[brian_]

Now Tesla got it a little more correct with a chassis that is basically two axles connected to a battery box.
...
Notice how they put that chassis as low to the ground as it could go? The handling advantage to doing that is huge.

That's not a Tesla chassis; the "axles" (front and rear subframes carrying suspension and drive units) are not even attached to the battery case. Tesla likes to display this "skateboard", but it is really just the drive unit(s) (with all of the messy wiring omitted), the battery pack, and the suspensions with their subframes. The vehicle structure is entirely above that stuff, and omitted entirely from the display. The suspension struts stick up into empty space (I don't know they they didn't just leave them out), and the mass of aluminum panels that they actually attach to isn't there. People like to suggest that this is nearly a drivable chassis, but it is very far from it.

The "rolling chassis" representation is much closer than the typical "skateboard" prop, although it's still missing the upper half of the structure (it would roll, but would still buckle under load). Image attached.

I do agree with Chris's point, that keeping the battery low is good. Motors are always mounted around axle height.

 

[brian_]

Now the four motors idea is obviously a little more "pie in the sky" but I know the military has been doing it with the Shadow RST-V as well as a couple of individuals. The advantages if it is feasible are huge. I could wind up with a truck that handles better than most cars do on the road. It's that feasibility thing that is the hangup. Having a second stability controller isn't a deal breaker, but if I need 5 controllers, that would pretty much kill the idea unless I could find some really, really cheap controllers out there. I would doubt that.

It's not so unreasonable. As already mentioned, Tesla uses one motor per axle (rather than using a single central motor and mechanically distributing power to front and rear), and any sane manufacturer would do the same. Coordinating the controllers is not unreasonably difficult when you are the programmer; hacking someone else's controllers to work together is another matter. The simplest approach of commanding two controllers (one per axle) to produce the same torque - unaware of each other - seems reasonably workable. That's far from producing ideal handling or traction, though.

Separate motors for left and right wheels is an obvious improvement from a single motor for the axle and a mechanical differential. So far it has not been typically been done due to cost: two gearboxes are more expensive than one gearbox with a differential, two motors are more expensive than one motor of the same total power, and two inverters/controllers are certainly more expensive than one of the same total power. Then there are packaging challenges. Despite that, the axle of some hybrids which is driven only electrically is driven by a two-motor system - this has been done by Honda and BMW. Although details are sketchy, it sounds like the coming Tesla "Roadster" (which is a coupe, not a roadster ) uses two motors to separately drive the rear wheels.

A popular line of controllers for low-voltage (up to 150 volts or so, in contrast to the 300+ volts of modern production EVs) DIY and limited production vehicles is from Curtis. Their AC controllers include "Dual-Drive", which is their name for a feature of the programming which coordinates two controllers for motors separately driving the wheels of one driven axle... normally for forklift trucks, because that's their main business. The same logic would work for front and rear drives, with appropriate control inputs and tuning parameters. I haven't heard of an off-the-shelf controller set up to coordinate with others for a full set of four motors.

Assuming the use of AC motors, there is no way to avoid a separate inverter for each motor. Each inverter needs at least basic controller functionality dedicated to it, although some functions could be left to a single master controller.... and you would need one master for traction and stability control. With current products available to individuals for DIY projects, that sounds like custom-building to me, presumably with a master controller communicating with four wheel controller by CAN.

 

[brian_]

Also I am wondering if I can make 4 smaller motors work. This would allow me AWD and to go with split axles and do some interesting things with the suspension and also let me reduce the un-sprung weight by moving the brakes to close to the motors.

You can have inboard brakes regardless of the number of motors. Inboard brakes have been used many times in the past, with conventional mechanical powertrains.

I suggest not bothering with inboard brakes. They do reduce unsprung weight, but not enough to be worth the resulting headaches, including high forces on CV joints, difficult access for servicing, putting brake heat into drive components, and packaging of inboard components.

With an electric drivetrain, regenerative braking will substantially reduce the workload of the brakes, so they can be lighter than for a non-EV of the same mass and use. This makes the unsprung weight advantage of moving brakes inboard less significant.

There are two well-known inboard brake applications which made it to mass production and are well-known:

1. Jaguar, E-Type and later: these were notorious servicing problems, and were abandoned in favour of a conventional outboard design decades ago.
2. HMMWV / Hummer H1: the geared hub of the portal axle left little room for a brake, which was mounted inboard instead; it worked well, and had the advantage of the hub's gear reduction, but it had to be kept small (comparable to the diff housing) to avoid clearance issues. This chassis is now obsolete.

 

[brian_]

Dual (siamese) motors that are mechanically linked (and thus act more or less like one motor) are fairly common.

Yes, but even then when they are AC they still use two inverters/controllers.

Examples:

• HPEVs AC-3Xx2
  Although the two motors are build on the same shaft and in the same housing, the wiring diagrams show one motor connected to a primary controller and the other connected to a secondary controller (using Curtis 12xx controllers).
• AM Racing AMR Dual Stack 250-90 AC Motor
  "Requires dual controllers for operation."

 

[brian_]

Chris, you might find the configuration and packaging of the prototype Bollinger 4X4 interesting. It has been discussed in this forum.

Image from Core77; additional images in article.

 

[brian_]

Yeah, I'm not sure if the weight advantage would be worth it, but the access issues would depend on where I put those motors and could be easier to access in some cases.

