Why have 'axle ratio' choices with 8,9,10 speed transmissions?

"Grindintosecond" (Grindintosecond) 04/30/2014 at 13:49 • Filed to: None

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With so many gears coming and available now, then why on earth do we need to select towing axle ratios when the transmission has the ability to just drop a gear down and then your ratio is improved for that towing purpose?

Looking at the reasoning of my thoughts: As I understand it, the towing vehicle is towing and not cruising and needs the engine to be in a torque range to be efficient at moving that big trailer. Understood. The past had 3 and 4 speed transmissions and with that you had to have a final drive ratio that got you to reasonable highway speeds yet was not lugging the engine along in low RPM out of that power range. If you were not towing, you selected a more MPG friendly drive ratio instead of that near 4:1 for the 30 foot goose-neck piled high with hay.

Today: 8,9,10 speeds! A recent trip to the RAM trucks site, where they have the 8 speed transmission, has selections of final drives. Why do we have that? Why do we need that? Is the tiny bit of RPM change from dropping from 8th down to 7th, or even 6th not enough to put the engine in the efficient towing range of the torque curve? At this many speeds, we should see a set final drive, and the bazillion gears in the transmission select the one for the best towing power range.

[EDIT] over some confusion over what i'm asking, here is the ratio of the ZF 8 speed automatic in the new RAM trucks.

1st-4.696 , 2nd-3.130 , 3rd-2.104 , 4th-1.667 , 5th-1.285 , 6th-1.00 , 7th-0.839 , 8th-0.667

Given the extremely narrow variances between gears 5-8, won't the final drive choices be absorbed into the flexibility of the transmission? Especially with GM working on a 10 speed?

So why do we still have to have a different axle ratio in the end if the transmission can make the change internally without impacting overall performance?

For Sweden > Grindintosecond 04/30/2014 at 13:51

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Because some trucks haul hay and some trucks pull 10,000 kg equipment trailers. They don't need the same final drive ratio.

Grindintosecond > For Sweden 04/30/2014 at 13:53

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That is my point! with 10 speed transmissions, the total drive ratio is selected that way, so why do we need the final drive to be different? just drop from 10th to 9th or 8th if your really heavy.....see my issue?

For Sweden > Grindintosecond 04/30/2014 at 13:54

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I see it, I just don't agree. Until splitters become an option on pickups, final drive ratio options will exist.

Grindintosecond > For Sweden 04/30/2014 at 13:55

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Explain 'splitters' so i know the term properly.

Party-vi > Grindintosecond 04/30/2014 at 13:57

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I think he's referring to gear-splitters, as-in hi/lo range for each gear, ala CDL big rig and dump truck transmissions.

Grindintosecond > Party-vi 04/30/2014 at 13:59

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Ok i understand that then but you are talking about enormous weight and very small rev range engines. For general pickups, like the ones here limited to towing 24,000 pounds max (usually with diesel), or those towing their boat to the lake or a 2 horse trailer, that is where my discussion lay.

For Sweden > Grindintosecond 04/30/2014 at 14:00

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A splitter is a device big trucks, like semi-trucks, use to create a low gear set and a high gear set. That's how they can have 15+ speeds.

Party-vi > Grindintosecond 04/30/2014 at 14:03

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Final drive ratio can increase or decrease power to the rear wheels to provide a mechanical advantage. Changing transmission gear ratios cannot (to my knowledge).

TJDMAX > Grindintosecond 04/30/2014 at 14:04

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Well first of all...No ram truck has an 8 speed trans. They are all 6 speeds from what I can tell.

Secondly you have to look at the final drive ratio of the trans. A 6 speed or 8 speed are probably very close in terms of final drive, the difference between gears is just less so it makes it easier or the engine to stay in its "sweet spot" while accelerating etc.

And when you think about the difference from a 3.32 ration or a 4.10 ratio and compare it to maybe like a .61 or .71 final drive ratio coming out of the trans, you'll see that the difference in the axle is much larger and therefore makes a lot more difference in the end engine rpm that the truck will see.

TJDMAX > Grindintosecond 04/30/2014 at 14:05

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again....no heavy duty pick up that i'm aware of has anything more than a 6 speed at the moment. definitely no 10 speed transmissions.

Grindintosecond > Party-vi 04/30/2014 at 14:07

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changing the final drive ratio also changes the rpm of the engine. engine rpm through transmission = driveshaft rpm, altered by final drive to the tires.

