Changing an Eight-wheeler from Solids to Pneumatics
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Interesting Modifica. lions Made in Converting a Scammell. The Layout of the Rear Axles Altered SOME months ago we described the conversion by the Michelin Tyre Co., Ltd., Stoke-on-Trent, of a number of Scarrene11 six-wheeled articulated lorries from solid to pneumatic tyres. The converted vehicles have proved to be most satisfactory in practice, and it was represented to the Michelin concern that there is a definite demand for a similar conversion of the Scammell eight-wheeler.
This work was, therefore, taken in hand and the first of the converted eight-wheelers bee now been in service for some time, whilst others are also on the road. This first vehicle belongs to Roden Drammer and Co., Ltd., Tunstall, Staffs, and the entire conversion was designed and caeried out by the Michelin Company at Stoke-on-Trent.
As desig-ned, the &amine11 solid-tyred eight-wheeler has four wheels in line On a divided axle at the rear. This arrangement is, for three reasons, undesirable in connection with pneumatic
tyres. Access to an inside wheel in the event of a puncture would be difficult, and, owing to the location, of the original divided axle, too great a share of the load would be thrown Zm the driving wheels, whilst most important of all, the legal limitations of axle weights would make it impossible to take full advantage of the load-carrying capacity of the pneumatic tyre.
For the reasons, it was decided to redesign the trailing-axle suspension and to use two separate axles at the rear, one behind the other. In this way, the three main axles are equally loaded. These axles are equipped with wheels carrying 88-in. by 8-in, twin pneumatic tyres, which, at a pressure of 90 lb. per sq. in., can safely bear 7i tons per axle. The front wheeLs carry 32in. by 6-in. tyres, and thus the permissible gross load with the tyres fully laden is 24 tons 8 cwt., but the legal maximum gross load is 22 tons, so that the new equipment is well up to its work.
So far as possible, use is made of the original parts. In that connection, it is worth mentioning that owing to the original wheels having roller bearings, the cost of converting the eightwheeled chassis is little more than that of converting the six-wheeler. The original brake gear is used for the forward trailing axle, and only new brake camshafts and levers are required for the rear axle.
The original springs with half the leaves removed are used, together with the spring-centre brackets, which are bored out to fit one of the new axles. Sinailarly, the forward spring brackets are retained, but are fitted in a different position along the frame. In place of the rear brackets, there is a pair of new ones, each of which carries a balance beam ; at the back end of this the second new axle is mounted in spherical seatings.
At the forward end of each beam there is a hardened roller, under which fits the rear end of the spring. This arrangement, together with the spherical seating on the balance beam, not only provides the necessary compensation between the two trailing axles, but also permits of a good deal of angular movement of either. As a practical demonstration showed, the converted eight-wheeler is well able to traverse the most irregular surfaces likely to be encountered in the course of ordinary road use.
With a platform-type body, the estimated legal pay-load is 12 tons 15 cwt., but apart from the legal restrictions, the conversion to pneumatic tyres raises the maximum permissible load to 15 tons.