An undershift occurs when a sliding gear, synchronizer sleeve, or dog clutch does not move far enough along its shaft to fully engage the mating component. This leaves the engagement teeth or splines only partially meshed, concentrating the driving load onto a smaller contact area. If left uncorrected, the partial engagement often leads to the gear popping out under load or suffering severe edge loading and tooth breakage.
We found heavy wear on the leading edges of the dog teeth, pointing to a chronic undershift on third gear.
Mechanics often trace an undershift to a bent shift fork, worn external linkage, or an improperly adjusted detent mechanism.
Do not mix it up with
- Overshift: An undershift means the gear did not move far enough to engage fully, while an overshift means it traveled past its intended locking position.
Related terms
Dogs
Shop name for the chunky engagement teeth on a gear or sliding collar that lock a gear to its shaft.
Edge loading
Tooth load piling up at one end of the face width because of misalignment or deflection.
Tooth breakage
A gear tooth cracking off, most often by bending fatigue starting in the root fillet on the loaded side.