Hypothetical overdrive question

[FLATBEDFORD]

FLATBEDFORD, a two speed truck rear is just like a transmission, there is still only one set of ring and pinnion gears but the case also contains a set of straight through drive gears. This basically is the same as having the trans in one gear or another and has nothing to do with the rear end except for the fact that the gears happen to be placed there rather than in the trans (although on some the ratio change is done in the driveshaft plane while in others it is done in the axle plane, either way it is still teh same result).

Hawkrod

The two speed rear makes the final drive ratio numerically lower, or in "overdrive" with the transmission still in direct drive. Is this less powerful and efficient? If yes, than why would this system be used in a heavy duty truck where power and efficiency are very important?

[willowbilly3]

You said:
No, I did not miss anything, I am talking about the same thing. I think you missed the point of that specific post. willowbilly3 and I are discussing the issue of the fact that by changing rear ratio to a numerically lower gear requires the use of more power and that is why the automotive manufacturers do not do it. Modern drivelines behave much better than the Model T, but manufacturers did go to numerically lower ratios in the 70's and 2.50 or lower was not unusual. The problem is that a car with a 2.75 final ratio and a 1-1 trans will not do as well as if that car had a 3.23 and .85-1 overdrive for nearly the same final ratio. The car with the 3.23 and overdrive will usually perform better on fuel consumption, emissions and performance. That is the point of the discussion and what is truly important when you get to the bottom line.

I said:

Hawkrod, the last half of that statement, exactly why is that true? I'm not saying I don't believe it but inquiring minds need to know how the same top gear overall ratio would effect emissions and fuel consumption by arriving at it in a slightly different way? What are the mechanical principles involved?

You said:

Torque multiplication. Too much is useless and breaks stuff and too little uses power.



Well how about this one. I asked you a pointed simple question and got a vague answer that didn't help me understand what you had said about mileage and emissions at all. I was open and not searching for a certain answer, I just wanted a solid factual answer and you wouldn't give it. Let's just start right here with the question asked. Explain WHY the emissions and mileage would vary if the combined ratio in each gear on both transmissions was the same. Explain WHY the torque would be different with the same combined ratio arrived at with the 2 different transmission/rear diff combos. Just how would the applied forces be different if the overall combined ratios were the same? Is there any part of that question you don't understand? Are there any of my words you do not comprehend? Is it not totally clear what I "meant"? If you are having a hard time understanding any of my words or questions I will be glad to clarify anything you need clarified.

[Hawkrod]

Well how about this one. I asked you a pointed simple question and got a vague answer that didn't help me understand what you had said about mileage and emissions at all. I was open and not searching for a certain answer, I just wanted a solid factual answer and you wouldn't give it. Let's just start right here with the question asked. Explain WHY the emissions and mileage would vary if the combined ratio in each gear on both transmissions was the same. Explain WHY the torque would be different with the same combined ratio arrived at with the 2 different transmission/rear diff combos. Just how would the applied forces be different if the overall combined ratios were the same? Is there any part of that question you don't understand? Are there any of my words you do not comprehend? Is it not totally clear what I "meant"? If you are having a hard time understanding any of my words or questions I will be glad to clarify anything you need clarified.

I am sorry that you did not recognize that my answer was not vague and was actually specific. Just because an answer is simple does not make it wrong. You asked why and the correct answer is torque multiplication. Because a vehicle does not operate in a static condition it takes advantage of the torque multiplication offered by rear end ratios. It really, truly is that simple and that truly is a complete answer to the question. As to why mileage and emissions are affected, it is due to the fact that torque multiplication allows the engine to operate in a the proper power band to optimize performance. Again it really and truly is that simple. The applied ratios are only the same at operating speed and assuming no load variances. You ask "Is there any part of that question you don't understand?" and I would answer no, I understand that question but that was not your original question to which I had been responding. You decided to change the question, it is not my responsibility to make you understand the answer or to even provide an answer. Honestly, I thought I was answering and am very surprised you see it otherwise. Hawkrod

[FLATBEDFORD]

[willowbilly3]

I just don't see the differences in torque being much different. The part I asked you directly and pointedly about is WHY the torque would be different if the over all ratio is the same. WHY or HOW the mileage and emissions would be affected If the ratio between the flywheel and the driven tires is exactly the same on both combination. You seemed to indicate otherwise and won't elaborate why. And no this isn't the original question, it is an attempt to get you to elaborate on statements that you made in the course of the discussion.
"
"Torque multiplication. Too much is useless and breaks stuff and too little uses power. Hawkrod"

You don't think that answer was pretty vague? That's first grade stuff and does nothing to answer how the differences in torque would occur between the 2 combinations as you indicated.

