Ping: Rope-Pusher - over/direct/final drive, and efficiency

[theholycow]

Rope-Pusher,

A few times, in response to things I've posted, you have pointed out that monkeying with final drive ratio can produce the same result as a taller high gear or shorter low gear. While true, I never saw a need for it when it can be done in the transmission where they're already doing plenty of monkeying anyway.

Someone on another forum made a good point: Direct drive through the transmission, locking the input and output shaft directly together, is more efficient than putting power through meshed gears. This would seem to indicate that the most efficient configuration is to make direct drive your highest gear, put all 4 or 5 other gears below it, have no overdrive, and instead have a really tall final drive ratio.

Is that a correct assessment?

If so, why have overdrive transmissions become common instead of that configuration? My guesses:
1. Marketing.
2. Packaging or engineering issues for really short gears in the transmission or really tall gears in the rear end.
3. In the past, maybe the efficiency of that gear would have helped with 0-60 times, but these days we all get to 60 before we reach the direct drive gear.

[Rope-Pusher]

THC,
Various reasons tht "High" gear in a transmission is often over-driven rather than 1:1 ratio.

One would be that in order to keep the same overall gear ratios, a trans with 1:1 top gear ratio (TGR) would need a lower numerical final drive ratio and all the speed gears below top gear would need higher numerical ratios. As you make say 1st gear ratio ("First Gear Ratio" - I knew you could say it!) higher numerically, it tends toward a lower torque capacity - i.e. it gets weaker). This is because with a big difference in tooth-count between the drive and driven gears, you end up with less teeth engaged in the mesh at any given time. In this case, the drive gear would get smaller and have fewer teeth and the driven gear would get bigger and have more teeth. If you wanted to change the ratio without reducing the strength, you would need to use gears made from unobtanium, or change the width of the gear faces, or change the distance between the shafts (so the number of teeth engaged in the mesh would stay the same). As you can see, the cost and size of the transmission might have to increase if you did any of these things.

Also, by keeping TGR overdriven, the exact ratio used can be tailored for an individual application (less overdrive for a vehicle with more aerodynamic drag maybe?) and still be able to change the final drive as well - it gives you more flexibility.

Oops - students are entering for the next hour - gotta go.

[Rope-Pusher]

...and as for transverse FWD Amish transaxles, they way they are laid out doesn't allow for ANY "gear ratios" that don't use gears. In fact, some have multiple overdrive ratios in the "speed gears" and some even have multiple final drive ratios (they run two intermediate shafts).

[theholycow]

Great point!

I knew about the multiple final drive ratios due to VW using them on one of its transmissions that I read about, which makes gear charts a little confusing...

[Rope-Pusher]

Great point!

I knew about the multiple final drive ratios due to VW using them on one of its transmissions that I read about, which makes gear charts a little confusing...

Yeah, we had people doing fuel economy estimates and vehicle data publication that had a difficult time dealing with multiple final drive ratios. In some cases, you just fake it to get what you need (pick one FDR and then edit the speed gear ratios to represent the overall gearing correctly). In others, you just have to bite the bullet and edit the document / program format to accommodate multiple FDRs.

[kamesama980]

I think there's also a lot of "change is hard" attitude. old 3 and 4 speeds had direct drive for the final gear, back then 5th gear was added on top using the same or similar housings, shafts, and other parts. for a LONG time 5 speeds with overdrive were those same gearboxes. (toyota used the W box for a good 30 years. it started out as a 4 speed and has had about half a dozen 5 speed variations with different ratios and slight design differences but the basic box is the same from a 1980 toyota pickup/hilux to a 2000+ lexus IS 5 speed or NA supra.

Also a matter of saving more gas by the lower revs than it's worth in engineering costs to eek out another .5 mpg by making the top gear direct and going to a lower final.

That being said, I heartily agree. 1:1 top gear would be nice with a super low rear end. I know the 4 speed manual version of my pickup came with a 3.08 rear end. I just ran the math and at 2500 rpm, my trucks doing 63.1 mph, the 4 speeds doing 63.0. HMMMMMM..... My 5 speeds known for very high 20's mpg, the 4 speeds get 30+ so maybe it's not so little a difference.

