3.2 Dimensions and constraints
Many of the dimensions of the chassis have been determined by the choice of the Ford Sierra as the donor car. The larger rear track has meant a widening of the chassis by 153mm (6”) and the wheel base a lengthening of 306mm (12”) affording more room in the passenger compartment, footwells, transmission tunnel and engine bay. The chassis has also been made deeper by 76.5mm (3”) to solve the clearance problems of the larger engine sizes planned and to keep the silhouette similar to the original Locost.
These changes are in relation to the measurements given in Ref. 5 and could be beneficial to the torsional stiffness of the chassis if used correctly as they increase the enclosed area of the torsion boxes inherent in this type of frame.
The exterior dimensions of this chassis were not to be altered as moulds for the fibreglass components had already been taken. This meant any modifications had to remain on the inside of the chassis rails but still leave adequate room for engine installation.
The boxes formed by the bulkheads effectively split the chassis into a further series of springs. These can be represented by simple models as shown below in Fig. 11.
Fig. 11 Chassis as Springs Between Bulkheads
To examine how this affects the stiffness of the chassis we can look at the following example:
One stiff spring with stiffness Ks =1000
Two flexible springs with stiffness Kf1 = 500 and Kf2 =200
Due to continuity a load applied to the series springs, the loads in each spring are equal. Applying Hook’s Law (f = k, = F/k), then each spring has its own deflection. The total deflection is the sum of the deflection of each spring, so the total stiffness is K = Ks Kf1 Kf2 / (Ks Kf1 + Kf1 Kf2 + Kf2 Ks). From the numerical example then the total stiffness is K = 125. This value is much lower than any of the original values. Therefore, it can be seen that stiffening the stiff spring further has little effect on the overall stiffness. The flexible springs must be stiffened first.
3.3 The competitors and their structures
3.3.1 Dax Rush The Rush is front engined and rear wheel drive, claiming to offer more interior space than any of its competitors and has been designed to accept a wide variety of engines from Ford's OHC/DOHC and V8 ranges to the Sierra Cosworth Turbo or Rover V8. The chassis is a multi-tubed triangulated space-frame chassis with integrated structural rear hoop. Additional stiffness is provided by bonded & riveted aluminium panels. It uses Sierra 'running gear’ with the 4 x4 specifically using Sierra Cosworth 4 x 4 'running gear' [Ref. 7].
Fig. 12 Dax Rush [daxcars.co.uk]
3.3.2 Caterham
Caterham Cars Ltd was founded in Caterham, England over 40 years ago and currently produces what can be considered as the ‘Original’ Seven using the Lotus 7 design [Ref. 8]. The chassis is a multi-tubed triangulated spaceframe design with riveted and bonded aluminium panels. The development of the Caterham has often been at the leading edge of the class. The running gear is all of a modern design with current generation engines and drive trains fitted. The Caterham is no longer available in kit form but only as turn key cars. The Caterham Seven is shown in Figs. 13 & 14 opposite.
Fig. 13 Caterham Seven [caterham.co.uk]
Fig. 14 [Ref. 4]