40s or towing on factory rubber?

[Pescatoral Pursuit]

With factory gears.

A lot is made about the strain on the factory drivetrain (and brakes) with the addition of larger tires, but how does it actually compare to towing within ratings?

Leaving aside the notion that if one were to drive full time towing or with larger tires they should regear:
At what weight does towing with factory tires, (edit) or (edit) driving on 40s, place the same load on the drivetrain?

Please do your best to hypothesize within the constraints of the question. If you think that tires and towing presents different types of loading, then quantify exactly how the different components would experience that load differently, and only if you think it would produce a measurable effect on either wear or strain.

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[rharr]

who care about hypothesis, you already gave people crap when they said 3.73 on 40 was a bad idea even for a eco d.

At this point if you want to tow, then tow, if it sucks with your 3.73 you know what needs to be done.

 

[Pescatoral Pursuit]

I'm asking for intelligent discussion. I understand you're not prepared.

who care about hypothesis

 

[sharpsicle]

Let me make sure I understand the ask:

You want to know when the load of towing x pounds equals the load imposed by going up to 40s without towing?

To better frame the question and the answer, what application is the answer going to be applied towards?

 

[ShadowsPapa]

With factory gears.

A lot is made about the strain on the factory drivetrain (and brakes) with the addition of larger tires, but how does it actually compare to towing within ratings?

Leaving aside the notion that if one were to drive full time towing or with larger tires they should regear:
At what weight does towing with factory tires, and driving on 40s, place the same load on the drivetrain?

Please do your best to hypothesize within the constraints of the question. If you think that tires and towing presents different types of loading, then quantify exactly how the different components would experience that load differently, and only if you think it would produce a measurable effect on either wear or strain.

Going back to the engineers making these things capable of towing what the ratings say - it's all about tires and final gear ratios and heat rejection.
The determination was made, and this NOT my guessing, but having read the info, that the best tow rating was with the 32" tires and gear ratio of 4.10:1 and putting the engine in the right RPM band.
This is from one of the engineers involved:

So if you up tire size, you obviously take it way out of their intended place. You would need to get back to the final ratio - engine RPM in given gear vs. road speed. And that must be done for each given gear the transmission may be in while towing - could be 6th, or 7th, not just the final 8th gear ratio. On hills it's going to drop back - and you need it placed in the right RPM range for each gear. Can't just say fine, I'll hold 7th and be fine.

The weight of the vehicle has a big impact as well as it's not just the trailer the engine has to move, it's the whole thing - truck, payload and trailer. So you can't just drop 32s on a Rubicon and say there, same thing..........


And of course bigger tires = reduced braking, so there's that.
You'd need to be an engineer to do some complex calculations to get the in between figures. But there's reasons that Rubicon and Majove aren't rated the same as Sport S with max tow.
We get back to it being the whole package - putting that engine in the right RPM band to get the work done, minimize heat rejection, figure in the heat of the transmission also needing to be taken care of and so on.

I still wonder why it is that forums try to figure ways around engineers and their results - which have been tested.

 

[sharpsicle]

Going back to the engineers making these things capable of towing what the ratings say - it's all about tires and final gear ratios and heat rejection.
The determination was made, and this NOT my guessing, but having read the info, that the best tow rating was with the 32" tires and gear ratio of 4.10:1 and putting the engine in the right RPM band.
This is from one of the engineers involved:

So if you up tire size, you obviously take it way out of their intended place. You would need to get back to the final ratio - engine RPM in given gear vs. road speed. And that must be done for each given gear the transmission may be in while towing - could be 6th, or 7th, not just the final 8th gear ratio. On hills it's going to drop back - and you need it placed in the right RPM range for each gear. Can't just say fine, I'll hold 7th and be fine.

The weight of the vehicle has a big impact as well as it's not just the trailer the engine has to move, it's the whole thing - truck, payload and trailer. So you can't just drop 32s on a Rubicon and say there, same thing..........


And of course bigger tires = reduced braking, so there's that.
You'd need to be an engineer to do some complex calculations to get the in between figures. But there's reasons that Rubicon and Majove aren't rated the same as Sport S with max tow.
We get back to it being the whole package - putting that engine in the right RPM band to get the work done, minimize heat rejection, figure in the heat of the transmission also needing to be taken care of and so on.

I still wonder why it is that forums try to figure ways around engineers and their results - which have been tested.

