I'm using the overall dimensions of the 312B as a rough starting point. At least I know a guy will fit in it. At this stage of the game I'm doing a very rough fleshing out of roughly where components will go, how much room I have to play with, etc. As I continue I'll step through the design process, both the concepts and hard numbers. I'm open to suggestions, by the way.. particularly regarding aero.
- Weight: 1000 lb, including driver
- Wheels: 13" bead diameter x 10" wide (maximum)
- Wheelbase: ~94"
- Track: ~60"
Engine
There are a number of options available here. At the moment I lean toward the Honda CBR1000RR, just as I've had experience with some of the internals on the 600 F4i from FSAE, including the gearbox. Word on the street is the '08 models boast 180-ish HP at 12,000 rpm, but that's just a wikipedia stat.
Differential
At first I was 50/50, but now I lean 80/20 in factor of an ATB differential over a Salisbury-type. If I was doing FSAE again I would probably lean toward the Salisbury in order to get some locking on coast to tighten the car up entering the corners. Otherwise with the ATB they seem to really shift to oversteer when on the brakes or in lift-off transients. Plus, they're traction-limited in enough corners you can steer the rear around with throttle.
With the F1000 I'm hoping the much longer wheelbase will mitigate the longitudinal weight transfer effects a bit... keeping good rotation while trailbraking but not wanting to spin the car around. And then the ATB wins with power application, and keeps up the differential action at all times so I'm not scrubbing any speed off going tight mid-corner and on track-out.
Exhaust
I much preferred the drivability of a long-header 4-2-1 exhaust to the 4-1 merge on a FSAE car. The '04 and '05 cars (and I believe '03) all suffered from the same issue, in that there would be no torque at all up until probably 7-8k rpm, and then bam the torque would flick on like a lightswitch. If you were out of the powerband you were junk. The 4-2-1 on the '06 and later, with longer primaries, seemed to drop the start of the powerband down into the 5k rpm range. Much better for a "street" course.
Twin 2-1 exhaust pipes on either side of the car would look very cool. I may have to consult with the folks at Burns Stainless to get their input. Would be nice to be able to drive this thing at an SCCA autocross and not just at Mid Ohio or Beaver Run.
Frame
Welded tube spaceframe. Composite tubs are not allowed in this series, nor would I probably be interested in making one. I'll have to check the SCCA rules but I believe 1.25" OD x .095" wall is recommended for roll hoops. At the moment with the placeholder frame I have 1.5" OD x .125" wall. Easier to start extreme overkill and back down.
Having spent many frustrating hours welding the 2007 frame, in a variety of awkward positions, actuating the TIG pedal with anything from my foot to elbow to ass, I'm trying to ensure the joints aren't completely asinine this time! Plus I'll be able to spend extra time making sure I have good fit-up, etc. Good fit up is the difference between a joint taking 10 minutes, and over an hour!
Rigidity and safety are at the top of my list. I'll take a weight knock for both. Rigidity shouldn't really cost much in weight if I design it clever enough, and given that this will be going faster than the 60mph of the Silverdome parking lot, I want something safe.
Wheels
13" bead diameter obviously. I'd really like to get single-piece wheels for rigidity, but that may be tough to get a 13x10" and 13x8" single piece with the dimensions I want. If anything I may have to go with a BBS 3-piece, with Jongbloed as an alternative. Kodiak was impossible to deal with and the Keizer may as well have been made out of cardboard. Camber and toe compliance otherwise are going to cost a heap of grip and perceived response linearity.
Brakes
Disc brakes on all corners. Beyond that I'm pretty open to sizing. I've sent notes out to Wilwood and AP for ideas regarding rotor, caliper bore, and master cylinder sizing. Since this is basically a double-weight FSAE car and since I'm not a brake expert, my initial thought is to run small 4-pistons up front, and 2-pistons rear. I may be able to get away with 2-pistons up front, but then again I don't want to skimp on brakes.
It's likely the front rotors will be ventilated. Rears may be solid disc. I had initially wanted to run inboard brakes on the rear to drop some of the unsprung mass and help with any TLV to get extra grip out of the corners. I may still do that. Cooling ability will be the deciding factor. Easier to get ducts to work when outboard and away from the exhaust...
Tires
Couple options here. I'm tempted to run Goodyear FM tires. They have bias tires in R160 up to R600 or so, and radials in R250 and R430. R250 in itself isn't as grippy as the R160 but the radial construction may pay off in terms of footprint efficiency, wear, and rolling resistance. Goodyear doesn't have an awful lot appropriately sized in 13" bead diameter. One other option would be to mount up the D2692 FSAE slick on a 8" wide wheel front and rear. They aren't particularly responsive, though maybe bumping the inflation up to 18-20psi would stiffen the carcass up. That compound on this type of car would probably be ridiculously sticky. That may be a good one in the bag o' tricks.
The other option is to run one of the zillion Hoosier tires available in the appropriate size, in R25 or R35. They even have a 13" radial. We'll see what Jeff Speer @ Hoosier has to say when he gets back to me.
Suspension
Pushrod-actuated double a-arm front and rear. I have some interesting ideas regarding the kinematics. On the front I'll take a look at running a negative FVSAL, putting the instant centers outboard of the respective wheels. The idea being I can run a lot of static negative camber for the radial tires, but on the brakes the tires will go more positive toward 0 camber and give me a heap of braking grip. On-throttle the tires will go more negative camber which may help get an extra bit out of the outside tire, but I'm not sure. I wonder if flipping the IC's flips the jacking effect... ie you wouldn't have jacking on the sprung mass unless the roll axis was under the ground there. Who knows.
The Cane Creek Double Barrel is probably perfect in this application if I want to use close to a 1:1 motion ratio. Looks lighter and more compact than the Ohlins on the Stohr car, and hopefully isn't as expensive while still giving 4-way adjustability.
Allright that's a lot. Until next time...
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