Why Don’t Buses Have More Axles?

I was in yet another discussion on Friday where the topic arose that buses do more damage to our streets than any other vehicle, because their full weight falls on just four contact points.

Which has me wondering, what keeps bus manufacturers from putting more axles on a bus, like a semi-trailer? Would it have some adverse affect on the performance of the vehicle? Are there impacts on passengers? Turning radius? Or is this an untapped market opportunity?

14 responses to “Why Don’t Buses Have More Axles?”

  1. To answer the question in reverse: Why do trucks and trailers routinely have multiple axles in a given position, and/or dual wheels on an axle?

    I suspect the answer is simply that their rated gross weights, in many cases, far exceed that of busses. There may be impacts on suspension as well–even small trucks such as U-haul moving vans (the only sort of large vehicle I have experience driving) are quite bouncy in the back; far more so than would be tolerable with humans instead of boxes as the cargo.

    But that’s just guessin’.

  2. How about cost?

    If the per axle load of the bus if legal, why add to the cost of the bus by adding more? That would just encourage customers to go elsewhere.

    As I write, I would guess that manufactures may offer a range of axle options and TriMet chose the cheapest that was allowed by the cities/state. To do otherwise would to waste taxpayer money. (as if that mattered when it came to rail)


  3. To do otherwise would to waste taxpayer money.

    That’s a somewhat myopic view. If TriMet saves money, but it increases the cost to the City to maintain the streets by a greater amount, taxpayers lose out.

    If that were the only issue, it would just make sense for the city to reimburse the transit agency for the cost of the extra axles.

  4. I’m told that road damage is proportional to (weight/axles)^5. Here we see our local buses sqeezing asphalt out of the roads and up onto the sidewalks at bus stops. I’m sure that the bus company does not lose sleep over this, as road repair is paid for by another pot of funds, and thus “not their problem”.

  5. Well. it’s the lighter weight stainless steel bus to the rescue, then…..right? Plus carbon fiber frame, aluminum wheels, nanotech, lightweight components. (soon you could just put a sail on it on windy days:)

    Another innovation truckers are starting to use is a large, wide tire to replace the dually’s. Not sure what this does as far as traction and potential hydroplaning (I guess the tires could be designed to reduce that) but the rear axle would have far fewer tire edges to cut into the pavement.

  6. Interior space is a big reason. Wheels take up space that could otherwise be available to passengers. As we move to low floor buses (for good reasons) the wheels take even more space. Note that long distance buses, which are typically higher floored, have traditionally had “tag axles” that provide an additional single wheel on each side at the rear.

    The other question I would have, is which end of the bus causes the damage? On stopping, there is weight transfer forwards, so even though the engine is in the rear, there are dual wheels at the rear, and only single wheels on the steered axle. Perhaps the front wheels cause cause the squishing of the asphalt pavement as much or more than the rear wheels. If so, then the solutions are articulated buses or lighter weight buses. These would obviously cost more.

    During the streetcar era, the streetcar lines typically were required to pave the streets on which they operated. This was an obvious incentive to motorize. So beware of unintended consequences. Should the City operate the bus system, so that the cost is internalized?

    New technology may be the answer, but the early adopters are sometimes burned, and then accused of wasting taxpayer money on development costs.

  7. The major damage done to roads by buses is at bus stops. This is caused by the lateral forces transmitted to the road surface from stopping and accelerating buses.

    All heavily used stops should be paved with concrete which is far superior to asphalt which tends to accordion, especially in hot weather.

  8. Of course, busses stop at far more locations than bus stops–traffic lights and stop signs being a prime example.

    While it’s within reason to install concrete pads at bus stops–adding concrete at every traffic light on a bus route might be a bit much.

    Might this be an argument for increased use of signal preemption on busses? Not only might that help with service times, but if we can reduce the number of starts/stops, it would reduce both road wear and fuel consumption.

  9. It would be interesting to measure how hybrid buses distribute the lateral forces that cause road wear. Hybrid buses brake and accellerate more evenly, or so I’ve come to believe.

  10. the lateral forces transmitted to the road surface from stopping and accelerating buses

    I thought it was because of all the people getting on and off, which causes the amount of weight to often change.

    Also, I believe it was Rode Island or someplace at one time required Greyhound and other buses to only have two axles.

  11. Found this Wikipedia article, but it doesn’t say much.

    And there are places which toll and regulate vehicles according to number of axles (as a proxy for factors that probably matter more such as GVWR)–though I doubt that’s a concern for government-owned transit (as opposed to private motor carriers).

  12. If you look at the back of a U-Haul truck, or any of the companies that rent trucks to the general public, you will only find one rear axle. The reason has nothing to do with the load in the trucks, and everything to do with the fact that, in most states, the presence of two rear axles means that the driver must have some level of commercial drivers license to operate the vehicle.

    Yeah, this is not directly related to buses, but may be of passing interest to a person reading this thread.

  13. Cost is one concern, but the other big problem is maneuverability. Having just one rear axle means you’ll turn much more easily than having multiple rear axles, where the tires tend to fight each other during a turn (this also causes much greater wear on tires during turns).

    The long haul (18 wheeler) trucks on the road are designed for long distance hauling in basically a straight line, so they sacrifice some maneuverability for less wear and tear on the road & tires, and also better traction.

    One way around this issue is having rear wheels that can turn. But, that would add significantly to the cost of the vehicle.

    You can use articulated buses to get extra axles under the vehicle without adding to turning difficulty, but again it’s at a fairly significant cost.

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