Corner Travel Index or CTI measures a vehicle's maximum axle/wheel articulation at the four corners of the vehicle, often referred to as “flex”. The CTI is used in the off-roading industry to quantify the axle/wheel articulation in order to compare the performance potential of a wide variety of vehicles and vehicle builds. CTI totals the articulated wheel travel of each individual tire on the vehicle (in inches) and multiplies that total by 10 (ten).
Significance of CTI and Axle/Wheel Articulation
editA vehicle with a higher CTI offers greater axle/wheel articulation and therefore greater potential off-road performance over extreme terrain and obstacles. A vehicle with a higher CTI will maintain more constant wheel contact with the ground while traveling over extreme terrain and obstacles. This ensures that all wheels deliver torque or traction to the ground surface maintaining the vehicle’s ability to move.[1]
Measuring and Calculating CTI
editCTI can be measured two ways, the two wheel method (Estimated) and the four wheel method (Complete). Estimated is the easier value to generate as it only requires lifting the vehicle once. Complete is required and preferred for a truly accurate measurement of the CTI.[2] [3] Calculations use the following tire positions.
Procedure
edit1. Set all tire pressures to 15 psi. This sets a standard for consistent measurement across a wide variety of vehicles and builds.
2. Simultaneously lift the Front Passenger Tire (FPT) and the Rear Driver Tire (RDT) off the ground and STOP lifting when the first tire not being lifted is just about to leave the ground (two tires must be touching the ground). This can be done with forklifts or with a CTI platform lift system (see pics). Measure the vertical clearance, in inches, between the ground and the lowest point of the Front Passenger Tire. Measure the vertical clearance, in inches between the ground and the lowest point of the Rear Driver Tire.
Measurement must be taken from the lowest point of the tire.
Record the values:
Clearance under the Rear Driver Tire (RDT) = 18 ¾” (example)
3. The two wheel method or Estimate of CTI can be calculated at this time:
4. Simultaneously lift the Front Driver Tire (FDT) and the Rear Passenger Tire (RPT) off the ground and STOP lifting when the first tire not being lifted is just about to leave the ground (two tires must be touching the ground). This can be done with forklifts or with a CTI platform lift system. Measure the vertical clearance, in inches, between the ground and the lowest point of the Front Driver Tire (FDT). Measure the vertical clearance, in inches, between the ground and the lowest point of the Rear Passenger Tire (RPT).
Measurement must be taken from the lowest point of the tire.
Record the values:
Clearance under the Rear Passenger Tire (RPT) = 18½” (example)
5. The four wheel method or Complete CTI is calculated as follows:
Corner Travel Index (CTI) vs Ramp Travel Index (RTI)
editThe Corner Travel Index (CTI) originated as an alternative to the complex, confusing and inconsistent system known as the Ramp travel index.[4] As off-road enthusiasts looked for better and more consistent ways of measuring axle/wheel articulation it became clear that the CTI was far superior to RTI for the reasons outlined in the side by side comparison below.
| Ramp Travel Index (RTI) | Corner Travel Index (CTI) |
|---|---|
| Driver Skill can affect RTI value[5] | Driver Skill has NO effect on CTI value |
| Lower tire pressures can improve RTI value [6] | Tire Pressure is fixed at 15 psi |
| Different ramp angles yield different RTI values; For example the same vehicle yields the following RTI values on different ramps angles 20 degree ramp yields 1033 RTI 25 degree ramp yields 836 RTI 30 degree ramp yields 706 RTI[7] |
Flat Platforms. No ramps. |
| The location of measurement relative to the ramp is confusing; is it the center of tire contact on ramp, the perpendicular plane the passes through the axle or the lowest point of tire. |
Measurement is simply vertical clearance between the ground and the lowest point of the tire. |
| Calculation of RTI is not an easy formula. | Calculation of CTI simply adds four numbers and multiplies by 10. |
| Safety…precarious vehicle positions are often required to make proper measurements[7] | Vehicle is safe, level and balanced at all times. |
| Vehicles traveling FORWARD up the ramp often yield a different RTI value than the same vehicle traveling BACKWARDS up the ramp. | Direction of approach to lift platforms is irrelevant. |
| Counter intuitively, a vehicle traveling FORWARD up the ramp primarily measures the articulation of the REAR suspension. A vehicle traveling BACKWARD up the ramp primarily measures the articulation of the FRONT suspension. |
The front and rear suspensions are tested simultaneously with no vehicle motion. |
| Gear backlash/clutch release/suspension bind etc. often cause the vehicle to rock backwards on the ramp when releasing the break/clutch or shutting off the vehicle. | No rocking backwards. Vehicle is at rest prior to lifting wheels of vehicle with lift platforms. |
| Vehicles with different wheelbase lengths yield different RTI values[8] | Wheelbase is irrelevant to the calculation of the CTI value. |
| Moderately portable system. | Moderately portable system. |
| Approach angle to ramp affects measurement | Angle is not a factor in CTI measurement |
| Weight distribution in vehicle can affect RTI measurement. | Weight distribution will not change CTI measurement total. |
See also
editReferences
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