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Auxiliary electric rear-wheel-drive

Auxiliary electric traction to address the limitations of modified front-wheel-drive cars
  (+8, -1)
(+8, -1)
  [vote for,

This is purely a performance-oriented modification. I am not here interested in improving fuel economy or reducing emissions. The motor-vehicle problem is in any event one of spatial and economic systemics, not one of automotive technology: if you're looking at electric motors in cars for clean transport you're on the wrong mode of transport in a wrong-shaped city.

Be that, however, as it may. The older cars out there these days, the ones ripe for the taking of outrageous mechanical liberties, tend to be front-wheel-driven. They have distinct traction advantages as long as they are as anaemically powered as originally intended. As one applies effort, cleverness, manaical laughter, and cash, however, one soon reaches a point of equity, after which rear-wheel-drive emerges as preferable. For instance, I need a mere 92bhp to light my Golf MkI's nearside front tyre, even though it is a stickyish Yokohama, and then to stand still in billowing pea soup.

Imagine, then, the dismay of anyone trying to put 492bhp, not to mention 892bhp, through the front wheels.

Now, it is known that an electric motor, suitably breathed-upon, can produce the sort of torque delivery to put all that right, when coupled to the rear wheels. The effect is particularly remarkable when the motor is fed by banks of goofium-phlogiston batteries, which, as we all know, have negative mass. But I digress. Let me proceed to the idea.

There is an electric motor powering the rear wheels of a front-wheel-driven car. I see a motor and co-axial differential mounted to a typical torsion beam set-up and driving the rear wheels by chains; there are lots of possible arrangements. I also see this drive incorporating a free-wheel, so that the wheels might readily overrrun the motor at speed, when the advantages of rear-wheel-drive are less pronounced.

There is some sort of augmented or additional alternator, be it in place of the usual alternator, attached to the front differential, or whatever. There are some batteries or capacitors, whose capacity to provide "bonus" power is carefully considered against their consequences in weight.

There is obviously some sort of motor controller, but here's the thing. It is not referenced to the throttle pedal but to the speedometer drive and to the front suspension. The electric motor runs faster as the front wheels turn faster, and receives more current as the front wheels are unloaded. That way, electric drive is added when drive to the rear wheels is needed, that is, when rapid initial acceleration results in the transfer of weight from the front to the rear wheels.

There it is: a use for all those tatty front-drive cars that cannot be given away.

Ned_Ludd, Jul 26 2007

Hybrid Toyota GT Car http://gizmag.com/go/7646/
Like what you said, and then some [marklar, Jul 26 2007]

2500w hub motors http://www.made-in-...ub-Motor-2500W.html
Just a link to what they look like [acurafan07, Jul 27 2007]


       Wheelies in a K-car.   

       If the front end comes up, you're stuck with full-bore until it drops again. Letting off the gas is no help.
lurch, Jul 26 2007

       I've been wondering if your K-car would indeed so lurch, [lurch]; that is, trying to figure out the feedback situation. I can't get it quite straight in my head. What say the rest of you?   

       It might be an argument against any batteries/capacitors in the system: i.e. cutting power to the engine cuts current delivery from the alternator. Otherwise, the speedo reference could be replaced by an output reference on a front-diff-mounted alternator?
Ned_Ludd, Jul 26 2007

       Simple fix! A switch at either the top end of the throttle pedal travel, or near the bottom of the clutch pedal travel, if you've got a clutch.
Ned_Ludd, Jul 26 2007

       There are many concept cars that have electric rear wheel drive and engine powered front wheel drive, I don't think any of them are baked though.
BJS, Jul 26 2007

       Never mind my "Cube" comment for anyone who saw it, this is for a completely different purpose.   

       My old man used to have an Acura TL (hence the username) that had 258 hp through the front wheels. With the stability control off, that thing could light up both front tires at once with more force than most RWD muscle car launches. Problem was, the thing usually chirped the tires from a standstill even with the stability on and even at fairly low throttle. This system would have made that car so much better to launch, and could have probably shaved a second off the 0-60 time. Great idea [+].
acurafan07, Jul 26 2007

       This has come up a few times in comments to other ideas in this category, but it looks like you're the first to post it whole. [+]   

       Not sure about your choice of control inputs though - I'd be looking to get some data out of the onboard computer and run it through a microcontroller.   

       Electric motors produce their highest torque from a standing start and at low revs - consider this.   

       Throw away the fancy diff/chain arrangements, mount a motor per rear wheel direct drive, and do all your control electrically.   

       Google Calculator says 1HP is 745 watts. How many HP do you want to lay down on those rear wheels?
BunsenHoneydew, Jul 26 2007

       Thanks, [acurafan07] and [BunsenHoneydew].   

       As the whole idea is around the traction limitations of fairly low-tech fwd cars, I'm assuming no onboard computer to tie into. If there had been one it has long taken a delicate trajectory over the right shoulder, like the electronic immobiliser system in the abovementioned Golf. The motor controller would have an electronic component, but that would be quite independent of the normal operation of the car.   

       Likewise the motor/differential arrangement: the chain drive allows the use of a motor that is conventional except for a hollow shaft. Then it may be mounted with its cg on the rear suspension pivot axis and thus contribute very little to unsprung mass.   

       I've also given some thought to [lurch]'s above concern about self-perpetuating feedback. If the speedometer drive input serves to limit the electric motor speed to that corresponding to say 97% of the front wheel speed, the whole system kicks in and stays in only when there is some loss of traction at the front. Backing off on the gas would collapse the traction loss and cut power to the electric motor. The front suspension input would then determine the amount of current available to drive the motor over a fairly steep curve, in the process allowing the electric motor to be completely inactive while the front wheels are loaded normally.
Ned_Ludd, Jul 27 2007

       Okay, it's baked by RQ Riley, but I'm bunning it for "goofium-phlogiston batteries."
elhigh, Jul 27 2007

       As long as the negative-mass batteries more than offset the weight of the electric motors and related controls, sounds good to me! [+]
ed, Jul 27 2007

       You know hub motors might work for this application better since they don't take up as much space and you wouldn't need the chains and such. It wouldn't add much power, but two 2500w hub motors (each rear wheel) would add about 7hp. You could gear them to top-out at the end of the car's 1st gear to boost speed off the line. Just a thought.
acurafan07, Jul 27 2007


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