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 Post subject: Rear-engined, AWD
PostPosted: March 19, 2016, 4:18 am 
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Fellow gear heads,

Objective - create a rear engined, AWD, vehicle using stock components.

The Ford RS200 Rally Car used this layout to great - if not extremely expensive effect, as well
as several exotics (Porsche, Audi, Lamborghini).
So, is this simplifying a home builders approach too much?
Using - for example- a donor manual transmission RAV-4 drivetrain (pre-2008, engine mounted transversely at the front, AWD), flip the whole drivetrain around so that the engine is at the rear. Swap over the steering assemblies with what was the rear wheel assemblies, and vice versa. Add a shift lever extension. And finally, reverse the rotation of the engine (reverse ground cam, reverse rotation starter motor, re-time, and possibly modify some of the accessories).
Poor man's alternative to an otherwise very expensive proposition?
What am I missing here? Any sexier (but just as available) alternatives to the RAV-4 drivetrain?


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 10:43 am 
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Subaru with reverse gears would also work. You can get a reverse pinion for the internal transaxle diff and then get reverse parts for an R160 diff from a FWD version.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 11:58 am 
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You might also look at some of the older off-roaders like the Vic Hickey designed "Baja Boot". Interesting drive line setup.
Attachment:
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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 12:35 pm 
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Its very easily done with most of the 4x4 utility vehicles running gear. With Landrover, Daihatsu,Suzuki etc the 4x4 racing fraternity just rear mount the engine with the front of the engine facing the rear of the car, flip the diffs 180 degrees in the axle case and cut new props. Very easy indeed.

Bob

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 1:05 pm 
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I remember seeing somewhere that a guy was doing the "Backwards Drivetrain" thing, but instead of flipping the diffs, he added a set of spur gears ahead of the X-fer case. This not only gave him the correct rotation, it had the added benefit of a quick change gear ratio! Can't remember where I saw it though.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 3:27 pm 
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"The backwards drivetrain thing" - love it!
In the interests of full disclosure, I ought to mention that I have rebuilt a 140 HP Corvair engine with all new components, forged pistons, rods, magnafluxed crank, lightweight everything, mated to a Crown VW adapter. Original intent was to put it in an experimental aircraft, but as this is one of the few stock designs that was built reverse rotation to every other manufacturers engines (except perhaps some Hondas, I hear), there may well be some scope for "the backward drivetrain thing" there too. Anyone know what the stock Corvair bolt pattern will mate to, transmission wise?
As its a GM product, maybe the S-10 4X4 tranny?


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 4:23 pm 
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Corvairs were such nice cars, it would be great if you could put it back in one.

A VW Passat would be a good match for a reverse rotation setup or Subaru. They both had longitudinal engine installations. reverse rotation engine is a big help. Honda B-series is reverse rotation.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 5:27 pm 
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Tickityboo wrote:
"The backwards drivetrain thing" - love it!
In the interests of full disclosure, I ought to mention that I have rebuilt a 140 HP Corvair engine with all new components, forged pistons, rods, magnafluxed crank, lightweight everything, mated to a Crown VW adapter. Original intent was to put it in an experimental aircraft, but as this is one of the few stock designs that was built reverse rotation to every other manufacturers engines (except perhaps some Hondas, I hear), there may well be some scope for "the backward drivetrain thing" there too. Anyone know what the stock Corvair bolt pattern will mate to, transmission wise?
As its a GM product, maybe the S-10 4X4 tranny?


Its not a good idea to run a gearbox in reverse, the cut of the gears and thrust loadings on the shafts are all designed to run one way. Its far easier to flip diffs , there are other counterclockwise engines like some of the Honda`s , there may well be a suitable trans in their line up

Bob

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 5:57 pm 
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The Corvair manual trannys were Saginaw 3 and 4 speed boxes, but the differential sat between the bellhousing and the trans so there's no telling what the bolt pattern at the bell housing/transaxle mating surface is. Possibly Saginaw?

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 19, 2016, 6:06 pm 
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I just remembered! The 'Vair bellhousings were backwards! The large opening bolted to the tranny, and the small end, to the engine as the cover for the cam gear. Drill a large plate with the bellhousing bolt pattern on the periphery, and the new transmission pattern in the center, and you should be good!


Attachments:
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 Post subject: Re: Rear-engined, AWD
PostPosted: March 20, 2016, 12:10 am 
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ngpmike wrote:
I just remembered! The 'Vair bellhousings were backwards! The large opening bolted to the tranny, and the small end, to the engine as the cover for the cam gear. Drill a large plate with the bellhousing bolt pattern on the periphery, and the new transmission pattern in the center, and you should be good!


That must have been the approach taken for the Corvair / Crown VW Adapter then - I have two of these, found on e-bay


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 20, 2016, 9:07 am 
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That's it! The cavity on the front there is where the cam gears run. I've owned a number of 'Vairs over the years, but it's been a long time.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 20, 2016, 2:51 pm 
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The big problem with the "backwards drivetrain thing" is the torque split of the center differential. 4WD and AWD truck transfer cases, afaik, all have torque splits of between 40f/60r and 35f/65r. Once you turn them around you have a torque split in in the neighborhood of 60f:40r in a car with rearward weight (read: traction) distribution. The end result is going to be power induced understeer all over the place.

Offroad vehicles with backwards drivetrains like the Baja Boot, or any number of Ultra4 buggies use a selectable transfer case with no center differential at all. That's a wonderful thing if you're offroading, but it's unacceptable in a road going car with sporting pretensions.

To do it properly you'd need a custom transfer case, or a Lamborghini donor car.

For a different approach, you might want to check out this thread. This one has a 50/50 torque split, but it's probably around a 35/65 weight distribution.
http://www.locostusa.com/forums/viewtopic.php?f=36&t=17516

That approach, but with a FWD+haldex style drivetrain (with a programmable controller) probably has the most potential, assuming you can find one that's suitable.


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 21, 2016, 9:30 am 
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gregk wrote:
The big problem with the "backwards drivetrain thing" is the torque split of the center differential. 4WD and AWD truck transfer cases, afaik, all have torque splits of between 40f/60r and 35f/65r. Once you turn them around you have a torque split in in the neighborhood of 60f:40r in a car with rearward weight (read: traction) distribution. The end result is going to be power induced understeer all over the place.

Offroad vehicles with backwards drivetrains like the Baja Boot, or any number of Ultra4 buggies use a selectable transfer case with no center differential at all. That's a wonderful thing if you're offroading, but it's unacceptable in a road going car with sporting pretensions.

To do it properly you'd need a custom transfer case, or a Lamborghini donor car.

For a different approach, you might want to check out this thread. This one has a 50/50 torque split, but it's probably around a 35/65 weight distribution.
http://www.locostusa.com/forums/viewtopic.php?f=36&t=17516

That approach, but with a FWD+haldex style drivetrain (with a programmable controller) probably has the most potential, assuming you can find one that's suitable.



Not necessarily true, the Subaru, Nissan and Honda automatic transaxle systems normally operate at 90/10 F/R, which when flipped around would give you a 90R/10F till needed.

The Subaru manual transaxles are all 50/50 split, with torque appropriation between 20/80 and 80/20, as there is no way to fully lock the center diff.

I once heard rumors of a set up like this.. with a B20B manual driveline from a 98 CRV, and an F20C, but the guy used straight cut gears in his transmission, and I'm not sure what he did for the now front, rear end.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 21, 2016, 11:19 am 
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On some of the older 4x4`s the bias if any was done by fitting a different diff ratio front to back. It was left to the viscous coupling or center diff to compensate.

Bob

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