ROSI True. Optimize. Grind.

Concentric trueing

The tire stays on the original axle. So the exact combination of axle, rim and tire is ground together – just the way it will later run on the track.

Replaceable PETG bushings

The axle runs in printed PETG plain bushings. Low-vibration, easy to maintain and simply reprinted and swapped if worn – in my use no visible wear after many tires.

Contact pressure knob

A fine-adjust knob regulates the pressure of the tire against the sanding pad. Reproducible, controlled, adjustable any time.

Mostly 3D printed

Designed for home 3D printing. Only a few standard parts are added – motor, O-ring, a handful of screws, sandpaper. See the parts list.

Two pulley variants

Two versions for different axles: the variant for the Carrera 1:32 rear axle slides onto the axle and engages the original gear – completely without screws or heat-set inserts. The second variant is a two-part clamp for axle shafts, tightened with 2× M2×6 screws into 2× M2 heat-set inserts. Both seat without play and transfer power without damaging axle or gear.

Axle in PETG plain bushings

The axle runs in printed PETG plain bushings, not metal sleeves. Low-vibration, easy to maintain and simple to swap if worn.

Fine-adjust contact pressure

Set the pressure precisely: increase it slightly and wait until the motor returns to its idle current, while moving the sanding pad back and forth to avoid grooves – as shown in the video.

Compact build

Everything packed into a small footprint – stable on the table, minimal desk space, all key controls reachable from the top.

Few off-the-shelf parts

One RS550 motor, one O-ring, one 2 mm rod and a handful of M2/M3/M5 screws. See the parts list below.

Motor & drive

• 1× RS550 motor (12 V 550-10000) – runs happily at ~4 V
• 1× Ø 50 mm O-ring as drive belt
• 1× Ø 2 mm metal rod for the swing-arm axle

Screws & nuts

• 3× M5×15
• 5× M3×10
• 2× M3×8 (motor mount)
• 2× M2×6 (only for the clamp-on pulley)
• 2× M2 heat-set inserts (only for the clamp-on pulley)

Sandpaper

• Grit 240 – rough
• Grit 400 – intermediate
• Grit 1000 – finish (finer to taste)

Alignment

• Tenth-mm shims to align the columns
• Use as many as needed so the axles run exactly parallel to the sanding pad