Ernie's diff....iculties

It depends a bit on the configuration inside.

Looking at your pic, it appears you have the two abrasive discs fitted next to each other.... this means they effectively cancel out two of the 4 friction surfaces (each side). You can fit the plates alternately... so...

cup, abrasive plate, cross plate, abrasive plate, cross plate.

This will also tighten the static slip. OR, if it was as I just wrote it, you can swap it as I "think" it was, to loosen it.... not that you'd want to.

And you can do one side of the diff one way, and the other differently... whatever it takes to get your desired static slip torque.

When new, the abrasive plates are doubly grippy as a used one, so ZF set the diffs up with new parts to whatever the spec is, but of course, as it wears, it loosens.

If I build a diff with new abrasive plates, I always set it tight, and try to strip, clean, reassemble after 2-4 hours if it's a race car.

To shim it tighter, you must NOT have your home made shim facing onto an abrasive plate, so if the configuration won't allow this, you need to use thicker crossplates or abrasive plates... which are available from Modena Engineering in Melbourne (Frank Capo). (I'm sure he sells via a US agent, but I don't know who.)

Up the road, or on track, the static slip is not the difinitive adjustment to traction... the ramp angles of the cups do this, but the static setting is the common factor that determines total slip when factored into the equation.

This area of set up tuning is wholly time consuming and not an area commonly explored. But a well set up diff can make the difference.... especially in one make racing series (Challenge cars/Porsche cup etc).

Incidentally, look VERY closely at the cup ramp angles, and the corresponding angles on the spider shaft tips. These can be assembled incorrectly and the diff will not work correctly.

 

Thanks again for the info Phil.

I was talking with billybob exactly about this when we had it appart. Just like you said, the discs only are making contact with two sides instead of four. Both sides of the discs are abrasive so alternanting them as you said is totally possible. My only question, and what billybob and I talked about, was will it cause excessive wear on the sides that are not in contact with the cross plates? The side that would be in contact with the back of the cup? Since we weren't sure I put the dics back the same way they came out, and as illistrated in the exploded part diagram.

Also I'm thinking that if we only alternate the plates on one side, and leave the other side with both plates on the inside, won't that only increase the grip on the side that has the plates alternated? I'm thinking it will, and then possibley cause more traction on the one side and not the other, causing unbalanced transfer of power and possibley screw up the traction. So if I do this, I will have to do both sides the same way won't I?

Yeah I ran into this, LOL. I had the small removable gears, that go on the top (3rd pic down in post #4 http://ferrarichat.com/forum/showpost.php?p=135740888&postcount=4 ) on the cross brackets the wrong way and they wouldn't mesh properly with the ones that were on the correct way (2nd pic down on post #4). So they can only go on one way. Also the pics were taken with them on the correct way.

 

1) No, the cups are hardened like the cross plates.

2) (your other "Only" question ) No again. The pressure on the plates will remain uniform.

 

LOL smartass

Okay so now it looks like I'll be taking the diff out again. Yippy. Do you think should I alternate only one side or both?

 

Just measure your staic slip and see what you got, then decide.

You can measure it in situ BTW.... Just use the torque wrench across the axle.... jam a screw driver in the brake disc on one side.

 

Will do.

Well since the tranny is out of the car I guess I'll put the axle flanges on, lock one side down, and then test it that way. I'm looking to get 50 lb/ft right? I ask because it says absolutely nothing about the slippage in the book.

 

Yeah, i'd go for 50 lb/ft. std is about ~35 lb/ft.

 

Most excellent! Thanks for your insights into another "black" art. I will be playing the game and see what I get.

 

Okay Phil,

Today we tested the preload on the differantial, and came up with zilch. We put the axle flang on one side and then locked it down, and turned the other side. Well guess what, I could turn it by hand. So thinking I did something wrong we locked the other side down, and again were able to turn it by hand. Then we tried it on Billybob's car, which we know works because he races it. We locked down one side lifted the other and the same deal, we could turn it by hand, no real force needed. Then we figured that maybe we needed to lock the big gear on the diff. So we put it in 1st and then tried it. This time it would not budge. We got the torque up over 120 lb/ft and it would not move at all. Took it out of gear, and again could move it by hand again.

So what gives?

 

Dunno.

The 348 I had in recently felt fine and normal, the only Ferrari that has an open diff statically is 246 and early Dino GT4.

Maybe both your cars have worn diffs?

PAP, or anyone else care to jack up one side only, and measure the torque required to turn one rear wheel only..... You can use a torque wrench on the big 36mm hub nut....

edit... Not "only Ferrari" of course, just popular model era... ~1970 and newer...

 

Yup. From the sounds of it neither car has a working positraction.

Car in "N", hold one wheel, the other should turn only with great difficulty.

 

Hey Brian, can our positraction be impacted by the type of gear oil being used??

 

Yes but not to that degree. There needs to be some preload on the plates for proper operation and that preload WILL be felt regardless of lube type.

The lube type will affect it's dynamic operation without question.

 

Thanks. That's good to know. So *some* small impact on static load (and obvious impacts on dynamic operation).

 

Some oils (or bad diffs) cause the diff to chatter or rattle when you perform this test, or dirve around in small circles.

 

I went around my bros and jacked up the RH side of my car. I put the car in N,and i could spin the hub by hand. Oops

 

Me too. I just jacked up the right side a bit with blocks around the left rear wheel, put the car into Neutral and it took just a bit more than 40 pounds of pressure (guessing) to move the elevated right wheel for my '94 348 spider.

 

You too No Doubt?? FF......Rifledriver...........whats goin on here?? Do we all have worn diff centres???? Hmmmmmm........

 

Unless we are checking it incorrectly? Or.........its the Redline oil we are using? Oh oh...........

 

Man, I hope not. Vern would **never** let me live it down! He'd probably put some "superlight shockproof No Doubt" tagline on every post he made from here on out if my gear oil recommendation turned out to jag off our positraction!

 

Did you guys hold the other wheel like Brian said?

 

Yes. With blocks. With the left wheel on the ground. It didn't budge, but the other right rear wheel was elevated and did move by hand.

 

I actually done it FF's way first,with the LH wheel on the ground and the RH wheel jacked up off the ground. And it spun freely. I then jacked both wheels off the ground and got my brother to hold the LH wheel as i still spun the RH wheel freely. I didnt worry about swapping sides because it would make no difference. Hmmm.......not good.....not good at all me thinks

 

Well, it's got me snookered.

No doubt said it took him ~40 pounds of effort... well over a wheel diameter gripping by the tyre, thats about 40lb/ft VERY crudely... so his would seem in the ball park.

Even at 50 lb/ft static slip, it can be turned by hand via the road wheel, but not "Freely", it'll take a little effort.

Pull out the wheel cap badge, and use a socket on the 36mm hub nut to measure the torque required.....

Try a few other models too... 308/328/512/Trossa/550/355/360 whatever.... all very similar.

"Maybe" some model 348 have an open diff with reactive lock up only... I havent seen or heard of it though. And as I said, the one I had in recently had a preloaded diff for sure.

 

Well then what's going on, because that's four cars that had the same thing happen? Maybe they do have an open diff?