Having dealt with the deficiencies of the rear subframe, 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 arms:
These are pictures of the bushes when installed on the car:
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.
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 panhod rod bushes to reduce the amount of slip between the axle and the body. The bushes are shown below:
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 attemted 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.
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 subframe 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.
To get an understanding of the Jaguar chassis designers thought processes and to formulate a way forward to provide a resolution to the issues, we undertook an evaulation of the suspension bush compliance and Dynamic Index.
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:
The above traits and charteristics 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.
