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  #1 (permalink)  
Old 04-16-2002, 07:51 AM
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Question Limited Slip or Posi-Trac or Trac-Lok or Detroit Locker or...

I thought that I was knowledgeable in this regard. I thought that limited slip meant that more power would be transferred to the tire with the least amount of traction (would only leave one black stripe during a burnout), but my conversation with someone this weekend clouded that theory.

My old Galaxie would burn rubber to the county line and back, but only with one tire. Was that not a limited slip diff? I thought it was. Maybe I was wrong all this time.

I wanna be able to put as much power to the road as possible, so having both tires working their hardest should be important to me, right? And if I lay drag in some Holiday Inn parking lot (ahem) I don't wanna leave only one stripe, do I?

What are the differences between the differentials mentioned in the title. How do they work differently? Are some differentials known by more than one name (i.e. is Limited Slip and Trac-Lok the same thing?) Which applications are better suited for 99% street use? I understand (or think) that the Detroit Locker diffy will actually lock both sides to work together, which can cause problems in turns. Is this "locking" feature something that can be turned on and off with some differentials?

Thanks guys.

Keith
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Old 04-16-2002, 08:33 AM
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Default Let's uncloud this

This is a subject that most people don't understand. Your old car would leave a 2-mile stripe because it had an open rearend. Open means that there isn't anything valuable except for spider gears in the differential case. When one starts spinning, the spider gears turn (in the same way as they would if you were making a hard left or right-hand turn), and you transfer all the power to one tire. Very bad...especially on slick surfaces such as rain or snow...or when you're racing.

There are many rearend names out there. Limited slip and positraction rearends are essentially the same in their function. Limited slip rearends usually are viscous coupled...i.e. they act like a torque converter. If one tire spins, it creates a viscous situation which in turn applies a moment force to the other side...causing it to turn also. As you would think, the amount of traction to both tires is directly related to ring gear or axle RPM.

Positraction units usually have mechanical locking units...Most of them use clutch packs. In the same way, they lock up due to mechanical friction instead of fluid friction. The more one side starts to spin, the more friction it creates between the clutches, and it all hooks up, causing both tires to spin at the same rate. You can vary this clutch friction...causing the clutches to hookup immediately, or have some slip to them. You can set them up so tight that you would have trouble turning.

Locking rearends actually have some sort of locking teeth that pop in and out of each other. When turning, they disengage each other allowing the tires to turn differentially. When going in a straight line, they will lockup, transferring 100% power to both tires. They are very noisy however. They pop and crack and shutter when they disengage/engage. They are also bad for most types of road racing, because you would be in an open-rearend situation (spinning one tire) at the apex of a curve...then it would immediately lockup without warning, and you could get some power-on oversteer.

Detroit Lockers are of this type...and there's also a locker that you can engage/disengage with an air pump controlled in the cockpit.

Most other diffs (Auburn, Trak-Lok, etc.) are positraction units and have clutch packs in them.

There are also "spools" available, which pretty much connect each axle to each other...allowing NO slip at all between them. Good: you get power to both tires 100% of the time. Bad: It's very hard to turn.

Hope this clears some fog. It is a pretty messy topic.

Brent
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Old 04-16-2002, 08:38 AM
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I also wanted to note that it's called "limited-slip" because there is a limited amount of slippage before both tires pull in unison. Most high-performance AWD cars (Porsche 911, Lambo, etc.) have limited slip diff's between the rear and front axle. If the back tires spin at a high rate, this is either sensed with a fluid or more than likely with a computer sensor, and it will lock up causing the front tires to pull with the rear tires.
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Old 04-16-2002, 08:41 AM
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Excellent descriptions! This website rocks!

Keith
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Old 04-16-2002, 02:38 PM
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blykins,

Thanks for the info. Clears things up for me as well.

Regards,

John
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Old 04-16-2002, 03:00 PM
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So I can get a Ford 9in with limited slip/positrac?

GS
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Old 04-16-2002, 05:18 PM
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Arrow Limited (Knowledge)

I had a rear end blow out last summer. My unit was a 1974 Bronco Ford 9" open rear end. That meant I could smoke one tire whilst the other held fast and kept me on the road. So, lotsa noise, smoke, and control.

Fast forward... now I have the full meal deal with a brand of traction control known as Detroit "Tru Trac". It is advanced from the standpoint that it contains no clutches or the like. So reliability and maintenance are not an issue. (Want the story? visit: Awesome Differential Support )

Anyway... the car is way different now. Alot more to handle. Kinda scary actually. But, the rear will never be an issue again.

Hope this helps the topic string.

Zderf
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Old 04-16-2002, 05:40 PM
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Gotsnake;

Most definitely.....Let's see,I have a 65 fastback with a 1976 Versailles 9" rear with a limited slip/posi-trac unit out of my dad's 1983 Ford F-150. All 9" rear center sections should be the same from about early 60's on,the only difference you may encounter is axle splines....Ford made two sizes for the 9" rear,28 and 31 splines and there are of a different diameter. Just make sure your axle gears have the same spline size as you axles....

Good place to look is in the junkyards for a Ford 4x4 from 1970 or so till around the late 80's,note quite sure what year they did away with the 9" and started with the 8.8.....The rear tag will read something like this-----3L50---the three five zero would be for the gear ratio and the L is for limited slip....If it does not have the L then it is not a limited slip/posi trac unit....

BTW,the "stock" limited slip/posi trac unit works very well........ and is easy to set-up and rebuild if needed....

David
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Old 04-16-2002, 06:14 PM
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Default Ford 9"

Guys if your using, or plan to use, a Ford 9" rear you've chosen one of the best rears around for strength.

