William, I’m not quite certain whether this reply will answer your question but I hope that this is simple enough for everyone to understand....

On Z, A and B classes (and maybe some lowfloor classes too) as well as the Gold Coast trams, the brakes are applied by springs and released by an electric motor pumping hydraulic oil to lift the brake pads from the discs on the same axles as the driving wheels. These motors are run off the tram’s 24v batteries.

But it takes a couple of seconds for the brake pads to move clear of the disks and allow the tram to move. If you are located in the right part of these trams, you will hear a little whining noise for about three seconds while the pumps are running.

However at the same time that the pump motors are activated, the electronic power controller is feeding more and more power to the motors. So The power to the motors is increased by quite a few volts by the time the brakes let go and so the tram starts with a jolt. It would be much like the driver of a W releasing the airbrake when he is in third or fourth notch, rather than first notch. In both cases, the brakes will hold the tram solid against quite a high voltage applied to the traction motors.

Unlike a PCC where the foot pedal controls the rate of acceleration and ultimately the tram will reach maximum speed even with the lightest touch of the pedal, on the trams we are talking about the so-called ‘first notch’ of power will cause the tram to run along at its lowest possible powering rate. (This was useful in the Bourke Street Mall climbing slowly from Elizabeth Street to Swanston Street.) That means if the driver pauses his accelerator in the minimum speed position for a couple of seconds, the brakes will fully release and the tram can then be fed more power without a jerk.

As for the Melbourne E class which I ride in Melbourne most days when commuting to wherever I’m videoing for the day, I have never heard the whine of the electric hydraulic pump motor so I strongly suspect that it does not have any and that maybe some form of electromagnet releases the springs instead.

So with all the “Could not care less” drivers who whack the accelerator to the floor, you will always get an unpleasant jerk when starting off. Similarly with braking, I discovered that on the Melbourne foot control cars that you can ease the brakes back even in the hydraulic mode and get a rather smooth stop, but I have no idea just how that is achieved. And few drivers ease off the footbrake on stopping. I really notice that when trying to hold the video camera still (but now have a suction cup holder, not that that is important to you folk.)

I’m not interested in arguing whether software should be better or anything like that. I am just stating the situation as I understand it to be.

As for the Xtrapolis trains, never having driven one I can’t say whether they have the equivalent of a ‘first notch’ position.

Richard

On 22 Mar 2019, at 1:27 pm, William Jackson apairofjacks@...> wrote:

I’ll back Richard 100% here. Is there any way of overcoming the holding brake?
William – Noting that the suburban Xtrapolis trains have the same issue

From: 'Richard Youl' via TramsDownUnder [mailto:tramsdownunder@googlegroups..com]
Sent: Friday, 22 March 2019 9:22 AM
To:tramsdownunder@...
Subject: Re: [TramsDownUnder] Re: Paris Tramway T3a on youtube

If you happen to be replying to me, you response proves that a week in Melbourne is WAY overdue.

Regards,

On 22 Mar 2019, at 08:04, Prescott lenkaprescott@...> wrote:

I must say that I've never experienced a jerky start or stop on a tram. Look forward to the day when I find one! It must be limited to a few cases where either the manufacturer's or the operator's techs haven't done their job properly.

Tony P