First Mods for a C5 Corvette: Wheels, Autocross Tires, Performance Alignment, Tire Trailer | Project Chevrolet C5 Corvette Z06 Part 2

J.G.
Update by J.G. Pasterjak to the Chevrolet Corvette Z06 project car
Mar 16, 2021

Part 2 of the magazine series.

How awesome are Corvette Z06s? You can have lots of fun driving one on track or at an autocross without even bolting on a single high-performance part:

[Announcer voice]

Previously, on Project Z06….

[Smash cut to montage of Corvettes squealing around race tracks, furrowed-browed engineers diligently studying engine components and blueprints, and prototypes being unveiled with a flourish. Faith No More’s “Midlife Crisis” blasts loudly throughout.]

Yeah, that pretty much covers it. If you were paying attention last issue, you learned that Chevy intended C5 Z06 owners to track or autocross their cars with little to no modification. This time around, we’ll explore that option with our own 2004 model. We’ll also discuss the non-performance mods we’ve found essential to creating the true Z06 experience.

Wheely Tired of This Discussion

Okay, our first mod is technically a performance mod. But it’s also a basic maintenance job, one that every sporty car will have to face: new tires.

We wanted a tire that could take on lapping days as well as SCCA Street-class autocross competition. We also wanted it to carry us to and from events if need be. 

The tire we chose for that mission was the BFGoodrich g-Force Rival S. Its 200-treadwear rating makes it legal for any class requiring such a tire, and it’s also well-suited for street use on a car of our Vette’s capabilities. 

After we mounted a set on our Corvette, the Rival S tires did well in their opening act as street rubber. They easily, comfortably handled the 1000-plus-mile trip home, even when faced with some wet and slushy conditions. (We should note that according to Tire Rack, this tire shouldn’t encounter near- or below-freezing temperatures.)

No matter which modern supertire you choose for your C5, there’s going to be an issue: No one really makes a 17-inch-diameter tire that’s wide enough for the front end. 

A stock C5 Z06 comes with 17x9.5-inch front and 18x10.5-inch rear wheels. Base Corvettes have similar diameters but are an inch narrower front and rear. 

When the C5 was new, 17-inch wheels were big, and 18-inch wheels were huge. Now they’re on minivans, and finding appropriate rubber requires a little mixing and matching.

Luckily the SCCA allows some fudging when it comes to wheels in their Street classes. Wheels are allowed to go up or down 1 inch in diameter from stock so long as they retain the same width and don’t vary offset by more than 7mm. This means we could fit 18x9.5-inch front wheels along with 275mm-wide Rival S tires. Had we stuck with 17s up front, we’d be limited to the 245mm Rival S. 

But where to find 18x9.5-inch wheels? Well, how about the rear of a base Corvette? Brilliant.

Our front wheels (top) may not match the backs, but the whole set carries those sticky BFGoodrich g-Force Rival S tires, so they look pretty darn good to us.

If we take a look at the handy chart on the Corvette Central website (check out corvettecentral.com/wheel-offset), we can see that our stock front wheels measure 17x9.5 inches and feature a 54mm offset. Looking down the chart, we learn that the base C5 Corvette came with 18x9.5-inch rear wheels featuring a 57mm offset. Those rear wheels, with a 3mm spacer, would be SCCA-legal for the front of our Z06. 

Now for the bad news: We didn’t have much luck finding 18x9.5-inch wheels that match the existing Z06-style alloys. We went with a set of reproduction alloys from OEWheels that feature the five-spoke design found on the standard C5. They look cool enough when paired with the stock rears that we don’t mind them so much. (OEWheel has just started offering the same style in an 18x10.5-inch size, however, so you can now get a set of matching wheels that are legal for the SCCA Street class.)

Thanks to this mixing and matching, our Z06 now wears a pair of 275/40R18 Rival S tires on 18x9.5-inch wheels up front, along with the 315/30R18 Rival S tires on 18x10.5-inch wheels out back. It’s a solid setup and one that took home more than a few trophies at the SCCA Solo Nationals in 2017, so there’s plenty of precedent for success.

In the midst of this wheel scavenger hunt, we also added a set of 3-inch-long ARP studs. This wicked-strong hardware would provide plenty of threads.

