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Thread: Final crawl gear ratio

  1. #1
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    Final crawl gear ratio

    I'm working on my drive train (as well as lots of other stuff) and trying to determine where I want to end up.

    From what I read 45 or higher is recommended for rock crawling. You can run your number here http://www.manoian.net/jeep/gear_ratio_calculator.html
    and it gives you a few different things you can move around like RPM to show how many feet you will travel.

    If anyone will post a range or number and mention if you like/dislike the crawl ratio that would be awesome. Any opinions regarding the scale if you know it, are appreciated.. Buggy guys???
    Last edited by CJ007; 02-26-2011 at 04:06 PM.

  2. #2
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    Re: Final crawl gear ratio

    What motor you going to use? highest torque range of the motor at what rpm
    what size tires?
    Per the cal I am 50.83:1
    Last edited by CrashDawson; 02-26-2011 at 03:28 PM.

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  3. #3
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    Re: Final crawl gear ratio

    Quote Originally Posted by CrashDawson View Post
    What motor you going to use? highest torque range of the motor at what rpm
    what size tires?
    Per the cal I am 50.83:1
    Thanks man

    Do you feel that 50 is good for what you do? I willl prob stay in the 4.0 or less trail range, it's not a DD though. Looks like about 210 @ 1800 (83 CJ 4.2L 258) on 35's. Although, it should be a little higher as I have my engine at the machine shop, going 20 over and will do something with cam/crank.

  4. #4
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    Re: Final crawl gear ratio

    I have an '05 TJ with 4.56 gears and 35's and I think my RPM's are around 2600 at 65 mph. With the stock NP231 transfer case with a 2.72:1 low range my crawl ratio was about 54:1. With the new Rubicon transfer case with the 4:1 low range my new crawl ratio is about 80:1.

    In my personal opinion with the 35's and running on the street I would have preferred to go a little deeper, at least 4.88's. The crawl ratio with the stock NP231 left a little bit to be desired when I was rock crawling. With the new Rubi t-case I absolutely love my new crawl ratio.

    I should also mention that I have a manual transmission so that changes things too. If you have a manual I would recommend going to at least a 70:1 crawl ratio. With an automatic 50:1 might work but I'd still recommend going a little bit lower.
    Finally paid it off and then parted it out, onto the next one!

  5. #5
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    Re: Final crawl gear ratio

    There's a lot of extra stuff to consider. One thing is the torque you're expecting to put out, and another is the torsional resistance you're expecting to offer it. If any of the parts in between that aren't up to the challenge, you're going to start snapping stuff. By gearing down, you just make that easier. You haven't said, but I hope you're building some nice axles.

    Another thing to consider is what gear choice you want to use to "bump" with. If you need to use some wheel speed, you're probably not going to be in your lowest gear.

    I have a very low crawl ratio and a manual tranny. It's about 157:1. I have a very heavy buggy, a powerful V8, 42" stickies, and thanks to my inexperience, I've already managed to snap Dana 60 chromoly axle shafts twice.

    However, I *LOVE* the ride. It's extremely easy to control. I use low/low first and second gear most of the time.

  6. #6
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    Re: Final crawl gear ratio

    Quote Originally Posted by TanTJJim View Post
    With the new Rubicon transfer case with the 4:1 low range my new crawl ratio is about 80:1.

    I should also mention that I have a manual transmission so that changes things too. If you have a manual I would recommend going to at least a 70:1 crawl ratio. With an automatic 50:1 might work but I'd still recommend going a little bit lower.
    That sounds good, I didn't realize there should be a significant difference between the manual vs auto and was not sure how low (high number) to go. I picked up the T18 with 6.32:1 granny and have the stock D300 2.62:1. I currently left stock RP but have a rear set of 4.56 and will go ahead and pick up the front. It will be about 75 with the new RP which should be good.

    On the highway I do ok, I pretty much am fine with that aspect but I'm sure it will improve as well.

  7. #7
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    Re: Final crawl gear ratio

    The torque converter of an automatic lets the engine spin faster than the transmission input when you're crawling along at low speed. As far as wheel speed while crawling, it's kinda like an extra 2:1 reduction, except the extra torque is being wasted as heat.

    What axles are you using?

  8. #8
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    Re: Final crawl gear ratio

    Same ol factory stuff. I did lock, go to one piece and stitch weld the AMC20, also ordered heim for the anti wrap I'm building and doing a truss.
    I did a spring over last year and have lock for the D30 too. Just had soooo many breaks and been adding as I go, but finally trying to get it a little stronger and slow it down.

  9. #9
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    Re: Final crawl gear ratio

    Now, I'm the first to say that I'm not experienced with Jeep axles. I'm just looking stuff up in my "Differentials" book. According to this, the AMC20 axle shaft has a 1.17" minimum diameter which is the minor diameter at the splines. They recommend a 33" tire maximum with a standard 2-piece shaft or a maximum 35" tire with an upgrade 1-piece shaft. The D30 is about the same with a 1.16" minimum diamater shaft but more upgrade options.

