Tyre Testing at Fort Dunlop
We recently paid a rather hasty but very educational visit to Fort Dunlop, at the instigation of their Mr. Woodgates, who had raised his eyebrows at some of the remarks about tubeless tyres made by our correspondents and had suggested it was time the Editor saw for himself the careful tests made to ensure the reliability of Dunlop products.
Accordingly, one Thursday I drove North in the Austin-Healey 100-Six I was testing and, skirting Birmingham, arrived at mid-day at Fort Dunlop, a vast rubber empire which has grown from small beginnings, this factory having been built during the middle of the first World War.
Having been met by Mr. J. D. Sinclair I was taken in a Rover to the Dunlop Proving Ground, where new tyre designs are tested and comparisons made with rival makes of tyre, etc. M.I.R.A. is used for long-duration tests but Dunlop find it convenient to have their own facilities close to the factory.
Here all manner of surfaces, flat and banked, enable noise measurement, adhesion tests, and the study of over and understeer at constant side force on a banked section to be carried out. The banked area is 800 ft. long, with a transverse bank of 1 in 5 to generate a steady sideways force on a vehicle driven straight along it. A skidding area varying from smooth asphalt to polished pebbles and setts has been laid out, on which straight-line braking and cornering breakaway tests are made. It was found necessary to lay the rock asphalt area of the skid pad by hand to ensure a uniform surface; it is lubricated by clean water only. Cars approach along a 1,300 ft. road and the pad measures 400 ft. by 150 ft. An accelerometer in the car records speed at point of brake application and all figures taken relate to repeat tests to ensure complete accuracy. Stops from 50 m.p.h. or higher are possible in safety on good tyres. Cornering is done on a skid pad and also on two special surfaces, one of worn granite setts supplied by Liverpool Corporation after 25 years’ wear, the other of sea-polished Dorset pebbles. I was able to watch the Dunlop test driver demonstrate these tests on a 150 ft. radius bend, in 3.4 and XK140 Jaguars, the point being that he schools himself never to correct a skid, holding the steering wheel constant until the car slides under front or rear-end breakaway. Thus comparisons relating to different tread patterns, tyre pressures and cars can be made. A poor tyre will allow breakaway at 28-30 m.p.h., whereas a good tyre holds up to 40 m.p.h.
After an excellent lunch I was taken in an Austin A95 to the Dunlop Research Centre, which occupies a two-storey building in what was once an aircraft shadow factory, which should give some idea of its extent. Here the Test House Manager, Mr. B. I. S. Williams, and his able assistant, showed me some of the machines used for testing Dunlop tyres in every conceivable fashion, after I had seen examples of tyre and wheel failure displayed in a showcase, in a corner of which reposes a large lump of salt from Utah! The first machine I saw was a rotating arm on which tyres could be driven round a circular track. On this, solid tyres of natural and synthetic rubber were compared during the war, with the result that Dunlop soon convinced the authorities of the need for natural rubber on military vehicles. I was interested when Mr. Williams remarked that many “solids” are still in service and so this machine is still useful.
Next I was shown a versatile machine, built like most of the test equipment by Dunlop themselves, on which a tyre could be driven in contact with a roller to simulate all manner of road surfaces, wet or dry, and turned through 24 deg. while the load on it was varied hydraulically on the beam carrying the wheel. From the figures recorded on dials on the control panel it is possible to present to the tyre designer a graph providing not only data relating to tyre wear and life, and adhesion, but to “feel” when cornering. Batteries o (of) 18 rather similar-looking machines are in use for measuring rolling resistance of tyres, the wheels being belt-driven from electric motors through torsion-shafts and torque recorded by means of slip rings on the motors. In these tests tyres are run from idling to full speed, at pressures which can be varied as the test proceeds, commercial vehicle tyres being loaded up to 3 tons and car tyres up to 1½ tons.
Similar roller-machines test the life of tyres, impact damage being inflicted before the test starts, if required, to record the effect of this on tyre failure. A smaller machine subjects motor-cycle and scooter tyres to a load of 10 cwt., running at up to 50 m.p.h. At the other extreme is a rig that tests aeroplane tyres up to 70-80 m.p.h. under a 10-ton load. A new machine has been installed to record the adhesion properties and rolling resistance of motor-cycle tyres when banked over, as in racing cornering, and I saw a development of the rolling-resistance machine which tests car tyres up to about 70 m.p.h. under 1½ tons load, torque being measured in this case by the reaction of the electric driving motor in its mounting, in which it is free to rock.
