Diffs – Free up the bearings

The large 15mm bearings that hold the differentials in place are another great spot to spend some time freeing up.  There are two issues to work on.  One is the plastic part that holds the bearing has a hole that is a bit too small for the bearing, creating a binding effect between the holder and the bearing.  The other issue is the bearing itself.  The 15mm bearings are rubber sealed and full of thick grease, and that means a slower spinning bearing.  With a little bit of work, these issues can be cleared up.

The plastic diff bearing holders are sized too small.  The diff bearings are 15mm bearings.  The holes in the plastic are only 14.96mm.  This means the bearings will fit in the holes very tightly, and the fit will be so tight that the bearings will be somewhat crushed by the holders.

The reason for this is probably related to plastic shrinkage.  A lot of engineers in the RC hobby tend to ignore the fact that plastic shrinks.  When the engineer sees that a 15mm bearing will be used, the engineer creates a 15mm hole in the plastic part in the design on the computer.  Unfortunately, a 15mm hole will shrink to 14.96mm as the plastic cools.  What the engineer should have done was size the hole a little bit too big (for example, maybe 15.04mm) and then let it shrink back down to 15mm as it cools.

Anyway, you can adjust the size of the hole back to 15mm using a reamer, hobby knife, sandpaper, or Dremel bit.  If you use a Dremel bit, just spin it gently with your fingers.  Don’t use a Dremel tool, since that will remove far too much material.

For this example, I chose to use a 15mm reamer tool that I ordered through a local tool store.

Slowly remove a little bit of material.  Then retest the fit with the bearing.  Your goal will be to have a bearing that falls out of the holder with nothing more than a light tap, or even just the gentle pull of gravity.  Continue to remove material until the bearing can fall out easily.  Be careful not to remove too much material.  You don’t want there to be any play between the bearing and the holder.

Once you have the plastic parts conditioned to perfectly fit the bearings, the next step is to focus on freeing up the bearings.  There are two reasons the stock bearings are slow.  One is the rubber seal and the other is the thick grease.

Start by removing both rubber seals from the bearings.

With the seals out, use a RPM Bearing Blaster and some Trinity Buggy Blast (electric motor spray) to flush all of the thick grease out of the bearings.

Place a few drops of thin bearing oil on the ball bearings.  Gently rotate the bearings to spread the oil evenly.

The next step I recommend is to modify the rubber seals, so the seals don’t drag on the bearing races.  If you plan to race outdoors, you’d be smart to skip this step.  Rubber seals are handy for protecting bearings when racing outside.  I only race indoors on carpet, so I don’t need the rubber to completely seal the bearings.

Remove a little bit of rubber material from the inside of the rubber seal.  This will increase the inner diameter so the rubber cannot seal against the inner bearing race.  Don’t modify the outside diameter of the seal.  You want the outside diameter to remain big enough to cause the rubber to catch on the bearing’s outer race.

When you are done, carefully insert the rubber seals back into the bearings.  Technically, the rubber parts will be more like rubber shields than rubber seals at this point in time.  This will allow the bearings to spin more freely.

The last step is to place the bearings into the bearing holders, and then tests fit the diff assembly in the car.  Try spinning the diff.  If you did all of the steps perfectly, the diff should spin easily for 5-15 seconds.  If the diff does not spin freely, recheck your work and test it again.  Good luck with it.

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