Rear suspension handling for speed

My front suspension seems to be working decently, but my rear still needs a little help. I had a good amount of bucking (rear end bouncing off the ground) on the medium to large whoops on the last trip to the desert. I'm trying to figure out what will help this in general, and specifically on the XJ. I know a relatively soft spring rate in the rear is desireable, but my springs already seem pretty soft for rockcrawling. Who has some experience with this?

I didn't have my front set up like it will be soon, so I wonder how much that will play into it. We got done at night, so I didn't stop to adjust my front limiting strap before the mad dash back to camp. I shorten it for the trail, then lengthen it when I stop to unlock the front hubs. I never did that last weekend, so I ran back to camp with the droop limited about 3".......not sure what effect that could have on the bucking. I plan on a front shock rebuild that will firm up the front valving, and I have a set of stiffer coils that are going in.....but until I try it I don't know how much that will help the rear. I also just moved the lower rear shock mounts, so I'm going to soften up the rear bumpstops enough to use another inch of rear compression.

Any ideas?

I think someone should remove this post asap and put it in the speedfreaks forum.

rules are rules.

Dang, I thought I logged into , gotta put my glasses back on so I don't type jeepspeed again :laugh3:

couple basic things comes to mind to look at. spring/shock rates, how the rear travels as the suspension moves. Spring/shock may be ok for rockcrawling since the system has more time to react then when bombing acrosst the desert. Also if your suspension has to move straight up rather then in the direction of impact, the rear end will buck. Our Baja cars have ran swept back rear arms and flat a arms and the flats setup bucks the rear end pretty good. Sweeping the arms back at about a 10-15deg. angle makes a ton of diff. but....

don't know about the front not having all the travel. Just thinking out loud but if the front drop is extended to the limited travel say while coming off a bump then as soon as the limiter hits the front will start to come down to level the vehicle out. So rather then coming off level it's now coming off nose down which may cause the rear kick ot be more noticable. Also if the front ends coming down then would the rear end lift as the suspension moves down, eating up some of your rear travel?

what bumpstops are you using in the rear? stock rubber? poly?

Richard, you already know what the answer is
It's on this page: http://www.bilstein.com/html/applications/9100/




either air-bumps or bypass shocks :bling:

Lose some weight???????????????????????????????????????????

Richard - I don't have the answer but - I have the same problem with my 89 on highway and logging road bumps.
The front absorbs the bump and the rear hits hard as the Jeep pitches up slightly. 89 Xj 3520 lbs ZJ up country front springs and Doestech 3000 front shocks. Rear D44 with 240# trailer tow pkg springs, energy suspension bushings and MJ shackles. About 3" travel to the bumpstops. With the stiffer sway bars and bushings the handling on smoth roads is great (which is most of the time on this DD).

Anything you do is going to have to be a compromise.

I am guessing your priorities are
-rockcrawling
-fire road handling
-JV "back to camp" racing

To totally solve the rear bucking you would need to change to softer springs in the rear, or progressive rate springs that start really soft. To control the bucking i would increase the bump setting (shock stiffer in compression) but soften the rebound (shock softer in extension).

Sometimes running the shock too stiff in rebound (sometimes called jounce) leads to "suspension jacking". This is where you go over a bump the shock compresses, and because the rebound setting is too stiff the shock does not have time to return to it's original position before encountering the next bump. This results in the shock becoming increasingly compressed, the stroke is reduced and you end up riding the bumpstops. Like NASCAR used to do at Talledaga.

Softening the rebound too much might have adverse results in slow speed crawling situations. A stiff rebound (in slow crawling type stuff), can act a little like a sway bar.

The ideal situation would be to run
- progressive springs that are pretty soft initially but stiffen to 500lbs/inch within 4 inches of travel
- multiple adjustable shocks, or 4 way adjustable. These would have both bump and rebound settings for both hi-and low speed. so you could retain your settings for cralwing, but when hauling butt the rebound could be a bit softer than what you are currently running.

Of course the obstacle to this would be $$$. I imagine that long travel 4 way adjustables are very spendy.

If a total suspension redesign is out of the question, then I would concentrate on making the front stiffer than it currently is. Look at the vehicle as a whole. Too stiff in the rear? Two options - soften the rear OR stiffen the front, or a combination of both.

The mention of not adjusting the limit strap leads me to believe that the front is a touch too soft, and with the limited travel meant you were running off the bumpstops more (I did read the other Pre-running suspension thread again), and thus making the rear "work" more than it needed.

YMMV, and take into account that most of my suspension knowledge comes from fast cars on billiard track surfaces where max suspension travel is 2".

Your plan of stiffer front springs and rebuilt front shocks, in conjunction with the longer limiting strap position, will probably solve most of your problems.

