Crawl Ratios? Fill me In - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 11-30-2002, 01:03 AM
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Crawl Ratios? Fill me In

I'm new to learning about calculating crawl ratios... how do you figure crawl ratio? I'd say add up the First Gear Ratio of Tranny? Xfer Case Ratio? and then your Gear Ratio? plus some kind of tire calculation? I am very dumb to this so please guide me in the right direction. [img]images/graemlins/smirk.gif[/img]
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post #2 of (permalink) Old 11-30-2002, 01:44 AM
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Re: Crawl Ratios? Fill me In

Very close except you multiple not add. You will need to multiple axle ratio X transfercase Low X transmission Low (in automatics the torque conveter is generally considered to be a multiple of 2 to 3 in addition to the mechanical reduction) X any other crawler boxes of dual transfer cases you may have. THis will give you your crawl ratio. No atjustments are made for tire sizes or engine type power or torque or the RPM at which these are developed.

For this reason Crawl ratios are not nessisarally equal. A crawl ratio of 60 to 1 behind a Ford 300 inline six with 31 inch tires might give beter real life crawling abillity then a 100 to 1 behing a Ford 302 with 35 inch tires. Note: large tires requier a lower crawl ratio to acheave the same result, as smaller tires. Also an engine that tends to produce torque lower on the RPM scale will also need less crawl ratio to acheave the same result. In general the longer the stroke on a engine the lower the RPM at which it will develop max torque.

My jeep has a low gear in the transmission of 4.02, a low in the transfer case of 2.62, and a axle ratio of 3.54: this gives a crawl ratio of 37.3. I have run this with both 235/75R15 and 33 inch tires Behind a 258 inline six. Sometimes I have wished for a lower crawl ratio but not that often. What crawl ratio you need depends on what kind of wheeling you do for example I do little rock crawling and therefore have less use of super low gears. Howerver someone who does nothing but mub bogging might never use a ratio as low as mine. Many people say that past about 100: to 1 is excessive for allmost all types of wheeling.

bandhmo
post #3 of (permalink) Old 11-30-2002, 09:07 AM
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Re: Crawl Ratios? Fill me In

There is a calculator out there to figure out what effect tire size has on your crawl ratio. It figures the tire speed at a given ratio. I'll post it later if someone else can't find it, its on my favorites on my laptop, so I'd have to go boot that up and plug it in to the internet.
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post #4 of (permalink) Old 11-30-2002, 09:25 AM
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Re: Crawl Ratios? Fill me In

Here's the calculator I was talking about:
http://www.tommyjeep.com/apps/ratio/index.htm
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