Are you like a savant or something? Because if you are I apologize and will just let the whole thing slide.

[sideoilerfe]

Why don't you simply build 2 identical trucks: one with the overdrive transmission/ gear ratio you claim will work better and one with a direct 1:1 ratio on 4th gear and fill both trucks fuel tanks to the cap and drive them until they're empty then compare the "real world "results.

That is the best solution.

[Hawkrod]

snip
Are you like a savant or something? Because if you are I apologize and will just let the whole thing slide.

Unfortunately, yes I am. But you were already well aware of that from the last time we had a discussion and you posed an issue and I responded and you did not like the response. You are well aware that I have Asperger's syndrome ( http://en.wikipedia.org/wiki/Asperger_syndrome ) which is an autistic spectrum disorder. It affects the way I communicate and the way I word things. We discussed this already so you also know that it is not intentional on my part and is simply part of who I am. It is not a bad thing to have Asperger's although it does create issues with communication but it also came with a gift of significantly above average intelligence. My son has the same issue (it is hereditary) and he has the same gift. It is people like you in this world that don't like people that are different than youself that make it a problem. Most people are understanding and once they take the time to get to know somebody with Asperger's are willing to accept that they are not being assholes, they simply use the language much differently than you do. People with asperger's are very literal and usually do not use emotion in communication (and when they try to they usually fail miserably). In that thread you also got mad because I did not respond to one of your questions about brakes but you failed to provide the needed info and faulted me for not giving you answer. You seem to have a very large chip on your shoulder and need to understand that I will answer questions to the best of my ability. I may not be able to get you to comprehend the answer but that really does not make it my problem. If you don't like my answers then ignore it instead of attacking me for offering help. Hawkrod

[dablack00]

Ok, I have tried to read through all the posts, but had to skim some. ADD I guess. anyway, I think WB was having a hard time asking the right question. I think I know what he was asking and maybe if I ask it in a better way, others might be able to answer. NOTE: I'm a mechanical engineer and sometimes design gear driven tools for space use. Of course these are hand tools and I'm not concerned with effeciency so much as living in extreme conditions with no lubricant, but I don't know the answer anyway here is the question (I think).

Ok, we have two identical trucks. One truck has an OD trans with the fallowing ratios. 1st gear is 4.0, 2nd gear is 2.4, 3rd gear is 1.5, 4th gear is 1.0 and 5th is .8. These are real trans gears. Now this truck has a 3.5 rear gear so the overal gear ratios in each of the gears is (in order) 14, 8.4, 5.25, 3.5, and 2.8 (this is just the trans ratio multiplied by the rear ratio.

Now our second truck has the trans ratios of 5, 3, 1.875, 1.25, and 1. So this trans doesn't have OD. The final ratio is 1:1. This truck has a rear gear of 2.8. The over all gear ratio for this truck is exactly the same as the other truck.

So, which truck will get better milage?

They have the exact same torque multiplication ratio from the engine to the wheels. The only different is driveshaft speed.

Is this a more clear question? Is this what you were asking WB?

Now, like I said before, I have no idea which is better and why we do it the way we do.....

[averagef250]

That's why I suggested driving a first gen dodge diesel. You can get them both ways, 3.07 gears and a direct auto or 3.55's and an overdrive auto. The overdrive A518 ratios are better, but not by much. Pit two identicle trucks against each other, but with different trannies and the difference is clear as day, the one with lower gears wins in overall driveability.