HOWEVER, If you plan on drag racing with a Ford 9" with Limited slip (I think Ford calls it Trac-Loc) you're going to toast the clutches.

What destroys this rear is that its not like a GM or Chysler "Posi" unit in that it only has a few clutches. I found this out when I was heating my slicks and one of my friends told me that the drivers side tire simply STOPPED in the middle of the burn out. I called one of the really good 9" builders in the Atlanta area and told him what happened and he said "Drag racing, huh. HEATED the tires and clutches went away, huh"...

I brought the rear over to him and the NEW clutches were toast. He said in a Ford 9" rear you either use a "locker" or a spool. I asked him about the PowerTrax unit and he said he had never installed one. So I bought the unit and a new ring and pinion (changing ratios). We put the heavier Daytona Pinion support and bearing in it (figured why not we've got it out) and he told me that the Powertrax looked really nice and asked me to let him know how it worked out. It worked out great. Car handles fine and has NO CLUTCHES to burn out. Both tires heat up correctly and my "mini motor" powered Cobra has been running steady high 10's with it. And think how much easier my little 274" 6 cylinder motor is on the rear compared to some "big iron" motors.

I talked to the Powertrax folks and they told me that it takes around 50 miles to break in and from then on it should be bullet proof.

Hope this helps and, if your drag race your Cobra, Don't use the Ford center section in your 9".

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  #10 (permalink)  
Old 04-16-2002, 07:31 PM
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Default Quirky 8.8

I have the 8.8 with the trac-loc. After about 12,000 miles of abusive driving, it still works like day one. But it has always had a quirk. Maybe they all do.?
While coasting or decellerating, the rear is fully unlocked. But give it ANY gas, and I mean ANY, and it locks up like a spool. I can drive like an old man turning out of a driveway, and if I give it normal gas while turning out, it will drag the outer wheel squalling around the corner. Not accelerating, just holding speed with throttle.
On the track, the car pushes hard in the corners if any gas is held on the turn. Go from a little gas in the turn to neutral gas, and the car turns in hard. And I don't mean hard on and hard off the gas oversteer, understeer. I mean a small change in gas and I can feel the rear unlock and the front tires do their job.
Do all trac-locs perform this way? I don't remember the old GM posi's acting this way.
How does the tru-trac (no clutches) handle on the track?

Bob
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Old 04-16-2002, 07:52 PM
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I know the Trac-Loc in my father's '97 Mustang Cobra is mashed together pretty tight all the time. It probably varies from car to car...there are actually springs and shims that keep a preload on the clutch packs. I know when backing out of a gravel driveway, you can just let the clutch out quickly and it will spin both tires. There has to be a certain amount of preload for the clutches to be tight...and there has to be a certain amount of tolerance there so you can actually turn a curve without bouncing the hindend around.

From your description, it sounds like it may be quirky. A Trac-Loc diff doesn't really "lock." It just has constant pressure between the clutch discs inside. The faster you spin the diff (i.e. acceleration) the more you get centrifugal motion pushing the clutches together. Of course this makes them even more tighter. But they should always have a certain amount of preload. And of course in a curve, the clutches are gonna slip a little, or you'd have trouble going around the curve. As for your turning in when you let out of the gas, that's about normal for most cars. Anytime you're in the gas, weight is transferred to the back tires...causing you to have to input a little more steering than usual. As soon as you let out of it, the weight falls back on the front of the car, putting more emphasis on the front tires, causing them to bite a little harder and turn you in quicker. That's why 911 drivers used to have so much trouble with trailing-throttle oversteer...all that weight is shifted to the front tires when they lift...causing a severe turn-in.

What kind of caught my attention is that you said you could "feel" the diff locking and unlocking. That seems a little out of the ordinary. Usually you can't feel them at all like say a Detroit Locker... Is it noisy?

Hopefully I've helped answer a few of your questions. I may have misunderstood you about the turn-in though. I've been known to do that from time to time. : )

BTW, are you using a friction modifier in your rearend grease? There has to be an additive in there to give the clutches a little friction...else they would slip a lot.

Brent
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Old 04-19-2002, 03:15 AM
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There are a host of options on the market at present. My understanding is that a clutch type limited slip is inclined to generate clutch debris which can cause problems. Especially as allready mentioned with less plates and higher wear concentration. The traction lok is available as a full lock off road type package or a more streetable version like your looking for. The plus with these is the clutch arrangement is a dog type that is made with a high tech material (one of the "ium's") that gets by the debris/wear problems alot better. I'm looking at installing one of these in a Jag (Dana 44) pumpkin. I'll be interested to know what you come up with and how it works.
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Old 04-19-2002, 08:09 AM
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Lightbulb The above cautions on 9 inch posi diffs is why ....

I went with a Strange Engineering (aluminum alloy housing version) "Street Pro" center section (diff) and the "Street Pro" 33 spline axles as well. Both of these components are made for very hard street use, drag racing AND road racing use.
We are using the 9 inch housing out of, I believe, a 1986 T-Bird, painted in black epoxy.

I don't see where a guy has much to lose, especially if he's on a budget, to find a low mileage good condition 9 inch T-Bird (or other) rear, use it till it smokes, then convert to the Strange Engineering stuff. DO be aware that the T-Bird rear is too wide for MOST cobras, the West Coast cobra is a bit wider (like 4 inches)

You can order Strange Engineering gear from www.jegs.com
They also have LOTS of other goodies to drool over !
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Old 04-19-2002, 08:28 AM
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now in the offroad work some peolpe usa an Air locker its an open diff when turned off but once you throw trhe switch it locks up but good. and use for those in a "cobra" type car?? or only for us die hard offroaders???
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Old 04-19-2002, 09:42 AM
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Great analysis, Brent. Glad to have your expertise on the forum. Very enlightening post.
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