Apr 19, 2006 update to the Mazda STS2 Miata project car

Dialing In Suspension - Pt 2

With our car properly prepped and a test plan in place, we headed off to Mineral Wells. First we setup a skidpad on a relatively flat and clean surface with a radius of about 100’. We’ll use that to establish steady state performance (“mid-corner”) for average sized sweepers. We also set up our standard multi-lap practice/test course that we used in our previous tire testing, although the cones are not in exactly the same place so comparisons between the two would be invalid. We’ll use the slalom and offset runs by themselves to fine tune transitional handling and the full course to see how it all works together.

Skid pad testing is very tricky to get meaningful data. Our process starts with three laps in one direction, then three laps the other, and finally, a full set of 3-5 laps in the original direction. The first two sets are to warm the tires, while the last are the data collection. Once the car finishes the last laps, temps are taken across the tread surface starting at the outside (most critical) and moving in. Front first (again, most critical) and then rear. Only one side is done since the inside tire temps are not that useful. Also, we don’t take a set of runs going the other way without a complete cool-down and warm-up phase again. That’s because tires that have been run hard on the inside will show misleading temps when immediately run on the outside. The temps will reflect some of their inside temp variation without a cool-down cycle.

Another key point is to find where the appropriate outside temp should be taken. Experimentation with the probe will let you find the “hot spot” area. And you should be consistent in using that area as you collect data. Note also that the hot spot can move when you make camber changes, so plan accordingly. And speaking of probes, you really should be using a probe type pyrometer. Infrared surface guages do not properly measure what is happening down in the belt area. And the latter is what you really want to know.

If we are making small changes between runs, we skip the warm-up laps. Sometimes we are even looking specifically for what happens when the tires get really hot. We also collect lap times, in addition to temps. Even temps are a wonderful thing, but in the end, only the clock really counts. In fact, driver impressions are also important as to whether the car is understeering/oversteering, or relatively neutral. For a FWD car, we are looking to get as close to neutral as possible. For RWD, we want a slight push (understeer) so that power can be applied early exiting a turn without totally upsetting the balance.

Our goals on the skidpad are to first establish ballpark tire pressures, then look at camber and finally, sway bar balance.

Our first set of laps at 36 lbs all around yield the following data:

129/123/103 Front: (outside/middle/inside) 106/103/103 Rear: (outside/middle/inside) Ooops! No info on the timer. Somebody forgot to connect the lights to the computer!! Try again.

126/119/113 114/114/110 11.358, 11.471, 11.520, 11.481, 11.366, total = 57.196

Note that the front temps are higher than the rear. The car is definitely “pushing” in feel and the temps show this. The rears are pretty well-balanced so we’ll leave that pressure alone for now. The fronts are a bit hot on the outside, so we’ll try playing with air pressure and camber. We set the fronts to 42 lbs and get this:

128/119/108 105/96/94 11.330, 11.367, 11.434, 11.424, 11.385, total = 56.940

These temps look a bit worse, but the times are right in line. So more air seems not to help. Let’s try lower next, at 34 lbs in front.

124/116/109 106/100/97 11.365, 11.395, 11.484, 11.443, 11.528, total = 57.215

Not much change here, either. Let’s go even lower at 30 lbs in front.

127/118/113 110/106/105 11.696, 11.864, 12.026, 11.967, 11.947, total = 59.500

Ok, now that’s a change! It seems like these tires are not that sensitive to pressure above a certain threshold, but below that they fall off like a rock. We’ll settle on 35 psi as a final setting.

Next, we’ll try our “max camber” setting for the front to try and bring down that outside temp and work the tire more evenly. As we mentioned in Part 1, we know that a simple turn of one adjuster will give us this setting, but we forgot to figure out how many turns to adjust the tie-rods to reset the toe once the camber has been reset. In the field, on tires that don’t have longitudinal grooves, the easiest way to do this is to scribe a line around each tire and measure the distance between them with a tape measure. Make sure the tape is straight and not hitting some other body part. We measure, adjust the camber, and then alter the toe to get the toe measurement back where it was. Back on the skid pad we get this:

121/123/113 100/102/92 11.297, 11.380, 11.342, 11.373, 11.323, total = 56.715

Hmmm…temps are a bit better and the times are marginally better. Let’s repeat this one to see for sure.

137/131/124 116/113/103 11.398, 11.532, 11.494, 11.439, 11.508, total = 57.371

Temps still look good, but times have dropped off. Maybe too hot now? Clearly, the extra camber did not net better skidpad times, even though the temps were a bit better.

Let’s move on next to sway bars. In addition to our stock rear sway bar(12mm) with rubber bushings, we also brought with us the 2-hole adjustable Racing Beat 5/8” bar set in urethane. We install that and set it on the softer setting.

121/121/115 106/103/94 11.470, 11.617, 11.638, 11.520, 11.345, total = 57.590

We are expecting a big difference in feel from this one, but are disappointed with almost no change in results. So we do a repeat:

139/142/130 121/120/113 11.417, 11.326, 11.272, 11.312, 11.392, total = 56.719

Well, that’s better. Temps definitely look improved as the differential between front and rear is now much smaller. However, the times are no better than our previous best. In fact, going all the way back to our initial setup, our actual times aren’t significantly faster. The only solid fact we have established is that the tires don’t like to go much below 34 psi. It seems like something is overshadowing our test variables. Time to move on to the full course and see if we can gain any more insight there.

Our first set of laps is in the same configuration as our last skid pad laps and we put down the following:

26.540, 26.147, 26.140, 26.231, 26.407, total = 131.465

The car was quite loose in the slalom, which is what we have experienced before running that rear sway bar on anything other than a hi-grip surface. So, for our next set of passes, we crank up the front shock compression to full stiff to see if we can balance out the transitional response.

25.997, 26.066, 26.315, 26.038, 25.954, total = 130.370

Car is now better balanced in the slalom, but still a bit twitchy. Let’s try going back to the stock rear bar.

26.253, 25.852, 25.960, 25.777, 25.492, total = 129.334

While a bit pushy in the two sweepers, the car is now much more stable in the transitions giving better overall times.

Next we try going back to our initial camber setting to see if there is any change on the whole course.

26.193, 25.810, 25.690, 25.881, 25.999, total = 129.573

Not much difference in time, though we notice that our braking has less chance of locking up the inside front tire.

With time running out, we did one final test and that was to slap on our venerable RT-215’s (we had been testing on the 205/40-16 RT-615’s).

26.641, 26.402, 26019, 25.749, 26.000, total = 130.811

The results backed up what we had seen on the Civic a month earlier during a previous test session. The RT-215 was harder to drive consistently fast, yet could yield single laps that were quite fast.

In conclusion, we tried a number of things yet reached only a few conclusions. Perhaps we were already close on a number of variables? Or perhaps some one thing was overshadowing other things? More to come in Part 3…

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