Toyota 4Runner TRD Off-Road Suspension Flex Test

·9 min read

You’ve seen this particular Toyota 4Runner before because it was the subject of the first Suspension Deep Dive I wrote for Autoblog. It’s still hanging around my driveway and available to make the occasional repeat appearance because, well, it’s mine. I chose the TRD Off-Road for a couple of reasons, some of which will come into play on my Flex Index ramp.

First, it’s the only model other than the TRD Pro that comes with a push-button locking rear differential, electronic crawl control and multi-terrain select. Second, it can cost as much as $10,000 less than a TRD Pro, particularly if you’re content with cloth seating and no sunroof, as I am. I used some of the money I saved to buy the third item: an option called KDSS, the Kinetic Dynamic Suspension System. Importantly, this clever option can only be fitted to a TRD Off-Road; it’s not even available on the TRD Pro.

I absolutely wanted KDSS because that incomprehensible acronym stands for a very useful feature: automatically disconnecting front and rear stabilizer bars. This capability is a boon to off-roading because it does wonders for articulation. But KDSS isn’t just for hardcore off-roaders. It also improves everyday stability because the ability to disconnect the stabilizer bars allowed Toyota the freedom to specify much fatter stabilizer bars than you’d find on a 4Runner that lacks KDSS. This means my KDSS-equipped 4Runner will handle better on the way to the trailhead and be more stable if I load gear on the roof or buy a roof-top tent.

I didn’t have my Flex Index ramp when I put together the Suspension Deep Dive of the 4Runner, but the ramp is the perfect way to show what KDSS can do. It also allows me to safely crawl underneath and show how the system works its magic in a frame-twist situation that simulates a common off-road scenario.


There was plenty of clearance when I nosed up to the ramp, but I knew that beforehand. That’s because this is a 20-degree ramp, and my Toyota 4Runner TRD Off-Road 4x4 has a full 33 degrees of approach clearance. But it's even better than that as far as ramp clearance is concerned.

The approach angle is typically measured under the chin of the front bumper or the leading edge of the skidplate — whatever is lowest. Sometimes a low-hanging tow hook will define the number. But the zone that arguably matters more is this area directly in front of the tires because off-road drivers tackle most obstacles by putting a wheel on a convenient ledge or rock instead of straddling it. The front fascia of the 4Runner (and the Tacoma) is cut up and back at the corners so there is extra approach clearance in this critical area.


My 4Runner traveled a good ways up the ramp before a sliver of daylight appeared under its left-rear tire. I chose to engage my rear differential lock to climb slightly past the tipping point, however, because I know that 4Runners tend to roll back slightly after you set the foot-operated parking brake. It’s a function of how drum parking brakes take up the slack and self-energize on a slope.

It took a couple of tries to get it parked at the precise point where the tire just barely touched the ground after my weight was subtracted from the vehicle (ie: I climbed out) but could be easily lifted clear of the concrete with a finger or two. So positioned, it was time to make my measurements.

Warning! Math Content Ahead

The wheel lift height measured = 20.83 inches.

Divide that by the sine of 20 degrees (the ramp’s angle) to get the ramp climb distance.

Ramp climb distance = 60.9 inches.

The wheelbase of the 4Runner is 109.8 inches.

Divide ramp climb by wheelbase and multiply the result by 1,000 to get the Flex Index.

4Runner TRD Off-Road w/KDSS Flex Index score = 555 points

That’s a healthy score for an affordable mid-size SUV right out of the box. KDSS is clearly making a difference. Let’s see what it’s doing.


The bumper-to-ramp clearance diminishes during the climb, but there’s still quite a bit left at the top. The nearest point is still 6 inches away even with the front tire buried in the wheel well.

But there’s something else to notice here. The front stabilizer bar was previously located down low near the skidplate, but it has moved up considerably now that I’ve driven up the ramp.


This side-by-side image shows the difference. On the right we have the fixed neutral position that KDSS maintains when driving on the street. The KDSS hydraulic strut (green arrow) is rigid and unmoving in normal driving. Its partner on the passenger side (yellow) is a solid fixed-length rod. Together they hold the pivot point firmly in space so the stabilizer bar can twist and develop roll resistance that suppresses body roll.

