What Happens to Roll Centers When You Lower a Toyota MR2 Turbo | Project Toyota MR2

Update by J.G. Pasterjak to the Toyota MR2 Turbo project car
Jan 15, 2021

Shoutout to the SCCA for adding a major national-level Solo event at our official test track, the Florida International Rally & Motorsport Park in Keystone Heights, Florida. Wait, what? It’s in three weeks? Man the wrenches!

Our original plan was to install and sort a new suspension in our 1991 Toyota MR2 Turbo before the mid-March event that usually kicks off the national Solo season, the annual Tire Rack Dixie Tour at South Georgia Motorsports Park. But just last week, the SCCA announced that Pro Solo 1 event—which brings the drag-race starts of Pro Solo to a single-course autocross format—at the FIRM for February 12-13. So our build schedule got compressed a bit. Hopefully we can have a somewhat sorted ride ready for XS-A competition in just a few weeks.

Any suspension upgrade should start with a basic look at what you're trying to accomplish and what problems you're trying to solve. With an SW20-chassis Toyota MR2, those problems are manifold, and the path to fixing them can be a little more complicated than just applying traditional methods.

We know we want to increase spring rate, adjust corner weights and control body motion. So a set of coil-over shocks is the easy solution there, right? Well, yes, but it’s only part of the equation.

For coil-overs, we chose the Variant 3 kit from KW Suspension. All V3 shocks are hand-built at the KW factory in Germany and are independently adjustable for both rebound and compression. The threaded shock bodies on the kit, which retails for $2289, also provide easy adjustment of ride heights for proper corner balance.

The KW V3 kit for the MR2 is designed to work with the OEM upper mounts as well, meaning you have fewer potential points of rattling and clunking than with some coil-overs. We’ll be using a bit more aggressive upper mounting solution for our application, and we’ll get into the details of that in future updates.

Once the KWs are in place, we’ll have better control over body roll due to the increased spring rates (good), better control of those springs through superior damping technology (good), independent adjustment of that damping for specific balance fine-tuning (good), and reduced ride height to lower the center of mass of the car (not necessarily good). Wait, what? Lowering is bad? Do explain, Mr. Car Magazine Smart Guy.

Actually, we’ll defer to an MR2 suspension expert, Alex Wilhelm of Wilhelm Raceworks, to explain the finer aspects of the SW20’s fairly sketchy suspension dynamics.

Let’s look at a few charts that Alex has produced through physical and virtual modeling of the MR2’s suspension.

Chart 1 shows the toe change in the rear of our MR2 when the suspension extends or compresses. With 2 inches of motion, the inside-rear wheel has toed out—turned into the corner—almost 0.2 inch. Meanwhile, the outside compressed wheel has toed in—also pointing toward the corner—a similar amount.

This toe change can produce a “roll understeer” effect where the rear tries too hard to follow the front—right up to the point where all the tires are overwhelmed, physics takes over, and the rear breaks loose unexpectedly.

Toyota addressed this weirdness in the 1993-and-up MR2s, as we can see in Chart 2. Dynamic toe change is still present in compression, but the extension toe change has been mitigated somewhat. Still, it’s not great.

Now let’s look at the front. In Chart 3, we can see that the front also suffers from some dynamic toe changes, with the inside wheel going to toe-in and the outside wheel going to toe-out during cornering. With just 2 inches of compression, total toe-out increases by nearly a quarter of an inch.

Next, let's take a look at what happens to these dynamic tendencies when the car is lowered 1.5 inches, which is the minimum amount that the KW kit affects the car. In Chart 4, that the toe-in tendency in the rear under compression has been mitigated by the slight lowering, but the toe-out under droop is almost unchanged.

Chart 5 shows similar tendencies. Dynamic toe change under compression has been slightly mitigated, while droop response is largely the same.

Can we conclude that lowering the car has little—or maybe a slightly positive—effect on dynamic toe? Well, not so fast, because there’s another secondary effect that changes everything.

In stock form, the roll centers of both suspensions—the points in space that the suspension allows the car to rotate around—sit a few inches above ground and a few inches below the center of mass for the front and rear ends.

When the car is lowered just 1.5 inches, the front roll center dives fully below ground, while the rear one moves down to just above ground level. This greatly increases the distance between the roll centers and the centers of mass. The additional distance creates more leverage, which greatly increases body roll. As a result, any minor gains in dynamic toe change made through lowering alone are instantly erased by increased body roll that aggravates the same dynamic toe situation.

Bottom line: We’re making a big improvement in potential by upgrading to a quality set of double-adjustable KW Variant 3 coil-overs, but to fully realize that upgrade, we’ll need to make some other changes to mitigate the negative second-order effects of lowering.

But that’s a discussion for the next update.

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