8.75 Sure Grip spider gears break

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JTT

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While racing my vintage road racer Dart last weekend I broke the spider gears of the Sure Grip clutch type diff carrier again. This is already second unit within one year that breaks the similar way. The rear end is 8.75, and the ratios that I've had are 3.91 and 3.55. Car has a 273 engine and A833 gearbox. It's difficult to imagine that there would be too much torque... :)

The first one that broke was a Chinese copy, but the second one was an original NOS unit from 60's. Any suggestions for a solution? Spool is not allowed...


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what are you doing when they shear?
did they break once you get back on the power after threshold braking prior to turn-in?
if so, do you down shift prior to turn-in and braking? Are you religiously rev-matching when down shifting?
I have seen guys bust the rears in Cortinas and MGs because they fully lift when down shifting/threshold braking, don't rev-match, then go wide open through the apex. Even with a low tq motor, shock loading the drive line like that over and over is going to highlight any weaknesses..

I race a '68 Zink C4, so breaking a rear end isn't exactly a concern for me with the massive 49hp she builds :)
 
Do you have to keep it sure grip, what about getting a locker or a true track for it?
 
A limited slip is really the wrong type if differential for road racing. The detroit locker or even an open differential would serve you better.
 
I was just gonna suggest, "Detroit locker" but the others beat me to it!
 
I do double de-clutch always when shifting down, but I seems that driving over the kerbs is pretty hard for the diff. I've always had a clutch type diff in my race cars, with good results. Except with this one... Probably I'll try Truerack next if I can't find better gears for the Sure Grip. Open diff is no-no in racing...
 
I have a Detroit we ran for one practice session in a oval track car, I'd be willing to sell, pm if interested
 
A limited slip is really the wrong type if differential for road racing. The detroit locker or even an open differential would serve you better.

Sorry but I will have to disagree with you on that. When road racing, you are trying to be as smooth as possible. You are trying to use every single inch of the track to maintain momentum. The absolute last thing you want in a turn I s the car to become unsettled as the rear abruptly locks up as with a Detroit locker. That can push you off your line and cause over/under steer in a heartbeat. In racing you really don't want to make contact, ESPECIALLY in vintage racing where yea, everybody wants to win, but the cars are really the star of the show and the point of vintage club racing is really to run the old girls. Not a big deal on dirt track where breaking traction is intentional. With an open diff, if you break traction in a turn, once again the car becomes unsettled as it has the same effect as lifting off the throttle. Your car also slows as applied power is lost, so you now have a car moving off it's line, and decelerating at an unexpected time. Not good when you have other cars tucked in behind you wanting to eat your lunch like the Porches in the picture.

My best guess is they are breaking from the shock load of locking/unlocking when you go 1 or 2 off on a curb. As suggested, I would contact DrDiff..

Mopar 8 3/4" (8.75) Clutch Type Sure-Grip ("Powr-lok")


Also don't come on here showing a pic of a sharp little Dart like that, talking about road racing, pushing hard enough to break rear ends, and not atleast post a YouTube video of some racing :)
 
Thanks for the tips. I also believe that they break due to shock load when cornering over the curbs. Luckily I have now he whole winter to find a solution... :)

Some video can be found for instance here:
 
Sorry but I will have to disagree with you on that.

Certainly your prerogative. This is why there are many different types of limiter slip units. You do not want a very strong unit such as a Power Lock for road racing. The Track Lock would be a more viable unit for that.
 
That is a gorgeous track! My car is really better suited for a more open track with big sweepers. I stay wide open in 4th most of the time. This is an old video shot with an older GoPro that tends to "straighten the curves" a bit, but I like it because you can see just how different my line is compared to the big hp cars..

 
Have you considered the detroit tru trac? The helical gears design is smooth operating, no clutches or cones to wear out. I have one in my 9" in my s-10 and i'm very happy with it
 
Thanks for both videos, was entertaining.
I can't recommend a replacement but i think the clutch is to harsh for what you are doing. Straight line drag racing....only a spool is better!!! For what you are doing you need the traction device to allow some slippage on the inside tire, but not act like a open diff.
I guess with shims You could make that "clutch style" lose a little bit of its grip to get both tires around the curves stuck to the ground. Probably be easier on your tires as well.
 
i need one of the axel gears for the old style power loc, its about a 1/2 " thick, cant do anything with my car till I get one, can't afford to buy the dr diff kit at this time but have some funds to buy a used one in floral city florida 34436

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Sorry but I will have to disagree with you on that. When road racing, you are trying to be as smooth as possible. You are trying to use every single inch of the track to maintain momentum. The absolute last thing you want in a turn I s the car to become unsettled as the rear abruptly locks up as with a Detroit locker. That can push you off your line and cause over/under steer in a heartbeat. In racing you really don't want to make contact, ESPECIALLY in vintage racing where yea, everybody wants to win, but the cars are really the star of the show and the point of vintage club racing is really to run the old girls. Not a big deal on dirt track where breaking traction is intentional. With an open diff, if you break traction in a turn, once again the car becomes unsettled as it has the same effect as lifting off the throttle. Your car also slows as applied power is lost, so you now have a car moving off it's line, and decelerating at an unexpected time. Not good when you have other cars tucked in behind you wanting to eat your lunch like the Porches in the picture.

