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NULE ON AXLE DESIGN
No Technical Reason for Tyrewear Claims—Mr. Haigh
N replying to Mr. Jim Hays's article on the Burgess suspension system, which appeared in your issue dated May 14 last year, and in which he claims that tyre wear decreased by the use of the device, let me first state that do not wish to decry the mechanism as a suspension unit, it to show that tyre life is not improved.
The vehicle described was a semi-trailer with a Burgess nit fitted at the rear, hence no drive was taken through Le rear tyres. In order that a true comparison may be ade with a vehicle employing orthodox suspension,*only a Ad axle can be considered.
First, let us analyse the action of a wheel when dropping to a 10-in. pothole. Because of the shortness of the wings on the Burgess Unit, the sideways motion of the heel cannot be ignored. Mr. Hays asserts that the tyres >now the read contour and, therefore, are in constant antact with it. Assuming that the vehicle conforms with le maximum permissible Width dimension of 7 ft, 6 ins., ie spring length, allowing for the 10.7-in, width of lour irres and the clearance between the two inners, is approxilately 20 ins., so that the radius of the arc on which the -heel swings is 10 ins.
As the springs pivot about the trunnions, a 10-in, drop f one wheel would raise its partner an equal amount, iaking a total distance between them of 20 ins., but, as he pothole is only 10 ins. deep--that is a difference in eight of 10 ins, between the tyres—the equivalent drop will e only 5 ins. The distance which the spring eye moves awards and, consequently, palls the tyre, for a 5-in. drop, 3 1.34 ins.
If the pothole be 2 ft. wide, a generous allowance, the listance forward which the wheel travels is only 12 ins., so hat the tyre would be considerably distorted against the brasive action of the ground surface.
How Potholes Affect Wheel Movement 'For a solid axle with orthodox suspension the drop is iefmitely 10 ins., but the tyre scrub is not so great as the sbove. When one wheel drops into a pothole, the arc on vhich it travels is regulated by the track of the vehicle, that is the tyre, in falling, pivots about the wheel not segotiating the dip.
With the single tyres, as specified for most cross-country vehicles and which, incidentally, are better than twins under :hese operating conditions, a track of approximately 6 ft. .s used. This track would limit the sideways movement of the falling wheel to .7 in., or, roughly, half that of the wheel with the Burgess Unit.
With regard to Mr. Hays's statement that tyre wear is aggravated by spring rebound, there can be little doubt that this is true, especially in the extreme case where wheels are badly balanced and cause tramp. But, as Mr. Hays made no claim that the Burgess Unit cured tramp, we will assume that it is equal in this respect to orthodox suspension.
Over rough ground, such as that prepared for military tests, which consists of a series of bumps and hollows about 9 ins, high, or deep, set so close together that a wheel has barely cleared one obstacle before encountering another, the suspension has little control over the wheel oscillations, so that neither orthodox nor Burgess springing could be said, with accuracy, to be the better.
Both normal and Burgess Units would ensure equally good tyre wear over smooth ground, or over terrain with long undulations for, as stated by Mr. Hays, it is only when the tyres are forced away from the road surface and rebound on to it that scuffing can take place.
The real test would occur over a surface which contained an occasional high bump or deep pothole. .First, consider the Burgess-equipped machine negotiating a bump. At low speed the tyre would .ride it perfectly, and any excessive lift with which the leverage system could not cope would
be accommodated by the two springs. At high speed, assuming that only one wheel strikes the obstruction, the inertia of the vehicle would, first of all, cause the wheel to rise, with the springs acting purely as stiff levers, the springs being deflected upward as the wheel rose higher.
On descending, the tyre would be forced downwards by the rebound of the spring and the vehicle weight acting on the leverage system. When it touched the ground, its motion would not be merely vertical but slightly lateral, so that tyre abrasion would take place in two directions at once. Further downward motion of the body would cause the springs to deflect and both tyres to scrub sideways, as the wheels would once more be on level ground.
To reduce this argument to mathematics consider that, with the Burgess system, half the rear-axle load, is concentrated at the centre of the two wheels. The reaction on each tyre, then, is half this load, say, L. If the wheel rises 10 ins. (3 ins, above a 7-in, obstacle) the potential energy of the raised wheel is 10L lb. ins.
