Optimum crawl ratio?

[nickw]

I searched but came up empty handed....

Over on boards such as IH8mud/Pirate4x4 there is typically a lower is better attitude towards overall crawl ratio's.

Does anybody have an opinion on ratios for an expedition rig? I remember reading an article a couple years ago about a exp. Land Rover that had I think 60:1, which the owner though was much to low. He was happier in the 40-50:1 range.

Is there a general consensus on this?

 

[Scott Brady]

Crawl ratio is still important with an expedition rig, because of the weight, but it needs to be useful in more than just the rocks. Anyone who has ever tried to reverse out of a mud hole with a 4:1 t-case will know what I mean.

IMO, I would shoot for the following:

Automatic: Between 30:1 and 38:1

Manual: Between 45:1 and 60:1

An automatic gives you much more flexibility, but on the low side needs lots of extra cooling.

 

[Rexsname]

Scott,

Can you please elaborate on the need for extra cooling with the automatic? I would have thought that with less slipping it would run cooler.

REX

 

[Spikepretorius]

Diesel vs petrol? Same crawl ratio?

 

[crawler#976]

3.0:1 to 3.5:1 gearing in a t'case w/ an automatic would be optimal for me. Between the torque converter and the slightly deeper gearing it would perform well in most situations.

Dual t'cases or one of the 3 speed t'cases combined with a manual tranny offer a lot of versatility. My old beater had the option of 2.28:1 or 5.19:1 with both cases engaged. I frequently compared the manual tranny and dual cases to my wheelin' buddy's Jeep that has an automatic and a 304 V8. We were able to do similar things at similar speeds without using excessive throttle to accomplish it.

Mark
_____________

Rex, when using a crawler box and an automatic the engine rpm is high with little or no forced airflow thru the radiator to assist fan cooling.

 

[BigAl]

I think more important than the actually crawl # is how you get there. Use one of the popular gear calculators to determine what you rpms at 65mph will be in high gear. You don't want to gear the axles so low that your running at 3000 rpm on the road. I'm at 53, (4:1 first x 2.72 tcase x 4.88 axles). I don't do alot of highway, if I did, a 4:1 tcase and 4.11s would have been better.

 

[njtaco]

Isn't tire diameter a consideration? Taller tires need a steeper crawl ratio, don't they? I think this is sort of what BigAl was getting at...

 

[DaveInDenver]

Crawl ratio is still important with an expedition rig, because of the weight, but it needs to be useful in more than just the rocks. Anyone who has ever tried to reverse out of a mud hole with a 4:1 t-case will know what I mean.

IMO, I would shoot for the following:

Automatic: Between 30:1 and 38:1

Manual: Between 45:1 and 60:1

An automatic gives you much more flexibility, but on the low side needs lots of extra cooling.

Your numbers are seem good with one caveat. I'm running ~48:1 in 1st-low on my truck with 33" tire and it's OK, but with my truck being on the heavy side for a mini and on the low side of power, this is pretty often not low enough. So I think also the engine power band needs to be considered. I'd like to have the option of a ~3.25:1 single case transfer case ratio.

I'm planning on a single 4.7:1 case because a double case is going to be tough to squeeze in at this point. My center console is located just right and the back shifter will come up about 1" inside of it. So an offset shifter will be necessary and I just simply don't want to screw with it right now. As my club has said, I'm not doing a double case because of cup holders... Pretty much, yeah.

This will be too low for some stuff, I understand that, but I have to dump the clutch with enough frequency that I'm willing to live with it. Really the only time I'll be truly SOL will be in reverse, I can work with 3rd, 4th, 5th for forward situations. I also carry chains most of the year, which also be an option.

 

[crawler#976]

Dave, I think you'll be happy with that, especially with a 2.4L. It makes up for a lack of torque and HP. As I mentioned above, it compared favorably to a V8 powered Jeep with an auto as far as it's controlabilty when climbing.

