New steering system concept to minimize aerodynamic drag

[Blue Bomber Man]

Heres a more recent model that I was trying to make more accurate then the first screenshots. It still needs some modification before I actually cut into the model for the steering system, but it shows the general lay out of the vehicle.

This version the vehicle is about 10' 6" long, maybe 4' "6 wide (guessing on the width, I dont remember off hand, probably wider). Once I update the model tonight the nose will be wider, allowing the wheels to move further forward. This allows for a wider wheel base and more area that could accept the drivers seat. I'm also considered moving the vehicle to a two seat design because it would only require a very slight increase in the vehicles side.

In this design I really wasn't trying to copy the silhouette of the Aptera but as it evolved the shape made a lot of sense.



This version doesn't include any enclosures for the wheels yet. Not going to bother designing them until I have everything else squared away for the general shape.



The rear wheel will probably get wrapped up in a similar way to the Aptera's.

[TestDrive]

Assuming the hash marks in your original are 12" apart - I added a few dimensional references, which make me think the car would have to be longer than 10.5 feet.



Yellow Dot - If the rear of the car pivots off of this point (and the wheels don't steer independent of their respective body sections), the vehicle will steer decently. Otherwise the rear wheel is going to be dragged laterally across the pavement at each turn.

Dark Grey arrows - Overall tire radius is 12" - implying ~9" wheels???

Green & Light Blue arrows - Seat base is 15" x 15". I'd guess that even an anorexic, 5' 2" person would find that seat at least a little cramped. The butt of my overweight 5' 11" (180cm) frame needs/wants 20" wide x 18" deep (51cm x 46cm).

Red arrows - Sitting in my desk chair with height set so thighs are parallel with the floor and knees @ 90*, the distance from the intersection of seat base and seat back to the tips of my toes is 39". Sitting in the seat of my Ford Escort (knees @ ~150*, the distance is 44".

[pasadena_commut]

Why not just steer with the back wheel? Then the suspension on the two front ones could be much simpler since it wouldn't need to include steering components. The back one is already mostly out of the wind flow, so minor changes to accommodate any additional steering related parts shouldn't make much difference.

Admittedly one would have to drive such a beast a little differently. For instance, to leave a parked position by the curb the car would likely have to back out. Going in would be easy though, stuff the nose in going forward, crank the wheel over the other way hard, and the tail slides right over to the curb.

[Blue Bomber Man]

That is not a bad idea for doing it, and I might look into doing it that way as well.

The main complications I for see in doing it that way are these:

-Steering would probably have to be by wire
-Steering by wire would probably want an electric steering system
-Would probably require the vehicle to be frontwheel drive and thus need two motors
-Controlling a two motor setup is likely more expensive

Direct Benefits:

-Simplified body construction
-Possibly tighter turning radius (depends on much you can force the vehicle to turn without the rear wheel skipping)
-Indirectly allows for a more powerful drive system.

If I could keep that same system rear wheel drive it would be extremely maneuverable though. This could be done by either doing a hub motor setup (difficult to find one powerful enough that is actually on the market: 15kw+) or by enclosing the motor in with the rear wheel setup; a setup that requires strong considering in this application.

TestDrive:

Here is a marked up image:



I haven't move any components around yet for the previous adjustments I noted I wanted to make in an earlier reply.

Also on my Mazda Protege from the front of my seat to the back of the pedals is ~24". I am only 5'8" but that is with my legs fully extended. So I assume if I design 6-10" of travel into the seat position that it could easily accommodate larger people. If I decide to move on with the front section steering there would be a footbox space cut out of the front section to allow the legs to clear into it some to provide more room. As long as the whole section didn't need to turn more then about 15-20 degrees this would not impede movement of the outer front section walls as they swing inward.

I definitely want to put some more thought into designing a rear swinging assembly similar to what pasadena_commut was suggesting. It would depend on me verifying that I could maintain rearwheel drive which I think would improve the steering in this situation.

As a side note: I am using Screenshots then pasting them into MS paint. I'd like to be posting these images cropped but I don't see an option for that in paint, any suggestions?

