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List of Bedford Bus Models 1931-87
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Author:  9711T [ Thu Apr 04, 2019 1:03 am ]
Post subject:  Re: List of Bedford Bus Models 1931-87

The attachment shows the specifications for the Bedford VAM3 and VAM70 as supplied in New Zealand in 1973 June.

Bedford VAM 197306 p.01.gif
Bedford VAM 197306 p.01.gif [ 957.92 KiB | Viewed 1507 times ]

The VAM3 had the Bedford 4-speed wide ratio gearbox and a two-speed axle.

The VAM70 had the Turner-Clark 5-speed overdrive gearbox and a single-speed axle. The ratio set was somewhat different to that which had been used in the VAL14, as follows:

1st: VAM70 6.06; VAM14 6.06

2nd: VAM70 3.50; VAM14 3.00

3rd: VAM70 1.80; VAM14 1.70

4th: VAM70 1.00; VAM14 1.00

5th: VAM70 0.799; VAM14 0.80

For a given axle ratio, the 2800 rev/min Bedford 466 would have achieved with its 3.5:1 2nd gear the same speed as the Leyland O.400 at 2400 rev/min with its 3.0:1. But it would, or should have had better starting capability, thus with fewer occasions on which 1st was needed.

In its 1.8:1 3rd gear the 466 would have been 10% faster than the O.400 with its 1.7:1, although probably at the cost of a noticeable increase in noise, vibration and harshness. On the other hand it would probably have “hung on” a bit longer in 3rd when running uphill.

One is left with the impression that Bedford was working to a “just acceptable” target. A 6-speed overdrive unit would surely have been better, but probably not readily available in synchromesh form. With a 6:1 1st, the geometric ratio set would have been 6.00, 3.83, 2.45, 1.57, 1.00 with a half-step to 0.80.

An aspect of the VAM is that as far as I know, it retained the original TK-type air assisted hydraulic braking system, as used on the VAM5 and VAM14, through to the end of production. The VAM3 started with the TK-type vacuum assisted hydraulic system, but was swung over to the air-assisted version at some stage in its production life.

On the other hand, whilst the SB was changed to the TK-type system late in 1961, it saw a further change (late 1960s or early 1970s?) to the air-actuated hydraulic system using I think the same Clayton Dewandre actuator that first saw the light of day on the BMC FJ truck, but was probably best known for its use on the Ford D800 truck.

I have also attached the Bedford SB specifications from 1963 February and 1964 September:

Bedford Passenger 196302 p.10.jpg
Bedford Passenger 196302 p.10.jpg [ 984.04 KiB | Viewed 1507 times ]
Bedford PSV 1964 p.10.jpg
Bedford PSV 1964 p.10.jpg [ 203.27 KiB | Viewed 1507 times ]

In 1963, the SB1, SB3 and SB5 all had the Bedford 4-speed wide ratio unit as standard.

Optional was a 5-speed direct top constant mesh unit with ratios: 6.93, 3.78, 2.24, 1.47 and 1.00. Quite a bit of convergence, as the geometric sequence would have given a 4th gear of 1.63:1.

The SB8, with Leyland O.350 engine, had the close-ratio version of the Bedford 4-speed gearbox as standard. Its ratios were: 6.50, 2.86, 1.576, 1.00. The corresponding geometric set would be: 6.50, 3.48, 1.87, 1.0. So there was significant convergence.

The optional 5-speed direct top unit had ratios: 6.48, 3.54, 2.09, 1.37, 1.00. A geometric 4th would have been 1.6:1.

The 5-speed overdrive option was the same for all engine variants, 6.3, 3.36, 1.83, 1.00, 0.82. In this case 3rd was further away from direct (4th) than was the case for the 4-speed gearbox, very much so for the SB8.

In 1964, the SB3 and SB5 gearbox options were unchanged from those obtaining in 1963. By then the SB1 had been withdrawn. And the SB13, which had replaced the SB8, had the same gearbox options as the latter.

My take on that is that the ENV and Turner-Clark 5-speed direct top gearboxes could be had with reasonable ratio sets, but the overdrive top versions were more problematical.


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