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Improving the XJ trail performance, #2.

In this Web Page I will cover three Issues; Tire Clearance/Bump Stops, Shock Length & Valving, and Shock Length Upgrade Choices.

If you think I should add something, or think I missed an important point, e-mail me to communicate your concern.

If you plan to lift the vehicle in the future, and like the way it rides with the existing shocks, there is no reason to upgrade (better to save the $$$ and upgrade in the future with a shocked matched to the lift height.)

With that said, adding adjustable shocks (shocks capable of softer valving off road) are worth the additional cost and are a great second step to improve road and trail performance. The firmer adjustment settings reduce sway on the road and the soft settings make the ride more comfortable off road.

If you followed this far, and still must have larger tires, you need to realize that 31 inch or taller tires will hit the fenders before the axles bottom on the stock bumpstops. You can let the fenders fight the tires, or install taller bumpstops to keep the axle (and tires) further away from the body.

Tire Size and clearance makes a big difference in the rear, as big tires will rub the fenders unless the bumpstop height is increased. If you do not care, tall tires will wear the fender lips away in the rear (or worse). The front fenders allow more room, and will fit a 31 on the stock bumpstop, but the tire will rub the anti-sway bar and lower control arms when the steering wheel is at full steering lock.

Some input from Bruce <blacksheep@awwwsome.com> on the XJ-List, "As has been recently stated, the rim width and backspacing play a major roll in determining what size tire will rub and where it will rub under what circumstances. 31" tires on 8" wide rims with 4.5" of backspacing will *look* bigger than 31" or 32" tires on 7" wide rims with stock (5.25") backspacing. It sure is fun figuring out what you want to do and even more fun once you get it done. It's a never-ending series of modifications and improvements. Good Luck."

The factory seven-inch alloy rim backspacing is 5.25 inches to keep the wheel centerline tucked under the body. This backspacing is the best fit for 10.5 inch or narrower tires. The picture of the 31x10.5 front tire shows how far the tire will tuck if the axle is pushed one inch higher than the factory bumpstop. This tire is touching the top of the fender liner, and will still turn, while fitting inside the fender flare.

Wider tires may require less rim backspacing to minimize the chance of tire bulge hitting the rear springs. This also pushes the tires out to where they will hit the fender flares, unless you modify the bumpstops. Rims with less backspacing (more outward offset) will move 31x10.5's out to where they will hit the fender flares at full compression. The least backspacing I recommend is 4.5 inches (with 4.75 inches currently working great with 33x12.5's on my XJ).

The best way to identify if the tires in your configuration will fit is to remove the springs and cycle the axle up until you find the limits. Removing the factory bumpstops, before testing the wheel travel limits; will provide a picture of the safety margin built into the factory design. This exercise will also identify where you may find extra unused shock travel (more on this later).

Everyone talks about lift springs, but when designing a suspension system the spring height and spring rate is one of the last things to consider. A first step is to be concerned with the end limits of the suspension; bumpstops and open travel length.

Bumpstops serve a dual purpose; they keep the body away from the axle, and they work as springs to slow down the velocity of the body in relation to the axle through a small travel displacement. Slowing the body and axle motion through the last half-inch of compressed travel is more comfortable to you and saves the suspension & drivetrain from damage.

When you have tires hitting the body you block out available articulation of the suspension in addition to increasing the possibility of damage to the body and tires. As an example, you had four inches of compressive shock travel before adding a tall tire. After the addition of the tall tire you end up using only three inches of the shock's compressive travel before the tire (or relocated bumpstop) hits. This interference will block the spring from compressing by an equal amount and effectively reduces the spring force at the displacement when the tire hits. If the spring rate is 200 pounds per inch it resists 800 pounds at four inches of compression, and is limited to 600 pounds at three inches compression. The last inch of spring travel is now blocked by tire rub, effectively making the spring force softer, allowing the tires to more forcefully crash into the body and cause damage.

A stock height XJ will accommodate 31-inch tall tires... but will spend even more time bottoming-out due to the effectively softer springs. However, you can prevent this possibility of damage by changing the bumpstop. Since you have lost the last inch or so of axle travel to tire interference, why not match the bumpstops to the reduced compression travel by an equal amount, and protect against damage?

There are kits that provide shims to lower the front bumpstops, as the earlier XJ (and all ZJ) stops unbolt. Use large pliers to pull the rubber bumpstop cushion out of it's retainer cup and you will find a bolt in the center of the cup holding the bumpstop assembly to the body (early XJ stops require a pipe-wrench to remove.)

