[Functional Artist]

Here is some more interesting info I found, it's for designing racecars but, most info should still pertain

Chassis explained

As you design a racing car, it is important that you know the requirements of your engineering work. The nature of the race car's normal operation and fatigue life depend on the structure and material composition of the car. Therefore, topics such as metallurgy and structural design are important for the designer to grasp. The whole concept of engineering considerations is that you keep in mind four aspects, where they are appropriate:

Any good chassis must do several things:
•Be structurally sound in every way over the expected life of the car and beyond. This means that nothing will ever break under normal conditions.
•Maintain the suspension mounting locations so that handling is safe and consistent under high cornering and bump loads. This means that there is no flexing of the body, or at least to reduce flexing on lowest possible value.
•Support the body panels and other components so that evevrything feels solid and has a reliable life span.
•Protect the driver from external intrusion.

Structural stiffness is the basis of what you feel at the seat of your back bottom. It defines how a car handles, body integrity, and the overall feel of the car. Chassis stiffness is what separates a great car to drive from what is merely OK.
Contrary to some explanations, there is no such thing as a chassis that doesn't flex, but some are much stiffer than others. Even highly sophisticated Formula 1 chassis (actually, Formula 1 has monocoque structure) flex, and sometime some limited and controlled flexing is built in the car.
The range of chassis stiffness has varied greatly over the years. Basic chassis designs each have their own strengths and weaknesses. Every chassis is a compromise between weight, component size, complexity, vehicle intent, and ultimately, the cost. And even within a basic design method, strength and stiffness can vary significantly, depending on the details.
There is no such thing as the ultimate method of construction for every car, because each car presents a different set of problems.
Some think an aluminium chassis is the path to the lightest design, but this is not necessarily true. Aluminium is more flexible than steel. In fact, the ratio of stiffness to weight is almost identical to steel, so an aluminium chassis must weigh the same as a steel one to achieve the same stiffness. Aluminium has an advantage only where there are very thin sections where buckling is possible - but that's not generally the case with tubing - only very thin sheet. And even then, aircraft use honeycombed aluminium to prevent buckling. In addition, an aircraft's limitation is not stiffness, but resistance to failure. Aluminium problems are overcomed something with Audi Aluminium Spaceframe (ASF), very expensive and for now made in limited models.

Ladder Chassis (Body on frame technology)

This is the earliest kind of chassis. From the earliest cars until the early 60s, nearly all cars in the world used it as standard. Even in today, most SUVs still employ it. Its construction, indicated by its name, looks like a ladder - two longitudinal rails interconnected by several lateral and cross braces. The longitude members are the main stress member. They deal with the load and also the longitudinal forces caused by acceleration and braking. The lateral and cross members provide resistance to lateral forces and further increase torsional rigidity. Since it is a (little bit more than) 2 dimensional structure, torsional rigidity is very much lower than other chassis, especially when dealing with vertical load or bumps.
This technology you can find today in some basic auto racing categories. Most known is kart. On picture below you can see chassis of an Superkart car without bodywork.

Backbone chassis

Backbone chassis is a type of a car construction chassis that is similar to the ladder design. Instead of a two-dimensional ladder type structure, it consists of a strong tubular backbone (usually but not always rectangular in cross section) that connects the front and rear suspension attachment areas. The tunnel or backbone becomes a primary load bearing member.

Backbone chassis is very simple: a strong tubular backbone connects the front and rear axle and provides nearly all the mechanical strength.

Inside backbone is space for the drive shaft in case of front-engine, rear-wheel drive layout like in the case of Lotus Elan. The whole drivetrain, engine and suspensions are connected to both ends of the backbone. A body is then placed on this structure.
It is almost a trademark design feature of Czechoslovak Tatra heavy trucks (cross-country, military etc.), but this type of chassis is also often found on small sports cars. It also does not provide protection against side collisions, and has to be combined with a body that would compensate for this shortcoming.

http://www.formula1-dictionary.net/chassis.html

On an electric car/kart the battery pack is, usually, the heaviest (single)component after the driver.

