How to choose our gear ratio for sand driving ?

[abdalla]

Yesterday I read this reply from ME4WD

main reason why the rubi's are not here because of the gear ratio of 4:1 which is made for a rock crawler not for sand, try to have the rubi moving on the sand (the H2 have a c.r of 4.7:1) and that's how they use to be good sand diggers rubis will be almost as bad as these on soft sand

I don't know if its only me but I was under the impression that we choose our gear ratio according to the size of the tire, I always refereed to the R/P - Tire Size table like the one in Quadratec

Link: http://www.quadratec.com/jeep_knowledge ... cle-26.htm

So we used to go for 4.56 with 33" tires, 4.11 with 31" and so on....

But after reading ME4WD comment i got confused again, it seems that there are other factors to this equation...

So, what are these factors and what is the best gear ratio for sand driving??

[khaiwi]

What was said is true and the understanding u had about the R&P with Tire size is true too.

However, you have the Gearbox ratio, the transfer case ratio to endup with the total engine to tire ratio..... More confused???

[abdalla]

lol Yes much more ... lets keep the gear box ratio and the trasfer case ratio as constants, i havnt heard of anyone changing these over here correct ?? but even though you havnt answered my Q Chief

[ME4WD]

Good question, check the table here to see the tire size in relation to R&P size (keep diff ratio different than gear ratio, I will explain this later here). When ever you change tire size from the original size to another there will be a direct effect on power and drivability. See the chart here to help you select the optimum gear ratio to put your performance back where you want it.

Using the chart below, first determine your original gear ratio and tire size and follow to the intersection that should be your approximate RPM at 65 MPH. Next find your new tire size and follow that across to the closest RPM to your original and that will be first choice. If you are interested in a change that will give more power or better fuel mileage use the color coding as a guide.


if the chart is not shwoing click here http://www.me4wd.com/images/R&P-Table.JPG
BLUE: Highway Driving, Good Fuel Economy
GRAY: Daily Driver, Best Overall Performance
YALLOW: Most Power and Towing, Reduced Fuel Economy

Now let's look at the Gear Ratio: One important aspect of gear ratios is numerically large gears are the low ones. Gear ratios are normally stated in the form x.xx:1, e.g. 6.32:1. This translates mathematically to 1/6.32, and since 1/2 is more than 1/6, 6:1 is lower than 2:1. The ratio means that for every 6.32 revolutions from the input the output will rotate once. This happens to be granny low first gear on a Ford T-18 transmission. When you are in first with the T-18, the engine does 6.32 revolutions for every 1 revolution the transmissions outputs. Fourth gear on the T-18 is 1.00:1, so for every revolution of the engine, it outputs one revolution.

Follow the link here to activate an applet that calculate the gear ratio of you car (Open your car manual and get the proper transmission intervals a.k.a. shift points(always mentioned in you car spec. maunal / flyer), http://www.offroaders.com/tech/mph-crawl-ratio.htm here is the formula used


I hope this is now all clear to you.

[abdalla]

Thanks alot ME4WD,

So when i get a higher crawl ratio is this better on the sand or worse, cause right now am running 31" and the R/P is 3.55, and am willing to change the R/P to 4.11, useing the applet in both cases (3.55 & 4.11) the crawl ratio was higher in the 4.11 case so i guess this means its supposed to be better in the sand, right?? (i used to think this was true until i read your post about the Rubicon) ...

[ME4WD]

Your confusion is logical. Bear in mind when you say "higher" or lower, there are two meanings either we say MATHMATICALLY HIGHER (and your understanding is correct --->mathematically higher = better sand offroading) or we say HIGHER crawling ratio (this is the regular way how we refer to just as in the case of the Rubi and it means BAD SAND OFFROADING) simply on sand you are on higher RPM's than when doing rock-crawling which require very low RPM but massive torque in opposite to massive momentum when on sand 8)

[abdalla]

Ok, i think am starting to understand (sorry if i'm a bit slow )

so lets take an example ok...

in the case of your JK ... your running 35" and 4.88 R/P i guess (?)

and lets say someone is running 33" and 4.56 R/P which is common in the club

now if your TJ had 31" tires what is the approprite R/P ? how would you figure it out ?

Thanks alot man 8)

[ME4WD]

I'm running 37" on 4.88 (factory performance is gained using 4.88) more torque would be gained with 5.12R&P but to keep acceleration / momentum inline with ideal torque I chose 4.88.

A TJ with 31" uses / should use the OE R&P which is 3.73. (31" are on the edge of the table, so if you increase tire size by one inch, I advice going for 4.11R&P).

[abdalla]

Okay so 4.11 is fine with the 31" tires, but isnt this the same as the Rubicon's case, the Rubicon AFAIK has this exact setting

[khaiwi]

Dear Abdallah,

Why ur confusing urself, u r back to the initial table describing the R&P ratio against the tire size with color code for the drivability mpg etc.....The gearbox ratio and the T-case ratio is what is different in the orther models affecting the total engine to tire ratio.

So, ur not touching anything other than the diff R&P ratio to regain the lost power. Keep in mind the heavier the tires and wider or more far from the center of the diff the extra stress u add on the axles, U joints etc.....

Cheers,

[abdalla]

ahha Ok ... everything is clear now 7emdelah

Thank you guys for your patience & knowledge... You've been of great and help.

[DaddyBear]

I am not sure if this is on track or not. I read the article that was supplied at the link. My impression was that they were talking about driveablility.

From a pure performance perspective: Reducing your R/P (Differential gears) from say 355 to 410 is simply a torque multiplier. This will enable you to "launch" better. Yes, in one gear, you will loose some top end speed, but not enough to make that much of a difference. In fact, if you adjust the R/P enough you will actually end up in a bigger gear, hence moving faster.

Adding a super charger is in essence a torque multiplier. Yes it does add horse power, and that is what is looked at most, but it is the low end torque that makes them so impressive. If you want horse power, turbos are the way to go. The torque of a super charge gets you out of the sand and gets you into the horse power range and in reality gets you to another gear, making you faster!

If lowering your gears is a sand digger, adding a super charger would be too. In fact, any engine upgrade would in theory make it a sand digger, too much power? You can never have too power! You just need to find the next weak link:)

[iguana]

You are right daddybear. However, when you get into extreme drive, you will need both, Torque and HP ... and beleive me you will never get enough ... you always want more .. More torque will mean more digging if you don't know how to handle you car .. But when you get used to it, you will love it .. Heaving a strong engine does not mean that you can go anywhere with your car in the desert, you still need to add the driver's experience.

I saw people doing great things with stock suzuki and Rav4 and others getting stuck every 5 minutes with a fully modified Wranglers

[DaddyBear]

Iguana,

Believe me, I understand there are several ways to skin a cat. If you compare Honda RC55s and Kawasaki ZX7s super bikes, two different approches. Hondas' use controled power throught the corners while the Kawasaki's use pure power through the strait aways. Both have won their share of championships. Just two different points of veiw to having fun:) Drivers may want to select the approach that matches their skills, hence succesfull vehicles mathced to their drivers????

[halwa]

Its not about what you drive, it all about how you drive. A good tyre setup on a well ballanced power to weight ratio vechile, you can go anywhere.