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True crawl ratio

K

K5dreamer

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FML my last post got lost when i hit submit. ok, short version.

I read through a few threads on the lack of accuracy of claiming a crawl ratio based on drivetrain ratios. And how tire size impacts the actual crawling performance of your rig. Lots of good information and debate, but no one actually put it into a formula that was clear and easy to use. I have spare time at work.... lots of it, so i decided to do just that. I set it up to give you a number similar to an RTI score, in three digit form, that could be easily expressed. For instance my RPI score (rotations per inch) is .114, or 114, or one hundred fourteen. I also did a formula to calculate your speed at idle with the vehicle in gear. Enjoy.

Variables
CR - crawl ratio
C - Circumference
F - transmission first gear ratio
Tc - Transfer case ratio
A - Axle gear ratio
Td - tire diameter
P - Pi or 3.14
I - rpm at idle
MPH - miles per hour
RPI - rotations per inch

Formulas
CR = (F)(Tc)(A)

C = (Td)(P)

CR
--- = RPI
C

((I)/(RPI))
-------------- = MPH
1056

EDIT - reduced the last equation to make the math simpler. reduced 60/63360 to 1/1056.
 
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As an example, my M1009 with 33in tires has an RPI of 114, and assuming an in gear idle of 1100 rpm, would be rolling at 9.14 mph. whoops, wrong first gear on the TH400, actual RPI is 192 and 5.34 mph.

Upgrading to 39in tires, NV4500, LoMax 3:1 205, and 4.11 gears would give me an RPI of 568, and an 1100 rpm idle speed of 1.83 mph.
 
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So heres the thing about alot of stuff, no matter how much you calculate some setups just seem to work really well. For instance my setup, 42s 4.56s th350 gears and a doubler, to some folks that seems like it may not be low enough, even when I was running 44s the only time I really didn't like my gears was on the street. It was slow for sure.

In mud it was more my stock HEI I ran out of ignition 4500 rpm and it would start to drop out
 
i dont see why that would be seen as not having enough gear. 4.56's and a doubler, (im assuming a 203/205 doubler), is pretty deep gearing, even with the TH350's 2.52:1 first gear. youve got nearly 2.5 times the gear that my M1009 has, and ive only had a few times with my truck that i actually was hung up by not having enough gear.

However, your RPI of 349 is only 1.8 times as high as my RPI because youre tires dwarf mine. :)
 
decided to see what the resonable limit was to this with gearing and whatnot. This is what i came up with. using a 39in tire.

-Granny gear NV4500 - 6.35:1 first gear
-LoMax 205/203 doubler - 6:1
-D60 and D70 axles deepest gear i could find - 7.17:1

-Crawl ratio - 273.18:1
-RPI - 2.230 (four digits needed instead of the three i thought would cover it.)
-MPH @ 1100 rpm - 0.47 (< half a mile per hour)
 
Now a real world setup, that could be driven to and from the trails, and is a bit more balanced. stepping up to a 40in tire. (or 42's if you factor in flat spots from compression under the trucks weight)

-Granny Gear NV4500 - 6.35:1
-LoMax 205/203 doubler - 6:1
-D60 and D70 axles - 4.88:1

-Crawl ratio - 185.93
-RPI - 1.480
-MPH @ 1100 rpm - 0.7 (still < 1 mph)
-RPM @ 60MPH - 1795 (spot on target for a 6.2 diesel)

oh dear, i sense a much bigger hole being burned in my pocket over the next few years.
 
K5dreamer said:
First off, you missed the most important part here, about how bored i am at work. that would have taken away all this work ive done to entertain myself.

Second off..... link dosnt work here at work. ill check when i get home :D
Click to expand...

LOL I will see if there is something I need figured out and you can come up with a formula for it :D
 
I had a really good working set-up with my old Jimmy. 6.2, SM 465/205, 4.56's, 39.5's. On paper it seems a bit underwhelming, but it would idle straight up pretty much anything. Idle speed at ~600 rpm meant it was still very slow and controllable.

The amount of tq AT idle really made up for a lack of deeper gearing, and the governed idle helped a ton with wheeling 'elegantly'.

I did eventually end up with a NV4500/203/205 behind the 6.2, same gears and tires...but I never did wheel it with that combo.

Rene
 
I kind of hate you guys sometimes. a 6.55:1 first gear, 2:1 tc, and 4.56 axles is underwhelming..... I really hate to think about how bad you guys laugh at me when im not around with a 2.48 1st gear, 2.6:1 tc, and 3.08 axles lol.

For the record TRustyK5, the RPI on that first setup is 482 :)
 
Yeah, but your TC helps a lot. 58:1 CR with 40's is a bit underwhelming relative to most other rigs out there crawling and stuff.

FWIW Stephen from ORD had a quick 'guide' he used for crawl ratio vs. tire size. If the crawl ratio was equal, or close to equal the circumference of the tire in inches, it was a pretty good set-up.

So, for 40's a CR of about 120:1 was good. My NV4500/203/205/4.56's was at 101:1...near enough for me.

Rene
 
You gotta factor in another 2:1 for the torque convertor...

Rene
 
K5dreamer said:
Now a real world setup, that could be driven to and from the trails, and is a bit more balanced. stepping up to a 40in tire. (or 42's if you factor in flat spots from compression under the trucks weight)
Click to expand...


The tire will have the same circumference regardless of flat spots or how far they're aired down :deal:
 
Just figured my RPI for the Jimmy:
SM465,203/205,4.56 gears and 37's= 1.01 (or so)?
Math was never my strong suit, cool formulas bro! :waytogo:
Who wants to figure my crawl speed at idle? I apparently am too bad at math to get it. :doah:

EDIT: tried it again, came up with 1.03 mph (@1100rpm) is that right?
 
38377k5 said:
The tire will have the same circumference regardless of flat spots or how far they're aired down :deal:
Click to expand...

hahaha wow, thanks for correcting that, brain fart and a half. observed radius may change, but you are correct sir, circumference is unchanged.
 
blowedupmotor87 said:
Just figured my RPI for the Jimmy:
SM465,203/205,4.56 gears and 37's= 1.01 (or so)?
Math was never my strong suit, cool formulas bro! :waytogo:
Who wants to figure my crawl speed at idle? I apparently am too bad at math to get it. :doah:

EDIT: tried it again, came up with 1.03 mph (@1100rpm) is that right?
Click to expand...

Dont sell yourself short, I got an RPI of 1.03 for that setup. 6.55:1 / 4:1 / 4.56 gears. MPH @1100rpm would be 1.01 mph.

interesting. same numbers, but flipped. its late, maybe i screwd something up lol.
 
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