TT02 FRD: Changing the Rear Gearing

After initially running my TT02 FRD with the recommended 10.5T motor, it seemed a little slow even with the timing bumped up.  Eagle hasn't released an alternate gear set specifically for the TT02 FRD, but they do have a countersteer gear set for the regular TT02.  By adding the TT02 FRD Joint Cup Shaft to the countersteer gear set, you are supposed to be able to use those gears.  The stock gears on the FRD are 39/15.  The CS gears are offered in 27/16 , 25/16, and 22/16.  I received all the parts that are required, and they can be made to work, but it is not a simple "drop in" operation.

The Joint Cup:

The CS Gear Set:

I wanted to run the 27/16 option, because I felt that would be the best match up with the 10.5T motor.  The other ratios might work will with 13.5T.  However, the gear carrier on the rear axle does not work for gears over 25T (the range is marked on it, but not in the part description).   The 27T will not mesh correctly with the 16T gear.  So, lets get to the gear change...

The CS gear set comes with a short joint cup that is useless in this application, however you need to get the spacer off of the shaft to use with the correct joint cup.

The bearings need to be reused from the stock gear set.  To remove them, you will have to pick out the o-ring retainers and remove the drive cup pins.

Now comes the tricky part.  The joint cup shaft ends up having play on the end where the gear meshes and the end that connects to the driveshaft.  You will need a lot of shims to set this up correctly.  First shim the gear against the inside spacer to establish the correct gear mesh.  I have added the shims already in the pic below.  You can see the play on the part of the shaft that connects to the driveshaft:

It may take a few trial fittings to get this correct, but be patient, and get it right.  You want a perfectly smooth mesh with no "notchyness" to the gears.  Once that is done, you need to shim the remaining space on the outside, otherwise the drivshaft cushions will want to drive the gear in too tight and ruin the mesh.  In the pic below, you can see all the shims that it took to get mine setup right:

There should be close to zero play in the joint cup shaft once shimmed correctly and it should rotate very freely.  Here is everything installed:

And finally, the real tricky part to installing these gears... Since the joint cup is longer than the original one the driveshaft ends up being too long by 1-2mm.  There are some set screws in each end of the driveshaft to watch out for, but there is a little bit of aluminum on each end that can be removed before getting to the screws.  I removed about 1.5mm total using my lathe, but you could file or dremel them if those are the tools you have available.  I ended up removing one of the foam cushions as well to get the fit just right with no play.

It took some effort to get it setup correctly, but it is a worthwhile modification.  The car now has the extra speed when needed, and these gears are super smooth and quiet.  It is the quietest car at our track now, even compared to the other TT02s.  Stealth mode...

Here is a video clip of me testing it with the new gears.  Once the other two cars leave the track you can hear just how smooth and quiet it is.