Why does reverse gear make a higher pitched noise?

I've always wondered this. At one point I'm sure it was explained to me, but I've since forgotten the reason.

Whether manual or automatic transmission, cars in reverse seem to make a much higher pitched whir (almost electric sounding) than when in forward gears.

Does anyone have a layman's explanation for this?

Thanks

Evan J.

Reply to
Evan
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I think that it is a matter of construction of the sprockets. The forward gear is used extensively, so the sprockets have high-tech teeth to reduce the wear. The ones for the reverse gear are simple sprockets, making a lot of noise.

shakiro

Reply to
shakiro

Reverse is usually a very low gear, or the input shaft spins many revolutions for one turn of the output shaft. I suspect it is the relative high speed of the gear on that input shaft that is causing the whine.

Reply to
John S.

Two reasons - one, reverse "gear" actually has an extra gear engaged compared to most other gears (to reverse the direction of the output shaft) also the gears comprising the reverse gear assembly are likely straight cut where the gears for forward drive are helical cut. Straight cut gears are cheaper and easier to make and also don't end load the bearings like helical cut gears do. Helical cut gears however are much quieter running as you can see.

If you do a google search for a Muncie M-22 you will find an explanation of this, that's a notoriously noisy yet durable tranny, reason being that the helical cut of the gears is much shallower than standard.

nate

(music to my ears...)

Reply to
N8N

Two reasons - one, reverse "gear" actually has an extra gear engaged compared to most other gears (to reverse the direction of the output shaft) also the gears comprising the reverse gear assembly are likely straight cut where the gears for forward drive are helical cut. Straight cut gears are cheaper and easier to make and also don't end load the bearings like helical cut gears do. Helical cut gears however are much quieter running as you can see.

If you do a google search for a Muncie M-22 you will find an explanation of this, that's a notoriously noisy yet durable tranny, reason being that the helical cut of the gears is much shallower than standard.

nate

(music to my ears...)

Reply to
N8N

It's the type of gears being used.

In most modern transmissions, the forward gears are "helical cut" - Vastly oversimplified, a helical gear is essentially a slice taken from a screw that has as many threads as there are teeth on the gear - Stack up several of them and line up the teeth, and you'd re-create the screw. Most (but not all...) reverse gears are "square cut" - Using the same concept as for helical gears, they are essentially a slice taken from a splined shaft that has as many splines as the number of teeth on the gear - Stack several of them up with the teeth aligned, and you get a "shaft" with grooves that run straight along the length of the shaft, as opposed to the helical cut, where the grooves spiral around it

Helical gear teeth mesh *MUCH* more quietly than square-cut because basically, they "slide onto" each other to mesh and un-mesh, while the teeth on two square-cut gears "crash into" each other as the teeth mesh and un-mesh.

The hum/buzz/whir/etc you hear from reverse is the high-speed "crashing together" of each tooth of the gear coming into contact with its counterpart on the other gear.

Reply to
Don Bruder

Two reasons: 1) its usually a slightly different ratio than other gears, and 2) in some transmissions reverse uses straight-cut gears instead of helical gears like all the forward gears do (except in race transmissions which use straight gears everywhere- which is why you can hear the transmission over the sound of the open exhaust system when NASCAR broadcasts show an in-car audio/video segment).

Reason #2 only applies to manual trannies, automatics use all helical-cut planetary gears.

Reply to
Steve

Thanks everyone. Never in a million years would my pathetically feable automobile knowledge have led me to these explanations.

Reply to
Evan

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