mountainbiker2

No. On the axle nut. I was just saying you don't need a axle nut for the test results. Of course, you need one to conduct the test. I did the test just like your video. No parking brake was on. If you ask me, I think your diff. is bad.

In my other Corvette, the break away point was much higher. Like 120 or something. I added two bottles of additive to help bring it down. All this locking up, causes understeer. I believe. I also tested this Corvette in neutral. It's the only way.

Steve

froggy47

Well, I dunno, not quite ready to concede that it's bad. When I do the test (in gear) after the torque wrench clicks (or in the case when I used the breaker bar) I could absolutely feel something (the clutches) slipping but under pressure so as it stopped as soon as I let off pressure & would be stiff until I put enough pressure back on it so that it slipped again.

If mine "freewheels" both directions in neutral - also in the video - and if you are right, it's shot, how can I possibly be getting good results at the champs? Would seem to me if the clutches are that totally worn out I'd be spinning the unweighted wheel every corner - which is not the case.

froggy47

Wish Ernie would ck back in (not just cause he agrees with me) about why you guys get a torque reading in neutral.

Joshboody

Ok, this is what I'm thinking. 2 legs have to move, either 2 wheels or wheel and prop shaft. BUT potentially you need some resistance to put pressure on the clutches. In neutral you may have an open diff cause no resistance. In gear you maybe able to put pressure on clutches, but you'd be turning the engine also and wouldn't be able to determine torque for engine and torque for clutches.

Now, if you applied torque to both sides (both wheels up) you could determine force to turn the engine. Then you can check 1 side and if torque is higher, then you can conclude this is the force required to overcome clutches. If its lower then cannot measure clutch force. Could try higher gear in this case... starting in 6th would prob be better.

Anyway, this hurt my brain and could be wrong.

EDIT: but if diff does NOT need resistance to put pressure on clutches, then neutral should be ok and proper way to check... this would mean Froggy has bad diff though.

Another EDIT: Potentially the clutch pressure can only be applied by the prop shaft (diff carrier)... in this case we CANNOT check clutch pack by this method of turning a wheel.

aaronc7

Came across this thread and made me curious about my own car. I don't have anything useful to contribute other than possible another data point. 2003 Z06 w/ 42k miles. Turns out I don't have a socket quite big enough to get actual data....but in gear, of course the thing won't budge. In neutral, it takes some effort but I am able to get it to spin by using both hands pushing/pulling on the outside part of the spokes.... hard to guesstimate here but maybe 50 ft-lbs breakaway? There is definitely some resistance in neutral though.

Solofast

Well, not really. Even with an open differential you won't spin the inside tire all that much because the Corvette is pretty wide and unless you have the rear tire really unloaded (pulling big g's when you are accelerating at the same time) it won't do the one wheeler peeler thing. You would be surprised how hard it is to unload the inside tire and then get it to spin. You also have to be HARD on the gas to do it, at or before the apex, and if you are unwinding the steering at the same time at all, that will quickly plant the inside tire. Maybe you aren't getting on the gas early enough... Just a thought...

More often than not in these cars you notice that the rear end is a bit loose under hard braking entering the corner before you get nasty wheel spin on exit. Also autocross cars tend to notice it more than road racers because RR cars are going faster and are in a higher gear than in an autox.. (don't get me into the trap of saying autocrossers drive harder, (they probably do), but the fact that they are in a lower gear does make it harder for the diff to control the higher torque)...

If it picks up enough to spin and then grip when you set it down you've been waiting way too long between freshening up the diff.

As noted above, diff clutches are a wear item, just like a clutches, brake pads and rotors. EXPECT that you will have to freshen it up periodically.... Such is life...

UstaB-GS549

Here is another data point FWIW.

2002 Z06 41,000 miles

In neutral wheel off the ground it took about 75-80 ft lb. to break the wheel loose when using 33 mm socket on axle nut. I started at 50 and went up in 5lb increments. Once it broke away it had very little resistance.

I think you really have to have car in gear.

talon95

I think a diff that turns easily like Froggy's doesn't have any pre-load on the clutches. Maybe a broken spring like someone posted early in this thread?? Since the clutches also lock up further from the engine applying a torque, it could be it still partially works??? I think that's at least possible. I'm not sure if the test done in gear is showing that it's still working partially, or I was originally correct and you're just turning the motor.