    If you're going to run 35's *AND* significantly increase torque multiplication, you may find it very easy to exceed the capability of these shafts.

    They make a point of noting that if you upgrade a D30, you'll approach where a D44 is, stock.

  10. #10
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    Re: Final crawl gear ratio

    My opinion is that unless you're doing big waterfalls that require lots of wheelspeed, lower is better...to a point of course. As stated, you only need approx. 50% as low of a crawl ratio with an auto. It's also a bit tough because hp/torque plays a huge role...obviously a V8 will overcome a poor crawl ratio much better than an I4...assuming they're both stockish and running properly. Many will tell you that 100:1 is a good crawl ratio for a manual and 50:1 is ideal for an automatic as a general rule, but honestly I think a little lower than that is ideal...unless, as stated, you are hitting big falls that require a ton of wheel speed. If you're running intermediate trails and want ultimate control, I'd recommend 75:1 with an auto or 140:1 with a manual for most AZ situations. That's just my opinion...so take it for what it's worth.
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  11. #11
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    Re: Final crawl gear ratio

    Great info! At this point I will hope I don't exceed the capability too easily. I plan not to run anything over 4.0 but if it proves to be too much then I will probably look at the 8.8.

    I have been trying to build up and stick with stock. But I can tell you, I'm done with the T4. This is the second one, and although it has held up, it actually feels weak to me on trails and inclines. I picked up the T18 a few weeks ago and had a chance to drive Joshj52's CJ with a T18 last week. The shifting is a little stiff but man is it solid and low. The weight difference alone is about 70 pounds.

  12. #12
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    Re: Final crawl gear ratio

    im at like 148 to one with my sm420 dana 300 with 4to1 and 456 gears and that was with 37in tires i just put on 39.5s so it's prob changed some.
    350 howell inj dana 44 front with arb and cromos 9in in rear with a spool dana 300 tera low 4:1 sm420 with 456 gears and 39.5s swampers 4 linked all around.oh yeh it was a cj7.it was a jeepthing but all that s**t broke http://www.alloffroadaz.com/index.php

  13. #13
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    Re: Final crawl gear ratio

    that chart say im 128to1 with the 39.5s
    350 howell inj dana 44 front with arb and cromos 9in in rear with a spool dana 300 tera low 4:1 sm420 with 456 gears and 39.5s swampers 4 linked all around.oh yeh it was a cj7.it was a jeepthing but all that s**t broke http://www.alloffroadaz.com/index.php

  14. #14
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    Re: Final crawl gear ratio

    Quote Originally Posted by BellyDoc View Post
    If you're going to run 35's *AND* significantly increase torque multiplication, you may find it very easy to exceed the capability of these shafts.
    I can pretty much guarantee you know more than I do, so thanks for all the info

    Quote Originally Posted by howyadoin View Post
    If you're running intermediate trails and want ultimate control, I'd recommend 75:1 with an auto or 140:1 with a manual for most AZ situations. That's just my opinion...so take it for what it's worth.
    You guys opinions are worth a lot, that's why I'm here ( and to b.s. ) I think the best I could do would be 115 with a 4:1 on the transfer and most likely axle upgrade. Do you guys know if there a way to know if I have enough torque? I assume some of you guys run a doubler/crawl box. I don't know aything other than the name on those...

    Quote Originally Posted by blazerv10 View Post
    that chart say im 128to1 with the 39.5s
    Thanks blazer. I am starting to get it...

  15. #15
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    Re: Final crawl gear ratio

    One of the things about gearing is that it's a torque multiplier. If you supply 100 foot pounds of torque, going 1000 RPM, and you do a 10:1 gear reduction (by whatever combination of gears you want to put together - it doesn't matter) then the output is 1000 foot pounds of torque going 100 RPM. That's the whole thing with crawl ratio. You can run a wimpy little engine, and turn big tires going up and over steep obstacles. However, if you clamp one of those big tires between some rocks so that it doesn't turn anymore, then your gear ratio becomes an axle-snapping machine. With low gears, the engine will continue to turn, the driveshafts will continue to turn, and the axle shaft will twist until it's gone almost a full revolution without the tire turning... and then the axle shaft will snap.

    Here's a good video that shows how an axle snaps. Watch what happens at just before the 1-minute mark. The passenger side rear tire is locked up against an undercut boulder. The driver's side continues to turn and gets remarkably far around before you hear the *SNAP!* All that time, the shaft on the passenger's side is getting twisted. It's certain that this Jeep has a locking rear diff, and based on the crawl speed, it also probably has deeper than stock gearing.

    Last edited by BellyDoc; 02-26-2011 at 08:40 PM.

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