Apta (Draft) Specification For: 30-ft, 35-ft, 40-ft, 45-ft, 60-foot (articulated) Transit Buses

IP 33.Wheels and tires 33.1Wheels

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IP 33.Wheels and tires



Painted steel

Wheels and rims shall be hub-piloted with powder coated steel (maximum 3.5 mil) and shall resist rim flange wear.


Brushed aluminum


One-sided polished aluminum

Two-sided polished aluminum rims


Tire pressure monitoring system
All wheels shall be interchangeable and shall be removable without a puller. Wheels shall be compatible with tires in size and load-carrying capacity. Front wheels and tires shall be balanced as an assembly per SAE J1986.


Tires shall be suitable for the conditions of transit service and sustained operation at the maximum speed capability of the bus. Load on any tire at GVWR shall not exceed the tire supplier’s rating.


The tires shall be provided under a lease agreement between the procuring agency and the tire supplier.


The tires shall be supplied by the contractor.

[The procuring agency should designate preferences for the tire size.]

IP 34.Steering


Hydraulically assisted steering shall be provided. The steering gear shall be an integral type with the number and length of flexible lines minimized or eliminated.


Electrically assisted steering shall be provided to reduce steering effort.

34.1Steering axle


Solid beam axle and grease-type front bearings and seals

The front axle shall be solid beam, non-driving with a load rating sufficient for the bus loaded to GVWR and shall be equipped with grease type front wheel bearings and seals.


Oiled-type front bearings

The front axle shall be non-driving with a load rating sufficient for the bus loaded to GVWR and shall be equipped with sealed, oiled-type front wheel bearings.


Independent suspension axle

The front axle shall be of an independent suspension design, non-driving, with a load rating sufficient for the bus loaded to GVWR and shall be equipped with grease type front wheel bearings and seals.

All friction points on the front axle shall be equipped with replaceable bushings or inserts and, if needed, lubrication fittings easily accessible from a pit or hoist.

IP 35.Drive axle

The bus shall be driven by a heavy-duty axle with a load rating sufficient for the bus loaded to GVWR. The drive axle shall have a design life to operate for not less than 300,000 miles on the design operating profile without replacement or major repairs. The lubricant drain plug shall be magnetic type. If a planetary gear design is employed, the oil level in the planetary gears shall be easily checked through the plug or sight gauge. The axle and driveshaft components shall be rated for both propulsion and retardation modes with respect to duty cycle.

NOTE: The retardation duty cycle can be more aggressive than propulsion.

The drive shaft shall be guarded to prevent hitting any critical systems, including brake lines, coach floor or the ground, in the event of a tube or universal joint failure.

35.1Non-drive axle

The non-drive axle is the drive axle without the drive gear with a load rating sufficient for the load to GVWR.

IP 36.Turning radius


Table 1

Bus Length (approximate)

Maximum Turning Radius
(see Figures 1 and 2)

Agency Requirement

30 ft

31 ft (TR0)

35 ft

39 ft (TR0)

40 ft

44 ft (TR0)

45 ft

49 ft (TR0)

60 ft

43 ft (outside front axle, TR0)

17 ft (inside rearmost axle, TR4)


Figure 1


Figure 2

IP 37.Steering

37.1Turning effort

Steering effort shall be measured with the bus at GVWR, stopped with the brakes released and the engine at normal idling speed on clean, dry, level, commercial asphalt pavement and the tires inflated to recommended pressure.

Under these conditions, the torque required to turn the steering wheel 10 degrees shall be no less than 5 ft-lbs and no more than 10 ft-lbs. Steering torque may increase to 70 ft-lbs when the wheels are approaching the steering stops, as the relief valve activates.

Power steering failure shall not result in loss of steering control. With the bus in operation, the steering effort shall not exceed 55 lbs at the steering wheel rim, and perceived free play in the steering system shall not materially increase as a result of power assist failure. Gearing shall require no more than seven turns of the steering wheel lock-to-lock.

Caster angle shall be selected to provide a tendency for the return of the front wheels to the straight position with minimal assistance from the driver.

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