Grassroots Motorsports

Dialing in Suspension - Pt 1 Apr 17, 2006

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. 

Dialing In Suspension - Pt 2 Apr 19, 2006

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... 

Dialing In the Suspension - Pt 3 Apr 21, 2006

Here's the final installment of our Dial In session at Mineral Wells for the STS2 Miata.

Two days after the main test session, we got a chance to do some more testing. We were holding the actual Evolution Dial In class, and had a small number of students. So there was ample downtime to try a few things here and there in our Miata. Also, one of the participants had brought out an STS Civic that was similar to ours so we'd get a good benchmark on how we were doing in our STS2 Miata.

Here's a sample skid pad run (me driving) in the Civic:

10.979, 11.194, 11.112, 11.073

And in the Miata:

11.405, 11.382, 11.285, 11.368

The bad news was that the Miata was several tenths slower in basic steady-state cornering on an 11-second course! Simply maddening! And the tire temps on the Civic showed a much higher spread front to rear, showing that it was not as well-balanced as the Miata. But the Civic just went round and round with very little steering effort, while the Miata felt heavy.

To eliminate variables, and because the other driver wanted to try out my 16" 615's, we swapped tires and I bolted on his 15" 215's.

11.429, 11.467, 11.254, 11.276, 11.368

No significant change. Problem was in the car, not the tires. Let's try that bigger rear bar again and put it on full stiff this time.

11.418, 11.255, 11.275, 11.233, 11.320

In the afternoon, we get some time to do slalom runs and work on shock settings. Adding compression on both ends makes the car much more predictable and it starts to come alive in transition. Some of the mushiness of the 615's goes away. We play with the rebound settings at various extremes but find no improvement.

Finally, it hits me. We've been driving around the real problem. When pressed hard, the car is loose in the slalom because we are forcing it. It does not want to dive in on the cones. In short, we don't have enough front toe-out!

Front toe can be a wondrous thing. It controls the initial turn-in response by using the inside front tire to steer inside the turn before enough weight transfer occurs to reduce its influence by loading up the outside tire. A little bit of toe-out can go a long way, so we had set it to .125" total toe-out. In our Civic we run .25" total, so we adjust the Miata to that same setting. Instantly, the slalom performance is improved and the car dives in on each cone.

We then take some full course runs and are consistently within three tenths of the Civic. Happy with that, we take the car back over to the skid pad and turn this:

11.210, 11.046, 11.166, 11.141, 11.200

Finally! Now the car goes around just as fast as the Civic. And it has a much better steering feel, easily making the turns.

Unfortunately, our test weekend is now over. In the end, we learned a lot about things that made little difference, and found a few that made a big difference. Now that we are in the ballpark on those latter items, we'll need another session to try some more stuff. We didn't really get enough time to play with shocks after we fixed the front toe, so we'll have to find time to do that. But we are definitely faster now than before the weekend began.

In the future, we still need to work some on ride height, rake, and perhaps some rear alignment settings (toe and camber). 

STS2 Comes of Age May 18, 2006

Newsflash: STS2 car takes the Super Challenge win at Atwater Pro Solo!

In the time since the last update, we've done some major dyno testing of various components (details in another article) and competed in several national events with very good results. We've also had a chance to see how STS2 has continued to evolve as a class across the country. Finally, we've started making event-day adjustments to optimize for various course type and surfaces.

After taking a break to compete successfully in our STS Civic at Walnut Ridge (Tour and Pro), we moved on to Houston and Atwater for the National Tour events in the STS2 Miata. Talk about a contrast! In Houston, the surface was low grip asphalt that punished you for being slightly off-line due to the gravel build-up, and the course featured a good mix of slaloms and sweepers. Atwater, on the other hand, was almost entirely fast sweeping turns on super high grip concrete. The results, though, were the same in one regard. We took a second place at each. In Houston it was a result of several small driving errors that cost us. In Atwater, though, it was failing to get up to speed quick enough on the amazing grip that took its toll. The first day we were 6 tenths out, but only 2 tenths on the second day. Too bad it wasn't a three-day event!

The Atwater Tour was a watershed event for the STS2 cars. With a total of 10 entries, all of the major target cars were represented (CRX, Miata, MR2 and RX7). More importantly, it was the first time that STS2 beat STS at a national event. Class winner Mark Scroggs did it both days, and I did it the second day. Woo-hoo!