Yeah this makes a lot of sense. I think there are too many variables and caveats to really answer a question like this, considering you're comparing potentially different applications of load and force (linear vs rotational, heat vs mechanical, and so on).

Another reason it's difficult is engineers themselves develop ideas and theories and then test them to see the results. It's not a plug-and-play mathematical formula. They evaluate and develop multiple times based on testing.

So you'd have to define the actual metric (i.e. how are we defining 'load'? Where is this 'load' measured and how?). Then you'd have to develop a hypothesis and test it multiple times. But again, that's going to be challenging given all the variables. What's the end goal anyway?

And the more I read the original question, the more patronizing it reads, so that's a bit of a turn-off.
"Please do your best..." "...quantify exactly how..."
It feels a bit tone deaf to talk to a forum of enthusiasts as though they're your students. If you're asking for help in understanding something, it would help to present it that way.

EDIT: Thinking about this even more, we all know that towing is more than just pulling weight. It's control of the load, which may or may not have drivetrain implications based on how the weight is loaded, shaped, and distributed. Adding to that, trailers themselves vary in efficiency when being pulled. I could have two trailers A and B, same weight, but B has less resistance than A, and as such the stress on the drivetrain to pull B is less than to pull A. Same weight. Tow rating is unaffected, because it's more about controlling the load, regardless of it being easier to pull trailer B. This is why often times boats are easier to tow than camper trailers, even if they're the same weights.

All this to say, I don't think this is the right question to be asking or the right comparison to be making.

 

[ShadowsPapa]

EDIT: Thinking about this even more, we all know that towing is more than just pulling weight. It's control of the load, which may or may not have drivetrain implications based on how the weight is loaded, shaped, and distributed. Adding to that, trailers themselves vary in efficiency when being pulled. I could have two trailers A and B, same weight, but B has less resistance than A, and as such the stress on the drivetrain to pull B is less than to pull A. Same weight. Tow rating is unaffected, because it's more about controlling the load, regardless of it being easier to pull trailer B. This is why often times boats are easier to tow than camper trailers, even if they're the same weights.

Jeep changed the bushing stiffness, sway bar stiffness, other changes in order to able to meet the handling and stability parts of the SAE J2807 towing test which are related to steering performance (understeer) and sway control.
It's not all about towing weight and having the HP to do so. You've got to control it. And they passed the testing only with the configurations found in the factory releases.
Go changing those things and you drop your towing ability (safely, that is).
But people are all about springs and such.
We go back to - they were rated/tested at the factory heights, factory tire sizes, factory track bars and sway bars, factory bushings and more.

As far as "load" you run into people saying they try to keep the RPM down while towing - not realizing that they have increased the load on the engine, and increased heat generation and made it worse, not better.
Towing a load of xxxx pounds along a grade of z% may require YYY HP -but at a higher gear and lower RPM it's actually more "load" which increases heat (determined by throttle opening - the same load being pulled at the same speed gives a higher load on the engine with reduced throttle opening, it's working it harder).
That means more heat rejection and you want to try to keep that at a minimum. Better off keeping the RPM up a bit in a lower gear which means lower engine load, less heat.
And we go back to the weight added to the truck by larger, heavier tires, wheels and other gear as well as other factors.
You can't make mods and add weight, change height, change the functionality of the sway bars and other things and keep the same tow rating.

 

[Toro34]

i know you want to have a detail and exact discussion, but here is my 2c:

trailering safety is more a function of stopping, than getting going.

going from 32" tires to 40" tires is an increase of ~23% (rounded up) this will increase your stopping distance/brake performance around the same amount

in addition, for every 10lbs more you add, your performance will decrease by 1% (40s usually weigh about 80lb more than stock, ymmv) so let's keep it simple and figure you now have a 30% decrease in brake performance.

again, not exact, but it should also work with getting the tire going. you are now decreasing your acceleration by 30%

regear will give you a mechanical advantage with acceleration, but your braking is still 30% down, add a trailer to that, and obviously, you're taxing the brakes that much more

CAN you tow a trailer with a Glad on 40s, sure, no one will stop you. SHOULD YOU? That is up to you. there are way too many variables involved. Distance, trailer weight and type, cargo, number of passengers etc...

Again, just my opinion, but if you're trying to tow anything of significance, get/rent a heavier duty truck.


https://www.beefedupbrakes.com/how-heavier-wheels-and-tires-impact-braking-distance/

 

[Toro34]

With factory gears.

