This same article was posted in one of the MR2 groups of which I am a member. A number of people commented that they also had an X1/9 at some point. As many of you know I have had many X1/9s over the years to include both street and race cars. It was the X1/9 that was my first real race engineering project. I learned much from analyzing the X's chassis and suspension. I have mentioned here before that I noted the similarities in the X and the Mk1 MR2 when I bought an MR2 racecar several years ago. One of the things I learned was the minor difference in the MR2's rear suspension geometry that resulted in the reputation for "snap oversteer" that has followed the car throughout its history. I wrote up a quick overview of the finding for that group. It is as follows:
Yes, the X1/9 was launched in 1972 and us deliveries started in 1974. Fully 10 years before the AW11 was launched by Toyota.
First, Japanese manufacturers had a history of copying European and American designs throughout the '60, 70's and '80s. The Datsun Roadster (1600/2000), the 240Z, the MR2 and even the Miata, are just the best known. By the way, the AE86 is a knock off of the Fiat 124 Sport Coupe, if you didn't know. So we have history here.
The AW11 chassis is slightly longer and wider than the X1/9 with the extra width found between the inner control arm pivots.
The two cars' front suspension is nearly identical in both architecture and geometry. The unibody configuration is quite different in the trunk area but the suspension mounts and their designs are nearly identical. The additional track aside the geometry; roll center height, control arm range of motion, strut inclination, scrub radius, caster and camber curves, are all so close that the change in roll center geometry due to the additional track width is the only notable difference. Even the steering rack ratio is nearly the same.
The rear suspension is where the notable differences occur. I can't determine the engineer's thinking as to why the geometry is different. It is typical for a front engine car to have the front roll center below the rear. This is to produce more front mechanical grip to counter the front weight bias and give the car better balance and a more rewarding feeling to the driver by offsetting the basic understeer attitude the configuration produces. This is NOT a good idea in a rear/mid engine car.
In the AW11 (and the SW20 which has a similarly configured roll axis) the rear roll center is slightly higher than the front which tends to promote rotation on corner entry. That is: when driven near its limits, turning into a corner, the higher rear roll center forces the car to rotate more aggressively. It also tends to promote oversteer on corner exit. It has less effect on mid-corner steady-state balance.
The X1/9's rear suspension is a A-frame lower control arm (so no track rod) with the rear toe link attached to the arm. The MR2 is a similar layout to the front; a control arm with a track rod and the toe link attached to the chassis. The layouts are similar in antidive, scrub radius and camber curve but the roll center height is the primary difference.
On the Mk1b, Toyota attempted to correct this resulting handling characteristics by building in additional rear toe-in roll steer (bump steer) and removing the rear swaybar. The changes reduce the Mk1a's rotational characteristics but it was done by dialing in additional additional understeer and not correcting the geometry.
I should note that the Porsche 911 and 914, a whole list of Ferraris and Lamborghinis all have a roll center configuration with the rear roll center below the front.
I have won 4 SCCA National Championships in an X1/9 and two in an AW11. I can tell you that there are few cars that drive better than a well setup X1/9. The chassis is excellent. It is easy to drive at the limits and is both fun and rewarding. It does NOT have the reputation of "snap oversteer" that the AW11 and SW20 have.
I will note that "snap oversteer" isn't caused by a design flaw but is by the driver's lack of skill in being able to read the car's dynamics, drive it within its limits and not cause the car to spin. The MR2 is just a more demanding car that punishes mistakes.
That Toyota's engineer's designed the MR2's geometry as they did could have been a intentional to make the car have a more lively feel but I doubt it. Toyota had no experience with mid-engine or rear engine cars and thus probably didn't understand the reason behind Fiat's choice of rear roll center location. Note that Fiat has a LONG history of building sporting rear engine cars, the 500, 600, 750, 850 and X1/9. All with long, successful racing resumes
As I developed my AW11 racecar I noted the similarities in the two chassis and geometry and realized the deficiencies of the AW11. As a race engineer I immediately began looking to correct the geometry and realized the best solution was to completely redesign the suspension system.
I did little to the design of the front suspension other than to allow for a significantly lower ride height that provided for a roll center location that was just above ground level and to correct the camber curve.
For the rear I made the change to the roll center location to place it just below the front roll center and again corrected the camber curve. Correcting the camber curve is ensuring the camber curve never moves to the positive side of the curve during the full range of motion.
I retained the same basic layout of control arms and track rods and used tubular units with Rod Ends to reduce unsprung weight and eliminate the bushings.
If you haven't seen them before, two racecars:
A current picture of my garage:
Very glad you came out of it OK. The X is a tough little car.