Momo quick-release jig (was Houston 420R Build)

[williamwashere]

Everyone, I’m about to write a longer build post describing the process I used to finish my 420, and I’m about to install the engine now.

 

During some downtime I wanted to share one of the tools I created for the build. In case someone is already working on theirs, I made a 3D‑printed jig to locate the holes for the Momo quick‑release steering wheel as accurately as possible.

 

I thought I’d share that here as my first post.

 

Hopefully this can help someone else while I continue with my build!

 

I'm fitting a Lifeline formula-car quick release to the Caterham 290 mm suede Momo (Mod 26). The Lifeline boss bolts on with three M6 bolts on a 50.8 mm (2") circle. The wheel's centre is a solid 5 mm plate with no holes at all, just a tiny machined cone dead centre on the back, so the three holes have to be placed from scratch. I couldn't find a template online, so I made one with Claude (the AI) doing the geometry and the CAD.

How it was made:

The idea was a jig that locates itself: a printed plate that sits on the back of the wheel's centre plate, with three lips that hook over the plate's top edge and its two lower curved edges. With all three seated, it can't slide or rotate.

That only works if the jig matches the plate outline closely, and getting the outline was the hard part.

 

Attempt 1: I traced around the plate with a marker and scanned the paper. The jig built from that was too big, by about 1 to 1.5 mm per side on the lower edges, so it rattled.

 

Attempt 2: Claude generated a 104 mm paper disc with a ring of 28 small coded markers (ArUco markers, like little QR codes) around the rim, plus two 150 mm lines on the same page so I could check my printer's scale with a ruler. Both measured 150 mm. I cut the disc out, put it behind the plate with the markers showing around the plate's edges, and took one overhead photo.

From that photo, the markers give the exact scale and correct for the camera not being perfectly square. Claude then traced the metal edge, found the centre cone, and applied a small (about 0.5 %) estimated correction for the paper sitting slightly behind the edge being measured. As a sanity check, the left and right halves of the measured outline mirrored each other to about 0.1 mm.

The jig was generated straight from that outline with:

• 0.3 mm clearance at the lips

• three 3 mm pilot holes on the 50.8 mm circle

• a 3 mm sight hole in the middle

 

Printed on a Prusa Core One, flat face down, lips up, no supports. The fit on the wheel is excellent: it drops on and doesn't move.

How to use it

1. Check the hole pattern against your own quick release boss before anything else. The 50.8 mm figure came from retailer data, not a Lifeline drawing.

2. Decide the clocking. The jig puts one hole at 12 o'clock and two below. The Lifeline only goes on the column one way, so make sure that orientation gives you a straight wheel with your boss.

3. Seat the jig on the back of the wheel (the side facing the column) with all three lips hooked over the plate edges. It should sit flat with no rock or slop.

4. Look through the centre hole. The cone in the wheel should be dead centre. If it isn't, stop; the jig isn't seated or your wheel differs from mine.

5. Clamp the jig to the plate. Protect the suede.

6. Drill 3 mm pilot holes through the three outer holes. Go gently; it's a plastic guide, not a hardened bushing.

7. Remove the jig, open the holes up to M6 clearance (6 to 6.5 mm) in steps, and deburr both sides.

self_locating_jig.stl

[JohnCh]

Very cool! To aid discoverability, I moved this to the appropriate forum and also updated the title reflect the topic. Feel free to tweak that further by either clicking and holding on the title or clicking the ellipse at the top right of your post, then select Edit.