I am thinkin' about a backbone chassis design that incorporates the battery box as a structural member rather than something to be supported.

This way most of weight of the batteries could be centrally located & spread out along the length of the frame.
The driveline (the battery cables, speed controller & most of the wiring connections) could be mounted & ran thru the inside of the tunnel/tube & would be protected in the true spirit of a backbone chassis
__________________

 

[brian_]

In current terms, a pickup truck would be a much better example of a ladder frame than an SUV, since most current SUVs are unibody.

The information about aluminum in the second post is quite outdated and has some blatant errors. I suggest checking other sources for a more balanced view.

Backbone chassis

... this type of chassis is also often found on small sports cars.

It was, but not in recent decades. I can't think of a single one still in production, offhand; although of course I may have missed one or two, they're not common.

I am thinkin' about a backbone chassis design that incorporates the battery box as a structural member rather than something to be supported.

This way most of weight of the batteries could be centrally located & spread out along the length of the frame.
The driveline (the battery cables, speed controller & most of the wiring connections) could be mounted & ran thru the inside of the tunnel/tube & would be protected in the true spirit of a backbone chassis

I understand the logic, but it's hard to find enough space in a backbone box of reasonable dimensions to fit the entre battery; for instance, the Volt (which has only 16 kWh of capacity) fits only about half of the battery in the tunnel. To be workable, the backbone/box would need to be as long as possible, from nearly the drive axle to the other axle or even beyond.

The Bollinger 4WD, which is still really a concept at the stage, has a box beam backbone for most of the structure... but hangs the battery boxes off each side as seating area floor sections, with just motors and electronics within the backbone.

 

[Functional Artist]

In current terms, a pickup truck would be a much better example of a ladder frame than an SUV, since most current SUVs are unibody.

The information about aluminum in the second post is quite outdated and has some blatant errors. I suggest checking other sources for a more balanced view.


It was, but not in recent decades. I can't think of a single one still in production, offhand; although of course I may have missed one or two, they're not common.


I understand the logic, but it's hard to find enough space in a backbone box of reasonable dimensions to fit the entre battery; for instance, the Volt (which has only 16 kWh of capacity) fits only about half of the battery in the tunnel. To be workable, the backbone/box would need to be as long as possible, from nearly the drive axle to the other axle or even beyond.

The Bollinger 4WD, which is still really a concept at the stage, has a box beam backbone for most of the structure... but hangs the battery boxes off each side as seating area floor sections, with just motors and electronics within the backbone.

Thanks for the feedback

I am just adventuring & exploring with different chassis/frame ideas & concepts
...& sharing some of the interesting info that I came across

Most of the vehicles with backbone chassis that I have seen, usually incorporate a differential & half shafts into the rear axle

But, I'm thinking of using a live axle on a light & balanced backbone chassis style kart

Still researchin' & bouncing ideas around

 

[Functional Artist]

Here is more frame/chassis info


Spaceframe

The two most important goals in the design of a race car chassis are that it be lightweight and rigid. Lightweight is important to achieve the greatest acceleration for a given engine power. Rigidity is important to maintain precise control over the suspension geometry, that is, to keep the wheels firmly in contact with the race course surface. Unfortunately these two goals are often in direct conflict. Finding the best compromise between weight and rigidity is part of the art and science of race car engineering.

As ladder chassis was not strong enough, and provide small rigidity values, motor racing engineers developed a 3 dimensional design - Tubular space frame.

The spaceframe chassis is about as old as the motorsport scene. Its construction consists of steel or aluminum tubes placed in a triangulated format, to support the loads from suspension, engine, driver and aerodynamics. A true space frame has small tubes that are only in tension or compression - and has no bending or twisting loads in those tubes. That means that each load-bearing point must be supported in three dimensions.

