Mk Ultra - 3D printable 1/10 4wd rc buggy - drive shaft remix

Goals of this project:

1. Making the rc car more accessible by using widely available parts.
   • The original differentials were hard to get, and although alternatives exist, they weren't easy to find.
   • The Schumacher belts were hard to get were I live.
2. Reduce wear-and-tear parts.
   • The printed layshaft and belt pulleys tend to wear out or deform during heavy use.
3. Reduce overall build cost.
   • One of the orginal differentials cost me 26,00 €. For less than that I was able to purchase all the HSP-Parts needed for this conversion including the bearings.

Changelog:

2025-01-24:

• Added pictures of the different versions of the bevel gears
• Updated section about bevel gears (HSP-03015)

2024-11-17:

• Updated dead links in the BOM

2024-11-05:

• Updated optional - spindleRear02033
  • Accidentally uploaded a development interation previously

2024-10-31:

• Added optional - spindleRear02033
• Added optional - spindleRear02033 section to the optional parts
• Added cups and dogbones to the BOM as optional
• Updated alternative drive shaft section in the notes

2024-10-29:

• Added diffSpacer
• Added V2 of Chassis01 and Chassis03
  • Removed a small piece in the front of both parts that is too thin to be printed properly with a 0.4 or bigger nozzle
  • Merged old Chassis01 and optional - chassis01 - lowProfileServo
• Updated optional - lowProfileServoMount section
• Updated the instructions regarding diffspacers
• Removed the differential kit from the BOM due to subpar quality
  • Some diffs were missing an O-Ring
  • Two bearings failed after just one run
• Integrated select images directly into the text for better context alignment

2024-10-25:

• Added optional - spindleFrontS106015S
• Added alternative drive shaft section in the notes
• Added spindleFrontS106015S section to the optional parts
• Added drive shafts to the BOM as optional
• Added S106015S to the pictures

2024-10-22: 

• Added V2 of linkBall, link01, link02 and linksRework
  • Reduced the ball diameter by 0.1 mm 
  • Reduced thread rod hole diameter by 0.1 mm
• Updated linksRework section in the optional parts

2024-08-29: 

• Added pinion compatiblity to the pictures
• Updated motor section in the notes
• Updated the pinion gear description in the BOM

Notes:

All the parts were recreated in Onshape, using the outer and hole dimensions from the original STL files.

The chassis sits 3 mm lower to have enough space for the HSP-Differentials.

To make the drive shaft fit, I had to shorten the car slightly. The original comes with optional LWB arms, which I recommend using to achieve a similar wheelbase. Using the standard SWB- or optional MWB-Arms will probably result in the shocks rubbing against the body. To dampen the back-and-forth play of the shaft I recommend putting small rubber pieces inside the drive cups.

There seem to be different versions of the bevel gear (HSP-03015). Eventho they have the same part number the position of the hole for the pin is 1mm off. 

I recommend getting the version with teeth that extend from the base of the head all the way, at least for the rear. This version seems to be widely available on Amazon, but is hard to find on Aliexpress, at least for a reasonable price that justifies the wait. 

The front bevel gear shaft needs to be shortend by about 8 mm.

I used the 110-120 adjustable CVDs for this project. Due to the HSP differentials being wider than the FS Racing ones the rear drive drive shafts needed to be shortened by 13 mm instead of the suggested 8 mm and the front drive shafts needed to be shortened by 3 mm. The flatspots were extended by the same amount the drive shafts were shortened. Pictures below are showing the final adjusted length for the front and rear drive shafts.

Since multiple people raised concerns about the adjustable drive shafts not being reliable I started looking into different options. I found alternative universal joints for the front and a dogbone setup that fits the rear. Both alternatives require modified spindles which can be found in the optional parts list. Be aware that due to the HSP differentials being wider than the original models diffs these drive shafts will not fit on the original MK Ultra.

There is room for a 540, 3650 or 3660 motor. 3660 motors come with a 5 mm shaft keep that in mind when selecting the pinion gear. On 3650/3660 motors the mounting holes center-to-center distance is 25 mm and on the 540 I used it is 25.4 mm. Both are compatible with the motor mount. According to my calculations the 17T pinion gear only works with a 3650/3660 motor. 

Due to the modified steering system I recommend adjusting/limiting the endpoints. During testing 77% turned out to be the optimal value for me.
If you have low profile servo you can use the optional servo mounts and put the link on the other side of the horn, closer to the center instead of adjusting the endpoints. 

The body was modified with a cutout for the new steering system and is slightly shorter in the rear. I made it symmetrical since the extra space for the spur gear is no longer needed.

There is one thing I dislike more than writing documentation and that is not having documentation, so I created step-by-step guide detailing what goes where and which screws to use. The manual only covers the assembly of the drive shaft conversion, not the entire RC car. After completing these steps, I recommend watching RC Printer’s build guide for detailed instructions on assembling the rest.

Optional parts:

optional - centerbrace - receiverMount:
A variation of the centerBrace that allows you to mount the receiver on the second level of the car.

optional - chassis02 - motorScrewCutout:
Some motors have screws on the outer casing, this chassis has cutouts to accommodate those screws.

optional - linksRework:
I personally had a lot of trouble with the printed linkages so I designed my own. The linkages are available as a set and as individual parts. Version 1 was designed with zero clearance so I recommand applying a little bit of grease. After the link is pressed onto the ball, it should be worked in by hand. In Version 2, the ball diameter is reduced by 0.1 mm to decrease friction while still ensuring a snug fit. These links can be printed with 0.2 layerheight instead of 0.1 like the original suggests. When using these the suggested end-to-end measurement changes:
Front camber links: 67.6mm
Steering links: 69.6 mm
Rear camber links: 77.6 mm

optional - lowProfileServoMount:
A low profile servo allows for the steering linkage to be mounted on the other side of the servo horn. On this side the link can be mounted closer to the center of the horn, so adjusting the steering endpoints is no longer needed. The original servo mounts had to be redesigned because the rear one would interfere with the chassis.

optional - spindleFrontS106015S:
Spindles for beefier alternative front drive shafts.

optional - spindleRear02033:
Spindles for rear dogbone setup. The 77 mm dogbones start binding slightly at the theoretically highest arm position. This will have no effect during driving since the chassis would touch the ground way before this position is reached. The next step down in size is 74 mm, these were too short. 

Known inconveniences:

• Reaching the bottom screw of the motor mount requires a long hex driver.
• The pinion gear can not be swapped without unscrewing the motor.
• CenterBrace or chassis03 needs to be removed to take out or install the steering rack.

Original parts needed:

• Armfront01
• Links01
• Option - Armrear-LWB
• Option - Spindlefront-110-120mmAdjustableCVD
• Option - Spindlerear-110-120mmAdjustableCVD
• Pivotfront01
• Servomount01
• Wing01

Print settings:

The print settings were taken from the original:

• Layerheight: 0.2 (0.1 for Links01)
• Perimeters: 2 (1 for the body)
• Infill: 100% (5% for the body)
• Supports: only chassis03

Everything was printed out of PLA, except for the motor mount. The original post recommends a more heat resistant material like PETG, ABS or Nylon.

Videos:

Links:

Instructions - Assembly guide with pictures

BOM - List of all the required and optional parts for this remix

Mk Ultra Cults - The original model

Mk Ultra Build Guide - In-depth video by RC Printer

Mk Ultra Discord - Highly recommended if you have questions

"Happy printing, and God bless, my friend."
— Bob Ross

All metal drivetrain instead of beltdrive.