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View Full Version : Light Weight Diff Pulling On One Side


ronald55555
2012.02.27, 01:11 AM
I've had a light weight ceramic PN diff on my MR-03 that I'm very happy with, which is why I decided to buy another one to replace the old tired diff on my MR-02. But ever since day one, I've been having handling issues with this new diff. When turning left, the car is very loose, but when turning right, the car is very tight. I've narrowed the problem down to the diff, as I've already tried to swap the diff with another car, and the same problem persists even when the diff is in another car. The opposite is also true, when I put my old known good diff in the mr02 without changing anything else, the problem was gone. I've already tried the following in an attempt to fix the diff:
- rebuilt it twice with 2 difference spur gears (once with 64pitch, and once with 126 using 6 balls on the 1 way holes)
- polish the plates with 2000grit on both rebuilds
- lubed well
- replace all bearings on rear axle and wheel
- replaced tires on car

I adjusted the diff to be as loose as possible without slipping under power, and was smooth as butter after both rebuilds. I guess the only components I have not replaced are the balls, shaft, and plates. Anyone else have any suggestions? I really don't want to have to end up basically replacing every part on a diff that I just bought brand new.

color01
2012.02.27, 01:39 AM
Hey Ronald,

The one thing I didn't see you mention was the rear left wheel hub -- is this too tight? Typically if you get it wrong, the rear axle/differentiation will feel erratic and the handling will be quite messed up. Because the diff axle is aluminum, when you crank down on the nut it's possible to cut a groove into the aluminum with the steel set screw, and if the groove isn't in the right place it might make the wheel hub too tight no matter how you try to back it out incrementally. The set screw may try to "pull" the wheel hub back into its too-tight positioning.

ronald55555
2012.02.27, 10:31 AM
Yep, I made sure the hub wasn't binding and there's a very tiny side to side play

yasuji
2012.02.27, 10:47 AM
Yep, I made sure the hub wasn't binding and there's a very tiny side to side play

what about the rt rear wheel.... do not tighten it all the way down
just till the wheel just touches the outer drive

briankstan
2012.02.27, 10:51 AM
I agree with Grant, if the wheel nut is to tight against the bearing it will cause this as well.

Have you sanded the disk to make sure they are flat? I like to use 400 grit on them give good bite to the balls and makes them nice and smooth.

Another thing that I've seen cause this is a small defect in the spur gear from the injection molding. Where it will rub on one of the disk. Check and make sure there aren't any bumps of plastic that are rubbing the disks.

ronald55555
2012.02.27, 11:40 AM
Thanks for all the suggestions guys, but unfortunately I've already tried all of that. I loosened the wheel nut to the point where it wasnt putting pressure on the wheel. The discs were sanded to 2000 grit against a height gauge block I have in the machine shop that I know is dead flat. Interestingly enough, the gear that originally came with the diff had a defect on the ejection mark, but I have shaved that off and made sure any other spurs I've tried were cleaned up as well.

yasuji
2012.02.27, 11:46 AM
Thanks for all the suggestions guys, but unfortunately I've already tried all of that. I loosened the wheel nut to the point where it wasnt putting pressure on the wheel. The discs were sanded to 2000 grit against a height gauge block I have in the machine shop that I know is dead flat. Interestingly enough, the gear that originally came with the diff had a defect on the ejection mark, but I have shaved that off and made sure any other spurs I've tried were cleaned up as well.
hmmm seems odd...
i dont sand the discs or anything just put new outta the bag....
can u send it back to us @ rckenon? i will take a look at it!

hrdrvr
2012.02.27, 12:24 PM
Make sure the bearings on the shaft are oiled well too. I have found a drop of oil on the spur bearing, and the out drive bearing makes a big difference in diff consistency. I run all other bearings in my car dry, but these get loaded so much, they need oil.

ronald55555
2012.02.27, 07:44 PM
I probably should've mentioned that this diff is the MR2048, with the orange shaft. I also have a MR2049 on my other car that I have no issues with. I believe the only differences between those 2 are the shaft colour and gear pitch. I played around with the mr2048 when I got home, and found that it is very sensitive to the drive side wheel nut tightness. I have been running it with the nut a full turn loosened from the point of being tight. If I tighten it any further, the wheel binds with the shaft and would not turn freely on the shaft. Whereas on my mr2049 diff, I've always been able to run it with the nut snug with no issues. I suspect there might be a tolerance issue with the location of the c clip. I do see a slight imprint on the inside face of my wheel that might be from it binding against the c clip.

I never had to sand my other diff, it worked flawless out of the box, same with all of the other PN parts I have. It was just more of a last ditch attempt to fix this one.

Grant, I'll send you a pm.

ronald55555
2012.02.27, 08:28 PM
I just took everything apart to measure the shaft with a caliper, and it looks like the distance between the c clip groove and the outer shoulder (part of the shaft where the bearing in the wheel sits), is about 0.5mm shorter than 2 other diffs I have (PN MR2049 and an old Kyosho diff). The make the wheel nut tighten down on the wheel bearing, instead of bottoming on the shoulder of the shaft like the other 2. This explains the sensitive wheel nut tightness, but I'm not sure if this explains the pulling to the left issue since I was running it with the wheel nut pretty loose and not binding.

Traveler
2012.02.28, 05:22 AM
I switched to a thinner bearing (3x6x2) in the right rear wheel when using this shaft to avoid binding.