Re: Gear/Towing Theory

[AustinL911]

So here's the Sitch:

I currently run a 6MT JTR with 35s and the stock 4.10s. I have 5.13s sitting in the garage waiting to go in. I have analyzed GrimmJeeper's calculator in just about every scenario possible.

Final Drive with stock 4.10s in 4th gear (1:1) is 4.10
Final Drive with 5.13s in 5th gear/OD (0.81) is 4.16

For the sake of comparison's sake, let's say they're exactly the same. Will the truck pull exactly the same in 5th with the new gears as it did in 4th with the old gears, even though it's an OD gear? Will parasitic drag or some other variable make it worse as far as efficiency goes?

Obviously towing is going to be better overall (which is what I'm going for) but I was just wondering about the theory part of it.

Sponsored

 

[sharpsicle]

I mean, the RPM at speed will be the same between the two. Is that what you're asking? The only way it would "pull" any different is if for a given speed, it were at a different RPM, or if it were a different engine. But with the same engine at the same RPM at the same speed, the output is going to be the same. That's really all gears are; a conversion of RPM to ground speed, so if those things are equal, the expectation should be equal as well. The gear's 'number' is trivial at that point. "OD" only means that you're beyond the 1:1 ratio, so you could gear anything to be or not be "OD".

If you shorten gears, you'll just use more of them as you accelerate, and it's logical that where you used to be in "4th" you'll now be in "5th" for the same results since the gears are now closer together. The advantage isn't 'more power' as much as it is that you have more gears available closer together to keep you in the ideal powerband.

 

[AmishMike]

Re-gearing is a good idea if you need more in 1st and 2nd gear. Other than that, you are removing some of your OD. If you NEVER use 6th, then got for it.
I have a stock Rubicon on 35's with the automatic. I have towed up and down the east coast from Florida to Mt. Washington. I will not re-gear. I had to adjust to this hi-revving V6 and get used to 4&5th gear on the highway when towing. It is very different from the big-block pickups I am used to.
I second sharpsicle, it will make it more of a close ratio gearbox, shortening the effective range of each gear.

 

[AustinL911]

I mean, the RPM at speed will be the same between the two. Is that what you're asking? The only way it would "pull" any different is if for a given speed, it were at a different RPM, or if it were a different engine. But with the same engine at the same RPM at the same speed, the output is going to be the same. That's really all gears are; a conversion of RPM to ground speed, so if those things are equal, the expectation should be equal as well. The gear's 'number' is trivial at that point. "OD" only means that you're beyond the 1:1 ratio, so you could gear anything to be or not be "OD".

If you shorten gears, you'll just use more of them as you accelerate, and it's logical that where you used to be in "4th" you'll now be in "5th" for the same results since the gears are now closer together. The advantage isn't 'more power' as much as it is that you have more gears available closer together to keep you in the ideal powerband.

Basically, I'm wondering if the transmission being in OD will have any effect on the power output at the wheels due to losses in efficiency (or something else I'm not thinking of). If the final drive is exactly the same, will the power to the wheels be exactly the same? The engine will be turning the same RPM, the wheels will be turning the same RPM, but the driveshafts will be spinning faster with the 5.13 setup than they will with the 4.10 setup. I'm wondering if this has any ultimate effect on power. Hope that makes sense.

 

[AustinL911]

Re-gearing is a good idea if you need more in 1st and 2nd gear. Other than that, you are removing some of your OD. If you NEVER use 6th, then got for it.
I have a stock Rubicon on 35's with the automatic. I have towed up and down the east coast from Florida to Mt. Washington. I will not re-gear. I had to adjust to this hi-revving V6 and get used to 4&5th gear on the highway when towing. It is very different from the big-block pickups I am used to.
I second sharpsicle, it will make it more of a close ratio gearbox, shortening the effective range of each gear.

Like I said, the gears have already been purchased and are waiting for install. But I stumbled upon the final drive numbers while looking at Grimm Jeeper for the 500th time and it sparked this theoretical question in my head. I know the gears will be a drastic improvement and make the transmission far more useable to me, but as mentioned above I'm more curious about final drive theory.

4th gear with the old setup (4.10s) and 5th gear with the new setup (5.13s) should be nearly identical as far as engine RPM input into the transmission and wheel speed on the output side of it, but the 5.13 gearing will be ~25% more transmission output shaft speed to achieve this. I'm curious if this affects anything noticeably.

I hope I'm making sense here.

 

[AustinL911]

 

[sharpsicle]

Basically, I'm wondering if the transmission being in OD will have any effect on the power output at the wheels due to losses in efficiency (or something else I'm not thinking of). If the final drive is exactly the same, will the power to the wheels be exactly the same? The engine will be turning the same RPM, the wheels will be turning the same RPM, but the driveshafts will be spinning faster with the 5.13 setup than they will with the 4.10 setup. I'm wondering if this has any ultimate effect on power. Hope that makes sense.

There's no such thing as "being in OD" in this truck. You're either above or below the 1:1 ratio based on the gear you're in and how you're geared. And all that gear number is, it's just a sequential number. You're looking at 5th because it's after 4th, not because 5th has anything special when compared to the other gears. And being on either side doesn't really change anything. So no, it will have no effect.

If you gear it such that today's 5th is equal to yesterday's 4th, then that's pretty straight forward. It will act just like that. The "pull" in the new 5th will equal the "pull" of the old 4th.

