Suspension Design

With uprights and a good deal of my running gear selected, I need to come up with a suspension geometry for the racer. With the Miata in my garage and using Miata uprights, baselining the Miata suspension as a reference became my priority. Taking a look at my Miata page:

http://www.cardomain.com/ride/796587/6

The plots show the trends and behavior of the Miata suspension. I was surprised by how little body roll was required to drive the outside wheel's camber to zero. Looking at the geometry of the system though, it makes sense.

Now that I have a good basic tool for suspension analysis, I can refine the system and begin developing my racer suspension. I'm going to plan on keeping things simple for the moment with a traditional double wishbone layout. I may add some anti-dive and anti-squat geometry to give a more stable platform for aero mods, and in the longer term, I might consider trying a camber compensation system like that on the Dax Rush. The suspension will be set up though from the beginning to take advantage of a third spring arrangement. Third spring arrangements have appeared on a variety of Le Mans prototypes and on the Corvette C6 racecars. The third spring works to moderate ride height but does not significantly affect roll response. In that way you can seperate ride height control to some degree from roll stiffness. The assumption with the system is that more ride height stiffness is required than roll stiffness.

Using the tools now available, I have roughed-in a suspension layout with unequal length wishbones to ratchet in camber as roll angle of the car increases. The camber versus roll gain rate will be fairly aggressive by road car standards, but it will not be exceptionally aggressive compared to race car suspensions. It will be about 50% more aggressive than the camber change in the Miata. The upper wishbones angle up noticeably as they climb to reach the top of the uprights.

Detail development will wait for better definition of the suspension attachment frames and for availability of the suspension uprights themselves.

Another detail item that I will watch in the design is the width of the front tires compared to Miata tires. The uprights were designed with castor and kingpin angles appropriate to a Miata. Changing the width of the tires a great deal will change the behavior of the nose under load.

I can feel in my Miata an increased sensetivity to bump steer and a less neutral steering feel when the car wears tires that are 205mm wide rather than 185mm wide like stock. You can feel the crown of the road very noticeably. 205mm width still allows good car control and the Azenis defenitely grip better than more pedestrian stock tires, but the steering is noticeably affected. Unfortunately, the maximum performance street tire sizes often don't include 185s and if they do, the tires are often speed limited because it is expected that they will be fitted to a Miata that will not exceed 130 mph. 195 may be the best I can get in an aggressive street tire to fit a set of 15" wheels.

Parts

The uprights and hardware from the Miata have begun appearing in a large number of custom cars. The parts are relatively lightweight and there is a large and affordable supply. The only other donor that might suit this job better is the Toyota MR-2. Being mid-engine, that car's supension parts might have a little better geometry for this layout. Finding spares though would be a nuissance so I took the easy way out. When I want the perfect upright, I'll go back in and design it from scratch. Until then, cheap and easy is good.

You can see that the uprights are fairly simple cast iron parts. It's overkill for the loads, but it is my life that's riding on the parts. The parts are shown assembled with the brakes and wheels in proper position below.

Lower Ball joints up front are from a 1992 Ford Escort. The mounting method is simpler than the Miata ball joint. The upper ball joint and the tie rod attach both have the same taper. The Miata upper ball joint is an integral par tof the a-arm, so it's not really an option. The upper ball joint taper in the upright matches the tie rod taper and they both fit a 1993 240SX tie rod as shown. The Miata tie rod might still work best for the steering but it has a kink by the spherical that would be weak for use as an upper ball joint. The Miata tie rod also costs five times as much as a 240SX tie rod at autozone.