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Choosing Gearing That Works For Your 4x4

Posted in How To on June 1, 2011 Comment (0)
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Choosing Gearing That Works For Your 4x4

High horsepower and torque are great, but gearing can really make the difference as to how you get the power to the ground, and how effectively you can negotiate the terrain you’re on. Whether it’s slow-speed ’crawling or high-speed racing, gearing makes the difference in how you get your vehicle to go where you want it. With a 4WD vehicle, gearing concerns arise in essentially three areas: transmission, transfer case, and axle gearing. Each of those can be tailored to optimize the type of ’wheeling you want to do and how you best utilize your motor torque.

When building a vehicle completely from scratch or when doing a complete drivetrain overhaul, you may have the ability to choose a wide range of ratios in these three areas. Of course, there is the option of two types of transmissions: manual and automatic. Manual transmissions may have the benefit of a low, granny first gear that is great to add additional low gearing on a 4WD. Automatic transmissions also vary a bit with respect to first-gear ratio. On the other end of the cogs, we can find manual and auto transmissions with overdrive top gearing (less than 1:1 final ratio) for highway use.

When planning your drivetrain, first decide how you want to use the vehicle. Will it be used often on the highway or will it be used only off-road or some combination of the two? This will determine at what end of the gearing spectrum you want to concentrate on and how to choose for low- and/or high-speed performance.

Bigger tires mean better ground clearance and the ability to roll over bigger obstacles, but adding larger tires means the rotational speed of the engine drops for a given road speed. Going from a stock-ish 28-inch tire up to a 35-inch tire is an increase in size of 25 percent over stock. This means that an engine turning a comfortable 2,000 rpm at 65 mph drops to a more lugging 1,600 rpm with the bigger meats. This knocks the engine out of its efficient powerband, spoiling fuel economy, acceleration, and the ability to hold good speed on hills.

You may choose to build the vehicle to have a very wide range of usable speeds. This can often be done, but can depend on how many transfer case components you can accommodate in your chassis. But, with the wide range of transfer cases available today, along with aftermarket accessories, you can build a setup to accommodate a broad range of uses.

Axle Gearing
When choosing what hardware to use in your drivetrain, the first question to ask yourself is whether the 4WD will see much or any extended highway use. If so, then you’ll have to ensure that you have sufficient gearing to comfortably drive at speeds of 65 mph or faster. If the vehicle is mostly a trail rig you want to use for slow trail-running or crawling, then you can bias the gearing to optimize low speeds and place less emphasis on highway needs.

To help compensate for the larger tire diameter and restore a good bit of the engine efficiency, the gearing in both axles can be changed. A simple calculation can be done to determine the new axle ratio needed to restore the original cruising rpm while running the new tires. It’s also not uncommon to slightly over-compensate on the gearing as the heavier tires rob a bit more power and can benefit from a little more drop in gearing (high numerically) in the axles.

Manufacturers design vehicles with an overall drivetrain (or final drive) ratio that can provide a good compromise, providing reasonable acceleration (lower gearing) and good highway speed at a practical engine speed (higher gearing). The final drive ratio is what you end up with when you factor in the transmission and transfer case gearing, and the axle gear ratio. Tire size also plays a big role in the final results you get.

What do the proper gears do for your rig? They help maintain decent fuel economy, good acceleration off the line, better highway passing performance, and greater torque and smoother application of power to the tires. When dealing with an existing vehicle, it’s most common to swap only axle gears when needed to accommodate larger tires or other changes. When you’re building up a complete drivetrain, you may have the option to choose other gear ratios as well. We’ll discuss later the implications of those choices with respect to strength.

Using the proper ratio axle gears in regards to tire size helps maintain decent fuel economy, good acceleration off the line, better highway passing performance, and greater torque and smoother application of power to the tires.

Changing axle gears to a numerically higher ratio to compensate for the addition of larger tires should not harm engine or transmission life. As long as these components are not rotating excessively fast, their lives will most likely increase with the gearing change. Engines will operate within their optimal powerband, providing smoother power delivery and avoiding excessively low end lugging that can wear bearings over extended time periods.