Keep us updated.

I've found 2 things that helped/eliminated rear end bucking.

1 was when I valved my F/R SAW's too stiff last year. My 1 trip out to JV with this valving was by far the best my XJ ever worked at speed. Unfortunately they slowed me down on the trail, which was a compromise that I wasn't willing to make. After I loosend the shocks up a bit the rear end bucking came back, but they worked better in the rocks so I was willing to live with the bucking.

2 was my recent round of Mods., but I'm not sure what did it. I lightend the rear end quite a bit, I got softer rear springs (to make up for the lighter rear end) & light valved rear shocks.
I now have little to no rear bucking, but if I hit the large woops with enough heat, I can bottom the rear out. By 'enough heat' I mean I've gotta really be on it.

Paul

how big are these whooops?...like the ones from dirt bikes? are you going airborne over them, or riding them out.

BrettM said:

what bumpstops are you using in the rear? stock rubber? poly?

Richard, you already know what the answer is
It's on this page: http://www.bilstein.com/html/applications/9100/




either air-bumps or bypass shocks :bling:

Click to expand...

Yeah.....air bumps and/or bypass shocks would be great.......but that's a lot of $$$.

My rear bumpstops are Daystar extended XJ poly. They were pretty good at first, plus I drilled a few holes in them to soften them a little, but the bottom loop cracked and broke, so I cut that loop off. Now they're not as cushy, but I'm going to drill some more holes to soften them up enough to use another inch of compression.

Grant said:

Anything you do is going to have to be a compromise.

I am guessing your priorities are
-rockcrawling
-fire road handling
-JV "back to camp" racing

To totally solve the rear bucking you would need to change to softer springs in the rear, or progressive rate springs that start really soft. To control the bucking i would increase the bump setting (shock stiffer in compression) but soften the rebound (shock softer in extension).

Softening the rebound too much might have adverse results in slow speed crawling situations. A stiff rebound (in slow crawling type stuff), can act a little like a sway bar.

If a total suspension redesign is out of the question, then I would concentrate on making the front stiffer than it currently is. Look at the vehicle as a whole. Too stiff in the rear? Two options - soften the rear OR stiffen the front, or a combination of both.

The mention of not adjusting the limit strap leads me to believe that the front is a touch too soft, and with the limited travel meant you were running off the bumpstops more (I did read the other Pre-running suspension thread again), and thus making the rear "work" more than it needed.

Your plan of stiffer front springs and rebuilt front shocks, in conjunction with the longer limiting strap position, will probably solve most of your problems.

Keep us updated.

Click to expand...

Good advice, Grant. Yeah, it's always a compromise. At first with these shocks it felt fairly balanced, but the valving was just too soft and it swayed too much and bottomed out too easily.....but it sure rode good. I could really scoot down a rock trail, but it did sway side to side more than I like.

I can see that stiffening up the rear shocks and not the front, plus running the limiting strap too short would force more of the suspension action to the rear. I got stiffer springs for the front, I just need to get them on, plus get the front shocks revalved a little stiffer. I got new limiting straps for the front, and positioned them, the shocks, and the bumpstops to work together for full travel.....I just need to finish it and see what difference that makes. It makes sense that stiffening the front can help.

For those who are interested in shock valving, the SAW's I have came with what is called a stand off disc in the valve stack. Shock valving consists of a piston with holes in it, some for rebound and some for compression. On the top and bottom of the piston are sets of thin metal discs that block off the holes, or ports, in the piston. When the piston moves through the oil, the oil forces the discs to deflect so the oil can flow through the ports. The disc stacks are of various sizes and thickness, larger next to the piston and smaller away from the piston, very similar in principal to a leaf spring. The faster the shock moves, and hence the oil through the piston, the more those discs deflect to let the oil through. Back to a stand off disc, which is a small disc under the valve stack, next to the piston, which allows some oil to flow through the ports without deflecting the valve stack. This softens the initial action of the shock, making them feel more comfortable. I thought these shocks allowed too much sway, and were a little too soft. All we did in the rear shocks was remove the stand off disc, but left the valve stack the same, and it made a huge difference in the feel of the shock. I felt before that I needed to run the sway bar on the highway, but with the stand off disc removed just in the rear shocks I now have no need for the sway bar. The stability difference is night and day, plus they feel firmer.

I'll let you know how the stiffer front coils and shocks do when I get a chance to run them.

jjvande said:

how big are these whooops?...like the ones from dirt bikes? are you going airborne over them, or riding them out.

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Hard to know what your point of reference is. I would say they are small to medium whoops. There are some really big ones out there, but they require a slower speed. I can get airporne if I drive fast enough, but I was maintaining a speed that mostly kept the front on the ground, but the rear was bouncing a couple of feet into the air.