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Old 09-08-2010, 02:15 PM
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Default straight vs helical cut gears

When I had my first dirt bike, the manufacturer boasted of helical cut gears for added strength and the ability to handle loads. Certainly, gearbox noise on a motocross bike is not an issue.

Now I understand that straight cut gears are stronger, but are substituted for helical gears for noise reduction, as in new Harleys.

OK, they why is my fifth gear sound louder than the other gears? If I cruise in 4th, I hear more engine (exhaust) noise. If I cruise in 6th, it's as almost as if I'm in a glider type of quiet (full coverage helmet, stock pipes). In 5th gear, I can hear the gearbox. Tranny's fine, I just am trying to navigate around all the hype. What's the deal?
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Old 09-08-2010, 02:31 PM
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Bart van der Meulen Bart van der Meulen is offline
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every gearbox has its own 'critical' revs. That means that the vibration and sound of gear tooth gripping into one other is heard at a 'critical' level. That means that your gears into the range of revs that is indicated for that gear is different. That's why your engine revs is lower at a certain speed in a high gear than in a lower gear. What you hear is the 'critical' revs of that gear at that particular speed. Your engine has also a 'critical' speed in revs. And on top of that the resonance of the sounds is amplified or weaken by the frame, the tank, the covers etcetera.
That means that 'sound' of gears (or vibrations) is very difficult to determine at all speeds. Don;t you worry, an board of the ocean freighters the gigantic gearboxes (over 10,000kW) starts to 'sing' at certain speeds of the ship and certain revs of the main engine. It's a normal phenomena with gearboxes.
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Old 09-08-2010, 03:09 PM
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Quote:
Originally Posted by TORQUEY View Post
Now I understand that straight cut gears are stronger, but are substituted for helical gears for noise reduction, as in new Harleys.
Helical gears are stronger and quieter than straight cut gears. The strength increase is due to the increase in contact area of the diagonal tooth.

Straight cut gears are more efficient as they do not have the sliding frictional component that helical gears have.

Like the above poster mentioned gear whine is nothing to worry about. It's just the nature of the beast.

The type of lube can dampen the noise quite a bit. Try running some Bell Ray severe gear in the tranny if it bothers you.
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Old 09-08-2010, 03:34 PM
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i agree that if the noise bothers you a heavier gear oil will help
as mentioned im sure bell ray would work fine. I use red line heavy gear oil in mine
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Old 09-08-2010, 05:57 PM
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Quote:
Originally Posted by Bart van der Meulen View Post
every gearbox has its own 'critical' revs. That means that the vibration and sound of gear tooth gripping into one other is heard at a 'critical' level. That means that your gears into the range of revs that is indicated for that gear is different. That's why your engine revs is lower at a certain speed in a high gear than in a lower gear. What you hear is the 'critical' revs of that gear at that particular speed. Your engine has also a 'critical' speed in revs. And on top of that the resonance of the sounds is amplified or weaken by the frame, the tank, the covers etcetera.
That means that 'sound' of gears (or vibrations) is very difficult to determine at all speeds. Don;t you worry, an board of the ocean freighters the gigantic gearboxes (over 10,000kW) starts to 'sing' at certain speeds of the ship and certain revs of the main engine. It's a normal phenomena with gearboxes.

Excellent commentary on sound to be sure and I thank you. What seems odd, though, is that 5th seems louder throughout its range than the other gears. All is fine though. Thanks again.
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Old 09-08-2010, 05:57 PM
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Points well taken by all. Thanks you.
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Old 09-08-2010, 07:40 PM
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Spur gears are easier to make so cost less.

Helical gears stronger, run quieter.

Harley's 5th gear was a straight/spur gear. This is the reason for the gear whine. I guess HD thought people would be using 6th a lot more often and not notice the noise.

However after a lot of complaints HD switched the 5th spur over to a 5th helical. Not sure what year this was done, 09?

Another example of HD cutting costs only to have it bite them their ***.

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Old 09-08-2010, 07:41 PM
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Wow... what an education a person can get just reading this forum! Amazed by the experts..we have onboard!!
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Old 09-08-2010, 07:53 PM
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Straight gears create more noise but they do not load the cases as helical gears do. I believe spur gears are stronger they can carry more load but creat more heat (90/140 oil in those trannys). Helical give the impresion of more load bearing cause of the surface area of the contact, but in turn loads the cases and you will see more broken cases then gears(these tranys mostly use ATF fluid.

You want a trans you can beat on and not worry about get spur gears.
Other wise with the heli cut gears your trans is only as strong as the cases and they are aluminum.
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Old 09-08-2010, 08:29 PM
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Straight gears create more noise but they do not load the cases as helical gears do. I believe spur gears are stronger they can carry more load but creat more heat (90/140 oil in those trannys). Helical give the impresion of more load bearing cause of the surface area of the contact, but in turn loads the cases and you will see more broken cases then gears(these tranys mostly use ATF fluid.

You want a trans you can beat on and not worry about get spur gears.
Other wise with the heli cut gears your trans is only as strong as the cases and they are aluminum.
Where does that leave the typical modern HD tranny with one helical and five straight cut gears...just curious.
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Old 09-08-2010, 08:29 PM
 
 
 
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