Now, consider an orthodox suspension system. Low
speed operation would result in perfect negotiation of the bump as in the case of the Burgess Unit. The tyre itself would absorb part of the rise, whilst the /spring and frame would take care of the remainder. At high speed the wheel would be thrown into the air as before, hut the height to which the wheel would rise would not be 10 ins. The weight distribution on each tyre would be 21, with the same loading as fot the Burgess Unit, so that, after negotiating the 7-in, bump, a weight of twice the previously mentioned aniount would have to be lifted, and the height to which it would be thrown would be approximately. half, or 14 ins.
(In 'actual fact the additional height would not be 14 ins. as some energy • above that required to raise the wheel with th-e Burgess system would be necessary to lift the orthodox • wheel on to the obstruction.)
The total height, then, to which the normally suspended wheel would rise, would be 84 ins., and its' potential energy at the -peak would be 17L lb. ins. It would appear, from a comparison of the two potential energies, that tyre wear with orthodox suspension would• be the greater, but it must
be borne in mind that, so far, we have assumed the on do.x unit to be fitted with only two tyres.
If we consider the orthodox wile to be fitted v " twins " the, effective potential energy per tyre is ; lb, ins. and, therefore, less than. the Burgess.
For a pothole, the height to be dropped is the same . each system and, therefore—still assuming equal t equipment—the effect would be identical, worked out the basis Of potential energy as for bump. .
For the purposes of the argument, we have assumed t the wheels, in each case, definitely leave the grou whereas Mr. Hays assures us that those on the Burg, equipped machine followed its contour. From the t3 reaction figures given, however (those on the Burgess 1..) were half those on orthodox suspension) it is reasonable conclude that, instead of the Burgess wheel following ground contour the more accurately, exactly the. oppo: should occur, as equal effort would lift the orthodox wh only half the height of the former.
There is one more point to consider, namely, the eff of road camber. The relative positions of the two fran and the road would be the same, that is, normal to • road-camber radius which passed through the longitudi: centre line of the vehicles. In the " at rest " positions the wheels lie parallel to the webs of their respective Ira: side members.
Road Camber and Tyre Distortion On the Burgess Unit the wheels adjust themselves to t road camber by means of the trunnions, thus ensuring t minimum of tyre distortion. The tyres on the orthod, axle, on the other hand, do not adjust themselves but, wi single equipment, the conditions would be the same as thc for the Burgess suspension.
With twin tyres, however, the inner tyres would slightly raised above the outers. Assuming a normal ro with a camber radius of 200 It., the difference in height the extreme edges of twin 10.5 by 13 tyre treads would slightly over 4 in., which could be expected to acc.elera tyre wear. But this is not so, far the difference in heig of the edges of the tread of a similar tyre fitted to a fro axle haying the customary 2 degrees camber angle is aga just over 4 in., which, as is well known causes no noticea.b excessive tyre wear.
In conclusion, I would venture to suggest that ti designer of thw Burgess Unit was solely concerned wi abnormal articulation, which he has overcome admirabl 'and that he gave no more thought to tyre wear than, sa. the designer of any independent suspension system.
Practical Tests Over-ride Theoreticz Deductions—Mr. Hays
IN his criticism of the Burgess suspension system, M i Haigh's statement, that tyre life s not improved, is base IN his criticism of the Burgess suspension system, M i Haigh's statement, that tyre life s not improved, is base almost solely on theory. Having had ,the advantage witnessing gruelling tests and watching performances ovt a period of two years, my statement is based on the soli foundation of practical experience.
Actually, as a tyre expert of many years' standing, was, at first, interested only in the performance of ti tyres. The conditions under which they operated an the way they stood up to the work aroused my interef to such an extent, that I felt it my duty, as a tyre mar to study the problem and discover the reasons. Therefor( I watched the vehicle operating on all types of road, mad and unmade, and carrying both high and low loads many kinds.
In the first place, I readily agree that the wheel swinf through an arc, and that when a pothole is encounters the tyre will move downwards and inwards. TheoreticaTh one would expect to see signs of severe tread wear bu after two years, there were no such signs. This I coul only assume to be due to the fact that the tyre follows the road contour smoothly, no matter how irregular 11 lace might be, this being accounted for by the free illation of the wheels.
