how come a 19" rim doesnt measure out to be 19"?

A 20" is not 20", its like 16" and a 16" is like a 12" ect ect.

if thats correct then whats a 4"?

Scar_head, would you please try titling your threads so that we know what they are about? Your title didn’t help at all.

A 19" rim is just a name for the mod bike trials rims. It is a way of saying the tire dia. is similar to a 20", but it’s also a way of notifying us that it’s not interchangable with 20". Does this help?

No but its true, a 19" does not messure 19" its like 17 or 16", i forget, il go messure…

A 20" wheel is 20" in diameter with the tire. A 20" rim is a rim for a 20" wheel, so it’s less than 20" in diameter.

The size of the tire may vary, so 20" wheels need not be exactly 20".

At some point someone started making 19" rims. My guess is that people wanted to use bigger tires in their mod trials bikes. The frames where made for 20" wheels, and in order to make room for a bigger tire the rim was made smaller.

That way the outside diameter of rim and tire was still approx. 20".