"OD" was a thing in the old days because vehicles would often define their top gear of 'final drive' as being 1:1, so when they'd go beyond that, they'd call it "OD" and give you a little switch to enable using that gear for cruising speeds. Not really a thing anymore. Now it's just a convenient way to say which side of the 1:1 ratio that gear is on. Don't sweat it.

 

[Pescatoral Pursuit]

So here's the Sitch:

I currently run a 6MT JTR with 35s and the stock 4.10s. I have 5.13s sitting in the garage waiting to go in. I have analyzed GrimmJeeper's calculator in just about every scenario possible.

Final Drive with stock 4.10s in 4th gear (1:1) is 4.10
Final Drive with 5.13s in 5th gear/OD (0.81) is 4.16

For the sake of comparison's sake, let's say they're exactly the same. Will the truck pull exactly the same in 5th with the new gears as it did in 4th with the old gears, even though it's an OD gear? Will parasitic drag or some other variable make it worse as far as efficiency goes?

Obviously towing is going to be better overall (which is what I'm going for) but I was just wondering about the theory part of it.

All things being equal, it determines how much and where the stress is distributed back through the driveline.

A higher rear end gear ratio will concentrate more stress from the wheels to the pinion gear, with less stress forward of that.

The lower rear end gear ratio will distribute the stress more evenly back through the entire drive line to the degree it encounters other gear reductions/ multipliers along the way.

Figure one tire revolution is a unit of work/ stress. The more times you can divide that unit of work/ stress by the number of revolutions of each other driveline member, is the division, or reduction of work/ stress which you apply to them.

 

[Pescatoral Pursuit]

Basically, I'm wondering if the transmission being in OD will have any effect on the power output at the wheels due to losses in efficiency (or something else I'm not thinking of). If the final drive is exactly the same, will the power to the wheels be exactly the same? The engine will be turning the same RPM, the wheels will be turning the same RPM, but the driveshafts will be spinning faster with the 5.13 setup than they will with the 4.10 setup. I'm wondering if this has any ultimate effect on power. Hope that makes sense.

You are on to something here based on where the transmission routes power through it in different gears. Probably going to have to get that info from someone knowledgeable in this transmission.

 

[AustinL911]

You're looking at 5th because it's after 4th, not because 5th has anything special when compared to the other gears.

Nope. I'm looking at 5th because, mathematically, it ends up close to what 4th used to be. I typically tow in 4th with the current 4.10s and try to stay around 65mph. I can't remember where I heard it, but I seem to remember hearing that the majority of One's towing should be in a 1:1 gear, however, I can't remember where I heard that, what the reasoning even was, nor can I find anything in the OM that corroborates that. Obviously moving to the new gearing isn't going to support that prior (probably incorrect) thinking, unless I want to now run 60mph

If you gear it such that today's 5th is equal to yesterday's 4th, then that's pretty straight forward. It will act just like that. The "pull" in the new 5th will equal the "pull" of the old 4th.

That's pretty much what I've settled on. I should be clear, I'm not worried about this being a problem at all. It just got me wondering if spinning the transmission output/driveshaft faster to achieve the essentially the same thing ended up causing any loss of efficiency, or perhaps heat build up in the transmission, or, or, or....

In theory, there probably is a limit to this, but in practice, I assume it doesn't make a noticeable difference in anything.

Just thinking outloud, on the internet.

 

[ShadowsPapa]

You can pick nits and say there's a fraction of a fraction of a percent less drag because 1:1 ratio is direct drive in the transmission and it means the input and output shafts are locked together and not going through the cluster and other gears. They'll still be turning, but you won't have the friction of the gear teeth sliding over each other with a load on 2 pairs of gears. No gears would be involved if it was in 4th gear which is your direct drive "gear". Input and output are locked together.
As far as the transmission, the most efficient "gear" for the sake of the friction losses inside the gear box - 1:1 is the least drag from the teeth on the gears sliding under heavy load.
In the case of the 6 speed manual in the Jeep, that's 4th.
You'd need some fine equipment to measure that.

For the engine - already answered - the engine likes to wind up so anything that keeps you from loading it at lower RPM is better. Heat rejection is lower at higher RPM for the same load and road speed (in other words, better for the engine)

 

[DesertDog]

For what it’s worth, I have all three offerings prior to the 392. Unfortunately, the Pentastar isn’t going to tow much, no matter what, unless you live near sea level with no Hills. I I have an eco-diesel that needs no gearing at all, with 35”s, it climbs mountains in Utah all day, pulling 4500 pounds. My Pentastar (with a six speed) barely pulls itself. My wife has the 2.0 ETorque in a JLU Rubicon and it pulls great. I wish that would’ve been offered in the Gladiator.

Now. As much as I like, driving the eco-diesel, it is in for warranty work for the third time. The last time, the turbo charger blew and it took two months to fix. This time, it is back in the shop, and I don’t know when I will see it again. In the meantime, I have a Ram 1500 from Avis until it’s fixed. It’s a 3.6 VVT and I haven’t found much info on this motor but I sure hope it’s offered soon in the Gladiator. It pulls WAY better than my Pentastar.
Anyone know more about this motor?

 

[ShadowsPapa]

Unfortunately, the Pentastar isn’t going to tow much, no matter what, unless you live near sea level with no Hills.

There's a lot of us here who would disagree with that.
I've not problem, even on long 3% grades, i do just fine.
Granted that's not mountain driving but it's just not a problem if you let the engine run as designed.
You've got the MT - and that's a big negative towing with a gas engine. With the automatic, it's just fine. It can find a ratio it's happy with, and on the take-off, you have the TC multiplying torque to get you going.

Sponsored