We wanted to develop the suspension set for the new Supra A9x from the day it was available, but things really started to happen, when we have watched this video: https://www.youtube.com/watch?v=XSH3RgGSmF0&t=288s
Robert from Apex Nürburgring explains the issue with bump steer in the supra and potential problems that it causes in the video above. On the Nürburgring where the car's speed goes up to 270-280 km/h or 175-185 mph, the suspension works in its full travel range. Robert is driving daily the best handling cars in the world like Ferrari 488 Pista, Porsche GT2RS, McLaren 620R, BMW M3, Toyota GT86, and many others. He knows how to drive and based on this we have developed our set in cooperation with Apex.
The full set includes
- Rear Lightweight Tubular Subframe
- Front Caster Adjustable Control Arms
- Front Lower Adjustable Control Arms
- Front Bump Steer Adjustable Tie Rod Ends
- Front Sway Bar End Links
- Rear Adjustable Traction Links
- Rear Upper Adjustable Lateral Straight Links
- Rear Upper Adjustable Lateral Bent Links
- Rear Adjustable Toe Links with a lockout kit
- Rear Spring Wishbone Bushings with lockout kit
- Rear Knuckle Bushings
- Rear Sway Bar End Links
- 66 printed pages of install and suspension setup instructions
Toyota Supra A9X and BMW Z4 G29 were originally designed to provide the best driving experience for normal street use, with a reasonable amount of pushing the car to its limit. That is why it provides factory tail-happy handling to encourage the driver to have some fun and go a little bit sideways from time to time during everyday use. The issues start to arise when we are starting to look in that car for performance, that tail-happy behavior is starting to give problems with instability of the car.
The main cause for that is the rear-end bump steer. In simple words bump steer is telling us how the toe is changing through the suspension travel. In stock Supra bump steer is quite significant. For example, during alignment, you set your rear toe to 0,11° toe in (black center dot on the graph below). This is using stock camber values. Under hard acceleration, Your rear end is going down, around 40mm. Then due to bumping steer your toe is changing to about 0,3° toe in (blue left dot on the graph below). On the other hand under hard braking, your rear suspension is going up, again around 40mm. This time it will give you about -0,1° toe-out (yellow right dot on the graph below).
As you can see, the Supra/Z4 bump steer has a significant impact on the toe during driving. When on alignment you will set your wheels to be toe in, it does not mean that in some circumstances, often difficult to predict you will be driving with rear toe out which is causing the rear end to be very lively. In daily low speeds, it is not so bad, but on track usage or highways, it could be the cause of car major instability.
Another issue with that suspension (5 links especially) is that it has a lot of interdependencies between its properties. It makes it very complicated to modify its setup without deterioration of its kinematics. On the graph below you can find an example of how a change of camber (new alignment setup) by 0,5° can increase bump steer even more (green line – 0,5° increased camber, red line – stock camber setup).
If we would look again at an example of hard braking and rebound of rear suspension by 40mm, we are getting about -0,2° of toe-out (pink dot on graph above) instead of -0,1° for stock setup (yellow dot on graph above).
In some cases like going fast over a crest, you can have up to 80mm of rebound which in that case would give as much as -0,5° of toe out per wheel. This is -1° of total toe out! Such a big change into toe-out is giving relatively a huge chance of losing grip and control of the car.
Graphs above are coming from suspension kinematics software, whereas an input we use suspension geometry which we gather from 3d scans of a real car. A simulation model was also validated by measurements of bump steer and camber gain on the real car. To do so, we put Supra on leveled jack stands, taken off suspension springs, and disconnected the sway bars, and then we have the ability to move wheels up and down with a small car jack. For measurements, we have used strings, a camber gauge tool, and a steel ruler. Measurements accuracy was about 0,05 degrees for toe, 0,2 degrees for camber, and 1mm for suspension travel. Our measurements have shown a good correlation between simulations and reality.
That is why to prevent such problems we have prepared 3 suspension setup presets (Track, Street, Drag) to provide the best possible performance out of the box, for every type of driver. These 3 suspension setups developed by us consist of our basic alignment and ride height suggestions, and what is the most important, predefined lengths of every arm, to provide proper kinematics for every designated use of the car.
On the graph below you can find a comparison of 2 previously shown bump steer curves (red and green lines) with a curve of our Street setup preset (yellow line).
As you can see, the bump steer curve of our setup provides much-reduced bump steer to prevent toe out through all suspension travel. This will give you stable handling without unpredictable dangerous situations, which were caused by sudden transitions from toe into toe out.
Summarizing: VERKLINE goal in preparing that 3 setup presets is to improve stock Supra A9x/Z4 G29 and additionally prevent all issues which would occur due to modifying, e.g. camber or ride height to fit the car to a designated use. Our kits will include precise guidelines on how to set up the car using our wishbones and rear subframe.
– Toyota Supra A90
– Toyota Supra A91
– BMW Z4 G29
Notice: Sold for off-road, track, and racing applications only. Mounting the bushing may increase some noise, vibration, and harshness (NVH) in the car. We strongly advise mounting the subframe in a qualified and experienced workshop