Tubular space frame chassis employs dozens of circular-section tubes (some may use square-section tubes for easier connection to the body panels, though circular section provides the maximum strength), position in different directions to provide mechanical strength against forces from anywhere. These tubes are welded together and form a very complex structure, as you can see in the left picture.

For higher strength required by high performance sports cars, tubular space frame chassis usually incorporate a strong structure under both doors, hence result in unusually high door sill and difficult access to the cabin.

In the early 50s, Mercedes-Benz created a racing car 300SLR using tubular space frame. This also brought the world the first tubular space frame road car, famous 300SL Gullwing. Since the door sill dramatically reduced the accessibility of cabin, Mercedes had to extend the doors to the roof so that created the "Gullwings".

Since the mid 60s, many high-end sports cars also adopted tubular space frame to enhance the rigidity / weight ratio. However, many of them actually used space frames for the front and rear structure and made the cabin out of monocoque to cut cost.

There are also some inherent advantages to using spaceframes at the amateur level of motorsport as well. Spaceframes, unlike the monocoque chassis used in modern Formula 1 or CART, are easily repaired and inspected for damage.

Triangulation

How does triangulation work? The diagram below shows a box, with a top, bottom and two sides, but the box is missing the front and back. The box when pushed collapses easily because there is no support in the front or back.

Of course, race cars (or any other car for that matter) need to be supported in order to operate properly, and so we triangulate the box by bracing it diagonally. This effectively adds the front and back which were missing, only instead of using panels, we use tubes to form the brace. See below:

The triangulated box above imparts strength by stressing the green diagonal in Tension. Tension is the force trying to pull at both ends of the diagonal. Another force is called Compression. Compression tries to push at both ends of the diagonal (Shown above in the horizontal yellow tube). In a given size and diameter tube or diagonal, compression will always cause the tube to buckle long before the same force would cause the tube to pull apart in tension. As an experiment, try pulling on the ends of a pop can, one end in each hand. Then, try crushing the can by pushing on both ends. The crushing is much easier, or at least humanly possible, compared to pulling the can apart.

Spaceframes are really all about tubes held together in compression and tension using 3D pyramid-style structures, and diagonally braced tube boxes. A true spaceframe is capable of holding its shape, even if the joints between the tubes were hinges. In practice, a true spaceframe is not practical, and so many designers "cheat" by using stronger materials to support the open portions of the structure, such as the cockpit opening.

Torsional rigidity applies to spaceframes too, but because a spaceframe isn't made from continuous sheet metal or composite panels as in monocoque design, the structure is used to approximate the same result as the difficulty to twist "cigar car".

Another reason torsional rigidity is mentioned here is that it greatly affects the suspension performance. The suspension itself is designed to allow the wheels/tires to follow the road's bumps and dips. If the chassis twists when a tire hits a bump, it acts like part of the suspension, meaning that tuning the suspension is difficult or impossible. Ideally, the chassis should be ultra-rigid, and the suspension compliant.

It is important to ensure that the entire chassis supports the loads expected, and does so with very little flex.

Advantage of spaceframe is that is very strong in any direction compared with ladder chassis and metal monocoque chassis of the same weight. Disadvantage is that is very complex, costly and time consuming to be built. Impossible for robotized production. Besides, it engages a lot of space, raise the door sill and result in difficult access to the cabin.

Monocoque

In contrast to Spaceframes, the monocoque chassis uses panels, just like the sides of the box pictured below. Instead of small tubes forming the shape of a box, an entire panel provides the strength for a given side.

A common shape for 1960s racing cars of monocoque construction was the "cigar". The cylindrical shape helped impart something called Torsional rigidity. Torsional rigidity is the amount of twist in the chassis accompanying suspension movement.

