How to prepare a race car for a test day | Project LS-Swapped 350Z

Tom
Update by Tom Suddard to the Nissan 350Z project car
Sep 28, 2021

Finally, our race car was done. After all those late nights, early mornings and ruined weekends, we finally lowered our LS-swapped Nissan 350Z off the lift and onto a set of used Pirelli slicks–BimmerWorld take-offs–in preparation for our first test day. 

But then a realization hit us: We have no idea what we’re doing. Sure, we’ve been attending test days since before we could walk, and we’re well versed in testing a car for reliability and repeatability. But we weren’t just testing for those things with our 350Z. We’d be testing for competition and building a setup notebook, and that’s a skill set we haven’t yet fully developed.

How to Find a Suspension Expert

Fortunately, there are people who have. They’re called race engineers, and they have names like “Andy Hollis.” You probably know him as the expert behind more than a decade of scientific tire testing in the pages of GRM. What you might not know is that when Andy isn’t winning his own races, he’s in the paddock acting as an engineer to help pros like Tom O’Gorman win theirs, too.  

[Ultimate track tire guide | 200tw, 100tw, street-legal track and R-comps]

That’s why Andy was the first person we called as our 350Z neared completion. “Oh my god, we just finished building one of the fastest cars that we’ve ever built, with more adjustments than we can count, on tires we’ve never messed with. How do we sort out the chassis and make it fast?!”

Andy agreed to take us under his wing and show us the basics, but with a disclaimer: “We aren’t going to make the car fast,” he said. “Instead, we’re going to give you a working understanding of what each adjustment does to the car as well as how they affect each other, so you’ll able to adjust the car and make it fast wherever you’re racing.”

Talk about a paradigm shift. We always knew it was a mistake to ask for the “fast” setup for any particular car, but we didn’t realize that truth ran so deep. Andy explained that the fast racers don’t just have the best parts: They have the best parts and know how to use them.

We’ve spent extra time and money every step of the way to add adjustability to our 350Z’s suspension, and now it was time to learn how to harness that power. Our MCS shocks, for example, weren’t just expensive because they’re shiny, but because their adjustment knobs function much more linearly than lower-cost options. That meant we’d be able to tune the chassis more precisely. Same for all those SPC control arms, which replaced fixed-length stock parts with adjustable ones at every opportunity.

[Big brakes and adjustable suspension parts for our LS-powered Nissan 350Z]

We knew we’d be turning knobs, but we didn’t really know what we were getting into. That reality hit hard when Andy started assigning homework. He planned a trip to our Florida home in a few weeks’ time, and we needed to have the car ready when he arrived.

What If You Don't Have a Race Engineer?

Nobody named Andy Hollis on speed dial? Don't worry: You can still learn how to do all this stuff at home. The best way we've found is with High Performance Academy, which just launched a new suspension course that covers everything we learned from Andy Hollis and more. The online course is presented as a series of videos broken up by skill, with helpful animations and real racers talking through each step. The course usually costs $149, but you'll get 20% off this–or any other–HPA course with the promo code 1GRM20.

[Learn more about the High Performance Academy Suspension Tuning & Optimization course here.]

How to Tune With Anti-Roll Bars

Our first assignment? Anti-roll bars. Our 350Z was still running a stock front bar and a 370Z rear bar, the latter we added because its extra bend allows it to clear our twin exhaust pipes. What’s wrong with that? They weren’t adjustable. Andy reminded us that anti-roll bars are an important tuning tool, and we’d be leaving time on the table if we weren’t able to adjust them.

[What is an anti-roll bar and how does it work? | Handling Basics]

So we called Progress Technology, a household name in anti-roll bars for the import market for decades. Plus, the company makes every one of its bars in the U.S., meaning we didn’t have to worry about a lack of inventory or quality as the pandemic continues to disrupt our world.

Progress sent over a 35mm tubular front 350Z anti-roll bar ($238) and a 25mm tubular rear 370Z anti-roll bar ($225). Both bars feature three adjustment holes for each end link, meaning we could change the bar’s effective rate quickly and easily. For example, our math says these holes alone allow for a 35% change in effective rate between the softest and firmest setting up front. 

