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.
CTIS system is mandatory equipment in Argentina. They are optional in Brazil and few South American countries.
I visited Argentina for the first time in 2016 and was awestruck by the beauty of CTIS attachments. I started taking photos of them and posted to my Instagram.
These came & went on Canadian trucks. The cold and snow saw the break and get torn off too often.
There’s also a passive system known as “Cat Eyes”
Link Mfg – Cat’s Eye®
They have a central glass bulb that starts to literally look like the iris of a cat as pressure drops. It helps do a quick visual of tire pressure during your circle check.
These came & went on Canadian trucks. The cold and snow saw the break and get torn off too often.
There’s also a passive system known as “Cat Eyes”
Link Mfg – Cat’s Eye®
They have a central glass bulb that starts to literally look like the iris of a cat as pressure drops. It helps do a quick visual of tire pressure during your circle check.
Mercedes, glad you enjoyed the Harley-Davidson Museum!
Just a heads up, this year for the Homecoming Rally, we will be opening Juneau Ave headquarters for a historic tour and yours truly will be one of the tour guides.
Come on out!
Mercedes, glad you enjoyed the Harley-Davidson Museum!
Just a heads up, this year for the Homecoming Rally, we will be opening Juneau Ave headquarters for a historic tour and yours truly will be one of the tour guides.
Come on out!
Wayward giant ducks on the highway
and DUKWs ashore! Absolutely quackingtastic
Wayward giant ducks on the highway
and DUKWs ashore! Absolutely quackingtastic
I recall a similar system mounted on the end of the axle like these, but instead of a central pump, it used the rotation of each tire set to power a pump located right there in the center cap. Pressure settings were made on the device, and excess pressure either disengaged the pump or was simply vented. I’m not sure if it was an invention PR release, or something that was actually made and sold.
It seems like a much simpler solution than plumbing through the center of the axles. Even if you wanted central control, it wouldn’t take much to add that to a rotation-driven pump set.
I recall a similar system mounted on the end of the axle like these, but instead of a central pump, it used the rotation of each tire set to power a pump located right there in the center cap. Pressure settings were made on the device, and excess pressure either disengaged the pump or was simply vented. I’m not sure if it was an invention PR release, or something that was actually made and sold.
It seems like a much simpler solution than plumbing through the center of the axles. Even if you wanted central control, it wouldn’t take much to add that to a rotation-driven pump set.
Thank you, Mercedes! I’ve always wondered. Cool stuff but they sure are ugly…
Thank you, Mercedes! I’ve always wondered. Cool stuff but they sure are ugly…
‘Pressure Systems International’ is such a great way to backdoor your acronym into all your competitor’s stuff.
‘Pressure Systems International’ is such a great way to backdoor your acronym into all your competitor’s stuff.
‘Vigia’ is a fitting name for a system like that.
‘Vigia’ is a fitting name for a system like that.
That’s a pretty neat tech. I wonder why it’s not more common on off-road modded trucks and production off-roaders like the Raptor? It seems like a CTIS system would see as much or more use than, say, an electronically disconnecting sway bar. Since some of these trucks are over six figures and three tons I wouldn’t think cost or weight would be a barrier they couldn’t climb.
Might be pretty hard to protect those systems in an off-roading environment, no? Seems like just driving through some brush might break something.
Pretty sure the military Humvee uses a centralized air pressure system. Yep, I just searched, and the system is called, you guessed it, CTIS.
As the article mentions the first such system was used on amphibious assault vehicles for the Normandy invasion and then throughout the Western Front during WWII so I have to assume durability was a solved problem even at that early date. Also a similar system was used on the H1 Humvee and, I believe, on its successors.
The ducks and the H1 both use portal axles and the airline passes through the stub shaft. It would be tricky with a regular solid axle or independent suspension. Where there is a will, there is a way but I think a regular joe would balk at the cost of such a system when a small onboard compressor and 10 feet of hose would do the trick.
Darn it Thomas, now you have me looking for a portal axle retrofit for my TJ.