Let us consider the example given by Mr. Haigh regarding tyre striking & pothole 2 ft. wide and 10 ins, deep. stated, the tyre will have an inward pull of 1.84 ins. ring a forward movement of 12 ins., but it must be nembered that the tyre is rotating, the forward and :eral movements coinciding, and this factor, together th the tendency of the rubber to distort temporarily :her than to overcome the high friction between its sum-' and the ground, much reduces the possibility of wiping." Hence, no signs of undue tread wear show :ex considerable service.
Premature tyre failures, on commercial vehicles operating der severe conditions, are usually brought about by ists due to carcass fatigue or impact, which result from quent shock loads, These shock loads occur when a re hits a pothole similar to that .just described. With . orthodox axle the tyre leaves the road and plunges into e hole, the tyre being free of the ground for a fraction a second. The loaded platform, at this point, suddenly prived of support, follows, until it is arrested by the tyre. It is true that the springs absorb a considerable amount shock load, but the impact on the tyre is still most vere. One of the accompanying sketches shows the lative positions of the vehicle and road at this point, id it will be seen that the centre of gravity has fallen the depth of the pothole, and its point of application considerably nearer the lowered wheel, increasing the ad on it appreciably.
The most important point to remember about the =gess system is that the movement of the wheels has as effect on the platform. I have already emphasized Le fact that if one wheel drops 10 ins the movement of Le affected frame member will be 5 ins., but the actual iovement, both vertically and horizontally, of the centre I gravity, will be only half that occasioned with orthodox ispension.
An important Advantage of the Burgess Suspension System It will be seen, therefore,' that the tyres are not acted pon by such violent movements of the load, shock loads eing reduced by half. To • summarize this question, a &licit with a high load, high centre of gravity, and rthodox wheel suspension will more seriously affect tyre sear than the Burgess Unit.
At no time did we experience " tramp " during the trials 4 the Burgess Unit. This, I believe, was due to the .doption of two springs, when, as the periodicities of ibration of the springs did not coincide, the effects of ebound on one spring were damped out by the other.
'This Unit was tested on a trailer carrying a load of 0 tons over ground prepared for military trials and the losest observation failed to detect any wheel leaving the ;round; the movement of the platform was smooth
throughout. If two wheels of one bogie struck obstructions simultaneously the effect was similar to that of one wheel; On an orthodox axle, striking a similar object.
When one wheel of the Burgess Unit strikes an obstacle it rises freely, without bouncing, the springs acting as resilient levers, raising the load one-quarter of the vertical rise of the wheel. It is this free oscillation of the wheels, coupled with the raising or lowering of the load by leverage, that we must consider in order fully to appreciate the advantages of this system of suspension.
For instance, trailers fitted with orthodox axles were used for carrying loads of drain pipes over an aerodrome site, and it was found that a large percentage of the bottom layers were fractured in transit, whereas no pipes were fractured on any of the loads carried by Burgess Units..
Regarding the question of road camber, the Unit was designed to give the wheels a camber of about If degrees, and the condition of tread wear, throughout the life of the tyres, was perfectly even.
It is true that when manufacturers designed the Burgess suspension, they were solely concerned with articulation. It was my own observations which discovered the improvement of tyre life, which I tried to explain in my earlier article. The 10.50 by 13 tyre was, to my mind, the most unsuitable for use on unmade sites, As a matter of fact, this tyre is rated to carry a load of 40 cwt. at 75 lb. per sq. in. pressure, which is excessive when compared with the 10.50 by 20 tyre carrying 45 cwt, at 75 lb. pressure. The tyres in use carried a consistent overload of 40 per cent.
Reduced Wear on Tyres Not Primary Object of Design Under excessive loads, low-loading tyres quickly show signs of tread " wiping" which is a direct contradiction of Mr. Haigh's concluding remarks regarding the reasons for decreased tread wear. The 'general performance of this tyre had not been too satisfactory, owing to the small diameter of the beads and the fact that it had not been long enough in service to be fully developed. It was with special interest, therefore, that I kept these eight Firestone tyres under observation and considered their ultimate performance exceptional. On trailer axles the Burgess Unit undoubtedly reduces rolling resistance. If this be so on a trailer axle how will a driven unit be affected? It is logical to assume that less effort will, be required to move heavy loads; difficult terrain will be negotiated with greater ease; wear and tear will be reduced; fragile loads will be carried with less breakages, and passenger loads with greater comfort; tyre life will be improved.
I had no intention of attacking or criticizing axle designers, but rather to bring to their notice the interesting points of my investigations, in the hope that they may get a new angle on suspension design.