A lot is made about the strain on the factory drivetrain (and brakes) with the addition of larger tires, but how does it actually compare to towing within ratings?

Leaving aside the notion that if one were to drive full time towing or with larger tires they should regear:
At what weight does towing with factory tires, and driving on 40s, place the same load on the drivetrain?

Please do your best to hypothesize within the constraints of the question. If you think that tires and towing presents different types of loading, then quantify exactly how the different components would experience that load differently, and only if you think it would produce a measurable effect on either wear or strain.

I just re-read your question, and did some additional Googling.

i think you're looking for the number (Y) where a a stock truck towing X lbs. = truck on 40s towing (X-Y) lbs ?

This thread will get you going, https://www.irv2.com/forums/f85/the-physics-of-towed-vehicle-braking-19087-2.html

 

[ShadowsPapa]

I just re-read your question, and did some additional Googling.

i think you're looking for the number (Y) where a a stock truck towing X lbs. = truck on 40s towing (X-Y) lbs ?

This thread will get you going, https://www.irv2.com/forums/f85/the-physics-of-towed-vehicle-braking-19087-2.html

Yeah, there's some math out there for braking, steering and control is another matter no one can tell someone what will happen in an emergency as far as maintaining control, understeer handling on corners, exit and entrance ramps and so on. Nothing more scary than taking that exit in a clover leaf and turning and having the truck go straight - not taking the curve, or trying to swing to the inside of the circle. Have had both happen. that trailer wants to keep pushing the rear of the truck in a straight direction - you are trying to drag the front of the trailer around a corner. Thankfully my truck at that time had very heavy-duty brakes, a well optioned Ford as I found out when I tried to replace the front hubs and they were a special order.

 

[Chief_jeep]

I think a lot more goes into this than just the gearing. The brakes would be my biggest concern if I was planning on towing on 40s with the d44. You could easily upgrade the brakes. My next concern would be how much stress you are putting on your drivetrain and everything else associated with 40's and towing. I'm sure it could do it, but for how long, I'm not sure.

I am curious what towing looks like with 40s, upgraded axles, gears, driveshaft, etc. When you go to a dana 60 or larger the braking is upgraded as well. From personal experience, I don't think I'd be comfortable towing on 40s without a solution for keeping the oil cooler even with a 60/70 setup.

 

[Pescatoral Pursuit]

it's all about tires and final gear ratios and heat rejection.

Tires as a component of engine rpm at final speed, not as factor of rpms on initial start??

So if you up tire size, you obviously take it way out of their intended place. You would need to get back to the final ratio - engine RPM in given gear vs. road speed

This would only apply from a stop where a different gear cannot compensate for additional load. If I'm going 30mph and the load calls for 2500rpm, the trans will find the correct gear.
We should be working from the beginning (initial and acceleration) rather than final (whichever the established constant speed is, where dynamic forces are minimized/ mitigated with inertia.)

Can't just say fine, I'll hold 7th and be fine.

Gear selection is the servant of engine power/ mph. I think you're trying to have the tail wag the dog here.

 

[Gvsukids]

This sounds like a good time to ready the ?.

 

[Escape.idiocracy]

Threads like this…. Esh.

Reminds me of the part number for the Mopar factory plastic rivets thread.

I suppose this debate could be tied to the same folks that don’t think re-gearing needs to be done at all because, it’s ok to run 70mph in 4th gear, the maths say it’s fine ?

strap 40’s on, hammer down, when the oil/coolant get close to derate back off take the 40’s off load a trailer until the same effect. Rock on ?

 

[Reddog]

I'm not sure there is a good answer to your question, at least without doing a great deal of dynamic modeling with sophisticated software unavailable to most of us. I was trained as a civil / structural engineer, and I can tell you for a fact there is no easy or satisfactory answer to your question. Just guessing but there are probably 50 or more variables that must be considered and without the modeling software, forget it. Having said that, I still get that you want some information here. Not being critical here, but if I were going to tow with what I assume is a more or less stock Jeep, I doubt I would ever consider putting 40" tires on it before doing so. Many of the commenters here have touched on the main issues already so not going to add anything other than if you intend to tow a relatively heavy (say 2000 lbs) load with a Jeep on 40"', at the very least I would regear to at least 5:12's, install significantly larger brakes, make sure the towed trailer had electric trailer brakes, and be gentle on the throttle while towing. Remember, 40" require a significant lift which increases your center of gravity and roll-over moment, among other things. Good luck to you and be safe.

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