Lately I've gotten a few e-mails from folks asking if using shocks valved for a different vehicle will be an upgrade VS shocks valved to their vehicle. I'm honestly never really sure how to respond to these questions.
I think fundamentally theres some confusion about the generations of vehicles. A new generation vehicle implies that it is not the same as the old one... otherwise it would be the same generation? The 1st gen tundra is essentially a tacoma they made wider, its offered with the same anemic motor and drivetrain as the 1st gen tacoma. Meanwhile the 2nd gen tundra is a 430 horsepower v8 monster with 10.5 and 9" ring gears. Just because a vehicle shares a model name, that doesn't mean that it's just the same truck with a slightly worse drivetrain and more computers and still drum brakes *cough cough*. Of course there are a few exceptions.
The 4th gen and 3rd gen 4runner are slightly more similar than a 1st/2nd gen tundra, to be completely fair. Both use the same size rear diff, although the 4th gen gets an "upgrade" to an 8" front vs the suzuki samurai sized 7.5" front from the previous model. But that's where the similarities start to fizzle out. The 4th gen is a bit of an odd duck actually, when toyota first released the supercharged 4.7 it had a better 0-60 time than the Porsche SUV of that year. By todays standards the 4.7 makes less power than a modern v6, but at the time it was a pretty crazy leap. The v8 was also only offered in AWD, I'm genuinely not too sure why they decided to do this. They used a torsen style center diff which biases power (not torque, which is a common misunderstanding that bothers me almost as much as people saying they have 4:10 gears). These 2 models have different wheelbases, different corner weights, different spring rates, and different suspension geometries both front and rear (suspension encyclopedia with OEM/kit geometry for most popular applications coming soon). I'm not sure why im picking on the 4runners, because this is a common idea among several applications... it's really just the easiest to explain why this can cause issues, and might ultimately be a downgrade.
Lets first take a look at the rear shock on a 3rd gen 4runner, im using the 5100s as an example because thats the easiest thing for me to get product photos for...
3rd Gen rear OEM fit shocks
The bushings in the middle of the display i've highlighted get pushed into the shock eyelet, the top is what's called a "tennon". A shock tennon is common in linked suspensions because with bushings it can easily handle the side loads and weird arcs the suspension travels in. One main thing you will notice is that typically a shock tennon designed for a leaf spring vehicle vs a linked vehicle will be different sizes due to the difference in force side loading the damper. If you take a tennon for a leaf spring truck and put it on a linked rear end, you are likely to snap it off... of course you wouldn't want to try that because the valving would be wrong to beging with.
The bushings for a 3rd gen rear shock have an ID of 19mm, there is no sleeve, no vulcanized rubber, just 2 bushings you shove together. The bushing durometer in these is fairly compliant to deal with the suspension geometry in the rear and how it relates to the shock mounting locations, and the shock eyelet can freely rotate on the stud. The rear suspension of a 3rd gen 4runner is not ideal (although pretty good for its time), and takes some engineering to make a product that lasts. I would be remiss without mentioning that there are several companies that offer great products to improve the ride quality of these rigs, including kits to reduce the binding Toyota designed into the system for highway driveability, and weld on parts to correct panhard angles.
4th Gen OEM fit rear shocks
This is the same series shock designed for the 4th gen 4runner. If you look at the bottom mount it has a vulcanized rubber bushing with a sleeve incorporated into it, not only is this much less bushing material, but it's also much higher durometer. That sleeve has a 19.35mm ID... in comparison to the 3rd gens mount which is a 2 piece bushing for a 19mm stud. Now the size difference isn't substantial, if you overtorque the nut and let the washer squish it in place it'll hold totally fine as long as the stud isnt too wallowed out. The bushing with the vulcanized sleeve is essentially creating a much larger stud, you can see that there is not much bushing material between the sleeve and the eyelet, which is fine for the application its designed for because of the specific mounting location and suspension geometry it does not require as much deflection or much twist.
If you scroll back up and look at the application specific shock for the 3rd gen you will see that it has a 2 piece bushing that presses into the eyelet with no built in sleeve. The axle side is a stud mount, and the stud itself acts as the sleeve. The bushings will rotate directly on this stud instead of the 4th gen where the sleeve minimally rotates. If you install any 4th gen rear shock on a 3rd gen, and it snaps because of the increased side load from the bushing design in an application it was not engineered for, no shock company will warranty that product. This is similar to folks putting heavy "upgraded" springs on 5100/6100 series shocks, then complaining when it snaps or blows the seals and their warranty gets denied. In applications where Bilstein offers slightly heavier springs, the shock has been completely redesigned to work with the extra force.
Alot of this is sort of a moot point though, because they are also valved differently. It should not be a suprise to anyone that because the 4th and 3rd gen only share their name and rear diff size in common, the valving is completely different. Why anyone would choose to run a shock from a different generation that sort of fits, instead of a shock designed for their specific lift amount and geometry, is something i can't quite figure out.
The 4th gen OEM product is also 2" longer when compressed. Installing this on your 3g4runner will require a bumpstop extension or else you will be bottoming into the shock itself. At this point a coil spacer lift would be a good option, so you can keep your stock or just slightly higher spring rate, without having to go to some extremely heavy setup that rides poorly. You can move the rear travel around slightly, although the rear link geometry of the 3rd gen is such that your anti-squat and to a lesser extent your roll center become exponentially worse the more droop you achieve with the stock link geometry.
Even though earlier in this article i was complaining about how new generations are often completely different vehicles, i have to say the shock fitment for a 4th gen is carried over for 5th gens as well, and frankly the 2nd and 3rd gen tacomas are almost the same vehicles. That being said some shocks that fit multiple kinds of vehicles based on the same chassis will not use the same springs. The FJ cruiser springs come to mind as an example, the 4th gen 4runner and FJ shock may be the same part number but the springs will not work across the models as the FJ ones are a higher diameter or have an extra rung to compensate for weight distribution.
The biggest source of my confusion about this, is that there are shocks in stock valved for the 3rd gen and for 2" of lift. ARB 60028 have a compressed length of 13.3 and are for 1.5-2" of lift and theres over a hundred on the shelf right now. The 4th gen 5100 has a compressed length of 14.9", the 3rd gen version is 12.91" compressed. That means with the 4th gen shocks you are eliminating 2" of bump travel from the 7.9" of travel the rear of the 4runner gets, this damper is not useable on the vehicle without at the minimum a large bumpstop extension that will reduce your up travel. With stock links and a shock with a 2" taller compressed height, you are greatly reducing your suspension travel overall, and you don't have much to loose.
So to summarize:
- The 4th gen shock is too long to use without reducing your up travel
- The 4th gen shock cannot be warrantied when installed on a vehicle it isnt designed for
- The rod end design is not engineered for the 3rd gen mount/geometry
- The 4th gen shocks valving is not correct for the 3rd gen, that is why it is not sold as a 2" lift shock for a 3rd gen
- The 4th gen uses a slightly larger metal sleeve than the 3rd gens bushing inner diameter
- There are 2" lift 3rd gen shocks in stock right now that are both designed and valved for this application