I don't see how any inboard location could be more accessible than an outboard brake when the wheel is removed.

Either way, calipers and pads are usually not too bad. The real pain is changing the rotor (disk), with the axle shaft having to be disconnected to remove the rotor.

If the particular motor used was a dual shaft, I could even put them on the opposite side of the motor.

If you use a reduction gearbox between the motor and the axle (and you certainly should), then a brake on the motor side of the gearbox (whether between the gearbox and motor or on the other end of the motor) would have the same torque advantage as the motor. That help a small disk and caliper achieve high braking force, but doesn't help heat absorption or dissipation.

 

[brian_]

That framing below the beam looks a lot what I was thinking but it would be too wide in the front to steer the truck. I'm wondering if he started by thinking about batteries below the axle and then talked himself out of it for some reason.

The lower framing in that wooden mock-up is completely unworkable, in both width and ground clearance. It looks like it was built to support the mock-up, not as part of the vehicle.

... knowing that he went into this with the same thought in mind about what would be done differently if you were doing this from scratch, gives me some hope and a good starting place.

This suggests that the Bollinger design is a new and unique creation. It's an interesting approach, but it's a backbone frame with battery boxes hung outboard. Backbone frames have been used many times on many vehicles, including my Triumph Spitfire and most Lotus models for decades. Fuel tanks, ancillary equipment including battery boxes, and cargo boxes are hung outboard of frames on virtually every heavy truck on the planet.

I'll attach an image of a Spartan K4 motorhome chassis, which uses a box-beam backbone chassis.

 

[brian_]

Okay I might have figured out a way around the 4 motor thing.

https://chargedevs.com/features/torque-vectoring-and-electric-drives-qa-with-gkns-advanced-engineering-director-full-interview/

This company combines two clutches with 1 motor to create torque vectoring on an axle in a compact design.

The eTwinster is an odd child of two streams of development:

1. mechanical drive systems for AWD vehicles wanting individual wheel torque control evolved from a differential to just two clutches - one per wheel - under computer control
2. hybrid systems for AWD vehicles evolved to multiple modes of drive for the axle at the engine end of the vehicle, and electric-only drive for the other axle... using a single motor and a differential.

While Honda and others have decided that it is time to just use one motor per wheel, GKN supplies systems to auto manufacturers and has found that the total system cost can be kept lower by using the two clutches to connect one motor (via one set of reduction gears) to the axles. This has the wear, maintenance, and inefficiency of the clutch system, but at least only needs one motor and one inverter.

Efficiency is a major issue if considering this design for primary propulsion. It has no differential, so the only way the vehicle can turn is if the clutch for the wheel on the inside of the turn (which must turn more slowly) slips. In a tight turn of about 6 metres radius (at the outside tires) with a vehicle having a 1.8 metre track width, the inside tire is only turning at 60% of the speed of the outside wheel: that means that the outside wheel clutch is locked and the inside wheel clutch is slipping 40%, so that 40% of the power to the inside wheel is wasted as heat in the clutch! This is why differentials, not clutch pairs, are used for primary drive of all road vehicles.

What is not mentioned here though, is if motor control and torque vectoring are done with the same controller.

Each motor (only one per axle with the eTwinster) needs an inverter. Each inverter needs a controller. Control of the clutches is entirely unlike control of an inverter, and overall control of the system is needed, but any combination of control functions could be packaged together in one unit. In practice, the auto manufacturer will want a tidy package from GKN, but the unit will only be functional when properly integrated with a vehicle's control system, which needs to coordinate this drive unit with the engine, transmission, and braking system.

Also they don't mention the price, but they do mention that it is already used by the BMW i8, Porsche Spyder 918, PSA 3008 Hybrid, and Volvo XC90 T8...

I don't know anything about the PSA 3008 Hybrid, but the XC90 T8 is the cheapest vehicle of the remainder, and it is the most expensive Volvo in existence. This hardware will not be inexpensive.

... it is already used by the BMW i8, Porsche Spyder 918, PSA 3008 Hybrid, and Volvo XC90 T8 on either the front or rear axle. So salvaging a front axle and control from one vehicle and a rear axle from another is possible.

A very good point especially if I take axles out of two different cars. those motors are going to be dealing with a different weight and different size tires just to start.

It would be good to avoid the complexity of adopting hardware from two different vehicles - why not just use two identical units?

Another problem is that two eTwinsters (or other similar electric-only axle drives from hybrids) will not likely be strong enough for adequate performance and durability for a pickup truck. They are used - and sized - to drive the secondary (lower-powered) axle of hybrids.

 

[brian_]

Thanks Brian, I didn't think about the efficiency. I was assuming that power got transferred to the only engaged wheel. I realized that there would be loses in the clutches but thought that would be minimal.

There is no loss when going in a straight line, but a bit of turning means a bit of slipping of one wheel; I had described the most extreme case.

I didn't think about using the same model axle for both front and rear, but that could be done just by reversing the power on one of them I guess.

You don't even need to reverse one, if you use them in the same orientation. If, for instance, the motor is ahead of the axle in the front, it can be ahead of the axle in the back, too.

I won't be towing or offroading the truck but may do some hauling, so yeah durability of the axles was something I probably should have thought about.

Anything involving sustained significant power is a potential concern.

It's not like I wasn't expecting this, but I'll need to rethink my rethinking.

Isn't the design exercise much of the fun?