The Progress bars installed without issue, but there was one more point of adjustability we needed to add: anti-roll bar end links. These have a fixed length on almost every stock car ever built, but that design gives up an important point of adjustment for race cars. By installing adjustable end links, we can ensure our anti-roll bars aren’t attempting to twist the chassis at its static ride height during corner-weighting. And once we get more experienced, we can use them as another tuning tool to pre-load the anti-roll bars. We ordered a set of adjustable end links on eBay for about $200.

How to Check for Tire Clearance

Next on the to-do list: tire clearance. Andy’s a pro at making the most out of every second of track time, and he made it clear that the test track isn’t the place to find out a tire rubs. How do you test for tire rub without taking a car on track and pushing it to its limits? Remove all the springs and cycle the suspension to the limits of its travel.

And if that sounds like a lot of work, buckle in, because we’re just getting started. Tasks we once considered major surgery become routine items on the checklist when you play at Andy’s level. Fortunately this test was a success, and a little bit of touchup fender rolling allowed us to get reasonable steering angles without any rubbing, even at full compression.

How to Remove Suspension Bind

We removed the springs and shocks for Andy’s next to-do item, too: Search for suspension bind. To have an easily tunable chassis, it’s important to isolate variables so you’re only making one change at a time. It’s tough to tune spring rates, for example, when the suspension bushings are binding and creating additional spring rate that you can’t control. With the springs and shocks on the bench, we cycled each corner of the car’s suspension through its travel to check for bind.

And, well, the news was mixed. The rear suspension moved smoothly, meaning the only real bind we anticipate is some slight deflection of the lower shock mount bushing.

Up front, though, the news was less great. The design of the rear bushing of the front lower control arm, put plainly, is terrible, bind-inducing and makes us cry.

We’ll eventually need to upgrade this bushing to a spherical bearing, but in the meantime we’ll just keep its additional spring rate in the back of our minds when we find that the front suspension isn’t behaving as expected or desired. This limitation will present an artificially high floor in the effective spring rate we can run up front: Go too soft, and the spring will be overshadowed by the relatively stiff (and unpredictable) bushing instead.

​​​​​​

​How to Check Suspension Geometry

We’d already completely disassembled our suspension and laid beneath the car, pushing it up and down by hand, but that was just the warmup for what came next: figuring out our 350Z’s suspension geometry.

It’s rare for a suspension to move in a single direction at once, as almost every car will experience dynamic camber and toe changes as the wheel moves up and down. If we were going to understand our car and make it fast, we needed to know what else was happening with each change in ride height. We measured toe and camber at various ride heights and consulted a few experts for their own advice, too.

The results? Good news: Our 350Z didn’t have any bad habits, like the huge dynamic toe change that we experienced with our Toyota MR2 project car, for example.

How to Check Motion Ratios

We also needed to figure out the motion ratios for the springs and shocks on our 350Z. The motion ratio is the ratio of shock or spring travel to wheel travel, and it’s important when valving shocks and, more pressingly, choosing spring rates.

Let’s say, hypothetically, that you want an equal spring rate at each end of the car. If the front spring is farther inboard (meaning it has less leverage) than the rear spring, it will have a different motion ratio, and thus a different effective spring rate, than the rear spring. We’d need to compute accurate motion ratios to build a spring rate selection spread sheet.

So how do we do it? There are two basic methods. You can accurately model the suspension in a virtual environment, then do the math on all the various levers and actions until you arrive at a motion ratio Or, well, you can just measure it.

We used a long ruler to measure the corresponding distances between the upper and lower shock mounts as we moved the front wheel through its range of travel in 10mm increments. After repeating the process in the rear for both the spring and shock (we’re not running coil-overs in the rear, meaning our shocks and springs have different motion ratios), we crunched the numbers and punched our motion ratios into the spreadsheet. Front ratio is .67. Rear spring ratio is .68. Rear shock ratio is .97.

