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Hey all, the previous owner of my '79 J10 installed a Rough Country 3-inch lift kit (replacement springs in front, blocks in rear), and so I've got what I believe are stock springs on the 3-inch blocks in the rear. I am getting a lot of sag under heavy load.

Here are a couple pics, one is with a rick of red oak firewood from a few months ago, the other is from today with nothing in the bed. I estimate somewhere between 3-4 inches of sag under that load. I am guessing the firewood was close to a full 1/2 ton, though I have seen similar sag with lighter but still heavy loads of 500-600 lbs.


Here is an image underneath with nothing in the bed, and you can see there's a rougly 5 inch gap between the bottom of the bump stops and where they would land on the axle.


If those blocks weren't there, then my axle would have been resting on the bump stops with that load of firewood. It is rare that I need to haul a heavy load, but I am guessing that can't be good?

Timbren was suggested to me as a potential solution, but looks like that gap is too large for any of their stuff to work. Are there any other helper options worth considering, like coils? Air bags maybe? I guess I'd have to pump them up whenever I needed to haul a heavy load.

Other than helpers, I could consider new springs, but they don't seem to come cheap. I am seeing an option on TGW and BJ's for ~$450. Does anyone know of other options for replacement springs?

The springs are probably soft from age. squatting that much is maybe excessive, but I don’t have a truck with stock springs to compare to. Springs do fatigue over time and not support as much as they once did.

Nice looking truck!

My .02 cents, get rid of the blocks and the worn out rear springs. Check with a company called Tough Country. Get their E-Z Ride rear springs with the proper lift to match the front.
Block lifts are horrible.

If funds allow, I would do new front and rear springs. A good quality company like BDS, Skyjacker

If you still have the label inside your door, what does it show the GVWR is for your J10? Yours being a '79 means it likely won't have the payload on that.

Most J10's had a max payload of around 1200 pounds. And that includes driver and such, not just in the bed of the truck.

To get the real payload, you need to weigh the truck, and subtract that from the GVWR. Or weigh the truck with the load in it, and see if you are overweight (with you in the truck).

The GVWR can also be read from the VIN. All 1979 J10s (models 25 and 45, also in the VIN) have a GVWR of 6250 lbs. As Stuka mentioned, you need to subtract the weight of the vehicle without payload to determine what the factory allows for a payload. Even if you add booster springs, the rest of the truck (frame, axles, bearings) is built for the GVWR, not beyond it. Typically the first to go from overloading is the rear wheel bearings.

You can read and download the '79 TSM at the Tom COllins site: https://oljeep.com/edge_parts_man.html Check page A-6 for VIN decoding.

Yes, leaf springs sag with age. Those look well past done to me - check the reverse arch in the center. Somebody is making replacement springs for these trucks - search old posts. I'd expect the cheapest is to replace the rears with OEM replacements from the aftermarket. https://www.amazon.com/Dorman-97-485-Sp ... B09HZJN2TV - search for 97-485.

There are companies that will make replacement rear springs for you, to whatever spec you want. Look up Alcan and Deaver.

Also, the truck sits pretty much level now. With replacement springs with the proper arch, it will sit higher in the rear with no load. Is this what you want?

You put 1000 lbs of anything in a truck with even new springs and it is going to show.
Unless you back up the springs with helper airbags that you pump up when loaded ... you are going to get some sag with heavy loads. It's the nature of a spring.

My understanding was the spring picture is not loaded, and the spring is flat/reversed anyway. Maybe not?

The flat overload spring shown in the link I provided may have been original equipment for these trucks. If so, it's been removed or from the PO's rear springs, or is always compressed. This leaf would not affect ride height or bump stiffness, but would provide some load capacity at maximum spring compression.

OldFarmTruck22 wrote: ↑Fri Mar 24, 2023 8:51 am You put 1000 lbs of anything in a truck with even new springs and it is going to show.
Unless you back up the springs with helper airbags that you pump up when loaded ... you are going to get some sag with heavy loads. It's the nature of a spring.

I agree, but he is getting alot of sag with not alot of weight...those springs are old, and new springs would likely yield better results

I mean that looks pretty typical to me. It's a half ton truck and you put half a ton it it, and it still has a fair bit of travel. Not really sure where the issue is.

Also it may be offset because of the stiffer front springs. The spring rates will be totally different and not proportional now because of the front springs--rather than loading the rear a little and unloading the front fairly evenly, the front is going to be mostly unloaded to start with, so ALL of the movement is going to be in the rear. Additionally if you move the weight forward in the bed in an attempt to add weight more evenly between the axles, the front lift springs are going to have a higher proportional spring rate to the rears than stock, and the rear is going to move more than it would have on a stock truck.

