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 Post subject: Re: AWD Locost?
PostPosted: July 14, 2011, 6:59 pm 
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Warpspeed wrote:
I always assumed it must use the Ford Sierra 4WD drive train,


Yes, I saw that on the Dax site. But it looks like the transfer case would be smack in the middle of the driver's left leg unless there was some non-obvious foolery going on that's not shown.


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 Post subject: Re: AWD Locost?
PostPosted: July 14, 2011, 7:43 pm 
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TRX wrote:
Yes, I saw that on the Dax site. But it looks like the transfer case would be smack in the middle of the driver's left leg unless there was some non-obvious foolery going on that's not shown.

When I planned my own 4WD system, I made sure the front driveshaft was located on the passenger side.

The reasons for doing it that way, were to avoid conflict between the front diff/driveshaft on one side, and the steering rack/steering shaft on the other.
Both shafts need to pass very close beside the engine block, and there is not only not much room in that general area, but the physical locations of both front diff and steering rack are pretty much fixed by front end geometry considerations. It is all going to be an extremely tight fit, especially in a low vehicle.

The passenger can get away with having a bit less leg room than the driver anyway.


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 Post subject: Re: AWD Locost?
PostPosted: July 14, 2011, 10:05 pm 
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Quote:
Any more info on that ?
Maybe a link ?


Your in luck, I think the source was in Australia. Sorry I don't recollect the link. I think that as I keep up with the forum these days, things I learned earlier are dropping off the back end of my memory, so to speak. The info is somewhere on this forum though... It wasn't extremely cheap, but something like $1000. Not even sure whose dollars though, LOL...

http://www.subarugears.com/index.htm

Here's a picture of one from that web site installed on a, I think, Porsche subframe. You can see it would be lightweight - the motor is full of holes! :) ( how does Gonzo get those ROTFLMAO in his posts?... )

At least the shift linkage might be easy... It really doesn't look too long. maybe you can mount the spare tire in front and use it for a soft bumper... :)


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 Post subject: Re: AWD Locost?
PostPosted: July 18, 2011, 9:50 pm 
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Now that is neat! My only question would be how the torque is split front to rear? The best you could hope for is 50:50 with the Subie diff right? I don't think you could get it rear biased in that configuration.


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 Post subject: Re: AWD Locost?
PostPosted: July 18, 2011, 10:51 pm 
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Making the engine rotate backwards would be the next challenge.
Starter should be easy, alternator does not care, distributor should not be too difficult either if you use aftermarket engine management.
Getting four reverse rotation cams ground up from billets would not be too difficult either, although the cam belt tensioner would be on the wrong (tension) side.
Reversing the water pump and the oil pump would require a bit more thought.
Clutch plate sprung centre needs to work in the opposite direction, maybe just turn the plate around ?
Probably quite do-able if you were really keen.


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 Post subject: Re: AWD Locost?
PostPosted: July 19, 2011, 8:39 am 
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Isn't that what they make the reverse ring and pinion for?
People already put these in Porsches, just like in that photo....

I don't know what the torque bias is on a Subaru but with the engine in the back, it should be reversed...

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 Post subject: Re: AWD Locost?
PostPosted: July 19, 2011, 12:44 pm 
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Pssssst! Marcus, I do it like this, only with no spaces between any of it-
: r o f l :

comes out like this :rofl:

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 Post subject: Re: AWD Locost?
PostPosted: July 19, 2011, 4:19 pm 
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horizenjob wrote:
Isn't that what they make the reverse ring and pinion for?
People already put these in Porsches, just like in that photo....

I don't know what the torque bias is on a Subaru but with the engine in the back, it should be reversed...


Correct. I'm pretty sure Subarus are rear-biased though, at least the more performance oriented ones. So front bias would likely be the result if one were to use this setup, and at best 50:50. Not exactly ideal for a locost. Such a shame as this does seem like a brilliantly simple solution and otherwise would be the best setup out there IMO.

It's not even all that expensive really when it gets down to it. $1600 for the ring and pinion on top of what, maybe $3k for a good WRX(but not STi) engine and gearbox? I wouldn't know exactly but I can't imagine it being more than that. So $4500ish + a front diff, driveshaft, axles, etc. That's certainly pricier than most RWD locost drivetrains but not prohibitively expensive.


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 Post subject: Re: AWD Locost?
PostPosted: July 19, 2011, 7:35 pm 
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There are several alternative types of aftermarket centre diffs available for the Subie, but all of them are symmetrical 50/50 types by design.

There is one way, and one way only to get a constant fixed biased torque split, and that is with an epicyclic torque splitting gearset, which the Subie does not have.
The only way to get a variable split, is with some type of variable friction clutch driving one end, the subie does not have that either.

So you can run an open centre diff, or a viscous, or a clutch pack, or a torsen, or just lock it solid.
But there is no way to bias a Subie centre diff to favor one end more than the other.

