THE VEHICLE THAT OT A LITTLE BETTER
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Our Contributor Contends That Ther Vehicle Should Not Be as Economic Examination of a Chassis, he says. ( as to Its Ability !ason Why a Low-priced Mass-produced Expensive Machines. The Superficial Relied Upon to Provide Accurate Data lp to Hard Service
By. A. W v1ech.E., A.M.I.A.E.
IREAD your leading article, entitled " The Operator's Practical Vehicle," and the first part of S.T,R.'s Article, " The Vehicle That Is a Little 'Better," in the issue of "The Commercial Motor " dated November 19, and would venttire to say that, with regard to the economics (,I the case, . the latter .do not satisfy the suggestion, -idvanced by your contributor, Mr. Pickles, that S.T.R. should contribute to the recent series, " Let the Operator Advise the Maker." Furthermore, it is .my opinion that the conclusiorie drawn by S.T..R., from his inspection of the two vehicles mentioned in the article, are erroneous: and misleading and that, instead of a:deising the operator, it biases him in favour of more expensive i:ehicles; without stating any case for cheaper Machines.: .7.:::.s.
I think Mr. Pickles intended that a series of comparable operating costs should have been obtained from Owners of fleets of mass-produced or cheap vehicles, 'and more expensive'rnachines of the same capacity. In this way, a ,definite
guide would have been obtained on„WhiCh both operator and manufacturer could have based their future programmes.
Instead of this we have an article, based on theory, -in whieb 10 supposedly advantageous features are discussed, and a decision made to purchase a vehicle coating £73 more than its rival, purely on these deeisiens, most of which can be shown to be incorrect. S.T.V. leads off on the wrong foet by stating that no one ,seems surprised at 'being asked £2,000 for a RollsRoyce. He does concede that he is not concerned with the problem' of the car owner, but he fails to justify the cost of the Rolls-Royce, even in analogy. He forgets to mention that the production of this particular ear is small and the methods employed in its manufacture so reminiscent of the tool room, that a price lest than that quoted would leave no margin of profit. "Were the demand for RollsRoyce cars raised to a sufficiently high level, the price could be greatly reduced without impairing the quality of the vehicle.
Quality is Not Inferior 'In Mass-produced Vehicles Price is a (Unction of demand, and it can be shown that the quality of mass-produced chassis is definitely not lower than that of individually produced machines.
The whole problem, 'then, resolves itself into one of design. S.T.R. was not impressed by the layout of the cheap vehicle, hut he cannot condemn them all on the evidence of one badly laid out chassis. Obviously, he cannot mention the particular make under consideration, but he could have found other cheap machines, of different make, which are laid our with forethought and ability equal to those of the more expensive model.
Surely, S.T.R. is not suggesting that the only able cliesigners in the country are employee by the manufacturers of the more expensive vehicles? The available evidence rather shows that the designer of mass-produced vehicles must give'more thought to his* product than his rival, for he must consider both suitability' for the units' function in the chassis, and its ease of continuous: production.
Now let us consider the 10 points of superiority enumerated by S.T.R.
(1) He states that the hack-axle -casing was more substantial and better made, but, unfortunately., he leaves that bald statement to explain itself. Does he mean that the casing weighed more? Did he actually check over the dimensions Of the twocasings and so prove that one was bigger than the other, or did he merely glance at the two and note, perhaps, an extra rib, and jump to the conclusion that the more expensive model-was the better?
It is obvious that he has not calculated the stresses set up in the uriits for, tad te done so, he would have quoted
figures to support his argument. The designers of the'
axles, however, have stressed the components. ft is highly -probable that the cheaper unitis bnilt to cope with these stresses with an adequate factor of Safety, and 'that • the other axle has haen built up to give the impressiot of .additional strength, entirely disregarding the fact that the A24 added material, besides being unnecessary for withstaml ing the -.stresses impoteel by operating conditions, is redundant weight. It is extremely unlikely that the manufacturer' of the cheaper vehicle, even in war-time, would turn out an axle which would flex and cause the trouble S.T:R. 'quotes'. The explanation of the greater robustness of the axle on the more expensive vehicle probably lies' in the mentality • of the designet who unlike his .'rival, must add just that little mbre than is required to satisfy the imposed conditions, -and so 'convince himself -that the job looks right. 'The statement that the one casing Was better made than the other conveys very little to the reader. In the first place, the outside appearance of a casting gives no indication of what is inside. It may-have been 'that the outside of one was ,rougher looking than the other, or that a flash had been fettled badly on the cheaper. unit Surely, the outside finish of a casting (S.T.R. only complains of the casing) is no indication of the reliability of the entire unit. How is it to influence the function of the driving mechanism? In what way is the internal accuracy 'affected? In Other words; how can the outside appearance show whether a unit is Well made internally or not, which is the point at issue?
