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Gunman (Automotive) (OP)
7 Dec 05 9:42
Any thoughts on how to quiet a straight cut gearbox?  simple is better (as opposed to Comanche helicopter style electronic noise cancelation, etc.)

Everything should be designed as simple as possible, but not simpler.

evelrod (Automotive)
7 Dec 05 14:37
Use thicker lubricant.  Is that simple enough?

Gunman (Automotive) (OP)
7 Dec 05 14:57
Yep, simple ideas like that is what I'm looking for.

Everything should be designed as simple as possible, but not simpler.

GregLocock (Automotive)
7 Dec 05 15:15
Clad it in rockwool and lead.

More serously, you are facing an uphill struggle. That's why virtually every production gearbox uses some form of constant contact (ususally helical) gears.


Greg Locock

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Gunman (Automotive) (OP)
8 Dec 05 8:59
Yeah, I realize it wouldn't be easy.  We've just had good results using this gearbox on the track, but get complaints on its noise when used on the street.  This results in using two different boxes, and other issues.

Cost vs. noise vs. power handling...its a viscious circle :)

Thanks for the input

Everything should be designed as simple as possible, but not simpler.

carnage1 (Electrical)
15 Dec 05 1:46
Undercoat the car, coat the drivshaft in undercoating then rebalance to eliminate it as a resonator, make the rear transmission mount into a damper to avoid the gearbox using the car body as a sounding board, or turn up the stereo. Any of these things will help your problem some. (maybe not the last idea)
The most effective thing you could do is cover the transmission with a damping layer such as the three layer aluminum and urathane stuff you can get. the problem with this is the possiblilty of trapping to much heat and damaging the transmission.

evelrod (Automotive)
15 Dec 05 13:56
gunman, you said SIMPLE...for a combination street/track driven car (poor idea as you end up with an underperforming, often dangerous, racecar and an uncomfortable, noisy, often dangerous, street car, IMO)...

SIMPLE yourself a case of those little yellow foam ear plugs and put ATF in the gearbox!!!!!


Gunman (Automotive) (OP)
16 Dec 05 9:43
Yellow ear plugs as a standard feature...hmmmm :)

Actually, I think we're either going to leave that gearbox option noisey on the street, or find a different box.

Thanks for the input!

Everything should be designed as simple as possible, but not simpler.

NormPeterson (Structural)
17 Dec 05 10:21
Is a compromise on helix angle a possible solution?  Not quite straight-cut, but using a much shallower angle than normal for street-intended devices?  More or less, I think that was the approach taken by Muncie with their M-22 box (mid 60's).

Rob45 (Automotive)
6 Jan 06 14:16
Make that "virtually every production *CAR* gearbox" uses helical gears,  and I'll agree.
For those of us who deal with straight-cut gears on a daily basis, however,  Gunman's question is a good one;  unfortunately, there are few simple answers.
I deal with transmissions for which I can identify in the interior noise spectrum the .5, 1st, 2nd, and 3rd harmonics of both input gearset and output gearset tooth passing frequencies,  and sometimes they are VERY annoying!

One thing to be done is to have straight-cut gears that are as near perfect as possible,  then hone or polish them to improve the surface finish.

Having as rigid a case as possible helps to reduce the radiation efficiency of the gearbox.  Think ribbing.

Next,  the gearbox shafts must be as stiff as possible and the gear assemblies rigid to avoid harmonics of the gear-tooth passing frequencies from generating noise.

The mounting of the gearbox to the chassis should be as compliant as possible to eliminate sound transmission paths, and clutch rods or clutch cables should also have rubber mountings to the transmission/clutch housing and to the body, again to avoid being a transmission path.

The shift lever should incorporate rubber isolator(s) for the same reason.

Then if the gearbox has an oil cooler,  it can have sound barrier/absorber panels affixed to its surfaces;  but not if there's no cooler!  Mechanical gearboxes generate more heat than you'd think.

