I just completed the dreaded butt sag repair now the the straps that run from the the post behind the rear door frame to the end of rear frame are in the way of mounting my external cooler in the rear driver wheel well.
Any suggestions on alternate mountain solution?
I'm looking at the space between the back of the rear seat. Maybe I can build a bracket the at mounts the cooler at a tilted almost 90deg angle. The cooler is a Derale cooler and fan combo.
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I ran my butt sag straps from inner most side of the wheel well - tight (flush) against the fiberglass. You could put it in cavity behind seat but would need ducting to lift air from below car and exhaust it down and back. I'd get it to one side so the engine shroud fan doesn't suck warm air back in.
Tangent, sorry - is the Butt Sag problem specific to any particular builder? CMC is what I found in the search, others I might have missed? thanks
You'll find it in FF/CMC since most were DIY home built and builder didn't follow all the instructions in the build manual. The sub frame used for the VW pan based IM thru pan based VS/JPS seem to differ very little. There is little steel support (maybe 1.5 square tube) to the rear - the engine compartment side fiberglass pieces are pop riveted to that tube. The rivet holes elongate allowing the body to "hinge" at the door opening creating the gap. The build manual has the builder fiberglass in a piece that goes across the entire rear and attaches to the tube cross member. If done properly it adds great strength and helps prevent that elongation. Many builder don't fiberglass it in properly. There is a front steel brace that goes above the front H beam for support there - and also fiberglass pieces that get bonded to the wheel wells for front strength.
This photo is sub-frame from a burned out CMC. The X'ed bits are stock - they were added. You can see the circled weak areas. I drilled out the rivets and jacked up rear fiberglass - added liquid nails like product and used large SS washers and bolts thru the fiberglass and tube. I made the square bits behind the doors a bit higher with angle iron. I attached steel straps from it to as bar back as I could.
Same sub frame but with the optional show roll bar mounted. Note: no pieces added to rear for extra support.
Missing the second image, Greg.
On my CMC I not only fiberglassed the rear piece in, but I also fiberglassed the side pieces to the tube, wrapping over the top and inner side of the tube. This seems to have prevented butt sag, on the car at least. I seem to be experiencing some personal butt sag.
@bkelly wrote (in part): "The straps that run from the the post behind the rear door frame to the end of rear frame."
Wow..... That's one helluva triangulated strap!
On the CMC cars, it isn't so much a problem with the frame deflecting downward from the back of the door rearward, as much as the fiberglass rear section was never attached to the frame at the proper height/attitude in the first place. All or most CMCs suffer from this and they're all different.
What we all did is detach the body's inner fender vertical wall from the rear portion of the frame by drilling out those big rivets, lift the body up at the rear corner til the door seams match and then drill new holes through the fender wall and frame and either rivet it or bolt it to the frame a bit higher than it was.
Running a diagonal brace (I used 1" X 1" square tubing on mine) from the door rear pillar to that frame piece is a great idea, but you needn't go all the way to the rear of the frame horse shoe - Back as far as the wheel centerline or slightly less is fine, which would give you plenty of room for a cooler mounted in the wheel well behind the brace.
I have hopes of installing a T-4 engine so wanted that rear section strong so I could add a rear engine mount and not just rely on it hanging off the trans axle. I've seen it also done with cable and a turn buckle. I can't remember if these are photos of my car or another. I thought I had gone higher up behind the rear door piece.
Hi bkelly. I had the same butt-sag issue with my CMC and came up with a different solution. I jacked up the body, drilled out the rivets ,drilled and tapped the sub-frame and bolted the body to the subframe. While I was at it I made up some brackets and welded them to the subframe and frame horns and bolted in a 1" square brace to tie the subframe and frame horns together eliminating sag and taking the twist out of the frame horns at the same time. I also made up a couple of fibreglass body mounts which I bonded to the rear of body and bolted to the rear subframe to give extra support at the back end.
Nice work, Jesse!
