The Unsprung Weight of the Tramcar.
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There has beentta lot written.from time to time concerning the capacity for damage to the roads, to say nothing of undue noise, on account of the unsprung weight for which certain chassis designs provide. Now that the double chain drive, in all but a. few motor factories, has gone by the board, and that the big double-reduction or worm-driven axle, with its massive casing, appears to have won a prominent place in the schedules of most manufacturers' outputs, this question of the proportion of weight, which may be rightly allocated below the. chassis springs themselves, is becoming a more insistent one than ever. We have little doubt that discussion will be particularly rife upon this subject, when the time comes for the re-modelling and redesigning of types on the basis of the great road trials which started on the Continent in August last.
Perhaps, in an endeavour further to consider this problem of hammer action on the road, as good a preliminary study as any can be secured by carefully observing the effect of tramway traffic. Few of us perhaps realize the extraordinary intensity of the blow delivered by the pony truck of a. double-deck tramcar running at high speed over badly-laid tracks, cross-overs and points. Such intensity, of course, is entirely due to the fact that the unsprung weight, of the pony truck and its motor equipment is excessive, usually being in the neighbourhood of 27 cwt., although this is not a whit too much for the work it has to do.
No rails will stand such wear long, and the granite setts and concrete base on which the track rests soon break up under such continuous percussion. Not even the worst examples of heavy self-propelled traffic units approach the crudity of this method of mechanical street haulage. Even the steel.tired but railless tractor has its more or less efficient springing system, and not a few of them have the advantage of partly-resilient or wood-insert treads.
The motor omnibus, the motor lorry, and other' familiar types, of course, have no unsprung weight at all in the proper sense of the term, the rubber tread effectually absorbing much of the blow on the road surface. The worst case of unsprung weight in. respect of road traffic is certainly that of the maximum-traction truck of the heavy double-deck tramcar on a badly worn 4 ft. 81 in. steel track.
Careful investigation of this class of wear and tear will do much to solve the problem of a much less serious nature which is presented to the commercialvehicle manufacturer when he desires to know how much he may safely load on to his axles below the chassis springs, all the time remembering that such load will usually be carried on india-rubber treads.