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How To Choose Drivetrain Gearing

Posted in How To: Transmission Drivetrain on July 1, 2012
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Put simply, gearing makes things turn faster or slower, or trades speed for torque. With it we can make our vehicles fly across terrain at crazy fast speeds or crawl super slow over boulders. Big motors with high horsepower and grunting torque are great, but gearing can really make a big difference when it comes time to put power to the ground.

When we’re building a 4WD rig, we have three main areas of gearing to be concerned with. These are transmission, transfer case, and axle gearing. The ratio at each of these points in the drivetrain affects our overall performance and how efficiently we use the power we have to get us over the obstacles we want to tackle. Final drive ratio is the sum ratio through all three gearing changes in the drivetrain.

When a vehicle manufacturer builds a production street-licensed 4x4 it is catered to the average consumer. Much of the gearing choices that go into the design are dictated by the needs to maximize fuel economy and meet the requirements of EPA regulations. Typically, extensive off-road performance is not heavily considered in these choices, yet is often quite important to us as four-wheelers.

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 stockish 28-inch tire up to a 35-inch tire is an increase in size of 25 percent over stock. This can push highway engine speed out of its efficient powerband, spoiling fuel economy, and acceleration.

We have choices when building a vehicle drivetrain from scratch or when modifying our drivetrain to choose or change gear ratios to best suit our needs. There are choices of automatic or manual transmissions with some having very low First gear ratios and some having Overdrive top gears. We may be able to accommodate single or dual transfer cases, and finally, axle gear ratios affect power delivery and have implications for axle strength. How you choose these should depend on expected use of your vehicle.

Axle Gearing
When choosing what hardware to use in your drivetrain, the first question to ask yourself is whether or not the 4x4 will see much or 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 more. 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.

A change in axle gearing can help compensate for larger tires and restore a good bit of the engine efficiency. 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 (numerically higher) in the axles.

When dealing with an existing vehicle, it’s most common to swap only axle gears when needed to accommodate larger tires. When you’re building up a complete drivetrain, you may have the option to choose other gear ratios as well. We’ll consider those choices below.

Gearing and 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 ten pound force exerted over a one foot distance is 10 lb-ft (pound feet) of torque. Given these physics, a given torque acting through an axleshaft can exert less road force through 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). This illustrates why a vehicle feels so sluggish with the larger tires.

Whenever power is transmitted through a manual transmission, 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.

T-Case Gearing
We’ve seen that we choose our transmission based on preference of manual or automatic, and then with some idea towards 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 1:1 high range.

Another way to get lower T-case gearing is through the installation of an aftermarket gear set such as this Marlin Crawler 4.7:1 low range gearset. The stock Toy transfer case has a 2.28:1 low range, so this gear swap greatly reduces the low range gearing.

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 four-wheeling. 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.

Kits are available for many transfer cases to replace the internal low range gearset with one that offers a much lower ratio (3:1 to 5:1). Another option is to add a “doubler”, or second transfer case reduction. This gives you 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. Kits are available on the market for this adaptation for some vehicles.

See the chart below for the basic differences between regearing a T-case and adding a doubler.

Strength Considerations
With all the gearing choices, we have multiple ways to build a drivetrain to get the torque or speed we need. But, what are the strengths or weaknesses of making these choices?

Stepping up in performance, you can choose one of several complete aftermarket transfer cases on the market, such as the Advance Adapters 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.

Consider a torque example of running through a gear change. Say we have an axle ratio of 4:1, then for every four times our driveshaft spins, the axleshafts spin one revolution. Our output torque at the axleshafts 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 gearsets, 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.

An example of a dual case setup is this conversion from Offroad Design (ORD) that mates the reduction portion of an NP203 to an NP205 T-case. A setup such as this makes it 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.

Today we have numerous choices when it comes to completing a fourwheeling drivetrain. You can have go-fast speed and granny slow movement as well. It pays to run the numbers of your proposed setup to determine speeds and torque numbers to ensure you get the optimized gearing you want.

Low-Gear Reduction
No other modifications required
Preserves original drivetrain setup

Cannot pick “stock” low range (only new super low)

Dual-Case Setup
Two or three low range transfer case ratios
Improved front driveshaft angle

Slight increase in driveline backlash
Requires driveshaft length modifications
May require shifter/floorboard/crossmember/speedometer modifications

PhotosView Slideshow


Offroad Design
Advance Adapters
Paso Robles, CA 93446
Marlin Crawler
Fresno, CA 93703

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