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 Post subject: Re: Rear-engined, AWD
PostPosted: March 21, 2016, 12:29 pm 
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1055 wrote:
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.


Notice how I specifically said "4WD trucks' in the first paragraph? And a transverse drivetrain wouldn't be backwards. But, this is precisely what I was talking about when I said FWD+Haldex. Sure, in most cases the parts aren't actually made by Haldex, but the concept is the same, and I'm lazy. All of the transverse engined "soft roaders" are set up like this (the Subaru 4eat and CVT as well, but with packaging differences). There's no center differential, the front wheels are directly driven, and on the back of the transaxle there's a PTO. A driveshaft runs from the PTO to a rear diff with a computer controlled multiplate clutch that can apply a modest amount of torque to the rear axles. The Subaru 4 speed auto has the clutch pack in it's tailhousing instead of on the differential, but the concept is the same. That said, with a rear engine Subaru set up you'd have such a rear-heavy car that I'm not sure there would be much point in having AWD.

Personally, I'd look into the drivetrains from the Mazdaspeed 6, or VW r32's or 4motion Jetta. I don't know if they could be modified for use in an AWD middy, but they'd be much more interesting than an automatic soft roader drivetrain.

bob wrote:
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


Sporty UTVs are built that way as well to get a rearward torque bias with a locked center. It would be death sentence to an LSD though.


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 21, 2016, 12:58 pm 
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gregk wrote:
1055 wrote:
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.


Notice how I specifically said "4WD trucks' in the first paragraph? And a transverse drivetrain wouldn't be backwards. But, this is precisely what I was talking about when I said FWD+Haldex. Sure, in most cases the parts aren't actually made by Haldex, but the concept is the same, and I'm lazy. All of the transverse engined "soft roaders" are set up like this (the Subaru 4eat and CVT as well, but with packaging differences). There's no center differential, the front wheels are directly driven, and on the back of the transaxle there's a PTO. A driveshaft runs from the PTO to a rear diff with a computer controlled multiplate clutch that can apply a modest amount of torque to the rear axles. The Subaru 4 speed auto has the clutch pack in it's tailhousing instead of on the differential, but the concept is the same. That said, with a rear engine Subaru set up you'd have such a rear-heavy car that I'm not sure there would be much point in having AWD.



You also included AWD in that sentiment.. with the exception of the Subaru Baja (arguably a truck..), Honda ridgeline (also arguably a truck) or GMC typhoon also a useless truck in 'truck' form.. there are no AWD trucks in NA. My comment was purely towards AWD, and that's why I cited Nissan, Honda and Subaru AWD systems.

A rear engine, factory built transaxle would have to be mounted 'backwards' in order for the PTO output shaft that drives the rear (now front) wheels to work.. Which is why the gentleman used the b20b drivetrain with an f20c..

the same argument about your weight bias over the rear wheels could be made about front engined, awd cars and the lack of weight over the rear wheels. That weight is typically made up by the passenger compartment and fuel tank, which you would have to mount forward if you were putting the engine rearwards anyways. So say a 10 gallon tank, plus font driff, driveshaft, axles, etc.. you're at 80 lbs in gas alone, probably an additional 200 lbs in components up front? Its not quite the 400 of most modern small engines/transmissions, but its still a fair chunk, and could also be compensated for with stiffer springs out back and softer up front to keep things on an even keel.

I'm curious as to why you say there is no center differential on a Subaru? are you not familiar with their manual transmissions?

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 21, 2016, 2:20 pm 
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1055 wrote:
You also included AWD in that sentiment.. with the exception of the Subaru Baja (arguably a truck..), Honda ridgeline (also arguably a truck) or GMC typhoon also a useless truck in 'truck' form.. there are no AWD trucks in NA. My comment was purely towards AWD, and that's why I cited Nissan, Honda and Subaru AWD systems.


I'm not going to argue about semantics and definitions. I was referring to longitudinal engines with longitudinal transmissions and external transfer cases (of which, there are AWD examples available), just like what was used in the Baja Boot that caused the OP to get excited about "backwards" drivetrains. And since the OP was specifically talking about using S-10 parts, I specifically mentioned trucks.

1055 wrote:
A rear engine, factory built transaxle would have to be mounted 'backwards' in order for the PTO output shaft that drives the rear (now front) wheels to work.. Which is why the gentleman used the b20b drivetrain with an f20c..


You were thinking about your example, I was thinking about my example. Again, not going to argue.

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I'm curious as to why you say there is no center differential on a Subaru? are you not familiar with their manual transmissions?


I'm very familiar with Subaru manual transmissions, but you are specifically talking about the 4 speed automatic. The five speed auto has a center differential with a 45/55 torque split, the manuals have center differentials with either a 50/50 split or, in the case of the STI, either a 35/65 or ~40/60 split depending on year. But the 4 speed auto, the one that you specifically mentioned, the one with the 90/10 split, does not have a center differential. It has a multiplate clutch.

1055 wrote:
the same argument about your weight bias over the rear wheels could be made about front engined, awd cars and the lack of weight over the rear wheels. That weight is typically made up by the passenger compartment and fuel tank, which you would have to mount forward if you were putting the engine rearwards anyways. So say a 10 gallon tank, plus font driff, driveshaft, axles, etc.. you're at 80 lbs in gas alone, probably an additional 200 lbs in components up front? Its not quite the 400 of most modern small engines/transmissions, but its still a fair chunk, and could also be compensated for with stiffer springs out back and softer up front to keep things on an even keel.


