Rear leaf springs, firm SMB couch and bounciness

wl1193 · 02-22-2012, 11:58 AM

Hi. I believe my rear leaf springs are original and if they are, they have about 103000 miles on them. They are higher in the front and they slope downward as they go towards the front of the van. They look kinda flat. I replaced the stock shocks last year with Bilsteins.

My 2006 van is 2 wheel drive and it is based on a SMB EB50 layout. I bought the couch and platform mattress from SMB. The couch is very firm. The rest of the interior was done by a local builder. I added cabinets, two hose batteries, microwave, cabinets. No built in fridge, propane, water etc.

I weighed my van yesterday with me, an almost empty tank of gas 9the fuel light was on) and a normal daily driving load. If i add wife, daughter, fuel and gear I expect to add another 700 pounds to maybe 800 pounds. I am going to assume the scale is accurate.

The front of the van weighed 2820 pounds. The max GAWR is 3700 pounds.
The rear of the van weighed 3580 pounds. The max GAWR is 5360.
The van weighed a total of 6460 pounds. The max GVWR is 8600 pounds.

While driving at freeway speeds, when I drive over a small bump or dip in the road, my teenaged daughter sitting on the couch gets launched several inches into the air. I dont get the same bounce while riding in the front. The couch is almost on top of the rear axle. I think the firmness of the couch, the leaf springs and the location of the couch all play into her getting airborne.

I have traded pms with a very knowledgeable person on this board, another knowledgeable person in the Jeep world and sent off a few emails to different spring shops. I would like a little more info from other members as well. Just trying to get more ideas.

I would like to reduce the bounce for my daughter. So far I like the idea of removing one leaf to soften the ride for her (thanks Bryan). How much capacity carrying would I lose? any downsides of removing one leaf? Other suggestions have been helper springs, road master, adding thinner leaves to replace a stock leaf, velve ride shackles.

Running 225/75/16 Michelin tires right now.

Any suggestions?

Thanks in advance.

Will

BajaSportsmobile · 02-22-2012, 02:57 PM

You have to know what the rear suspension is doing to figure out your problem. In the simplest terms, "Sprung Weight" is everything above the springs - chassis, body, engine, passengers and load, and "Unsprung Weight" is everything below the springs - axle, brakes, wheels and tires... there is a little more to it than that, but this will suffice.

If everything is working correctly, when you hit a "small bump" the springs allow the Unsprung Weight to travel up and over the bump, absorbing the energy and then releasing it, keeping the tire in contact with the surface and not transfering the energy to the Unsprung Weight which should not move.

If the spring is too stiff (to much spring rate) it will not absorb the energy or deflect enough to allow the Unsprung Weight to travel up and over the bump, and will transfer the energy to the Sprung Weight, sending it upward. Your daughter is part of the Sprung Weight and she is subject to this upward force. Because she is lighter than the van, the force moves her more than the van and she becomes airborne. Additionally, if the spring is too soft and absorbs only some of the energy from the bump before the suspension bottoms out, the residual energy at bottom out will transfer to the Sprung Weight with the same effect.

So, you have to first know, is this being caused by too soft a spring (bottoming out) or too stiff a spring (no wheel movement).

The easiest way to check this is to put a dab of chassis grease on the bottom of each of your rubber bump stops and dab of grease on each of your shock shafts and hit your favorite test bump.

If the grease transfers from the rubber bump stop to your axle, well you are bottoming out on the bump stop. If the grease is pushed all the way down to the bottom (top depending on shock configuration) of the shock shaft, it is bottoming on the shock. If neither is bottoming out, your springs might be too stiff.

All of this does not account for the valving in the shock, they all have some compression valving, but rely mostly on heaver rebound damping to control spring oscillation. I doubt any shock used on a SMB has too much compression valving and most have too little rebound control.

Try the grease trick and we can go from there.

P.s. I've learned a little something about suspension from 25 years of Off-road racing and suspension tuning.

I recently in stalled some FOX Shox on my SMB, these are a new offering, and they made remarkable improvement in the handling and ride quality - night and day difference.

TexomaPowerboater · 03-20-2013, 12:09 PM

Any updates? Wl1193 - did you solve the bounce problem?

BajaSportsmobile - how are the Fox Shox's holding up? Do you still like them?

I've got a little bounce problem myself. Not sure what route to go - add a leaf, add an air bag, new shocks, ????? Same exact problem as Wl1993 first reported. Ride is fine upfront, but in the back the bounce was enough to launch the wife into the ceiling of the van.......haha.

BajaSportsmobile · 03-20-2013, 01:22 PM

I am very happy with the FOX Shox. I am going to be revalving them to add more compression and rebound damping to the front and more rebound to the rear to do some fine tuning.

REF · 03-21-2013, 07:10 AM

Lots of us have swapped to Deaver springs, huge improvement! My 2003 stock leaf springs were not made for the added weight of the conversion. Depending on your set up, my stock springs had 3-4 leafs? Deavers have 7-8 leafs. Lots of other threads about this , I also went with bilsteins, so far so good, sounds like those fox are nice though. I would caution against helper springs or adding a leaf, as its just a band-aid. I had originally gone with airbags to remedy my harsh ride, while they serve a purpose, they did not improve the ride quality.

zmock8 · 03-30-2013, 01:04 PM

Who makes Fox Shocks for OE replacement? Will they work for a 2WD E350? Any comparison to the Bilstein stock replacement? I am not all that impressed with the Bilstein comfort series..... Thinking about having someone local build some custom King shocks....

TexomaPowerboater · 04-25-2013, 11:51 AM

Some of this bounce is really just due to physics and the fact that its a 3/4 ton or 1ton van and if your in the back seat you are sitting on top of an axle with heavy duty springs. Think of it like a seesaw. The more you move to the middle the less you will move and thus, less bounce, while the guys on the end can get launced or land hard. I do think springs and/or shocks can help some.