Auto midi on trial
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Greater Manchester Transport has 22 more Seddon midibuses on order and it was the first of these which I drove last week. Like the rest of the order this bus has automatic transmission, and is the first operational midi to be so fitted.
A vehicle with an Allison four-speed box was shown at the Glasgow show last November but during trials the PTE was not satisfied with its performance. Since then it has been discovered that another Allison box with improved change characteristics and developed for use with motor caravans was more suited to the output of the Perkins 4.236 engine and this is being specified.
These rn id ibuses are designed for use on city centre services and have 19 seats and accommodation for 15 standees. Outwardly they differ little from the earlier midis operated in Manchester and have the high window line designed originally for the coach version of the midi.
I was able to drive the midi back into Manchester from Oldham, a run over varying road conditions of some 15 miles or so. The selector for the Allison AT540 box is to the right. Though each of the four-speeds can be held manually, in practice the D or Drive position is the one most commonly chosen. In this position gear changes re outstandingly smooth in ny road condition.
Certainly this rates as one f the pleasant bus autoatics I have driven.
A point that needed atching by a driver new to he bus is the absence of ngine braking at roads higher than 20mph or so. It is not until this speed is reached during deceleration that the gearbox begins to change down. Until one makes allowances for this feature progress tends to be a little jerky with too frequent brake applications.
Change-up points at part throttle are as follows (with change-up points on full throttle in brackets): 1St to 2nd, 5 mph (8 mph); 2nd to 3rd, 12 (16); 3rd to 4th, 24 (28). Even using kick-down -available in top gear at speeds of less than 25 mph — virtually jerk-free gear changes are feasible. Driving the vehicle, which is fitted with a 6.83 : 1 ratio, I discovered that no special skill is needed to obtain such changes.
When CM road-tested the little Seddon last year we strongly criticised the internal noise levels. Although the Manchester bus had a padded engine cover and 1 am told that development work has been carried out in this area I felt that in most conditions engine noise was still too high.
During stop-start operation — for which the vehicle is intended — constant acceleration produced really unacceptable noise levels. Only during steady speed cruising — not common in city centre services — was the noise reasonable.