BTW, when I found my C5Z diff had lost some of the break away torque, I did try the fluid trick. I initially filled with Redline without any additive, then added just enough to keep it from making noise. That gained about 10 ft-lbs vs. the M1 I had in it originally. Went from about 60 to 70. Just thought I'd throw that in as a data point for anyone looking to nurse their aging diff along.

froggy47

I went thru the 2004 FSM section on rear axle/diff and the trouble shooting they lay out is regarding leaks, noises, chatter, etc. Not a thing about testing diff clutch breakaway which sort of surprised me.

ErnieN85

ok if you guys are done. I got my info and a demonstration of how it is checked from Lou Gilotti's race team
so I'm pretty sure that it is correct
once again with one wheel on the ground and one off and the car drive line locked (mine is an A6 so in park)
you should not be able to put a rorque wrench on a lug or the axel nut and turn it with less than 100 lbs if so they rebuild it.
mine is right about 100 but I don't have their money so it will wait a bit.

froggy47

That is good enough for me to stop sweating, however, could you ask those guys why they think mine spins free in neutral & some other guys have resistance? I do believe these other guys, well at least mountainbiker as I know him personally.



Just kidding you other guys.

talon95

But are they running a factory diff? Like can be seen in those videos we posted, there are multiple types of clutch diffs.

talon95

Yea I know, that was the first place I looked. I wasn't sure if the electronic version I have was different than the printed version, but mine didn't have anything. I've searched the interwebs before also and only found conflicting information.

0Anthony @ LGMotorsports

When we check it on the race car....

The car is in neutral and the rear tires are off the ground. We "lock" one wheel in place...typically by placing a bar in the rotor against the caliper or similar device so that side will not spin.

We use a torque wrench with a axle socket on the end and spin the wheel that is free.

Depending on how the diff is assembled/built/worn what you are going to get. You should have a "break away" and a rolling torque value. The break away is what most will look for as that is the amount it takes to actually get the diff to break from locked. Each driver is different but Lou liked about 80-90 ft-lbs for a break away number. If I remember right, rolling torque was around 50-60 ft-lbs.


We have seen other cars that the driver wanted a diff that would open quicker and some that liked it almost like a spool...that is the whole reason for adjustable diffs and different ramp rates.


For those interested..yes you can make slight changes in how the diff opens with a fluid change. If you run a fluid with little to no limited slip additive, it will have a much higher break away torque....hence the 'chatter' and skipping during slow corners. They act like a locker. If you add more than what they say it will make it break away sooner, so there are some things you can do with just a fluid change to change the diff setup as well. What I can not tell you is how that effects wear in the long run. For the race cars, we were changing diff units out every 5-6 events as we started to see failures after that in either the diff itself, the case or a gear.

0Anthony @ LGMotorsports

The Corvette WC cars, yes were a clutch OEM style diff. We tried a Torsen and Lou didn't care for the feel of the diff in the car. We tried a very early Giken style diff and had some development issues with it in the Vette's but later we had it in the Camaro WC cars and ran it with very good success and it can be checked the same way.

The ALMS Corvette we ran in 08 and 09 used a clutch style and actually could be adjusted from outside of the car, and we used the same method to adjust and test it.

Xian

Thanks for the chiming in with the above details. Much appreciated!

Someone mentioned earlier 1 way vs. 1.5 way vs. 2 way diffs... wouldn't that yield different values depending on which direction you were turning the wheel/tire? Does the C5Z have a 1 way diff?

Christian

Rookieracer

Thanks Anthony, I was hoping you would chime in.


Did you check hot or cold?

0Anthony @ LGMotorsports

We typically did them warm....not right off the track (unless we were having a possible issue with it) but rarely did them stone cold.

froggy47

So there it is, with as much authority or more than most threads get.

I am not going to lose sleep over it, but it does "bug" me a little why some guys get a resistance (clutch torque - or something else?) when they try the test in neutral. As you saw in my 2nd video, mine spins like a "front wheel" when in neutral.

As I said, I have no other driveline symptoms so I'm not sweating it, but some driveline expert may have an opinion what that resistance is.

I'm "guessing" it's not a clutch break away resistance at all, but something else between the tranny and the axle nut.

Disc brake drag, parking brake drag, bind in the CV joints, old fluid in the diff, duct tape winding around the axle, dunno.

FWIW my fluid has been changed a LOT, sometimes every year. Maybe why it turns so easy?

Let's shift the discussion to that, I think we nailed the test procedure down with Anthony's posts. Thanks a bunch dude, hope to meet you some day.

Joshboody

We need to review the specific diff design. I briefly looked and could only find older positrac, which seems to have constant pressure on the clutches... thus neutral check is ok.
But many limited slip designs pressurize clutches during accel/decel... less so means more open diff characteristics.
Has GM design changed?

Checking when hot could make a big difference with expansion of all the components.