1985 720 - Suspension & Tire Questions - Sunrader Option

[Jopacabra]

So I've got a 1985 720 2WD dually with a Sunrader motorhome mounted to the back of it. I've gutted the motorhome and am building a much sturdier and lighter weight version. It does have the "E" engine tag, as discussed in this post:
 

 

I think that the suspension in the rear is pretty shot, and I'm looking around at my options for replacement and upgrade. I'm also interested in potentially throwing on some larger tires to give me a bit more clearance.

 

Here's my usecase:

 

I live in New Mexico currently, and I take the truck out on the weekends to camp and climb in a lot of backcountry terrain. Mostly this involves going down a few (1-5) miles of heavy washboarding before crawling through some rough-but-fine terrain that's just dirt road with a bunch of bowling ball sized rocks and a bit of gravel strewn around.

 

As of now, I can make it down all of this totally fine, but the washboarding especially transmits an insane amount of vibration into my camper. Up front in the cab I feel fine, barely anything, but in the back it's vibrating enough that I worry it may eventually damage my camper build and the 1kW (75 sq ft ish, so a lot) of solar up top if I keep doing it.

 

I am okay with having to go slow, but as of now slow enough to not fear the vibration is like 10mph. I'd like to shoot for 20mph, realistically, while still feeling pretty solid about the level of vibration in the back. I also want to be able to eventually tackle some slightly more adventurous terrain (I'm thinking of a hump that is roughly 2 ft tall that takes me farther back in one of my favorite places. It's not so bad, and I think I could take it, but I have never been adventurous enough to try) down the line.

 

I'm also concerned with gas mileage. With my current setup (with 32/36 Weber carb), I get about 18mph on a good day. I really like that, and I'd like to minimize impact to that, as well as my top speed (roughly 75mph on flat interstate) for long road trips.

 

What are my options for suspension upgrades?

 

I read through the discussion in the linked thread, and it seems that there isn't much to do with the leaves besides replace them with newer versions of the same, if I can find them. 

 

I've been reading around and people seem to say adding coilover shocks of some sort is great for washboarded roads, a al Bilstein 5100s. Is this something that's possible for the 720?

 

What are my options for tire upgrades?

 

The tires on there definitely aren't stock, but they are very small for off-road use. I'm having trouble finding information about what kind of tire is reasonable to put on the 720 rims, and what facets of tires really affect off-road capability and gas mileage. Do you guys have some thoughts on this?

 

Some photos of everything to kick it off:

Front suspension:

 

Rear suspension:

 

Current tires and ride height / angle (photos from not-New Mexico):

 

I'll take a better photo of the tires and such this evening. I forgot to take them when I was thinking about posting this.

 

Any kind of help, links, info, etc. would be appreciated.

 

Cheers,

Jo

[datzenmike]

The 2wd Cab/Chassis option with a camper isn't really intended for off road to begin with.

 

The E has a hefty spring package on it already, intended for extreme loading. Adding more will only stiffen the ride. Have the top two leaves re-arched by a spring shop. Tell them what you want. This will increase the road clearance of the body but little else, and retain the same stiffness. Replace all the shock absorbers they DO wear out. This will dampen the bounce over washboard and prevent any harmonic bouncing. Tires are actually part of the suspension. Look at something 'softer' or softer sidewalls that will give more. It is a trade off as softer/thinner sidewalls are more damage prone.

 

Nice truck BTW.

[Jopacabra]

 

37 minutes ago, datzenmike said:

The 2wd Cab/Chassis option with a camper isn't really intended for off road to begin with.

 

Yeaaaaaaah, but I'm a hoodlum who wants to give it my best go. Of course I can never get performance off-road action with an old camper, but I'd like to get it as good as things will get. One day I hope to build something crazy on a flat bed or even a bare frame, but that day is not today. Until then, I'm not willing to quit being an idiot, I just want to mitigate the risks as much as I can.

 

40 minutes ago, datzenmike said:

Have the top two leaves re-arched by a spring shop. Tell them what you want. This will increase the road clearance of the body but little else, and retain the same stiffness.

 

Thanks for the reply. I didn't realize that was possible! I'll give it a google around my area and make some calls. What's a "good price" for rearching? Google seems to say "upwards of $200", but I don't want to be the gumby who says yes to a terrible deal.

 

Sidenote on this, is there any detriment to load capacity from rearching? I imagine you're heating and adjusting the spring... Is that likely to change the maximum weight? I want to be very careful with it, as I've heard these guys were pushing their weight limits off the lot. Any advice, opinions, or thoughts on that? I see you're *the guy* around these parts.

