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Discussion Starter · #1 ·
We all know that you figure crawl ratio as a product of: 1st gear x t-case low range x diff ratio. But, has anyone ever seen
EFFECTIVE Crawl Ratio discussed, taking into account the other obvious variable, tire size. Obviously a vehicle with a crawl
ratio of 70:1 with 33s is going to behave a whole bunch different than a similar vehicle but one that is running 38s. I've never
seen this aspect discussed when crawl ratio is discussed, but to me we are leaving out a very important piece of the equation.
Just a thought.

Brad
ORC Land Use Section Editor
Get involved or lose it all, the choice is yours!
 
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Discussion Starter · #2 ·
I've seen this discussed on one of the mailing lists I'm on a few months ago. For the life of me, I don't remember the formula that was decided would work for this.

Anyone else who's on the Jeep-L and ORML remember this?

_____________________
John
95.5 YJ with "stuff"
http://SonsofThunder4x4.com
 
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Discussion Starter · #4 ·
Okay, I think I've got it. First of all, we need a baseline tire size for a factor. Since serious off-road tires begin with 31", I suggest that. So, your "effective" crawl ratio would be based on the following:

TD = Tire Diameter
GR = Gear Ratio
TR = Transmission Ratio in 1st gear
T-C = T-Case Ratio in Low
CR = Crawl Ratio
ECR = Effective Crawl Ratio

CR = GR*TR*T-C

ECR = CR*31/TD

So, let's say you have 4.10 gears, a 2.61:1 T-case ratio, & 3.06 first gear.

CR = 4.10*2.61*3.06 = 32.75

With 31" tires, your ECR = 32.75*31/31 = 32.75

With 33's, your ECR = 32.75*31/33 = 30.76

With 35's, your ECR = 32.75*31/35 = 29.00

With 235/75R15's, your ECR = 32.75*31/29 = 35.00

Does that seem to be on the right track? Not sure if 31 should be the baseline or not, but I think the formula is simple & solid. Sound about right?

TEX

/wwwthreads_images/icons/wink.gif Got Mud?
G.U.M.B.O. Mud Racing
 
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Discussion Starter · #5 ·
I'm no expert on the matter, but I did take a couple of calculus bases physics classes. The ratio, not concerning is just the ratio that the engine is turning in relation to the wheel. The wheel will turn as many times as the axle does(unless the axle is broke). But, turning a bigger tire takes more torque. So the question would concern a loss of torque from larger tire size. the actual torque develolped would depend on the torque the engine puts out plus the torque developed by gearing it down minus the torque sopped up by the tire size. If I remembered the actual formula for torque I could be of more help. This should at least point you in the right direction. TEX, you are right about the tire size having a lessening effect on the crawl ratio.

Hope this helps some.

Robert87yj/wwwthreads_images/icons/tongue.gif
engine rebuild w/4.0 head done, now for the MPI
 

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The gearing wont change, but a larger tire will move the vehichle faster and take more torque to turn. Just to make things more confusing on everyone, dont forget that airing down the tires changes things once again....

 

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Harry......the infamous Harry......was involved with a thread on this on another board. He asserted that instead of tire diameter......tire radius (x2) would be a more accurate plug in due to various inflation pressure. CJDave was also involved in this thread. Some poor guy created a "crawl ratio calculator" and hung it out there for critique ..... well Harry .....the infamous Harry......did just that.....poor bastard......never had a chance.

GeeAea

Figures don't lie ....... but liars sure do figure.
 

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FWIW, torque to the ground can be thought of as a measure of the "moment" about the axis of rotation and is the product of the Force applied times the perpindicular distance from that force to the axis of rotation (the units is what gets confusing)

/wwwthreads_images/icons/cool.gif Big Ed
'88 YJ, 4" susp,3" body,33's,283 Chevy V8,TH350,4.11's,D30,D35c
 
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Discussion Starter · #9 ·
When comparing crawl ratios between freins I alway's just figure the speed for a given rpm(usually 1500rpm) for each vehicle, the formula is:

tire diameter x RPM/crawl ratio x 336 = mph

Jeff
'83 Toyota long bed
5" lift, YJ front springs
 

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I think a better way to calculate the Effective Crawl Ratio(ECR), would be to calculate the distance traveled per rotation. This would be Tire Diameter(TD)*PI(3.1415)/Gear Ratio(GR)*First Gear(FG)*TCase Low(TL). A smaller distance would indicate a better ECR.

(31"*3.1415)/4.1*3.09*2.61=2.62 inches per rotation
(33"*3.1415)/4.1*3.09*2.61=2.79 "
(35"*3.1415)/4.1*3.09*2.61=2.97 "
(38"*3.1415)/4.1*3.09*2.61=3.22 "

Correct me if I am wrong.

I have used a similar formula in the past to calculate the effective gear ratio after tire swaps. Percentage increase in circumference=percentage decrease in gear ratio.

Tom

85 CJ7-4.2L,T-176,D300,2.5"lift,32"BFG MT,Durabak,York-Air,HEI
 

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Jeff has the right idea; actual crawl ratio isn't affected by tire size..... crawl speed at idle (or some set rpm) takes tire size into account and would be a more useful measure..... after all, the real question is, "How slow can you go without slipping the clutch?"

-Dana

"Very funny, Scotty. Now beam down my clothes."
 

