poor mans dccd, the 5mt center diff locker thread

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Alphius
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Re: poor mans dccd, the 5mt center diff locker thread

Post by Alphius » Wed Sep 03, 2014 4:23 am

My first post however was not entirely correct:

A limited slip differential of any type basically transfers torque. If you apply enough torque to slip one side of a differential, the LSD will transfer torque to the side that isn't slipping. The method varies by type of differential.

Returning to our example car, front tires on ice and rears on pavement, if you add an LSD center diff it will be able to transfer some amount of torque to the tires which are not spinning. Say your center diff can transfer 20 ft-lb before slipping. You apply torque and the center diff sends 20ft-lb of the torque going to the front wheels to the rear wheels. In this situation, if the front tires slip at 10ft-lb of torque, the rear tires will have 20ft-lb of torque transferred to them by the center differential. This gives you a variable torque split that is defined by the available traction at each wheel. In this example, you will be pushing against the ground with the front wheels with 10ft-lb of force, but the rear wheels will push against the ground with 40ft-lb of force and hopefully the car will still move forward. This example leads to a 25:75 torque distribution.

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Re: poor mans dccd, the 5mt center diff locker thread

Post by Alphius » Wed Sep 03, 2014 4:26 am

To make it even shorter:

An open diff always applies the same torque to each output. It takes very little torque to spin a tire freely in the air, so an equal amount of torque (very little) is applied to the remaining wheel and the car does not move.

A limited slip diff can take some multiple of the torque delivered to the spinning wheel and apply it to the wheel with traction. Very little torque is used to spin the freewheeling wheel, however if you multiply that very little torque by some factor and apply it to the remaining wheel you may have enough torque to still move the car forward.

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Re: poor mans dccd, the 5mt center diff locker thread

Post by legsportwag » Sat Sep 06, 2014 2:15 am

Well said alphius. Thank you for clairifying for everyone.

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Re: poor mans dccd, the 5mt center diff locker thread

Post by James614 » Sat Sep 06, 2014 10:28 pm

We could have a week long discussion about differentials and the dynamics of torque at the wheels. But I think you summarized it very well.
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Re: poor mans dccd, the 5mt center diff locker thread

Post by cj91legss » Sat Sep 06, 2014 10:38 pm

I think we should, I'd like to understand it more. After I get things more squared away with the vehicles I'm going to read the link Alphius posted.
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Re: poor mans dccd, the 5mt center diff locker thread

Post by czei » Mon Sep 08, 2014 7:33 pm

Alphius wrote:Read the link for a better description. Here's a short version:

To understand the torque split, you have to understand Newton's third law of motion: every action has an equal and opposite reaction. To understand this as it applies to traction, think of how the wheels move the car. The wheels push against the ground with a force equal to the torque the engine is producing to move the car forward. Assume our Subaru has an open center diff with a torque split of 50/50. The front and rear wheels will each push against the ground equally with exactly 50% of the torque. On dry pavement, it is likely that the engine cannot produce enough torque to overcome the wheels friction with the ground, and the car moves forward. If the engine produces 300ft-lb of torque, the front wheels and rear wheels each apply 150ft-lb of torque to the ground and the car moves forward.

Now let's assume we have our open center diff Subaru but with one important difference: the front wheels are on ice and the rear wheels are on pavement. If you apply torque, the front wheels will spin and the car will not move. In this situation, the max torque that can be applied to the ground is limited by the least amount of traction available to any wheel. So, if it takes 10ft-lb of torque to spin the front tires on the ice, your open 50/50 diff will still transfer 50% of that torque to the rear wheels. In this situation, 50% of the torque is 5ft-lb. The center diff applies that torque to the rear driveshaft, but it's not enough torque to overcome inertia and move the car forward. In this example, the center diff is still applying 50% of the torque to the front and 50% to the rear, but the maximum torque applied to the car in total is limited by the traction of the front wheels.

Make sense?
If this is true, then if the front or rear of the car were jacked up, in the absence of any brakes the car would be unable to move?

How would the behavior change if the center diff were new vs. open?

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Re: poor mans dccd, the 5mt center diff locker thread

Post by Alphius » Thu Sep 11, 2014 8:48 pm

Yes, you've got it. If you take an open center diff car and lift the front wheels off the ground it will apply 0 torque to the rear wheels and the car will not move. 50% of 0 applied torque at the front wheels is 0 applied torque at the rear wheels.

If the center diff was a limited slip it would be able to transfer torque to the rear wheels at a specific rate. In the case of the stock center diff that's 4kgf-m/100rpm rotational difference.

When the VLSD gets weak it starts to transfer less and less torque. There is an empirical way to measure the remaining effectiveness of the center diff by measuring the force required to turn it at a certain rate. The other failure mode for the VLSD is to lock solid and that gives you torque bind.

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Re: poor mans dccd, the 5mt center diff locker thread

Post by legsportwag » Sat Sep 20, 2014 4:36 am

Found one more pic.
Image

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Re: poor mans dccd, the 5mt center diff locker thread

Post by legsportwag » Mon Sep 29, 2014 6:29 am

Rob 550 tune dropping the hammer on the diff locker ;-) enjoy!
Image

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Re: poor mans dccd, the 5mt center diff locker thread

Post by James614 » Mon Sep 29, 2014 6:39 am

That is awesome! You need to get to a track and see what your 60' times are like with that thing.