 

41 minutes ago, datzenmike said:

Replace all the shock absorbers they DO wear out. This will dampen the bounce over washboard and prevent any harmonic bouncing.

 

Okay, great! That sounds like exactly what I'm looking for in the short term. Do you have any recommendations on brand / style / parameters / anything? I don't know what I'm looking for, what is "good enough", and what is "way overkill for a camper that will never see rock crawling".

The shocks I referenced above were recommended to someone building up a Jeep. Are these the sorts of things I'm looking for?

 

https://www.4wheelparts.com/p/bilstein-2-5100-series-struts-and-2-5100-series-shock-package-shockingbil5100w-frtstrut/_/R-BBGL-SHOCKINGBIL5100W-FRTSTRUT

 

Seems to say this will fit my truck, but I don't trust those little "fitment checker" tools. What specifics can I look for to make sure the fitment will go smoothly without any fabrication? What else (replacement bushings, etc.) do I need aside from what will come in a kit?

 

49 minutes ago, datzenmike said:

Nice truck BTW.

 

Thanks. Threw in the carb and finished the paintjob pretty recently, and I really like the looks. I went with the hype and did the whole dang thing in truckbed liner. I think I would not do that again, just because of the cost... It looks okay in those shots, but at night the paint looks blotchy and wack. Either way, I'm gonna gather some friends and beers and paint some pretty neat detailing some time soon.

[captain720]

Have you thought about some airbags for the back? Airing down could be essential, maybe you need an airbag system with an onboard compressor to air up and down tires and bags?

 

What about just mounting a rack on the back with a dirt bike on it? 

[Jopacabra]

1 hour ago, captain720 said:

Have you thought about some airbags for the back? Airing down could be essential, maybe you need an airbag system with an onboard compressor to air up and down tires and bags?

 

What about just mounting a rack on the back with a dirt bike on it? 

 

I've thought about air bags, but I don't know much about them, but they seem interesting. What qualities do they impart to the suspension system?

 

Definitely want to put a small onboard compressor in... It's on the list, just a while ahead.

[captain720]

When they have a moderate amount of air in them they have a "shock" like cushioning effect and they can also make a heavier load safer. They don't add onto your vehicle weight capacity in a technical sense but when you air them up they level the vehicle eleminating squat, improving handling, and properly aligning headlights. I want to try a set on a 720 so that it will sit level with a 4 wheeler in the back. It's like a leaf spring add on or re-arch but you can adjust it. They also can be used to level it, say you have a fuel tank on one side that is heavy and not on the other, add a few extra psi to the low side. You don't want to get too far out of balance or it's sketchy but a few psi different for leveling is helpful

[captain720]

They can also reduce forward load transfer under heavy braking if you are not squated down in the back so far

Edited September 29, 2021 by captain720

[Jopacabra]

21 minutes ago, captain720 said:

When they have a moderate amount of air in them they have a "shock" like cushioning effect and they can also make a heavier load safer. They don't add onto your vehicle weight capacity in a technical sense but when you air them up they level the vehicle eleminating squat, improving handling, and properly aligning headlights. 

I did some googling, and it seems that the verdict is you either pay obscene amounts of money for nice ones, or you're liable to have them wear out quickly and potentially burst unexpectedly. Thoughts on that?

[captain720]

In my experience we always just buy a "size up" so a higher weight rating than you plan to carry and my family's never had a problem, but we've only used them on 12 and 24 valve Cummins trucks. I have seen them fail but the weight points they have failed at it makes sense, if they get old and the rubber cracks though they are done so just keep an eye on that usually a last a long time they don't see a lot of sun. They come factory on a lot of new motor homes, works great on my parents big coach.

 

If you are offroading a lot it will accelerate wear on them just like every other component

 

 

 

[Jopacabra]

That's interesting. So, you would recommend them over traditional shocks?

Seems that replacing some shocks is going to be a half-hour affair... What's the time and money commitment with air bags, and how do I determine if an airbag is compatible with my setup?

[Jopacabra]

Well, for anyone who comes after me, I have discovered that the rear shocks on the dually 720 that was used for the 1985 RWD Nissan Sunrader is definitively NOT the same as those that were used for the same year RWD 720s. Go figure. I think I will need to remove them and take some measurements / consult some oracles.

[captain720]

Well unfortunately I don't know as I havn't done it on a 720 but a few brands do make them. It is common for exhaust shops to also install airbags so you may have a local shop that could be consulted for advice and or parts. I do not recommend air shocks, the regular shocks you inflate as most shock mounts can't handle the stress. Use regular shocks and an airbag that goes frame to axle 

[Jopacabra]

Ah, okay, I'm beginning to understand. The airbags are a replacement for the leaf springs, which actually suspends the frame. The shocks are just devices that add some resistance to flexing the leaf springs.

