Trying an IN with Sevcon
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by Martin Hayes
CHEAP WAYS of converting Britain's estimated 32,000 buses fitted with pneumocyclic semi-automatic gearboxes to full automatics are now common place. Latest addition is the device produced by Sevcon (CM, last week). Nearly 30 buses fitted with the device are now under trial, including a Leyland National operated by Eastern National. I recently had an opportunity to try this vehicle in central London.
This is equipped with the standard five-speed, wide-ratio semi-automatic gearbox and Sevcon unit, mounted under a passenger seat. After early trials Eastern National also modified the throttle return arrangements, fitted a throttle dipping facility and replaced the normal two-speed governor with an all-speed device. effort to eliminate fussiness and smooth gearchanges. The Sevcon unit usually offers manual engagement of first (on five-speed boxes) and automatic control of the •top four ratios but retaining full manual override facilities. To prevent drivers using the box manu ally, but while retaining a third gear hold facility for descending a 1 in 7 hill, in its territory, Eastern National has blanked off second and fourth gears.
On the road in the congested West End any automatic shows up well, and the Sevcon was no exception. Col league Martin Watkins found the Northern General Fleetline he tried in July exceptionally smooth. The Leyland National I drove was good, but not as smooth as LNs I have tried with Leyland's own G2 control system. That, however, is only fitted to vehicles with the close ratio gearbox and costs considerably more • than £300. The vehicle I drove, it must be remembered, had a 3.852 to 1 axle ratio and wide-ratio gearbox. This has a very high step between 2nd and 3rd gears—amounting to 1.76 to 1—which the Sevcon handles successfully.
I was very impressed by the kickdown performance and the lively way the vehicle moved.
Although both Eastern National and Sevcon were very satisfied with the vehicle, I thought that some minor improvements in gearchange characteristic ought still to be possible.