The Double-Cambered Drum or 15-ton machine was evolved to discover why the beads of bomber tyres failed during dicey landings in the last war. Mr. Williams got the idea from holding four pennies and splaying them out, and a huge double-camber drum was built against which the test tyre, revolving upright, is held, thus subjecting the tyre wall to heavy side loads. So great are they that this machine is used for testing the strength of Dunlop wheels.
Next I was put into a 3.4 Jaguar and driven to the old High Speed machine, on which for so many years prior to the war Dunlop tested to destruction the special tyres they built for Land Speed Record cars and other record-breakers. On this, tyres were tested to the equivalent of 420 m.p.h. and, although the rotating drum has had to be skimmed down, it is still capable of testing racing tyres to 250 m.p.h. Such tests are observed through plate-glass windows, with provision for cine-filming tyre growth. Two foot pedals outside the test houee allow the tyre to spin free or brake the wheel to rest. One of the disc wheels and smooth-tread tyres from the Railton-Mobil which holds the Land Speed Record at 394.2 m.p.h., was in the observation room. In testing such tyres speed is worked up gradually, in steps of 5 m.p.h., with free spinning for half an hour to cool the tyre, so that its growth and running temperature at all speeds up to maximum can be recorded.
If fresh attempts on Cobb’s record, perhaps by jet-propelled cars, are contemplated Dunlop will build a new High-Speed test rig. Meanwhile, they use a fascinating plant for testing aeroplane tyres. In this two tyres are run in contact, which can be achieved suddenly or gradually, load being applied by air pressure. The wheels are driven by a Rolls-Royce Merlin aero engine, its giant twin silencers like great chimneys outside this isolated test-house. On this rig the largest aero tyres are tested under a variety of conditions, up to 350 m.p.h. The last machine I was shown is perhaps the most interesting, in view of suspension developments. It is a platform in the Chassis Laboratory on which any size of private car can be mounted and any of its four wheels rotated by rollers, one electric motor driving the front wheel rollers, another motor the rear wheel pair, while the car is lightly tied to eye bolts beside the rig. Wet or dry conditions can be reproduced, the steering can be operated, as all manner of “bumps” are fed to the wheels by bolting slats of various sections to the rotating drums. The platform is raised for close observation of suspension rnovernents and a glass panel with grid-lines drawn on it enables tyre scrub in a straight line for various suspension and steering settings to be measured as the car is drawn across it.
This machine is naturally of great interest to car manufacturers, who are permitted to take data with the aid of it, not only on tyre problems but on suspension problems as well. It is considered by Dunlop to be essential now that we are on the threshold of universal adoption of i.r.s., although it has also proved of great value in recording noise transmitted to a car’s occupants via the tyres. A Dunlop spokesman confirmed our astonishment that British manufacturers have been so reluctant to introduce i.r.s. and they consider it will soon come into general use here. Go-ahead manufacturers will undoubtedly treat themselves to similar test rigs, based on the one Dunlop technicians have designed and built for themselves.
Reverting for a moment to the tyre destruction machine, I was told that a “blow-out” cannot be heard above the noise of the machinery so when this happens a siren sounds and an indicator light tells the observers on which machine a tyre has run its course. “But,” said Mr. Williams’ assistant, “when a giant cover goes, it’s quite something!” Wire-mesh guards are used to catch flying fragments . . .
Incidentally, they are naturally very proud of the Jaguar successes at Le Mans on Dunlop tyres and a model of the winning D-type Jaguar is already on show in the entrance hall at Fort Dunlop.
Apart from their indoor tests, Dunlop use a track at Halfpenny Green, their aforesaid Proving Ground, and M.I.R.A. for other tests. They also have a Road Research section, which has access to every sort of vehicle from motor-cycles to a Leyland Beaver, and a Jaguar XK140 for fast work. Commercial vehicles apart, no speed limits are imposed on test drivers on the open road, but vehicles are often run in convoy to ensure identical traffic and weather conditions during long-distance tyre-comparison tests. Special tests can take interesting forms, as, for instance, checks on tyres and their effect on fuel consumption for London Transport, to whom an increase in their tyre bill would be justified if a saving can be effected on their annual enormous fuel consumption.
Before I left Fort Dunlop I was shown two short films in their very pleasant cinema, taken by the Dunlop Film Unit. One shows the virtual indestructibility of Dunlop tubeless tyres, even when sadly abused, and another employs a Triumph TR3 to emphasise the importance of front-wheel balance on fast cars.
Altogether, as I drove away in a cloudburst in the Austin-Healey on “Road Speed” tyres I felt that, wet as I got erecting the hood, I was unlikely to become still wetter through having to change a wheel — and that we owe much to Dunlop (or the present-day immunity to tyre trouble, and the good handling-characteristics of modern fast cars. — W. B.