Mar 22, 2006 update to the Mazda STS2 Miata project car

Brakes

In our initial national competition outing in California, we noted a terrible braking issue. In the wet, the brakes were almost unusable since the fronts would lock up with very little pressure, and were difficult to modulate. This was also an issue in the dry, though not as severe.

A rainy Saturday at home gave us the perfect opportunity to ignore the yardwork and spend some quality time in the garage. The first thing to check out was wether or not our brakes were actually functioning as designed, and then we’d look into additional modifications to improve performance. Too often people dive straight into mods when, in fact, there is some fundamental maintenance problem.

So we pulled all four wheels and used hand pressure to move the calipers back and forth on the sliders. Some squeaking ensued. Hah! A possible problem due to drag. I guess that what happens when the car sits on jackstands for three years. Each brake assembly was then taken apart, caliper slides lubed, pad slides cleaned and lubed, and everything carefully reassembled with special attention paid to the pad springs and clips. Its also a good idea to work the pistons back and forth some to dislodge any dirt or rust that has gotten past the boot.

As we took apart the the final brake assembly (left rear), it all came apart in our hands! Turns out that the retaining tabs for the pad sliders had broken at some point and the sliders had relocated themselves in such a way as to keep the pads from functioning properly. In fact, the pads on this side looked very new in thickness, while those on the other side were half gone. Oops! I wonder how long that has been going on?

New clips and pad sliders were sourced from our good friend and DP Miata competitor Steve Hudson’s large stash of take-off parts and everything was reassembled. Special care was taken at the rear to properly set the pre-load on the piston. This can be accessed by removing a plug at the rear of the caliper and inserting an allen wrench. The manual specs it at 1/3 of a turn from first contact. Don’t go too tight or the brakes will operate by themselves when they get hot and everything expands! (Note: This same adjuster is used to pull the piston back to allow room for fresh pads. Never use a c-clamp for this purpose or you will ruin the caliper. Ask me how I know…)

Since it was still wet out, a quick test drive was in order. Hmmmm…much improved performance! And use of the e-brake gives very even side-to-side performance from the rear brakes. Well, right until the point where you lock them both up and the car goes sideways. Don’t try this at home, kids!

Properly lubed up, adjusted and working correctly, the brakes were now much easier to modulate. But the fronts were still doing too much work compared to the rears. Time to attempt some mods to fix that.

Early Miatas have a two-stage proportioning valve that sends most of the brake pressure to the front when you exceed a moderate amount of pedal. There is a chart in the service manual which diagrams this. For the street, this is a Good Thing, since it causes any panic stops to lock-up the front wheels only, keeping the car from fishtailing. For racing, when exceeding moderate braking is common, its a disaster. In CSP, where brake proportioning valves are unlimited, we merely plumbed it out with a pair of brass flared unions. This is not legal in Street Touring, though, so we’ll have to look elsewhere.

The other mod we had done for CSP, was installation of Carbotech’s carbon-kelated pads in the 913 compound. This, combined with the prop valve removal, gave very balanced braking that was aggressive yet easy to modulate. In STS2 trim though, we transfer less weight forward because the real street tires grip way less than the short, fat Hoosiers. In fact, we probably generate less forward weight transfer now than the car did in Stock category trim way back when. We need to get the rears to do more work to deal with all the extra weight they now carry under braking.

Since we are limited in STS2 to Stock-legal brake mods, we can only alter the pads. Being the pack-rat that I am, I find the original pads that came on the car stuffed in a box in the attic above the garage. These are basic cheap no-name parts store organic pads. Using these in the front only will give even better modulation and provide less coefficient of friction relative to the rear Carbotech’s. Installation is a breeze and in no time we are out testing again.

The very first stop is disappointing, but subsequent stops are very nice indeed! Mixing pads is a bit of a black art since the friction produced by each compound changes as the pad heats up. In our case, it is clear that the rears need a bit of temperature for them to give the the proper balance. In fact, we notice that the rears start to squeak when they get properly warmed up.

The next day was a regional event in San Antonio in mixed weather (wet morning, dry afternoon) so we double-entered to run in both conditions. In the wet the brakes were incredible once warmed up. Totally “night and day” compared to what we experienced in Fontana. In the dry, they were also much better balanced but now required a bit more pedal pressure. Nothing drastic, but not the lightning quick response we had before. Still, they were better balanced, easier to modulate, and similar in feel to the STS Civic we also drive.

One note on brake -warming: All too often we see people moving from grid to stage, surging along on and off the gas/brake to warm their brakes. This is much easier to accomplish by just applying constant pressure to the brake (left foot) and using the right foot to work the gas pedal against the brakes to move the car forward. You’ll easily get more heat, and you’ll won’t risk the ire of the Safety Stewards or run into someone. In our case, since we are just looking for heat in the rear, we just set the parking brake up a couple of clicks and drive to the line.

So, now we have decent brakes again. In the future, we will likely try some other compound mixes to see if we can improve even more. We’ll likely try the products from Hawk since they support Pro Solo. And those of you with ABS, we salute you!

Next up this week is a quick corner weight session and baseline alignment as we prepare for an all-out suspension dial-in session this weekend.

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