LS- Utah
Some jumps

I am running 10 PSI on the stock fronts, and 10 in the rear. The recommended psi is way too low in my experience. When I first got the quad, I ran the rec. psi, the front beads were always full of dirt and rocks, because they fold so bad. Same with the rear, but somehow I got a sidewall puncture. Didn't bend the rim, but scratched them up pretty good on the lip.
I have raced a few times with the stock rims, and jumped -A LOT. No problems so far. I am running Shredders on the rear, 10 psi with no prob. I still have my stock (September 1998) front tires, but they will be needing replacement soon~!

I am about 190 lbs.

 

It depends on what type of terrain you ride on, how much you weight and how much pressure you got in your tires...
I'm probably around 160lbs (70kgs), ride in dirt/moist sand...and run 15 psi in front and 4 in the rear (I know it souns weird....but it works just fine...what would you expect from Rikens in front and 22"Bearclaws in the rear...Holeshots MX soon to come in). With this I never ever did the slightest damage to any of my rims...

But considering you ride in rocks, weight 240 and run low pressure...The only thing (if you don't want to change rims) is to get bigger tires. Or reinforced...or Hyper rims (carbon fiber...they are said to be "unbreakable&quot

 

The only thing I've had happen to my stock rims is having chunks taken out by rocks, and I jump mine with only 4 lbs in the rear.

 

I have to agree with Gabe. The stock 400EX wheels are great, and shouldn't bend like you're saying they are. It does sound like low air pressure to me. Try running about 10 psi if you jump a lot and very high. You wheels should hold up better then. Good luck.