Why 19" and 20" wheels?

Does anyone know why there are 19" and 20" wheels for unis?
Surely they are so close in size, that it would make little difference to trials or other disciplines which size they had (assuming the correct tyres were available).
Or am I missing something?

The 19"_wheel is usually used for trials and street,where there’s a lot of jumping which requires a bigger tyre. If you were to put a bumper tyre on a 20" wheel,it would make upward jumps more difficult due to extra weight and less clearance due to bigger diameter.Also,all things being equal a smaller wheel will be stronger than a big one.

But that’s only my opinion.I’m not certain what the manufacturers reasons are.

19 in tires and rims are mega expensive in comparison

The reason is quite simple. In competitive bike trials there was a class with a maximum wheel diameter (including tyre) which was specified as a standard 20" wheel and tyre. Riders realised that a fatter tyre would give an advantage, but within the class rules it was impossible to put a fatter tyre on a 20" rim. Hence the 19" rim was born, with a slightly smaller diameter than a 20" rim, enabling riders to use fatter tyres with the same total diameter.

Why does this matter to us? Because if you want a big fat tyre in 20" size, generally they’re made to fit on 19" rims to meet trials bike rules. No reason why you couldn’t make a fatter tyre to go on a 20" rim, but the market would be limited, hence nobody makes them. As a side effect a 19" rim with a fat tyre will run in a frame made for a 20" wheel - a 20" wheel with a fat tyre might not fit.

I am working On a wide 33mm BMx rim and a 2.4 tire