Jaguar XKR Suspension Upgrade.
Firstly let’s consider what we’re looking from a sports car. There’s a pretty good overview here and a white paper here and here.
The Jaguar XKR is a highly respected sports tourer which has been applauded by the international motoring press and owners alike.
Although the XKR is indeed a highly capable car, we identified significant handling issues such as:
- Poor chassis stability during normal driving and gentle acceleration.
- Inconsistent chassis response and feedback.
- Slow turn-initiation response.
- Poor rear traction under hard acceleration.
- Poor rear traction stability under hard braking
- Excessively harsh ride
- Excessive under steer
The above traits and charateristics lead to an erosion of driver confidence, a reduction in dynamic performance and of course a loss of driving enjoyment.
Our initial conclusions are that the overall ride quality is poor which displays all of the down sides of heavily sports orientated suspension but with none of the expected dynamic performance benefits. The overall chassis performance is so poor that the ability to easily use the engine’s power is severely limited which makes spirited driving challenging and unenjoyable. We found that under extreme circumstances such as confronting a slight road hump on the apex of a corner, or one rear wheel encountering a pot hole or adverse road camber, the suspension becomes uncontrolled to an unacceptable degree.
Jaguar XKR Suspension Upgrade Evaluation
By way of a brief explanation of the meaning of “Dynamic Index”, in a nutshell it’s the relationship between the mass of the vehicle and it’s wheel base. This is the primary influential factor on the vehicle’s dynamic handling performance.
The DI can be calculated and analysed scientifically, and the results will provide an indication of a car’s behavioural tendencies. Typically, a high DI will display slow reactions to command inputs, a low DI will display extremely fast reactions to command inputs, and a neutral DI will be optimum.
Unsurprisingly, cars that are renowned for exceptional dynamic handling capabilities can be demonstrated to have a neutral Dynamic Index.
We measured the XKR as having an overall mass of around 1800kg. This combined with a relatively short wheel base and the position of the centre of percussion, the calculated DI is found to be exceptionally high. This explains a lot about the way the XKR handles by design, and why the designer was forced to use relatively high spring rates in an attempt to overcome an intrinsic design issue. Possibly a classic case of form over function?
We also evaluated the suspension bushes and discovered that the rear sub-frame void bushes allowed approximately 15mm of compliance which extrapolates to around 4 degrees of unwanted geometry deflection under normal every day driving conditions! This is very apparent when driving, as the car is easily unsettled by uneven road surfaces, hard acceleration and braking.
Other bushes relating to the front suspension were also identified as requiring a compliance reduction to prevent unacceptable geometry deflection during cornering and heavy braking conditions.
Jaguar XKR Uprated Suspension Evaluation
The next area to be considered are the springs and shock absorbers, and the kinematic geometry.
As mentioned previously, the spring rates are exceptionally high, presumably in an attempt to compensate for the poor Dynamic Index. The damping rates seem adequate and have the CATS system incorporated.
The geometry that is recommended by Jaguar seems to suggest that they are aware of the poor DI and have attempted to improve the turn-in by manipulating the geometry, at the expense of tyre life and straight line stability. The additional kinematic geometry induced by unwanted geometry deflection tends to exacerbate this issue.
Installation of XKR Uprated Rear Sub Frame Bushes
As identified previously, the rear sub frame bushes are a “void” design which allows for an unusual amount of movement, which in turn causes an unacceptable degree of geometry deflection.
We designed and commissioned some poly urethane replacement bushes to correct this issue. Here’s the standard bush compared to the uprated bush:
Here’s the uprated bush when installed:
The images above show the difference between the standard bush and the poly urethane bush. None that these are development bushes and that the production models will not be yellow!
Additionally, we installed poly urethane panhard rod bushes to reduce the amount of slip between the axle and the body. The bushes are shown below:
The Jaguar XKR uprated bushes are available here.
Installing Uprated Front Suspension Bushes on Jaguar XKR
Having dealt with the deficiencies of the rear sub-frame, we turned our attention to the front suspension.
To deal with the immediate concern of the excessive compliance inherent in the bushes of the front anti roll bar, upper and lower wish bones and front sub frame, we developed some poly urethane replacements.
These are the uprated front anti roll bar bushes:
These are the upper and lower front suspension arm bushes:
These are the upper and lower front suspension arm bushes when installed on the car:
You can review the range of bushes available here http://shop.tdi-plc.com/catalog/suspension-technology-c-1_788_826_827_832.html
Having completed the development of the poly urethane suspension bush upgrade on the XKR, we spent approximately 3 months on performance evaluation and durability testing.
The significant improvement in performance allows the driver to have considerably more confidence in the car now that the input commands and responses are totally predictable and controllable.
We are very pleased with the end results and hope that anyone upgrading their XKR suspension will experience a new level of driving enjoyment.
These suspension components have been released for sale and can be viewed at: https://www.tdi-plc.com/catalog/index.php?cPath=1_788_826_827_832
Jaguar XKR Uprated Springs
Following on from the very successful implementation of uprated suspension bushes for the Jaguar XKR, we turned our attention to the springs.
We are always careful about our choice of strategic partners, and in this case we were happy to choose Eibach as our preferred supplier of road springs. Eibach are one of the best springs manufacturers in the world with experience at the very highest levels of Motorsport.
We wanted to lower the car slightly to provide a lower centre of gravity without compromising the utility for everyday use. The Eibach springs reduce the ride height by approximately 10cm.
This is the difference between the standard and Eibach spring:
This is an Eibach spring fitted to a shock absorber.
These are the ride heights before the spring change:
This is the new lower ride height:
Outcomes:
The revisions to the geometry, suspension bushes and springs is a transforming outcome. Significant improvements in chassis balance and grip provide a much more intuitive and enjoyable driving experience.
Jaguar XKR Uprated Suspension – Track Testing
We were invited by Jaguar World magazine to attend a test day at Mallory park recently.
The weather was atrocious but the XKR performed exceptionally well and was driven by a professional racing driver who was impressed with the Jaguars improved handling dynamics.
Jaguar XKR Suspension Upgrade components are available here.
Torque Developments International boasts a comprehensive understanding of chassis dynamics, enabling it to evaluate, enhance and optimise high performance and racing car dynamic behaviours.
Torque Developments provides precise and effective chassis solutions for applications from a simple spring and damper combination to a clean-sheet suspension design, as well as production car optimisation for specific Motorsport purposes.
Chassis Dynamics Capabilities
- Inertia matching: Optimising the vehicle’s wheelbase to its mass and moment of yaw
- Spring and damper optimisation
- Anti roll bar optimisation
- Suspension arm/link design and optimisation
- Geometry calibration
- Limited slip differential optimisation
- Tyre evaluation
- Roll cage implementation
Chassis Dynamics Facilities
- Chassis design/ evaluation software
- Corner weight measuring equipment
- Vehicle dynamic performance measuring equipment
- Arago V3D chassis alignment equipment
Please contact us if you would like more information on Jaguar XKR Suspension Upgrade or to discuss your project.