Why do buses have bogie wheels?

Many buses, especially ones on the highway, have an extra rear axle equipped with single rather than double tires, called “bogie wheels” (as I understand it). Why?

I did some research and learned that this was to be able to handle the weight and to brake better. I don’t understand this – there are trucks of all kinds of weights including smaller than, similar to, and bigger than buses. But trucks use single or double wheels on all their rear axles, whether that gives them a total of 8 rear tires or 4 or 2. Why do so many different buses then need exactly 6?

I even read stuff in “Bus Conversion Magazine”. I think I am way beyond deserving an explanation.

I’m going to try to explain this simply. Not because I don’t think you can comprehend, but because I can confuse myself when I try complex explanations.
You can’t really compare a bus to a semi. A 18-wheeler uses the 12/34/34 rule. It means that on a truck scale, 12,000 pounds can be on the steer (front) axle. 34,000 pounds can be on the (tandem) rear axle (of the tractor.) 34,000 pounds can be on the trailer’s tandem axles. The weight isn’t determined mainly by the number of tires, but by the number of axles. Some newer “18-wheelers” only have ten wheels: 2 steer wheels, 4 drive wheels (the same total width of traditional dual drive wheels) and 4 trailer wheels(the same total width of traditional dual trailer wheels) .
A bus doesn’t usually carry as wide a variety of weight as a semi. It has 2 normal wheels on the steer axle. It usually has dual rear tires on the drive axle because duals are more traditional, cheaper, and more common. The bogies are single tire because that is all they need to meet the letter of the law. If the rear axle is overweight, they can lower the bogies and all of a sudden, a 2 axle bus is now a 3 axle bus. They only need the axle(s) to support the extra weight to be legal.

Oh. Well, that makes sense. So, that axle with fewer wheels on it is also retractable?

Not necessarily,
One extra axle behind the drive isn’t going to cause any steering problems and these buses are on pavement almost 100%
Many extra axles(tag axle(s)) on other trucks are retractable and its not unusual to see 3 air tags on trucks permitted for up to 120K
The suspensions on these buses are most likely air and as the bus loads the weight will push the chassis down against a air valve and air is added into the bags (air springs) and support the load on all axles.

I was told they are called “tag axles” and are retractable to avoid tolls based on how many axles are on the ground.

So, the answer, like always, is “Money”.

So then, they can “bogie down”? :stuck_out_tongue:

I was told (not sure by whom, at least not by a source I’d 100% trust) that it was a space issue.
Most highway buses have the engine in the rear, and it sits between the rearmost tires. So fitting dual tires there would mean a narrower engine compartment, and all the problems that would bring.

I doubt this is a primary reason, but it is one more reason why one set of tires on that axle is enough.

Interesting idea, so I looked at a few pictures online. But it appears that they make the single tires narrower - that is, they make the space available for the engine narrower, and leave surplus space outboard of the wheels.

But I could only find a couple examples on which I could see clearly enough to tell.