A mounting issue

If specifying steel wheels, on steer, lift and or drive axles, might ask the OEM (Original Equipment Manufacturer) if they have an option for longer wheel studs. Each manufacturer uses multiple stud lengths based upon the thickness of the materials they secure.

Wheel material and hub type are a couple of the considerations that combine to require 3 or more different stud lengths. The one that is the proper length for your chassis will be used, and if it is the shortest possible then you probably have steel wheels and iron hubs. Maybe your customer wants to upgrade to aluminum wheels. The studs will not permit what should be a simple swap.

The shop will charge around $600/axle to replace the studs, so there is a solution. However, a cheaper/easier action is to specify long studs when you order the vehicle. Given this is usually a $0 cost code, might be advantageous by providing flexibility.

Rule of thumb: you should see at least 2 full threads exposed after the nut has been torqued. Steel wheel flanges on average are 3/8” thick, aluminum flanges 1”. You can see where the problem is. If you order aluminum wheels from the factory, no special consideration is required.

Something's in the air

If the chassis has an air compressor (air brakes), then a dryer should be specified. FMVSS 136 Phase 1 mandated ESP/ESC starting Aug 2017 for tractors, the system is optional for straight trucks. Beyond this, today’s vehicles have expensive and complex ABS and ATC systems including more components subject to failure. The dryer protects the entire air system including the reservoirs and valves from solids, liquid, vapors, and contaminants. A heater is highly recommended if operating in freezing temps.

We highly recommended pull cords, so that it is relatively easy to see if moisture is present. Pull the cords on the walk around to see if moisture is present and if it is, then it is probably time for a filter change.

Compressors come in all sizes, depending on air consumption, but regardless, they all bypass oil by the piston rings. Standard filters remove some oil, but the lions share is removed via the coalescing filter. It has a special layer of desiccant that is made for this purpose. It does not matter where the air is supplied to the compressor, before or after the turbo, oil will be present in the system.

Given these filters are usually mounted inside the frame rail, they are out of sight, out of mind. Most OTR (Over the Road, 3 axle tractors) require a filter change every 3 years. Things like increased service brake count and size, lift axles, applications like refuse, etc, can speed up this cycle to every year or sooner.

Today’s filter elements spin on/off, so changing is relatively easy. The previous AD-9 filter (over a million sold) also has a coalescing option but takes more time and effort to replace the element, given the cartridge is inside the dome and is bolted in place. Proper maintenance reduces valve replacement and prevents freeze up. One wrecker bill and down time, pays for the filters, many times over.

Fuzzy math? Fuzzy is as fuzzy does...

Remember the power divider function? On a set of two or three drive axles the incoming power is most often split in half going into the first axle in the set. On a tandem drive this is equal between the two axles, or 50/50. On a tridem set if can be 50/25/25, since the power divider in the second axle splits the 50% it receives from the forward drive. In other cases, such as with planetary gearing, the power is divided equally between the three axles.

How can you tell if tri-drive axles split the torque 50/25/25 or 33/33/33? One way to discern is a GCW rating comparison between the tandem and tridem? A tandem group made up of two 23K. rated axles, versus a tridem group made using three of the same 23K rated axles. The tridem will have a higher GAWR (23K x 3 axles, or 69K) over a 46K rated tandem. But does the manufacturer permit additional GCW (Gross Combination Weight or pulling weight). If the answer is “yes”, then there is a real good chance it is truly an equal 1/3 split.

For example, Sisu axles are rated at GCW of 425K for tandems but increase to 625K for tridems. The P600 axles from Meritor tandem are 420K with 560K for tridems, using 5 planetary gears. There is a special way these suppliers split the gearing to achieve this increase in difference. Both split the power 33/33/33.

If the tridems share virtually the same GCW as tandems, then the torque is split 50/25/25 with the forward drive axle picking up half the power through the divider. Given this, the forward drive does the majority of the work, and wears tires much faster than the other outer ends.

All Dana, Meritor, and Detroit tridem axles are geared this way, 50/25/25 split. You will find the same GCW rating regardless of whether the configuration is tandem or tridem. The advantage of the tridem over a tandem in these configurations in GAWR, or the weight the vehicle can carry (not pull), in addition to improved traction.