As "secret" testing of the XJ13 continued, the car was taken to the Silverstone race circuit for two days in August of 1967 ...

The following quoted text is © Jaguar Daimler Heritage Trust and should not be copied without their permission.

Ninth test August 15th & 16th 1967 - Silverstone

222 miles
Sustained testing at Silverstone over 222 miles and subject to special report by M Kimberley (see later)

After Silverstone tests the car was stripped of its g/box, brakes & suspension units. The entire car being thoroughly checked over for structural failures etc. No deficiencies found in any structure
Distortion tests were carried out on the front vertical link assys and as a result certain stiffener webs were added together with mods to accept the girling caliper and disc
Extensive modifications were carried out to the body to accept tyres size 5.25 10.50 x 15 at the front and 6.50 13.00 x 15 at the rear
New wheels to accept these were drawn and made and are now fitted to the car, less inner tubes
Modification to the 5DS25 ZF gearbox including a new differential assembly with 34.5mm drive shafts replacing the original 29mm shaft which had previously twisted and a more positive method of locking the drive shaft bearings in their housings to eliminate end float. The aluminium end covers being 4.2-1 ratio altered to suit. Cast iron end covers are also available but not fitted at present
The fuel system has been completely re-piped to overcome the possibility of a further failure. The new pipe being wire braided and teflon lined
The car has been received by Lucas to suit the OPUS ignition system and the battery replaced by a standard E-Type one

INTERIM REPORT No.9 BY MJ KIMBERLEY TO WM HEYNES

"RESULTS

Fastest laps
D Hobbs - 1 minute 35.7 secs (110.1 mph)
R Attwood - 1 minute 38.1 secs (107.4 mph)

Ride
Satisfactory and wheel bounce coupling obviated

Handling
Both drivers reported inherent oversteer characteristics. Hobbs indicated rear wheel steer and Attwood roll oversteer
Tyre temperatures showed front outer wheel to be vertical, but rear outer wheel camber changed from positive to negative (at max roll) during testing. Front inner wheel camber was slightly negative, but rear inner wheel camber too negative. Adverse rear wheel camber changes are known to occur (See Report No. SP1/13/1) and new parts are awaited from ZF to rectify this.
Time did not allow the various combinations of springs front to rear to be assessed. It is felt that some improvement could be obtained with respect to 'driver feel' by the use of variations in front to rear roll stiffness ratios etc.
Both drivers complained that tyre adhesion diminished after two to three laps....Straight line running now good.

Brakes
Drivers complained of vibration and lack of deceleration at high speeds. Brake fade was experienced, and pedal movement increased with use.
Generally, the brakes were poor, and efficiency was decreased with use. The drivers lacked confidence in them - both drivers stating that lap times could be reduced by two to three secs with improved brakes.

Engine
Engine performance good after thee plugs changed, due to oiling at start of test

Transmission
As anticipated, 7,700 rpm obtained in 4th gear before Woodcote (D Hobbs), but drivers feel that lower ratio would be an advantage.Misc
Fuel System - Tanks and pickup system satisfactory. High pressure feed pipe to PI metering unit connection leak.
Gear Change - New gate satisfactory although Attwood dropped from 4th to 1st on one occasion - interlock mechanism to be checked.
Steering Wheel - Rim section diameter requires increasing.

CONCLUSIONS IN GREATER DETAIL
Although time was not available to utilise the variations in tyre secions, spring rates etc in order to obtain improved ride and handling, and reduce lap times, lack of confidence in the braking system was a severe handicap. The brake system is being stripped and temperature traces analysed to determine reasons for deterioration during running.
Rear wheel steer and uncontrolled camber change can be obviated as soon as parts are available. From the tyre temperatures shown on the data sheet, it can be seen that uncontrolled camber changes occurred at the rear wheels. Without these, the design objective od an upright rear wheel at maximum lateral G would have been obtained at the rear as well as the front. D Hobbs reported that coming out of Chapel Curve with 'tail out' attitude the car 'flicked' to the straight-ahead position. This was confirmed by the reports of two observers.
Tyres were not suitable, but Dunlop will have new tyres shortly in improved mixes, however, in the meantime, it would be most useful to try alternative makes to obtain comparisons. Wheel rim sections are very restrictive on this car and considerable improvement would be obtained with an increase of 1 1/2".

A reduction in lap times could be achieved by ...
Improved brakes ... 2 to 3 secs
Lower CW&P ... 1/2 to 1 sec
Increasing cornering ability with improved tyres and location of wheel ... 2 secs

Although this brings the lap time down to approx 1 min 30 secs the power weight ratio of the car will need to be improved considerably. Comparable power weight ratios:-
Ferrari P4 - 0.210 bhp per lb weight
Lola Chev - 0.207
Ford Mk4 - 0.206
XJ13 - 0.177

The car in its present form is much heavier than necessary and hence the power weight ratio can be improved considerably.

It is noticeable that the driver's comments regarding handling varied considerably between MIRA and Silverstone, thus indicating that the development of a car of this potential is best carried out at a suitable circuit. However, it could be conversely argued that the 'D'Type whilst winning Le Mans, never performed well at Silverstone.

INVESTIGATION RESULTS

Investigation revealed the following major points. For full details see report by E Brookes.

Brakes
The wrong master cylinders were fitted and pedal loads would have been impossibly high (280 lbs for a 1G stop). A large piece of aluminium was located behind the recuperation seal giving intermittent failure of the front brake system.
Although the DS11 brake pads were badly flaked, tapered and distorted, this would be due in part to the above mentioned system faults. Mintex M48 and Ferodo 2429F pads will be available as alternatives for future tests.
Two fractures were discovered in the pedal box but these were not detrimental to the operation of the system.

Tyres
Although D15 tyres were requested and supposedly supplied, it has since been discovered that the rear 5.75 - 1200 x 15 tyres were non D15, thus reducing lateral stability. Dowty Vibrator showed 3.8 c/sec side shake frequency with non D15 5.75's compared to 4.7 c/sec using 7.00 Mx15 D15 - and increasing 'flat tyre' feel remarked upon by D Hobbs.

Transmission
The 29mm spline drive shafts were on the point of failure and the location bearing outer race thrust washer indented. This condition increased lateral 'float' and uncontrolled camber change.

Hubs
The front hub outer bearings were also on the point of failure, due, according to the Timken and Shell Service Engineers, to pre-load. Hubs now to be reset with .002" end float. Dr Tait's calculations confirmed the above engineers' reports."