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Project Mazda RX-8 - Adding Grip And Style

Track-Testing BC Racing's BR-Series Coilovers.

David Pratte
Aug 18, 2010

Project RX-8
Last month we upgraded the wheels and tires on our Titanium Gray RX-8, and the results around Toronto Motorsports Park were nothing short of spectacular. We're talking a 4-second reduction in lap times and a significant reduction in understeer, not to mention drastically improved appearance thanks to the more aggressive offset and design of the Volk Racing G2 wheels.

Modp_1010_01_o+project_mazda_RX8+before Photo 2/12   |   Before

To continue the handling and style transformation, we needed to add a proper coilover kit, one that would allow us to independently adjust the ride height and spring preload without reducing shock travel. And that's exactly what we got with BC Racing's affordable BR-series coilover kit, though not without some extra effort to dial-in the spring rates and overall handling balance.

We started the suspension upgrade process by spending a few hours bolting up the BC Racing coilovers. We were impressed by the kit's build quality, given its entry-level pricing of just $1,250. But with dampers all the magic happens inside the monotubes, so we were determined to reserve judgment until after some street and track testing. If the spec sheet is any indication, these BR-series coilovers should offer lots of tuning potential because of their 30-way single-adjustable damping, independent ride height and spring preload adjustment, front pillow-ball mounts and patented concave lower locking rings that prevent any unwanted slipping to the ride height settings. It's also worth noting that the dampers are rebuildable and come with a one-year warranty.

Modp_1010_02_o+project_mazda_RX8+after Photo 3/12   |   After

Having done some homework on what the hot setup for STX (Solo 2) class RX-8s is (as we mentioned in the intro story, we're planning on competing in this class at the Solo Nationals), we opted for 450 lb/in (8 kg/mm) spring rates up front and 280 lb/in (5 kg/mm) spring rates out back. The OEM spring rates are 156 lb/in front and 113 lb/in rear, so we've increased the front rates by 188 percent and the rear rates by 148 percent. We planned to combine these spring rates with adjustable antiroll bars, but we haven't quite narrowed down our choices yet, so we'll report back on this once we install and test whatever bars we go with.

With the BC Racing coilover kit bolted up, we moved on to the tedious but vitally important process of setting the ride height and dialing in the alignment. Having researched what most RX-8 racers consider the lowest possible ride height without overly compromising suspension geometry and shock travel, we set ride height to 13.5 inches all around, measured from the center of the wheel to the top of the fender lip. This drop was a bit on the aggressive side, according to some experienced RX-8 racers on rx8club.com, but we like the way it looks so decided to give it a try. Since the BC coilovers have independent ride height adjustability - meaning we shouldn't lose any shock travel no matter how low we go - we also felt this would be a good way to see just how sensitive the RX-8's suspension geometry is to moderately aggressive lowering (but certainly not super low, as you can see from the pics).

Modp_1010_05_o+project_mazda_RX8+lowered_RX8 Photo 6/12   |   We've shown some restraint when lowering the 8 because going too low would've upset its suspension geometry, not to mention make it less street-friendly.

After that, we dialed in -2.75 degrees of camber and 1/16-inch toe-out on each front wheel and -1.85 degrees of rear camber and 1/16-inch toe-in on each rear wheel. These alignment settings were chosen based on tire temp data collected during the wheel and tire track test. To measure the alignment, we used a digital camber gauge and a string box so that we could measure toe settings at each wheel separately. For a more detailed discussion of DIY wheel alignment techniques, check out this month's Tech Talk on page 14.

Track temperature and weather conditions were basically identical to when we tested the wheel and tire package, so our best lap time of 1:25.82 would be the benchmark to measure the BC suspension and alignment against. On the very first test lap, our RX-8 was now showing a very strong tendency to oversteer, which was fun but definitely not the fastest way around the track. With lap times having only dropped by 0.5 seconds, we had to find a way to tone down the tail wagging.

Modp_1010_06_o+project_mazda_RX8+bump_stop Photo 7/12   |   A zip-tie that's pushed up against the bumpstop is a surefire way to tell you're running into shock travel problems.

Our first thought was that we were getting onto the rear bumpstops and that this was causing a sudden increase in rear spring rate, resulting in the back end stepping out. To see if this was the case, we put zip-ties on each shock's main shaft. As the shock compresses, the zip-tie will be pushed up the shaft, so we can visually inspect to see how far up the shaft the zip-tie traveled as the shock compresses.

We also decided to disconnect the rear antiroll bar, which is as simple as unbolting one of the endlinks. This proved to be a very effective adjustment because the oversteer problem was now more or less gone and the overall balance of the car felt much more neutral. However, the zip-ties on the shock absorber shafts showed that the rear shocks were indeed running out of travel and getting onto the bumpstops. After talking things over with Pete at BC Racing, we decided to trim the bumpstops by 0.75-inch and increase the spring rates to 560 lb/in (10 kg/mm) front and 392 lb/in (7 kg/mm) rear. Higher spring rates will reduce roll and the shock travel required to absorb a given bump, with some loss of ride comfort as the trade-off.

Modp_1010_07_o+project_mazda_RX8+springs Photo 8/12   |   We've addressed this by trimming the bumpstops and installing stiffer springs.

After installing the stiffer spring rates, we hit the track again and the results were much improved. The car felt more stable in every turn, and the sensation that the rear shocks were bottoming out in a few of the bumpier corners was gone. As a result, our best lap time dropped to a 1:24.61, a respectable 1.2 seconds quicker than the stock suspension. This isn't quite as big an improvement as we've seen when testing coilovers on many other cars, but not many cars come with as good a suspension as the stock equipment on a Sport model RX-8 like ours.

We've definitely moved Project RX-8's handling in the right direction thanks to BC Racing's coilover kit, and the ride quality on the street is very livable, even with the stiffer spring rates. However, we're still getting onto the rear bumpstops in places, so we'll likely go to a 8 kg/mm rear spring and a really big front sway bar to help further sort out overall chassis balance and the limited rear shock travel we're working with.

Modp_1010_11_o+project_mazda_RX8+ARK_MFD_digital_display_and_accessories Photo 12/12   |   A few of the goodies we'll be testing in the next Project RX-8 installment.

But before we further sort out the handling, next issue we'll be installing an ARK MFD digital display so we can monitor oil and water temp closely, two critical parameters when it comes to the health of the hot-running Renesis rotary engine. We're also going to ditch the big, heavy OEM battery that clogs up the engine bay and reduces airflow efficiency through the radiator and install a lightweight Odyssey dry cell in the trunk.

Sources

BC Racing
Casselberry, FL
407-637-2941
http://www.bcracing-na.com
By David Pratte
216 Articles

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