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Why does a car with manual gearbox whine when driven in reverse ? You know what I mean ? ...... the type of noise (from the gearbox ?) that you get in reverse but not forward. Just curious.
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You have at least one more idler gear,which usually is fitted with a slightly bigger tolerance(so that engagement is easier) and it doesn't need to be used very much.Also it puts the output shaft(and all the other bits fitted to it)effectively on the overun where the tolerances and wear patterns are different.
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It's because reverse gear in most boxes is straight cut. Helical gears, used for the forward ratios run smoothly as the contact point of torque transmission is arranged to overlap between one tooth and the next.
659.
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Yup most reverses have a straight cut gear, whereas forward gears have a helical cut. They are hellical cut for the very reason of quietness.
Tho it has to be said, I have not had a whinning reverse gear in my last three cars.
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TourVanMan TM < Ex RF >
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They used to be straight cut now many have synchro on reverse.
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A previous car of mine, a manual Volvo S60, didn't make the usual reverse whine, all you could hear was the engine noise regardless of how fast you reversed.
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Which is why a "straight cut" Mini used to whine going forward as well as backwards.
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My Corsa used to whine like anything in reverse; current Ibiza makes no noise at all.
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Straight-cut gears are used in competition gearboxes/engines (racing) as they are much tougher than helical cut gears.
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The change from straight to helical cut is not just a matter of substituting a different (and more expensive) gear. The point about a straight cut gear is that you can slide the actual gear along its shaft for engagement. I don't think a helical gear will do that, because the teeth have to engage at an angle. So helical gears have to be constant mesh, necessitating a hub with a ring of teeth to engage with a corresponding set of notches. Early constant mesh gearboxes left it at that - they were a lot easier to change than the old straight-cut "crash" boxes, but still did not have synchro mesh. The final refinement was to incorporate a synchronising clutch into the hub, a feature now extended to include reverse gear as well.
I remember early LandRovers before the introduction of helical cut teeth in the transfer box. In low range all the gears whined, even top.
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The other advantage of straight cut gears for high power applications is that, unlike helical gears, they don't transmit any end thrust on the shaft.
JS
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It is in fact possible to use a helical gear for a reverse pinion without problems of engagement or endthrust. As long as the pinion is not stepped, ie. it has the same number of teeth on the driven side as well as on the driving side, the endthrust forces almost cancel. Sliding a helical pinion into mesh with a helical gear is no problem at all - as any Meccano set owner will tell you.
The Swedish SAAB 99/900 gearbox used precisely this arrangement to good effect - a sliding helical pinion with equal input and output tooth counts. It was almost silent in reverse, and had to engage "nicely" because of the gearlever ignition lock.
659.
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Indeed, and another reason for the noise is that the teeth of reverse gear tend to be ground to a lower tolerance than the forward gears which are in mesh at high speed even if they are not transmitting torque - owing to the sliding mesh action of reverse, there are no meshing teeth when you are going forwards, hence, no noise.
The argument is that most people only use reverse for a short period at low speed, and so aren't too concerned if reverse is a little noisy, so, a cheaper solution is good engineering, and appropriate.
Although it is mentioned elsewhere in this thread, I'm not entirely convinced of the argument that straight cut teeth are any stronger than heical. For a given width of gear, there is more facewidth for the helical, so contact stresses should be less, and I don't see any argument that bending stress will be any less for a straight cut gear. I think the main reason they are used for the forward gears of these 'boxes is that they are much easier and cheaper to make, and for sporty gearboxes, straight cut teeth offer lower friction.
Number_Cruncher
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I overhauled the gearbox from a 1947 Triumph Roadster many years ago. All the gears were double helical, looking like tractor tyres. Some pairs had a clear line round the centre portion, on others the teeth on opposite sides overlapped, which presumably was even more expensive to machine. Not only was there no end thrust, but the gears were precisely located by interelation of the teeth alone, so could not slide along the shafts.
It still managed to whine however, and occasionally shed teeth.
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My old Strada 130TC Abarth had a ZF gearbox with a nice whine in all forward gears from new. Allegedly the gears weren't helical. Lovely to drive - just a shame that it was so badly put together.
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Remember reading somewhere that Citroen boxes used double helical gears, hence the ">>" logo still used on Citroens today. Doubt whether modern PSA gearboxes still use that technology....no doubt someone on here will know!
Incidentally, why don't autoboxes have straight-cut reverse gears?
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>>Incidentally, why don't autoboxes have straight-cut reverse gears?
Because they don't have dedicated reverse gears (or dedicated any gears come to that!) as such.
The gears in an automatic gearbox are typically a cascade of two sets of epicyclic geartrains, and all the ratios are produced by varying which parts of the geartrain are driven or held via hydraulically controlled clutches and brake bands. So, in an automatic, typically, all the gear pairs are involved in producing all of the ratios at different times, and you can't point to a gear and say that's 1st, 2nd, 3rd or reverse, like you can in a manual 'box.
Number
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