The left image shows what happens when the system encounters a mogul or some other kind of off-road frame-twist situation, the exact scenario that is simulated by my ramp. This change happened automatically as I drove up. I didn’t push any buttons.

The once-rigid strut (green) has gone limp. But its partner on the other side is in the same place as before because it’s a fixed-length rod. This difference allows the stabilizer bar to wobble in place instead of twisting. No roll resistance can be developed; its anti-roll function has been neutralized. From the vehicle’s point of view, it’s like the stabilizer bar isn’t even there.


And that’s why the suspension flexes enough to shove the left-front tire up into the fender wheel on the way to scoring 555 Flex Index points. One thing I can’t help noticing is there's still enough room for larger-diameter tires. Yes, a ramp like this is a good way to perform clearance checks.


Here I’ve drawn a line to show how the stabilizer bar is oriented up behind the skid plate. It’s pretty much parallel with a line between the tire contact patches instead of the body, as it would be if this 4Runner didn’t have KDSS. By flopping loose it stays parallel to the footprint between the front tires. Since it doesn’t twist, it can’t generate any roll resistance. What we get instead is unrestricted wheel articulation.


This side-by-side image shows what’s happening at the rear. As before, the right-hand side is what the KDSS strut looks like when the vehicle is being driven on pavement. The strut stays rigid and unmoving, so the stabilizer bar twists in corners to develop torque that limits the amount of body roll. That’s why stabilizer bars are also called anti-roll bars or anti-sway bars.

On the left, my short trip up the ramp has caused the KDSS strut to grow instead of shrink. That’s consistent with a mogul or frame-twist situation (also why we call something like this a frame-twist situation), and indeed the fact the front and rear are doing the opposite thing is precisely why KDSS not only works, but does so automatically.

I lack a cutaway to explain the plumbing, but suffice it to say the front and rear struts are connected. When both are trying to compress in a corner, the fluid from each cylinder buts heads and reaches a stalemate where they meet in the middle of the vehicle. The result: no fluid flow, locked cylinders and functioning stabilizer bars. But each is moving the opposite from each other in a frame twist situation. The fluid flow is complimentary, so it circulates from front to back freely, which allows free strut movement and wobbling, ineffective stabilizer bars.


It’s easier to see how the rear axle and the rear stabilizer bar have remained parallel. There’s no twist in that stabilizer bar because the strut has lengthened relative to its fixed partner on the other side. The only reason neither is parallel to the ground is because of what’s happening to the tires themselves.


Here’s what I mean. The right rear is compressed much more than usual. And as we’ve seen a couple of images back the left rear tire is essentially unloaded (and round) all the way ‘round.


My Suspension Deep Dive revealed that the 4Runner can be said to have two bump stops that work in stages. The first stage is a large rubber overload spring/bumper (yellow) that gives extra support when the vehicle is loaded. In this frame twist scenario we can see that the supplemental overload spring/bumper has grounded out. The main bump stop hasn’t touched down yet. There’s a little bit of margin that might come into play if this was a more dynamic situation. But it’s also a reminder that spring stiffness can also define the limit of off-road articulation.


Here’s how the 4Runner measures up to other vehicles I have measured recently. It’s in good company. It clearly bests the Gladiator Mojave, but the Land Cruiser — which also has KDSS — eats it for lunch. That’s intentional on Toyota’s part. I used to be an insider there, and the Land Cruiser was always engineered to be the top dog. The 4Runner would never be allowed to out-perform it. It’s a bit like Porsche’s approach to the 911 and the Cayman.

For my money, quite literally, the 4Runner still wins. It’s a lot cheaper than a Land Cruiser, and its narrower dimensions allow it to fit through tighter squeezes and be less susceptible to the plant-caused paint ruination that we in the west call Desert Stripe. More than anything, the 4Runner’s articulation can be quite strong if you buy the TRD Off-Road with the clever and transparent KDSS option.

Contributing writer Dan Edmunds is a veteran automotive engineer and journalist. He worked as a vehicle development engineer for Toyota and Hyundai with an emphasis on chassis tuning, and was the director of vehicle testing at (no relation) for 14 years.

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