Direct top gear allows the driveline and rear axle to last longer, but the transmission takes more abuse. Medium duty trucks have used direct transmissions for a very long time. Millions upon millions of allison 540 series autos have made it behind everything from FT powered Fords to DT466 powered Binders. They are 4th gear direct and suck. Imagine the acceleration potential you have with 4.10 gears and 40" tires moving 26K pounds down the road. The newer trucks run 1000 and 2000 series allisons which are 5/6 speed and overdrive.

Wear and tear wise overdrive transmissions hold up better because the highest loading they see is not in overdrive, it's in direct gear. A vehicle should be geared so that it's more comfortable to pull it's rated GVW up a grade in direct gear than it is in overdrive. With a direct transmission one gear down from top gear will no longer be direct, the transmission will be subject to more wear since it sees it's highest loading through the countershaft.

The older medium duty trucks that used overdrive stick transmissions used very mild (.8, .85) overdrive ratios to compete with the close ratio direct gear transmissions. This didn't work so well. The old clarks, spicer 3053's and NP540-O's wore out faster than anything else because they could be used to pull big in over. Modern overdrives are all built with very steep overdrive ratios ranging from .73 up to .69 or so. This gives a hell of a jump between direct and over and the purpose is to get the driver to use direct instead of over for pulling large loads.

[Hawkrod]

Maybe this will help: http://www.valvoline.com/carcare/articl ... 4&scccid=1 Hawkrod

[willowbilly3]

Ok, I have tried to read through all the posts, but had to skim some. ADD I guess. anyway, I think WB was having a hard time asking the right question. I think I know what he was asking and maybe if I ask it in a better way, others might be able to answer. NOTE: I'm a mechanical engineer and sometimes design gear driven tools for space use. Of course these are hand tools and I'm not concerned with effeciency so much as living in extreme conditions with no lubricant, but I don't know the answer anyway here is the question (I think).

Ok, we have two identical trucks. One truck has an OD trans with the fallowing ratios. 1st gear is 4.0, 2nd gear is 2.4, 3rd gear is 1.5, 4th gear is 1.0 and 5th is .8. These are real trans gears. Now this truck has a 3.5 rear gear so the overal gear ratios in each of the gears is (in order) 14, 8.4, 5.25, 3.5, and 2.8 (this is just the trans ratio multiplied by the rear ratio.

Now our second truck has the trans ratios of 5, 3, 1.875, 1.25, and 1. So this trans doesn't have OD. The final ratio is 1:1. This truck has a rear gear of 2.8. The over all gear ratio for this truck is exactly the same as the other truck.

So, which truck will get better milage?

They have the exact same torque multiplication ratio from the engine to the wheels. The only different is driveshaft speed.

Is this a more clear question? Is this what you were asking WB?

Now, like I said before, I have no idea which is better and why we do it the way we do.....

Thanks yes. I really thought I had spelled it out pretty well that the overall ratio would be the same all the way through all 5 gears in both vehicles but I keep getting responses that made it look like I was only changing the rear or not changing any of the other gears but top gear. Then in an attempt to clear that up I gave an example of using 3 and 4, 3 to one rear end and 4 to one first gear as opposed to a 4 to one rear and a 3 to one first gear. You would have a combined overall ratio if 12 to one no matter what speed you went in either combination The engine would have the same force on the wheels. I can't see how the torque would change. Hawk said it would and even said the mileage and emissions would change. I just wanted to know what forces would be in place that would make that change. Hawk would not answer that question no matter how many times I asked it or how many ways I worded the same question. He simply avoided giving me any direct answer and if that makes me have a chip on my shoulder, well it just grew there from the frustration of getting this guy to back up what he offers. That was a whole sidetrack that was brought in an attempt to address that part of the power transmission would not be affected. People claimed it would but would not explain why. That was all I wanted to know was why or how the torque would change any significant amount in the lower gears as was stated, which isn't what the original question was at all but was where peope who disagreed wanted to take it.

Hawk and Dustin. You both are great guys and have a lot of good wisdom to offer but just saying something is so isn't good enough for me. It's kind of like flow chart trouble shooting, you might get all the way through and still never understand how a system functions. I want to know how systems function and neither of you gave me any data that proved anything to me except that you really don't know. And I still don't either.

