Letters from Readers, December 1967

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N.B.,—Opinions expressed are those of our Correspondents and Motor Sport does not necessarily associate itself with them.—Ed.

“Showtime Soliloquy”—Ferguson on the defensive

Sir,

You refer to views on 4-W-D and non-locking brakes attributed to Herr Rudi Uhlenhata, of Mercedes-Benz, which will certainly be questioned by most eminent engineers in the motor industry. As Herr Uhlenhaut’s comments seem from the context in your article, to have been directed against the Jensen F.F. we may, perhaps, be permitted to express our disagreement with the conclusions which he draws from what seems to us a somewhat superficial approach to the subject.

First of all, we would point out that, as far as the Ferguson Formula for All-Wheel-Control is concerned, the advantages of 4-W-D and antiskid braking are not separate and unrelated, as Herr Uhlenhaut appears to consider. They are complementary features of a system which provides against loss of control of a vehicle through either wheelspin or wheel-lock. While it is certainly one merit of the Ferguson Formula that a car equipped with it does not, in “the really slippery places of the world,” require chains, special tyres or tyre-spikes to obtain sufficient adhesion for safety and traction, the system is not designed primarily to cope with such extreme conditions. The main purpose, which it is generally conceded that it achieves, is to minimise the very real risk of loss of control through lack of adhesion and traction in ordinary and commonly encountered road conditions in which chains, spikes or snow-tyres are not used.

On wet or greasy roads which can be experienced at all times of the year in most parts of the world, on those with a thin film or patches of frost or ice, common enough in winter anywhere outside the tropics, and on surfaces polished by prolonged hot weather and coated with an amalgam of oil and rubber, which are met almost everywhere at some time, the Ferguson Formula guarantees a very large margin of extra safety in acceleration, cornering and braking compared with a 2-W-D vehicle. Even cross-winds and steep cambers can cause dangerous instability with 2-W-D cars, but such hazards have for less effect upon the toad-holding of a Ferguson Formula car.

Turning to non-locking brakes, it appears that Herr Uhlenhaut has not grasped the fact that a key feature of our 4-W-D transmission is that it goes most of the way in providing this most important safety factor without the addition of any sensing device or additional controls. Consequently, what he says about the defects of “the pioneer systems, such as the Maxaret,” does not apply to the Ferguson-Dunlop antilock arrangement.

It is a fundamental advantage of the Ferguson Formula that, without any additional anti-lock apparatus, no single wheel can lock or spin without the others doing so. The 4-W-D control also prevents the front or rear pairs of wheels from locking or spinning without the other pair doing so. Theoretically, two wheels on one side, or one front and one rear wheel diagonally, can lock or spin in freak conditions, but these are so rare that they can, for all practical purposes, be ignored. It was to prevent all four wheels from locking that a sensor, and associated controls applied to the normal servo system, were added. This anti-lock apparatus does also minimise the effects of the freak one-sided or diagonal locking referred to above, and ensures that the maximum stability possible in such circumstances is preserved. It is true that, with the introduction of the Maxaret, one of the biggest of the many problems encountered in developing our anti-skid system proved to be the achievernent of good results on slippery surfaces without sacrifice on dry ones. The problem has, however, been overcome. This is borne out by tests carried out recently on our own Jensen F.F., one of the first production cars, using precise measuring apparatus similar to that employed by the British Road Research Laboratory.

To obtain a direct comparison, measurements were taken first with the Maxaret switched into the sYsystemtem, and then with it switched out. [Graphs of the results are available to those interested.—Ed.]

There are three important points to note about these tests.

(1) With the Maxaret switched in, steering control is always maintained because no wheels are locked.

(2) On the dry surface, with the Maxaret switched out, because the special 4-W-D system prevents either the front or rear pair of wheels from locking without the other pair, the car stops in a straight tine, although steering control is lost because all wheels are locked. With a 2-W-D car under similar circumstances, not only is steering control lost, but on most ocasions the car will slew violently out of line owing to the rear wheels locking before the fronts.