Automatic transmissions generally benefit due to less slip and heat build up as the result of running at too low of engine rpm. For those trannies with lock-up torque converters in the higher gears, a properly geared rig will more quickly shift up to these gears and lock up the converter, reducing heat buildup in the tranny. Manual transmissions can also benefit as clutches will wear less quickly and provide better performance with big tires if the proper axle gear ratios are used.

Whenever power is transmitted through a manual gearbox, the output rotation speed differs from the input speed by the gear ratio of the selected gear. This will affect the available torque. For instance, in first gear, the output spins slower than the input and the output torque is proportionately higher than that at the input. If we have a 4:1 first-gear ratio, output torque would be four times as great as the input torque.

Late model vehicles use the speedometer reading as one input to the on-board computer or ECM (engine or electronic control module). Whenever you add larger tires and change the expected speed, as seen by the ECM, this will most likely affect your vehicle engine performance. Swapping to gearing that corrects the overall axle ratio to compensate for the larger tires will also correct the signal to the ECM.

Gearing & Torque
Torque is defined as the product of a force multiplied by the length of the moment arm to which that force is applied. For instance, a 10-pound force exerted over a 1-foot distance is 10 lb-ft of torque.

A torque converter provides a fluid coupling between the engine and the internal automatic transmission gearing. This photo shows a disassembled converter. You can see the internal vanes, or blades, of a converter that are used to provide the fluid coupling. The converter also serves as a torque multiplier (by a factor of 2 or more) as you accelerate from a standstill.

So if we double the length of the moment arm, we double the available torque. This is why a stroker engine with a longer stroke distance often delivers greater torque than an engine of similar displacement but bigger bore diameter. By the same physics, a given torque acting through an axle shaft can exert less road force using a larger diameter tire than a smaller diameter tire. Imagine lifting a bucket of water with your outstretched arm with your hand one foot from your shoulder (small tire) versus two feet out from your shoulder (large tire).

T-case Gearing
We’ve seen that we choose our transmission based on preference of manual or automatic, and then with some idea toward having a low first gear and/or overdrive gear. Then, we usually choose axle gearing based on our tire size and ultimate high-speed goal, be that highway or otherwise. To this point we’re really unconcerned with the transfer case because we’d be running it in high range and most all T-cases have a high range ratio of 1:1.

When we’re ready to think about shifting into low is when we’re faced with a lot of design options that allow us to more effectively use low range for our backcountry pursuits. Most factory transfer cases come with a low range ratio that is about 2:1 to 2.7:1. These serve a lot of needs, but today we have many more possibilities.

Dual Transfer Cases
With a single two-speed transfer case you get two speed ranges. However, add a doubler, or second transfer case reduction and you can have three or four speed ranges for an even greater range of gearing. Essentially, a second gear reduction is placed in the drivetrain to further lower the final drive gearing.

Here’s an example of the dual case setup from Offroad Design (ORD) that’s suitable for a wide range of full-size applications. They offer a conversion kit that mates the reduction portion of an NP203 to the venerable NP205 case. You can use their kit along with a couple of salvaged T-cases to build a doubler setup. ORD can also provide a two- or three-stick shifter mechanism to control both cases.

This is typically done in one of two ways. First, a gear reduction portion of some transfer case is adapted to fit between the transmission output and the input of the main transfer case. A transfer case has two major portions: the gear reduction for high/low range and the 2WD/4WD coupling portion. Only the gear reduction portion is cannibalized and adapted into the drivetrain.

As an example, with a 2:1 transfer case supplemented with another 2:1 gear reduction, we can choose 1:1, 2:1 or 4:1 gearing. By modifying this configuration with the addition of lower aftermarket gearing in one of the reductions, we can go even lower and have four speed ranges.

Thus, it’s possible to have ultra low gearing for boulder crawling, medium low ranges for trail running, sand, or mud; plus you have high range for fast driving and highway speeds. Here you end up with a truly versatile drivetrain.