Monocoque, from Greek for single (mono) and French for shell (coque) (monoshell), is a construction technique that supports structural load by using an object's external skin as opposed to using an internal frame that is then covered with a non-load-bearing skin. Monocoque construction was first widely used in aircraft in the 1930s. Structural skin or stressed skin is other terms for the same concept. A welded unit body is the predominant automobile construction technology today.

Modern car monocoque chassisToday, 99% cars produced in this planet are made of steel monocoque chassis, thanks to its low production cost and suitability to robotized production.

Monocoque is a one-piece structure which defines the overall shape of the car. In fact, the "one-piece" chassis is actually made by welding several pieces together. The floorpan, which is the largest piece, and other pieces are press-made by big stamping machines. They are spot welded together by robot arms (some even use laser welding) in a stream production line. The whole process just takes minutes. After that, some accessories like doors, bonnet, boot lid, side panels and roof are added.

Monocoque chassis also benefit crash protection. Because it uses a lot of metal, crumple zone can be built into the structure. Another advantage is space efficiency. The whole structure is actually an outer shell, unlike other kinds of chassis, therefore there is no large transmission tunnel, high door sills, and large roll over bar etc. Obviously, this is very attractive to mass production cars.

There are many disadvantages as well. It's very heavy, thanks to the amount of metal used. As the shell is shaped to benefit space efficiency rather than strength, and the pressed sheet metal is not as strong as metal tubes in spaceframe construction or extruded metal, the rigidity-to-weight ratio is also the lowest among all kinds of chassis bar the ancient ladder or backbone chassis.

Although monocoque is suitable for mass production by robots, it is nearly impossible for small-scale production. The setup cost for the tooling is too expensive - big stamping machines and expensive moldings.

 

[brian_]

Most of the vehicles with backbone chassis that I have seen, usually incorporate a differential & half shafts into the rear axle

By that, I assume that you mean an independent rear suspension and rear wheel drive. I agree, that's typical of the vehicles which have used backbone chassis.

But, I'm thinking of using a live axle on a light & balanced backbone chassis style kart

"Live axle" actually meant one which is driven - of any suspension design - but the use of the term has been distorted and now most people mean a driven (live) beam axle... and omit the "beam" part.

A backbone-style chassis is a strange combination with a beam axle (whether live or dead, with "dead" being not driven), since there is no need for structure near the centreline of the vehicle at the axle line with a beam axle, and yet there is a need for structure outboard for the suspension arms with a beam axle. The backbone puts all of the structure in the wrong place, so it would need to be extended nearly to the wheels on each side both at the axle line (for the springs and shocks) and further forward (in the case of a rear axle) for the control arm mounts.

I think that a backbone chassis with a beam axle is going to look more like a spaceframe in the back transitioning to a skinny (backbone) section in the middle of the vehicle, which would just be flexible (in torsion) and inefficient compared to a more normal spaceframe.

I realize that a live beam axle has a simpler driveline than an independent suspension. The simplicity results from fewer joints, with as few as two in the propeller shaft (driveshaft); a driven independent suspension typically has two joints per halfshaft, plus two or more in the propellor shaft (unless the engine or motor is at the axle line so there is no propeller shaft). Other than a couple of CV joints, what is the appeal of a live beam axle? It reacts to drive torque undesirably, and has high unsprung weight.

In an electrically-driven vehicle, the use of a live beam axle requires either:

1. mounting the motor on the axle (like a typical golf cart), causing very high unsprung mass; or,
2. mounting the motor remotely from the axle and using a propeller shaft (even though an independent suspension would allow the motor(s) to be directly coupled to the final drive).

The only live beam axle EVs in production for road use are adaptations of engine-driven vehicles with live beam axles - it's almost never the preferred configuration, especially for a sports car.

 

[Functional Artist]

Wow, Well put!
Thanks again for the feedback, that is some great info!

