I just replaced my differential bushings this weekend and had a thought. When you jack the rear of the car up from the diff all the weight at the rear of car is resting on the diff bushings. Maybe that's not a good idea to do with stock bushings as they are oil filled.

I cut my old bushings apart and inside is a large cross shaped piece of rubber which the pin passes through. This is then sealed inside an oil filled bladder. The weight of the car doesn't really pressurize the bladder and blow the oil out because of the large rubber piece inside, but if your bushings have any age jacking it here may accelerate their failing. Maybe not. Just something I thought of this weekend and I'm not going to do it anymore.

If your diff bushings are solid I wouldn't give a second thought to jacking from under the diff.

 

/agree



I have always used the ***** that hang down on the ends of the rear subframe. I have never felt right about jacking on the diff.

How bad was the diff bushin job? a few hours or 2 days?

I have engine mounts, diff mounts and an engine torque brace sitting in my garage waiting for a free weekend.

 

I did it in less than 2 hours; simple. I bought another mount and pressed my new bushings into that one, this way all I had to do was remove the old mount and swap in the new one. Went exactly as jimlab said it would.

I just put an ad up for a used diff mount and the offer to press your new bushings in for free before I ship it. Check the third gen classifieds

 

SO you have an extra set of diff mounts on your hands eh? How much to send em my way so I can do the same thing.

I'd even send mine back after I am done if for some reason you want to hang on to a set.

Shawn

 

i've done it alot.

the only reason i do is b/c the fsm says it's the correct location for the rear support.

 

Ive always done it on V-8s thats why, but not that you mention it , it "is" possible. Maybe someone should grab stock bushing and see how much pressure it can take. he he he fun experiment when it blows.

 

The diff is only jacked up for a few seconds until the car is on jackstands...

 

First, when jacking up the car by placing the pad of the jack under the differential, you're only lifting (approximately) half the weight of the car, so we're only talking about ~1,400 lbs. Second, that's really not much different than the load they see under acceleration in first gear.

 

1st gear with a V-8 or 1st gear modded? lol How many mods are we talking about here. j/k

 

I'll figure out what it is and let you know. Jacking the diff is going to be far more load than first gear I bet, especially when you consider that the PPF isn't loaded when jacking the car; all the load is in the diff bushings.

 

Even stock, 217 lb-ft. * 3.48 * 4.1 * 0.85 (losses) = 2,631 lb-ft. at the axles at peak torque. Suffice it to say that there's a considerable amount of force twisting the drivetrain, even on a bone stock car.

What do you mean the PPF isn't loaded when jacking the car? It has to be loaded, it moves with the differential as a unit. You're transferring some amount of force to the PPF (and the rest of the drivetrain) by jacking up the differential, just like the engine transfers force through the drivetrain to the differential bushings when it moves, because they're all linked together.

 

I mean exactly what I said The entire drivetrain attaches to the car through the engine mounts and the diff mounts; that's it. The PPF merely transfers torque between the tranny and diff along with locating the pinion in space so the diff housing doesn't rotate around the axles or the engine/tranny "tip" forward or backward. The PPF is the beam that resists bending (and some torque) between the tranny and diff so that the whole assembly can be hung at its ends by just the engine mounts and diff bushings.

When there is power passing through the drivetrain then yes the PPF can see large loads as it transfers torque back and forth from the tranny and diff along with resisting the bending force that the torsional axle loads place on it. When you put a jack under the diff which is directly under the diff bushings the diff bushings receive all the load from the weight of the car, the PPF doesn't do much but sit there. The PPF is not somehow transferring weight to the front engine mounts just because you're jacking up the rear because your jack is already directly under the diff and its bushings which is at the far end of the solid "beam" the engine/tranny/PPF/diff make. If you were jacking the car from midway down the PPF itself then sure, but with the jack directly under the diff mounts all the weight is sitting on them, the PPF will be doing nothing of consequence. If it were the front end of the car would raise up slightly too because it would make a big cantilever.

When you put a jack under the diff and raise the car all of the weight sitting on the jack is passing through the diff mounts, it's not shared by anything else.

Hairs can be split to show that yes the PPF must see some load since it still has to locate the diff and keep it from rotating but the tiny amount of load it sees is nothing of consequence when compared to how much the diff mounts see. That's also why I feel launching the car hard isn't as hard on the diff mounts as jacking it is. When you launch the torsional loads are shared by the engine mounts and the diff bushings since the PPF transfers engine mount torque back to the diff as well. When you jack the rear of the car from under the diff all that's involved is the diff bushings.

Again I may be worrying about something that isn't a real problem but for me it can't hurt so I'm not going to do it anymore.

 

If you think jacking up the car by the differential doesn't load the PPF to any degree, try unbolting the PPF from the front of the differential while you've got the car jacked up that way.

Is that why drag racers crack the snout of the differential off instead of breaking the engine mounts? Most of the load is downstream of the engine, especially where the power is trying to turn a 90-degree angle to the axles.

