COUN­TER­IN­TU­ITIVE GEAR­ING

4 Wheel & Off Road - - NUTS & BOLTS -

Q Why are lower gears the op­po­site of what they’re sup­posed to be numer­i­cally? Ask­ing for a friend.

KEVIN S. Via nuts@4wor.com

A It seems coun­ter­in­tu­itive, and it is con­fus­ing to be­gin­ners, but lower gears are numer­i­cally higher than tall gears. In other words, a 4.56 gear is lower than a 3.07. Why? The num­bers rep­re­sent the num­ber of ro­ta­tions the pin­ion (at­tached to the drive­shaft) has to make for ev­ery revo­lu­tion of the ring gear (at­tached to the axles). So with a 3.73 gear, the pin­ion (and there­fore the drive­shaft) has to ro­tate 3.73 times for ev­ery ro­ta­tion of the ring (and there­fore the tires).

Why is this im­por­tant? It has to do with torque mul­ti­pli­ca­tion. The more ro­ta­tions the pin­ion has to make, the less ef­fort it takes to ro­tate it. So, it has to spin more but it takes less force to do it.

You could also look at it from the op­po­site di­rec­tion. When we in­crease the di­am­e­ter of the tires on a ve­hi­cle from stock, we in­crease the tires’ lever­age on the axle (and ring gear) be­cause the con­tact patch with the ground is far­ther from the axle than it used to be. By swap­ping to a lower gear ra­tio, we com­pen­sate for that longer lever by in­creas­ing the num­ber of times we have to ro­tate the drive­shaft. It also means that the tire doesn’t travel as far with ev­ery drive­shaft ro­ta­tion, but it takes less ef­fort (force) to do it.

The ra­tio of a gearset is dic­tated by the num­ber of teeth on the ring-and-pin­ion gears. In the case of most stock gear and some af­ter­mar­ket ones, tooth counts will be stamped on the side of the ring gear. You sim­ply di­vide the num­bers to get the ra­tio; e.g., 41 di­vided by 10 is 4.10.

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