Frame flex and type and kind of spring?

[Ultimark]

I wondered about fixed mounting of a box or tray directly to chassis rails around 50 years ago, it was pretty new to me then but I had only been driving 3 tonne trucks over paddocks with probably around 1½ tonne on the tray at the most; basically things didn't flex and wheels came off the ground. Then I was introduced to military trucks which had unbelievable flex and I learnt a bit about having the load carrying part able to move in relation to the chassis rails.

Fast forward 50 years, I'm back in trucks and have sort of come full circle. I have a single cab chassis Isuzu NPS 300, current model. The tray on the back is spring mounted with four springs either side. We have a slide on camper bolted directly to the tray in four points, essentially the tray and camper stay flat, although with chassis flex there is movement of the tray and it's load in unison. So far and with some considerable chassis flex and reasonably good axle articulation, not a squeak, whimper or sign of twist in the camper. The camper by the way, is 11 years old as it was on another vehicle; it's been on the NPS for a year now.

The accompanying pictures are lifted from a YouTube video done by Robert Pepper; L2S-FBC. It was a comparison between a Ford ute and our Isuzu light truck.

In the first picture the chassis is certainly twisted, the front and rear axles are diametrically opposed and pretty much on their limit of articulation. The cabin is going one way, essentially being twisted by the front axle, while the rear of the chassis rails are twisted the other way by the rear axle. In-between is the tray and camper pretty much dead flat, although they too were moving around. But they weren't flexing, which, as far as I'm concerned, is good.

In the second picture only the front of the chassis is noticeably twisting. This can be seen by looking at the cabin in relation to the rest of the tray and its load, one can see that the rear axle is dead in alignment with the rear of the chassis rails. I'm actually watching the right rear wheel in anticipation of applying throttle just before the right rear wheel kisses the mound. I was able to slide the rear axle around the mound and away we went.

 

[BruceAndKrista]

The chassis of an FG is comparable to a wet noodle; it will twist if you look at it sideways...

LOL - also somewhat resembles a wet noodle when looked at from the side ?

The primary purpose of any type of kinematic mounting system is not to allow for more articulation, but to reduce (as much as possible) any chassis twisting from being transferred into the body.
Unlike a goods truck, where the body is quite often hard mounted to the chassis, flexing inside the box is not of any major concern, but in a camper, where you have fixed furniture, flexing of the box can cause a significant amount of damage, which is obviously not desirable.

Right, we for sure don’t want the camper body to flex - but this should be the job of the sub frame I think... (?)

As to which type of kinematic mounting system... this has been discussed to death on this forum over the years.
Our Australian Rural Fire Service use this type of 4x4 truck ( both Fuso and Isuzu) extensively, where they carry heavy dynamic loads, often on rough dirt tracks.
A full spring mount setup is the most common system used by the RFS and the majority of the commercial expedition camper manufacturers in Australia.

You guys get all the fun toys!

It's your truck and your choice, but I cannot count how many times I have encountered people who believe that they are smarter than the professionals and that they can build a better wheel.
Just sayin'...

Yep, that’s why I’m here - I’m for sure dumber than the professionals, just trying to understand the logic and reasoning before I start building.

 

[SkiFreak]

Right, we for sure don’t want the camper body to flex - but this should be the job of the sub frame I think... (?)

Well... yes and no.
In order for the subframe not to flex when the chassis is being articulated it would want to be a bloody strong subframe.

As I eluded to previously, most truck body builders give little or no consideration to the chassis flexing, because for a cargo truck or tray back that is primarily driven on tarmac, flexing is normally not a concern.
From my experience, even heavy duty tray backs made of steel still flex quite a lot when directly connected to the chassis.

In my case, I have what could only be called a very weak subframe (less than 150KG) that even with a full spring mount setup wobbles all over the place when there is nothing on top of it.
However, that is by design, because my camper body is a monocoque fibreglass box, which has exceptional tortional stiffness.

I would argue that the majority of camper boxes have minimal tortional stiffness, in which case the subframe does need to be stronger to compensate for that.
But, even with a strong subframe, for a camper body to resist flexing there needs to be some form of disconnect between the chassis and subframe.

The obvious caveat here is the terrain that the camper will have to deal with.
If the intent is to not go on anything rougher than a forestry dirt road, a hard mounted subframe will probably work okay. However, if the camper will experience deep rutted tracks and the like, that changes things significantly.

It should also be noted that a kinematic mount system is just one part of a good solution.
Add to that good wheels (singles) and tyres (at the appropriate pressure for the terrain) along with better suspension, which will all work together as a "system".
That kind of setup will ultimately improve the overall ride characteristics and significantly reduce stresses on the camper box.