(3) This method of measuring stops gives the true mean deceleration in terms of “g,” for the complete stop, which will he seen to average around .75 on the dry surface. It will be appreciated that considerably higher instantaneous peak readings—.9 or even 1g—would be recorded with “U” tubes or similar instruments in obtaining such mean deceleration.

You also report that at the Guild of Motoring Writers’ Silverstone test-day some drivers of the Jensen F.F. “appeared to be troubled by feed-back from the brake pedal and at least one driver abandoned the car beside the circuit for this reason, unless I have been misinformed.” We are assured by Jensen Motors Ltd. that you must have been entirely misinformed. One driver did express surprise at the pulsation when he stamped hard on the brake pedal. When told that this was exactly what was supposed to happen, and why, he was perfectly satisfied. He was, apparently, the only driver of the car at Silverstone who was not aware that this “feed-back ” is a deliberate design feature and informs the driver that he is on the limit of braking -adhesion. The Jensen F.F. instruction manual contains a passage which reads as follows :—

“In all normal conditions the brakes will respond in the conventional manner. When, however, excessive pedal pressure is being applied, which would otherwise lock the wheels, the anti-skid system will intervene and a repeated firm ‘pushback’ at the pedal will be felt. The braking effort at the wheels will then he controlled in such a way that the car is slowed down in the shortest possible distance without skidding or loss of steering control.

“When braking violently on very slippery surfaces, if the push-back at the pedal is ignored, and further heavy pressure is applied, the wheels may ultimately lock. The driver can, however, apply much more pressure to the pedal before locking occurs than with a conventional car, and therefore has a greatly increased margin of safety. In addition, the push-back at the pedal provides a valuable ‘early warning’ of treacherous conditions.”

This unique early warning is a key feature of the F.F. system and is of the utmost importance. On receiving this warning any sensible driver will take extra care, not only in braking, but also in acceleration and cornering.

N. F. Newsome, Coventry. Director, Harry Ferguson Research Ltd.

[The remarks of Herr Uhlenhaut’s which I quoted were general comments made over lunch to some assembled motoring editors—I may have misinterpreted them, as apparently I did the opinions of some of those who drove the Jensen F.F. at the G.M.W.’s Silverstone Test Day. To put this right I append the views of Mercedes-Benz (Gt. Britain) Ltd.—and would conclude by remarking what a sorry thing it is that this so-advanced and apparently foolproof Jensen F.F. has eluded those of us who would so enthusiastically submit it to a road-test. —En.]

Mercedes-Benz (Gt. Britain) Ltd. reply as follows:—

“Herr Uhlenhaut was in no way attacking the well-established designs of Harry Ferguson Research Ltd. or the Jensen F.F. in his informal comments at the Motor Show to motoring correspondents on design trends. What he did say, however, in reply to a specific question about the future of four-wheel-drive was that this principle was unlikely to be widely adopted in the future in view of the extra-expense and complication involved. He felt that non-locking brakes were a much more positive contribution to road safety and therefore more likely to be adopted. When asked why they were not already fitted to Mercedes-Benz cars, he said tests were still continuing and mentioned longer stopping distances in the dry of some pioneer systems.”

*     *     *

Code Of The Road

Sir,

I refer to Mr. Dudman’s letter in your November edition and wholeheartedly agree with his suggestion for a code of some sort to indicate a radar trap to fellow motorists.

I personally owe a clean licence to a lorry driver who was good enough to let me know what to expect two miles down the road, I certainly would not have spotted it!

Since then, I too attempt to warn fellow motorists by two rapid flashes and a thumbs down signal. If any of your readers have a more practical suggestion, I would gladly take it up, as a driving licence is a precious item when it can be taken away so easily for so little when one’s livelihood depends on it.

Short of the A.A. and R.A.C. circularising, on a regional basis, a list of the more popular spots chosen by our ‘ Black-enamelled friends,” or local radio stations informing us (if you can afford V.H.F. radio that is), this method of signalling seems to be the only way.

Long Eaton. A. D. Crooks.

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