Some seem to go outwards and some go inwards. Why are they not all the same?
What do you mean by “outwards” and “inwards”? I can’t fathom how those words apply to describing the wheels of a bus.
I mean that the center part of the wheel is either convex or concav.
The OP is talking about the offset of the mounting hub relative to the centerline of the wheel. Front wheels are often given a strong negative offset, meaning that the wheel and tire are almost wholly inboard of the hub (more precisely, the centerline of the wheel bearings). Rear and drive wheels are often given a strong positive offset, meaning the wheel and tire are well outboard of the hub.
There are various engineering and handling reasons to use these high offsets, when most cars use something close to zero offset, or no more than an inch or two. I am not 100% certain, but I think most large vehicles have only one wheel type that can be mounted either way - “in” on the front and on the inside rears, and “out” on the single rears or outer dual rears. Aside from everything else, this compromise makes it easier to maintain spares, rotate tires, etc.
I think the reason is that truck wheels are often mounted in pairs on each hub. The inboard wheel has the center outward, and the outboard wheel has the center inward, so that the centers of the wheels are touching each other. If only one wheel is used on a hub, such as on the front of trucks and buses, then the center is outward.
I don’t know anything about buses but with cars and trucks that’s called “offset” and is used to make the tire stick out the right amount. So, for example, a convex wheel will make the tire stick out more so it won’t rub the suspension. I imagine the different wheels on the buses depend mainly on the bus design and how much room they have in the wheel well for the tires that particular bus uses.
That’s what I meant to say and perhaps wasn’t very clear about.
I think front wheels are at least nominally designed to use the same wheel, which is why they look so oddly mounted. The choice of positioning the wheel centerline in or out with respect to the hub centerline has many engineering choices (and compromises) involved, but it boils down to tucking the front wheels in for tighter steering response and a smaller turning radius, and pushing rear/drive wheels out to get a wider and more stable track without costly and heavy axle extensions.
this also gives access to the tire stem in dual wheels.
Well, that’s not really a driving factor. Wheels can have the stem anywhere it’s out of the way of the tire and lugs. If you look at the next dualie you stop next to, you’ll see that the inner wheels have a long, L-shaped stem that’s accessible from the outer edge
I’m probably overthinking this, but is it possible the OP is really asking why some buses with tandem real wheel axles have one rear axle with double wheels, and one axle with single wheels like in this image, so that the differing hub types are adjacent to each other and readily apparent?
The bus in your picture will be fitted with ‘rear steer’. The rear pair of wheels are on a castor mounting so that the bus pivots on the front pair and the rear pair steer round to follow them. If the bus turns left, the rear wheels will turn right.
The purpose is twofold - it gives a tighter turning circle and there is less wear on the tyres.
The wheels on the bus go round and round, just like all the other ones I’ve seen.
because the axle with double rear wheels is the only one driven by the engine. the one in front of it with single wheels is a “tag” axle which is used for load leveling. Some trucks have the ability to raise the tag axle off the ground when it’s not needed.
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the one in front of it with single wheels is a “tag” axle which is used for load leveling.
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Not really for load leveling, but for reducing the tons of load per axle to reduce road wear, and for adding braking power to increase safety.
They’re not really common on city buses, but fairly common on “long-haul” buses and very common on dump trucks.
Last year, I was living near a freeway construction site, and we could tell by sight if a truck was hauling dirt or if it was empty, just by the position of the tag. They pick up the tags when empty to reduce wear on the tires and brakes, and to reduce bridge tolls, which can be worth $5-10 per trip.
All through the town!
Don’t get me started on the wipers.
This. Dual drivers give better traction. Singles are usually adequate, on large vehicles, for load spreading, although you’ll note that most semi rigs have double duals on the rear.
It’s not uncommon to see dump trucks and other really heavy-load vehicles have duals on the back, sometimes idler duals in front of that, and then a third axle with singles that can be lifted out of road contact, to provide yet more load-spreading with maximum-weight cargoes.
(I’m honestly not sure why all of that engineering is put into being able to raise the load wheels - there is obviously some tire-wear to be saved, and probably a bit of fuel economy, but it seems like the amortized cost of that engineering and its operation would greatly exceed minor tire and fuel savings. I suspect there is something about vehicle types permitted on certain roads and other obscure road regulations, which are accommodated by this dump truck equivalent of “no hands, ma!”)
they already have air suspension so the additional hardware isn’t all that cumbersome.
I don’t think dump trucks have air suspension. Buses, regular freight haulers, sure… but gravel and rock don’t care.
There’s also a big difference between air-adjustable springing and height adjustment, which tweak settings but don’t change the suspension action much, and having suspension that is both ultra-heavy-duty AND can be swung up out of the way.
It might be simpler than I’m envisioning - never looked closely at the machinery - but it still seems like a lot of troublesome engineering and maintenance and so forth for trivial reasons. Unless “more than three axles” are not permitted in places some dump loads have to go - or some such.
You have that backward. Positive offset pushes the hub out putting most of the tire and wheel inboard and negative offset pushes the hub in putting most of the tire and wheel outboard.
As explained here.
Why, yes. Yes, I do. :smack:
Positive: Wheels tuck in. Negative: wheels stick out.