Half-Tooth Kart Sprocket Family in 7075-T6 for CNC Machining
Thirteen rear sprockets with fractional tooth counts — 67.5 through 73.75 — to reach gear ratios whole-tooth sprockets cannot, each with its own production drawing.
A two-stroke racing team could not reach the gear ratios they wanted. With a fixed 12-tooth front sprocket, the rear count jumps in whole teeth — and the ratio they needed fell between them.
The problem
Changing the front sprocket to get a finer ratio step changes the whole drive geometry and does not land where the team wants either. The only way to hit ratios like 67.5 or 72.25 is to build the fractional count into the rear sprocket itself: tooth spacing that no catalogue part offers.
Three constraints came with it:
- #219 chain pitch — nominally imperial, 7.774 mm in practice. Tooth form has to be cut to the chain that actually runs on it, not to a rounded number.
- 7075-T6/T7 plate, machined — so the geometry had to suit CNC profiling.
- 14,000–16,000 RPM. Lightening the sprocket was wanted, but not at the cost of balance or wobble at racing speed.
The approach
- Tooth form built from the chain, not from a table. Pitch circle and tooth spacing derived for each fractional count so the chain seats correctly on every size in the family.
- Lightening that stays balanced. Material removed in a rotationally symmetric slot pattern, keeping the mass distribution even about the axis rather than chasing the lowest possible weight.
- A lead angle on the tooth crest added in revision, so the chain drops into the tooth root cleanly and rolling resistance drops with it.
- One drawing per size. Thirteen sprockets, thirteen dimensioned production drawings plus machine-ready geometry — because a family part is only useful if each member can be ordered on its own.
The result
A complete sprocket family — 67.5, 67.75, 68.5, 68.75, 70.5, 71.5, 71.75, 72.25, 72.50, 72.75, 73.25, 73.5 and 73.75 teeth — modelled, drawn and ready for CNC, with revisions carried consistently across every size.
Where this applies
Drive components where the catalogue stops short: sprockets, pulleys, timing components, any family of parts that has to stay dimensionally consistent across a dozen variants. Parametric modelling earns its keep here — one change propagates to all thirteen.
I design machined parts and part families for CNC, with the drawings and geometry a shop can run. Request a quote or see all services.