Counter rotate diff by hand??
Thread Starter
Drifting
Joined: Nov 2015
Posts: 1,707
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Finalist 2020 C4 of the Year - Unmodified
Counter rotate diff by hand??
Hey all, dumb-question-if-not-asked, had Her in the air working on the brakes (one of the caliper pins rusted out) and was the rear to make sure brakes were spinning free, etc.
Came to my mind to check the diff too. It's a Dana 36 with automatic if it matters. Putting in neutral, no problem spinning the wheels.
Now the question: I can't remember from before when I changed the diff fluid years back, when in park should I be able to counter-rotate the diff by hand? As in, grab a tire and spin, the other side should rotate the other way. Can't say I've had any indication before how stiff the diff is supposed to be, and the FSM only mentions noise as problem indicators. My concern is if the diff is locked or getting close.
Thanks!
Always cracks me up when I see pics of the diff internals, my 1/8 scale R/C car uses the exact setup! I've rebuilt those many many times, so tempted to try the big boy.
Came to my mind to check the diff too. It's a Dana 36 with automatic if it matters. Putting in neutral, no problem spinning the wheels.
Now the question: I can't remember from before when I changed the diff fluid years back, when in park should I be able to counter-rotate the diff by hand? As in, grab a tire and spin, the other side should rotate the other way. Can't say I've had any indication before how stiff the diff is supposed to be, and the FSM only mentions noise as problem indicators. My concern is if the diff is locked or getting close.
Thanks!
Always cracks me up when I see pics of the diff internals, my 1/8 scale R/C car uses the exact setup! I've rebuilt those many many times, so tempted to try the big boy.
Thread Starter
Drifting
Joined: Nov 2015
Posts: 1,707
Likes: 328
Finalist 2020 C4 of the Year - Unmodified
I found a tidbit on youtube, someone checked how much torque was required to counter-rotate a limited slip. The diff he tried was fairly small with spring for the clutch plates, took about 40ftlbs. Guess the correct term is "break strength".
Testing mine, without his little dohickey, had to be up around 90 ftbl, no break at all. Not a good feeling. I've noted She tends to spin more in hard turn-takeoffs or plain takeoff these days, lot more than in the past. Just not sure what is causing that. Bad tires? Super Awesome Rebuilt Engine? Diff too tight or worse, locked? *sigh*
Time get serious I suppose.
Testing mine, without his little dohickey, had to be up around 90 ftbl, no break at all. Not a good feeling. I've noted She tends to spin more in hard turn-takeoffs or plain takeoff these days, lot more than in the past. Just not sure what is causing that. Bad tires? Super Awesome Rebuilt Engine? Diff too tight or worse, locked? *sigh*
Time get serious I suppose.
Thread Starter
Drifting
Joined: Nov 2015
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Finalist 2020 C4 of the Year - Unmodified
It's more that 90 ft-lbs, more importantly I haven't been able to break it at all. Includes leaning on the tire with 150lbs. Turning tires while in neutral or drive, rotate freely. Both tires in same direction, of course.
On the flip side, I'm not noticing any noise (which doesn't say much as my ears are damaged a little) nor any squeals while turning.
Guess this winter, got me a project. Drive until then.
On the flip side, I'm not noticing any noise (which doesn't say much as my ears are damaged a little) nor any squeals while turning.
Guess this winter, got me a project. Drive until then.
Thread Starter
Drifting
Joined: Nov 2015
Posts: 1,707
Likes: 328
Finalist 2020 C4 of the Year - Unmodified
Guess what I'm looking for is the "torque bias ratio" (TBR) of the Dana 36. I'm guessing this changes with actual gear ratio.
Google seems to be an *sshole about phrase searches these days, can't find squat on the web.
Is TBR typically listed in differential specifications?
Google seems to be an *sshole about phrase searches these days, can't find squat on the web.
Is TBR typically listed in differential specifications?
