# Perfect rock ratio

Discussion in 'OffRoad Design' started by biltntboght, Jan 10, 2002.

1. ### biltntboghtRegistered Member

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Guys, What is the perfect final gear ratio on a rock crawler? And how do I figure out what I have?? Do I add first gear on trans.+ transfer case + diff+ tire size???

2. ### 55Willy3/4 ton status

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I think those are the numbers..go to <a target="_blank" href=http://www.4lo.com>Online Calculator</a> To figure out what you have, it even list what the tranny and case ratios for each make and model would be.

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55 Willy's

3. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Crawl Ratio is a multiplication-thing....

Take your 1st Gear Ratio x 4LO ratio x Axle Ratio = Crawl

If you have an automatic tranny, you can usually multiply the above answer by 2....the torque converter gives you some additional crawl advantage at low RPMs. Once it hits it's stall speed, that advantage goes away.

As an example:

1st Gear (2.52) x 4LO (1.96) x Axle Gears (4.56) = 22.52 Crawl or roughly 45 if you've got an automatic.

Tire diameter is not part of the calculation....

Hope that helps.

-Greg72

'72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's

4. ### biltntboghtRegistered Member

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Thanks now I know how to multiply to get my crawl ratio. What is the the ratio most suitable for rock crawling? And wouldn't tire size be a factor? I'm going to use this Blazer 95% off-road and wonder if 4.10 gears, 35 in tires, 6.2 diesel and automatic tranny is a realistic set-up to start with??

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You would get allot more help with this in the main forum. But, since your here, that should be a good point to start from. Generally, for rocks, the lower, the better. However, "perfect rock ratio" is a matter of style, engine torque curve, tire size, and the trucks weight (among other things but that is the main stuff). You've got a good starting point, I wouldn't change a thing till you spend 20-40 hours out in various types of terrain just to see how you like it. If you feel the gears aren't working out, you'll have a rough idea *how much* it will take. Your 6.2 is not a power house by any means. However, they are very dependable and tend to run forever if you take care of it. Also, it is injected so you don't have to worry about a carb flooding out on extreme angles. Just don't expect too much in the way of power. 4.10s and 35s should be about right for the 6.2 as far as I know.

Russ

85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.

6. ### Triaged1/2 ton status

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If you have a O/D tranny you will want lower gears than 4.10. Go as low as you can and still have good speed on the hwy. I am going to be running 4.56's and a non-O/D tranny (th350) and that is going to hurt my top speed on the hwy. but will be nice when offroad.

Right now I have 3.73's and have to rev the motor just to get over rocks when in 4LO.

'71 Blazer CST w/ a 400sbc, 4" lift, 36" Supper Swampers, and alot of rust
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7. ### K10ANDYKHAMNIC1/2 ton status

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i agree Dan

always gets me home K10

8. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Todd,

Get a copy of the July 2001 Four Wheeler magazine....p 50.

It's a really good article that explains how crawl ratios are calculated, what sort of values are good, and how to take other factors into account when trying for your "ideal crawl ratio"

It's a good start to making sense of all this stuff.....

-Greg72

'72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's

9. ### biltntboghtRegistered Member

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Thanks for the help everybody. I'll have to get a copy of that issue.

Todd

10. ### DesertDueler1/2 ton status

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I run a 700r4, NP 208, 4.56 gears, and 35s. Mine crawls over just about anything at idle. I never had to use the gas on the AZ run this weekend except through the easy parts where you could go faster. The idea of a low crawl ratio is it keeps you from breaking things because you had to use the go pedal. If you have to use alot of gas to get over things you will break front 10 bolts or dana 44s real quick. You will also need to be locked front and rear for real rockcrawling btw.

86Jimmy 4"&35"MTRs,87 burb 4"&33"MT,69Firebird w/525 hp,and 70GTO live in PHX,AZ<a target="_blank" href=http://www.geocities.com/modifierperformance>My vehicles</a>

11. ### Stephen1/2 ton statusModeratorVendor

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I'm at 62:1 with the auto, so figure about 124:1 with the converter. With a 42" tire, circumference is 130" or so. So I go about 1" for every revolution of the motor. I do have a 468 so I have some good torque available but this works for me.
Previously I was running 55:1 (110:1 with t/c) with a 37" tire, circumference = 116". so the 1" per engine revolution theory holds up for that configuration also.
With a manual, I know Tim Sprouse (butch) was at 6.55x2x4.10 = 53.7 with 35's and it didn't work so well. So he did the Doubler and went to about 107:1 and with 35's it seems like it worked well. So once again we have 110:1 gears with a 110" tire circumference and a v-8.
That might be a good place to aim with your gears, 1 engine rev. = 1 inch traveled at the ground.

Making the world better, one truck at a time.
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