Motorcycle Tire Safety Standards: What It All MeansThink you’ve got it rough? Try being a motorcycle tire that’s DOT certified. We have covered the particulars of motorcycle tire size designations and preventive maintenance in detail (Top 10 Things to know about Motorcycle Tires), but like motorcycle helmets and face shields, motorcycle tires that are intended for use on public roads in the United States have some deeply technical federal safety standards.
- Tire and rim matching information. This requires each manufacturer of motorcycle tires shall ensure that a listing of the rims that may be used with each tire that the manufacturer produces is provided to the public.
- Requirements. Each motorcycle tire shall be capable of meeting any of the applicable requirements, when mounted on a model rim assembly corresponding to any rim designated by the tire manufacturer for use with the tire.
- Endurance. Prior to testing in accordance with the procedures of S7.2 (the endurance test procedure explained below), a motorcycle tire shall exhibit no visual evidence of tread, sidewall, ply, cord, innerliner, or bead separation, chunking, broken cords, cracking, or open splices.
When tested in accordance with the procedures of S7.2: (a) There shall be no visual evidence of tread, sidewall, ply, cord, innerliner, or bead separation, chunking, broken cords, cracking, or open splices. (b) The tire pressure at the end of the test shall be not less than the initial pressure specified.
- The endurance test method. The tire is mounted on a model rim assembly and inflated to the inflation pressure corresponding to the maximum load rating marked on the tire. The single maximum load value is used when the tire is marked with both single and dual maximum load. After conditioning the tire-rim assembly in accordance with S7.1.2 which calls for certain temperature (95 or 70 degrees F for 3 hours prior to testing depending on which test is being administered), adjust the tire pressure to that specified immediately before mounting the tire rim assembly.
The tire-rim assembly is then mounted on an axle and pressed against a flat-faced steel test wheel that is 1,708 mm (67.23 inches) in diameter and at least as wide as the tread of the tire. The test load is applied and the test wheel is rotated as indicated in the test specification for the type of tire being tested. For motorcycle tires, rotation speed is 80 km/hr & 250 r/m at 100 percent of the maximum load rating for 7 hours, 108 percent of the maximum load rating for 16 hours and 117 percent of the maximum load rating for 24 hours.
Each successive phase of the test is conducted without interruption. Immediately after running the tire the required time, the tire inflation pressure is measured. The tire is then removed from the model rim assembly and inspected.
- Strength-also meets the requirements for high speed performance if the criteria for endurance testing are also met. The high speed performance requirements apply only to motorcycle tires and to non-speed-restricted tires of nominal rim diameter code 14.5 or less marked load range A, B, C, or D.
When tested in accordance with the procedures of S7.3 (explained below) a tire’s average breaking energy value shall be not less than the value specified in Table II for that tire’s size and load range. For motorcycles, this means that for tires with load range A, the breaking energy must be not less than 16 Joules or 150 in./lb., load range B, 33 Joules or 300 in./lb. and load range C, 45 Joules or 400 in./lb.
The strength test for determining these values under S7.3 consists of conditioning the tire and rim as described before, then forcing a cylindrical steel plunger with a hemispherical end and of the diameter specified for the tire size (7.65mm or 5/16” for motorcycle tires), perpendicularly into a raised tread element as near as possible to the centerline of the tread, at a rate of 50 mm (2 inches) per minute, until the tire breaks or the plunger is stopped by the rim.
The force required and the distance of penetration achieved just before the tire breaks, or if it fails to break, just before the plunger is stopped by the rim is recorded. The plunger application is repeated at 72° intervals around the circumference of the tire, until five measurements are made. However, in the case of tires of 12 inch rim diameter code or smaller, the plunger application is repeated at 120° intervals around the circumference of the tire, until three measurements are made. The breaking energy for each test point is then computed by one of two formulas, depending on whether the results are desired in Joules or in./lb.
- Treadwear indicators. Motorcycle tires shall have at least three such indicators which permit visual determination that the tire has worn to a tread depth of 0.8 mm (one-thirty-second of an inch). Tires with a rim diameter code of 12” or smaller shall have at least three such treadwear indicators.
- Tire markings: The standards also specify what information must appear on the tire and even dictate the position of the information on the tire’s sidewall. The required information includes the symbol DOT, which shall constitute a certification that the tire conforms to applicable Federal motor vehicle safety standards. This symbol may be marked on only one sidewall.
Also required are the tire identification number, marked on only one sidewall; the tire size designation as listed in the manufacturer’s documents and publications; the maximum load rating and corresponding inflation pressure of the tire; the speed restriction of the tire, if 90 km/h (55 mph) or less; the actual number of plies and the composition of the ply cord material in the sidewall and, if different, in the tread area; the words ‘‘tubeless’’ or ‘‘tube type’’ as applicable; the word ‘‘regroovable’’ if the tire is designed for regrooving; the word ‘‘radial’’ if a radial tire; and the letter designating the tire load range.
- Maximum load rating. If the maximum load rating for a particular tire size is shown in one or more of the manufacturer’s publications, each tire of that size designation shall have a maximum load rating that is not less than the published maximum load rating, or if there are differing published ratings for the same tire size designation, not less than the lowest published maximum load rating for the size designation.