diff tuning - what do you think.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
diff tuning - what do you think.
I'm digging in my diff to
1. Change gear
2. Tune the posi (which is the first time i'm doing it)
I stumbled on the question of how much preload the clutches should have. Gary and Tom seem to tune them untill they can just turn them by hand or with a spanner wrench. However, after doing some tests it seems to have to little preload on the gears this way (below 10 lbs/ft when measured with a torquewrench on a bolt welded to an old yoke)
The workshop manual for these cars says that with one wheel on the ground on the other in the air, the torque to turn a wheel should be 70 lb/ft when new and 40 lb/ft when used. Of course, this is with the drag in the wheel bearings, diff and trans, so at best it's only an indication.
After some further investigation i found that, when the posi was setup loose and the springs inserted, the preload on the clutches was 25 lb/ft.
When i shimmed the gears to get the backlash just within specs (.006") I got a reading of 25 lb/ft. But 25 lb/ft seems not enough.
When inserting the springs additionally with the gears at .006 backlash, the preload was 50 lb/ft, which made the posi rather difficult to turn with the dialtorquewrench ( about 50 cm long).
The last thing i did was go one step up in shim size (+2.5 thou) and this time backlash on the gears was .002" and preload came out to 50 lb/ft. Which would put me in the ballpark of what I would think was technically right.
I must say that with the last setup, it was the limit of getting the trustwashers in between the pinion and the case. I read in an hotrodder article that they suggested shimming the posi to a point where you could just get the washer behind the pinion.
However I'm a bit afraid that it is maybe to much and will inhibit the operation of the diff in low speed tight turns.
On a side note : i did a calculation of how much torque a wheel on wet pavement could create and this was way above the preload, so purely based on that assumption i would say the wheels would have enough force to make the clutches turn and not bind with a preload of 50 lb/ft
What do you guys think ?
1. Change gear
2. Tune the posi (which is the first time i'm doing it)
I stumbled on the question of how much preload the clutches should have. Gary and Tom seem to tune them untill they can just turn them by hand or with a spanner wrench. However, after doing some tests it seems to have to little preload on the gears this way (below 10 lbs/ft when measured with a torquewrench on a bolt welded to an old yoke)
The workshop manual for these cars says that with one wheel on the ground on the other in the air, the torque to turn a wheel should be 70 lb/ft when new and 40 lb/ft when used. Of course, this is with the drag in the wheel bearings, diff and trans, so at best it's only an indication.
After some further investigation i found that, when the posi was setup loose and the springs inserted, the preload on the clutches was 25 lb/ft.
When i shimmed the gears to get the backlash just within specs (.006") I got a reading of 25 lb/ft. But 25 lb/ft seems not enough.
When inserting the springs additionally with the gears at .006 backlash, the preload was 50 lb/ft, which made the posi rather difficult to turn with the dialtorquewrench ( about 50 cm long).
The last thing i did was go one step up in shim size (+2.5 thou) and this time backlash on the gears was .002" and preload came out to 50 lb/ft. Which would put me in the ballpark of what I would think was technically right.
I must say that with the last setup, it was the limit of getting the trustwashers in between the pinion and the case. I read in an hotrodder article that they suggested shimming the posi to a point where you could just get the washer behind the pinion.
However I'm a bit afraid that it is maybe to much and will inhibit the operation of the diff in low speed tight turns.
On a side note : i did a calculation of how much torque a wheel on wet pavement could create and this was way above the preload, so purely based on that assumption i would say the wheels would have enough force to make the clutches turn and not bind with a preload of 50 lb/ft
What do you guys think ?
Last edited by Belgian1979vette; Jun 24, 2011 at 02:29 AM.
Burning Brakes
Joined: Sep 2008
Posts: 805
Likes: 5
From: Naperville Illinois
Belgian, could you post a picture or describe how you are measuring the preload on the carrier?
As I understand it, tuning the posi removes the slop from the setup so that there is a constant and correct amount of pressure on the clutches (GM took out the slop with the plates & springs). I left the GM setup alone--I felt that wear on the clutches, side gears, spiders or spider thrust washers would eventually leave the posi too loose, whereas the springs will maintain pressure on the clutch packs regardless of wear. To each his own.
At any rate, pinion depth and pinion bearing preload are set independently of tuning the posi.
The depth of the pinion is set by the pinion shims; the pinion bearing preload is established by torquing down the pinion yoke nut with a crush sleeve or solid spacer between the 2 pinion bearings (I highly recommend the solid spacer, FWIW). Pinion bearing preload/rotational torque is measured in inch-pounds by turning the pinion nut with the appropraite torque wrench. IIRC mine was about 20, I think the book spec might be a little lower than that?
Reading your post again, I wonder if you might be referring to something else when you use the word pinion? pics would surely help.
Chris
As I understand it, tuning the posi removes the slop from the setup so that there is a constant and correct amount of pressure on the clutches (GM took out the slop with the plates & springs). I left the GM setup alone--I felt that wear on the clutches, side gears, spiders or spider thrust washers would eventually leave the posi too loose, whereas the springs will maintain pressure on the clutch packs regardless of wear. To each his own.
At any rate, pinion depth and pinion bearing preload are set independently of tuning the posi.
The depth of the pinion is set by the pinion shims; the pinion bearing preload is established by torquing down the pinion yoke nut with a crush sleeve or solid spacer between the 2 pinion bearings (I highly recommend the solid spacer, FWIW). Pinion bearing preload/rotational torque is measured in inch-pounds by turning the pinion nut with the appropraite torque wrench. IIRC mine was about 20, I think the book spec might be a little lower than that?