That 50/50 probably does not work out too bad with a front engine, but with a rear engine and strong rear weight bias, there is going to be far too much fury at the front.


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 Post subject: Re: AWD Locost?
PostPosted: July 20, 2011, 12:45 am 
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Warpspeed wrote:
The only way to get a variable split, is with some type of variable friction clutch driving one end, the subie does not have that either.
.


One 'locost' way is to have a taller front diff ratio with a one way sprag bearing/clutch in the drive to it.

When the rear wheels start to spin faster than the fronts the rear drive 'catches up' to the speed of the front drive and the sprag clutch stops it going faster and at that point you have rear wheels spinning faster than the front by a percentage according to the diff ratio difference.

Last time I looked DP Cars was playing with this same idea.


Last edited by cheapracer on July 20, 2011, 1:29 am, edited 1 time in total.

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 Post subject: Re: AWD Locost?
PostPosted: July 20, 2011, 1:04 am 
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That means zero front drive torque at all, until the rear has actually broken traction. That would give some fairly sudden and dramatic changes in handling.

Even with a fully open centre diff and different front/rear diff ratios, although the torque would certainly split unequally as desired, the spider gears in the centre diff would very likely quickly wear out or overheat.

They are not really designed or capable of running with sustained high speeds, just constant heavy static loading, they are simple steel on steel bearings, and really designed to just slowly creep slightly on slow turns.

It would be like running unequal sized rear wheels. The tractive torque would certainly be more on the side with the smaller wheel, but the small gears in the guts of the actual "differential" would be spinning like mad all the time.
It might work with something like 48/52 ratio split, but with a much more dramatic torque split like 30/70 the small gears in the centre diff housing would be going pretty fast all the time, and might spit the dummy fairly soon at high power levels.

This is the reason and purpose of an epicyclic torque slitting centre diff.
The whole thing in it's entirety turns at the same speed and splits the torque, without requiring any constant rapid internal motion of the small planet gears.
It can be built small and compact, with the usual metal to metal bearings, and because the small planet gears just creep, it does not tend to overheat or wear out fast.

If you want 30/70 split, no problem with an epicyclic.
Have 30 teeth on the sun gear driving the front wheels, and 70 teeth on the ring gear driving the rear wheels, (or multiples of these numbers).
What you cannot do with an epicyclic, is have a very near even front/rear split, because the ring gear obviously needs to be sufficiently larger than the sun gear.

I am convinced an epicyclic centre diff with it's constant torque split is the best possible solution for a road or race car driven on paved surfaces. And above all, it is very consistent and predictable in it's handling characteristics.


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 Post subject: Re: AWD Locost?
PostPosted: July 20, 2011, 1:42 am 
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Warpspeed wrote:
That means zero front drive torque at all, until the rear has actually broken traction. That would give some fairly sudden and dramatic changes in handling.




No, what it means is that you have some nice rear wheel drive traits that are then nicely aided by the fronts when it gets a bit much for the rears. I just looked at the DPCars site and he is doing one now and will test it as a straight drive before he decides if the planetry set is needed or not.

My glass is always half full :mrgreen: :cheers:

You may want to study up on the Granddaddy of them all, the Ferguson Factor Jenson FF.


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 Post subject: Re: AWD Locost?
PostPosted: July 20, 2011, 1:53 am 
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Should be interesting to drive.
Because a sprague clutch is either completely decoupled or wedged totally solid, no gradual build up of slip as it comes in.


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 Post subject: Re: AWD Locost?
PostPosted: July 21, 2011, 3:47 am 
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Warpspeed wrote:
Because a sprague clutch is either completely decoupled or wedged totally solid,


Correct but .......

Warpspeed wrote:
no gradual build up of slip as it comes in.


......you're not viewing the vehicles dynamics as a whole.

Anyway will be interesting to see what DPCars result is although I think his 65/35 ratio split is a mistake, going to need an awful lot of speed difference (read rear wheels spinning) before the sprag takes up, I think he has the practical application of continuous torque split slightly confused. Literally his car will have to be wildly wheelspinning at the rear at near double the road speed before the fronts take up - poorly thought out I believe.

I myself would start at no more than around 5% or less ratio difference looking for a nice gentle bit of slip at the rear to help rotate the car in difference to having wheelspin. In common terms say a 3.9 front and a 4.1 rear ratio and in DPCars case where he is using chains and sprockets, possibly closer.

Do you get your sprag clutches from Czechoslovakia by chance?


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 Post subject: Re: AWD Locost?
PostPosted: July 21, 2011, 4:39 am 
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cheapracer wrote:

You may want to study up on the Granddaddy of them all, the Ferguson Factor Jenson FF.

Yup, I already have...
And the Jensen/Ferguson system uses an epicyclic torque splitter, my preferred system.


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