Other kits add a bolt on puck (requiring some drilling) to the lower spring pad. I glued large poly bushings to the lower spring pad, inside the spring. These have never moved in over 150k miles. The stock bumpstop hits these bushings an inch before they would have hit the bare spring pad. An inexpensive alternative, to the bushing that I used, is to glue a Hockey puck to the center of the axle pad inside the spring.

As a note, some people add longer shock bodies that bottom before the axle will hit a bumpstop, and neglect changing the bumpstop height. The longer shock body will keep the axle further away from the body and tall tires away from the fenders. This can, however, lead to vehicle damage (In the past I used a longer rear shock with rubber "pre-runner bumpstop" cushions until I found the upper shock cross-member cracked. The damage motivated me to correct the axle/body bumpstop height.) Shocks are typically not designed to act as bumpstops and can be damaged when pounded closed. Look at the shock warranty, most manufactures exclude this type of damage.

The rear bumpstop can be shimmed away from the body, or they can be replaced with MJ stops that are taller.

The rear bumpstop extension can be a simple piece of boxed tubing added as a shim between the frame and stock bumpstop. Any good welding shop will have 1.5 & 2-inch square hitch tubing that will work. The supports shown are modified unistrut with bolt lengths chosen to bottom-out, making the support rigid. Aftermarket bumpstops are available in rubber or poly in a variety of heights.

A little help with factory XJ/MJ rear bumpstop part numbers and lengths from: "David A. Windle" <windle@flash.net> on the XJ-list. "All info is from my local dealer who had no idea they could be used on both vehicles. No specific information was available as to what model(s) each of the different sizes was supposed to fit. The tallest ones are a direct bolt on. The holes and the curve at the base of the stop which snugs to the frame are identical."

Modifying the shock mounts, to optimize the wheel travel with bumpstops, should be a priority after any change in tire height or lift. You can change the shock mounts to move the unused compression travel into the extension portion of axle movement and gain back the lost shock travel. Change the shock mount to match the bump stop (see the shock mount picture) and eliminate the loose fitting bar pin at the same time.

 FWIW, it's easy (cheap) to fabricate your own bar pin eliminators and optimize the shock mounting to exploit the full shock stroke & length for max droop. The photos are of 1 1/2-inch extensions on the front, and older (crude) 1-inch extension on rear of my 88XJ.

A little cutting and drilling on angle iron, a nut & bolt, a bronze bushing, and some washers, all available at a good hardware store, is all it takes. The bronze bushing keeps the angles apart and prevents the shock bushing from being over tightened. The bushing is about the width of the metal shock eye. Washers contain the bushing and allow the angle iron width to be a narrow one-inch allowing it to seat into the small factory shock mount well.

The static shock length is shortened the height difference from the angle flat (the centerline of the old shock bar pin) to the centerline of the new bolthole. A 1 1/2-inch extension requires 2-in. web angle; 2 1/2 in. needs 3-in. web angle, etc.

The angle iron may take some trimming to prevent interference with the shock but all of the work can be done with a drill motor and hacksaw. The result will eliminate the bushing thump by pre-loading the shock bushing.

If you are wondering why I picked 1.5-inch extensions for the stock length front shocks, look back up the page to the picture showing the front spring removed. The stock length RS9000 shock is an inch too short even when the axle is compressed an inch past the normal bumpstop height (Yes, 2 inches too short). With stock length shocks, the factory designed the system to be safe with up to two inches of bumpstop crush. This crush expectation is excessive. I lengthened the stock shock mounts to gain the lost shock travel but still maintained some crush safety in the bumpstop.

Bill Van Tuyl (from the XJ-list) had a good suggestion to make the front shock mounts wide enough to allow outside offset at the lower end of the shock. This will eliminate the shock from hitting the body when the axle is extended with tall lifts. The same can apply to the rear mounts if a longer shock and more inboard mounting is desired.

Wheel and shock travel is dependent on the length of the shock. Sounds simple, but the need to match the shocks to provide more wheel travel is often overlooked when assembling the components for a lift system. Improved articulation (wheel travel) is the goal of building the suspension so that it can utilize all the possible shock travel.

Making significant changes in the lift height of an XJ, without matching the shock length, sometimes results in a taller vehicle (a higher CG that is more effected by crosswinds, etc.) with the same or less wheel travel than stock. When larger tires are added, the interference between the fenders and tire may result in the tire hitting before the shock is fully compressed. This may leave unused and unavailable shock travel, resulting in less wheel travel and worse articulation than a stock vehicle. You can also add too long of a shock with the result of losing all the extra lift spring travel to a mismatched longer shock body.