How to Check Suspension Travel

Suspension travel is a precious resource for a race car, as every inch of it represents space for a wheel to smoothly and progressively press into the pavement. Reaching a suspension’s limits, either by bottoming out or by fully extending and pulling the wheel off the ground, quickly throws all your tuning out the window and creates a nearly infinite spring rate. This usually breaks traction, makes the car hard to drive, and generally slows you down.

So before we could go further with the 350Z, we needed to figure out what limited its range of travel, and whether or not that range was correct for track duty.

Yeah, the springs needed to come out again. As did the shocks.

We measured suspension travel without shocks or springs, then with the shocks, to reach the following conclusions: Our MCS dampers, with an effective range of travel of about 135mm at each corner, were the piece of our suspension puzzle with the smallest amount of travel, and therefore the piece we’d focus on. By multiplying these numbers by our shock motion ratios, we know we have about 201mm of front wheel travel and 139mm of rear wheel travel to work with.

For many cars, this is where the story would end: You’d figure out your travel, measure its upper and lower limits, and keep those in mind when finding your optimal ride height. In many cases, this means you can’t lower a race car as much as you’d like before bottoming out too frequently to be fast.

But we have a trick up to our sleeve. Our shock travel is independently adjustable from the rest of the suspension’s range. Credit to MCS and DIFtech (who built our top hats front and rear) for this trick: The rear top hats can be flipped upside down to change the location of the spherical upper bearing, while all four top hats have exceedingly thin spherical bearings that only take up a small amount of space on the shock shaft. And because we’re running MCS dampers, we have a ton of threaded area above the shock’s shaft shoulder.

Having trouble visualizing this? Here’s the short version: By mixing and matching spacers and top hat orientation, we’d be able to perfectly match our range of shock travel to the range of suspension travel we expect to use.

What range did we expect to use? We had no idea. But Andy recommended that we start with the front lower control arms and tie-rods level at static ride height, as this should be the position with the smallest amount of bumpsteer. Then we should set the rear of the car slightly higher—maybe about a quarter of an inch at the pinch weld. Andy recommended this based on past experience with high-power, rear-wheel-drive cars, so we figured it was at least a good starting point.

Andy suggested that we allocate two-thirds of our available travel to compression and one third to droop. Once we set the ride height, we measured, did some math, and came up with the “magic” combination of top hat orientation and spacer length to put our shock shafts exactly where we wanted them.

Rather than stack washers or order spacers with the correct dimensions, we made them out of some 304 stainless on our lathe. Don’t have a lathe? Don’t worry. These don’t need to be precision parts, so you can make them with a piece of round stock, a saw and a drill press.

One of the fastest ways to kill most aftermarket shock absorbers is to bottom them out, so we also installed urethane bumpstops on each shock shaft, too. These universal circle track parts from Summit Racing are likely too tall for the 350Z, but we’d rather sneak up on the limit of our compression travel versus slamming into it and potentially damaging a shock during our first session on track.

We also slipped an O-ring around each shock shaft while we were installing bumpstops. By moving these O-rings down before each session, we’ll see how far each corner is compressing and therefore how much travel we’re using.

How to Pick Spring Rates

Spring rates are an incredibly important tuning tool, and we’ll be testing different combinations extensively to find the fastest combination for our 350Z.

But before we could test, we needed a baseline. And the closer that baseline was to ideal, the fewer extra sets of springs we’d need to buy, and the less track time we’d need to spend testing them.

But how do you figure out spring rates for a fresh build like ours? There are two options: Ask everybody else, or do the math yourself. We chose to do both, eager to see if each method arrived at the same result.

First up: advice. We asked DIFtech (makers of our rear spring perch conversion kit), MCS and Bryan Settle, a racer that tracks an LS-swapped 350Z similar to ours. And, well, they each gave us different advice. We did spot a pattern, though: front spring rates around 1000 lbs./in.; rear spring rates a bit softer. At least it’s a start.

Time to up the difficulty: Let’s do some math. Fortunately we had Andy at our side, so we asked “What spring rates should we run?” and grabbed a pencil to write down his decree.

Well, it depends on what ride frequency you’re shooting for.”

Uh… what?

