Slightly OT, but here's
my comparision on ZF and Voith:
ZF:
Jerky gearshifts
between 1st, 2nd and 3rd gear due to falling revs (rpms), but
generally uniform and smooth acceleration after 3rd gear. This
makes it suitable for many routes. Smoother braking also is ensured
as the gearbox can shift gears efficiently back to 1st gear.
Braking may still be jerky though.
Voith:
Smooth transition when
changing gears (possibly due to DIWA making up for two less gears),
but mostly jerky braking as the gearbox cannot change back to 1st
gear quickly. Sometimes, the pick-up may also be affected, although
it gradually allows the bus to travel at high speed with ease after
around 5-10 seconds. This makes it better for highway routes, where
a higher speed is needed for journeys.
Conclusion:
I would say that each
gearbox has its own advantages. ZF is more versatile, but Voith
should be utilised for highway routes. Both are okay for urban
routes with many stops, though ZF is better at handling smooth
gearshifts when stopping and going. But, it does depend a lot on
the BC to handle the bus to make it as comfortable as possible.
Very few BCs actually know how to handle a bus with Voith gearbox
with smoothness and speed. BCs are generally more familiar with ZF
gearbox, but may need more training to drive smoothly.
Buses that should get
ZF:
- Volvo B9TL Wrights =
Smoother acceleration and better braking.
- Mercedes-Benz Citaro
= Same as Wrights.
Buses that should get
Voith:
- MAN NL323F =
Strangely, the Voith gearbox suits the engine of the MAN
better.
Some remarks (i have already commented on this in an earlier
post):
1. torsion dampener quality affects upshifts and downshifts. The
Ecolife units deliver remarkable upshifts and downshifts across the
gears with minimal jerk/surge thanks to improved torsion dampeners
(over the Ecomats). While falling rpms are a problem, it is the
disparity in torque across the falling rpm which contributes to the
jerk as well.
2. in all transmissions, shifting back to gear 1 as part of
engine braking does not occur as a rpm surge from 2->1 will
damage the transmission /engine (recall that 388S has a tendency to
2->1 on slopes). Across all transmissions, engine braking drops
from the current gear to at most 2 or 3 for maximal
effectiveness.
to add - most transmissions have the gears 'follow' the braking
such that the best gear is selected should on-demand acceleration
be required. while braking is generally done by the hydrodynamic
retarder (integrated into the Ecolife/Ecomat/Voith), some operators
have had the EBS configured such that the transmission may
agressively downshift to provide the best engine retardation for
the speed.
3. the fewer gear changes may allow Voiths to reach speed after
about 10-15 seconds, but do not forget the amount of tradeoff at
the lower end.
4. If you have not taken the B3 ZF A22s, they actually povide a
much better ride than the B1/B2 ones due to the lack of 'surging'
as the engine exceeds 1400rpm, and not to mention the much less
agressive acceleration from halt. I cannot agree that the Voith
suits the A22s best because the engine is consistently pushed past
1400rpm by drivers who need acceleration at the lower end. To push
the D2066 engine past 1400rpm is a reciepe for high fuel
consumption!
note - to my knowledge, I have only seen the B9TLs demonstrate
full engine braking on downhill slopes; the significantly different
roar of the engine (ontop of the retarder) is an indication that
the VEB (volvo engine brake, google it) is activated. more often
would you see VEB or similar mechanisms activated on long-haul
trucks than citybuses due to their speed and tonnage.
