Helical vs straight cut primary gears

[mac]

I think that I,ve asked this before on the old forum,but don,t remember the answer.What is the disadvantage of the helical primary gears in my Wombat vs the straight cut gears? Will it be a big deal to keep them with the reeded top end and a stock CW exhaust?

[bchappy]

Straight cut gears allow more power to get through as there is no side drag. However, they make a little more noise than the helical cut. Unless you are racing and need all available horsepower the helical cut will work just fine in the Wombat.

[Zyx]

Back in the dark ages of youth, replacing the heli gears on the Ace100 with straight cuts from the Super Rat was considered mandatory. But that was then. All things equal, helis are a bit stronger perhaps, but at a price. Helical gears create a side load due to tooth slip contact which straight cuts don't. This creates more heat, and more wear on shaft spacers. But, proper lubrication makes these differences moot. Straight gears transmit more of the torque otherwise available simply because there is no loss to side load friction and heat. However, these differences are really too small to be significant to sport riders. Helical gear sets are quieter because each tooth mesh takes place over a longer duration of gear rotation.

Unless racing, I would not go out of my way to replace helical cut gears with straight. Reeded power output has nothing to do with choosing gears in my opinion. The differences are too small to be important. None of our bikes make so much power that it would jeopardize the primaries.

Biggest advantage of helical gear: reduced noise.

[taber hodaka]

Straight cut gears came out for the ace 90, as well a most of the other race items chamber, piston, cylinder, head ect. I think straight cut gears were a bigger advantage on the smaller ace 90. I really did not hear any difference in the gear noise or have a heat and wear problem. I think the straight cut gears let my engine overwind a little more increasing my top speed??. ----------Clarence

[Zyx]

You couldn't hear the gear whine because of the exhaust note. Straight cut (spur) gears don't wind any higher or faster, they only produce a tiny bit less parasitic drag. But, trading Ace 100 heli gears for Rat spur gears would get you a different ratio, 3.63:1 for the Ace 100 versus 3.71:1 for the Rat, which can and does change how it revs. Same would be true swapping 94 wombat helis to 95 combat wombat spur gears. The seat of the pants difference is in the lower gearing of the corresponding straight system in the race bikes compared to the higher gearing in the street bikes.

It is cheaper and easier to just change the rear sprocket by adding a tooth or two, or dropping one on the countershaft sprocket, to get the same 2% gear change result.

Example: Ace 100 @ 3.63:1 change to 3.71:1 from the Rat, a difference of 3.71/3.63 = 1.022. Roughly a two percent change. 1.022 x 50 tooth rear sprocket = 51.1 teeth, rounded up to 52. Not really so much of a change and a lot easier to do with chain and sprockets than with primaries.

[hodakamax]

I forgot all I learned in trig a long time ago but my primitive calculations show the straight-cuts have about 90% of mesh area compared to the helical ones but as they say in racing --Ever little bit helps and it all adds up. I dropped out of the Vector Mechanics class so I won't be computing the side load difference for you but it might be interesting to see if it has any significance at all. At least you got me thinking for the day!---

Maxie's input.

[racerclam]

What has been said about the helical verses the straight cut gears is true but what isnt mentioned is that the Hodaka helical gear is actually a cast iron piece of crap and I have seen them shed teeth in high performance use. The straifgt cuts are a high quality pirce of steel and one would never break one. However the small primary gear on the crank , I have had one split on my SC , its not from the quality of the steel but rarher from all the sharp cuts in its shape creating stress risers. I recommend straight cut when ever you can aquire them and especially the cushioned unit..

Rich

[hodakamax]

Not only did I forget all my trig, I also forgot that the straight-cuts were made of steel---Reliability is most important. Now you have me looking for straight-cuts for my 94A vintage racer---Good call Rich!
Hmmm---Will any of the cushioned units fit my 94A?

Thanks, Max

[racerclam]

Yes combat wombat , super combat and late rat will work

Rich

[hodakamax]

Thanks for the info Rich!--I found both cushioned and non on the large gear but small one 954001 doesn't seem to be available anywhere--I'll keep looking. I've got it on my wish list--
Again Thanks---
Max

[mac]

Thanks to everyone for all of the information.

Max,thats one of the reasons I asked,the primary gear seems easy enough to get from Paul (good price also),but the clutch pinion gear might be hard to source although I haven,t tried too hard yet.I would say that if your 94A made it through the ISDT RR I should be OK as long as I keep my cool and don,t get carried away in the heat of battle and abuse it.

I am going to race it,but the plan is to use it for cross country and not moto.I,m guessing that if you don,t really hammer on it and snick your shifts,as I,ve read Ed talk about,things will last longer.I,m use to riding Spanish bikes and have learned over the years that they hold up much better when you always use the clutch and snick the shifts.I use to get a little too excited in the heat of a moto and just hammer on the poor shift lever and trans.

Thanks again to all for your input, Mac

[hodakamax]

Hey Mac--I think your probably right--I'm not as hard on equipment as I was in the past. I don't remember any abusive shifts at the ISDTRR at all and getting back to reality, this totally rebuilt 94A will probably last longer than me!----Just thinkin'!-- (I was in the 70+ class).

Thanks for your input also. Everyone's input is what makes the forum so useful and more importantly, so fun!

Maxie