There are ways to relocate the shock mounts to fit a longer shock, without additional static lift height, if you are willing to relocate the exhaust and fuel lines. The picture shows the current upper-rear mount on my 88 XJ. The inboard mounting provides room for considerably longer than stock shocks (Rancho 9112's) without blocking-out the compression travel of the suspension. The result is longer shock, and wheel travel, without unnecessary lift height.

The BPE mount shown in this picture is the factory Jeep rear anti-sway bar bracket normally found on top of the rear spring pack. If you have removed the rear anti-sway bar, find (or remove) these brackets and you will have a free pair of BPE's. The bolts line up with the factory shock bar pin to body nuts. You will need to add some washers, a shock bolt, and bushing spacer.

Stock length shocks, when compressed, are 12.77 inches long up front and 12.73 inches long in the rear. The stock bumpstops are positioned to slow the axle movement and stop the axle before these shocks fully compress, to protect the shock from damage. Regardless of lift height, the tires will compress fully into the wheel wells with the same possibility of rubbing as stock height because the shock travel is the same as stock, even though the static ride height is taller.

Anything longer than stock X128/X129's will take work to match the bumpstops. Look at the column listing the closed growth length, in the following table, to see how much to add for the respective shock. Anything other than the stock shocks will also require installation of your old bar pins into the new shocks, or install bar pin eliminators, because the shocks are originally designed for a different vehicle's bolt-in application.

The front shock travel is not as much of an articulation limitation as the rear shock length. Ramp travel testing with a stock XJ will reflect this limit by lifting a rear tire (after the anti-sway bars are disconnected). Additional safe travel up the ramp can be made after disconnecting the rear shocks (up to the point the rear brake line becomes the limit, be careful if you try this).

For a stock height to three-inch lift XJ, and 28-31 inch tires, most people run the stock length (0-3" spec. Rancho 5128 or 9128) front shocks.

Up front, most XJ's with 30-31 inch tires work well with the stock length X128's (using stock mounts or with bar pin eliminators,) and no change in bumpstop.

In the rear, most XJ's can use stock 92-98 ZJ Grand Cherokee rear shocks (X194's) with MJ rear bumpstops. This is the easiest choice of shock with only the bolt-in change of the rear bumpstop or the addition of a bumpstop shim.

The X194 shock will work well for three inches of lift with 31's, because the extra closed shock body length is in the last inch of compression where the tires hit the fenders. You will bottom these shocks without taller bumpstops so make sure you address this issue to prevent shock damage.

Up front 31's will fit fine and only rub the control arms and sway bar (when disconnected) if you hold the steering at full lock. The stock bumpstop will bottom before the 31 inch tire rubs a fender -- given they are mounted on stock 7 inch rims with 5.25 inches of backspacing.

Actually, you can stuff 31's one inch past the stock front bumpstops, with stock rims, before they rub the fenders up front by any significant amount (yes, I removed the springs and cycled the front axle every way possible to determine this answer). Some of the earlier pictures show the front axle fully stuffed without the bumpstops in place to determine how much room is available.

The rear 31's will rub at the back of the flare liner inside each wheelwell, even with stock rims, if the rear axle is bottomed on both sides at once. The tires will not rub much when twisting as the axle angle usually angles the tire in enough to miss the flare. You can minimize this interference by shimming the stock bumpstop 1.25 to 1.5 inches lower -- by putting a like-sized block between the stock bumpstop and frame (you can also purchase taller bumpstops from Jeep). The taller bumpstop assembly will limit the axle (and tire) from getting too close to the frame and body.

Shimming the rear bumpstop will reduce the rear axle load capacity by the lost displacement of the rear spring. This usually reduces the rear load capacity by 200-300 pounds per inch of lost axle travel. This is only a problem if you find your load bottoms the axle often. The remedy is stiffer springs or taller lift.

Proper bumpstop placement will prevent 31's (or taller tires) from rubbing the body. If anyone wanted to safely run 31's on a stock XJ the rear bumpstop shim is all they need to prevent rubbing -- they will, however have very little compressive axle travel. The stock XJ will spend quite a bit of time with the axle on the bumpstops (ride rough too). Preventing this loss in load capacity is why there is an aftermarket for lifts & stiffer springs.