 

I think that for now my first step will be to remove one of the shocks and take it and some photos to someone in town who knows what they're talking about. They should be able to recommend a replacement part, hopefully one that is beefier.

From there, I'll focus on the tires. Likely, since information is scarce on what tire setups will be compatible with the vehicle as I have it set up and would eventually like it to be set up, I'll talk to someone in town about that as well.

 

After that, I'll do some performance tests and consider whether re-arching the leaf springs or installing airbags is a better move.

 

The main issue with going to a real shop is just that my time during open hours is limited, and I actually live in the vehicle, so I can't easily just drop it off somewhere. I also don't own another vehicle, aside from a bike, so that's a bit of a struggle. Plus, of course, I'm a broke ass PhD student, so I can't really throw too much money around in labor and inspection costs.

 

Appreciate all the replies so far.

[datzenmike]

Re-arching  doesn't affect the stiffness only the ride height.

 

The shock absorber absorbs or resists compression and rebound of the springs. It 'smooths' out rough roads. If you climb on the bumper and bounce up and down several times and jump off, a vehicle with worn out shocks will continue to bounce several times. Good shocks you will have a hard time getting it to bounce at all.

[bottomwatcher]

Airbags can be added while retaining the springs. I added airbags to my silverado. It levels the rear with the heavy camper. I really like them.

[Jopacabra]

15 hours ago, bottomwatcher said:

Airbags can be added while retaining the springs. I added airbags to my silverado. It levels the rear with the heavy camper. I really like them.

 

So they just sit between the springs and the frame? Neat! Will definitely look into that.

[bottomwatcher]

Used the airbags on the Silverado yesterday. Wasn't fully loaded but there was some SAS (saggy ass syndrome) 30 psi leveled it up and it feels more solid although not harsh when hitting potholes.

[bottomwatcher]

After. Subtle but noticeable while driving.

[Stoffregen Motorsports]

On 9/28/2021 at 6:49 PM, Jopacabra said:

I did some googling, and it seems that the verdict is you either pay obscene amounts of money for nice ones, or you're liable to have them wear out quickly and potentially burst unexpectedly. Thoughts on that?

I purchased the Air Lift kit for my Dodge Ram and have not had one problem with them. They do like to have @5psi in them at a minimum, but I regularly run them at 30 when towing. The kit I bought cost about $350 and is pretty generic. I wouldn't call that obscene. I did not get the compressor. Instead, I installed the schrader valves in the fender wells and carry a low pressure gauge with me so I can get an accurate measurement as even 1psi difference from side to side will change the ride height. They have been on the truck now for about a decade and work perfectly well.

 

Curiously, the mfg does recommend having them set at 5spi minimum, and I have tested them at zero and at 5 and I think the ride is actually less rigid at 5spi. This makes sense if you think about what the 5psi is actually doing - adding a bit of ride height and taking weight off the stiff spring.

 

The problem I see for you is over-working the rear axle. Trucks meant to carry lots of weight have full float (or at least 3/4 floating) axles, which puts the weight of the truck on a double bearing spindle, instead of on a single bearing axle shaft. Axle shafts carrying lots of weight can snap off and leave you totally screwed. It is possible to swap in a full float dually axle from a Toyota or another mini truck. I have seen them in the wild, and in junk yards, so I know they exist.

[Jopacabra]

So, after consulting with a local here familiar with the conditions around and tooling things out for them, I've decided to go with some airbag helper springs, a la this kit:

 

https://smile.amazon.com/LIFT-59529-Control-Air-Spring/dp/B000CFNOYC/

 

I also ditched the rear sway bar and couldn't be happier, even with >150 pounds of solar on the roof. The sway bar setup they used in these things rendered the leaves and shocks practically unused, so I highly recommend that to anyone in the future.

 

I also plan to go up about 2 sizes on the total tire diameter and slap on a 2 inch body lift for better approach and departure, but those are projects for a ways down the road.

 

 

On 10/7/2021 at 12:01 PM, Stoffregen Motorsports said:

The problem I see for you is over-working the rear axle. Trucks meant to carry lots of weight have full float (or at least 3/4 floating) axles, which puts the weight of the truck on a double bearing spindle, instead of on a single bearing axle shaft. Axle shafts carrying lots of weight can snap off and leave you totally screwed. It is possible to swap in a full float dually axle from a Toyota or another mini truck. I have seen them in the wild, and in junk yards, so I know they exist.

 

Surprisingly, I actually already have a full-float axle. One of the bizarre boons of this oddity!