Happy Trailers

Okay, we know that we just said you can drive to the track, have some fun, and then drive home–all on the same set of Rival S tires. And you can. You totally can. But a journey of hundreds or even thousands of miles may warrant some special travel tires–and some support equipment. After all, just one trip over a particularly gnarly pothole or an errant screw can damage your track rubber beyond the point of safety. 

Plus, have you looked at a Corvette’s trunk? It’s not that small–for a sports car–but it still isn’t designed to haul an extra set of tires. 

The answer is usually a tire trailer: a small utility trailer set up to haul track tires, tools and some additional support equipment. It’s a portable paddock that you can easily haul behind your track car.

Many of these trailers are repurposed, low-cost wagons hailing from a ubiquitous lawn and garden center. And you get what you pay for. We’ve had several of these budget trailers–key word being “several” because they never seem to last too long. They do manage to serve their purpose, but the dollar savings aren’t always what they’re cracked up to be once they’re built, outfitted and sorted. And no matter what, they seem to explode at the worst possible moment–like when you’re already running late to a track day. And it’s raining. 

Not surprisingly, there was a call for high-quality, bespoke solutions for transporting tires and gear to the track. The Staggs family, operators of Leroy Engineering, answered that call. “We were getting into autocrossing and doing track events, and we saw that everyone had these little tire trailers, but they were all kind of sketchy,” says Jonathan Staggs, one of Leroy’s principals along with his brothers, Tim and Christopher. 

The Staggs are electrical engineers by training, but they’d been moving their engineering firm more into specialized mechanical engineering. They were adding some manufacturing capacity, too. “When you’re a contract engineer for a living, you only get paid when there are contracts,” Jonathan explains. “Adding a product that we could produce and sell–and in a market that we were interested in–seemed like a good project to take on.” 

The fruit of their efforts was a series of bespoke trailers designed to eliminate the need to tow cross-country with a questionable hardware-store utility trailer. “We wanted to design something that would be a complete solution,” Jonathan adds, “something that the user wouldn’t have to do a lot of custom work on their own to make it work. And we wanted the quality to be at a level that people would never have to worry about reliability or performance.” 

We can confirm that all these goals were hit. While our Leroy Engineering Grid 325 trailer costs a bit more than a kit from a feed store backlot–the model line starts at $1595, with our heavily optioned example retailing closer to $2000–that extra expense is easy to justify just by sight. The trailer is visibly higher in quality and efficiency. 

Independent suspension with adjustable coil-over shocks means a low center of gravity for decreased aerodynamic drag. The low-slung design also makes loading easier and provides better rearward visibility. The trailer’s excellent road manners make it easy to pull and maneuver–even with a small, underpowered car.

Leroy Engineering is committed to its products as well. Customer service during the build and delivery process was topnotch. The company’s continuing support and interest in improving the product has been impressive. 

We actually invented a multi-axis CNC plasma cutter that cuts all the tubes for the trailers, so making design adjustments on the fly is not as complicated as if we had dedicated jigs and tooling,” Jonathan explains. “We think it gives us a real advantage going forward as we develop trailers to haul stuff like karts or other track support configurations.” 

After a couple events with the Grid 325 trailer behind our Corvette, we’ll never go back to a utility wagon again. What may seem costly up front becomes all the more worth it when we remember spending hours on the side of the road while making hasty repairs to our old, crappy trailers.

It may seem like overkill to put a double A-arm suspension with adjustable coil-overs on a tire trailer, but when you feel–or, more accurately, don’t feel–the Grid 325 trailer behind your car, you realize that overkill is the best kind of kill.

Line It Up With a DIY String Alignment

Wait. Before you bolt on that high-performance suspension componentry, don’t miss out on the performance gains that come from maximizing your alignment settings for track or autocross use. There are two ways to go about this. The first option: Take your car to an alignment shop that won’t mind adjusting your settings well outside the factory recommendations. The second option: Do it yourself.

For a proper at-home alignment, you’ll need an accurate, repeatable way to measure the relevant specs. For us, that meant building a string alignment rig.

String alignments have been around as long as racing, and they establish reference points for chassis alignment. A perfectly aligned rectangle is defined around the car with strings,
and those strings can then be used to take–and retake–alignment measurements.

Our alignment settings were 3.0 degrees negative camber up front–pretty much the maximum front camber possible–and 1.8 degrees negative camber in the rear. Was that enough out back? If not, we could dial in a bit more. 