The researching & planning that I am doing is to build an electric go kart/fun kart
...for some "hands on" learning
...on a smaller scale
...that doesn't cost thousands of $

But, not just "another" go kart
...something kool & different
...maybe even "Bad Azz" (we'll have to see how it comes out)

It seems like an interesting challenge

 

[Functional Artist]

Now, lets look into the "load" that the chassis/frame will have to support.
There are a couple of different kind "loads" to keep in mind.
The static load is the weight that the frame has to support when the kart is standing still.
The dynamic load is the weight of the kart plus the force of moving that weight & stopping that weight.

* This helps explain it a bit more

Static Load vs. Dynamic Load
The main difference between a static and dynamic load lies in the forces produced by the weight of an object. When static, the load remains constant and doesn't change over time. With a dynamic load, some outside factor causes the forces of the weight of the load to change. Some of the factors that can affect a load and make it dynamic include:

Movement: If the holder of a load is in motion, chances are that the force created by the weight distribution could change. This means that such changes in force must be taken into account when moving a load from one place to another.
Increased tension: Tension is created when two loads struggle against one another. This increase can make the forces of the weight shift from one load to another. The result is that the bigger load has a greater impact on the smaller load, maybe even causing it to become unbalanced.
An outside force: Air, water and ground movement can cause a load to shift. This shifting usually causes changes to the force of the weight as well. This means whatever is holding the weight needs to adjust to compensate for the changing force.


Examples of a Static and Dynamic Load
A good example of a static load is a truck with cargo inside sitting still in one spot. The force of the weight of the load has little chance of changing as long as the truck remains still. Once the truck begins to move, the load becomes dynamic, as the force of the movement can cause the load to shift, changing the effect of the force of the weight of the cargo. If the truck goes too fast, it could even cause the forces of the load to shift greatly, causing it to fall or to at least make it harder to drive the truck on the road's surface. Also, when stopping, the force of the weight of the load can shift forward, making it harder to stop the vehicle as quickly.

A bridge represents another example of static and dynamic forces in play. The weight of the bridge is a static load, as it doesn't change over time, as long as nothing moves across it or outside forces, such as the wind, don't move against it. A truck moving across the bridge places a dynamic load on the bridge by increasing the weight of the bridge as it crosses. A wind blowing against the bridge can also change the forces of the weight of the bridge, as it moves it from side to side, creating a dynamic load on the bridge. That is why it is important that engineers take all of the forces that might apply to a particular bridge in order to design a stable and safe structure. Another important force to keep in mind is torsion, with any twisting of the bridge in the wind causing additional tension on the structure, which in turn can affect how much of a load the bridge can handle.

 

[electro wrks]

Competition go-karts usually are limited by rules to the materials used in their frames and to not an active suspension or drive axle differential. This is to keep the costs down to encourage the greatest number of participants. But, they're not practical at all on any kind of rough driving surface. Here's an in depth analysis of this type of vehicle frame: https://www.witpress.com/Secure/elibrary/papers/OP07/OP07018FU1.pdf

If your intent is to have an active suspension and maybe a drive axle with a differential, the design is much more complex. It might be more practical to adapt an existing 4 wheeler(quad, ATV) where most of the design heavy lifting has already been done. If you're into it, the Bad Ass golf cart stuff looks like a great way to waste a lot of time and money!

 

[brian_]

It's probably helpful to think of dynamic loads as resulting from any acceleration of mass, rather than from movement (such as load shifting position in a truck); static loads are those seen under the influence of gravity but no acceleration.

Going over a bump accelerates the mass of the vehicle vertically (up, then down), so it causes dynamic loads.
Speeding up and braking are accelerating forward or rearward, so they cause dynamic horizontal loads through the suspension.
Going around a corner is accelerating sideways, so it causes dynamic loads through the suspension.