Grindintosecond > TJDMAX 04/30/2014 at 14:07

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http://en.wikipedia.org/wiki/ZF_8HP_tr…

also GM is working on a 10 speed

Ramblin Rover - The Vivisector of Solihull > Grindintosecond 04/30/2014 at 14:11

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Drive. When the trans is in 1:1 drive, not one out if a thousand other combos, it is at its most efficient. Some guys hit their load/ required road speed compromise at a different speed. Highway travel in drive at higher revs may make possible higher ultimate highway speeds with extreme loads, where others would suffer mileage loss they didn't need at that same rear end ratio. It. Matters.

tromoly > Grindintosecond 04/30/2014 at 14:14

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Driveline stress, lower axle gears reduce the torsional stress of the components upstream of the axle. Hence why some older dump trucks have a Granny 1st gear (read: 1.5mph. We've clocked it before), yet still have a two-speed rear axle.

TJDMAX > Grindintosecond 04/30/2014 at 14:14

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I didn't see that as an option on the ram site. But even still, it was offered on a v6 truck so its not really a "two" vehicle. But just for comparison look at the specs on the Allison transmission in the Duramax equipped chevy and gm 2500/3500 trucks.

http://www.duramaxdieselspecs.com/allison-transm…

Notice that final drive ration is .61 compared to .667 in the ZF box. So again, not a huge difference, thus the axle ratio makes a much more noticeable difference in the end.

Grindintosecond > Ramblin Rover - The Vivisector of Solihull 04/30/2014 at 14:16

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Where i see the ZF8 speed transmission has two overdrive gears. 6th is at 1:1, 7th is at .839, 8th at .667. there are enough gears available now to make that rpm change inside the transmission. This 8 speed is in effect a splitter on a big rig transmission a low range 4 speed and a high range 4 speed. together 8 speeds. i see no reason for a final drive adjustment when we have GM working on a 10 speed with probably 3 or 4 overdrive gears at that point, or possibly 1 overdrive gear and several gears so close to 1:1 the final drive really wouldnt matter.

Grindintosecond > TJDMAX 04/30/2014 at 14:19

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in that combination yes. it does matter, but give it two more gears, throw in an .831 between the 5th and 6th of that allison and you have the tow range fix. Thats what im getting at, its all about mechanical advantage of efficient power range, it seem the flexibility is becoming built into the transmission now.

TJDMAX > Grindintosecond 04/30/2014 at 14:22

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I think we are getting ahead of ourselves still. No Heavy Duty pickup on the market today has anything more than a 6 speed. Strength and heat being the two biggest issues why that is the case. The transmission in a semi truck is massive and so all of these 8 speed boxes are built for maximum efficiency for less powerful trucks and suv's. So we can't really say that we shouldn't have axle ratio choices yet because we don't have the set up for that in a heavy duty pickup that is designed to tow.

Arch Duke Maxyenko, Shit Talk Extraordinaire > Grindintosecond 04/30/2014 at 14:24

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Strength. This is a lot stronger:

Than this:

Where would you rather have your torque applied?

TJDMAX > Grindintosecond 04/30/2014 at 14:28

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So i have a few trailers that i tow regularly behind my truck (07 chevy 2500hd classic duramax and allison) and when i am in tow haul mode the trans never goes into 6th gear. it always stays in 5th, and even when going up a hill it will hold a gear longer because when towing 15K pounds (weight of the 5th wheel camper I have) its not a good idea for the trans to be shifting gears all over the place. Automatic transmissions aren't as strong as a manual trans like in a semi truck, so i don't see where it would be possible to make a durable 10 speed or even an 8 speed that would be strong enough to handle all the power and torque from a modern diesel pickup, still fit in the body, and not blow up when towing.

And again, the difference between a 3.32 and a 4.10 rear end ration is still much larger than the difference between a .831 and a .61. The final drive ration will always be about the same because there is a limit on how small it makes sense to go. Go with too small of a gear ration then the motor won't have any power, so keep that in mind as well.

Grindintosecond > TJDMAX 04/30/2014 at 14:28

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I believe the concept of what people need vs. what does the job has been skewed. the RAM 1500 5.7 v8 is the largest with the 8 speed. That truck will tow your boat to the launch site and tow the full sized camping trailer just fine. In fact the older 3 speed v8 trucks from the 80's and late 70's did the same thing just fine but everyone now thinks they need that big ram 3500 dually diesel to do it. We are seeing a trend to a lot of speeds and the transmissions able to do that will eventually be engineered to work in the larger and larger trucks. (3500's) they still offer ratio changes to the smaller 1500 trucks WITH 8 speed trannys, that's my question. why have that when the transmission makes the ratio required.

