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Autobanmod
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9,896 Posts
Discussion Starter #1
Straight cut 2nd gear is used in the Destroyer model but not in the "normal" models.
However helical gears are "stronger" by design, more surface, more meat.

So I wonder if I actually would weaken my transmission if I go for the straight cut 2nd gear set instead of the stock helical cut set.
Are the straight cut 2nd destroyer gears wider or made of better material to compensate the design disadvantage?


I don't plan to install an air shifter, nor am I worried about drag strip times. My priority is just durability / torque resistance.

Any input?
 

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Elephant Motors Rock
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3,505 Posts
ElrodontheVRod is a transmission guy, you might want to ask him. It was my impression that the straight cut gears are stronger but make more noise (whine) that can be undesirable to general consumers.
 

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Gone
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9,302 Posts
I thought the straight cut gears were about putting less stress on everything else since they simply spin instead of having a tendency to pull lengthwise. I don't know if I'm wording that right.
 

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Autobanmod
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9,896 Posts
Discussion Starter #7
http://en.wikipedia.org/wiki/Gear#Helical
The angled teeth engage more gradually than do spur gear teeth causing them to run more smoothly and quietly. With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face to a maximum then recedes until the teeth break contact at a single point on the opposite side. In spur gears teeth suddenly meet at a line contact across their entire width causing stress and noise. Spur gears make a characteristic whine at high speeds and can not take as much torque as helical gears.
I don't have a problem with thrust but with torque.
 

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durata membro
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17,603 Posts
I work on transmissions for a living,but I am far from an engineer.
There have been failures of the V&H gears too.I don't know why they fail,but with the ratio change and the cut of the gears I say the new gear set will transfer the power more efficiently.Plus you can't go wrong with the new gears because the set you have now are showing a little wear in a couple of spots.:)
 

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Autobanmod
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9,896 Posts
Discussion Starter #9
lol, yes it is indeed so that I have to make a purchase order soon...
But I do have a choice, Larry from Eagle HD told me that he can also get the helical gears. hmm, not easy.
 

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Straight cut a little more efficient and a lot more noisy. For the same width helical gear stronger and quieter. A little more expensive (assuming the same material and treatment) to make as more surface has to be machined. The downside is axial load-some of the energy used to "push gear sideways" thus a little less efficient.
 

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Premium Member
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1,053 Posts
Must be a reason HD are using straight cut gears for second in their drag bikes.

If the gears were "new" when they gave out this could be down to point loading as they need some running time to fully engage to then spread the load. Like all machined parts a little high spot takes all the load till its worn down.
 

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The Massive Pr1ck
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Wiki is wrong, straight cut is stronger. Helical has a smaller contact area engaged at any one time and therefore higher point loading.
 

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EXCELSIOR
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8,473 Posts
Always been my experience that Helicals are quieter, constant engagement though smaller contact patch, but have side loading. I was taught that straight cut of equal width are stronger but noisier. Go figure.
I have seen large pieces of heavy equipment where alternate helical pairs, aka. Herringbone gears, are used to achieve both results but I wouldn't confuse them.
 

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'04 1300
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3,071 Posts
A lot of us have the V&H non-Destroyer straight-cut 2nd gear set. Mine's a fair bit noisier than stock, but the big difference is the change in ratio. The 1-2 jump is a lot shorter, which I think in your application would result in less stress on the driveline. Considering the number of straight-cut V&H 2nd gear sets in use, I would consider them a safer bet than another 2nd gear option.
 

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No offense mikeinatlanta, but I'm sure you will change your opinion if you will decide to go to university and get two degrees in petroleum and mechanical engineering. In my opinion, a lot of peoples here are correct: herringbone is the strongest and like sburke91 mentioned, most of advantage comes from ratio change for the 2nd gear. There is many other reasons, why race bikes and cars switch to straight cut and power is not the main reason. Again, I'm not trying to offense you or anybody else here.
 

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Premium Member
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7,727 Posts
A lot of us have the V&H non-Destroyer straight-cut 2nd gear set. Mine's a fair bit noisier than stock, but the big difference is the change in ratio. The 1-2 jump is a lot shorter, which I think in your application would result in less stress on the driveline. Considering the number of straight-cut V&H 2nd gear sets in use, I would consider them a safer bet than another 2nd gear option.
I really have not noticed any increased 2nd gear noise. Maybe the Comp pipe is too loud.
 

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'04 1300
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3,071 Posts
I really have not noticed any increased 2nd gear noise. Maybe the Comp pipe is too loud.
I think it's just my particular set. It didn't make more noise originally, but developed it over the past year or so. No significant metal in the drain pan though, so hopefully it continues to hold together.
 

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Helical gears are quieter (less vibration) than straight cut because there is constant contact between the two meshed gears; one tooth doesn’t disengage until the second is completely in contact. Depending on the tolerance of the gear, straight cut can disengage one gear before the second catches. The resulting impact may fatigue the thread root. Typically straight cut takes up less space, and tend to be cheaper to machine.
 

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The Massive Pr1ck
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No offense mikeinatlanta, but I'm sure you will change your opinion if you will decide to go to university and get two degrees in petroleum and mechanical engineering. In my opinion, a lot of peoples here are correct: herringbone is the strongest and like sburke91 mentioned, most of advantage comes from ratio change for the 2nd gear. There is many other reasons, why race bikes and cars switch to straight cut and power is not the main reason. Again, I'm not trying to offense you or anybody else here.
Interesting you would first state no offense and then proceed to attack not the validity of what I stated, but my level of education, which you know nothing about. So yes, offense taken.

EDIT: All things being equal in theory, I agree helical has strength advantages, however, in real world applications, switching to straight cut allows using fewer gear teeth with a wider tooth root than what is possible with helical. This wider tooth root and reduced case loading are what is responsible for the added strength. Classic example of real world engineering vs theory.
 
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