Why are the front and back wheels different on buses?
March 26, 2010 10:43 AM   Subscribe

Why exactly do the front wheels on some/most buses jut out slightly, whilst the back wheels are recessed? Is this just a case of weight distribution to improve comfort for the driver (by damping vibrations) or something else?

I haven't noticed this on other vehicles but perhaps hubcaps cover it.

A photo of said effect.
posted by jhayes to Technology (16 answers total) 2 users marked this as a favorite
Not an answer, but my guess is that there are rather mundain design considerations driving the difference. For instance, the front wheels have to accommodate the braking and steering systems, so the volume on the inside of the front wheel is occupied by them. The rears probably have brakes as well, but most of the braking is done by the front wheels so they are smaller in back. The indentation of the rear wheels may be done to minimize the length of the drive shafts which may save weight. You know.... stuff like that.
posted by Doohickie at 10:47 AM on March 26, 2010

Possibly relevant.
posted by box at 10:48 AM on March 26, 2010

I was going to mention that, but I noticed the wheels in the OP's link were not duallies.
posted by Doohickie at 10:58 AM on March 26, 2010

Duallies. Even if you can't see it. The second axle on the first segment of that jointed bus has two wheels each side.
posted by notsnot at 11:22 AM on March 26, 2010

notsnot: No, I can clearly see that it doesn't have duallies.
posted by Sys Rq at 11:33 AM on March 26, 2010

In your picture, the middle axle *is* doubled up - the shadow is hiding it. What appears to be light under the bus at the middle axle is something attached to the underside of the bus in front of the wheel.
posted by yeti at 12:10 PM on March 26, 2010

Judging by the linked photo, it is unclear to me whether the middle set of wheels are duallies or not. I would expect them to be. (there appears to be a cowling or something hanging down that obscures the putative inner wheel). It is also the difference between a drive axle (where the wheels do not turn to steer the bus) vs. non-drive axles.

My hypothesis: the front axle has inset wheels so that when the wheels are turned they do not protrude from an already over-wide bus carriage. this is especially helpful when they need to do the hard turn to get around a parked car and up close to a curb to allow passengers to board -- if the turned wheel extended beyond the face of the bus, it would present two crappy possibilities:
  1. The driver would have to leave a large gap between bus and curb, making it difficult for the old ladies, or
  2. The driver would have to drive "over" the curb to then pull back away from teh curb and into traffic, making for a rough ride.

posted by misterbrandt at 12:16 PM on March 26, 2010

I played with the levels in photoshop and then highlighted the outline of the tire in shadow here. You can see that it's darker in the grove between the two tires and a little darker on the inside edge of the inside tire.
posted by yeti at 12:21 PM on March 26, 2010

Even if this particular bus doesn't have duals at the rear the vast, vast majority of them do and the manufacturer is not going to make special axles (front and rear) and wheels for single rear wheel buses. So it's got deep offset dual style wheels on all three axles because that's what is found on 99+% of the buses they make (even if the second axle only has single wheels mounted on this example) and the hubs on those wheels stick out on the front to 1) let the front and rear tracks line up 2) make the bus as stable as possible via a wide front track 3) all while using just a single type of wheel.
posted by Mitheral at 12:53 PM on March 26, 2010

I was going to disagree with the duallies, until I started playing with it in a couple of image editors myself. There is a second tire there that's difficult to see, just some weird effect of lighting and shadows makes it appear to be just the one tire.
posted by hungrysquirrels at 1:06 PM on March 26, 2010

Most buses have their engines mounted toward the back, hence the need for more tires to support the heavy load.
posted by schmod at 1:16 PM on March 26, 2010

It's so that you can have a low, wide floor at the front of the bus for passengers to enter conveniently, since modern buses aren't staffed with a separate fare conductor - only a driver who must also perform that role from the driver's seat.
posted by randomstriker at 1:40 PM on March 26, 2010

The odd lighting effect is a mudflap. The middle axle is dualed as it supports about 60% of the weight of the vehicle. The front tires are mounted that way so the steering knuckles are is as far outboard as possible.
posted by seanmpuckett at 1:45 PM on March 26, 2010

notsnot: No, I can clearly see that it doesn't have duallies.

Duelies, can see 'em with an image editor and levels changes. That light part is exhaust.
posted by davejay at 2:08 PM on March 26, 2010

Thanks all for taking the time to clear up my misconceptions about that.
posted by jhayes at 2:16 PM on March 28, 2010

Ha! Well, I'll be.
posted by Sys Rq at 5:25 PM on March 28, 2010

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