RamblinRover Luxury-Yacht > Grindintosecond 04/30/2014 at 14:35

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It still comes down at least partly to the final RPM (and thus torque) vs. road speed, in the highest gear. You don't want a situation where you're constantly jumping between the top three if a little lower final drive and a little more Rs at a steady top gear will do. It's also a wear issue. Even with the reduction in wear and shock permitted by a lot of gears close together, you still have a factor affecting the force of all those shifts and the strain on all those gears - the final drive. 20% lower final drive means lower strain at each engine to axle step, regardless how many there are, or how much the many speeds reduce the requirements on the engine to strain between. Tall final drive in top gear = better MPG, low final drive = more stable and longer wearing drivetrain with a heavy load, regardless how many steps it takes to get to each.

Party-vi > Grindintosecond 04/30/2014 at 14:38

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Assuming engine RPM is constant, changing the final drive would only change the rotational speed of the rear tires.

Assuming transmission choices are constant, it is true that a higher final drive ratio would assist with acceleration under towing and regular conditions while increasing engine RPM and a given highway speed. You're increasing the torque available at the rear wheels by getting higher-ratio gears. Think of it this way, by choosing a 4.10:1 final drive instead of 3.73:1 final drive, you can increase torque to the rear wheels by almost 10% without adding any more gears to the transmission. Also, with the amount of torque needed to keep a vehicle moving at highway speeds I don't think a 0.48:1 9th gear is going to work unless you can get some lower gears in the back.

Grindintosecond > RamblinRover Luxury-Yacht 04/30/2014 at 14:41

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Towing isn't done in top gear. You're last case is the root of my discussion. The ZF8 has two overdrives. With a towing final drive you get more stable and longer lasting drive train yes, and then you have the last two overdrives for the long MPG unloaded runs. so why have the final drive differences from the dealer anymore? In this case 6th gear is 1:1. tow in that (wich is the normal method anyway) and unloaded and not towing, the 7th and 8th gears make the MPG happen....no need for changes to the FD in buying the truck.

TJDMAX > Grindintosecond 04/30/2014 at 14:43

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Ok, before i continue i just want to say this. I am enjoying this conversation. I like having conversations like this and i'm not getting mad and i'm not trying to be rude so just don't get mad at me, i'm just sharing experience / thoughts.

First, I still don't see the ram 1500 with the V8 coming with an 8 speed.....unless i'm missing something on that website....wikipedia only shows it coming on the v6 trucks.

I love it when people compare "old" trucks to the "new" trucks. Yes, those old trucks did basically the same task that the new trucks do today, the question is how well, or easily, or safely, or quickly, or comfortably did they do the same task? Now i have never towed 15K pounds behind a 80's pickup so i can't compare but i know I really like the power and strength of my current truck.

I agree that people tend to buy trucks now a days that are way more capable than they need, but that isn't a bad thing. I would much rather use 30% of a 2500 or 3500 truck's capability rather than 80% of a 1500 trucks capacity. Less stress on everything and having that extra capability left means if you need the extra power or whatnot in a scary situation you have it.

Yes we are seeing transmissions get more and more gears, but again. Heat is the biggest killer of automatic transmissions and with the size of the vehicles not changing drastically (yes they are getting bigger but not big enough to fit a suitable sized 10 speed for a 3/4 ton or 1-ton diesel pick up) i don't see it being a thing anytime soon to have such a transmission in a "tow" vehicle.

GM completely redesigned the 2500/3500 trucks in 2001 for the sole purpose of fitting the Allison trans in the body of the trucks. That was 13 years ago...they are still using the same trans and without doing a huge major redesign i can't imagine a bigger trans going into that truck. Have you ever seen the sheer size of an Allsion? Dodge and Ford have equally massive transmissions and they are all 6 speeds. They are so big because the gears/clutches have to be massive to handle the power of the engine as well as the weight of the max towing capacity of those trucks and they need GALLONS of fluid to keep them cool at all times.

Grindintosecond > TJDMAX 04/30/2014 at 14:50

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I'm enjoying it too. I understand your position as well and heat is the big issue. I just see a 1:1 ratio followed by two more overdrives making a final drive selection at the dealer unnecessary in the end. tow mode for 1:1 and the next two gears for unloaded mpg cruise where a .667 8th gear negates the effect of a high final drive.

The link i posted, look on the first entry of the transmission, to the right of the 3.7 liter engine listed you'll see the 5.7 v8 with clickable notes 18,19,20 next to them.