My best guess is they are breaking from the shock load of locking/unlocking when you go 1 or 2 off on a curb. As suggested, I would contact DrDiff..

Mopar 8 3/4" (8.75) Clutch Type Sure-Grip ("Powr-lok")


Also don't come on here showing a pic of a sharp little Dart like that, talking about road racing, pushing hard enough to break rear ends, and not atleast post a YouTube video of some racing :)

Totally agree. The ratcheting of a Detroit locker isn’t ideal for road racing. Not that it can’t work or that people haven’t used them but there are better options. The TruTrac should be good, but I haven’t done enough with mine to really give a useful comparison. I also would get DoctorDiff’s 6 gear version instead of Eaton’s 4 gear version of the Trutrac. I have the Eaton and while I don’t have anything specific to point a finger at I actually have liked the clutch style limited slips better. Something about that Eaton I just don’t like.

Also, keep in mind that some of how hard a clutch style Sure Grip grabs is the oil and modifier you’re running. Different gear oils will change that, so you may consider playing around a little with the oil type, weight, brand and modifier to tailor how hard the SureGrip grabs. Obviously if it slips more it will be smoother, but it will also wear out clutches faster.
 
i need one of the axel gears for the old style power loc, its about a 1/2 " thick, cant do anything with my car till I get one, can't afford to buy the dr diff kit at this time but have some funds to buy a used one in floral city florida 34436

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@SGBARRACUDA Roy might have something, Curtis. He usually has some drive train stuff and he's kinda in your neck of the woods. Good to see you around.
 
Friction modifier does just that. modifies the friction.........without it, it cause the clutches to be a little grabby around corners.
 
Friction modifier does just that. modifies the friction.........without it, it cause the clutches to be a little grabby around corners.

Right, but not all of them are identical. Some oils types/brands and modifiers will result in different levels of grip/slip. For racing, he may want something other than the standard.
 
Have you thought about how your unit works?
At very light throttle the crosspins sit in the neutral zone and it acts sorta like an open diff.with very little preload on the clutches.
Under power the torque differential drives the crosspins up the ramps and locks it all up like a spool.
Under heavy braking, or compression braking, the torque differential drives the crosspins up the backside ramps, and again locks it all up.
If you happen to be in a turn, something has to give; either the unloaded tire has to slip, or the clutches have to. If the tire slips, then the clutch pack will stay locked up.
I doubt the clutches will slip except during shifting.
So then if you are down-shifting in a turn....... the clutches will go from full reverse-lock , to full slipping, to full forward lock. Now as they do this, the spiders climb down and up on the crosspins going from maximum backlash,to minimum backlash, and back to maximum. So they are being jack-hammered mercilessly, hundreds of times during the race.
Same thing happens when upshifting, but now you are backshifting to a lower position on the power curve, so it might be a lil easier on the spiders.
That 4-pinion Sure-Grip is actually overkill for your application, and as mentioned, not a good match.

But if you swap to a regular Cone-Type Mopar Sure-Grip, you need to customize it to your useage. Which means minimizing the spider walk, up and down the single cross-pin, and then setting the clutch preload. You do this with the oil you plan on using, with the SG additive already premixed into it, and all the guts prelubed. Then I recommend 120Ftlbs turning torque as a starting point .
I do this by using an old cut off axle clamped in a vice, splined end up, and slide the case down over it, assemble the guts, then drop another stub axle into the top and tighten it all up. On the stub-axles, I have welded some large nuts. Now I put the torque-wrench on the top nut and commence turning it. I'm not interested in the break-away torque, only in how much it takes to slowly make it slip. Adjustment is by thickness changes in the side-gear washers, in conjunction with spring preload. If the guts are used, you will have to die-grind the steps off the old patterns, so at their new backlash setting, they will all play nice together.
120ftlbs is just right for launching my 68 barracuda in a straight line, with both 295/50-15s blazing. But around turns on a clean dry surface, the inside wheel still slips at normal driving gas pedal application. And I can adjust the slippage with power application, making huge 360* powerslides so much fun, right on the torque peak.
I don't know the correct setting for your car; you may end up changing it several times. Alternatively; someone here on FABO has done the testing already, and may spill the beans. I can tell you this; 100ftlbs was not enough for me, and 140 was too much. Third time, 120, was a charm, for me.
Whatever you decide
Happy HotRodding
 
Detroit locker with softer springs is my favourite street setup, the lower rate springs take care of the overly abrupt engagement/disengagement while cornering.
While not smooth a Torsen gear diff is lively and fun to drive on the street especially if you like hanging the tail out around corners.
 
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