What followed was a very technical explanation by Andy, which we’ll poorly summarize like this. The ride frequency is the natural speed at which the body of the car moves up and down without any shock absorbers, and it’s calculated for each corner by dividing the sprung weight by the wheel rate. Stiffer springs naturally have higher ride frequencies, and this data point is an important tuning tool.

One problem with calculating our ride frequencies: We didn’t know the sprung weight at each corner. We only knew our total weight, which includes both sprung and unsprung, thanks to our corner-weighting scales. To find our sprung weight, we’d need to unbolt everything that wasn’t sprung, weigh it, and subtract that total from the corner weight.

Okay, we know we said to get used to tons and tons of installing and removing parts, but this task was a bridge too far, even for Andy. Instead, he recommended we measure static deflection to compute sprung weight.

So we did.

First, we measured the ride height at rest, then raised the car little by little until the spring just barely came unloaded. The difference between the two is static deflection, and by pairing that ride height change with the rate of the springs currently in the car and our spring motion ratios, we were able to calculate the unsprung weight, and then our sprung weight. Science! All this work culminated in a spreadsheet showing our expected ride frequencies across a range of front and rear spring rates.

We (meaning Andy) had built our spreadsheet. What next? Time to dive deeper into the science of ride frequency, specifically the relationship between the front and rear frequencies. Andy started by explaining the common “flat ride” theory. Picture a car driving down the highway that’s about to hit a bump. The front of the car will hit the bump first, compressing the front suspension, then unloading it. Then the rear of the car will hit the bump, compressing and then unloading the rear suspension.

If you’re driving to work, the goal is for the front and rear suspensions to unload at the same time, producing what’s commonly called a “flat ride.” This is more comfortable, since it means the car won’t change pitch as you’re going down the road. Flat ride generally requires the front suspension to be softer than the rear, since it needs to take longer to spring back to ride height.

This is all fine and dandy for street cars, but race cars have different goals. They also have much better (and more adjustable) shock absorbers. That meant we could use our dampers instead of our springs to control unwanted oscillation, and instead tune spring rates for the most grip and the most ability to put power down. Generally, this means a higher ride frequency in the front than in the rear for a car like our 350Z, or at least an even ride frequency front and rear. Testing will help us figure out which is actually fastest.

So what’s a desirable ride frequency? Andy said we should bracket our tests around “the typical non-aero ‘touring car’ frequency of 1.5-2.5.”

We decided to start with 800 lbs./in. front springs and 500 lbs./in. rear springs. These give us a front ride frequency of 2.14 and a rear frequency of 1.94, meaning the front frequency is nearly 10% higher. We’ll test stiffer and stiffer springs, but our goal is to run as little spring rate as possible without bottoming the car or inducing too much body roll.

Shoutout to Eibach, by the way, for supplying us with an entire trunk full of springs for testing.

How to Determine Alignment

We’d set our ride height, picked springs, and done math. It was time for the last piece of the puzzle: our alignment.

This is another major tuning tool we’ll need to learn how to use, but the first step was picking baseline settings for our first test session. We hooked our $50 Caliper Garage string alignment system onto the car, then dialed in the basic formula for a rear-wheel-drive car: 3 degrees of negative camber up front, 1.5 degrees in the rear, and zero toe all around.

Notice the funny ruler in our hand? That's TrakkRat's new StringStick, which the company calls "the most accurate analog device available for use in stringing alignment on any racecar." Having used it, well, it's awesome. If you've got $139 to spare, it's worth every penny. Otherwise, a ruler will also work.

Is this the perfect setup? Who knows—but it’s where we’re going to start.

Alignment finished, we loaded the car into the trailer and set off for the track. It was time to start testing.

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Comments
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APEowner
APEowner GRM+ Memberand SuperDork
9/27/21 9:13 a.m.

Excellent article! Well done. I'm looking forward to the next installment.

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
9/27/21 10:11 a.m.

Thanks! I'm glad you liked it.

stylngle2003
stylngle2003 GRM+ Memberand Reader
9/29/21 8:29 a.m.

Sweet.  Needed to do this for our 350z this winter anyway.  Thanks for the step by step!  Looking fwd to your results.

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