RamblinRover Luxury-Yacht > Grindintosecond 04/30/2014 at 14:50

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*but*. Driveline stress. Taking 8 steps to accelerate 10,000lb and 8 steps to accelerate 16,000lb remain profoundly different things. The 8-speed will last longer doing the latter with a lower final drive; though the engine will rev higher to do the latter at full speed, it will do so under less strain and further from full capacity. I'd argue the counter - with the possibility to include overdrives, is there any reason *not* to have an astoundingly low final drive? Low tow-weight people still won't need it, so why not ship a truck for *them* that will operate in drive most of the time and get good mpgs, while allowing them to go insane speeds if they want?

I'd state that there's a stronger argument for not offering more than six speeds to lighter capacity trucks than there is for not offering a range of final drives.

Philbert/Phartnagle > Grindintosecond 04/30/2014 at 14:52

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Torque multiplication limitations of both the final drive and the transmissions I would suspect.

TJDMAX > Grindintosecond 04/30/2014 at 14:59

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Ok i just calculated some numbers on an online engine rpm calculator. All numbers were done a 65mph with 33" tall tires.

3.32 Axle Ratio / .61 Final Trans Ratio = 1340 rpm

3.32 Axle Ratio / .839 Final Trans Ratio = 1843 rpm

3.32 Axle Ratio / 1 Final Trans Ratio = 2197 rpm

4.10 Axle Ratio / .61 Final Trans Ratio = 1655 rpm

4.10 Axle Ratio / .839 Final Trans Ratio = 2277 rpm

4.10 Axle Ratio / 1 Final Trans Ratio = 2713 rpm

So there are some numbers to get an idea of how final drive and axle ratio effect rpm.

Another consideration to thing about is that people with trucks change their tire sizes...

For instance, my truck came stock with 31" tires. Here is what those numbers look like with a 31" tire at 65 mph.

3.32 Axle Ratio / .61 Final Trans Ratio = 1427 rpm

4.10 Axle Ratio / .61 Final Trans Ratio = 1762 rpm

It is a whole lot easier to change out the gearing on an axle than it is in a transmission, so for those who own trucks and changed tire size, being able to get the desired rpm is going to mean changing out rear axle ring and pinions.

Yowen - not necessarily not spaghetti and meatballs > Grindintosecond 04/30/2014 at 15:03

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Something I don't think has been mentioned, no matter how many speeds the trans has. Offering different axles will always give you the option of different ratios throughout EVERY gear, not just having the gear available that in a less-speed transmission wouldn't be available.

As others have mentioned, it's also a more robust way of lowering or raising your final drive ratio.

Another point: some people mentioned that HD trucks don't go higher than 6 gears. But that's irrelevant to the question you are asking: what if more speeds were available ? I think the answer to that is, even if there are more speeds. You still get more reliability and robustness out of swapping rear ends to increase your towing capacity, rather than solely relying on the transmission to handle your ratio-requirements.

BigBlock440 > TJDMAX 04/30/2014 at 15:10

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http://www.ramtrucks.com/hostc/bmo/CUT2…

I just tried building a 1500, it lists the 8HP70 (8-speed) as an option.

HammerheadFistpunch > Arch Duke Maxyenko, Shit Talk Extraordinaire 04/30/2014 at 15:10

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Thats the best reason.

crowmolly > Grindintosecond 04/30/2014 at 15:22

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Pretty sure it's a strength issue. For a long time (maybe even still) it's not a good idea to tow/haul heavy stuff in overdrive. Way too much heat being generated. You could have first and second etc being steep granny gears to get it moving which might allow a 2.0 rear axle ratio or something like that but then you are dealing with a very large pinion gear. Certainly a good question.

TJDMAX > BigBlock440 04/30/2014 at 15:42

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I stand corrected. With the 8 speed you are only given one option for rear end gearing. a 3.21.

If you do the 6 speed auto you can pick either a 3.55 or 3.92.

The 8 speed combo will give you lower rpm so potentially better mileage, but not as good for towing.

biglovesam > TJDMAX 12/15/2014 at 14:17

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You let the manufacture pick the ring and pinion ratio and it won't matter how many speeds the transmission has, they want high RPM's ,poor fuel mileage, and repeat customers. I'm online now trying to search how it's possible that new semis have 2.5 ring and pinion with 600 horsepower. Give me 2000 foot pounds of diesel torque 2.0:1 rear ring and pinion 4 overdrives .73 .61 .39 .15 with 12r24.5s all in a 4x4 pickup truck or motorhome!