If you want to reinvent the wheel, that's your choice, but in my opinion there are better, proven options.

 

[rruff]

Right, we for sure don’t want the camper body to flex - but this should be the job of the sub frame I think... (?)

If by "subframe" you include the kinematic mount between the subframe and frame, then yes.

But... actually it isn't that weird to simply make the subframe stiff and strong and hard mount it to the chassis/frame... therefore making them both stiff. That's the way every flatbed on a Tundra is done AFAIK. Most service bodies are done this way... but they are not often on technical terrain. I've never seen that on a Fuso camper. A potential issue is a stress concentration where the subframe ends... in the front where the frame becomes flexible again... but this can be dealt with.

There is a major camper manufacturer in Oz who quit spring mounts in favor of 3 point. They said the spring mounts were imparting too much stress to the box.

On the "smarter than professionals" theme, I didn't want a stiff subframe (or any subframe really), so I'm using cab mounts in a 3 point arrangement, pivot in the rear. It's a much lighter load than you are planning I'm sure.

Regardless of what you do, getting good on-road and off-road handling and performance is tough. Minimizing sway and maximizing articulation will be at odds.

 

[SkiFreak]

There is a major camper manufacturer in Oz who quit spring mounts in favor of 3 point.

Which one?

 

[rruff]

Which one?

I think it's SLRV: https://slrvexpedition.com.au/products/commander-4x4/

Looks like a pivot in the front.

 

[SkiFreak]

Given your experience, I am sure you would agree that when it comes to kinematic mounting systems that there is not really a "one size fits all" solution; the appropriate system is heavily dictated by the type of vehicle.
A 3 point system is more complicated on a Fuso, because of the stepped frame, which adds some extra engineering challenges, but they can obviously be done.
Personally, I still think that the spring mount system is the most suited for a Fuso.

If you twisted a Fuso as much as that Commander I think you would encounter some very undesirable results.

 

[BruceAndKrista]

If you want to reinvent the wheel, that's your choice, but in my opinion there are better, proven options.

Not quite sure where I am misleading folks to think that I am against a spring mounted sub-frame.... I’m not at all, I’m here to try to figure out how to build mine. The only thing I see doing “differently” (which I don’t think is really that different?) is a solid mount at the front of the box, where the Fuso Bodybuilder manual shows the highest section modulus of the frame, and having the spring mounts aft, where there is the most frame flex. To me this doesn’t defeat the purpose of spring mounting, it just keeps the box more in line with the cab, while allowing the frame to flex.

 

[SkiFreak]

I have seen fixed points at the front, in the middle and at the rear, but most of the commercial setups I have seen have no fixed points.
The other thing you should avoid is localized stress points (point loading), as the Fuso chassis is not what could be considered strong.

 

[DzlToy]

It's your truck and your choice, but I cannot count how many times I have encountered people who believe that they are smarter than the professionals and that they can build a better wheel.

Boxes are hard mounted to trucks all over the world. Remember, OEMs build to price points, for the masses, for ease of assembly, to meet crash test and emissions standards and so on. With very few exceptions, neither cars, nor trucks are built for performance, longevity, off roading, etc.

Many people confuse a three or four point mounting system for a tray or box, with suspension. It is most certainly not. A flopping box is an undamped spring, plain and simple. Add a damper in the mix and there is an element of control, but this mounting system still isn't suspension. It is an easy way for an OEM to build an incredibly stiff suspension, invest very little in engineering and still allow the truck to carry heavy loads, all-the-while, allowing the customer to post pictures on insta-face of the truck and box twisting this way or that.

PROPER suspension allows the truck to stay level, does NOT introduce stress into the cab or box, abhors a flexy frame, prolongs cab and chassis life, reduces stress on passengers and components, improves the ride in all conditions, ad infinitum.

FUSO has ZERO interest in making an FG comfortable for the driver who finds himself driving down a 40 mile long washboard road. Therefore, no engineering resources are put into solving this problem. Just because an OEM spent millions of dollars building something their way or IMO, the wrong way, does not mean there isn't room for improvement.

..alluded to, not eluded to
pedals, not peddles

As an example, watch the video below to see how the truck 'floats' and the suspension does all of the work. Yes, I realise you do not own a trophy truck, nor are you rock crawling. However, what seems to be lost on many people is that they need to steal this technology. Talk to the guy who tunes suspensions for a trophy truck team and ask him if he prefers a chassis that is a wobbly, flexy noodle or one that is as rigid as possible. Ride in a truck that has 223:1 low range gear ratio (crawl ratio) and gently, crawl over obstacles, stress-free, without banging, clanging, bouncing, slipping the clutch or tearing up the trail. Then, tell me that no low range in an FG is a better option. Millions spent doing something wrong, does not justify the choice.

