1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164 
Performance Characteristics of British Passenger Chassis
Page 120
Page 121
If you've noticed an error in this article please click here to report it so we can fix it.
TO form deductions from a collection of data relating to the road performances of vehicles, it is desirable first to consider what one may expect to find, just as mathematical problems are easier to solve, or the steps in their solution are easier to work out, when the result, or at least the approximate result, be known in advance.
The graphic representations on this page have 'been drawn from data extracted from the reports of road tests conducted by The Commercial Motor over a considerable period, and apply to three representative 'classes of passenger vehicle, namely, the double-deck bus, the single-deck 32-seater coach and the 20-seater, the first and second being subdivided into oil and petrol models.
Machines Which Provided Data.
The information is given in a fable and, although much of it was obtained-from machines introduced a year or more ago, certain models of these, if not still being produced, are in use with probably many years of service ahead of them.
The charts show average acceleration, retardation, fuel consumption, and power-to-weight ratio for the different types. These are given, respectively, by the numbers of seconds required to reach 30 m.p.h. from a standing start, employing the gears in the conventional manner; the distances in feet traversed in coming to rest from 30 m p.h.; the miles covered per gallon of fuel under normal operating conditions, and the ratios of maximum engine outputs, in bap. to the gross laden weights in cwt. of the vehicles.
D14 These last are included, partly as a matter of academic interest, and partly because they throw light, to some extent, upon the others. Braking is not affected by the type of power unit, so only three averages are shown in the retardation diagram.
Following the precept of our opening paragraph, in the consideration of the double-decker. we remind ourselves that high speed is not of primary importance, but good acceleration is desirable. Reference to the chart, however, shows it not to lead in this
respect. For quickly accelerating a
mass of 10 tons, a big engine is obviously necessary, but there is a limit to the dimensions practicable. Furthermore, the weight is much greater than that of the 32-seater, which also needs high power because in its case speed is a leading consideration. Thus• explanation is afiforded.
Economy of fuel is an aim in every case; therefore, for the moment, this matter may be omitted for simplicity.
The outcome of the compromise arrived at in respect of acceleration in the case of the oiler, is shown by an average figure as the lowest of the five
types, whilst, with the petrol machine, the difference from the smaller class is negligible. Between the oil and petrol 32-seaters, faster acceleration is alio revealed on the part of the latter. Thui .one may deduce, that the average compressio.,p-ignition units of the recent past and present have not drawn level with the petrol type in respect of liveliness. ,
Naturally, the rates of advancement of the two types are not the same, therefore we have no proof that the latest oil engines are inferior to those using the lighter fuel. Moreover, individual cases show the former to be gaining ground rapidly.
Relation of Acceleration to Weight.
The large coach, with its big power unit for high maximum speed and its lower gloss weight, has the better acceleration, despite the fact that somewhat higher top-gear ratios are generally employed. As would be expected, the handy 20-seater wins easily in respect of this characteristic.
In the chart showing retardation characteristics the lack of superiority of the 20-seater is, at first sight. surprising. In the goods-carrying field, weight—as a rule—is inversely proportional to 'speed of retardation, Nit between the double-decker and the 20seater there is scarcely any difference. The first explanation that comes to mind is that the bigger machine has power brakes, whilst the smaller chassis has not. That was the case withthe vehicles from which our figures were obtained.
One might surmise that bus and coach brakes are of higher efficiency than those on goods vehicles of equal gross weight; because of their bigger speeds and the greater responsibility they carry. But facts point away from this theory.
We find, however, that our weightretardation rule is properly observed by the two chassis with pdiweroperated brakes, so it appears that it is a case of the evidence supplied by the 20-seater being rendered of little use because one factor has changed the conditions of the test. On a number of the latest small-capacity bus-orcoach chassis, power brakes are standardized.
Fuel-consumption Deductions.
Five stops per mile are the rule for C.M. consumption tests of stage-carriage chassis. This practice was followed in obtaining the figures from which the double-decker averages shown in the chart were computed. Thus they are typical of returns experienced on ordinary service. What does the chart reveal? Clearly shown is the fact that the oiler consumes fuel at roughly the same rate as the petrol vehicle in the next smaller class, but to substantiate the opinions of those who claim that the oiler is capable of double the miles per gallon of the petrol machine, one has got td compare the worst petrol figure with the best oil, to do which, of course, is not representative, and uninformative.
To explain the discrepancies between
individual performances is not difficult. There are plenty of variables to account for them.For example, there are engine capacities, cylinder numbers, bore and-stroke,proportions, compression ratios, characteristics such as silent and smooth running, gear ratias and many others, whilst the actual tests we have conducted have inevitably been done under different conditions of road and weather, hills, traffic, stops, wind, and so forth all constituting a disturbing influence.
We endeavour to control these interfering factors, but exactly similar conditions are almost always impossible to obtain. This brings to mind an interesting side issue, namely, the different policies adopted by different makers. In some cases they wish the best obtainable figures to be published, and negotiate for an easy route to be chosen for the consumption test; in others they desire a return upon which users will find, with satisfaction, that they can improve. In the latter, the course suggested will be difficult,
The Commercial Motor engineer acts in the capacity of a 'damper to absorb• these variations, but their complete elimination is obviously impracticable.
In view of all these circumstances, it is not only gratifying, but also creditable to the C.M. system that the figures are as consistent as they are. Taken individually, they form an interesting study, it being instructive to try and trace a constant relation between • various figures, as in the case of powerto-weight ratio and acceleration.
Seeking Performance Factor.
This brings one to the old problem of seeking a performance factor, but a component of this must surely be chassis price. Here, again, however, a difficulty arises, for a cheap vehicle may have a road performance which is superior on all counts to a more expensive machine similar in type and capacity. One would he puzzled to include in the factor items such as durability, ease of maintenance and quality of after sales service.