Torsen Type 2 Differential for Tarmo 5

UPDATE:

October 23 2026

This is an update to the Torssen Differential.  The previous version were weak.  The worked but would not last.  This version attempts to fix this.  I am still testing this design but have good preliminary results and it is better that what was published before. 

Description

This is a differential.  The mechanical part allows the left and right tires to rotate at different speeds which improves handling when cornering.  The Torssen Type 2 was chosen because the differential is intended for a off-road RC car so the differential ensure both wheels retain traction even on mud, snow or when one wheel is in the air. 

 Update:

I am leaving the version 1 STL for historical trail.  In my daily use the gears may have been too small. 

Differential video: 

Non printable parts.

3 mm rod:

https://www.amazon.com/dp/B0DCBCRB1C?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

Two 3x8x3.5 mm trust bearings.

https://www.amazon.com/dp/B082PSL3SG?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0&th=1

3 40m  m3 screw

1 18mm m3 screw

3 m3 lug nut

https://www.amazon.com/dp/B0D146SHYF?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

1 25mm M3 screw and locking nut

Printing:

The Gears:

The inner and outer gears were printed with 2.5 mm nozzle. Because the gears are so small, the more resolution the better.  I highly recomend to print  TorssenInnerGear - TorssenInnerCounter and TorssenInnerGear - TorssenInner  using PA6 ( Nylon ).  The part is 100% scale, you will need to print at 103 scale to make up for PA6 shrinking.  Do not use carbon fiber because it could clog the nozzle.   The inner gear experience lots of shearing force.    

The TorssenOutterGears can be printed in PLA because the have more surface are to distribute the torque coming from the inner gears. 

Print with 10 perimeters to make sure the parts are all plastic since they will experience a lot of torque and shear. 

The Case

The case should be printed with 0.4mm nozzle.  Use concentric infill.  Version 2 has tighter tolerance to reduce gears slipping.

perimeters: 8

infill  20%

Use Concentric infill to make sure the part is balanced. 

If you are going to use it on a Tarmo 5, print the 50 teeth gear.

If you are going to use it on my remix called Moster Tarmo 2.1, use the 46 teeth gear.

The C02 Parts

The C02 parts are redesigned to tightly fit and support the PA6 inner differential gears.  This part I recommend printing in PA6-CF for now because that is what I tested and it works.  I will come back and relax this requirement if a PLA part performs well in the future. 

Assembly

WARNING:  Notice the parts are mirror image of each other.  Don't print one twice thinking it will work. 

print 3 of each of the outer gears  ( 3 clockwise and 3 counter clockwise ) 

print the two inner gears. 

Cut the 3mm rods into 6 40mm rods ( or purchase them precut ).  

Print the new C02 part twice.  

Notice that the inner gears and the outer gears twist clockwise and counter clock wise.  Place them in the gear case and play around with them.  There is a wrong way and a right way.  If you do it the wrong way the gears will not fit.  All the outer gears that twist in the same direction go into one side of the case.  and the other three go in the other way around. 

Make sure to insert the M3 25mm screw going through the two inner gears.  For version 2, it should be:

1. M3 25mm then 

2. Inner Torssen gear, then a

3. Trust bearing, then a

4. Inner Torssen gear, then a

5. locking nut

The C02 and Torssen Inner gears are redesigned to allow more space to fit the 25mm screw.  In my Playing... Testing, I found the differential will not work for too many plays unless this screw is there securely. The trust bearing make sure there is no friction that might loose the screw.   To assemble this, you can use the NutHelper.  You insert it wit th nut so you can screw it without it slipping.  Because the hole is so small, none of my tools were good enough.

The case should just fit into place.  If not, check the rod length is at or slightly below 40mm.  Use a file if needed.  DO NOT try to use the screw and lug nut to reduce the gap to close the case.

TIP:  Use the C02 parts to rotate the gears.  This will help you get the rods to fall into place so you can close the case.   Remember to grease the gears.  I follow the original Tarmo 5 instruction and use Silicon Grease liberally.

Open to Ideas: putting getting the case to close with all the gears require finesse.  If you have an idea on how to design the case so assembly is less frustrating let me know.