Here Is What Those Weird Things Hanging Off Of Bus Wheels Are For

The vast majority of the American workforce commutes to work every day. So many of those Americans will find themselves stuck in traffic, trying to find anything to pass what feels like an eternity. As you try your hardest to find the fastest-moving lane, you’ve probably noticed something weird on the coach bus blocking your path. The wheels on the bus have these odd pod things sticking out and they do not go round and round. What are these things doing?

I’ve lived near Milwaukee, Wisconsin my entire life, yet I’ve never really taken a tour of Harley’s Menomonee River complex. Since 2008, the area has been home to the massive Harley-Davidson Museum and its hundreds of motorcycles and hundreds of thousands of historical pieces. Yet, I never stopped by until this weekend. It’s a great place to visit, even if you don’t care one bit about motorcycles.

You’ll get to read about that later. Right now I want to take a peek at something that caught my eye on I-94:

I’ve seen coach buses with little pods hanging off of their wheels and have long wondered what they’re for. My own Nova Bus RTS-06 doesn’t have these and neither does any bus — recent or old — at the Illinois Railway Museum. The Greyhound bus I rode out of Flagstaff last year didn’t even have them. But this Tornado Bus Company Prevost H3-45 coach had them.

Military Tech

What you’re looking at here is a tire inflation system, specifically the Vigia Automatic Tire Inflation System. According to a report written by Allan Bradley, R.P.F., P.Eng for the Canadian Council of Transport Deputy Ministers Engineering and Research Support Committee, the tire inflation systems used in trucks today are descendants of the Central Tire Inflation Systems used on military vehicles as far back as World War II.

Many sources mark the genesis of the Central Tire Inflation System, or CTIS, being the system found in GMC DUKWs later in production. I’ve written a little bit of DUKW history before, so check this out:

As Hagerty writes, after World War I there was a gap between the need for the military to deploy on beaches and the equipment on hand to make that happen. Normally, a ship would sail up to the shore and begin the lengthy process of unloading. In the early 1940s led to the development of a vehicle that could drive on land and motor through water.

In just 38 days, engineers from the Office of Scientific Research and Development and GMC presented their creation, the DUKW. The “D” stood for 1942 production, the “U” stood for utility (in this case, an amphibian), “K” was for all-wheel-drive, and “W” was for dual rear axles. Of course, it would get the nickname Duck.

At first, the military wasn’t really sold on the idea. Then in a stroke of luck, a Coast Guard ship ran aground while a DUKW was testing. A storm meant normal rescue means couldn’t reach the ship but the DUKW? It drove into the water from the beach and saved the ship’s crew. GMC would build 21,147 DUKWs between 1942 and 1945. From 1946 to today, you’ll find many DUKW survivors running Duck tours around America.

If you’ve ever been off-road before, you know you need to air down your tires for certain situations. The DUKWs were built with this in mind. Early DUKWs had air compressors and air hoses. Operators had to hop out and manually adjust tire pressures. Of course, this is not a great solution as it leaves the vehicles as sitting DUKWs as the operators adjust pressure.

The solution was a new invention.

The tires would be filled through a series of tubes and pipes fed into the hubs. The driver behind the wheel could air up or down with the flick of a switch rather than hopping out and doing it manually. While we call a system like this a CTIS today, back then they were called a Speir’s device, a nod to DUKW engineer Frank Speir.

A number of military and off-road transport vehicles after have been found with CTIS systems. Lynch Hummer, a dealer of AM General parts, shows how CTIS works on the Hummer H1:

Taking CTIS On The Road

The Council of Transport Deputy Ministers Engineering and Research Support Committee report notes that derivatives of those military CTIS systems have been in use in North American trucking industries since the early 1990s. Adoption has been increasing ever since.

While a CTIS makes total sense for the aforementioned off-road applications, the producers of tire systems for road-going trucks, trailers, and buses see additional advantages.

An under-inflated truck, trailer, or bus tire can negatively impact fuel economy as well as tire wear. A tire running in a condition like this can run a higher risk of a blowout. Of course, drivers are supposed to check their tires before driving, but there are still situations where tires fall under the correct pressures. The companies that make automatic tire pressure systems for commercial use see their systems as saving truckers money in the long run.

CTIS designs do vary. One way to deliver air to a tire through a CTIS is through a special hub with an air line going through it that connects to a rotating portion that goes into the wheel. Other designs may even pressurize the axle itself on the way to the tires. You’ll often find a system like this on a semi-trailer. Meritor is a popular brand of semi-trailer tire inflation system and can be found on over a million trailers around America.

Not as widespread, at least in America, is what you’ll sometimes see on semi-tractors and buses. This design is external and involves mounting a disk to the end of the vehicle’s axle. A rotor-type of device is then mounted to the disk. This device allows a stationary air line coming down from the vehicle’s body to interface with a spinning wheel.

This is the kind of system used on the Tornado Bus I saw on I-94. A close look at the shiny rotor cover revealed a brand name of Vigia.

Now, Vigia isn’t exactly a household name. Some digging reveals Vigia to be a brand name of Colven, an Argentinian firm formed in 1972 to market the inventions of Rafael Antonio Colussi and Néstor Juan Vénica. The men invented a device that monitored engine temperatures and could shut an engine down before overheating led to catastrophic damage. The Vigia Engine Protector is still on sale today and now also monitors oil pressure. Now, that isn’t anything too special today. Even my old bus and its Detroit Diesel has an engine protection system.

Colven slapped the Vigia name onto a new protector system when it launched the Vigia Tire Gauge in 1984. Like the Engine Protector, the Tire Gauge was designed to prevent catastrophic failure. In this case, the Vigia Tire Gauge keeps tire pressures on point and notifies the driver if a tire is unable to hold its air on its own.

I found an installation manual for a Vigia system similar to what I saw on the Tornado Bus and it describes a rather convoluted process. Thankfully, there are diagrams showing what’s going on. It starts with an air tank. Air goes from the tank to a control module. From there, air goes from the control module to circuits behind and in front of the tank. The control module also connects to the display in the cab of the truck. That control module is said to be able to control and set nine pressures. Each tire gets its own control and dual tires are usually treated as one big tire. The diagram above is simplified, but there’s also a filter and a bunch of couplers in the system, too.

The system keeps pressures in the specified range all by itself. When something goes wrong, the driver is notified, but the system keeps working.

For example, if a tire springs a leak, the system should notify the driver but also keep running to keep the tire at the calibrated pressure. There’s also some redundancy. If something damages the air line going to a tire, the tire just keeps whatever pressure it was inflated to.

While I could not find a pricing gauge, the downsides of adding ATIS to a trailer, truck, or bus include additional complexity and maintenance. Those nifty rotors don’t last forever, and neither do the seals or the additional air lines. All of those will now have to be kept up.

Many truckers on internet forums seem to like these systems on trailers, but they’re still pretty rare to find on semi-tractors. Of the companies that make ATIS for trailers, only a few bring the tech to semi-tractors. Colven is one of them, as is American firm Aperia Technologies. Pressure Systems International (PSI) announced its own system for semis, but it does not appear to be in production yet.

So, there you have it. The next time you’re stuck in traffic and see a bus, semi-trailer, or semi-truck with some funny looking stuff hanging off of its wheels, you now know what’s up.