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Tom......the only flaw that I can see.....and this was the main complaint that Harry (the infamous Harry) had with this guy's calculator is that tire diameter is rarely true in a low crawl situation due to various tire pressures...hence the use of radius from center to ground X 2 ....which is less than the diameter...if you want to be accurate anyway. Harry also mentioned lots of stuff about moment arms and such. I wish CJDave was here to go into the particulars. Anyway your crawl ratio at 8 psi......is somewhat different than at 35 psi.
GeeAea

Figures don't lie ....... but liars sure do figure.
 

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I think the crawl ratio is an arbitrary measure, independent of tire size. You can, however, calculate the effect a tire size change on your gearing by the formula: (old size circumference/new size circumference) x crawl ratio, which is one ratio times another. This will give you the effect the tire change will have on your gearing, even though the crawl ratio doesn't actually change. Notice that if you decrease the tire size you have a positive effect on the crawl ratio (the ratio "becomes" larger numerically). /wwwthreads_images/icons/smile.gif
 
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Discussion Starter · #14 ·
OK.. i'm gonna get involved in this one too... i'm in all those calculous based physics classes right now, and this looks like an applications area, so.......
This is all off the top of my head (not gonna dig out the books, then it would be too much like HW, and not enough like fun) The Circumfrence of a cirle is 2*PI*Radius.. In the case given (lower tire pressure, this # will change depending on if you are going up hill, flat, or are upside down...DOH.... All Related to the Force Normal to the ground) so, lets assume flat ground. the formula will be.....(RPM*Effective circumfrence)/(1st Gear*T.C. Low*Axle Gears)

Moments of inertia (caused by heavier wheels, tires, axles, etc) and such will have no effect on this number, this is mearly the Inches your jeep will travel, per minute. Why you might ask..... Because the moment of inertia is Absolutly unneeded.. if the crawl ratiois Tall enough (1000rev of the motor to 1rev of the axle, or higher) a mouse can turn over 44's..... Moments of inertia will come into play at considerably higher speeds. But, hey, we're crawlers, we can run steal wheels, and boggers, and not hafta worry about the Unsprung, Rotating mass, like the desert guys, and some of the mud folks... ok. thats about all i have to say... any errors??


Once again ( RPM*2*PI*Effective diameter ) / ( 1st Gear*T.C. Low*Axle Gears ) will give you inches, per minute

Hope I've helped
BJ

to hell with it, lets go wheelin'
 
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Discussion Starter · #15 ·
Ohhh yeah, i'd check for effective radius (not diameter as was prviously posted), by measuring from the center of the axle, to the ground, at whatever tire pressure it is you'd like to find out about

BJ

to hell with it, lets go wheelin'
 
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Discussion Starter · #16 ·
Brad -
I think I can contribute somethings to this thread. First is an affirmative item and second is an interrogatory. (Gotta do something with those five dollar words or they'll clutter things up!)

Some posters are correct about the diameter not being part of this. The important thing on this part of the question is the radius. Not just the D/2 simple radius, either. Whether your concern is air pressure or how much camping gear, recovery gear and spares and kids and wheeling dog and fat jeepchick all weigh together, the question is radius. I submit for one and all to consider the following: Effective Loaded Radius. When we stop at traihead and air down and then get back in, it is not phsycological when we think that our rigs have gained more power as we tap the gas to move out - they really have acquired more working torque. The EFL is shorter.

The measurement of torque is in various guises, ie., pound/feet, newton meters, paschals, etc., etc. but I will use pounds/feet here for discussion since I'm a old geezer and too lazy to get familiar with metric.

If, after we do the math of the gearing and we get to the axle/ wheel and say we have 7200 pounds/feet of torque, what we mean is that there is enough energy available to move 7200 pounds of mass on an arm connected to the axle shaft if the arm is exactly 1 foot long.
I think that this means that if the arm is two feet long then the weight can only be 3600 pounds.

This is my question and I hope someone here knows the answer.

If I am right, then my BFG 31x10.50xR15s which have an EFL of only 14.25" would give me an Effective Crawl Ratio multiplier/reducer of .842. (12/14.25) I'm just barely sure enough about this to bet the farm but welcome all questions, comments and even flames if they are necessary.
sln


 
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Discussion Starter · #17 ·
I've got to agree with Geeaea; I saw this question on the partsmike page, while I believe Mike was at SEMA, it got pretty brutal for awile 'er. You should take into account your effective radius; from ground to axle hub. I can't remember all the logic but, the reference to a tank track was made. There is no need to use the circumference in the calculation; use the distance from the moment arm to the ground. I think... if I can find the other thread I'll try and do some more wesearchin'.

 
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Discussion Starter · #18 ·
BJ,
I sort of remember some of that. Sounds good to me. Man that brings back the memories. My first professor did his thesis on rolling v. sliding friction. Most of our test ?'s came from some sort of friction example. A little off subject, sorry had to share.

Robert87yj/wwwthreads_images/icons/tongue.gif
engine rebuild w/4.0 head done, now for the MPI
 
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Discussion Starter · #19 ·
I know about those Friggin Friction tests... my Prof had a sick sense of humor too.... if jane is skiing down the slope w/ Friction of .85.. at 15m/s... how fast will she be going if she falls, and friction goes to 0.... stuff like that.

Back on topic.. the tank track, MUST be driven by a wheel (or Gear) somewhere along it's length... use the circumfrence of that Wheel ( or Gear) in the equation

BJ

to hell with it, lets go wheelin'
 
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