But, that is plenty low enough to break stock part - so plan to carry spares. Of course my application/experiance was on the extreme end of the spectrum, but I twisted virtually every piece of the drive train behind the t'case at one time or another - rear d'shaft(s), front and rear axle shafts, birfs, and a hub, and multiple third members.

 

[rusty_tlc]

Isn't tire diameter a consideration? Taller tires need a steeper crawl ratio, don't they? I think this is sort of what BigAl was getting at...

As I understand it you should compensate for tire size with differential gearing. Otherwise on the highway you are fighting against the tires. Unless you need "rubber overdrive". A lot of guys with 2F engines do this for higher highway speed without winding up the 6 banger so much.

 

[cruiser guy]

Diesel vs petrol? Same crawl ratio?

This is a good question! The diesel has MUCH more torque at the lower rpm's.

 

[Chas Stricker]

I had a GM 203 gear reduction in front of the stock 3-speed t-case and 4.11. It had the gears to crawl and still be able to do mud and snow. I agree that too low is not a great thing either. There are times when you'd be reving high.
Chas

 

[ntsqd]

snip....
I'd like to have the option of a ~3.25:1 single case transfer case ratio.

Me too. I had settled on wanting a 3.5:1, but I'd take a 3.25:1 Even the 4:1 looks too low.

I'm planning on a single 4.7:1 case because a double case is going to be tough to squeeze in at this point. My center console is located just right and the back shifter will come up about 1" inside of it. So an offset shifter will be necessary and I just simply don't want to screw with it right now. As my club has said, I'm not doing a double case because of cup holders... Pretty much, yeah.

I've decided to solve this issue differently with the same goal of retaining the console and solitary cup holder in the cab. I extended the 2-4 shift rail fwds so that the stock t/c shifter still controls it. Then I've designed, but not yet built a shifter for the second range box that will allow me to shift it with either a Morse cable or an air cylinder. I'm leaning towards the air cylinder & have the parts to make it work. The key is to make the cylinder a momentary device so it only has pressure when it's shifting the range box.

As Spike & Al have noted, engines with more torque don't need as much reduction. My '91 Suburban is an auto with 3.73 x 2.71 x 3.0 = 30.3:1, but with the torque of a V8 it feels like it's enough for where I've taken it so far. If you factor in the approximate 2.0-2.5:1 ratio of the converter then it's a pretty reasonable number.

I suspect that a simple way to find what a diesel would want for a crawl ratio would be to use some example rig where the engine torque is known and the crawl ratio works for how you want to drive. Figure out what the actual torque at the axle shaft is by multiplying engine torque by the total reduction ratio.
Then divide that by the diesel's preferred axle ratio, it's torque, and the trans' first gear ratio. That will leave you with the desired transfercase ratio.

 

[crawler#976]

Speaking of diesels, I know a few guys who actually use the big Dodge and Ford diesels hard for hunting. In both cases they tell me the torque is there, but the extremely heavy front end is a major problem in any type of soft terrain. They are both running 285/75-16's with no lift and don't want to lift the rigs since they are also used as tow rigs. The one gentleman is a professional lion guide/hunter/tracker, so he's usually got a four mule/ten dog trailer in tow...

 

[AndrewP]

There are way too many variables to have one "best" crawl ratio for any type of off-road travel.

For instance, diesel may have low end torque, but without deep gear reduction, even idling in low range over the Rubicon is still way too fast.

SWB can tolerate a faster crawl than LWB, and the torque converter of an automatic also lets you tolerate a slightly faster crawl as well, though as noted, at the price of more heat in the tranny.

In the US, the "expedition" type places are still mostly graded dirt roads, where a low range is occasionally needed, but true crawling ability is not. What ever comes stock is just fine for that. It still makes sense to match the differential gearing to the tire size.

For me, I have about 80:1 in my FJ40. In places like the Rubicon, I would still on occasion like to go slower. If I were doing Rubicon in a LWB wagon, I think 150:1 would be nice. For my other trucks out in open desert or even the other low intensity trips we take, 35:1 is just fine.

In all cases, matching the diff gears to the tires is the place to start.