Issue link: http://read.dmtmag.com/i/34501
MOUNTING THE ASSEMBLY ON THE VEHICLE When wheel assemblies are mounted on a vehicle, be sure that the valves do not touch the brake drums or any mechanical part of the vehicle. When mounting the MICHELIN® X One® tire utilizing a 2" outset wheel onto a vehicle, position the tire so that the tire sits on the outboard side of the wheel similar to where the outer dual would normally be positioned. Position the tire and wheel assembly so the valve stem is facing outward, away from the vehicle. Valves of dual tires should be diametrically opposite. Ensure that the inside valve is accessible so the air pressure can be checked and maintained. Tires mounted in dual must be matched so that the maximum difference between the diameters of the tires does not exceed 1⁄4" diameter or a circumferential difference of 3⁄4". For tires of the same bead diameter and size, the maximum allowable difference in tread depth is 4⁄ 32". Failure to properly match dual tires will result in the tire with the larger diameter carrying a disproportionate share of the load. Mismatched duals can lead to rapid wear, uneven wear, and possible casing failure. Tandem drive axle vehicles without an inter-axle Incorrect Dual Wheel Placement differential (or when it is locked out) necessitate that tires are closely matched. The inter-axle differential is a gear device dividing power equally between axles and compensating for such things as unequal tire diameters, the effect of front and rear suspensions, torque rod positioning and the like on the working angles of the universal joints. Normally in the unlock position, this provides minimized wear and tear on tires and the drivetrain. Tandem drive rear axles (twin-screw) require that the average tire circumference on one axle be within 1⁄4" of the average tire diameter on the other axle to prevent damage to the drive differentials resulting from different revolutions per mile on the drive axles. Since any one tire of the size used with these axles may lose as much as 2.5" in diameter due to normal wear and still be serviceable, it is readily seen that a wide difference in tire circumference may exist. Equal tire inflation (between adjacent duals) at the pressures recommended by the tire manufacturer should be maintained. IMPORTANT: Check to ensure that you know which mounting system you are working with and that the components are correct. For additional information, see Wheel Type on Pages 126-129 of Section Nine, Appendix. 30 Section Two: Mounting the Tire DUAL SPACING It is also important that sufficient space is provided between dual tires to allow air to flow and cool the tires and to prevent the tires from rubbing against one another. To make sure dual spacing is correct, simply measure from the outside edge of the outer tire to the outside edge of the inner tire of the dual assembly. This will give you the center to center distance of the duals across that axle end. Refer to the minimum dual spacing column in the application data books. TECHNICAL CONSIDERATION FOR FITTING TIRES When fitting tires of sizes different than those specified by the vehicle manufacturer, the following points must be considered: 1. GEAR RATIO A change in tire dimension will result in a change in engine RPM at a set cruise speed, which will result in a change in speed, tractive effort, and fuel economy. Therefore, the effect of a tire size change on the gear ratio should be considered in individual operations. Generally, changes of 2% for a given diameter or less will have a negligible effect on gear ratio, tractive effort, and indicated/actual speed. If a smaller wheel diameter is chosen, make sure that brake over wheel clearances are checked before continuing with the mounting. (Changes in diameter of more than 3% percent should be discussed with the vehicle manufacturer.) • The formula for calculating the top speed is: Top Speed (MPH) = Engine RPM x 60 (Tire Revs./Mile) x R Where MPH = Miles Per Hour RPM = Revolution Per Minute (Engine) R = Overall Gear Reduction • Since engine RPM and R will remain the same when changing from one tire to another, the comparison is simply a straight ratio of the Tire’s Revs./Mile. Example: Tire Revs./Mile 11R24.5 MICHELIN XDN2 = 473 455/55R22.5 MICHELIN XDN2 = 495 Ratio 473/495 = 0.96 (= 4%. This change requires a gear ratio change as well as a speedometer change or ECM (Engine Control Module) program adjustment.) Therefore, when the vehicle’s speedometer reads 75 mph, the vehicle is actually traveling 72 mph. is neither recommended nor endorsed. Exceeding the legal speed limit