How to notch the frame

[patmckinneyracing]

I'm gonna look at the thickness of the original frame material and decide from there. I really think 1/4 material is rather unnecessary, and re-using the factory material will not hurt as long as the bracing is there to support the notch.

I'm guessing the material on the factory frame is probably 1/8 inch?

 

[joe_padavano]

Yeah, I've read where some guys say it's not strong enough and that you should use a thicker piece of metal but I don't think it will be a big problem.

As a structural engineer with 30 years experience in the aerospace industry, I can tell you that the notching in these photos is VERY scary. The vertical bending stiffness of the frame rails comes almost totally from the top and bottom parts of the rails. By removing half of each, you've cut the bending stiffness in half. The RIGHT way to do it is to maintain the full cross section of the frame rails and physically move the rails inboard, the way drag racers do it. This thinning of the rails is a REALLY bad idea. G-body frames are marginal to start with, and any rust on the inside surfaces just makes them that much weaker. Throwing away half the bending stiffness is wrong.

 

[jrm81bu]

Yeah, I've read where some guys say it's not strong enough and that you should use a thicker piece of metal but I don't think it will be a big problem.

As a structural engineer with 30 years experience in the aerospace industry, I can tell you that the notching in these photos is VERY scary. The vertical bending stiffness of the frame rails comes almost totally from the top and bottom parts of the rails. By removing half of each, you've cut the bending stiffness in half. The RIGHT way to do it is to maintain the full cross section of the frame rails and physically move the rails inboard, the way drag racers do it. This thinning of the rails is a REALLY bad idea. G-body frames are marginal to start with, and any rust on the inside surfaces just makes them that much weaker. Throwing away half the bending stiffness is wrong.

That's kinda funny, MR is pretty full of drag racers that have done it the same way as above, with only a few doing it the way you describe. One persons "right" way is always anothers"wrong" way. Ya your losing some strength in making the frame narrow but if you go with thicker materials you will gain some of that back. Not to mention the mounting points for the differential or farther ahead on the frame than the point that the notch is making the strenght issue far less severe.

 

[joe_padavano]

That's kinda funny, MR is pretty full of drag racers that have done it the same way as above, with only a few doing it the way you describe. One persons "right" way is always anothers"wrong" way. Ya your losing some strength in making the frame narrow but if you go with thicker materials you will gain some of that back. Not to mention the mounting points for the differential or farther ahead on the frame than the point that the notch is making the strenght issue far less severe.

Thicker material on the sides does not provide much help to bending stiffness in the vertical direction. Feel free to look up the calculations for bending stiffness of a box beam. Also, last time I looked at those pictures, the spring pockets, where the weight of the car is carried, are attached directly to these thinned sections. Let's just hope that these drag racers don't ever design commercial airliners, bridges, or other structures where some engineering intelligence is required.

Dedicated drag race cars came from the factory with swiss cheese frames. You don't ever want to drive one of these on the street, however. Speaking of these factory swiss cheese cars, ever notice that the lightening holes are cut in the SIDES of the frame rails, NOT the top and bottom. Ya think the factory engineers might have had a reason for this?

 

[FE3X CLONE]

Joe, I agree that the absolute best option is moving the entire frame rail in board. However I also know that in the real world, your concerns over the thinning of the frame rail don't seem to come into play other than on really quick, hard hooking drag cars. And even then it's really only an issue with the body.

All of the suspension points on the frame are well forward of the notch so structurally the rear frame rails are there to attach the body and the bumper.

 

[joe_padavano]

Joe, I agree that the absolute best option is moving the entire frame rail in board. However I also know that in the real world, your concerns over the thinning of the frame rail don't seem to come into play other than on really quick, hard hooking drag cars. And even then it's really only an issue with the body.

All of the suspension points on the frame are well forward of the notch so structurally the rear frame rails are there to attach the body and the bumper.

Obviously a car that will only see a flat, smooth drag strip has different requirements than one that drives on real roads with real potholes. However, even drag cars care about chassis stiffness. Thinning the frame rails not only reduces bending stiffness, it also reduces torsional stiffness. Any engine power that is being used to twist or bend the chassis is engine power that is NOT accelerating the car at the strip. The stiffer the chassis, the less wasted motion. Grumpy Jenkins proved this with his first tube frame Vega Pro Stocker in the early 1970s. Now, even a six point roll bar will help tremendously if the diagonal tubes are positioned properly. Also, several folks now have commented about the suspension pickup points, but again the springs are ALSO a major load point on the chassis.

Finally, even if you aren't worried about the reduced stiffness of the narrowed frame rails, consider what the additional flexing will due to freshly-painted quarter panels over time.

 

[patmckinneyracing]

My big issue out of this is that I've seen many g-body guys do this mod and never heard of any horror stories later. I met one guy who did this same mod on his grand national that pulled 10.20's all day on slicks.

Joe is addressing some of my initial concerns with this mod as far as the rigidity and possibly compromising the frame itself, at the same time FE3X Clone's logic makes sense as well concerning the placement of all the suspension components ahead of the notch. I just don't want to be the one guy who does this mod and causes permanent damage to the frame.

 

[joe_padavano]

My big issue out of this is that I've seen many g-body guys do this mod and never heard of any horror stories later.

Sadly, that's the same logic NASA used when they cleared space shuttle Challenger for launch. "Well, we've seen O-ring blowby but we launched successfully 24 times." :shock:

 

[patmckinneyracing]

Maybe it's just me but that comparison makes no sense. If they saw blowby once, but none on the other launches, then that was their screw up.

If you look at the pics FE3X put up after he painted everything and had all the suspension components in, you can see the added bracing that was placed in the key locations. Essentially he "kinda sorta" followed the same concept of why drag racers put cages in their cars. His design with the added bracing and aftermarket suspension components will prevent any horizontal or vertical movement of the frame.

No, I'm not saying that notching the frame is the optimum way of putting larger wheels in the back. In my case the paint and mostly everything is done and I just replaced the body mounts (which were a bit of a b****). The body isn't coming off and I'm not about to spend more time modding this car than I have to. No it's not half a**ing it, as far as I can tell, there are alot of people who have done this on GBF, Monte SS, MalibuRacing, Turbobuick, and all the other forums, and have yet to hear of a single incident of structural failure.

If anything, this allows the rear end to "collapse" in the event of a rear end collision. IE the car will absorb the energy better.