final drive and rear differential different?

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final drive and rear differential different?

Postby reaper0995 » Sun Jan 14, 2007 12:42 am

ok, so clear this up for me. as far as i knew, final drive WAS the differential ratio....but the alltrac specs has it listed as a two different numbers...


http://www.alltrac.net/specs/GT4Specs.htm
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Postby SuperWhite92 » Sun Jan 14, 2007 12:53 am

In some cars you are correct. In our cars, however, the Front Diff Ratio will be equal to the combination of the rear and center differentials.
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Postby alltracman78 » Sun Jan 14, 2007 5:18 pm

What you meant was correct.
The gears in the transfer case after the center diff step down the shaft speed for the propeller shaft, and then the gears in the rear diff housing step it back up to the exact same speed for the rear axles.

However, the final drive is NOT part of the front diff.

* I just realized something.
We have ALL been wrong on the trans......

The front AND center diff are housed in the same unit.
Inside the trans. NOT the transfer case. And BOTH are OPEN diffs.

The control coupling, which is what allows the center diff to act as a LSD, is what is in the transfer case.

Regardless, the final drive is the ring gear that is bolted to the diff case that houses both the center and front diffs.

The gear stepping down and up is just as I have described it.*
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Postby 2RM » Sun Jan 14, 2007 10:44 pm

So does it mean that if we upgrade the rear diff to an RC unit we have to replace the trance accordingly? :shrug:
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Postby SuperWhite92 » Sun Jan 14, 2007 11:57 pm

2RM wrote:So does it mean that if we upgrade the rear diff to an RC unit we have to replace the trance accordingly? :shrug:


No, all the rear diffs and transmissions are compatible with one another.
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Postby alltracman78 » Mon Jan 15, 2007 1:29 am

alltracman78 wrote:The gears in the transfer case after the center diff step down the shaft speed for the propeller shaft, and then the gears in the rear diff housing step it back up to the exact same speed for the rear axles.


The transfer case ratio is the same for both trans.
There are only 3 ratios that are different.
1st gear
2nd gear
final drive/oil pump gears.

Everything else is identical.
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Postby RedCelicaTRD » Mon Jan 15, 2007 2:03 am

And has anyone noticed, assuming the info from the main page is correct, that the RC trannys have the same first and second gear ratios as the st165 trans? So if someone could get a 92-93 st185 trans and a st165 trans they could basically make thier own RC trans.
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Postby coyoteboy » Wed Jan 17, 2007 10:47 am

AFAIK the rear diff ratio is different to the front end as they use the difference to form a lock on the viscous coupling, if they were the same the VC would take too long to lock up. Its only slightly different but it is different.
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Postby RedCelicaTRD » Wed Jan 17, 2007 3:42 pm

The ratios are the same. The viscous will lock when you get wheel speed differences front to rear.
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Postby coyoteboy » Wed Jan 17, 2007 4:26 pm

No, I realise thats how it works in other systems, Toyota supposedly set up the GT4 with a dissimilar ratio to maintain a constant lock. Think of it like this - the time taken to lock the centre diff is determined by the speed differential in the VC as you know - in order to lock the coupling there has to be a speed difference or its FWD until slip. This is not ideal. Watch any car with that setup at the lights in the wet and you'll see front slip for a split second then the rears take up. Watch a GT4 and you dont get that. This is also partly why we have crap drivetrain losses in comparison with other 4wd cars.

I'll see if i can find the source of the info.

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Postby Conan » Wed Jan 17, 2007 6:55 pm

coyoteboy wrote:Toyota supposedly set up the GT4 with a dissimilar ratio to maintain a constant lock.

That would kill the viscous diff after two or three trips to Burger King :D If they wanted a constant lock, there would be a locked diff, not a VLSD ;)
coyoteboy wrote:This is also partly why we have crap drivetrain losses in comparison with other 4wd cars.

crap = little loss or big loss :shrug: try comparing it to the obvious impreza wrx, which has a viscous both in the middle and in the rear...
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Postby coyoteboy » Wed Jan 17, 2007 9:53 pm

Well I'd assume they'd design in the expectation of continous locked operation, besides I said SLIGHTLY different ratio so its more likely to be partially locked. Always seemed an odd option to me but I'm just repeating what I read. Also you do never see the front wheels slipping before the rears on our car which you do on others.

Crap=high losses, we're known to be around 25% when plently of others, from what I've heard on dyno days, are around 20%. Which is a hefty amount at 200hp.

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Postby Conan » Thu Jan 18, 2007 5:03 am

coyoteboy wrote:Well I'd assume they'd design in the expectation of continous locked operation, besides I said SLIGHTLY different ratio so its more likely to be partially locked.

I'm saying the constant load on the viscous will make it blow up.
coyoteboy wrote:Always seemed an odd option to me but I'm just repeating what I read.

If you could find the source, I would be very interested to read it myself ;)
coyoteboy wrote:Also you do never see the front wheels slipping before the rears on our car which you do on others.

What about the fact that ST185 has over 60% of its weight on the front wheels?
coyoteboy wrote:Crap=high losses, we're known to be around 25% when plently of others, from what I've heard on dyno days, are around 20%. Which is a hefty amount at 200hp.

It depends on the dyno really. I'm holding up to the opinion the Four is loosing about 1/3 of it's power just as the wrx.
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Postby SuperWhite92 » Thu Jan 18, 2007 5:23 am

Actually, I've found that under hard launches, the front tires do spin for a fraction of a second before the rears kick in. Drag launching is like Drop the clutch... torquesteer.... and go (or bog)
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Postby coyoteboy » Thu Jan 18, 2007 10:23 am

Conan wrote:I'm saying the constant load on the viscous will make it blow up.

And I was saying not if it was designed for that load, which is perfectly possible.
If you could find the source, I would be very interested to read it myself ;)

Will keep looking. The reason it struck me as possibly the case is because it wouldnt be the natural assumption and would take some thinking to get to that idea - hence not likely just a simple mistake.
What about the fact that ST185 has over 60% of its weight on the front wheels?

Same with most front engined, 4wd cars and you see them slip the front wheels first.
It depends on the dyno really. I'm holding up to the opinion the Four is loosing about 1/3 of it's power just as the wrx.


It'd be fairly easy to tell with a small amount of error in one way. Run an engine-dyno test and then a chassis dyno test on the same engine in a car.Its just that no-one ever bothers doing an engine dyno test in these circles. I'm sure someone out there must have.
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