[Rope-Pusher]

^ Thing is, I doubt there is enough of a difference going one way vs the other to get you 0.5 mpg. Amish transmixers are fairly efficient as it is. Even when it's not transmitting load, that countershaft is still spinning in it's bearings in a puddle of oil and all the gears on it ('cept mebbe Reverse) are still spinning and meshed with their cohorts on the output shaft that are spinning on their own bearings amid a mist of flung oil. If you took it out of the gearbox entirely and then measured fuel economy of 1:1 top gear and overdrive top gear with same overall drive ratio, you might see that 1/2 gallop per mile advantage, but what fun is a 1-speed?

For transverse FWD transmissions, it's kinda a mute point, since they don't have an option to transmit power thru to the wheels without going thru a gear mesh.


This is another fine mesh you've gotten us into, Stanley

[ClutchFork]

Very educational. Would there be less wear in direct drive? Then is it better to have 4th or 5th direct drive? Depends on whether the vehicle is used on the highway much (I rarely use overdrive other than on the highway).

But a more important gear ratio thing would be a slightly deeper first gear to get out of the hole better and say two times first gear's ratio for reverse so it's a creeper gear.

[theholycow]

Yeah, my question was based on the assumption that the average driver spends most miles in overdrive, probably on the highway, even though most of my driving isn't that way...though with OEM ratios I'm likely to be in overdrive off the highway too.

I haven't encountered a first gear that isn't deep enough to easily exceed available traction in normal driving...every first gear I've driven has been easily able to roast tires on dry pavement without a huge high-RPM clutch dump.

[ClutchFork]

Yeah, my question was based on the assumption that the average driver spends most miles in overdrive, probably on the highway, even though most of my driving isn't that way...though with OEM ratios I'm likely to be in overdrive off the highway too.

I haven't encountered a first gear that isn't deep enough to easily exceed available traction in normal driving...every first gear I've driven has been easily able to roast tires on dry pavement without a huge high-RPM clutch dump.

This is true, but a lower first gear would probably be a bit easier on the clutch (launching being the most wearing on the clutch presumably, unless you drive like you stole it through all gears ).

My son's S10 has same final drive as my Ranger, but his first gear is 3.94 compared to mine at 3.72, add that his tire size is about 1% smaller than mine and he gets even deeper or about 8% deeper for first gear. (I do have about 20% more hp and torque though). But my little 4-popper doesnt really giddy-up until about 3000 rpm and to make a quick launch that means a good bit of clutch slippage. A lower ratio in first would put the rpms higher at launch.

Ha, in contrast my 1984 F150 had a 2.37 final drive and with that, first gear could be taken up to 45 mph and that on a vehicle that redlines about 4000 rpm! That was like having no first gear.

[tankinbeans]

I'm still frying my big lump of mush trying to figure out how gearing works. It doesn't seem to matter how many places I read it; the concept never makes sense.

[ClutchFork]

I'm still frying my big lump of mush trying to figure out how gearing works. It doesn't seem to matter how many places I read it; the concept never makes sense.

For all its apparent simplicity, it really is quite complex. Factor in the final drive ratio, but also tire size, which affects it. Then you can compare between vehicles but that the power bands are different too, and what is being compared: quickness off the line (now driver and clutch components become a factor), rpm at set speed (easy one), pulling power, ...

[tankinbeans]

I'm still frying my big lump of mush trying to figure out how gearing works. It doesn't seem to matter how many places I read it; the concept never makes sense.

For all its apparent simplicity, it really is quite complex. Factor in the final drive ratio, but also tire size, which affects it. Then you can compare between vehicles but that the power bands are different too, and what is being compared: quickness off the line (now driver and clutch components become a factor), rpm at set speed (easy one), pulling power, ...

Glad to know I'm not alone.

[Rope-Pusher]

I never understood Goering.

Don't feel bad, many native German speakers didn't understand him either.