which is obviously outside my capacity to figure...

we know that increasing tire height raises overall gear ratio (numerically lowers it).. that is pretty easy to figure.. what isn't so easy to figure is the perceived torque to the terra..

the owners manual on my rig states w/20" rims (stock on mine) I can tow 7200#.. with 17" rims, I can pull 8400#.. this has to be based on 'perceived' torque due to effectively lowering the overall gear ratio, right?

So, how does a person translate the overall gear ratio to perceived additional torque? How does that work?

I did most my mods at the same time I went with 35" tires.. Including gears (from 3.92 to 4.56).. There is no doubt I have a lot more power to the terra, and I'm almost curious enough about it to slap on some 33's (which was stock) just to see if it feels as strong as I suspect it would with effectively lowering the gears again.. dropping to 33's would be about teh equiv to 4.88's.. but that is back to ratio.. I REALLY want to understand this perceived torque a little better!!

thoughts?

 

Overall diameter is what determines final drive ratio. If you have 35" tires, rim size is irrelevant as far as torque to the ground goes. The torque capacity of the wheels, on the other hand, does become an issue.......

I think.

 

What you are actually feeling the loss of is the force of the tire to the ground. Remember that torque is force x distance. The amount of torque being supplied through the axle shaft will not change with all else but tire/wheel size being equal. The taller tire has more distance from the center of the wheel to the ground therefore the distance is longer. With the longer distance and the same torque there must be less force to keep the torque the same.

By lowering the gear ratio (numerically higher) you are increasing the torque to the wheel and with the distance staying the same (tire size) the force applied to the ground would be higher.

The loss of towing capacity is more likely due to the strength of the wheel itself and the associated warranty and liability that goes with it, not to mention the weight capacity of the tires on the 20" rim.

 

HeyYou- I was summarizing, the manual actually lists the entire wheel/tire size in stock increments.. You're right as rain though, the TIRE determines overall drive ratio.. I never once considered the toque capacity of the wheels.. I always figured the wheels could take a lot more torque than these engines/trannies/gears could EVER toss at them..

Gere- and your point is something I never thought about either.. the mechanical LOAD strength of the larger wheel.. huh.. you're right.. the larger wheel wouldn't sustain the weight the smaller one would unless the design and material was addressed.. huh..

ya learn something new every day!!

thanks y'all..

 

20in wheels = aluminum mag wheels.

16/17in wheels = steel wheels.

The steel wheels are usually manufactured for the specific purpose of towing. Remember back in the 1990's that you couldnt hardly get mag wheels on a 3/4 or 1ton truck. Now there has been quite a few advancements in wheel and tire tech. For instance, now you can get E series tires for 20in rims. But stock configurations still limit the towing on vehicles outifitted with mag wheels.

 

I never understood how it is that a taller tire creates more distance to travel. From a physics standpoint, the axle doesn't have anymore distance to travel just because the tire is taller. One revolution of a 33in diameter tire is the same as one revolution of a 31 inch tire. Right? I totally understand that a 33in tire has a good bit more weight and also that the width of the tires if different add a lot more weight as well. I'm not arguing that there is more work done with a 33in tire only that there isn't any difference in distance traveled by one tire than the next. Also, its Friday and, at least in my office, Beer Friday, so what little math skills I had are probably out the door by now.

 

Larger diameter makes for a longer circumference. So, for one tire revolution, circumference is the distance traveled. For example:

Circumference equals 2 X Pi X R

R is radius. This can be factored down to:

Pi X diamter. (as 2 X R IS the tire diameter.)

So, a 30 inch tall tire will have a circ of 94.25 inches. (approximately.) So, the 30 inch tire travels 94.25 inches per revolution.

A 35 inch tire has a circ of just about 110 inches. So, the 35 inch tire will travel almost 16 inches further per revolution......

Make sense?

 

I was told there wouldn't be any math.

 

You were lied to.

 

I hate that SOB named Pi. Hand me another beer.