"Dominic" wrote in message
om...
In article ,
Tom Anderson wrote:
What's the point of trams?

I'm not having a go, i just don't really understand what's so great
about
them. Not from the heavy rail side (they're obviously much cheaper
and
more flexible, whilst smaller and slower), but from the bus side.

Yeah, why do we need trams? Here's my opinion - feel free to criticise
it:

1. Buses are as fast as trams - even with diesel engines. If Croydon
trams ran long sections on the road mixing with cars, or if London
buses ran on a proper network of properly enforced bus lanes, that
would become clear. The maximum acceleration of both buses and trams
is set by passenger comfort - I reckon both can reach that maximum.

While it is possible to design a bus with an engine that would give it such
performance, this is not done because the size, weight, initial cost and
increased fuel consumption of such a large engine increases the operating
and capital costs without enough of a gain in performance for such
technology to be justified. The nature of electric traction is such that it
can be run at significantly higher than its maximum continuous power rating
for a short period of time quite safely. This is very useful for
accelerating rapidly and then maintaining that constant speed for a longer
period.

2. Buses can easily rival trams at shifting passengers - just 2 of
these 180 passenger Van Hool double-artic buses carry more than a
Croydon tram:

http://www.vanhool.com/products_bus_...Categ oryID=1
They're a bit unwieldy, but so would Croydon trams be if they really
had to mix with the traffic!

The confinement of a tram to the fixed swept path of the tramway makes them
considerably more controlable in the sort of tight spaces encountered in
London than such a bus. There is no guarantee that the middle and rear
portion of the bus in that photograph will follow the same swept path as the
front. Anecdotal evidence suggests that even with single articulated buses,
there are problems arising from this. If you look at the way articulated
buses attempt to pull over at a stop, you will see that they almost
invariably end up with the front half pulled over and the rear still
blocking the carriageway.

3. Buses ride just as well as trams, if you put them on a well
surfaced road. Both can suffer from harsh braking when mixing with
cars and pedestrians. There's nothing wrong with rubber tyres - they
allow you to apply greater tractive and braking forces. That's why
many Paris Metro trains have them.

With magnetic track brakes as applied to all trams built for decades, the
braking force available to a tram (the rate of deceleration possible) is
appreciably greater than that possible with rubber tyres on tarmac. Buses
only ride well if they run on a well maintained road. As the bus companies
do not maintain their own roads, they tend to run on ill maintained roads.
Trams ride well on well maintained track, but as tram tracks belong to the
tram company, they tend to be well maintained. Acceleration of buses and
trams is not limited by friction but by the power of the engines. The
torque characteristics of an electric motor and their ability to overload
means that, if they want to, they can out-accelerate just about any diesel
engined bus at normal street conditions. So that's wrong on all these
counts.

4. Diesel buses are more environmentally friendly than electric trams.
Although buses produce more pollution at the point of use, trams
produce more pollution overall - the electricity they run on has to be
produced somewhere, and it's been through a lot of inefficient energy
conversions by the time it reaches the tram.

If you consider the full energy chain for each, you will see this is not the
case. First of all, the rolling resistance for a tram is a fraction of that
for the same weight of bus. Weight for weight, trams carry significantly
more passengers because electric motors are much lighter than diesel
engines, and they carry no fuel. This means that per passenger, they use
less energy to move about. Because of the higher efficency of large power
stations, and the relatively high efficiency of the national grid, per unit
energy from combusted fuel to point of use at wheel tread, there is a slight
advantage to buses, but this is of the order of about 5%, while the benefits
of steel wheel and electric traction are more likely to be of the order of
20% on a per passenger basis. If you then consider the energy requirements
to get fuel from where it comes out of the ground to the location of
consumption, and any refining process, you find that diesel fuel is
substantially worse off there (coal and natural gas require no refinenemt
for use in a power station, and nuclear fuel is used in such tiny quantities
for the energy released, these costs are minute in energy terms).

The common fallacy you are falling victim to is considering only the
thermodynamic efficiency from combustion to power at the wheel tread, not
looking at the power demands, or the requirement to get the right fuel to
the point of combustion.

5. Buses could have the "wow factor" and desirability of trams, if
they were made to look more exciting. Designs like the Wright Eclipse
Gemini are heading in the right direction.

But buses will always lack the permanent advertisement of their route
provided by the fixed infrastructure. If I look at the road in front of my
house, there is no evidence that there are two bus routes down it. If there
were a tramline down the road, there would be.

6. One final point - buses require no fixed infrastructure to be built
on their route. You can run them on the road - brilliant! If there are
roadworks, you drive around them!

Wrong again. you can't run a bus over a field, you need some fixed
infrastructure called a road. To give the bus adequate reliability, you
need more infrastructure such as a bus lane. If nothing but buses use the
bus lane ever, then the costs of maintaining that bus lane must be
attributed to the bus services that use it. the point services are diverted
from under tramways is so that there *aren*t roadworks on the tram's path.

Robin