What overall wheel weight requires regearing?

[RENEGADE Concept]

When choosing to increase the tire size on a kj, I was wondering if anyone had actually calculated what overall rim + tire weight required regearing to avoid problems?? Generally speaking people say with a lift you can run 31" tires without problems, but you should regear if you're going to use 32" tires. But that's not really taking rim weight into account. Or the fact that tire and rim weights vary a lot. And if it's really all about weight, than that's not a very refined rule.

So if "A" stands for rim weight. And "B" stands for tire weight. And "C" stands for the maximum weight a stock geared liberty should spin.... Than what is "C" in the equation A + B = C ??

 

[tommudd]

When choosing to increase the tire size on a kj, I was wondering if anyone had actually calculated what overall rim + tire weight required regearing to avoid problems?? Generally speaking people say with a lift you can run 31" tires without problems, but you should regear if you're going to use 32" tires. But that's not really taking rim weight into account. Or the fact that tire and rim weights vary a lot. And if it's really all about weight, than that's not a very refined rule.

So if "A" stands for rim weight. And "B" stands for tire weight. And "C" stands for the maximum weight a stock geared liberty should spin.... Than what is "C" in the equation A + B = C ??

Has more to do with height , but weight also figures in
IMO any time you get above 245-70-16s you have lost some power and mileage '
Now going to a 245-75-16s / 265-70-16s you can " get by " with stock gearing but putting more wear and tear and the engine and transmission.
I'm regeared with 265-70-16 , ( along with several others ) and it does great.
So you're sort of on the right track as far as weight , but figure in height as well.

 

[dude1116]

Has more to do with height due to rotational weight. The taller the tire, the more weight you're spinning in a larger circle. But weight DOES factor in.

 

[RENEGADE Concept]

Alright... I get what you're saying. And I can see the height factoring in... but we're only talking about an inch in diameter difference between 31 vs 32. Which seems pretty minimal. Still seems like it's gotta be mostly about weight.

 

[RENEGADE Concept]

If most of the weight being spun were in that outermost half inch of rubber than I would agree that height plays a bigger role.

 

[CactusJacked]

The height of a tire plays a big part, it changes the final drive gear ratio. When comparing the stock 3.70 rear gears with stock 29" tires, 31" tires are going to change the final drive the same as if you had 3.46 gears. 32" tires will act the same as having 3.37 gears (these aren't conventional ratios, but that's how it works out mathematically).

 

[RENEGADE Concept]

That's all very interesting and probably accurate, but I think my equation is still a fair question and worth pondering.

 

[CactusJacked]

No probably's about it. Tire height changes the engine rpm to tire speed ratio, the same as how changing rear gears does. Just pretend your tires are gears...

 

[dude1116]

No...it's not really a fair question because this is how tire size affects mileage and power:

Gear Ratio Guide for Larger Tires | Quadratec

But keep in mind we have O/D so our 4th automatic gear is not a 1:1 ratio. Since our Jeeps are lacking the power to hold O/D with larger (and of course heavier) tires, you'll either be pushing the pedal harder (more engine wear, LESS gas mileage...don't let the chart fool you into thinking you'll get better mileage with a larger tire) or have O/D off where you'll be running with higher RPMs and worse gas mileage (but more available power, of course).

 

[RENEGADE Concept]

Every question is a fair question.

Even if it would have to be answered differently - like specific to different tire sizes. It's still a fair question.

 

[dude1116]

Every question is a fair question.

Even if it would have to be answered differently - like specific to different tire sizes. It's still a fair question.

I would argue that it's not really a fair question because it can't be answered in a definitive answer. Unless you consider our answers fair. :icon_razz:

 

[RENEGADE Concept]

If you don't think every question is a fair question than you don't respect the term.

And who said the answer had to be definitive. Approximations from experience would have been welcome.

 

[turblediesel]

Think of it as a lever the length of your wheel\tire radius and then add in panic braking forces on a downhill stop from 80mph with an overloaded Jeep towing a big boat; that's the brakes. Now take it to the drag strip or go up some mountains in overdrive and see what drivetrain component breaks first.

The spinning weight has to be sped up, slowed down, and controlled up and down. That takes energy but extra leverage is the breaker (bar).

That's my understanding of it.

 

[TwoBobsKJ]

Think of it as a lever the length of your wheel\tire radius and then add in panic braking forces on a downhill stop from 80mph with an overloaded Jeep towing a big boat; that's the brakes. Now take it to the drag strip or go up some mountains in overdrive and see what drivetrain component breaks first.

The spinning weight has to be sped up, slowed down, and controlled up and down. That takes energy but extra leverage is the breaker (bar).

That's my understanding of it.

Great analogy - I'll be 'borrowing' that one in the future when the issue of gear ratios, torque, etc., comes up :waytogo:


Bob

 

[tommudd]

Been gone a few days traveling around this great country of ours,
BUT back to Renegades question

All engines/ motors (what ever you want to call them ) are designed with transmission ratios / gear ratios etc to run best at a certain RPM range going down a flat hiway.
Now if for example its best at 65 MPH at 2500 RPM , installing larger tires on it drops the RPMs down to say 2100. This makes the transmission keep searching for the best gear and the motor works harder since its out of its sweet spot .
Now add in we'll say 15 lbs extra for tire and wheel, of course the added weight is going to add to power loss and mileage as well since the motor is working even harder to try and pull the extra weight.
So height makes a huge difference and weight make some difference as well .
Since the rolling weight to get them rolling, plus keep them rolling is much more.
But most of the tires we run are all with in a few lbs of one another so we deal mainly with ( or talk about height) more

 

[tommudd]

Basically any size larger than stock size and weight is going to affect the power, the mileage and overall wear on the KJ period, unless you regear for that

 

[krisP]

even with 255/70/16 ive noticed slight overall power loss and the ****** gets confused at certain spots like around 45-50 mph. Height width and weight will all be factors. Im regearing for the transmissions sake and to get a better overall feel, not feel like slug

 

[tommudd]

even with 255/70/16 ive noticed slight overall power loss and the ****** gets confused at certain spots like around 45-50 mph. Height width and weight will all be factors. Im regearing for the transmissions sake and to get a better overall feel, not feel like slug

It really helps a lot, no matter if automatic, manual, CRD or whatever your taste maybe

 

[jeeplib05]

I get up to speed just fine and haven't lost much gas mileage
Maybe I just step on the gas a little harder than most but all seems well to me.. then again I don't plan to have my KJ forever, just until I decide to get something new, so I don't mind what goes wrong with it :gr_grin:

 

[tommudd]

See there is a difference, I build all of mine to last a long time, don't skimp on anything, build them right. More than likely the reason I used to have people wanting to buy whatever I was building at the time since they knew what went into it.