(2) With regard to the brake units criticized, S.T.R. has based his argument on 'cost, and takes no account of the principles of braking. Although he states that the brake manufacturer produces a number of models, in actual fact there are only three different 'types of brake in regular use in 'commercial vehieles. These are the fixed anchor, the floating anchor, and the two leading-shoe types. The means for operation—cam, wedge, etc., has no effect' on the braking efficiency and .can, therefore, be ignored.
Brake linkages to all three types can be designed to give the same retardation, but each will require a different
pedal pressure. Therefore,' if the same retardation can be obtained with all three brakes, efficiency need not he considered, but the shoe-facing pressures, and general stresses set up in the units, are of importance.
Respective Merits of Different Types of' Brake
In the fixed-anchor brake the leading shoes does three times the work of the trailing shoe, hence its facing is worn out more rapidly than that of the trailing shoe, and uneven forces act on the brake drum and wheel studs. The floating-anchor brake imposes greater stresses on the trailing shoe,, so that facing wear and uneven 'braking forces., experienced with the fixed-anchor type, are reversed.
The third brake, the two-leading shoe type, is the most perfect -unit yet designed. The shoe pressures are equal .and all retarding stresses (and, consequently, shoe-facing Wear) are even. Be-zause of the extra parts involved, and its comparatively recent introduction, this brake is, at present, the most expensive of the three' types IAA, when its superiority has been fully realized and its production . increased, it is likely that it will become the cheapest..
S.T.R. chose the expensive -unit on principle but, as cart be appreciated from the foregoing: it was purely a fortunate circumstance. which linked the best 'brake with the highest price. •
"To choose any unit simply because it Is inbre expensive than a competitive component, -is to invite reanufacturerS to regulate 'their prices to control. salesi.–!` If the demand
tor a cheap model is insufficient, raise its price and it will sell will become their maxim.
With regard to S.T.R,'s quoted remarks.—" The chassis is a good one but for the brakes. I have had a lot of trouble with them "—in most cases the trouble experienced is entirely unconnected with the brake units themselves. The most coundm complaint is sponginess or soapiness. This feature is usually caused by badly designed brake linkage: the rods are of insufficient diameter and stretch when the brakes are applied, the cross-shafts are too weak and twist under load, Or it may be that the frame crossmembers, to which the cross-shafts are attached, are either flimsy or incorrectly Anchored to the side members. Then; again, the cable abutments may be loose or the brake drum is expanding.
• All these faults have nothing whatever to do with the type or make of braking unit and, therefore, to prefer any b-iake, without first examining the linkage and its
anchorage is, in S,T.R.'s own words, skimping " the job. (3) In this division the question of shackle-pin diameter is dismissed with the remark that, because one pin has three times the bearing surface Of the other, comment on them is almost superfluous. First,'then, let us analyse his statement that the bearing surface on the one pin is three times that of the other. We are told that the ratio of their diameters is as 4 is to 3, therefore, if both pins are the same length their bearieg surfaces must be in '-"the -sameratio, or assuming that the ratio is 3to 1, one
pin must be times the length of the other which, to say the least,: is a little on the long side.
Bearing surface, however, is not dependent on shacklepin length. The pins are, according to the design of the suspension mounting, either fixed in the shackles or the brackets-but, in either case, the governing factor in determining bearing surface is the width of the springs.. We are told that the springs on the more expensive vehicles are wider than those on the cheaper machine so, as the minimum width of spring for a chassis of 6 tons capacity is about 23ins., let us assume that figure for the cheap chassis, and 3 ins, for the other vehicle. We have, now, 4 6 the bearing-surface ratio – x – or 1.6 to 1.
3 5 An Analysis of Maintenance Costs of Small Components Next, let us assume that the ratio of the vehicle cost is 6 to 5, and that the operator intends to run the cheaper machine for five years. The operating period of the more expensive machine then must be six Years. Furthermore, we must assume that the manufacturer of the cheaper machine has allowed for two replacements at shackle pins in that 5 time. The life of the small-diameter pins, therefore, is – years and that of the larger pins – years, assuming that life be proportional to bearing surface.
The first, replacement of pins on the expensive vehicle would take place after n years, which still leaves 3s years oaf the s-ehicle's life, and as the pins will not last for that time they must be replaced just before the machine is discarded.
Therefore; despite, the larger spins, servicing costs will not be reduced as both sets of pins-must be replaced twice, and S.T.R.'s argument that larger pins are worth something extra on the price will not hold water.
