Tyres Do Duty as Suspension System Interesting Details of Michelin
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Products as Used on Thornycrof Amphibious Machine AS an example of the contribution .made to the war effort by tyre makers, it is of some interest to record that the amphibious Thornycroft vehicle — Terrapin 11 — which was described in our issue dated November 30, has no springing system other than that provided by the tyres.
About the middle of 1943, the Michelin Tyre Co.,' Ltd., Stoke-on-Trent, was approached by the Ministry of Supply in connection with the' production of a tyre suitable for an unsprung machine. The vehicle would have a 2-ton tyre load, be capable of a speed of 40 m.p.h. on the road, and the tyres were to provide sufficient tractive effort to enable the machine to negotiate soft sand beaches.
While the prototypes of this tyre were on test, the company was asked if the tyre could be modified to suit an amphibian vehicle which would have a load of 4 tons per tyre, and which would be capable of a speed of 20-30 m.p.h.
After due deliberation, the company was able to advise the authorities that minor modifications would enable the tyre to do the job, although, under operating conditions, the vehicle might, momentarily be balanced on only two tyres. As the machine was unsprung, this would induce an overload on -the tyres of 100 per cent.
The eight wheels of Terrapin II are mouthed on driven stub axles, attached rigidly to the hull, and are so arranged that, on level ground, only the centre four tyres are making contact with the ground, the load imposed under these conditions being no less than 4 tons per tyre. • The Michelin 14.25 by 20 tyre was made to carry the rated 4-ton load at only 40 lb. pressure, giving the tyre a static deflection of. 25 per cent, of its section, with dynamic deflection, under running conditions, of anything up to 50 per cent.
Measuring 14i ins, in section and 49 ins, in overall diameter, the tyre fits a 20 by 10.00 wheel. This can be compared with the normal 13.50 by 20 tyre as fitted to large load carriers, which are scheduled to carry 3/ tons at 95 lb. pressure, giving a static de flee t i on of about 12 per cent.
The subsequent trials with the actual vehicle showed that, with the use of Woodhead beadlocks to prevent creep, the pressure in the Michelin tyre could be reduced to 20 lb. for negotiating soft sand. Contrary to some opinions, the thin walls did not suffer unduly when operating under rocky beach conditions.
On the Terrapin II steering is per. formed by driving and braking of the port and starboard drives, and the stress of cornering at low speeds, caused the tyres to become almost fantastically contorted.