What Is a Crash Gearbox?

by Lewis J. Fagleman

Crash gearboxes use straight-cut gears and are found in vintage cars, large trucks and racing cars. The speed of the engine must be matched to that of the gearbox before a gear is engaged.

Operation

When changing gears, the clutch disengages the gearbox from the engine then selects a new gear before re-engaging. With straight-cut gears, if the engine speed does not match that of the gearbox, the gears will grind together and cause potential damage to the clutch and gears themselves; thus the speeds must be matched before a gear is engaged.

Double Declutching

The clutch is used twice for every gear change. First the clutch is depressed, and neutral is selected; the engine is then revved to the appropriate speed before the clutch is depressed once more and a new gear selected. This is a complex technique that requires an experienced driver.

Advantages

Crash gearboxes produce less friction and loss of power. In racing, this is a great advantage, and with the ability to rebuild the gearbox after each race, the damage caused by incorrect gear changes is not considered a major problem. Crash gearboxes are also very robust and able to deal with much more power and torque than helical gears.

About the Author

This article was written by the It Still Runs team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Runs, contact us.

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Photo Credits

  • photo_camera old gearbox dumped in a rest area on the side of the road image by Undy from Fotolia.com