UTV Gear Reductions and Tire Size
Posted by Craig Stumph on
Tun Ratio Background and Definition
The following information is based on gear ratios from a Polaris Turbo S. Percent reduction is used to describe various gear reductions from stock ratios and can vary based on your machine. UTV gear ratios are sometimes referred to as turn ratios. Turn Ratios are used because it is an easy way to calculate gear ratios if the gearing is unknown. For this example turn ratio=gear ratio. A turn ratio is derived from turning the rear UTV tire one turn and counting the rotations of the Driven or secondary clutch rotations in each gear. Example, low, high, and reverse. We are mainly concerned with low and high because the reverse ratio is very close to low Range and low and high are used most of the time.
Tire Sizes
Increasing tire size is commonly used to increase off road capability. Larger tire diameter=greater tire contact patch. Larger contact patch generally=more traction. Larger tires also roll over obstacles and holes easier. Increasing tire size increases the torque required to drive the wheels. Gear reductions are used to increase torque to the wheels to compensate for larger tires. Clutch tuning is generally not as effective as a gear reduction.
Torque Increase and Gear Reduction
Gears have teeth that mesh together for a given diameter of gears. A larger gear (more teeth) is usually coupled to a smaller gear (less teeth) to increase torque. Increasing the larger gear diameter and reducing the smaller gear diameter increases torque proportional to the diameter changes. Think of a 10 speed bicycle and you get the idea.
Torque Increase Formula Percentage and Example
Assume that the Turbo S machine in this example has 100 lb-ft of torque to the transmission input shaft. Pro UTV Parts Ultimate Gear Reduction is used in this example.
Stock Turbo S Low Gear Turn Ratio= 20.25:1
Stock Turbo S High Gear Turn Ratio= 11.58:1
Pro UTV Parts Ultimate Gear Reduction Low Gear Turn Ratio=33.20:1
Pro UTV Parts Ultimate Gear Reduction High Gear Turn Ratio=14.30:1
Percentage Torque Increase Formula
Low Gear Torque Increase =(New Ratio-Old Ratio/Old Ratio x100
Low Gear Torque Increase=(33.2-20.25/20.25)x100=(12.95/20.25)x100=63.40%
High Gear Torque Increase=(14.30-11.58/11.58)x100=(2.72/11.58)x100=23.5%
Tire Size Increase
Increasing tire size reduces available torque to accelerate and climb obstacles. Increasing tire size also increases belt slipping. Gear reductions reduce belts slipping and increase torque. Gear reductions are good for reducing belt slipping. Gear reductions also increase hill climbing and crawling ability. Generally, increase torque at least the same as the tire size increase in high range. Here is an example of two tire size increases.
Old Tire Size 30”
New Tire size 35”
Tire Size Torque Reduction Percentage Formula
Increased 35” Tire Size Torque Reduction=(New Tire Size-Old Tire Size/Old Tire Size)x100
Increased 35” Tire Size Torque Reduction=(35-30/30)x100=16.7%
Old Tire Size 30”
New Tire size 37”
Tire Size Torque Reduction Percentage Formula
Increased 37” Tire Size Torque Reduction=(New Tire Size-Old Tire Size/Old Tire Size)x100
Increased 37” Tire Size Torque Reduction=(37-30/30)x100=23.5%
Summary
The Ultimate Gear Reduction of 23.5% for high range is enough for 35s or 37s for most users. For serious crawling a further gear reduction may be useful. This is accomplished by stacking a 13% or 19% Stage 3 reduction in the transmission or with a portal lift/gear reduction. Some competition users may use both portal and further transmission reductions.
CLICK HERE for gear reductions.
Contact Craig or Danny for gear reduction info at 801 367 1395 or sales@proutvparts.com
Craig is tech and Danny is sales.