Finally,  the driveshaft should incorporate damping material to avoid excessive ringing especially at the geartooth passing frequencies.

Remember: It's not how much noise the gearbox makes that's important,  it's how much gets to your ears!

 - R

Gunman (Automotive) (OP)
9 Jan 06 8:11
Thanks for the input Rob.

Everything should be designed as simple as possible, but not simpler.

ornerynorsk (Industrial)
12 Jan 06 7:41
Sawdust in the gear casing?????  It used to work for automobile dealers !
kenre (Mechanical)
12 Jan 06 8:19
Sawdust in the gear casing?????  It used to work for automobile dealers !

Ive heard banana skins work in diffs.

evelrod (Automotive)
12 Jan 06 12:51
Listen, guys.  I don't think there are very many members here that have actually done any of these things.  Mostly I believe that they are usually filed under "urban legands"...not so!
When my dad retired from the fire department in 1958, he bought into several businesses...pawn shops, used car lots...where I worked part time (and a couple gas stations).  One of the primary sources of income for me was to take the 'clunkers' that could not be sold on a lot and sell them through the classifieds...Let me say up front, that was a looooooong time ago and the mechanics(?) at the lot really did some baaaaaaaad things to cars that were sold. Saw dust, bannana peels, egg whites in the radiator for stop leak...  Lots of other things to get them to run long enough to make it across the border. The cars usually would not pass the Texas safety inspection and most were sold to be exported (illegally by the purchaser, I might add) to Juarez, Mexico.

All this is not legal or moral, then or now.  I do sincerely hope that these things are not still as common a practice as they were in the 50's...I have my doubts.

Rod, "Would you buy a used car from this guy?" sad
ornerynorsk (Industrial)
16 Jan 06 9:23
Truth be told, straight cut gears will always be noiser than helical.  I don't think there is much you can do about it.  The costs associated with all of the "band-aids" that are mentioned in the above posts might be better spent on investing in the better gears in the first place!
ivanheow (Automotive)
28 Jan 06 6:13
there is a part in the modify your mini book which talks about taking a few thuo off the tips of the gear teeth on a straight cut gearbox ,this was supposed to prevent the hih speed jet of air rushing out of the gap as the tooth tip fitted into the valley of the oposing gear .this was said to create a significant reduction in noise ! regards robert.
kenre (Mechanical)
14 Feb 06 9:10
Ivanheow, similar things are done to toothed belt drives, eg Supercharger pulleys, by drilling small holes between the teeth its meant to cut down noise. No idea on a gearbox tho.

Rob45 (Automotive)
20 Feb 06 11:54
There are good engineering reasons to use straight-cut gears rather than helical gears,  when noise is not an issue.
I'm not a gear design engineer,  but it seems to me that straight-cut gears are a bit stronger,  and if space is an issue,  as when squeezing 6-speeds into a gearcase designed for a five-speed (or 4-speed!),  you might wish to have the greatest amount of strneght possible,  in a gear that's smaller than was originally used.
evelrod (Automotive)
21 Feb 06 13:00
The exprience in trimming the teeth of a straight cut gearset in a Mini (I use the straight cut 'clubman' gearset in mine) is from David Vizard's book and is more a performance/reliability mod than noise.  Noise in a Mini is "normal" along with the ubiquitous oil leaks. winky smile

ivanheow (Automotive)
7 Mar 06 12:22
yes evelrod thats true ,still trying to get my turbo mini to be as quiet as id like ,but the straight cut boxes are so loud yuo can be about 1/2 a mile away from one and still hear it coming .
Rob45 (Automotive)
24 Apr 06 13:36
If you can hear the gearbox whine a half-mile away,  I'd say there's a serious problem with the gears!
Virtually every heavy truck (Classes 6 through 8 in North America) has a gearbox using straight-cut gears throughout, and which are usually dealing with 1650 - 2050 lb-ft of torque.
While noisy,  it's not the gearbox you hear when one drives by.

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