Bus Transmissions

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ryan125hst

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Does anyone know why bus transmissions seem so much noisier that automatic car transmissions?

Take this video as an example:

It whines loudly even when the bus is stationary. This whine seems typical of Voith gearboxes - I've certainly heard many buses making such a sound.

The retarder can also be heard when the bus is slowing down. This is something not found in cars however so more noise during braking is understandable.

And then there's this:

This Allison gearbox is making noises as it changes through the gears which are quite difficult to describe. I can only assume it's caused by the clutches and or breaks within the transmission being applied/released?

So why is this never heard on cars with automatic transmissions?
 
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Bletchleyite

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Because cars have better engine bay to cabin sound insulation, and also the engine bay is basically outside of the passenger cabin whereas the bus engine bay is part of it?
 

Journeyman

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Also I'm guessing bus transmissions are a much more significant piece of machinery dealing with much higher power inputs and outputs, and far greater stresses.
 

CN04NRJ

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Most are virtually silent now, even the latest Voiths. Shame as there's nothing like a tuneful ZF Ecomat for me :lol:
 
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Some more modern buses came with sound insulation for both engines and gear boxes. This would be thrown away at the first opportunity for some reason. Fitters told me it was for access.
 
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RJ

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Most are virtually silent now, even the latest Voiths. Shame as there's nothing like a tuneful ZF Ecomat for me :lol:
This is about the best you'll get in London nowadays, where ZF Ecomat is dying a death. However that ringing in 6th gear is so loud, one has to wind up the electric window in the cab!

 

ryan125hst

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Because cars have better engine bay to cabin sound insulation, and also the engine bay is basically outside of the passenger cabin whereas the bus engine bay is part of it?
I'd definitely agree from an interior perspective, but equally you don't hear a BMW making noises similar to a bus with a Voith gearbox while stopped at traffic lights. :lol: There must be something else that causes it.

Also I'm guessing bus transmissions are a much more significant piece of machinery dealing with much higher power inputs and outputs, and far greater stresses.
Some cars have high horsepower engines and use automatic transmissions. The Ford Mustang, for example, has a V8 producing 460 hp and can be purchased with a 10-speed automatic transmission. Clearly there's a big weight difference but that transmission will have to deal with a lot of power. I guess such as gearbox is vastly different to the three or four speed ones found on buses though!
 

Jordan Adam

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Because cars have better engine bay to cabin sound insulation, and also the engine bay is basically outside of the passenger cabin whereas the bus engine bay is part of it?
Also I'm guessing bus transmissions are a much more significant piece of machinery dealing with much higher power inputs and outputs, and far greater stresses.
Some more modern buses came with sound insulation for both engines and gear boxes. This would be thrown away at the first opportunity for some reason. Fitters told me it was for access.
It's actually nothing to do with the engine, weight or soundproofing.

Gearboxes like the Voith Diwa and ZF Ecomat use Straight Cut Gears, these cost less, are easier to maintain, more durable and more efficient but produce a distinct whining sound, the faster the gear rotates the louder it gets hence the variation in pitch. The "Whoop" sound ZF Ecomats make when changing gear comes from two of the cogs impacting. Newer "silent" boxes such as the ZF Ecolife use Helical Gears, these have a slanted teeth and as such produce next to no sound.

Below is an image showing the difference between a Helical Gear (top) and straight cut gear (bottom)


With cars straight cut gears are only really used now for racing. That being said many cars do still use a straight cut gear for reverse. Linked below is an example of a car with straight cut gears, as you will hear it makes the exact same sort of whining sound that a Voith, Allison AT545 or ZF Ecomat bus would.
 

Journeyman

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That whine sounds a lot like vintage British Leyland cars! Remember Minis and Metros? They whined furiously, especially in first gear.
 

ac6000cw

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That whine sounds a lot like vintage British Leyland cars! Remember Minis and Metros? They whined furiously, especially in first gear.
IIRC that was because they also had straight-cut first gear pinions (and no synchromesh on first gear either). I owned a Metro for a few years, back in the 1980s.

I think the use of straight-cut lower gears is (or used to be) pretty common on gearboxes that have to deal with a lot of torque in a compact 'box e.g. trucks, buses & motorcycles. Those sort of vehicles often didn't have syncromesh either for much the same reason, requiring the use of double-declutching to get good gearchanges.
 

L401CJF

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It's actually nothing to do with the engine, weight or soundproofing.

Gearboxes like the Voith Diwa and ZF Ecomat use Straight Cut Gears, these cost less, are easier to maintain, more durable and more efficient but produce a distinct whining sound, the faster the gear rotates the louder it gets hence the variation in pitch. The "Whoop" sound ZF Ecomats make when changing gear comes from two of the cogs impacting. Newer "silent" boxes such as the ZF Ecolife use Helical Gears, these have a slanted teeth and as such produce next to no sound.

Below is an image showing the difference between a Helical Gear (top) and straight cut gear (bottom)


With cars straight cut gears are only really used now for racing. That being said many cars do still use a straight cut gear for reverse. Linked below is an example of a car with straight cut gears, as you will hear it makes the exact same sort of whining sound that a Voith, Allison AT545 or ZF Ecomat bus would.
Beat me to it, I was just about to say straight cut gears!
 

ryan125hst

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It's actually nothing to do with the engine, weight or soundproofing.

Gearboxes like the Voith Diwa and ZF Ecomat use Straight Cut Gears, these cost less, are easier to maintain, more durable and more efficient but produce a distinct whining sound, the faster the gear rotates the louder it gets hence the variation in pitch. The "Whoop" sound ZF Ecomats make when changing gear comes from two of the cogs impacting. Newer "silent" boxes such as the ZF Ecolife use Helical Gears, these have a slanted teeth and as such produce next to no sound.

Below is an image showing the difference between a Helical Gear (top) and straight cut gear (bottom)


With cars straight cut gears are only really used now for racing. That being said many cars do still use a straight cut gear for reverse. Linked below is an example of a car with straight cut gears, as you will hear it makes the exact same sort of whining sound that a Voith, Allison AT545 or ZF Ecomat bus would.
Brilliant stuff! I thought it'd be something mechanical so thanks for this Jordan. I've also found out that reverse gear on cars tends to be straight cut which is why cars reversing quickly also make a racket.
 

Egg Centric

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Also I'm guessing bus transmissions are a much more significant piece of machinery dealing with much higher power inputs and outputs, and far greater stresses.

It's been explained that it isn't the power, but just to expand on this it's not just muscle cars like the Mustang which are more powerful than buses. My car - which as a 2006 hot hatch (130i) is quick but not fast - has more power than a bus and I don't think that that is unusual.
 

Richard Scott

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It's been explained that it isn't the power, but just to expand on this it's not just muscle cars like the Mustang which are more powerful than buses. My car - which as a 2006 hot hatch (130i) is quick but not fast - has more power than a bus and I don't think that that is unusual.
Would think torque is more relevant than power as power is torque times speed so likely a higher revving petrol engine is likely to be more powerful than a lower revving diesel engine. Torque is turning force so this is what the transmission has to deal with so power not really relevant.
 
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