For all my Francophile readers Happy Bastille Day! This is the day that every French rider in the pro peloton wants to win a stage of the Tour de France. And that is as close as I plan to ever get in covering bicycle racing in this blog. I might mention the rider’s name if a French rider wins on Bastille Day, also, but only as a recognition of the holiday.
I seem to be obsessed with the frame design for my next car. I’m trying to get 2 mutually exclusive and 2 somewhat synergistic characteristics to their happy maximums, with the synergistic characteristics balanced against the exclusive ones. The 3 characteristics I’m working with are light weight, torsional stiffness, and interior space. The exclusive or antagonistic ones are light weight and torsional stiffness, the synergistic ones are torsional stiffness and interior space. There are some minor things I have to consider as well, being able to see out of the car in all directions and to be able to get in and out of the car.
For instance the lightest and most torsionally rigid structure I could use would be a tube with a bulkhead on each end. However this leaves either a structure too small to use or one that can’t be seen out of, not to mention you can’t get in or out of a sealed tube, so many tubes must be used as at least part of the structure. Also SCCA rules require a tube rollover structure with specific sizes for the front and rear hoops that go over the passenger compartment, in this case 1.5” diameter by 0.120” wall, so again at least part of the structure must be a network of tubes. Where torsional stiffness and interior space are not synergistic is placing diagonal tube braces across openings in the passenger compartment or through the compartment, so I have to work around a big unbraced space in the middle of the frame and somehow route torsional stress around this space so that stiffness is maintained, without adding weight. Where they are synergistic is making the center volume of the frame as large as possible increases the stiffness with only a marginal increase in weight, so placing the roll cage hoops on the outside of the body (which is easy to do with a T Bucket) gives me more space and a stiffer frame.
So, how do I plan on working around the big empty space in the middle of the frame that I can’t run diagonal braces through? Make the surfaces I can brace as rigid as possible by either welding a sheet metal diaphragm across the surface defined by the tubes (floor, and firewall) or by using diagonal braces (top of the roll cage from front hoop to the rear hoop, and across the rear hoop). Also I plan on using a bolt-on belly pan as a stressed member to tie together the bottom frame rails front to back between where the suspension pokes through. And in addition I plan on making a torsion box out of the structure under the passenger compartment by making that a sealed box with internal braces. The belly pan and floor are the top and bottom of the box with the shear web between the top and bottom of the lower frame rail tubes as the sides and crossmembers that tien those tubes together and support things like the transmission and the driveshaft loop with the driveshaft loop being the partially-open rear of the box. This will tie into the rollover structure to help make a stiff unified frame structure. I also plan on using some of this structure to mount the seat belt and shoulder harnesses, by using a horizontal brace for the rear roll hoop that intersects the point the diagonal braces intersect to mount the shoulder harnesses and make that structure even more rigid.
To also increase the stiffness of the whole structure the suspension pickup points will be tied to the rollover structure by diagonal braces from the coilover mounts to the tops of the rollover hoops like a sprint car or midget racer, and also from the coilover mounts to the bottom of the rollover hoops, making the front and rear of the frame triangulated (aka RIGID). The bolted-on belly pan completes the bottom leaving only the top of those boxes to brace. Rear is simple as I can run additional diagonals from the top of the rear hoop to the coilover mounts, but the front is problematical as the person approving the frame design for the street (aka vehicle inspector) might rule the diagonals as excessively obstructing forward vision. If that happens I’ll just have to live with the loss in rigidity. What I want to do is have crossed diagonals like the top and rear of the passenger compartment, but to improve vision I could run diagonals from the center of the crossmember connecting the front coilover mounts together from side to side to the top of the vertical parts of the front rollover hoop. The cars I’m emulating don’t have any diagonals on the front braces because they would have to run through the engine compartment, so it’s not like I have to use them. I could get adequate torsional stiffness without them, just not as much as I could get with them. This is a case where “good enough” may have to do.
My diagonal braces are not running through the engine compartment because I’m trying to make the front big enough to run a full-sized radiator that a sprint or midget would not need to cool the engine. That is because they don’t sit and idle, which actually requires more cooling capacity than moving, and they run on alcohol which runs much cooler than gasoline and actually reduces engine temperature as it evaporates inside the engine. It’s nice when your fuel is also a constant-loss coolant that improves your horsepower over burning gasoline.
Something else that is increasing the height and/or width of the front end is the need to run the steering box at the front of the frame. This needs to either run under the radiator or beside it with more radiator lower than the steering box. Sprints and midgets use a steering box that sits behind the firewall and has a drag link to the front suspension on one side of the engine. This would not work on my car as none of these boxes are approved for street use. There is an alternative that uses a reversed (reproduction) Corvair steering box just in front of the firewall, but this would likely interfere with the engine and transmission, as those are wider now than in the past when the reversed Corvair box was common usage. The other alternative, rack and pinion, won’t work because I’m running a tube axle instead of independent suspension. R&P steering would cause excessive bump steer as the axle travelled up and down going over bumps and potholes. This is bad as the car would be steered in the direction of the tire with the heaviest load at that instant, making the car move from side to side unpredictably over bumps. I looked into using I.F.S. on this car, but the combination of the multiple A-arms, heavier spindle, and the bracketry required to mount them on the frame and maintain torsional stiffness would make the car 50-100 pounds heavier, and on a 1700 pound car that’s a hefty weight penalty. Also I’m running the tube axle because it maintains correct camber as the suspension travels, body roll does not cause a change in camber. Now if I wasn’t running the T body and just making my own I would lose about 50 or so pounds just from the body and could use the frame shear web as part of the body and get a bunch more torsional stiffness and use I.F.S. But then I would face a much harder time getting registered and licensed for street use. Not to mention losing a potential activity to participate in with the car, taking it to rod runs and competing in driving events, which is a major reason why I’m building this car.
PSA, Opus
Witch on a Bicycle 