system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 . why are the double decker buses in the United Kingdom and i believe in some other European countries equipped with only 6 wheels while double deckers in countries have double decker buses with 10 wheels?
gpt
Weight, length, use, and laws.

The basics: Everywhere, vehicles have a maximum allowed gross weight rating (GVWR), and for cargo vehicles, there is a maximum allowable weight per axle, and the number of axles and spacing counts. For example, in the US the maximum weight per axle is 20,000 lb, however, a 2 axle group (like what you see on a semi trailer or on a 6x2/6x4 tractor or truck) is only good for 34,000 combined (17k per axle), and 12,000 for a front steer axle. There are other specifics such as the Federal Bridge Law (used statewide in many states, and countrywide on Interstate highways) that require certain wheelbase to meet the 80,000 max GVWR allowed federally, FBL is what leads to dump trucks and mixer trucks having an axle on a trailing arm, like the aptly named McNeilus Bridgemaster, as well as the dump trailers that are towed about 10-15' behind the truck.

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Mexico defines the weights depending on vehicle type, whether it's a bus, truck/trailer combo, semitruck / semitrailer combo, semitruck/semitrailer/trailer combo ("double") and road type, so for certain roads the weights are higher than in the US, and for some are approximately the same. For the highest classification road. it's 6.5t for a steer axle, 17-19t for an 8 tire tandem, cargo. For passenger buses, it's 6.5t for steer and 21t for an 8 tire tandem or 17.5t for a 6 tire tandem.

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The 2 axle buses you see in London are urban buses, so the seats are just plastic benches, there are no amenities, and the people aren't carrying luggage. On top of that, they're 10-11m long units.

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The 3 axle buses you see elsewhere are longer and therefore heavier. For European city buses, smaller units make more sense for maneuvering, in other countries that's not a concern and 11-14m buses are common. Now, the highway buses are even heavier, as they are at least 12m long and tend to have heavier seats as opposed to plastic benches, amenities, and often carry luggage and cargo. 13.7-14m (40-45') buses are the norm in many places, and in South America, 15m (50') is a common length as well.

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In the specific case of Mexico

For roads where a double decker makes sense, a maximum length of 14m is permitted with 2, 3 and 4 axles, however, there is an exception that allows for 15m buses as long as the 3rd axle is steerable. There is a penalty associated with this configuration, as the maximum GVWR is reduced from 3.5 to 4 ton (depending on road type) when going from a 10 wheel, 3 axle config, to an 8 wheel, 3 axle config, where the 2nd rear axle is steerable.

For the highest rated roads (4+ lane highways), the max GVWR for a bus is the following

* 2 axles / 6 tires: 19t
* 3 axles / 8 tires: 24t
* 3 axles / 10 tires: 27.5t
* 4 axles / 10 tires: 30.5t

Only the 24t setup is granted an exemption to go up to 15m, IF the 3rd axle is steerable, all others are 14m. There is no exemption for a 15m, 4 axle setup, which makes 4 axle buses incredibly rare in the country, versus parts of Central or South America where they are practically a staple.

The law makes no distinction between single or double decker.

Other countries require 4 axles to go up to 15m, or the roads require them for better load distribution.