I've actually put riders in the 400lbs+ range on a max of a 40h rear wheel with great results. One thing to consider is that as you go to a 48h rim, you're removing a fair bit more metal from the web of the rim for the additional spoke holes. That actually reduces the "burst strength" of the rim, which is essential to the rim's ability to hold tires at high pressure. What we will occasionally see is that a wider tire at higher pressrue will crack the rim down the web between the spoke holes.

For a 300lbs rider, I would have no problem putting you on a properly hand built 36h wheelset, on a rim as light as our Dyad, which has been a long time favorite among clydesdales, touring cyclists, and tandem teams but still comes in at a reasonable 520g.

Weve got some pre-configured wheelsets here under our clyde section, and we're able to customize the build for you a bit as wel: Velocity - Wheels, Clydesdale

Feel free to email me with any questions, seely@velocityusa.com