piston tension. This can be accentuated by a lift.

Pro trucks have uneven perches to compensate for weight distribution and tend to be level as a result.

If you install Fox pro coilovers with the higher perch driver side you make the kdss lean worse. When you detach the swaybars you are level because the pro suspension is leveling you against the driver side weight distribution. Keep in mind that non kdss sways are very soft compared to kdss as well.

Hope that helps with the confusion!

 

To make sure I understand, you’re talking about how the TRD Pro springs look ‘uneven’ because of their different perch heights? How one has a more compressed spring than the other?

This just adjusts the pre-load on the springs, as one side of the truck often has more load (e.g. gas tank), it doesn’t affect how they mount to the frame and the shocks are likely the same height. I can confirm after I remove them. Non-pro OEM shocks might just have dramatically different spring rates to control height under load, instead of pre-load tension.

I don’t think I agree that these trucks sit uneven without stabilizer bars. KDSS and non KDSS use the same front coil overs so I’m not sure why you’re saying they have different perches.

Edit: I just went and snapped a picture of the OEM coilovers I removed, one of them has a much thicker spring than the other suggesting Toyota uses different spring rates to compensate for load instead of pre-load with perch heights

Edit 2: I was wrong, I guess they looked different at first but they might be the same spring. I'll keep looking in to it!

 

Doing some more research it does look like KDSS gets a different treatment that non-KDSS, though it's not obvious at first. KDSS gets the same RH strut (shock + spring) on both the right and left. Non-KDSS gets a different left and right spring and different shocks (although same L-R). Here's a table I found, seems to match online parts catalogs as well: 4wd w/ 17 wheels w/o dynamic suspension:
Left coil spring #48131-35620
Right coil spring #48131-35650
Left shock #48510-80507
Right shock #48510-80507

4wd w/ 17 wheels w/ dynamic suspension:
Left coil spring #48131-35650
Strange, but I didn't see a part number for right side for 4wd with 17inch wheels and KDSS
Left shock #48510-80508
Right shock #48510-80508

I wonder if that means the KDSS springs are just tough enough to compensate for the intrinsic downward force of the KDSS pistons?

 

I just reinstalled the OEM suspension components and it is absolutely true that, without the KDSS stabilizer bars, the KDSS suspension leans about 1" to the driver side! This makes sense now as the KDSS 4Runners use two of the same struts (the RH strut) instead of different LH and RH struts like the non-KDSS. So I guess Toyota intentionally allowed a LH lean to compensate for RH lean from the force of the KDSS pistons. Some measurements:

OEM suspension, no stabilizer:

LF 33 ⅝ RF 34 ¾
LR 36 ¾ RR 37 ⅞

OEM suspension, with stabilizer:

LF 34 ⅛ RF 34 ⅛
LR 37 ¼ RR 37 1/4

It seems like, if you wanted to use Toyota's approach, you would want to get two RH of the TRD Pro Fox shocks, which should result in a 1" or so lean to the LH, which KDSS will then 'compensate' for. I'm not sure if it will work out the exact same way but could be worth a try.

 

Yes exactly right!

Not all kdss trucks lean, I’m not sure why. But when they do, you can use the opposite approach of the pro and bump the spring perch (add preload) up about .5” on passenger side.

 

My 4Runner is pretty new, I think they're meant to be level with OEM parts and just have the intentional LH lean cancelled out by the KDSS pistons. If there's a weak spring or some kind of leak in the KDSS (i.e. KDSS loses pressure) then I bet there could be some unexpected lean.

It's pretty interesting that Toyota was able to get things so well balanced between the force of the extra weight on the LH side (gas tank, driver) and the force of KDSS compressing the RH side. Someone must have done the math!