Suspension Travel Measurement
'98 Tacoma TRD Xtra Cab
When I switched from Bilstein shocks to Rancho shocks for the rear suspension on my Tacoma, I happened to time my purchase just as Rancho was phasing out the RS9000 shocks and replacing them with the RS9000X. As a result, they were offering a "buy 3, get 1 free" sale on the RS9000. I came in near the end of the sale, and quickly hooked up with a friend who also needed a pair of 9000's for his Tacoma rear axle. As I asked around for information on the proper shocks for my lift application, I was given some part numbers that were used as "standard" applications for 2" to 2.5" lifted Tacomas. I eventually discovered that these assumed part numbers are not necessarily going to be the correct length for a given application. As a consequence, I installed shocks that were slightly too short, and I was limiting my rear travel by doing so. This results in a possible off-road scenario where the entire weight of one side of your axle (with wheels, tires and all) is hanging from the tiny weld on your lower shock mount instead of your shackle mounts. This is a BAD THING. The other problem with this is that a shock that is too short can rob you of precious traction by limiting the flex of your axle. Even an inch or two can make a big difference. On the flip side, an error in calculating the compression of your shocks can be equally as disastrous. A shock that fully compresses before the axle does will run out of room for the piston, and "bottom out", putting the full force of your compressed axle onto the piston assembly. This can blow a shock in a hurry.
Do yourself a favor and remove your old shocks and articulate the suspension in a controlled environment to determine how much compression and extension your particular setup will allow BEFORE you order or buy your shocks.
This can be accomplished by parking on a level surface, removing the rear shocks and jacking up one side of your truck until the tire comes off the ground. (Make sure the truck is properly jacked and braced for safety.) Measure the distance from the center of the lower shock mount (on the raised side of the vehicle) to the center of the upper shock mount. This is your extension measurement.
Now measure the same distance on the opposite side of the vehicle. Even though you have one side jacked up, the other side is not necessarily fully compressed, so you have to compensate for this. Measure the distance between the lower and upper bump-stops and subtract this distance from the mount-to-mount measurement you just recorded. This will approximate your compression measurement. I say approximate because the distance between bump-stops is vertical, while your shocks will compress at the angle they are mounted. The bump-stop measurement will be slightly less than the actual additional compression of the shock, so subtract accordingly for your final compression measurement. For example, for a shock mounted at a 22.5 degree angle, the additional compression travel will be 1.0824 times as much as the bump-stop measurement. Be conservative with your compression measurement (better to error on the short side to avoid "bottoming out" the shocks). Also keep in mind that a hard hit (bottoming out) will compress the bump stop some.
If you find yourself in a quandary because the brand of shock that you want to use doesn't have a part number that matches up well with your extension and compression measurements, do not fear. You can always order a shock that has a little extra extension (so as not to limit axle droop) and then fit your truck with taller bumpstops to compensate for a compressed shock length that may be greater than your actual compression dimension. In my opinion, it's better to cheat yourself out of a little compression rather than sacrifice a little extension. (However, by reducing the available compression, you may inadvertently decrease your trucks potential to stay level when in a precarious situation where camber (tipping) is concerned.) On the upside, an aftermarket bumpstop can improve the available "cushion" over the usually thin, hard rubber factory stops. This can make bottoming out a little less harsh.
Let's use the following dimensions for an example:
23.25" - Extension (driver side)
14.75" - Compression (driver side)
8.50" - Shock Travel (drivers side)
23.00" - Extension (pass. side)
15.50" - Compression (pass. side)
7.50" - Shock Travel (pass. side)
Based on these dimensions, we can find an example shock - Rancho part # RS99246 that would probably work. The extended length is 23.125" and the compressed length is 14.625". I realize the extended length is 1/8" shorter than the example dimension, but it may be noted that the extension figures listed in Rancho's RS9000X specification chart have a footnote that states "Bumper Stop Unit. Dimension shown at bumper contact". When you consider this footnote along with a footnote in another Rancho spec guide that states "Internal extension bumper may reduce perceived length by 1/2". I would guess that there is a little bit of fudge-factor in there (for the example cited) that will allow the leaf springs to fully droop before the shock reaches its absolute limit.