That’s a pretty neat tech. I wonder why it’s not more common on off-road modded trucks and production off-roaders like the Raptor? It seems like a CTIS system would see as much or more use than, say, an electronically disconnecting sway bar. Since some of these trucks are over six figures and three tons I wouldn’t think cost or weight would be a barrier they couldn’t climb.
Might be pretty hard to protect those systems in an off-roading environment, no? Seems like just driving through some brush might break something.
Pretty sure the military Humvee uses a centralized air pressure system. Yep, I just searched, and the system is called, you guessed it, CTIS.
As the article mentions the first such system was used on amphibious assault vehicles for the Normandy invasion and then throughout the Western Front during WWII so I have to assume durability was a solved problem even at that early date. Also a similar system was used on the H1 Humvee and, I believe, on its successors.
The ducks and the H1 both use portal axles and the airline passes through the stub shaft. It would be tricky with a regular solid axle or independent suspension. Where there is a will, there is a way but I think a regular joe would balk at the cost of such a system when a small onboard compressor and 10 feet of hose would do the trick.
Darn it Thomas, now you have me looking for a portal axle retrofit for my TJ.
I tell my kids they are when the trucks need to air-down the tires to go under low bridges.
I tell my kids they are when the trucks need to air-down the tires to go under low bridges.
I’m more interested in why intercity buses run staggered tires on the back . . .
Never noticed that before but my guess is… that’s all the surface area they need for the weight distribution and is cheaper to run than adding two more wheels
Yup. Going down 2 tires that aren’t strictly necessary will save about $1500 a year. Multiply by the number of busses in the fleet, and you are looking at significant money
I think some bus chassis are 6×2 with duals on the drive and singles on the tag axle
The back tag axle is also probably a steer axle to help reduce scrub and add maneuverability..
I’m more interested in why intercity buses run staggered tires on the back . . .
Never noticed that before but my guess is… that’s all the surface area they need for the weight distribution and is cheaper to run than adding two more wheels
Yup. Going down 2 tires that aren’t strictly necessary will save about $1500 a year. Multiply by the number of busses in the fleet, and you are looking at significant money
I think some bus chassis are 6×2 with duals on the drive and singles on the tag axle
The back tag axle is also probably a steer axle to help reduce scrub and add maneuverability..
I saw all kinds of central tire inflation systems on personal trucks and small tour buses in Iceland.
I have seen and heard a bus tire blow on i-94 in Mpls. Scarry.
I had an 18 wheeler tire explode a few car lengths ahead of me on the freeway when I was riding a motorcycle. Easily the loudest sound I’ve heard in my life, loud enough that my nervous system decided to start making decisions without waiting for the brain and I nearly jumped off the back of the bike at freeway speeds.
Yes so loud I though something hit the roof of my wrangler.
Driving thru Cincinnati merging toward Indianapolis a Military 8×8 was on the other ramp and one of the tires de-treaded. Flew about 50′ in the air and landed in my lane. Of course I saw it and was already in the breakdown lane, but it really makes you wonder how AI/Self Driving will react to these seemingly abnormal situations (tires raining from the sky).
I saw all kinds of central tire inflation systems on personal trucks and small tour buses in Iceland.
I have seen and heard a bus tire blow on i-94 in Mpls. Scarry.
I had an 18 wheeler tire explode a few car lengths ahead of me on the freeway when I was riding a motorcycle. Easily the loudest sound I’ve heard in my life, loud enough that my nervous system decided to start making decisions without waiting for the brain and I nearly jumped off the back of the bike at freeway speeds.
Yes so loud I though something hit the roof of my wrangler.
Driving thru Cincinnati merging toward Indianapolis a Military 8×8 was on the other ramp and one of the tires de-treaded. Flew about 50′ in the air and landed in my lane. Of course I saw it and was already in the breakdown lane, but it really makes you wonder how AI/Self Driving will react to these seemingly abnormal situations (tires raining from the sky).
“sitting DUKWs,” nice.
“sitting DUKWs,” nice.