On Sat, 17 Feb 2018 19:15:42 +0000, Tim+ wrote:

Johnny B Good wrote:
On Sat, 17 Feb 2018 18:07:34 +0000, Tim+ wrote:


One problem with that is that it moves the disc (and pads) out of
reach of easy inspection/service.

The real problem with that is the possibility of a high speed wheel
lockup under emergency braking snapping the drive shaft as a result of
the rotational energy stored in the wheel.

Compared to the kinetic energy of a car I would have thought that the
rotational energy of the wheel is relatively trivial..

It's the peak shock loading going via a couple of UJs that's the problem.

I would agree that it introduces a new €œweak point€� in the braking
system though. Off the top of my head, Jaguar E-type, Rover 2000/3,500
and Citroen ZX all had inboard discs,.

That may be but I doubt they were attached to the wheel via a UJ. Even
though they might have been described as "inboard" brakes, the section of
drive shaft involved would not only have been quite short but also beefed
up to take the strain.


Did they have a reputation for snapping drive shafts?

No Idea, gov.

The torque loading from translating the kinetic energy of the vehicle
into heat energy in the brake disks at maximum braking force just shy of
locking up the wheels is limited by the tyre grip to the road surface,
circa 1.5 G. Locking a wheel through overenthusiastic application of the
brakes can generate a very high shock loading on the UJs in a system that
places the brake assembly at the sprung end of the suspension system
rather than more directly at the unsprung wheel side of the UJs.

Even assuming the UJs can cope with a limited number of such shock
loads, the further away the brake assembly is mounted along a relatively
spindly shaft from the wheel, the greater the risk of damage from the
sudden torsional forces being applied.

A conventional brake is still required on an all electric vehicle that
uses regenerative braking just to cover the final 15MPH or so to 0MPH end
of the braking phase where the regenerative braking effect fades to
nothing.

In a direct drive design using wheel hub motors where unsprung mass is
an issue, the temptation is there to reduce the mass of the conventional
disk brake assembly to a minimum which will reduce the maximum speed
rating to just above the tail end of the effective minimum speed range of
the regenerative braking system.

However, in practice, rather than qualify them for say 20MPH, they're
more likely to be qualified for 50MPH to give some margin for emergency
braking on a long downhill gradient (provided the speed is held to no
higher than 50MPH in this case).

Whilst this will add a little more unsprung mass than strictly necessary
when assuming the regenerative braking system is never ever going to
fail, being mindful that even the best designed systems can suffer
catastrophic failure, they'll no doubt hedge their bets on this and add
an independant secondary emergency dissipative braking circuit[1] to the
hub motor circuit which can, along with the transmission power management
control and monitoring logic, log any problems to reduce the likelihood
of two seperate, but extremely unlikely (it is hoped) faults occurring in
both electrodynamic braking systems simultaneously by alerting the user
and the service engineer to any symptoms of impending problems in either
system (regenerative or dissipative) which need to be immediately
addressed. The 'weedy' lightweight disk brakes can act as a last chance
saloon backup in the event of such a double failure (hence the likelihood
of them being rated for 50 rather 20MPH).

The point I was trying to make was that, given sufficient development,
the all electric transmission direct drive system offers far more benefit
than deficit in a 'normal' electric road car. I think the issue of
'unsprung mass' is perhaps being a little over stated in this case thanks
to rare earth permanent magnet DC brushless motor technology.

[1] Such a 'secondary' dissipative electrodynamic braking system will be
needed anyway just to cover the worst case scenario of a vehicle setting
off with a fully charged battery from the top end of a long downhill
stretch of road. Apart from the initial burst of acceleration to reach a
sane cruising speed, the battery will be in no condition to accept a
prolonged recharge from the regenerative braking system which will then
have to call on the dissipative electrodynamic system to keep the kinetic
energy build up in check by converting it into waste heat as the vehicle
converts its potential energy into kinetic energy during its prolonged
descent.

This waste heat would simply be dissipated to the environment in mild to
warm weather conditions but could be put to good use as cabin heating on
early frosty winter morning runs. In any case, an element of
electrodynamic dissipative braking will probably still be required to
limit the peak charging rate of even a discharged battery under extreme
high speed braking conditions to avoid exceeding the battery's maximum
charging limit.

The control logic to manage the power flows will not only need to be
very sophisticated but also very robust. I'm sure the car manufacturers
will be able to come up with a safe and reliable solution, after all
we've been putting our lives into the care of such 'Fly by Wire' systems
for well over a decade now with commercial aviation.

--
Johnny B Good