Articulation will be the same as stock, with the loss of the inch or so of rear axle travel due to the bumpstop shim. A good shock bar pin eliminator will return the lost shock travel my making the shock assembly longer. Installing BPE's to a small lift rig will actually gain an inch or so of wheel travel, at both axles, if the XJ is set up this way. When disconnected you should easily ramp somewhere above 650, and not rub anywhere.

You can also safely install a longer shock in the back (like a Comanche or Grand Cherokee shock) if you use stock bar pins (and get a longer rear brake line) to gain even more rear axle travel due to the shimmed bumpstop. I would not run longer shocks without the added bumpstop height.

Personal experience: I ran 5118's before matching the bumpstop height. I trashed a good set of shocks, broke both upper bar pins, and cracked the upper shock mount crossmember. This was during Baja Chase/Pre-running over severe terrain at speed (much higher impact than rock crawling) and I have seen no further damage or loss of wheel travel since correcting the stop height.

If you are unsure of what size shock or bumpstop to consider, my best suggestion is to install the lift and tires, but do not purchase the shocks until you can measure how much additional bumpstop length you need, and longest shock length you can actually use. After you find the bind limits and where (or if) the tires rub, pick the best fitting stop modification & shocks, and buy them.

Matching the suspension to taller tires and longer shocks will take away some spring travel, and will require more work to eliminate all the interference problems. If done correctly, however, you will be able to run 31's (and narrow 32's & 33's) on the factory rims with no rubbing.

**The intent of these listings is not to sell you on Rancho shocks, but to provide you length measurements so you can see what fits and cross reference to the product of your choice.**

FRONT (stock mounting -- stud top & eye bottom)

Unfortunately there is a small selection of shocks to chose from with the factory design stud/eye mounting. The LS notes denotes "Large Stud" on these models (they have been confirmed to fit). What can be accomplished, is these shocks' length can be optimized with bar pin eliminators.

REAR (stock mounting -- eye top & bottom)

The dimensions are in inches for all sizes.

Again, my best suggestion is to install the lift and tires, but do not purchase the shocks until you can measure how much additional shock length you can actually use. After you find the bind limits, pick the best fitting shocks and buy them. One thing that you will also find binding is the stock brake lines so make sure that you plan to install longer brake lines or make some changes to the brake line mounting (this is also covered on the next web page).

Measure the current front compression shock travel (measure the exposed shock shaft length with the XJ sitting level). Compare this to the measurements given for total open/closed/travel lengths for the current shocks. If you currently measure 4 inches of open shaft, and you have shocks with 8.5 inches of total travel, you will have 4 inches of compression and 4.5 inches of droop.

Raising the XJ two inches (without moving the shock or bumpstops) will result in 6 inches of compression and 2.5 inches of droop. Sometimes it is difficult to use this extra compression travel and spring force (you can never fully load the spring before lifting a rear tire).

Adding the BPE will move this travel down about 1.25 inches (the height of the BPE), resulting in 4.75 inches compression and 3.75 inches droop travel, reducing the total spring force at full compression. You can add spacers under the BPE or even add extra stud cushions inside the fenderwell at the top mount to add more length to the shock to change the balance of compression & droop travel.

Once you tune the shock mounting to match the spring travel, spring force and ride you want then you can match the bumpstops to protect the shocks when they bottom. Take some time with a tape-measure and sketchpad to determine what you have and what you plan to achieve. Looking at the mounts will help you understand the issues involved. The real decision of what shocks will fit will be up to you.

Converting the upper front stud mount to use eye to eye X010, or smaller, shocks will result in less actual wheel travel than the factory mount compatible X009 stud to eye shock. The common front stud to eye adapter takes up a lot of length space under the fender that is better used with a longer travel shock body using the factory mounts.

Most recommendations for lift shocks split the closed body length with the lift gain. This means that with a four-inch lift try to use a two-inch longer closed shock body. The result is two inches more compression travel (than stock) and two more inches extension travel from the new static ride height.

First look at the closed shock length:

FRONT -- X009's (the longest Rancho shocks available) are 15.67 inches long vs.12.77 inches (stock XJ) when fully compressed. Lets see what will hit first: 15.67 (X009's) - 12.77 (X128's) = 2.9 inch longer shock.