(4) The fact that more substantial spring brackets are provided in the dearer vehicle, cannot justify any extra cost. The same arguments as :those put forward for the rear-axle casing can be used 'again, namely; that the brackets on the cheaper chassis are adequate for the job, and any increase in size on the other machine is so much
wasted material. • The psyChologicaI argument, that the realization of the unlikelihood of breakage of the larger units is comforting: can only be a figment of S.T.R.'s irnagination. Ildw, many drivers does he know who comfort themselves, at the wheel, : with such reassuring thoughts on their spring bracketS1
(5) The point made by.S.T.R., when cbraparing the road Spiings; was that the longer, wider and thicker units'fitted to the more expensive vehicle would be less susceptible to breakage than the shorter springs on the other model. Designers of springs practically confine themselves to rules of thumb which have been proved, both in. theory and practice, over a number of years. It is almost a certainty, therefore, that the two sets of springs in question were designed to the same formula-, and that the maximum allowable' stress, in both cases, • was the same. Owing to the larger bending moment induced at the centre of the long spring it is only . reasonable to expect that its cross-section Will he larger than that of a short spring, assuming that both are subject to the same loading. The stresses in the material will be substantially identical, so that breakage is no more likely in the one than in the other. One point can be conceded, however, and that is, if both springs did break; the one with the larger number of leaves would probablyget the vehicle, to its destination where the other might not.
The question -of better riding, which S.T.R. mentions, would be, perhaps, the deciding factor, for the longer spring would have a lower periodicity and, in consequence, give a more comfortable ride,
(6) The arguments put forward in Op, case of the shackle pins (item 3), can be applied tci the king-pins, to prove that maintenance costs will not be decreased, so that there is no reason to choose the vehicle with the larger pins.
A Heavy Frame is not any Criterion of Superiority (7) In this paragraph, S.1 .R. states that 0 more substantial, and better-built, frame reduces the risk of breakage, but it has been proved, in a practical way, that this is dot so. If we compare a frame side-member with a stick which is being chopped for firewood, the argument will be readily appreciated. A stiff stick will, snap under the axe blow, whilst a thin stick, having no inherent reaction to the blow, bends and assumes its previous shape after the blow. Likewise, a stiff frame, when subject to the hammering due to road shocks, is more likely to break than one which is less robust.
If a frame be designed to meet the stresses imposed by a 0-ton load, under all operating conditions, the lighter it is the better.
It has likewise been found beneficial to design the frames of goods vehicles so that-they flex a little, and adapt themselves, partially, to the distortion of. a platform body and thus asSest the sprMgiug. The components which usually suffer, because of frame flexure, are brake cross-shaft, the frame brackets rarely being affected.
Distortion can assume such proportions that the shafts have been known to bind, but the provision Of spherical end-bearings has effectively prevented trouble.. Naturally, frames can be marketed which are insufficiently braced • and impose unwarranted stresses on other parts of the vehicle, hut even a detailed 'examination of .the frames under consideration would not show that either was too stiff or :too flexible. :
No absolutely reliable method of designing a frame, to . give all desired features, has yet been introduced. Sidemembers can be stressed but cross-members and bracing are still dependent on experience. S.T.R., therefore, cannot say, with certainty, that one franie is better than the other.
(8) " The engine mounting is superior," says S.T.R., and leaves the statement with no further explanation, except that ithe stiff frame helps the mounting. I agree that a stiff front-end assists in removing stresses from engine feet, but S.T.R. has not specifically mentioned the front-end of either frame, so the one may be as stiff as the other. He gives insufficient data for argument, merely leaving himself open on the psychological statement that, although the one ch issis may be up to its job of supporting the engine, it is comforting to realize that the other is stronger still.
The obvious answer is that " It need not be stronger," and that the driver or operator is not concerned with comforting himself an points of relative strength if the weaker support be strong enough to do its job.
-(9) In his criticism of the gearboxes, S.T.R. states that the one is of sounder design than the other and, therefore,
concludes that it will cost less in maintenance. If the vehicles are to be employed until they reach a ripe old age, his statement Is probably correct, but he gives no indication of the expected life of either (I have already been forced to assume one), and until he does so he cannot expect his points to carry.
The cheaper box is probably constructed to last for a given length of time. If that time he sufficient for the life of the vehicle there can be no argument against its adoption. It will be noisier than the more expensive box, but absolute silence in a commercial vehicle is not a saleable quality with a large demand.
(10) I entirely endorse his remarks on wheel studs, knowing from experience that small studs can be a source of trouble, but here, again, it may have been proved that the smaller studs on the cheaper machine are entirely capable of coping with the loads imposed on them. In any case, the provision of large-diameter wheel studs is scarcely worth an additional £73, the price difference between the two machines.