A transmission design where gears are engaged by sliding them directly into mesh while spinning. Because there are no synchronizer rings to match shaft speeds, the operator must double-clutch and match engine RPM to road speed. Missing the RPM target causes the gear teeth to clash violently, creating a loud grinding noise and risking tooth damage.
I learned to drive a truck on an old crash gearbox where you had to float the gears by ear.
The term comes from the inevitable noise made by inexperienced operators grinding the teeth together during a missed shift.
Also heard as
- Crash box
- Sliding mesh transmission
Related terms
Crash box
Shop name for an unsynchronized manual gearbox, where shifts grind unless engine and road speed are matched.
Dogs
Shop name for the chunky engagement teeth on a gear or sliding collar that lock a gear to its shaft.
Clunk
A single heavy knock when a drive takes up its backlash on a load reversal, such as on and off the throttle.