Adapting an SUV IRS for a trailer?

Louisd75

Adventurer
So here's some of my thoughts.

Proper IS using long-arm trailing arms are best obviously, but pre-built assemblies for bolting in to a trailer are pricey. An order of magnitude kind of pricey.

Common trailer dead-axles perform worst off road. A solid axle is great for a driven-axle but seems prone to jounce and trailers seem to bounce around like crazy.

Torsion axles that use rubber have been failure-prone in the past. I cannot speak to existing products but Andrew White talked about an expedition trailer that failed on him in the middle of nowhere when a torsion axle gave out.

Likewise, newer rubber-block-suspended trailing arms seem more oriented towards low deck height instead of long travel or high ground clearance, and I have my long term concerns about the longevity of the rubber blocks.

The Nissan F-Alpha platform suspension is meant for vehicles intended to go well over 100,000 miles. It may not offer the travel of the heavy duty, purpose-built long arm suspensions, but looks like a good compromise of ground clearance, long term durability, cost, and field serviceability. It also looks alignable in a regular alignment shop and should remain well aligned as it articulates on pavement, as it had to in its original application on a passenger vehicle. Furthermore those trucks had 7000lb towing capacities on top of the original vehicle weight. It might be more an issue trying to find soft enough springs if the trailer weighs less than the original axle load had in the SUV.
Well, if I reeeeeeally had my heart set on independent suspension for the rear, I'd look for something off of the rear end of an old VW. Longish trailing arm, off the shelf parts and it's hard to argue that a design that's been in use since before WWII isn't reliable. It wouldn't take much effort to make it work. The downside is that it's going to take effort to make it work. You'll have to source a donor and figure out how to make it work. If you have the resources and abilities to make the VW trailing arm work, you've likely got the resources and abilities to build your own trailing swing arm from scratch.

As it is, I run leaf springs. They're simple, cheap and reliable. I removed one leaf from each of the spring packs to make it work better for the trailer weight. I chalk tested the tires to figure out a good pressure for them. I also have shock absorbers. I've never felt the truck get squirrely due to the trailer, haven't had any jounce issues since doing the setup stuff I mentioned earlier and so far the trailer has gone everywhere that my truck has gone. The IFS and rear axle of my truck both hang down lower than the trailer axle.

Here's how I did the shock absorber. The shock absorbers sit aft of the axle. They're angled for clearance, but they also more directly oppose the travel of the axle which moves aft and up as the leaf spring compresses.

 

Fierokid

New member
I love the thought of this but an easier solution (assuming the width is at least reasonably close)would be to pull a rear axle from a chevy uplander (front wheel drive, not an AWD as it's an independent rear) the lug pattern is either 5x114.3 (4.5) or 6x114.3 I haven't seen many 5 lugs but I've owned 2 6 lug of the buick variety.
 

vintageracer

To Infinity and Beyond!
Torsion axles work great for many applications including off-road trailers.

10+ year old Torsion axles depending upon trailer usage and storage can sag and do not work great on any trailer when this happens. The rubber within the axle tube deteriorates causing the axle to sag and the suspension to bottom out. When this happens it's time for a new axle!

Like anything else Torsion axles do not live forever!
 

Teardropper

Active member
********, yeah nothing at all wrong with torsion axles. Anyone who says otherwise is on something, or had a defective part that should have been warrantied.

I've been pulling teardrops with torsion axles for sixteen years and wouldn't use anything else. Independent trailing suspension set for the weight of the trailer.


Makes for a smooth ride that extends the life of the trailer long enough to make it onto your will.

T
 

b dkw1

Observer
The motion ratio on this shock is bordering on useless.

Some thoughts, The unit bearings are held together buy the outer CV. You will need to cut the cup off and leave the stub in.

Rubber torsion axles suck when they get real cold. The rubber gets hard and they have almost zero movement.

Shocks make the biggest difference, no trailer should be without them.

Easiest thing would be some VW type aftermarket trailing arms with air bags and shocks. Airbags naturally isolate vibration better than any other spring. Trailing arms have Zero track change and a good suspension travel path. Spring rate on bags is variable for load. Trailing arms can take many unit bearings as a lot of them have common mounting dim's. Easy to mount with a large cross member and 4 tabs.
 

Louisd75

Adventurer
The motion ratio on this shock is bordering on useless.
How so?

It almost directly opposes the travel that the axle sees as the suspension compresses. Leaf springs cause the axle to move aft as well as up when the suspension compresses.

Is it ideal? In a perfect world it would be mounted more vertical. In the real world that would require a stupid amount of lift, mounting to the axle so that it hangs down much lower or cutting a hole into the cabin of the teardrop.

It's not a racing teardrop and my experience of actually towing the thing with and without shocks shows me that they're working.
 

chunko

Observer
My 1975 Volkswagen Westfalia would disagree with this sentiment.. 45 years in service and its rear torsion axle is riding just like it always has.
Pretty sure those are torsion Bars. The torsion axle in a trailer uses a big chunk of rubber as a spring and damper in one.

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b dkw1

Observer
How so?

It almost directly opposes the travel that the axle sees as the suspension compresses. Leaf springs cause the axle to move aft as well as up when the suspension compresses.

Is it ideal? In a perfect world it would be mounted more vertical. In the real world that would require a stupid amount of lift, mounting to the axle so that it hangs down much lower or cutting a hole into the cabin of the teardrop.

It's not a racing teardrop and my experience of actually towing the thing with and without shocks shows me that they're working.
The axle moves in an arc. It really doesn't move back all that much compared to it's vertical movement. As the axle goes up the shock leans over more giving it a decreasing ratio or falling rate. If you cycled that set-up you would see that at full bump the shock would be doing almost nothing. Exactly what you don't want.

There are a couple ways to get around this without major butchery. Some of them are pretty complex, some are not. Trailers are one place rotary dampers would be really useful.
 

Louisd75

Adventurer
The axle moves in an arc. It really doesn't move back all that much compared to it's vertical movement. As the axle goes up the shock leans over more giving it a decreasing ratio or falling rate. If you cycled that set-up you would see that at full bump the shock would be doing almost nothing. Exactly what you don't want.

There are a couple ways to get around this without major butchery. Some of them are pretty complex, some are not. Trailers are one place rotary dampers would be really useful.
When the axle moves up three inches it also moves back almost two inches. The picture shows it unsuspended, it compresses almost an inch with the weight of the trailer on it.
 
When I bought my trailer, it has the IFS frame from a 3/4 ton suburban as the frame and suspension. As soon as I got up to highway speed, the trailer started bouncing from side to side so bad that the tired were coming off the ground. I wouldn't do it.
 
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