Not really, but if it makes you feel better, sure.

 

No, it's because the ring and pinion are helical bevel gears. The ramps of the teeth try to force the pinion away from the ring gear; in both the axial directions of the driveshaft and axles. If traction at the tires is sufficient and load becomes stronger than the diff case can handle then the case ruptures. The case doesn't fail from being twisted by high torque, it fails from getting pushed apart from the inside due to the forces of the teeth on the pinion and ring gear trying to seperate them from eachother.

Look at all the pics of failed diff cases. The case cracks and seperates just as if you grabbed the pinion and tried to yank it straight out the case because that is in effect what actually happens. The case fails from the inside out due to gear tooth loads, not from the outside in due to somehow twisting the case. Axle hop exacerbates this problem because axle hop is actually a torsional oscillation. This means the torque doesn't pass through the system as a nice smooth load, but instead is fed in as if by hammer blows from an impact gun. The sharp spikes of the oscillation are what makes axle hop so much more damaging to components. The PPF is expecially prone to failing during axle hop because it's supposed to prevent the diff case from rotating around the axles by being strong in bending. The torsional oscillation in the axles during hop makes the diff case also try to rotate and the PPF takes a beating because it's what resists this force.

Check out this pic. Notice how the gear teeth look like a screw and then you can visualize the pinion trying to force its way from the ring gear and rupturing the case. This is why the case seperates in roughly a straight line as opposed to a torsional failure which is normally about 45 degrees.

 

Damon, don't try to lecture me on differential damage. There are probably very few people on this forum who have done more drag racing than I have, seen more drag racing carnage than I have, or know more about the differential and rear suspension of the FD than I do. You are correct that the pinion gear tries to climb sideways out of the ring gear due to the design of the teeth, but the damage in the picture above was caused by the repeated twisting shock of drag launches on slicks, enabled by badly worn differential bushings.

If it were simply related to the amount of torque trying to spread the ring and pinion gears apart, you'd see a lot more drag racers with high horsepower cars posting pictures of their grenaded differential cases, even with differential braces or upgraded bushings. In fact, you'd probably see a lot more differential case failures, period. Obviously, no external bracing or bushings could prevent the spreading of the internal gears if it were simply torque related, but people who have strengthened the mounting of the differential don't typically break their cases, and those that go drag racing with old stock bushings do. Go figure.

Anyone with badly worn bushings can tell you that the shifter jerks around when they transfer from on power to off and vice versa. Obviously that's caused by excessive motion of the drivetrain including the differential, or do you dispute that?

 

I don't dispute that the drivetrain moves around all over the place due to worn mounts and never offered as much? You didn't like the length of my reply and now try to bait me with something I didn't say and you know I wouldn't agree with? I explained my opinion of axle hop feeding large transient loads into the entire direvetrain and that includes loads at the gear teeth as well. A divergent oscillation (which is what axle tramp and tire shake are) is much more destructive then a smooth peak load. Solid diff mounts do a great job at helping to prevent movement and thus vibration of the drivetrain.

I guess I'm not one of those "few people" who know anything about the differential and rear suspension then. Must be a very exclusive club Just tell me you think what I said was dumb and why, I take no offense to that. No need for innuendo too.

 

 

^ No need for popcorn.

 

My opinion of what a schematic diagram in jacking the rear of the car from the diff looks like. You must assume that the portion of the chassis that reaches around the front and under the engine is very, very strong. The drawing got complicated doing it differently.

 

And you accuse moi of baiting?

Damon, I didn't say that you weren't one of the people who know about the differential and rear suspension. Perhaps I misinterpreted your post, but it seemed to imply that I was, and you of all people should know that's not the case. And that I don't take kindly to people lecturing me.

While your explanation of the pinion gear's tendency to climb out of the ring gear was correct, I don't think we've seen a single failure that can be attributed solely to "too much torque going into the differential with too much traction, too often". I've seen ring gears that have had teeth sheered off by the pinion gear. That's what happens when "too much torque goes into the differential with too much traction, too often".

The stock differential case is plenty strong and resists gear spread well. Vosko and the few others who have broken their cases have done it because they had badly worn bushings which allowed the differential to twist excessively under load. Launching on slicks, wheel hop obviously wasn't the issue. Repeated strain on the differential was, and the result speaks for itself. The reason the case cracks where it does is because it's the thinnest part of the casting.

The same twisting force under load is what eventually fatigues the PPF and causes cracks to appear, and you don't even have to drag race for that to happen.

I must have missed the innuendo. Was it a good one?

 

No, no need for popcorn. You're one of the few people on this forum whose opinions I respect and who I actually like (and that is an extremely exclusive club), and if my response indicated otherwise, then I apologize.

 

Be carefull...your girlfriend might catch you.

 

Are you in the right thread? The thread about getting caught masturbating is over here