We've also started to make adjustments on the car during these events. With increased grip comes increased body roll and the need for more static negative camber to compensate. Due to the big sweeping turns, the rear of the car was coming loose before the front so we wanted to get camber in the rear to stabilize it. The easiest way to do this at the track is to adjust the ride height. A number of years ago we spent time on an alignment rack plotting out the rear camber and toe change curves on our Miata. From our notes, we knew that dropping the ride height would add about a half-degree of negative camber to each side of the car for each .25" lowering of the spring perch. By lowering the car .5" at the shock (about .75" actual ride height) we went from -1.75 camber to -2.75 in the rear.

Dropping the rear height also has two other effects. It has a small effect on toe, moving .05" towards toe-out (total). Normally, this would be in the opposite direction of what we wanted, but we already had .125" total toe-in, so it would not be a problem. Finally, the drop also changed the rake of the car. Rake is the relative ride heights of the car from rear to front. Miatas tend to work best about .5" higher in the rear than in the front. Less than that and the car will not rotate properly. Of course, in our case we were looking for less rotation so less rake worked in our favor as well as the camber.

At the Atwater Pro Solo, the courses were a bit tighter so the car was pushy at the limit. With the improved camber setting of the previous week we decided that the rear could now handle more of the weight transfer. To compensate we went to a larger rear sway bar for the Sunday runs and it really paid off. We had been running the stock 12mm bar set in rubber bushings. Instead we opted for the Racing Beat 5/8" bar, set on the softer of the two settings. This made an immediately noticeable difference with the car now carving the turns instead of floating around. We were able to get on the gas much sooner on corner exit and picked up boatloads of time doing so.

Speaking of the Atwater Pro Solo, it was also a turning point for our project car: its first win! And what a win it was. After once again finishing second in the class competition, we were starting to wonder if this would only ever be a second-place car? But with increased confidence in our demon tweak, we knew there was more time out there. Starting off with a "grudge match" win against the class winner (Mark Scroggs) in the first round, we continued to drop time throughout the Super Challenge, eventually running times that were a half second faster on both courses! The quickness and consistency paid off with an overall victory. Furthermore, those times were once again faster than the STS winner.

STS2 has now shown that it is for real. Next up: Mineral Wells. 

An STS2 win at Mineral Wells! May 22, 2006

We seem to have hit our stride now with the STS2 Miata, taking the class win this past weekend at the Mineral Wells Pro Solo. Competing against last year's class champ in a CRX, we put down runs right from the start that turned out to be untouchable and we once again stayed way ahead of the STS class. We did learn a number of things, so we'll report on those here.

Since we've done much of our testing at this site, we knew exactly what to do on setup. It's a medium grip asphalt on which the car tends more towards oversteer. We also knew the course would be full of transitions where oversteer would be our enemy. So off came the 5/8" Racing Beat rear sway bar and on went the stock 12mm bar. This setup worked perfectly right from the outset.

The lot at Mineral Wells has a bit of a slope to it and the Pro Solo course went straight up the hill. As such, starts were a challenge so as not to roll out of the staging lights. The technique we used was to "heel and toe" with our left foot on both the brake and clutch at the same time. Then, a normal clutch release movement launches the car. Others were using the e-brake instead.

During the practice starts it became obvious that we'd have an advantage over the FWD CRX right from the start. Those cars had major difficulty getting off the line and up the hill, while our RWD Miata nicely transferred that weight to the rear wheels, hooked up and went. In fact, our 200-ft times were consistently about a tenth or two faster than the FWD cars.

One piece of bad news that we encountered is that the Falken RT-615 is not a good tire at all for back-to-back runs in 100 degree heat! Our afternoon runs, both on Saturday in class competition and Sunday in the Super Challenge, were a few tenths slower due to the lack of grip. About halfway through the first run, the tires became greasy. Each succeeding run was slower. Hopefully that kind of heat will not turn up at any more Pro Solos this year.

Next stop is Peru, IN for both the Tour and Pro Solo. We've got an underdrive crank pulley to install before leaving to up the power another notch. Also, we started to put the car on a final diet last week just prior to Mineral Wells. I'll post pics of the "cheap seats" as soon as I get them downloaded. 

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