Can Somebody Explain Gearing

[Wisha Haddan H3]

Another way to think of gearing is as a force multiplier.

Your engine puts out a certain amount of horsepower and torque at a given rpm. Your choice of gear multiplies your engine torque by your gear ratio. Low gears trade distance for torque. High gears trade torque for distance.

Gear ratios (automatic transmission)

1st - 3.06:1
2nd - 1.63:1
3rd - 1:1
4th - 0.70:1
Reverse - 2.29:1

Here's a simplified example of a 2006 H3 with the automatic transmission accelerating from a stop light.

When you pull out from a stop, you start in 1st gear. 1st gear multiplies your engine torque by 3.06. It's a slow gear (can't go very fast), but the extra torque gets the truck moving.

As you pass the peak torque RPM (220 ft-lbs @ 2800 RPM), your transmission shifts to second gear. 2nd gear multiplies your torque by 1.63. You've traded some of 1st gear's torque for more distance per RPM, which means you can go faster.

Again, you accelerate past the peak torque (2800 RPM) and now shift into 3rd. 3rd gear is 1:1. You're not multiplying the torque anymore, so you've traded 2nd gear's torque advantage for the engine's natural speed. In other words, your engine RPMs pass directly through the drivetrain to the axles.

Now, let's say you get on the highway and you want to cruise at 70 mph. You aren't accelerating any more, so you don't need extra torque. But you want to lower your RPMs and save gas. So your transmission shifts to 4th gear.

4th is an overdrive gear with a 0.70:1 ratio. Instead of multiplying torque, an overdrive gear actually divides the torque, allowing even more distance per engine RPM. This permits even higher speeds, again at the cost of torque. If you come to a hill, your speed will drop. If you want to pass, you won't have the oomph to get it done. You'll have to shift down to 3rd or even 2nd to generate enough torque to do it.

[Wisha Haddan H3]

Now for the rest of the story ...

The transfer case basically has 2 gear sets: 4hi and 4lo (ignoring the locked/unlocked modes). 4hi provides a 1:1 gear ratio. It passes all engine power through from the transmission directly to the axles, without multiplication.

4lo provides a 2.64:1 ratio in the Base model or 4.03:1 in the Adventure model. So in 4lo, all the torque coming from the transmission is multiplied by either 2.64 or by 4.03 before it heads to the axles.

The axles each have a ratio of 4.56:1. This can't be changed without a complete axle rebuild. So whatever gear you select in the transmission and transfer case, it will be multiplied by 4.56 at the axle.

To put it all together, let's say you're wheeling and want to go up a steep hill. You put the transfer case in 4lo. Then you select 1st gear. Since we know the axle ratio, we can calculate your final "crawl ratio".

Crawl ratio = transfer case "lo" ratio x transmission 1st gear ratio x axle ratio

Base model (auto) - 2.64 x 3.06 x 4.56 = 36.8:1
Adventure model (auto) - 4.03 x 3.06 x 4.56 = 56.2:1

Base model (manual) - 2.64 x 3.75 x 4.56 = 45.1:1
Adventure model (manual) - 4.03 x 3.75 x 4.56 = 68.9:1

Now you have an incredibly low drive ratio. You can't go fast, because you've traded distance (at a given RPM) for some incredible torque. But that torque will get you right up the hill.

Tire size is also a factor in all this, but that's another story.

[marin8703]

damn Wisha very nice and clear explenations.

[fw888]

Originally Posted by Wisha Haddan H3

Now for the rest of the story ...

The transfer case basically has 2 gear sets: 4hi and 4lo (ignoring the locked/unlocked modes). 4hi provides a 1:1 gear ratio. It passes all engine power through from the transmission directly to the axles, without multiplication.

4lo provides a 2.64:1 ratio in the Base model or 4.03:1 in the Adventure model. So in 4lo, all the torque coming from the transmission is multiplied by either 2.64 or by 4.03 before it heads to the axles.

The axles each have a ratio of 4.56:1. This can't be changed without a complete axle rebuild. So whatever gear you select in the transmission and transfer case, it will be multiplied by 4.56 at the axle.

To put it all together, let's say you're wheeling and want to go up a steep hill. You put the transfer case in 4lo. Then you select 1st gear. Since we know the axle ratio, we can calculate your final "crawl ratio"

Crawl ratio = transfer case ratio x transmission gear ratio x axle ratio

Base model (auto) - 2.64 x 3.06 x 4.56 = 36.8:1
Adventure model (auto) - 4.03 x 3.06 x 4.56 = 56.2:1

Base model (manual) - 2.64 x 3.75 x 4.56 = 45.1:1
Adventure model (manual) - 4.03 x 3.75 x 4.56 = 68.9:1

Now you have an incredibly low drive ratio. You can't go fast, because you've traded distance (at a given RPM) for some incredible torque. But that torque will get you right up the hill.

Tire size is also a factor in all this, but that's another story.

very well said dude..

[usetosellhummer]

Nice job, I lifted mine from another site

[Wisha Haddan H3]

Thanks. I'm just glad I made sense, trying to pull all the info together. Hope it helps, and as always all corrections are welcome

[tomp]

Here is a eally cool tool for gearing etc: http://www.bncoffroad.com/ratio/