 

[Duncan]

Functional
You appear to be missing the point of a Kart

Karts don't have suspension - or more accurately the suspension is the chassis twisting - the stiffness of a Kart chassis is one of the performance variables and is tuned by altering various parts for different track layouts and driver weights

A super stiff Kart will drive terribly!
And a Kart with suspension will weigh far too much and will not pass scrutineering

 

[brian_]

I haven't dug into what you mean by "kart"; I've just been assuming some sort of non-roadgoing track vehicle with minimal or no bodywork... and not a classic "go-kart" or racing kart (which has no suspension and cannot use most of the discussed types of structure).

Some forum members have built small competition vehicles which might be similar to the intent in this case, for instance, the two by galderdi:
Autocross EV special
Aussie EV Autocross Special II

You might even be interested in the discussion in this thread:
fast mini buggy
It is for off-road use, but a track vehicle can be essentially the same thing, set lower and on different tires.

 

[brian_]

If the kart seats two, it follows all of the usual structural and packaging considerations of a two-seat car.

If it seats only one (just the driver), then the situation changes. If you want the driver centred on the vehicle centreline, then a backbone chassis doesn't make much sense (since the driver would need to straddle the backbone). It is possible to build a single-seater with the driver on one side of the backbone and something (typically an engine, but in this case likely the battery) on the other side - the backbone can be offset somewhat (typically to give the driver a greater share of the body width).

galderdi's latest autocross special has some modules of the battery beside the driver's legs, but in spaceframe, not a backbone chassis.

 

[Functional Artist]

Competition go-karts usually are limited by rules to the materials used in their frames and to not an active suspension or drive axle differential. This is to keep the costs down to encourage the greatest number of participants. But, they're not practical at all on any kind of rough driving surface. Here's an in depth analysis of this type of vehicle frame: https://www.witpress.com/Secure/elibrary/papers/OP07/OP07018FU1.pdf

If your intent is to have an active suspension and maybe a drive axle with a differential, the design is much more complex. It might be more practical to adapt an existing 4 wheeler(quad, ATV) where most of the design heavy lifting has already been done. If you're into it, the Bad Ass golf cart stuff looks like a great way to waste a lot of time and money!

Thanks, that is some very interesting info

Not into competition or golf carts
...I do functional art

 

[Functional Artist]

Functional
You appear to be missing the point of a Kart

Karts don't have suspension - or more accurately the suspension is the chassis twisting - the stiffness of a Kart chassis is one of the performance variables and is tuned by altering various parts for different track layouts and driver weights

A super stiff Kart will drive terribly!
And a Kart with suspension will weigh far too much and will not pass scrutineering

IMO The "point of a kart" is to have fun

Nope, not planning for suspension on this one
...all of that info was for learning, more about what does what & why

 

[Functional Artist]

If the kart seats two, it follows all of the usual structural and packaging considerations of a two-seat car.

If it seats only one (just the driver), then the situation changes. If you want the driver centred on the vehicle centreline, then a backbone chassis doesn't make much sense (since the driver would need to straddle the backbone). It is possible to build a single-seater with the driver on one side of the backbone and something (typically an engine, but in this case likely the battery) on the other side - the backbone can be offset somewhat (typically to give the driver a greater share of the body width).

galderdi's latest autocross special has some modules of the battery beside the driver's legs, but in spaceframe, not a backbone chassis.

You are full of useful info, thanks again

I am workin' on a chassis all ready

 

[brian_]

Nope, not planning for suspension on this one...

Then, as others explained, planned chassis flex is important. Tires form part of the virtual suspension too, but unless they are mushy off-road tires they won't provide enough compliance to work well.

A backbone of well-planned torsional stiffness (that is, not very stiff) can certainly work, but you need to be careful about where loads are attached to it.

 

[Functional Artist]

Then, as others explained, planned chassis flex is important. Tires form part of the virtual suspension too, but unless they are mushy off-road tires they won't provide enough compliance to work well.

A backbone of well-planned torsional stiffness (that is, not very stiff) can certainly work, but you need to be careful about where loads are attached to it.