A lot of Jeep guys tend to ignore spring rates, whether it be SJ, XJ, or ZJ guys. Just care about how much lift they want and how to make it level, less worried about ride and handling. I'm not trying to make anybody feel bad or saying that everyone's like that; I'm just pointing out that that's a difference vs. say a Mustang or Miata forum/build. And on the SJs and the back of the XJs, without relocating the spring perches and/or adding lift blocks, you kind of have to choose between the lift and the ride. You CAN build a nice riding, lifted pack if you put a ton of thin leaves in it, but that's not the quick, easy, bolt-in in 2 hours and you're done method. Coils are a little different in that you can toy with the spring rate by adding/removing coils for the same height spring, and change the thickness of the metal, all without changing anything about how the springs mount.

Thanks for the feedback, everybody. Great discussion.

I like where the truck sits now, and so if I did a full rear spring replacement, I would be getting more height I don't want (thanks for pointing that out, tgreese), unless I also take out the blocks (but then she'd probably sit too low).

Honestly, I am not even sure what amount of lift I have currently, given the sag of the rear springs, and the fact that at one point I removed the bottom leaves out of the front springs to try and soften up the ride a little. Maybe it's closer to 2 inches... are there any factory specs I could go by to figure out exactly how much lift I have, as compared to stock? e.g. measurements between axle and frame.

p.s. On the GVWR, I've got the original manual, and it's listed as 6200 lbs. No curb weight listed, though (I always though it was around 5000 lbs, but I don't know for sure).

rurcon wrote: ↑Fri Mar 24, 2023 2:05 pm ... Honestly, I am not even sure what amount of lift I have currently, given the sag of the rear springs, and the fact that at one point I removed the bottom leaves out of the front springs to try and soften up the ride a little. Maybe it's closer to 2 inches... are there any factory specs I could go by to figure out exactly how much lift I have, as compared to stock? e.g. measurements between axle and frame.

I am not aware of any source that lists the original ride height.

OEM-type front springs are available too. Both these import replacements like I posted (origin where?) and the USA-made replacements that BJ's sells. Supposedly the BJ's springs come from Deaver in Santa Ana CA. BJ's has conferred with Deaver to reproduce these springs; likely the specs are proprietary. You could remove the blocks and fit replacements front and rear that are nominally the original ride height. There should be plenty of room in the wheel wells for your tires.

Another possibility is to replace the springs with the 97-485 and use somewhat shorter blocks. If you can't purchase the blocks you want from the aftermarket, a machinist could easily shorten the blocks you have, or make replacements to order. You'd probably need new u-bolts of the correct length; there are various places to get these. Possible you can find them online, or your local truck spring place could make them to order.

I'd think you can guess at the load capacity without weighing the truck by looking at the suspension compression with good OEM-type springs. Fully loaded, you'd want the rear axle to be riding on the overload leaves, but still have some suspension travel. That is, not riding on the axle bumpers. Ideally you'd want the load located forward so the front springs carry some of the load. A short WB truck like yours (model 25) has a disadvantage in this respect.

Those 3inch blocks arent stock, so you could replace with a nice quality 3-4 inch full spring pack, and have a pick-up truck with a nice level to mild rake. With nice new springs that aren't saggy to boot.

Blocks in a suspension are long recognized as an unsafe band-aid, and an engineering nightmare.

Full spring suspension is the way to go.

BTW, in your avatar picture, is that how your truck sags when you pull your boat?? If it is, my friend you BADLY need a proper suspension to pull that boat and trailer. That ain't right by any measure.

OldFarmTruck22 wrote: ↑Wed Mar 29, 2023 10:08 am Blocks in a suspension are long recognized as an unsafe band-aid, and an engineering nightmare. ...

My understanding: blocks are ok on the rear axle. Of course, they only lift if the axle is spring-over. Blocks on a front axle spring-over are dangerous. The OP's low-end lift kit came with new springs for the front and blocks for the rear.

Jeep Corp routinely put a small block in spring-under Jeeps under warranty if the customer complained their Jeep did not sit level.

Blocks aren't the end of the world. Sure they're not how to do it "right", and if you're looking at offroading you'll have wheel hop issues, but meanwhile they're not the end of the world. Ford put a 2" block in from the factory on the 4x4 F100s and 150s in the '70s; that way the 4x2 and 4x4 uses the same rear spring. My dad's truck frame on the '74 F100 (that's what the frame is) has not had the rear end touched, ever, in 250k+ miles. My point here is that it hasn't broken in the back yet.

Lots of 3/4 ton and 1 ton trucks leave the factory with 3" blocks. I don't like them but they work and are safe.

Did the J10 truck come with blocks from the factory?

I didn't think so.
That rear suspension is shot.

My 90 when i got it just me sitting on the tailgate at 300 lbs would drop the Jeep like 3 inchs...

I just got new springs for it.. 1440lbs capacity over the 1100 or whatever lbs ones. That helped.. but what really heloed was the 2 inch add a leaf.. it didnt give me a lift oddly.. but it did increase carry capacity so it doesnt sag with weight.

Yes i know max weight is 1300lnz.. but what i mean is when i have 1000 lbs of concrete bags in the back. It only drops maybe 2 inchs or so.

Cheapest option is getting a helper add a leaf. But you really need new springs