Old and obscure feature of 175 braking

[Philip]

I remember years ago when class 175s still had their hydro-dynamic retarder brakes active, you could clearly hear it being applied inside the carriage. Usually you would hear the throttle/gears being dropped and the engine cruising in neutral (if that's the right term?) for a while before revving up when the HD brake was activated, but occasionally the reduction in throttle went straight into the HD brake noise, without the neutral 'pause' inbetween.

I thought this sort of thing was bad for gearboxes and transmission, going straight from throttle into reverse or braking in this case? If anyone drove 175s or even remembers travelling on them when the retarder was still being used, do you know what I mean and could anyone give a better explanation for the difference in transmission noise I'm referring to?

 

[hexagon789]

I remember years ago when class 175s still had their hydro-dynamic retarder brakes active, you could clearly hear it being applied inside the carriage. Usually you would hear the throttle/gears being dropped and the engine cruising in neutral (if that's the right term?) for a while before revving up when the HD brake was activated, but occasionally the reduction in throttle went straight into the HD brake noise, without the neutral 'pause' inbetween.

I thought this sort of thing was bad for gearboxes and transmission, going straight from throttle into reverse or braking in this case? If anyone drove 175s or even remembers travelling on them when the retarder was still being used, do you know what I mean and could anyone give a better explanation for the difference in transmission noise I'm referring to?

They don't really have a gearbox in the conventional sense. The hydraulic transmission means the engine is never connected directly to the wheels, instead you have an oil bath with two impellers.

When power is shut off, the oil is drained from the transmission and the unit will "freewheel". My best guess for what you describe is that if the driver goes straight from power to braking, the transmission doesn't have time to empty, so the hydro-dynamic brake can come into action sooner than having to wait for the transmission to refill with oil.

 

[Philip]

They don't really have a gearbox in the conventional sense. The hydraulic transmission means the engine is never connected directly to the wheels, instead you have an oil bath with two impellers.

When power is shut off, the oil is drained from the transmission and the unit will "freewheel". My best guess for what you describe is that if the driver goes straight from power to braking, the transmission doesn't have time to empty, so the hydro-dynamic brake can come into action sooner than having to wait for the transmission to refill with oil.

Makes sense, thanks!