(Eli5) Why do Buses & Trucks use 2 skinny wheels put together on each side of the axle? Instead of just using one fat wheel?
In: Engineering
You want to be able to turn. Inside and outside wheels travel at different speeds in corners.
One reason I’m not seeing mentioned here is snow and ice. In a truck, your fat ass takes a looooong time to slow down, even in perfect dry weather (take note, drivers, this is why you shouldn’t swoop in front of trucks two feet off their bumper when passing). In snow? Your time to maneuver is effectively doubled.
What’s the form of a snow tire? In its most simplest form, it’s a tire with deeper treads that avoid being filled completely with snow, and softer rubber to help grip the road better in cold conditions, where rubber will harden. That first one is especially important, as deeper treads help get grip on packed snow and ice.
With dual tires, the space between them basically acts as one very deep tread, allowing greater control in the snow. Which if you’re running super singles (which is what the “one big tire” would be called — they do exist), you don’t have.
Source: was truck driver for a year. Not recommended unless it’s your only alternative to homelessness.
Adding to the other responses. A single big fat tyre is also more prone to hydroplaning. Hydroplaning is a dangerous condition when the tyre floats over the water on the road instead of cutting through it leading to skidding and well…crash
Drive tires can have super singles, but the trailer tandems are in pairs for weight distribution and the ability to change tires on the road. The double pair of tires are the trailer tandems and they also slide forward and rear to distribute weight legally and evenly .
Sit on one big balloon, it pops
Sit on many small balloons, they hold your weight
A stick will snap. A rope will hold. Many small stronger than one big.
Actually the two axels on the back of a full size bus in the US, the front ones, the drive wheels, of the two are dual wheels and tires and the rear of the two are just one wheel and tire called a tag axle. Some makes, like Prevost, this tag axle can be lifted by air pressure thus shorting the turning radius and increasing pressure on the four drive wheels.
In addition to all the other valid explanations, there’s a lot of “because that’s the way we’ve done it” resistance to change.
Back before radial tires, in order to have a tire be wider, it had to have a taller sidewall. To carry more load, you have to have a larger contact patch.
So in order to double the weight capacity of a wheel, you could either make the wheel twice as large, or just stack a second wheel next to it.
With radial tires, width is independent of height, so eventually somebody had the idea of “why don’t we just make the wheel twice as wide?”
They did that, worked out all the kinks, and ended up creating the “super single”.
After that, then they found that a super single isn’t as good in rain or snow as a dually.
But the initial reason for dually wheels was the limitations of bias ply tires.
Tire sidewalls are like the “legs” of a tire. More legs means the system can hold more weight. Also keeping the tires all the same all the way around helps keep things standardized. You can buy and stock all the same size tire instead of having to get multiple sizes. So, in the end it’s about weight carrying ability and economic maintenance.
Big wheels mean wide thicc rims. Thicc rims not cheap to make, waste money. Nobody wants to buy extra unused thicc rims.
So use normal rims, normal tyres, side by side. Save money and save materials/metal.
As others have likely pointed out, the reason a truck might have two individual wheels instead of one wheel is for redundancy. If one wheel bursts or goes flat the truck would still have a backup wheel/tire. With a single wheel/tire a flat would be more dangerous and more difficult to fix
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