8 3/4 Zero Lash for Racing
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With zero backlash clearance, there is no room for oil where it is needed the most --no matter what kind of oil it is.
If the gears have to bend because an oil film is added it takes force and generates heat to do so.
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Light car and braced. High ratios have lots of pinion teeth. How many passes? My sons 3750lb turbo car sheared the 8 3/4 pinion shaft at 16lbs boost. Today my buddy Mike took his 64 Polara out for it's 1st outing with the new 735 HP power plant. Not only does it need a rear axle assy. The trans is toast, rare 65 case to boot. It broke an ear of his Hemi block. Still want to chance an 8 3/4 with big power and a heavy car?
Doug
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gtxracer
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Doug, correct me if I'm wrong but from the looks of the exploded trans and the explosion, was the trans the real culprit? If so, it wouldn't have made any difference what rearend was in the car.
Bob
Bob
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Can't wait for DVW answer. Rear end breaks, gears jam for a moment, engine HP breaks trans?
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1967coronet440
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Ouch, When building my car I originally was going to Just throw some axles in my 8 3/4 because I didn’t know any better. Luckily I posted here about axles and someone asked about power etc... with 700 hp a strange s 60 was in my future.
moparcrazy
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I do know better and realize that a Dana or a 9" is in the near future, I guess I'm just trying to figure out what I can get away with for now, the new
8.75
Moser
housing with back brace, weigh's 16 pounds more than a factory 8 3/4 housing, with out back brace. wondering at what RPM is your friend launching at?, my plan is to launch at 3000, I've done that plenty of times with minimal damage, Lol.
This car leaves on a foot brake a little over 3000 rpm. The rear end failed. That in turn caused the low roller clutch to fail. When the low roller(sprag) fails the front drum accelerates to engine rpm x rear gear ratio (2.28). With no load on the engine it spins up against the rpm limiter. Now we have 7000 rpm x 2.28=15960 rpm. A stock front drum is made from powdered metal. Generally they don't stay together much over 10,000 rpm. When the drum comes apart it saws every thing in its path. Fortunately this car had a shield. He has a Dana, but was going to install it later. This was pass #4 with the new engine. The car had been running mid to high 10's for the past few years. The 1/8 mile passes it made this day (6.40@106) translate to roughly 10.15@131
Doug
Doug
gtxracer
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Doug, Thanks for clarifying the real issue. The video didn't seem to indicate what went first. Been doing this since '72 and have seen the damage caused by all different types of failure.
Bob
Bob
BSB67
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Having an 8 3/4 in a 10.0 sec., full body b body with slicks seems a little risky, but having a factory sintered front drum seems, well,...........
Glad he's okay, and no one else got hurt.
Glad he's okay, and no one else got hurt.
66Satellite47
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Seems to me the problem with zero backlash is the thermal expansion of parts. That is going in multiple directions.
Thermal expansion happens and the parts -all- expand due to the increase in temp but maintain their relative proximity to each other. (expansion is not random).
The difference in size from cold to hot depends on how much temp. difference happens. I'm not sure what would be considered a very hot rear end in operation but I suspect that 150 degrees would be considered rather hot for a differential.
So (for example) a cold temp. of 70 degrees to 150 degrees is a rise of 75 degrees.
A one-inch long piece of steel will get longer by about .00045 Inches. (less than a half of a thousandth of an inch.) @ 225 degrees the expansion would still be less than one-thousandth of an inch. (.0009")
As everything in the differential expands, --in the same way,-- the change in backlash is affected as well but the amount is very slight in reality.
The ring gear will get bigger but so will the pinion. The whole unit will swell up a bit.
For an illustration example-- let's say the axial shafts got hot and therefore longer.
That would overload the wheel bearings (which are basically set close to zero).
Since the housing also gets hot the same amount of wheel bearing running clearance is not affected.
The real problem with zero backlash is there is no easy way for oil to get to where it needs to be. It has to be forced to do so and that takes energy which translates to heat.
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Thinking about this topic brought back a memory about a rear axial bearing failure I had.
On a 180mile trip home in my 70 Charger RT I heard/felt something going on in the back. I pulled over and found the drivers side axial housing to be way too hot to touch where the bearing was.
All I could do (being in the middle of nowhere) was to keep going. I slowed down and stopped every 20 miles to let it cool and check the heat as well as get a look at the problem area.
I made it home and pulled that axial out to find that the bearing was dry and rusty and burnt. It had apparently got so hot that the inner race expanded and spun on the axial shaft and therefore created enough heat to actually weld itself to the axial shaft. It then went back to spinning on dry and crumbling rollers.
I have lived my entire working career with industrial machinery and seen many catastrophic failures because a part of my job was to fix stuff.
The overwhelming cause of bearing failure is lack of proper lubrication.
And I must add--every condition is designed to allow room for lubrication to begin with. This is called running clearance in my world. As opposed to shearing clearance or-- interference fit.
On a 180mile trip home in my 70 Charger RT I heard/felt something going on in the back. I pulled over and found the drivers side axial housing to be way too hot to touch where the bearing was.
All I could do (being in the middle of nowhere) was to keep going. I slowed down and stopped every 20 miles to let it cool and check the heat as well as get a look at the problem area.
I made it home and pulled that axial out to find that the bearing was dry and rusty and burnt. It had apparently got so hot that the inner race expanded and spun on the axial shaft and therefore created enough heat to actually weld itself to the axial shaft. It then went back to spinning on dry and crumbling rollers.
I have lived my entire working career with industrial machinery and seen many catastrophic failures because a part of my job was to fix stuff.
The overwhelming cause of bearing failure is lack of proper lubrication.
And I must add--every condition is designed to allow room for lubrication to begin with. This is called running clearance in my world. As opposed to shearing clearance or-- interference fit.
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66Satellite47
Well-Known Member
I agree the oil clearance is a major factor. Not all the parts expand in the same direction, some are more fixed than others. Seems to me the ring gear is expanding toward the pinion teeth, the pinion teeth are expanding toward the ring gear. Granted with steel the change is minimal, but I wouldn't do zero.
The ring gear is also expanding away from the pinion as well as the pinion is moving away from the ring gear as expansion is happening.
You say "Seems to me the ring gear is expanding toward the pinion teeth". This says you are thinking about what is going on. --Cheers-
Question--Why do you feel the ring gear is expanding toward the pinion and not expanding away from the pinion?
I do not intend to put you on the spot with a debate.
My intent is to share my perspective and what I have gathered from my experiences.
66Satellite47
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Haven't spent much time thinking about it, but if both parts are growing, ever so slightly, and they are fixed in the horizontal (which may not really be true), the clearance is reduced. Not worth anything to me since I'd never set it to zero.
Don't parts grow in all directions when heated? When I put a pinion bearing on, I put pinion in freezer for half hour. Then warm bearing a little (180*) it drops on. Isn't there a .002/.003 interference fit. So zero lash would grind the gears up I would think.
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I would think the pinion would grow around it's circumference putting it deeper into the ring gear. Also, wouldn't it grow a bit longer too? Would also think the ring would grow too which would also make the pinion run deeper....
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I've found factory pinion bearings as tight as .005" before! That's toooo tight. There is a chart for bearing to shaft fits for all the different diameters but can't remember exactly what it is....but have never had any problems with them being .001-.002.
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