Yup, right again

"Another important force to keep in mind is torsion"

IMO I think, "torsional rigidity" is what is needed to be considered when designing & choosing materials for a chassis or frame. (especially on a backbone chassis)

...& as you say "where the loads are attached"

More info:
"Rigidity is the maximum resistance an object can offer before it deforms, in other words, it is the minimum force required to deform an object.

Torsional Rigidity : The minimum force required to deform an object by twisting through a unit dimension..(in this case, for twisting the dimension is in angle of twist)

Lateral Rigidity : Again, the same logic.. The minimum force required to deform an object by bending along the lateral axis through a unit dimension..in this case, the dimension of bending is normally in mm or other length measure scale.. (if the bending load is applied on the longitudinal axis, then the object will not bend, instead the load will act like a compression load)"


So, for a backbone chassis the design & the materials used for the backbone have to be rigid enough to
...support the weight of the kart (everything between the wheels-frame, motor, batteries, driver etc.)
...support that weight while moving, turning & stopping
but, also flexible enough to take some bumps, while supporting that weight @ ~20mph

* Think about the dynamic load that is applied to a kart frame when doing a "Power Slide"
...or while doing donuts
...that's a lot of load

 

[Functional Artist]

Also, can't forget about balance & symmetry

So, lets examine the weight on a kart & how it is spread out

Rear wheels ~30 lbs, (~15 lbs. ea.)
Motor ~20 lbs.
Batteries ~35 lbs. (~8 lbs. ea.)
Operator ~150 lbs.
Front wheels ~15 lbs. (~7 lbs. ea.)

The design in the (top) pic is pretty well balanced
~50 lbs. in the rear (motor & rear wheels), ~150 lbs. in the middle (operator) & ~50 lbs. in the front (batteries & front wheels)

The (middle) pic shows how unevenly the weight is distributed on the !ARRIBA! kart.
~100 lbs. in the rear, ~150 in the middle & only ~20 lbs. in the front

The (bottom) pic shows (top view) how nicely everything could be balanced.
...& symmetrical (left half is a mirror image of the right half)

 

[Functional Artist]

Starting to "zero in" on where were going with this

I need to design a frame that is
...strong & light weight
...rigid but, slightly flexible
...& also evenly balanced

Choosing materials:
What shape would work best to make this thing out of?
round, square, custom?


I first contemplated using 4' round tube but, it seems like it would be easier to attach to flat surfaces

Then, I was leaning toward 4" x 4" x 1/8" square tube (~6.5 lbs. per ft.)
But, the 12V 12AH batteries, being 4" wide, would not fit inside a 4" square tube (~3 3/4" ID.)

So, now I'm thinkin' 2" x 6" x 1/8" rectangular tube
...it's also ~6.5 lbs. per ft. (same as 4" x 4" square tube)
...but, I can cut out 4" (for the batteries) of the 6" wide tube & still have ~1" of steel wrap around to help maintain the "rigidity" of the backbone

Front Axle, could be made of 1" x 3" x 1/8" steel, ~18" wide
...proportionally reduced from the backbone dimensions (2" x 6" to 1" x 3")
I am thinkin' of leaving the axle as (1) piece, cutting 1" x 3" slots on each side of the backbone & running it right thru
...the left side axle stub would be 6", the backbone is 6" wide & the right side axle stub is 6" also (kinda of a 1:1:1 ratio)

Rear Axle Housing, could be made of 2" x 1/8" round tube ~14" wide
...it could cap off the rear end of the backbone

I am thinkin' of running the 1" axle thru it (like a true backbone chassis) & just having the bearings mounted right on the ends
...the left side axle housing would be 4", the backbone is (of course) 6" wide & the right side axle housing would be 4" (kinda of a 0.8:1:0.8 ratio)
...the sprocket could be mounted between the bearing & the wheel, on one side
...& the brake rotor/drum, between the bearing & the wheel on the other side