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 
That 30 m.p.h. Limit
Page 51
If you've noticed an error in this article please click here to report it so we can fix it.
THERE has been much satisfaction amongst many operators at the increase to 30 m.p.h. of the legal speed limit for heavy vehicles, i.e., those weighing over 3 tons unladen. British manufacturers with products in this class may also have a feeling of relief, mainly for the reason that the gulf between the normal speeds of such vehicles operating here and in many places abroad has been reduced.
It is understandable, however, that some operators should have certain qualms—those who operate heavy vehicles which may be placed at a disadvantage in the new circumstances. Nearly all their vehicles are capable of attaining the new limit, but more than this is required, particularly with the really heavy type.
With commercial road transport, it is the average speed which counts most. Whereas, with the 20 m.p.h. limit, the scheduled average speed may have been approximately 16 m.p.h., it will probably now, as a result of competition, be raised to about 25 m.p.h. Certain of the heavies, however, have their engines so governed as to permit drivers to overtake in reasonable time and with safety, older and slower-moving vehicles travelling at a speed in the region of 17-18 m.p.h., by employing third or fourth gear, depending upon the type of gearbox fitted.
Acceleration for Overtaking This requires a speed of some 20 m.p.h. In the case of the new limit, however, it is certainly desirable to permit a driver to overtake a vehicle travelling at say 26-27 m.p.h., again by using an intermediate gear. To enable this to be done effectively, it may be necessary to increase engine speeds and probably alter back-axle ratios.
In many instances, however, the r.p.m. of present power units could not satisfactorily be increased, certainly without utilizing crankshaft vibration dampers, and in a number of cases the fitting of these would be difficult if not almost impossible. The alternative of raising the back-axle ratio would, in the opinion of some well-known users, give only a small advantage and might be attended by other troubles, including poor hill climbing.
It might appear that the two-speed axle would, by permitting intermediate gear ratios, meet the case, but it must be remembered that models in the 14-ton-gross class are those which must receive primary consideration. Apart' from this point, two-speed axles do need careful handling and in cases where there may be a constant changeover of drivers there is always the serious risk of misuse.
Modifications in Design?
The question of the change to the new limit has been to the fore for a considerable time, therefore vehicle manufacturers should have been well prepared for its advent and 'have been ready to modify their designs accordingly, but so far there is little indication from some of them that the matter has received the attention which it merits.. No doubt those who have been backward in this will at least have new ranges on the drawing board, and it would be interesting to have news of them.
The manufacturers may also have useful suggestions as to how previous models might be brought up to date in this respect. There are certainly amongst the heavies a few which, quite apart from any higher-speed capacity, do not possess sufficiently powerful brakes to operate constantly at a much faster average than formerly. They will, no doubt, have to continue as "plodders," although they will then be under an economic disadvantage.
For the rest, the ability to overtake is of considerable importance. It avoids excessively long convoys and a consequent reduction of average speed, but to be effective, the action must be performed quickly, so that oncoming traffic is not impeded or undue risks taken.
As overseas models are often expected to run at higher speeds than those normally achieved here, we have no doubt that our manufacturers will be able effectively to cope with this home-market problem and the sooner this is done the better will our operators be pleased, and the greater will be the demand for new vehicles.