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Hypoid gear ratio and efficiency question

magneticanomaly

Titanium
Joined
Mar 22, 2007
Location
On Elk Mountain, West Virginia, USA
A friend from the auto industry and I were discussing the existance of overdrive ratios in vehicle transmissions. An overdrive is, at first blush, kinda counter-intuitive. Why gear up in the trans when you are going to gear down in the final drive? If you want a higher (numerically lower, faster) overall ratio, why not just use a final drive with less reduction, and put more reduction in the low gears of the transmission to get start- and grade-ability?

I suggested that the answer was to limit maximum torque though the driveshaft. He said he was told that very fast/numerically low ratios in hypoid gear final drives lose efficiency due to larger sliding component, and that limits the fastest practical final drive ratios. I am not convinced. I have seen 2.73 final drive ratios, and seen overdrive transmission gears in front of final ratios n the 3s, and 4s. I had a car with a wormgear final drive that got 34 MPG, so I do not see huge efficiency loss from sliding in a final drive.

We have guys on here who know a lot about gears.

Opinions? "the Answer"? Thanks!
 
Can't really answer your "Question" directly - a lot to unpack there,

But more a progression of anecdotes, from someone that used to drive "Stick-shift" in the UK and France , back in the day...

Fast driving on narrowish and sometimes windy(sp) roads at speed would require one to overtake another vehicle in a relatively comprehensive way; usually even if one is going 40, 50 or even 60 miles an hour the technique was to drop the car down into 2nd gear and floor it.

Obviously over-reving / rpm-ing the engine temporarily, great acceleration at higher speed and usually a pretty successful / bankable maneuver. In France I found myself playing "Chicken" with on-coming vehicles that would also be over taking. - Seems perfectly normal in France / normal way to get from A to B.

Unlike the Friends "Theme Tune" ~ Driving around in second gear is not a fun experience due to high rpms - engine noise + constant and exacerbated gear box / transmission noise and presumably wear and vibration.

Take that one step further - transmission for something like a old school stick shift Land Rover. It's small underpowered (back in the day unless V-8 twin carbs etc.) YET can pretty much pull anything as the transmission is 1/2 tractor transmission. So you could pull (for example) large (horse) trailers at reasonable speed on the "Motorway" / Freeway BUT the freak'n transmission is so loud you can't have a "Conversation" you'd have to literally shout to converse - A good 75% of the noise was from the transmission. [Older - less precise gears with a bit of "Tractor" thinking behind it.].

Twenty five years later (in the USA) I have Diesel Powerstroke F-350 that gets stuck in the mud (at my own ranch) after heavy rains (two wheel drive not 4 wheel drive )... So three different friends try to pull my truck out of the mud with their various pick up trucks and fail. Finally I grab a small-ish Kubota tractor (front end loader) - HYDROSTATIC transmission and pull the f-350 out of the mud - no problem ! Small three cylinder diesel engine with a massive transmission. Still kinda noisy. [I have a "Yuppie-truck" F-150 with a "Stealth" raptor engine block - I like that for regular driving around. Overdrive works pretty well but I have to admit there seems to be a perplexing 'gap" in the gear ratios / gear box ]. ~ Sometimes when pulling using a less powerful engine (more bog standard plain vanilla F-150) (from a dead stop) + pulling trailers that the OVERDRIVE would cause the rear wheels to spin in place, lose traction and very difficult to find a good "Bite point" from a dead stop [I.e. the transmission senses the torque demand and drops into overdrive... . This becomes particularly dangerous when parked at an icy intersection on a hill and you lose traction waiting to get T boned. More powerful engine with lower end torque seems (for me at least) MUCH safer and not drop into overdrive. I don't feel that I would have the capability to destroy a drive shaft in either situation; maybe an ultra souped up off road truck maybe ?

Conversely had friends in the UK that were insane rally drivers and of course they had the power-curves on standard converted vehicles totally maxed out for very high RPM and higher responsive torque and all round responsiveness but was always funny coming back from the pub as it would take at least 5 minutes for them to start the bloody thing - without the engine stalling out at low revs for twentieth time on a damp winters night.



@magneticanomaly Maybe bust out some transmission layouts for the specific vehicles or applications you have in mind ?

I have almost zero knowledge of automatic transmissions but have always been shocked by the materials and methods used in automatic transmissions for a regular "Saloon" car as it seems to indicate internal torque may not be so much of an issue using "Rubber bands" and polymeric materials in the drive train.