40" tires on a Tundra-based 'trophy truck', hauling the mail through the desert. A tiny piece of this technology makes your truck ride like a Lexus over pot holes, expansion joints and on wash board roads. Suspension seats and low tire pressure are not the answer. They are Band-Aids. Proper suspension tuning is the solution. (Original video was deleted, the one below is similar. Notice how level and stable the truck cab is for the terrain.)



Tony Pellegrino's team designed this Jeep JK suspension system (below), tossing out millions of dollars in crappy Fiat/Peugeot/Chrysler engineering. Does this Jeep ride better than a factory/stock Jeep? I would suggest that it does, despite Tony not having 400 engineers on staff.

40" tires on a 6000 pound Jeep.

 

[christopher p weinberger]

When living in Tanzania Africa I had a friend who ran the Scania truck division in country, I asked him how he came to be there all the way from Sweden.... he said he was originally sent by the home office to find out why so many truck frames were cracking within 1 yr. ..bad roads and drivers exceeding conditions was reason, solution?
They removed the suspension seats and replaced with hard mount. Problem solved.
There is a lesson here...take it for what it’s worth..

 

[SkiFreak]

As much as I agree with the majority of what you have said @DzlToy, not everyone has unlimited funds to build something with such a nice suspension system.
Generally, we compromise.

My truck has had a 5 link coil suspension conversion, so how it deals with offroad terrain is arguably very different to a truck with a standard OEM leaf spring setup.
That conversion was definitely not cheap, and something that most would never consider.

As I see it, there are levels between really crap and brilliant.
With a few thousand dollars you can definitely make a standard Fuso much more comfortable to drive and less prone to damage when going offroad.
As I said, along with a kinematic mounting system for the subframe...

Add to that good wheels (singles) and tyres (at the appropriate pressure for the terrain) along with better suspension, which will all work together as a "system".

Making something perfect is a challenging goal, but that does not mean something cannot be made better.

 

[SkiFreak]

There is a lesson here...take it for what it’s worth..

In my experience, people, especially those that do not own the equipment, will push that equipment to its perceived maximum.
Your example does not surprise me at all.

 

[FGwannaB]

Rabbit hole #3. After reading this thread I am certainly convinced I have bitten off way more than I can chew however, I really like to eat. So one bite at a time for me. In a nutshell the consensus seems to be: put some springs on it.
Springs are better than no springs.
Shoot for the middle ground and you'll probably succeed.
I'm going to plan my build with springs and a set up that I can most easily copy but also incorporate my own alterations.

 

[yabanja]

I wondered about fixed mounting of a box or tray directly to chassis rails around 50 years ago, it was pretty new to me then but I had only been driving 3 tonne trucks over paddocks with probably around 1½ tonne on the tray at the most; basically things didn't flex and wheels came off the ground. Then I was introduced to military trucks which had unbelievable flex and I learnt a bit about having the load carrying part able to move in relation to the chassis rails.

Fast forward 50 years, I'm back in trucks and have sort of come full circle. I have a single cab chassis Isuzu NPS 300, current model. The tray on the back is spring mounted with four springs either side. We have a slide on camper bolted directly to the tray in four points, essentially the tray and camper stay flat, although with chassis flex there is movement of the tray and it's load in unison. So far and with some considerable chassis flex and reasonably good axle articulation, not a squeak, whimper or sign of twist in the camper. The camper by the way, is 11 years old as it was on another vehicle; it's been on the NPS for a year now.

The accompanying pictures are lifted from a YouTube video done by Robert Pepper; L2S-FBC. It was a comparison between a Ford ute and our Isuzu light truck.

In the first picture the chassis is certainly twisted, the front and rear axles are diametrically opposed and pretty much on their limit of articulation. The cabin is going one way, essentially being twisted by the front axle, while the rear of the chassis rails are twisted the other way by the rear axle. In-between is the tray and camper pretty much dead flat, although they too were moving around. But they weren't flexing, which, as far as I'm concerned, is good.

In the second picture only the front of the chassis is noticeably twisting. This can be seen by looking at the cabin in relation to the rest of the tray and its load, one can see that the rear axle is dead in alignment with the rear of the chassis rails. I'm actually watching the right rear wheel in anticipation of applying throttle just before the right rear wheel kisses the mound. I was able to slide the rear axle around the mound and away we went.

View attachment 653345


View attachment 653346

Just finished watching that video. Nice rig!