(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
This is mainly because of having a back up in case it got one flat tire. Imagine if the bus drive on a nail or sharp object, it can damage one tire, but the tire beside it is still fine and allow the driver to get to his destination or the closest shop where ha can change or repair his tire.
The simple answer is that they don’t all use 2 skinny wheels, many do actually use one fat tire on each side of the axle.
Many reasons have been given here, some are correct, others are not. A single tire absolutely can carry as much weight as two skinny tires, that is not the issue. The issue is wether or not the fuel savings is enough to justify switching to the single tires.
Each has its advantages. The “super singles” are relatively new, while the duals have been around a long long time. People are often reluctant to switch to a newfangled system and will find many reasons not to. You should be skeptical of reasons given on why something doesn’t work when that thing is actually currently in use by many thousands of vehicles. That should be compelling evidence that it really does work.
I’ve driven hundreds of thousands of miles in semi trucks, but never in one that used super singles. Multiple times I have had flats and have been able to drive to a place to get it repaired. For that reason alone I think I would prefer duals. However, I might change my mind if the singles saved me money.
Duals are cheaper, can have new tread put on them and do not require new wider rims. Duals are also safer in rain and snow and if you have a flat can limp off the highway to the next service station.
Price, mostly. The price of two standard sized tires is less than one specially made tire.
Also, there are extra wide tires called ‘super singles’. and their performance leaves a lot to be desired. They have less traction on snow, ice, and water than 2 regular tires. And when one goes flat, it’s far more problematic than when a regular tire goes flat. They do give about a 0.5% better fuel mileage – which will translate into a bunch of money saved over the course of their lives. But professionals generally do not like them as much due to their traction issues and troubles when one goes flat.
Source: was a trucker for 18 years.
The “wide single” is available and is used on semis. It offers a 120# weight reduction(240#/axle), less drag and lower cost. It is becoming more common.
Another part of the puzzle is tire technology. Designing wide tires that could handle the weight and stress loads was more difficult in the past.
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
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.
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 .
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