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Thread Starter
Drifting
Joined: Nov 2015
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Finalist 2020 C4 of the Year - Unmodified
Posi are supposed to have an intended TBR in the design, including fluid, I found someone mention 40 ftlbs, but that was on a pickup, smaller diff.
Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
The Diff isn't locked. If it were locked, you'd notice it, big time. Ever do a full lock u-turn on asphalt in a 4x4 truck....while it's IN 4wd? If you have, that's similar to what you'd experience doing any sharp turns in your Corvette; lots of binding, tire scrub and squeal, like the car is fighting itself.
Thread Starter
Drifting
Joined: Nov 2015
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Finalist 2020 C4 of the Year - Unmodified
The Diff isn't locked. If it were locked, you'd notice it, big time. Ever do a full lock u-turn on asphalt in a 4x4 truck....while it's IN 4wd? If you have, that's similar to what you'd experience doing any sharp turns in your Corvette; lots of binding, tire scrub and squeal, like the car is fighting itself.
Guess it's time for me to figure out a test rig.
Safety Car
Joined: Jun 2017
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From: North Georgia, USA
Torque Bias Ratio applies only to diffs with helical gearing. Examples are the Eaton/Detroit TrueTrac or the Zexel Torsen. Those weren't installed by the factory in C4 Corvettes. If you had one of those diffs (someone "upgraded" to that), they operate as open diffs when the wheels are off the ground, and you should be able to counter-rotate the wheels with it in park and both wheels off the ground.
Honestly, I'm not sure what type of diff was installed in the Dana 36 for the C4 Corvettes, so I'm not sure the correct testing procedure. Breakaway torque is the specification for the Auburn diffs used in the 3rd Gen (and early 4th Gen) Camaros. That's tested with the transmission in neutral, one wheel on the ground, the other wheel jacked up in the air, and use a torque wrench to turn the wheel.
For Salisbury types (Eaton Posi and some of the earlier "Positraction" diffs), the clutches are engaged by torque applied at the ring gear, With no torque applied at the pinion, some of those diffs can be basically open, or and with other designs, the clutches can be preloaded in some way and there's also a breakway torque specified.
Honestly, I'm not sure what type of diff was installed in the Dana 36 for the C4 Corvettes, so I'm not sure the correct testing procedure. Breakaway torque is the specification for the Auburn diffs used in the 3rd Gen (and early 4th Gen) Camaros. That's tested with the transmission in neutral, one wheel on the ground, the other wheel jacked up in the air, and use a torque wrench to turn the wheel.
For Salisbury types (Eaton Posi and some of the earlier "Positraction" diffs), the clutches are engaged by torque applied at the ring gear, With no torque applied at the pinion, some of those diffs can be basically open, or and with other designs, the clutches can be preloaded in some way and there's also a breakway torque specified.
Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
The C4 uses a multi disk, spring applied clutch. AKA, Dana Trac-Loc. Testing is done at the axle nut using a tq measuring device (a tork rench).
You guys might want to have a look at THIS THREAD
But specifically related to the OP's concern is this quote from post 1:
My bet is that the OP has a tight/strong posi unit and just hasn't found it's break away point, yet.
You guys might want to have a look at THIS THREAD
But specifically related to the OP's concern is this quote from post 1:
You can check that a number of ways but I do it with a 36mm socket and a beam type TQ wrench on the outer stub axle nut. Jack one wheel off the ground put the car in neutral and start pulling on the TQ wrench until the Posi clutches break away…. That’s your initial reading or “break away”…. a new Spicer Trac Lok unit will take 75-85ft lbs to break loose and a hand built, properly built unit can take over 150ft lbs. For a lot of racing use with most cars I’d suggest a rebuild when it gets under 65-70ft lbs
My bet is that the OP has a tight/strong posi unit and just hasn't found it's break away point, yet.
Thread Starter
Drifting
Joined: Nov 2015
Posts: 1,707
Likes: 328
Finalist 2020 C4 of the Year - Unmodified
Thread Starter
Drifting
Joined: Nov 2015
Posts: 1,707
Likes: 328
Finalist 2020 C4 of the Year - Unmodified