A second means of adding lower gearing options to a transfer case is with the addition of a planetary gear set. Such an assembly fits between the transmission output and the transfer case input and usually provides either an additional 2.7:1 or 4:1 low-range gear set. If you choose to run a single two-speed transfer case with very deep low range gearing you may lose the ability to accelerate quickly out of steep descents or have too wide a range between high and low ranges to run a comfortable speed on medium difficulty trails. This is less of an issue with a manual transmission or manually-shifted auto as you can choose to run a higher transmission gear straight off idle, if you choose to do so. Standard auto transmissions will have to upshift multiple times after each stop to get to the higher tranny gears

So what’s the upside and downside to a single T-case with modified low gearing or a dual case configuration? In general, a doubler conversion requires more vehicle modifications, but the end result is the increased versatility of a wider range of gearing options. The basic differences can be summarized as follows:

Stepping up in performance, you can choose one of several complete aftermarket transfer cases on the market, such as the Atlas II. With this, you have the option of choosing transfer case low range gearing. You can go high or low ratio here and combine the choice with axle gearing to give you a good low range crawl ratio. However, you’ll need to make sure that when in 1:1 high range the axle gearing is appropriate for whatever high speed needs you have.

Low-Gear Reduction

Pros:

  • No other modifications required
  • Preserves original drivetrain setup

Cons:

  • Cannot pick stock low-range (only new super low)

Dual-Case Setup

Pros:

  • Two or three low-range transfer case ratios
  • Improved front driveshaft angle

Cons:

  • Requires driveshaft length modifications
  • Requires shifter/floorboard modifications
  • Slight increase in driveline backlash
  • Usually requires crossmember modification
  • May require speedometer cable/wire extension

Strength Considerations
All this cool gearing stuff can sure make trail-running fun and bring the torque back into our adventure. However, what are the strengths or weaknesses of making the various drivetrain choices?

When very low gearing is used behind an automatic transmission, downhill compression braking can be substantial and highly beneficial, allowing you to keep your foot off the brakes and reduce heating them up. One word of caution is in order when it comes to the use of low-low doublers mated to a manual transmission. If you have a 10:1 low-low-range, you’re geared four to five times lower than a stock T-case. Should you be coming down a hill in deep low-range and gain too much speed with the clutch pushed in, it’s possible to quickly exceed the maximum rpm rating of the clutch disc. In such a case, you can literally spin one apart and destroy it under these circumstances.

Looking back to our torque example, we can now factor in the effect of running through a gear change. Say we have an axle ratio of 4:1, then for every four times our driveshaft spins, the axle shafts spin one revolution. Our output torque at the axle shafts is the input torque at the driveshaft multiplied times the axle ratio, making it four times greater than the input torque.

With a stock transfer case having a low range of roughly 2:1, the low range cuts the final vehicle speed in half, and also doubles the torque at the axles. When installing aftermarket gear sets, such as a 4:1 conversion, you again double the torque to the axles. So, you must ensure that the components downstream can handle the torque load or you risk parts failure. If we have the opportunity to design our drivetrain makeup, we might possibly have the option to choose components to adjust where our torque gains are, and consider their impact.

Getting the gearing to do what you want to do in the off-road world is now easier than ever with all the great innovations in our industry. Finding the perfect setup comes down to thoughtful planning, physical dimensions of your vehicle, and spending some of your hard earned dollars to meet your ultimate ’wheeling goals.

Another way to get lower T-case gearing is through the installation of an aftermarket gear set such as this 3:1 low-range set from JB Conversions for the NP205 transfer case. A stock NP205 has a 2:1 low-range. JB produces a new ductile iron casting to house the lower ratio gears, gaining you both a lower ratio and increased strength over the stock setup.

Helpful Formulas For Gears
To figure out your engine speed based on gearing and tire size use:

Engine rpm = speed (mph) x final drive ratio x 336 / tire size (inches)

Where: Final drive ratio = axle ratio x transmission gear ratio x transfer case ratio (1:1 in high range)

View Slideshow

Sources

Offroad Design
970-945-7777
www.offroaddesign.com
Advance Adapters
Paso Robles, CA 93446
800-350-2223
www.advanceadapters.com
JB Conversions
Sulphur, LA 70664
337-625-2379
www.jbconversions.com
Marlin Crawler
Fresno, CA 93703
559-252-7295
http://www.marlincrawler.com
Novak Conversions
Logan, UT 84321
435-753-2513
http://www.novak-adapt.com
Yukon Gear & Axle
Everett, WA 98204
888-905-5044
http://www.ringpinion.com

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