Can Somebody Explain Gearing

[usetosellhummer]

some more jargin with t-case


An open diff is designed to allow the two wheels on the axle to turn at different rates during turns. As you may know, the outer wheel on the axle must turn faster than the inner wheel during cornering to cover the larger distance. An open diff allows this to happen with a set of smaller spider gears between the two axle shafts. . The down side is that it transmits power to the wheel that has the least resistance, or traction. This is good for normal, on pavement driving since it minimizes tire wear in turns, but In terms of off road performance, this is a bad thing. As soon as one tire starts slipping in the mud or sand, the tire that still has traction will stop receiving power. All of the power is sent to the spinning tire and unless you can add resistance to that spinning tire, you lose pushing power on that axle. If you spin a tire on both axles, you lose forward movement. Thus, with two open differentials you really only have a true 2wd system.

Limited slip devices:
LSD's are a step up from open diffs. These diffs use spring loaded clutch plates in addition to the spider gears of an open diff to transmit power to the axle shafts. when moving straight, the clutches allow the wheels to turn together normally. when one wheel wants to turn faster than the other, the clutches resist the faster wheel's movement. it must first overcome the friction on the clutch plates, which is a result of the spring force, before it can turn at a different speed. if the turn is sharp enough, the clutches will release, allowing the LSD to operate like an open diff for the duration of the turn. at the end of the turn, the clutches re-engage and keep the wheels turning at the same rate. Going back to our "stuck in the mud" scenario, the plus side to this setup is you still have a limited amount of power being transmitted to the tire that still has traction. until the amount of torque applied to that wheel can overcome the clutch friction, you still have forward movement on that tire. If you are stuck hard however, the amount of torque needed to move you forward can be more than the clutch friction and the clutches will break free on both tires, leaving you with an open diff. There is also the downside of having to replace the clutch plates every so often (~30K miles on some models) and that can get expensive. If the clutches wear out, you are left with an open diff. with an LSD, you have a psuedo 3wd or 4wd system (3wd if you have one axle with an LSD and an open diff on the other, 4wd if you have two LSD's). it's only a psuedo system because they can be defeated and rendered into an open diff in the right situation.

Lockers:
Lockers are the ultimate form of differential from a traction stand point. locked diffs do exactly what they sound like: they lock both wheels together so that they turn at exactly the same rate all the time. Unlike the LSD's, there is no defeating them short of breaking something. the tires will always turn together. a locked diff wll give you a "true" 3wd or 4wd system (1 or 2 lockers again) because all four wheels will always be turning even when there are tires that are slipping without traction. the only way you lose forwad movement is to lose traction at all 4 tires. Modern lockers now offer either a selectable option (you can turn them on or off as needed) or an automatic option. Auto lockers will still allow the wheels to turn at different rates during cornering so you don't totally lose the benefits of a diff on road but still retain the benefits of turning both wheels off road. Selectable lockers offer the benefit of open diffs on road and locked diffs off road at the touch of a button. There are two main types of lockers: lunchbox type, and full carrier lockers. the lunchbox style replaces the spider gears in the stock carrier. This means that the competent home mechanic can install one with basic tools without having to reset the complex measurements on the ring and pinion gears (on most vehicles). The down side is if your stock carriers are weak and some axles do have weak carriers. Be sure to check to see what axle you have and where it's weak points are before adding a lunchbox style locker (or any locker for that matter). The full carrier style locker replaces the stock carrier with the locker assembly itself. the ring gear is mounted to the locker itself, which is usually stonger than the stock carrier. This style however requires that the gears be reset by a professional mechanic and is usually best installed when regearing to account for larger tires or to achieve a lower crawling gear ratio so you don't pay for the gear reset twice. The benefit is a stronger axle due to beefier locker acting as the carrier. both styles do the same thing however.

[stagger_lee]

Well said Usedtosell! If Pewterbird ever Logs back in, he should have more than enough info to answer his questions I hope.

[Pewterbird]

Thanks very much. All good info.

Good stuff, nicely worded. Not easy to explain these concepts that are intuitive to many of us gearhead types.

[stagger_lee]

Pewter -

[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..