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Differentials - Posi, Spool, Locker, ?

one finger john

Active Member
Gentlemen, have differential types been covered before? If not, does any one diff have an advantage over another? Does one produce "evil" handling characteristics more than another? And really, what is "evil"?

Still thinking, John
In my business we use Lockers more than Spools more than Posi.

The lockers are great because in the front it will allow you to turn by slipping as one wheel goes faster than the other. Same applies for the rear. If we do put a Spool in then 99% of the time it is in the rear because it doesn't effect us much off road. With a spool the rear wheels will want to push the front tires forward more than letting it turn. You can still turn but and on a 4x4 or car you wouldn't notice it as much as you would on say a go cart. Just be aware that those forces are there. Posi units are less used because for what we do they are junk and the performance sucks. Now on a race car or street car it wouldn't be that bad and would perform really well I would think. Lockers seem to cost more than spool, Unless you have a dana 70 like I have, and the posi's being the less costly.
Evil is when you visit Mr Guardrail while racing at the dragstrip. :eek:

As my bucket is a street/strip toy, I started out with a DPI Black Gold diff. I was told it was a "modern interpretation" of a LSD and would be much nicer on the street, and very effective on the strip.

It was fine on the street, but had a bad habit of turning me right on the strip. And Mr Guardrail is evil. So we fitted a traditional Ford LSD and then proceeded to blow out the spiders with street driving. So we fitted that head with a minispool for the strip. Car goes straight now.

I dont actually know if you need an LSD in a street driven bucket, but I have the DPI so Im using it.

And I have driven maybe 100 miles on the street with the spool and its a waste of time.
I've never been a fan of any posi unit in a short wheelbase, light rod. They can tend to drive you straight ahead on a turn. I run a one wheeler in my 27 and in my 23, and when I light em up both tires leave streaks.

My Kid is using a TracLok in the 30 he is building and that should be ok because it weighs more and is a longer wheelbase (112") But it makes a racheting sound when we push it around the shop in turns, and we hope that is because we haven't added Fords additive or any gear lube yet.

spools dont act to good on the street...

just as todd says, they try to go straight when you turn, they wear tires quick, and can be hard on axles. spools can be used on the street, i have one in my shortbed chevy, its heavy and i have to work to turn it. for straight line racing they work great.

a locker, cog type units are also awkward on the street, i had one years ago in another chevy truck i had, the clunking when it locked and unlocked was annoying as heck.. for straight line racing they work great until something breaks, i broke 2 in my old truck years ago.

i had a 8.8 in my 84 mustang, its a clutch type limited slip, i rebuilt it and added an extra clutch in it effectively tighening it up, and it worked real well.
i also had a limited slip in prior to my 9inch in my current shortbed truck that is narrowed, and something was wrong with it, and it was dangerous to drive, if the tires broke loose it would point you to the ditch, and it pushed left and right if you did a burnout, it probably needed work but i dont know, had a guy tell me it was more than likely a 60/40 limited slip, and when it broke loose it was engaging and disengaging which was pointing me in different directions..

I myself on the street like the eaton units, and the auburn units. ive had good luck with both..

adding this... i'm with donsrods ...with my roadster, i have 2 rearends for my roadster, ones the frankland quickchange with a spool, the other is a 9inch with a 3.70 gear out of an old style bronco, more than likely a locker unit, and i'm kind of apprehensive to use either, i would just assume to use a single trac in a T, it would be more driver friendly.
I am using an 4:10 Eaton unit in an S-10 rear end assembly. My wheelbase is 108 1/2" and the weight is 943.5lbs. in front and 993.5lbs. in the rear for a total weight of 1937lbs.. With the adjustable four link, it launches very well and remains straight. It corners very nicely also.

Standard posis usually only lock with the hammer down but you can get a spider shaft angled so as the rear locks to some degree on a trailing pedal too. If it is half as prone to lock trailing as it is accelerating, say, this is what is referred to as a 1:.5 posi. Having a trailing throttle lock up tends to load the outside front a bit more. On the street in your T it gives you all the undesirable traits of a spool or a locker, initial push followed by loose soon as you touch the gas.
This is the essential difference, a healthy posi/limited slip in street trim only locks with the hammer down and the lockup tends to be comparatively gradual. Part throttle they slip-ish so you can do a u turn without juddering clacking or tyre chirps.
A posi relys on clutch friction to start the locking process and a small amount of wear will spoil the action. If you love to burn rubber a posi will go away after a while as the clutches wear and once they start slipping thats toolbox time.
I'm using an Auburn cone-type limited slip unit out of a Camaro in mine.

The car seems to hook up good and (so far) hasn't proven to be squirrely.

I recently had to drive about 20 miles in a pretty hard rain at Knoxville and had no problems.

I have noticed a push when you go hard into a tight corner, but that could be the tires. I haven't noticed any tendency to get loose if you get off the gas in a turn.

When I installed this Chrysler rearend, it came with a cone locker, it used to scare the pants of people in the intersections when I turned corners and then got back on the gas, hated that!! and a light car does not need a posi, as far as I can see/feel or want.. tried to find an open third member, each was a locker, no luck at all there... Next will be an open leg for me, so much smoother operating, and I am not drag racing anymore in my street T Roadster... Took soo many years to learn, the two do not mix well, too much temptation... :D
Todd and Brucer pretty well covered it......Ted is also spot on also......its hard to quit racing...................I found out here's a little insite on rear systems.....
1.Spools for racing....tries to push and not turn on street cars......
2.Posi's are good for light trucks and milder street cars........when one wheel spins the other operates to get you outta the hole(in the street), or outta the mud hole(if your off road).
3.Locker is just what the name implies....think of it as a heavy duty version of a limited slip (posi). A detroit locker can bark the tires when going around a corner....also 'crab'.
4.Open........good single tire burnouts............can pull up on a snail going around a corner and total your car out

I believe....and this is my opinion, that a well built posi unit can give you the best of all performance it built correctly. They have HD clutches and replacement springs for these things...but since I've not yet completed my bucket yet......I might regrete it....but I'm gonna run a S-10 like a few others are running here. I was gonna go the 9 in. ford route.........but I believe it'd be extreme overkill......because of the weight of the little cars........just don't need it.

After much research....I'm gonna build a 4-link airbag type setup with a on board compressor....................:eek:

Now all you guys that are running these things for years....something must be going right.

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