Note that the shock will bottom first, before the bumpstops slow the axle down to a safe speed. The safe solution is to add spring pad spacers of 3 inches to protect the shocks. The bumpstop change is gluing a spacer on the front axle, on the flat pad in the center of the spring (two or three stacked hockey pucks will work well). However, remember the two inches of shock travel that is lost with the stock length shocks and the factory bumpstop. A little time spent with the front springs off and the new shocks installed (cycling the suspension to see what shock travel is available) to see how much bumpstop to add will be time well spent to gain all the travel possible with the new suspension.

With the reduced compression travel (with this longer shock) the extra load capacity and compressive spring travel gained with a 3-inch lift is now diminished as the spring travel is blocked by the longer shock. These shocks are best suited to a static lift height of about 6 inches (2.9 * 2 = ~ six inches).

With 32 inch and larger tires, on stock rims, the tires will probably hit the spring tower when twisted up, before hitting a stock length bumpstop. I still recommend adding the bumpstop spacers even though the tires hit under the fenders where you do not notice any damage. (IMO, the XJ front wheel bearings are not designed for, or capable of, withstanding high g-force side loads that result from stuffing them into fenders.)

The other issue to consider is total open length of the shock:

In the front the stock shock open length is 21.14 inches the X009's are 26.8 inches. (26.8 - 21.14 = 5.66 inches longer). Will the suspension allow the use of this much extra length?

Take the XJ and put the body/frame up on tall jack-stands. Then let the front axle droop and pull the lower end of the stock shocks loose. Have someone stand on the axle end to push the axle away from the body and measure how much extra length you can use before something binds or the springs unseat to the point of being dangerous.

Usually the stock lower control arm will bind at about 1 inch longer than the stock shock length. Since you already loose about 3 inches of wheel travel due to the longer shock's compressed length, you end up with 2 inches less wheel travel when the stock lower control arms bind. Aftermarket bent arms bind at 3 to 4 inches longer than stock. These breakeven, with about the same wheel travel as stock shocks, when matched to good aftermarket control arms.

If you want to use all the extra shock length available with 9009's, you will need to notch the arms or cut back the shock mount pad to gain an inch or two of additional clearance (this is described in the next web page). The next droop limitation is the factory driveshaft spline bottoming (this is possible with 9009 length shocks on some XJ's).

The other alternative is to run shorter X168's, or stock length shocks where the total shock travel is matched using bar pin eliminators. Since 30 inch and smaller tires will not rub with the stock shock length, this may be your best choice if you don't like cutting or grinding at the bind points.

REAR -- X010's in the rear, 15.81 inches compressed vs. 12.73 for the 9129's.

15.81 (X010's) - 12.73 (X129's) = 3.08 inch longer shock. The shock will bottom first and the safe solution is to add a 3-inch spacer between the bumpstop and the unit frame. The longer shock will also diminish the load capacity of the lift by about 600 pounds (3 inch * 200 lb./in.) and a 30 inch tire will be stopped by the longer shock about 3 inches before any chance of rubbing occurs. These shocks are best suited to a static lift height of 6 inches (3.08 * 2 = ~ six inches).

The six + inches of extra open length will take major work to exploit fully. Put the XJ unit frame on tall stands (do you get the idea that you need new jack-stands?) and let the rear axle droop. Pull the lower end of the stock shocks off and measure how much extra shock length your leaf spring, block & shackle combination will allow. Be aware that full use the shock travel available with 9010-9112 length shocks will require replacing the parking brake cables with longer lines or relocating the cable bracket.

FRONT -- X017 (Soft, or dual mount) or, X168 (normal dampening)

If you plan to run 31-inch tires these shocks will work best.

You give up 2.39 inches of upward compression due to the longer shock body, but gain 2.26 inches in travel for a total open length gain of 4.65 inches (this time you get to do the math). These shocks are best suited for a lift of approximately 4.5 inches (2.26 * 2 = ~ 4.5 inch lift.)

If you run stock to 31-inch tires the shock will bottom before the axle hits the stock bumpstop or the tires hit the fenders. I recommend adding one inch of bumpstop spacer inside the spring pad to save these longer shocks from possible damage.

Again, testing the suspension limits on your own vehicle will provide the best fitting result.

The droop gains 4.65 inches with these shocks. As always, (because every lift system is different) I recommend you jack up the front end and lower the axle 4.65 inches below the stock shock's open length to check for control arm binding.

REAR -- The X116 and X114 work well with 4 inches of lift but you do need to add bumpstop height. Adding 2 or 2.5 inches to the stop height (respectively) will protect these shocks.

Michael resolved the problems he has encountered with his suspension... with excellent results! :~)

Happy Trails!
Ed A. Stevens
stvns@aol.com

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