LETTERS from READEARS

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Sir,

I was interested in the article by Sqdn./ Ldr. Marris on “Speculations on Fast Car Suspension,” in the April MOTOR SPORT. The author’s opening remarks indicate, I think, undue modesty, as, though I do not agree with some of his views, I certainly regard him as something of an expert, a distinction I hasten to disclaim. Regarding his résumé of the old-type suspension, I do not know whether it is generally known that a 1-elliptic spring has good resistance to twist if its loaded

camber is slightly negative, and so its anti-roll properties are fairly good, assuming semi-elliptic suspension front and rear. Moreover, steering pivot angles do not alter on bump and rebound. Turning tO advantages of the

author stresses the importance of understeer, with which I agree, as it ensures directional stability. A normally-Sprung car can possess understeer, however, although in considering i.f.s., relative front and rear slip angles come into the picture ; but it is still necessary to consider the behaviour of front and rear wheels. As the author implies, steering geometry may affect it. The advantages of i.f.s. are several, and include, for racing cars particularly, those of “keeping the wheels on the ground,” and minimising gyroscopic action, “tramp,” and, in certain cases, brake torque reaction.

The author then describes various i.f.s. systems, and their pros and cons. Of the vertical types, the twin trailing crank system seems to be the best. Of other types, the unequal wishbone is the most popular, and it is notable that the respective link ratios allow wide scope in determining wheel movement and roll centre, which can, if necessary, be nearly vertical and nearly at ground level, respectively. Backward rake of the wishbones, which is sometimes adopted, would help in this direction, though I understand that this is not the primary reason. I am not well acquainted with the swing-axle system at the front, unless the Andre-Girling belongs to this class ; also, in some cases, the lower wishbones extend to practically half track length. Regarding the equal and parallel wishbone system used on the G.P. Merchles, I think gyroscopic action is a greater disadvantage than scrub, which need not be serious.

As to iridependent rear suspension, unsprung weight reduction is more effective in this case, and I agree with the author that the De Dion axle, which is not a true independent system, has a good deal to recommend it, though true i.r.s. may, in time, supersede it. ‘ Concerning the author’s theories on sports and racing car suspension, it is

doubtful if the e.g. of a ,touring car is likely to be as low as a G.P. car, and, moreover, it invariably has more conditions to be considered, such as variation in load, ground clearance, etc., and So wishbones at the front, which raise the roll centre, seem advisable. The G.P. type of car is therefore an extreme case, in which front roll centre must be approximately at ground level to be low enough, and the author’s theory regarding the vertical system is undeniably correct and reasonable. However, the engineering disadvantages of this method still remain, and I think a strong case could be made for the unequal wishbone system. Effective slip angles will be very small, and, I think I am right in saying, are limited to about 6′ from vertical.

The use of anti-roll bars seems to be a compromise, and their use with an independent system is rather obscure, as is the question of forward engine mounting. I appreciate the fact that weight transference at front must be more than at rear to minimise skidding, though this is limited by outer wheel loading, inner wheel adhesion, and the resultant overturning tendency: also that the wheelbase must be as short as possible, within limits, for various reasons.

Lastly, while what may suit a large car may not suit a small one, independent suspension would seem to he equally advantageous to both types, though offset by increased cost and complication.

Partly for this reason I am not in favour of front wheel drive, though expert opinion on this question continues to disagree, as in the case of rear engine position.

In conclusion, may I say how well MOTOR SPORT keeps up its standard of interest, particularly with articles on individual makes and models, like those on the Rolls-Royce and the Austin Seven ” Nippy.” The article on “Ethics of the Quality Car” was most interesting, although somewhat controversial, in view of what has recently been said and written about American cars. I am, Yours, etc., R. W. J. CI.ARKE. West Heath,

Birmingham, 31.

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