For toe, our Corvette was straight ahead in the rear and just a bit out in the front to aid turn-in–around 3/32 inch of total toe-out. These may not be our final settings, but for now they provide good performance and good feedback.

7 Steps to a String Alignment

1. To begin building a string box around a car, you’ll need a way to suspend the strings and keep them equidistant from each other. We cut notches in a couple of pieces of ¾-inch EMT conduit to properly index our strings.

2. Now, how to mount the conduit? You can suspend the conduit on jack stands, but a more repeatable way involves constructing a rig that hangs from the car itself. We used 1-inch aluminum and connectors sourced from 80/20, a company that makes all kinds of cool fabrication bits and pieces.

3. The goal here is to build a string “box” around the car. The strings should be parallel to each other and the ground, and the same height as the centerline of the wheels.

4. Time to square the strings with the car. Measure from each hub–both front and rear–to the strings and adjust until the distances are equal.

5. To make our rig easier to set up each time, we made some index marks on our supports and string bars. 

6. To determine toe, measure the distance between the leading wheel edge and the string as well as the trailing wheel edge and the string. The difference is the toe setting for that wheel. A smaller distance in the front of the wheel than in the rear means toe-out, and the opposite means toe-in.

7. Now you can adjust the toe–a job that requires getting under the car to access the tie rods. We built some simple cribbing from two-by-sixes and two-by-fours to raise the car off the ground while still keeping the chassis properly loaded. For accurate measurements, these stands must be level; you can level them with thin pieces of plywood, masonite or even vinyl flooring tiles. 

8. To adjust the alignment, your wheels will need to move while the car rests on the stands. We created some slip plates from plastic cutting boards and vinyl tile, then lubricated them with hydraulic oil. We also built some limiters from aluminum channel so the car won’t slide off the stands. Safety first (or at least very high on the list), kids.

Monitoring Our On-Track Progress With Easy Data Acquisition

Our Corvette was now shod in sticky tires, and our specialized trailer was ready to safely transport them to and from events. Now we wanted to see how well the car performed. In other words, time for some data acquisition.

For some motorsports enthusiasts, the words “data acquisition” immediately inspire concerns about spending big bucks to accurately and repeatably gather objective performance data. But there’s really no reason to worry anymore. There are great low-cost options these days, and SoloStorm from Petrel Data may just be our new favorite budget-friendly data gatherer.

SoloStorm was designed by and for autocrossers, but it also adapts well to track situations and multiple laps. It runs on most any modern Android device. We use a $200 Samsung Galaxy Tab E.

SoloStorm can work with the device’s built-in camera, accelerometer and GPS, or you can easily configure it to work with higher-resolution, third-party devices. We connect ours to our Garmin Virb XE camera via Wi-Fi. 

For a more accurate data capture, we use a Bluetooth-equipped $130 Dual Electronics 10Hz GPS receiver. The key regarding the GPS signal is the update rate. Most phones or tablets will offer 1Hz or maybe 4Hz GPS, but you want 10Hz or better. We also get throttle data via a Bluetooth ODBII dongle that’s plugged into our Corvette’s OBD port.

Less than 30 seconds after a run, SoloStorm can overlay tons of data on top of high-resolution video so you can break down your on-track performance on a very fine basis. It’s an exceptional tool for both car setup and driver development.

After each run, SoloStorm can overlay various traces on top of your video. We normally look at throttle, g-loads and vehicle speed. Adding more outboard devices can incorporate more data, like steering angle and brake pressure. You can also watch those traces on a timeline to monitor specific behaviors. 

What makes the package extremely valuable and powerful is its ability to overlay multiple runs. This allows you to compare different laps, different setups or even different drivers. 

We’ve found it to be a valuable tool for both car setup and driver development. The ability to compare two or more runs and examine speeds at specific areas of a course across multiple attempts accelerates the learning process. Think of SoloStorm as an upgrade from mere trial and error to actual point-by-point data analysis.

The SoloStorm software sells for around $200, and you can assemble a complete package that gathers high-quality data and overlays it onto high-resolution video for less than $500. 

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Comments
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captainawesome
captainawesome HalfDork
3/15/21 2:57 p.m.

I really like those slip plates. Gotta fashion something up like that for sure.

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