Tag Archives: going back to rear drive midengine

I teased long enough, now I have to tell what I have been thinking about

I keep teasing with removable frame parts and a possible mid-engine AWD, and what to take off for maintenance.

Well, I went back to the engine behind the body and not inside it, and rear wheel drive, but I went back to putting the gas tank in the passenger side of the body. The exercise of trying to stuff the engine beside me inside the body worked well for trying to get the 32 gallon fuel cell inside and covered up so that the assembly was crash-worthy, and the engine accessible for things like changing the oil and the air filters. The engine will sit behind the body with the proportions very close to the original Model T.

Now the “how”. The frame will have a flat section made from 1.5″ round tubing that goes under just about everything and serves as the foundation for the rest of the frame. The roll hoops bolt to the bottom frame and hug the insides of the body to have the largest possible radius of gyration for that all-important d4 stiffness and still leave room for my skinny butt and broad shoulders, the shoulders having the most problems to find room for. I was cursed with shoulders that even with just normal cyclist’s upper body development barely fit through the 22″ wide door to the bathroom. So I’m going to move the rear hoop as far back as possible and still be inside the body and tuck the driver’s side right up against the inside of the skin. If I center the driver in the left half of the body my left upper arm will be a light press fit against the inside of the outside wall of the body, meaning I just need to cover the left side of the body in some kind of soft, smooth upholstery to make it the side of a really big bucket seat made from a T Bucket.

Then the bolt-on superstructure gets bolted to the frame to turn the slightly floppy 2-D ladder structure into a stiff in all planes 3-D space frame. The tube diameters will be a constant 1.5″ except for diagonal braces directly in the driver’s line of sight which will be 0.75″ diameter for better forward vision. The wall thickness will vary according to diameter and placement. The lightest wall thickness I can get in the 1.5″ diameter required by SCCA rules is 0.065″ while I’m sticking with 0.120 for the roll cage as required by the SCCA, and for the 0.75″ diameter forward diagonal braces. All the rest not required to be 1.5″ by 0.120″ will be 1.5″ by 0.065″ for less weight. The fun part continues to be the diagonal bracing of the cage, so that I can get in and out and sit and reach the steering and pedals. Those are going to be the 1.5″ 0.120″ wall DOM tubing because they are part of the roll cage, but where should they go? Well the rules say no further down than 1/4 from the top or up from the bottom. So the diagonal brace between the front and rear hoops will run from the end of the bend up at the upper corner of the rear hoop to the end of the bend where the front hoop bends up from the frame to the side of the body, because that front part of the frame runs through the firewall of the bucket body where it necks down to 27″ wide, and the front hoop comes out from behind the dash panel that is 39.5″ wide, while the rear hoop comes from the 34.5″ wide frame rail to the 45″ wide rear body with the diagonal brace running between them. The part of the frame rail between the front and rear hoops is part of the roll cage and has to be 0.120″ wall thickness, but everything else 1.5″ diameter can be 0.065″. The one frame member I haven’t decided on is the one that runs from the front spring mount across the top of the roll cage to the rear shock mount. Technically it is a diagonal brace for the roll cage and should be 1.5″ diameter and 0.120″ wall, but it could also just be the upper frame rail that just happens to connect to the top of the roll cage. I could make 2 for each side, one light and the other heavy, but the difference in weight would be 13.5176 pounds at the highest point of the car. So the question is, is that a significant enough difference in weight to make the lighter part the choice for the car, or should I go ahead and use the part made to roll cage specs? The difference in stiffness is essentially nil for stresses in the elastic range of deformation, so the real question is when my ass is in the driver’s seat and the wreck is hard enough to get the frame bent, do I want 0.120″ wall thickness resisting plastic deformation around my head, or am I comfortable with only 0.065″ wall thickness?

Also there is one other diagonal brace that is not required by any rule that would make the cage much more resistant to a diagonal collapse, running from the upper corner of the passenger side to the intersection of the front rail with the driver’s side forward hoop. I did a quick mockup with paper towel tubing of the other direction bracing the driver’s side upper corner, and my head is in the way of the diagonal, but it looks like my right foot is in the way of the other direction, or any other brace from the top of the rear hoop to the intersection of the front hoop and the driver’s side frame rail. I guess I could run something from the general area to the right of my head to the intersection of the front hoop and the passenger side frame rail to keep the rear hoop off my head in that kind of impact, or I could figure out how to run those stresses around the outside of the cage instead of through the middle. There’s a trick where I can brace the halo against diagonal deformation by using a second halo inside the one formed by the front and rear hoops top tubes and the tube that runs between them front to rear. Now I add an X brace across the rear hoop and all the stresses go around the cage instead of through it.

No, what I need to do is make 2 cages, one inside the body with the front and rear hoop bolting to the frame rails and running up the inside of the body to get as much room as possible, and a second cage outside the body that also bolts to the frame and has the diagonal bracing I can’t get from the inside cage because there is a driver or fuel tank or other widget the car will not run without that can’t be relocated to another place, and the diagonal bracing on the second cage is entirely outside the bucket body. The second halo would be formed by this cage on the outside of the body interacting with the cage on the inside of the body.

OK problems solved, now I just need the donor vehicle and about a hundred pounds of 1.5″ diameter tubing to show up in my driveway.