Rear Leaf Downtravel / Do I need new shocks?
 

Hey guys,

I just added JKS shackles and relocation brackets to my 1999 XJ rear suspension (OME leafs and OME 60053L nitrocharger sport shocks).

With the additional equipment, my resting length is 22.5" of the 24.5" max extended shock length (no BPE and stock axle mounting on Chry 8.25).

I assume this will result in insufficient downtravel capability (~2") but would appreciate your input.

Also, I've felt the OME nitrocharger sports have been stiff onroad when fully unloaded. With all my camping gear, its much better.

Therefore, any input on new shocks (if needed) would also be appreciated.

You probably will have more than 2" of downtravel, but still an insufficient amount. Keep in mind that the shocks don't travel in the same arc as the suspension in the rear due to their mounting angle. Suspension and shock travel is not 1:1.

The best thing to do is to remove the shocks, flex out the suspension completely and measure for collapsed and extended shock length. While you're doing that, you may want to measure for correct bumpstop length as well. Use your shock measurements to buy shocks by length specification, rather than by manufacturer recommendation.

Below are images showing the process you describe. The distance from the shock axle mount point to body mount point is 27" just before the tire comes off the ground. (shock extended length is 24.5")

AT RESTING HEIGHT WITH SHOCK FULLY EXTENEDED
http://i68.tinypic.com/2wdswu8.jpg

PICKING UP REAR SUSPENSION WITH HI-LIFT
http://i68.tinypic.com/rh04yu.jpg

LEAF SPRING MAXIMUM AND SHOCK FULLY EXTENDED
http://i63.tinypic.com/21nk6iu.jpg

Well, you're most of the way there, then. You should articulate the suspension though, rather than just droop it as you show above. If you compress one side of the suspension, that will force the other side to maximum extension. Once you've done that, you can measure one side for maximum extension length, and the other for minimum collapsed length. Likewise, when articulated you can measure the compressed side for proper bumpstop length. Correct bumpstop length will protect the body from tire contact, and protect the shocks from bottoming out.

So the maximum extension via the "droop" will be greater than the maximum extension on the opposite side of the fully compressed single rear wheel (with proper uptravel limits)?
And therefore only the maximum extension seen in practical use will be via compressing the opposite rear wheel?

If so, then I could shorten the distance slightly by raising the axle shock mounts, if I find it is needed, instead of buying shocks that are ~3-4" longer like I had previously thought using the "full droop" measurement.

Correct. The force on the compressed side will also force the extended side downward if the upward travel is stopped by the leaf spring or bumpstop inboard of the tire. This is another reason to have proper bumpstops--they act as a fulcrum for articulation. The force on the compressed side, combined with the fulcrum of the bumpstop will transmit an opposing force on the other side to extend. If the upward travel of the compressed side is stopped by the tire inside the wheel well, that won't be the case.

I hope that makes sense...

You probably don't want to do that. Raising the axle shock mounts will limit maximum shock travel and articulation. If you shorten the distance between the mounts, you have also limited the area that the collapsed shock can fit into, and therefore you might need even more bumpstop to limit shock travel so the shocks don't bottom out. Bottoming out a shock will destroy it.

You must consider collapsed and extended shock lengths. The larger the distance between collapsed and extended length, the greater articulation you will achieve within the limits of the springs. The limits are unknown until you articulate the suspension.

Gotcha thanks for all of your help! I really appreciate it. I will get to work on this.

My mind went to raising the mount only because the fully collapsed length (14.4") is MUCH less than the resting length (22"), keeping in my that it looks like I'll get ~5" uptravel before my tire hits the fender.

Absolutely. I could have easily just said, "yes, you need longer shocks." But, to do it right, there's more that needs to be considered. You really want your bumpstops to be your uptravel limiting factor, rather than the shocks, tires or even the leafs. It can be a bit of a guessing game...

Good luck and check back with updates and any questions you might have.

I have pretty well the same setup, OME CS033RA leafs (stiffest heaviest duty ones) as well as the Nitrocharger Sport 60053L shocks, and the JKS Shackles.

I agree on the ride, pretty stiff when empty which I don't mind (comfort vs handling), and they ride beautifully when loaded up. Because I like the ride quite a lot, I decided to change where the shock mounts rather than opt for a longer shock.

I moved the lower shock mount to the centerline of the axle tube (requires welding obviously), and installed a 1" spacer on the upper mount, netted me around 4" total decrease in shock length at ride height with the added bonus of extra clearance under the axle. The shocks have plenty of travel and now they are pretty close to centered in their travel at ride height. Bueno

Edit: Just read Tbone's post, the only thing that you're potentially limiting by raising the shock mount is your uptravel, but with most aftermarket shocks (10" or 11" travel) you'd be pretty far into the fender with the tire before bottoming out the shock, so it's really not an issue, especially if you like the ride of the OME shocks. If you're not set on the OME shocks and are anticipating having some huge axle articulation (probably not gonna happen with the OME leafs), the proper way to do it would be to buy longer shocks. For me, I was optimizing the existing shock's range of travel to the ride height.