A front engined AWD locost would have a weight distribution of around 50/50. Typical locosts are a little rear heavy, say ~45/55, so add a front driveshaft, front differential, and front axles and 50/50 is a resonable estimate. Then consider the longitudinal load transfer due to acceleration, lets call it 10 percent. That front engined AWD car would have a load (traction) distribution under acceleration of ~40/60 which means that driving the all four wheels would produce ~67% more traction compared to a RWD car.

KB58's midlana, with a transverse engine ahead of the rear axles, has a weight distribution of 33/67. If you put the engine behind the rear axles then even with the weight of a front fuel tank, front diff, etc. you'd be lucky to end up with a 30/70 weight distribution. Then consider the longitudinal load transfer due to acceleration, we'll use the same 10%. That rear engined AWD car would have a load (traction) distribution under acceleration of ~20/80, which means that driving the all four wheels would produce ~25% more traction compared to a RWD car.

+67% is a lot more compelling than +25%. +25% is not bad, but when you consider the well over a hundred pounds of additional weight for the transfer case, front diff, driveshaft, and axles, how much of an acceleration advantage would there actually be? Case in point, the 991 Carrera S (0-60 in 4.1 seconds) vs the 991 Carrera 4S (0-60 in 4.0 seconds). Would the performance difference in a locost be that small? I'm not sure, but I doubt it would be big.


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 21, 2016, 4:30 pm 
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gregk wrote:
KB58's midlana, with a transverse engine ahead of the rear axles, has a weight distribution of 33/67.



Look again, a portion of his engine is in front of the rear axles, but a portion is over or just behind the rear axles as well.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 28, 2016, 8:26 pm 
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While we are this far outside the box, what about hydraulics? I remember seeing a guy run a mud bog buggy(VW Bug driveline type but making big HP) with hydraulic front drive motors. He qualified as 4wd for a while until his competitors bitched about him skipping like a water bug across the top of the mud pit while their 1000hp V8's tried to dig in.

Since you could already have spindles built for CV axles, just put a hydraulic motor between them. Drive it with an identical pump running off the rear CV output.


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 Post subject: Re: Rear-engined, AWD
PostPosted: March 28, 2016, 8:43 pm 
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I haven't really been following this but a couple of posts back I saw a statement about no AWD trucks in North America, GMC Typhoon (?) aside. My 2001 Dodge Dakota owners manual refers to both 4WD and AWD options and it is clear that the AWD one is a full-time, no-low-range version. It's only concession to 'truck' is a lockable centre diff. But other than what the owner's manual said I know nothing .... and since my own Dakota is 2WD I never went back to re-read that section of the manual.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 28, 2016, 9:53 pm 
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test - Been having trouble getting on the forum, so this is a test. Anyone else? OK, been trying to post this for over 24 hrs. this is the first time it worked.

Back in the 70s there were many American trucks and SUVs built with AWD, they just called it "Full Time 4WD". Generally, a single range X-fer case with a non-biased diff that could be locked at the flip of a switch.

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 29, 2016, 7:53 am 
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ngpmike wrote:
test - Been having trouble getting on the forum, so this is a test. Anyone else? OK, been trying to post this for over 24 hrs. this is the first time it worked.

Back in the 70s there were many American trucks and SUVs built with AWD, they just called it "Full Time 4WD". Generally, a single range X-fer case with a non-biased diff that could be locked at the flip of a switch.
I think it was called "New Process" AWD by GM. I don't recall who actually made it, might have been in-house GM stuff.

:cheers:

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 29, 2016, 8:21 am 
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JD,

You got the name of the manufacturer right there.. New Process Gear (NPG). It was a Chrysler subsidiary until they partnered up with GM and became New Venture Gear.

:cheers:

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 29, 2016, 8:23 am 
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Warren Nethercote wrote:
I haven't really been following this but a couple of posts back I saw a statement about no AWD trucks in North America, GMC Typhoon (?) aside. My 2001 Dodge Dakota owners manual refers to both 4WD and AWD options and it is clear that the AWD one is a full-time, no-low-range version. It's only concession to 'truck' is a lockable centre diff. But other than what the owner's manual said I know nothing .... and since my own Dakota is 2WD I never went back to re-read that section of the manual.


That was me, I had mentioned the typhoon in my post.

Just visited the old Google machine about the AWD Dakota.. looks like it was a two year run, 2001-2002 and only available with the 4.7. Learn something new every day!

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 Post subject: Re: Rear-engined, AWD
PostPosted: March 29, 2016, 7:31 pm 
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carguy123 wrote:
gregk wrote:
KB58's midlana, with a transverse engine ahead of the rear axles, has a weight distribution of 33/67.



Look again, a portion of his engine is in front of the rear axles, but a portion is over or just behind the rear axles as well.

About 95% of the engine proper is ahead of the rear axle, with the crank about 8" ahead.

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 Post subject: Re: Rear-engined, AWD
PostPosted: January 7, 2017, 4:27 pm 
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I've been thinking about the Subaru Gear rear-engined AWD setup and it really seems promising. The Subaru motor isn't especially heavy, and you can get the radiator, fuel tank, turbo, intercooler and front differential all in front of the rear wheels to a greater or lesser extent. Being able to shift power from the rear to the front when the rear begins sliding will also offset some of the rear-engined layouts inherent tail-happiness. Has anyone done this?


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 Post subject: Re: Rear-engined, AWD
PostPosted: January 7, 2017, 9:08 pm 
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I know of a few "German look" super beetles being built like that, seem to work quite well, a true poor man's Porsche.


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