[McTimson]

In Paint, you can change the size of the image. If you look close, there are handles on the edges, they're just small, and not very obvious. Or, you can go to Image -> Attributes, and change the height and width there.

I like the back wheel idea more as well. The first image looks like it would be difficult to implement, but I guess you have ideas on how to do that. It seems like it would make very wide turns though.

[Big Dave]

Y’know, I can see why you would want to eliminate the current steering system to improve aerodynamics, but did you ever stop to think that the current steering system that everybody uses exists for a reason?

Have you ever driven a wheelloader? That is essentially what this is – a high speed wheelloader. Wheeloaders steer by articulation the chassis just like this. Conestoga wagons used that system. For low speed (<10 MPH) construction/mining this is A-OK. The problem is that a wheelloader is nearly impossible to steer a straight course with. No big deal at slow speeds, but a life-threatening problem at higher speeds.

The five-bar steering linkage used today (called Ackerman steering after the guy who designed it) makes the outside tire describe a much wider radius than the inside tire. The whole system is nearly self-centering.

This was a technological problem that was solved a hundred years ago and aerodynamics are not a good enough reason to throw it out.

Wagon steering was thrown out for a reason: It didn’t work.

Why doncha just stay with Ackerman steering (we know it works) and widen out the vehicle to 60" but don't widen the front wheels so far they would need to protrude out of the body work at full lock. This was used back in the 30s and 40s on some high-dollar European cars. It seemed to work OK. with a 60" wide car you have to make the tail 160 inches long. The additional length would give you room for a second seat in tandem, making for a much more commercially viable car. A nice VW TDI diesel transaxle gives you a very efficient drive train and most of the rough spots have been worked out of that.

[Blue Bomber Man]

Those little guys on the side have been hiding from me =) Thanks for pointing them out!

I think I am pretty sold on rear wheel drive, I'm convinced I could integrate an motor to the steering assembly. I also think that doing it this way allows for two person seating, or possibly a 2+1 setup if I tuck the 1's legs between the front two seats without increase the vehicle's size.

I do need to figure out more about lofting properly before I spend more time on this in SolidWorks. You'll notice that the above drawings have the front nose cutoff (the rear is intentional, but I couldn't round it off if I wanted to atm). This has to do with the cross section sketches that I loft the surfaces across. I can't seem to figure out how to close it at the end without making the loft fail for one of the few possible errors I seem to come across:

"Unable to knit surfaces together"
"Self-intersecting geometry"
or a couple others I can't think of at the moment. Anyone that has some experience with Solidworks I'd love to find out how to fix this or how to change my approach at creating the solid body. I cannot telling you how many hours I've spent beating my head over that topic. FYI it's SolidWorks 2007

[Blue Bomber Man]

Big_Dave:

Thanks for the detailed response. One of my original primary concerns was how the wheel base decreases as the vehicle turns, probably resulting in poor vertical stability. I thought about your other concern already about it wanting to be uncontrollable. My solution to this is have the pivot point ahead of the wheel so it is self centering.

The quick and dirty sketch below illustrates the same thing, only with the rear wheel pivoting instead. The initial drawings from the beginning of the thread also had their pivot point ahead of the front wheels (actually the pivot point was about 2-3" in front of the car, the steering system was going to require an arclike track to accommodate for that).

With the sketch below what do you think about how the car would handle (ignore issues of going in reverse, reverse would only be at low speeds and this setup would have some sort of power steering)?

[Big Dave]

Rear-wheel is even more unstable. Even the Ackerman mechanism could not stabilize rear wheel steering at speed. This time, use forklifts as an example. Very manueverable at low speed but they wander all over the place at speed.

Front-wheel steering with the Ackerman linkage rules for a reason.

I hate to see you beating your brains out re-inventing steering, when you could be bearing down on what otherwise looks like a good design.

Rear wheel drive is OK.

[dcb]

Originally Posted by Big Dave

Rear-wheel [steering] is even more unstable.

Yup, that's where drive by wire comes in (a-la stealth bomber or other "can't fly it without a computer" aircraft).

The rear wheel could also just caster, and manipulate the front wheel torque to steer, maybe.