Brakes - Design Work
Although it may seem from the previous page like the brakes issue is covered (all Miata), it's not quite there. The Miata is renowned for its great handling with forgiving behavior largely due to its good suspension and 50/50 weight distribution. My formula car though is mid-engined in layout. This places a great deal more weight on the rear wheels. The Lotus Elise, the current icon for good handling, has a 39/61 weight bias. I'm planning to go for a 40/60 weight split.
This has several beneficial design impacts. First, the extra weight on the rears will improve traction for the drive wheels. This will allow better acceleration. Second, the extra weight in back will make it possible for the rear wheels to carry a greater amount of braking load without locking up. This allows shorter stopping distances. Looking at the Miata and the Lotus Exige (hardtop Elise) as baselines, the Miata has a 73/27 front to rear braking power bias versus the Exige with a 67/33 brake power split.
I figure then that I need to move more of the car's total braking power to the rears. If I use all '92 Miata brake parts though I won't get there without seperate master cylinders front and rear. Looking at later model Miata parts, using larger '94+ rear rotors with the small '92 fronts brings the bias a small step the correct direction, 71/29 result. Using the large '01 and later sport package rear rotors, I can get 69/31 without any custom fabrication and that's probably as far as I can safely go without either ABS or a lot of time devoted to testing the brakes. If I can spare the cash to spring for the parts, the sport brakes will handle the physical bias of the system at maximum capacity. Several people have recommended adjustable proportioning valves for fine tuning the system then. This will allow me to control the onset rate of braking between the front and rear to prevent premature rear lock-up. I've seen at least one simple proportioning valve for under $45.
After a bit of additional reading and some questions on a Formula SAE forum, I find another direction to pursue for brake bias. Like uprights and brake systems, pedal assemblies for the driver are very complicated devices that require attention to detail and precision. In other words, it would be a lot easier to buy a pedal assembly than design one from scratch. Looking at Wilwood and Tilton's sites (Wilwood Pedals,Tilton Pedals), I find a variety of options. My first choice at the moment would be the Tilton floor-mounted assembly with three pedals and mounts for three master cylinders. One master cylinder would handle the clutch while the other two would make a front and a rear braking circuit for the racer. If I use a smaller diameter master cylinder for the rear brakes, I can increase the braking bias to the rear of the car without using larger and heavier brake discs in back. In addition, the pedal assembly includes a balance bar that can be used to adjust bias from front to rear and back as desired. This solution would allow me to select my basic front to rear bias and adjust to fine-tune when needed. A proportioning valve may still be a decent idea with this arrangement to avoid spikes if the rear master engages quickly at initial application, but there are probably some areas in the details where I just need to try things one step at a time and learn.
Brakes Chosen
The simplest solution after the latest look at hardware available and prices is to use the stock '92 rotors all around and use a smaller bore rear master to increase the rear bias. Wilwood's brake and clutch pedals are much more affordable than the tilton parts, so I will go that way. $500 should get the pedal assemblies that I need and a hundred dollars ought to get the raw materials to weld an adjustable mounting bracket. I'll be designing the car to allow drivers much smaller than myself while larger drivers will have to work to fold themselves into the car. To be fair though, the cabin is four inches wider than most competition formula cars and the seating is actually generous if you sit fully upright.
These pictures show the '92 Miata's stock rotors. The fronts are larger and vented. My increased rear bias will put higher thermal loads on the rears, but this car should have a vast excess of braking power until relatively high speeds.
Steering
A good solution for steering essentially fell in my lap. Asking about steering racks on the sports racer forums, I was pointed to their resource site with a good reference page on currently available steering racks. A quick look and comments confirmed that I'd be looking at $700 for a new steering rack made for a car such as mine. Lower cost units from straight-line cars could be as low as $250 but looked rather marginal for something that I would need to trust in a 1.5G corner at over 100 mph. Used on the other hand could be ver thrifty. On several positive reviews and a $50 purchase price, I got a 1972 Porsche 914 steering rack from ebay. Shipping brought the total to $67, an order of magnitude more affordable than a catalog item. It's a beautiful and smooth turning part. Thank you for the suggestions. There is a cutaway of the unit I bought at the link below:
Tom's Steering Rack Page
Suspension A-Arms
Also in the mystery category is how I'll hinge the wishbones front and rear at the upright. Spherical rod ends are my first choice for motion and adjustment. These may be mounted into the a-arms or the chassis frame depending on what will offer the best adjustability and ease of construction. The arms themselves will be custom welded parts once I get my welder. The Miata's arms are very beefy. I'd actually like my suspension arms to collapse in the case of a crash rather than transferring all the load into the frame.
The plan is to use pushrods and a rocker to feed suspension input into inboard mounted coilovers. I'm not really sure that it helps handling to do things this way, but it does reduce drag and I'll have the only car on the block with inboard coilovers. I'm also planning to design the setup with a third spring arrangement in mind to help moderate ride height changes.
Weight Control
If you've read everything to this point, I'm obviously making some compromises in the interest of speed and cost of construction. Down the line I'd like to find ways to reduce weight. Assuming 900 pounds empty and roughly 1100 laden, shaving 11 pounds is one percent improvement in driving weight.
I think the brakes can be shaved for 10 pounds per corner if I switch to motorcycle brakes. The Suzuki Hayabusa looks like a good donor bike. The front calipers have three pistons and I can use the lightweight discs. The main concern is whether the bike rotors would have enough heat capacity to bleed off 150 mph on a vehicle with double the weight and a low CG so the driver can really lean on the brakes (without going over the handlebars).
Replacing the uprights, hubs, and wheels of the Miata with custom parts could probably drop 15-20 pounds. If the wheels drop down to either 13" or 10" diameter, some additional savings might be made in the tires. If I'm optimistic that the corners drop a total of 35 pounds each, 140 pounds of weight savings would be a 12.7 percent improvement.
Other than the corners though, I'm having a hard time finding weight to shave. When Gordon Murray designed the Rocket, he targeted 775 pounds empty. The car grew to 815 pounds largely due to adding a second seat he indicated. The car cost $60,000 though in rough terms and I'm shooting for an order of magnitude lower cost. I'll be very happy and surprised if I slip in under 900 pounds.
I know that I could shave some weight in my overkill frame design, but that's safety margin in a crash and general stiffness while the car is up and running. I'm really going to downsize my triangulation beams though to save there. I have a couple friends also encouraging that I use aluminum tubing for the frame. I'm just not confident at this point that I'll be good enough to make a safe frame of welded aluminum.
When I get a little farther in suspension and frame design, I'll be able to re-approach my overall weight estimates to narrow in on reality. Amazing how it all adds up.