Hawk, yes this is the second time we had a go round because you want to act like you know but when someone tried to ask for more details to support what you profess to have knowledge about, then you get evasive or nitpick a word usage and avoid answering the questions. In evaluating all of this I am led to one conclusion, you really don't know and when you get put on the spot you start with the smoke and mirrors. I was polite until the point where you just pretty much would not answer the questions I had about WHY. It is just frustrating trying to deal with you on any real technical level because you make statements that you won't back up.
If I get in this conversation with a friend on the street and state that the torque will change and he says why, I'm pretty sure if I answer because Hawk says or because Dustin says, won't be very convincing.

[willowbilly3]

Maybe this will help: http://www.valvoline.com/carcare/articl ... 4&scccid=1 Hawkrod

Well, that's good differential gearing basics but says nothing about the combined ratio of the transmission and the rear end, which is where the discussion went south.

[Hawkrod]

I did answer the question completely and was not evasive in any way. You are lying because you do not comprehend, not because I don't know. Hawkrod

[dablack00]

That's why I suggested driving a first gen dodge diesel. You can get them both ways, 3.07 gears and a direct auto or 3.55's and an overdrive auto. The overdrive A518 ratios are better, but not by much. Pit two identicle trucks against each other, but with different trannies and the difference is clear as day, the one with lower gears wins in overall driveability.

Direct top gear allows the driveline and rear axle to last longer, but the transmission takes more abuse. Medium duty trucks have used direct transmissions for a very long time. Millions upon millions of allison 540 series autos have made it behind everything from FT powered Fords to DT466 powered Binders. They are 4th gear direct and suck. Imagine the acceleration potential you have with 4.10 gears and 40" tires moving 26K pounds down the road. The newer trucks run 1000 and 2000 series allisons which are 5/6 speed and overdrive.

Wear and tear wise overdrive transmissions hold up better because the highest loading they see is not in overdrive, it's in direct gear. A vehicle should be geared so that it's more comfortable to pull it's rated GVW up a grade in direct gear than it is in overdrive. With a direct transmission one gear down from top gear will no longer be direct, the transmission will be subject to more wear since it sees it's highest loading through the countershaft.

The older medium duty trucks that used overdrive stick transmissions used very mild (.8, .85) overdrive ratios to compete with the close ratio direct gear transmissions. This didn't work so well. The old clarks, spicer 3053's and NP540-O's wore out faster than anything else because they could be used to pull big in over. Modern overdrives are all built with very steep overdrive ratios ranging from .73 up to .69 or so. This gives a hell of a jump between direct and over and the purpose is to get the driver to use direct instead of over for pulling large loads.

I get what you saying. You are talking about four speed trans with fourth being direct and you are compairing them to five or six speed transmissions that have overdrive. You are talking about gear spacing and how modern transmission with more gears and better spacing work better than the old transmissions. The problem is, that isn't the question. If you go back to my example, we are talking about two five speed transmission, one with OD and with without. They have the exact same gear spacing and from the engine to the tires, the torque is the same between the two.

Now, I do understand about what you are talking about with wear and that is good info but not what we are after.

[dablack00]

Maybe this will help: http://www.valvoline.com/carcare/articl ... 4&scccid=1 Hawkrod

Hawk,

If you think this article has anything to do with what we are asking then you don't understand the question. This only deals with picking a rear gear. That is one part of the system we are dealing with. In the article they say a deeper gear (numerically higher) will put more torque to the ground given the trans gear stays the same. Please go back to where I explain the question. We are dealing with a transmission gear / rear gear combo. Between my two examples, the trans gears are changing.

WB and others that will read this. Hawk is doing the best he can to answer the question he thinks is being asked. There is no reason to call him names. He is getting just as frustrated with the question as you are with the answers (or lack there of). Lets keep this clean. We are all just trying to figure this out.

As of right now, I haven't seen an answer. Dustin did have some good info on wear problems from running on different gears while pulling high loads, but that isn't what we are really after.