On mine with the stock suspension, the front wheels unload under anything remotely hinting at a hard launch, and the viscous diff likes to throw it's hands in the air like just doesn't care and it will smoke the front tires with barely any perceptible transfer to the rear.

Which I find odd because I thought the viscous diffs transferred a certain amount of torque per 400 rpm, which to me sounds like it should transfer more torque as RPM differential increases. But what I've experienced seems to be that it work fine under low RPM differential, and tends to just give up if you really get on it. I dunno, maybe my center diff is just old and tired,although the car it came from was in awesome shape without a ton of miles.
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Re: poor mans dccd, the 5mt center diff locker thread

Post by legsportwag » Mon Sep 29, 2014 6:46 am

That's exactly how viscous centers are.. They get worse with time and usually after about 10 hard launches they are pretty well shot.. I'm loving this diff locker. Lights all 4 up consistently ;-)

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Re: poor mans dccd, the 5mt center diff locker thread

Post by James614 » Mon Sep 29, 2014 6:55 am

Yeah, I'm already budgeting for a 5spd DCCD trans swap (I've got a list of trans codes so I can make sure I get one with the front LSD) with a controller and a Cusco 1.5 way rear LSD. It's way expensive compared to the locker, but I'm an autocrosser so I need something that will only partially lock in the corners :-/
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Re: poor mans dccd, the 5mt center diff locker thread

Post by VenomSS » Wed Oct 22, 2014 10:28 pm

Thought I'd register here and add my experience with the EA centre locker. I did this a few years ago with a RX/II dual range centre. Worked great for about 18months. The problem is anytime an individual or pair of front wheels spin while the differential is locked? That's the locking centre diff carrier spinning in it's housing. It chews it out and it doesn't take long for it to be worn out with lots of play. Locking the centre diff on tarmac? Same thing, the centre diff spins in carrier because it's overcome by the driveline binding forces (excuse my lack of technical terms ;). The diff stays truly locked, it's just the whole thing spins. Hope that makes sense. Add to this having to fabricate the linkages (I had the dual range linkages to do as well), getting a longer custom tail shaft made up (expensive in Australia), and lastly assuming I did find a replacement of one of the rare locking centres there was no gaurantee it hadn't been abused either. I had a freind with two he'd bought to build a gearbox and both were worn out and not worth putting in a new gearbox build.

My conclusion after all that? Don't fuck about and go straight to DCCD. Stock shaft, stock linkages, and designed to be tough enough to be used on tarmac. I'm running my car with the DCCD Pro UNI controller, Cusco front diff and KAAZ rear diff. Awesome setup.

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Re: poor mans dccd, the 5mt center diff locker thread

Post by Legacy777 » Fri Oct 24, 2014 2:08 am

Welcome to the BBS, and thanks for the feedback!
Josh

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Re: poor mans dccd, the 5mt center diff locker thread

Post by James614 » Fri Oct 24, 2014 11:09 am

I don't really get how it spins in the carrier during wheel slip but not normally if its truly locked.
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Re: poor mans dccd, the 5mt center diff locker thread

Post by czei » Wed Mar 25, 2015 12:05 pm

VenomSS wrote:My conclusion after all that? Don't fuck about and go straight to DCCD. Stock shaft, stock linkages, and designed to be tough enough to be used on tarmac. I'm running my car with the DCCD Pro UNI controller, Cusco front diff and KAAZ rear diff. Awesome setup.
I'm familiar with the Cusco-- you just take a phase 1 or 2 transmission to a transmission shop and have them install it. Same with the KAAZ for the rear, although that might be simple enough for a DIY. But what transmission is DCCD capable AND compatible with the 90's Legacy?

Thanks...

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Re: poor mans dccd, the 5mt center diff locker thread

Post by Legacy777 » Wed Mar 25, 2015 5:46 pm

There are some 5spd DCCD transmissions from overseas, but are difficult to get a hold of. The other DCCD trans is the 6spd. The 6spd can be made to fit with a little bit of work.

My 6spd swap details thread goes into the info.

http://bbs.legacycentral.org/viewtopic.php?f=7&t=49917
Josh

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Re: poor mans dccd, the 5mt center diff locker thread

Post by Abeall2 » Thu Apr 16, 2015 8:09 pm

How much power do you think it can handle? & sry I'm a bit of a newb but what years are "RX's"
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Re: poor mans dccd, the 5mt center diff locker thread

Post by Legacy777 » Sun Apr 19, 2015 11:48 pm

The RX's were from the 80's. I don't know the specific years though.
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Re: poor mans dccd, the 5mt center diff locker thread

Post by mike-tracy » Sun Apr 19, 2015 11:50 pm

I've seen prob 10 RX's in total, they are very, very rare at this stage. Only 1 in the junkyard, 1 owned by a friend and the rest at WCSS.
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Josh Colombo wrote:
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Wait....I'm confused now.

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Re: poor mans dccd, the 5mt center diff locker thread

Post by ride_child » Mon Apr 20, 2015 12:01 am

i have an rx for sale! center locking diff dual range transmission!!
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Re: poor mans dccd, the 5mt center diff locker thread

Post by WRXish » Tue Apr 11, 2017 3:47 am

James614 wrote:They probably don't use salt in Montana ;)
Everyone there is on a salt free diet lol

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