Apr 17, 2006 update to the Mazda STS2 Miata project car

Dialing in Suspension - Pt 1

These next three installments will chronicle a typical setup dial-in day for our Miata. As manager/instructor of the Dial-In class of the Evolution Performance Driving School, I often help others learn the basics of vehicle dynamics theory and chassis engineering in a practical setting. We teach folks how to isolate variables and be consistent in procedures to gain meaningful data, and also give guidance on what sorts of changes will affect a car in what parts of each corner. We recently taught one such class at Mineral Wells, TX and came in a day early to use the exercises ourselves to work on the Miata’s setup. But the key to a good dial-in is proper pre-test prep.

As you may remember, prior to taking the car to CA for its debut, we had no time for a fresh alignment. We knew that our low-camber, low-caster alignment we had used for CSP would be suboptimal for the real street tires we now used for STS2, but we could do nothing about it. In preparing for our test session, we wanted to make sure of the current settings and also dial in a bit more camber and caster since our experience with the Civic told us these tires respond well to lots of camber.

Prior to the alignment, we put the car on the scales to corner-weight it. This process assures that the relative ratio of weight from front to rear is the same for each side of the car. This will serve to provide the same understeer/oversteer tendencies in both left and right hand corners. This is usually read off of a set of scales as the diagonal %. Spring perch heights are adjusted to achieve the desired effect. Given a choice, we tend to make more of our adjustment at the rear of the vehicle so that our front perches have the same physical range of motion.

There is a never-ending discussion about which to do first, scales or alignment rack, since they each affect the other. Our philosophy is to start with the one that is farthest away from correct and fix it first, then move to the other, and finally go back to the first to fine-tune. In our case, we were fairly certain that the alignment was even, so we put the car on the scales. Initially, it looked perfect with a 50-50 diagonal weight reading. But then we realized that the car was empty and we needed to put in the ballast representing the driver. With the weights in the car the diagonals moved to 51.8-48.2. Not as good and not what we expected since we had all the perch heights the way they were in CSP and we did not think we had changed the weight balance of the car that much with the engine swap.

Just then, we also remembered that the front sway bar had been off the car for the motor work and had been slapped back on without adjusting out the pre-load. Sure enough, it was pre-loaded. Disconnecting one end link put our weights back at 50-50. A few turns on one of the end link rod-ends and we could reinstall the bar without any affect on the weights. Lesson: always disconnect one end of each sway bar before corner-weighting!

Next, we went to visit Bill Kim at Soulspeed Performance again. Bill has an awesome alignment rack that is incredibly accurate and repeatable. Some of that is due to fancy features like a wheel runout detection/correction process, but much of it is due to good care and maintenance. He even has weights at the shop so we don’t need to bring our own ballast.

The initial readings were a real eye-opener! Turns out we had very little camber at all on the left side of the car! Either something had changed since the last alignment three years ago, or that other place’s equipment was faulty. As we started to make some adjustments to fix it all, we noticed the second issue: there was not enough adjustment in the front to get the settings we wanted. In CSP trim we had installed custom offset bushings at both ends of the car to compensate for all the extra static camber the car got when it was lowered a bunch. The rear’s range seemed ok, but we could only get to -1.5 degrees in the front. Not enough. So the car came off the rack and I took it home to swap out the offset bushings and reinstall the Mazdaspeed hard delrin bushings. I also took the opportunity to regrease these bushings to keep them clean and free floating.

Bill made time in his schedule for us at the end of the day since we were testing the following day. We set the front at 2 degrees of negative camber, caster at 4.5 and the toe-out at .125” total. We also checked out a “max-camber” setting of 2.8 degrees, which took the caster to 4.2. This would be a test setting that would be easy to try out at the track by simply rotating the front adjuster cam. One thing we forgot to do was to record how many turns on the tie-rod we needed to reset the toe to compensate. This would add to our workload during our testing but was not a dealbreaker. In the rear, we put 1.75 degrees of camber and .125” total toe-in. This was the max camber setting we could achieve with the offset bushings still installed in the rear. We may pull these out at a future date if more rear camber is needed.

In Part 2, we hit the track.

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