Reading your post again, I wonder if you might be referring to something else when you use the word pinion? pics would surely help.
Chris
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
Belgian, could you post a picture or describe how you are measuring the preload on the carrier?
As I understand it, tuning the posi removes the slop from the setup so that there is a constant and correct amount of pressure on the clutches (GM took out the slop with the plates & springs). I left the GM setup alone--I felt that wear on the clutches, side gears, spiders or spider thrust washers would eventually leave the posi too loose, whereas the springs will maintain pressure on the clutch packs regardless of wear. To each his own.
At any rate, pinion depth and pinion bearing preload are set independently of tuning the posi.
The depth of the pinion is set by the pinion shims; the pinion bearing preload is established by torquing down the pinion yoke nut with a crush sleeve or solid spacer between the 2 pinion bearings (I highly recommend the solid spacer, FWIW). Pinion bearing preload/rotational torque is measured in inch-pounds by turning the pinion nut with the appropraite torque wrench. IIRC mine was about 20, I think the book spec might be a little lower than that?
Reading your post again, I wonder if you might be referring to something else when you use the word pinion? pics would surely help.
Chris
As I understand it, tuning the posi removes the slop from the setup so that there is a constant and correct amount of pressure on the clutches (GM took out the slop with the plates & springs). I left the GM setup alone--I felt that wear on the clutches, side gears, spiders or spider thrust washers would eventually leave the posi too loose, whereas the springs will maintain pressure on the clutch packs regardless of wear. To each his own.
At any rate, pinion depth and pinion bearing preload are set independently of tuning the posi.
The depth of the pinion is set by the pinion shims; the pinion bearing preload is established by torquing down the pinion yoke nut with a crush sleeve or solid spacer between the 2 pinion bearings (I highly recommend the solid spacer, FWIW). Pinion bearing preload/rotational torque is measured in inch-pounds by turning the pinion nut with the appropraite torque wrench. IIRC mine was about 20, I think the book spec might be a little lower than that?
Reading your post again, I wonder if you might be referring to something else when you use the word pinion? pics would surely help.
Chris
I measured backlash on these with a micrometer pointed at one of the teeth (difficult to get it to a very straight up angle, but managed somehow. And then moved the spiders (?) by pushing on them in either direction with my fingers. Difference between the 2 readings is the lash.
As for the wear. Even with springs the clutches wear and the drag thus reduces over time. The fact is that the springs exert 25 lb/ft of preload on the clutches even if the gears are very loose and don't provide any preload on the gears.
My guess is, that the springs were mainly used because the factory didn't want to take the time to tune the posi properly, so in the case that the backlash between spiders and sidegears was at the high end of the scale (8 thou) you would only have about 15 lb/ft of preload on the clutches. The springs would put that to an more or less acceptable 40 lb/ft. However the down side imo of the springs is, that once the clutches release they push the sidegears into the spiders with force and against the force of the springs acting on them, taking up all the backlash there was. This sudden release creates the famous bang of our rear end i assume.
Without the springs you can have the same amount of preload, but the backlash will decrease. Since there are no springs involved, if the clutches release the contact with the spiders is not at such a high impact (less clearance to take up, and not having to overcome the spring force), thus avoiding the bang imo.
Last edited by Belgian1979vette; Jun 24, 2011 at 12:40 PM.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
To tune the posi:
Install one spider gear w/ posi clutches (the type of gear that goes on the yoke), but do not install yoke.
Install cross gears (the ones that do not attach to the yokes) and pinion shaft.
Push a large screwdriver or pry-bar between pinion shaft & spider gear, pressing the spider away from the pinion shaft.
Feel the play of the cross gears. You should just barely feel any play. You want about 0.0005" -0.001" (less than 1 thou).
Repeat for other side.
GTR1999 and Tracdogg2 (some of the best rebuilders in the country) recommend NOT using the posi springs. they say the posi will still lock under load. And without the springs the gears will not have play which reduces shock loads.
Install one spider gear w/ posi clutches (the type of gear that goes on the yoke), but do not install yoke.
Install cross gears (the ones that do not attach to the yokes) and pinion shaft.
Push a large screwdriver or pry-bar between pinion shaft & spider gear, pressing the spider away from the pinion shaft.
Feel the play of the cross gears. You should just barely feel any play. You want about 0.0005" -0.001" (less than 1 thou).
Repeat for other side.
GTR1999 and Tracdogg2 (some of the best rebuilders in the country) recommend NOT using the posi springs. they say the posi will still lock under load. And without the springs the gears will not have play which reduces shock loads.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
I was reading up on the subject. This article has a good explanation.
http://www.ultimatecarpage.com/forum...ad.php?t=43214
If I get things right, the pinion or spider gears vs the sidegears undergo seperation forces. These will compress the spring pack more when more torque-input is present. So, in fact, what you would like in street driven car is an open diff in normal, low speed turns, but lock-up when under power.
After reading this, i can actually see, why it doesn't need to much preload at all.
I will try again tomorrow.
http://www.ultimatecarpage.com/forum...ad.php?t=43214
If I get things right, the pinion or spider gears vs the sidegears undergo seperation forces. These will compress the spring pack more when more torque-input is present. So, in fact, what you would like in street driven car is an open diff in normal, low speed turns, but lock-up when under power.
After reading this, i can actually see, why it doesn't need to much preload at all.
I will try again tomorrow.
