Most of the after market nerf bars are going to attach to the frame. This is usually done by the front bolt attaches to the body mount (they usually supply a longer bolt to replace the factory body mount bolt) and the rear bolt is threaded through a spot in the frame. there is usually a brace bar to keep the bars level when stepped on. Westin, Smittybilt, TruckTec, and many other attach like this.

 

Go to ebay and search aries oval. Aries makes some that attach to the bolts in the frame, and then have secondary reinforcement that attach to the body. I had them, I would never buy them again. The ones that attach to the frame using the bolts undernneath the truck squeak. I thought my shocks were going bad, took it in and they said it was the bars rubbing under the cab. Truck had 4,000 miles, luckily they didnt charge me. I sold the truck 1 week later, bought another 2005 and put the mopar that attach to the body only. Got them on ebay for around 330. I would not purchase any other bar after the mopar, they are the best I have had out of 5 different sets on my 5 different 3rd generation ram trucks. Dont worry about the frame, or body. I am a big guy, no problem with the body attached bars. Trust me, I have installed many different kinds. THere is a reason that a lot of parts from mopar are more expensive, the nerf bars are an example, they are quality, they attach properly, durable, and look great. STAY AWAY FROM cheap nerf bars! Just my 2 cents!

 

this is how my Westins look from underneath:



I've noticed no squeaking whatsoever and they're strong as hell, though I'm only 180 pounds and I haven't gone hoggin' enough to have any fat chicks climbing in from the other side, so maybe I haven't tested them fully...

 

I am talking about oval tubes that dont bend back into the frame. Bascially a straight bar. So oval tubes will attach differently than regular tubes.

 

As you can see from HedleyLamarr's picture above, the frame rails are actually above the bottom of the cab and bed sides. To have bars attached to the frame, you'd have to do some fabrication which could also weaken the frame itself. I wouldn't want that done to my truck, but then again, mine is never offroad.

By bolting up to the cab mounts, your truck frame can flex independent of your cab. As your cab moves, so does your bars meaning that the bars will never come in contact with the bottom of your cab.

I was skeptical when I put on my Westins. At 190#, I jumped up and down on them violently and they didn't budge at all. My kids thought I had lost my marbles.

Mike