I'm trying to imagine what kind of torque is required to destroy a drive shaft ?

I know carbon fiber drive shafts are becoming more popular for high performance vehicles.

Magneticanomaly
I suggested that the answer was to limit maximum torque though the driveshaft


These days we have electric vehicles and in some cases higher precision gears ...

Interested to see where you're going with this.
 
In heavy trucks the move to low numerical 2.73 rear gears and direct drive transmissions is for mileage. Overdrives are kinda a bandage where the original elements of a driveline have been surpassed in engine development. With regard to autos, there was a design block on multi stage transmissions for quite a while. Big trucks have used spliters to divide transmission ratios for decades. The new mulit speed transmissions 8-12 speed in cars are the same principal 3 or 4 speed boxes with ratio dividers.

A direct drive high gear is always going to have lower losses than a under/over drive by virtue of no gear mesh. Conversely on heavy haul big trucks the high numeric differential gearing 4.56 etc is still needed to transmit and multiply torque to start heavy loads. So the OP idea that rear end gearing has a lot to with torque is correct in heavy trucks, in the auto realm, it is more likely due to a lot of factors. Development cost and deployment across platforms, packaging, long term planning considerations and, the biggest cost per unit.

Big trucks are speced to exact customer requirements, you don't go to the dealer and pick a truck off the lot. There are some dealer speced trucks for sale but, 90% of big trucks are ordered for particular tasks. A line haul truck is very different than a local delivery truck and a heavy haul truck is far different than either of the above. In the auto/light truck market you don't have that configurability, it this package or that.

As far as hypoid gear efficiency, the ring gear and wheels are going to turn at a constant speed all other thing equal. A car with 2.73 gears and a car with 4.56 going 60MPH will have the same revolutions' per mile. The car with the 4.56 gears will have turned the drive shaft or drive line many more time to go the same distance. Intuitively the more times the gears mesh the more potential loss of efficiency. How much greater a loss?

Steve
 
I have a feeling tho have not done the math, that it is far more practical. You need strong[large tooth] gears in a finite diameter range that all fit on one shaft

If you tried to do that with a 2:1 final drive I think the convenient ratios would run out quickly or the size of the transmission would grow.

IOW First gear would have to be 6:1 instead of 3:1. So the smaller gear can only get so small so the larger gear gets larger

Could also be the amount of torque is problematic for the case of the trans, as it it frequently case flex that causes gear failure.
 
I don't think hypoid efficiency is any factor at all.

I believe it's mostly to do with limiting torque through the driveline and keeping a passing or towing gear (direct) in a position to handle the most abusive situations with the engine at peak power.
 
nuther thing

If you know how manual trans work, you realize all the gears are always spinning and always engaged, shifting merely locks your gear of choice to the input shaft and has it transmit torque. So I would think trans losses have a significant fixed component. Even in 1:1 that countershaft is spinning and all those gears are meshing.
 
In fact hypoid bevel gearing was first introduced to lower the tailshaft line ,and allow a much smaller ,or none,transmission tunnel in car floors.....it was used in trucks because hypoid gearing permits a much larger pinion with longer wear potential.....the tradeoff being the necessity for extreme pressure lubricants due to the greater sliding action of the teeth......As a lowered tailshaft isnt an advantage in a truck,many truck axles have spiral bevels not needing special oils.......Now EP oils are used everywhere ,even where they are detrimental to components,because the EP chemicals can replace the need for a quality oil,and make lubricants much cheaper to make.
 
It's really an issue of achieving a broad range of gearing. As other noted, if you put all the gearing in the trans and used a 2:1 rear end.....you'd better not want to put a lot of power through the trans. That's because your trans would need a 10:1 first gear (or whatever) and that's gonna be a small, weak gearset. The ring and pinion in the rear end is far more capable of handling the torque multiplication...it's a lot larger.
 
GregSY has it right. In heavy truck, the transmissions were not capable of the power transmission. The answer is always cost. It is far less expensive the do the torque multiplication in the drive axle. 2 speed axles were crappy but kept alive for decades by truck makers that didn’t want to spend the money on a transmission that already existed for a matchup with a single speed axle.
It’s unlikely that a lot of development will happen now because of the drive for electric vehicles
 








 
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