Model Railway
Wheel-Rail Interface
& the importance of Back-to-Back dimensions
by Caiptean
These help
notes are for the benefit of railway modellers.
They were originally prepared
for members of the Hornby Forum, but equally
apply to other makes.
|
Bachmann 2 COM units on
finescale track © Ewhurst Green |
Wheel-Rail
Interface
On all railways the wheel-rail
interface (in industry parlance) is the crucial element in ensuring trains run.
However, its importance can often be overlooked or even misunderstood by some
railway modellers – including manufacturers. Standards used on railway models
vary considerably (even within a manufacturer’s own range) and this can lead to
issues and poor running especially on turnouts and crossings.
In terms of model rails incompatible
wheel standards will cause problems. For example, early Rovex
models with coarse plastic wheels will not run properly on (say) code 75 track.
However, even when the wheel profile is compatible with the chosen track problems
can still arise through the back-to-back dimensions of the wheels being
incorrectly set. The back-to-back dimension being the measurement between the
inside faces of the wheel flanges.
Derailments
Many modellers will have a turnout on
their layout upon which a number of items of rolling stock regularly derail. In
the end that turnout gets lifted /replaced, the problem appears to go away and
the conclusion that it was badly laid else faulty is arrived at.
Except, was it?
Obviously uneven track will lead to
derailments but so will defective wheelsets. I don’t mean wheels that have
chunk missing out of them; just defective in their profile is poor and quite
often their back-to-back dimension is too tight.
When passing through a turnout’s
common crossing there can be a tendency for wheels to try and take a diverging
route either jumping as the wheelset catches the crossing nose or simply
derailing. Wheelsets with tight back-to-back (i.e. less than 14.5mm) have
considerable sideways play on the track as the flanges are not sitting snugly
within each running rail; this excessive lateral play between the running rails
can often lead to poor running.
With this excessive lateral play the
rail vehicle itself can also move more than it should. So, it struggles to
follow the track and at certain locations the tolerances required to keep the
wheelset on the track are exceeded leading to a derailment. Often this may
happen with (say) two or three wagons which are then labelled as being ‘rogue’
or troublesome and so are ‘carded’. Relegated to the back of the layout they
are now rarely used without further investigation.
Model train wheelsets
As a railway modeller my layout has
seen more visitor-mileage than with my own stock and this provides an
interesting insight into what occurs with models. The coarse wheel profiles of
Lima and Jouef cannot run; neither can the older Triang and Hornby wheels but much of the rolling stock
today can. Indeed, even some of Hornby’s current pizza-cutter wheels give
problems (for example the model JA /JB - class 73 in TOPS parlance) so are
replaced with Ultrascale wheels (they produce excellent conversion
packs for many models).
Moulded plastic wheelsets are taboo for
similar reasons. Furthermore, plastic wheels also build up dirt.
I know the track is now spot on – some
adjustment of check rails, K-crossings on slips (etc) followed many hours of
running my own stock fitted with finescale wheelsets amply demonstrates this.
But then (say) that pesky leading axle on a visiting Bachmann class 40
occasionally derails on the common crossing of a single slip. Up-turned on the
modelling desk, it is found that the back-to-back dimensions of that ‘pesky’
wheelset was less than a millimetre too tight and once gently opened out to
14.5mm the locomotive runs faultlessly thereafter.
Another regular contender is (say) the
Hornby modified Merchant Navy class where the tender wheelsets are frequently
too tight. Recently the leading axle on one locomotive (i.e. on the pony truck)
was found to be wide to gauge leading to infrequent derailments until it was
adjusted.
Excess lateral axle play
Many model locomotives also have
excessive lateral play in their driving axles and for a layout where 3’ is the
minimum radii (a standard used by a number of us) this also leads to unreliable
running. Wheels are removed and (for a 2mm axle) 8 BA brass washers are
inserted to reduce the sideplay; the locomotive’s
running is significantly improved. To be precise 8BA is 0.0866 in or 2.2
mm.
Of course, there are many who rarely
encounter these issues and may even be bemused by them. However, next time you
have a derailment just get a set of callipers out and check the dimensions of
the derailed vehicle’s wheelsets – you might just be in for a surprise!
Split Axles
When it comes to split axles these
can present a problem if the back-to-back dimensions need adjusting for you
can’t! Essentially you are down to
replacement wheelsets which is rarely possible or retiring the locomotive.
Fortunately, split chassis locomotives have fallen out of favour and in some instances manufacturers have re-released versions of a
particular model with a new chassis.
Unfortunately, with the increased use
of lighting (particular with Pullman cars) split axles are finding favour is
some coaches. However, this rarely presents a problem.
Loose Wheels
Some Hornby coaches appear to
have a slightly deeper flange than (say) Bachmann equivalents with these deeper
flanges contacting the sleeper chairs on curves
formed of finescale track such as SMP or Exactoscale. In addition, on certain batches of coaches one
of the wheels is not a firm fit on its axle and can be prone to unwanted variations
in its back-to-back dimension (leading to derailments).
Accordingly, some modellers may
choose to replace the wheelsets on Hornby coaches with 14mm Romford wheels (or
equal equivalent), even resetting their back-to-back dimension set to 14.75mm. As a consequence, running of these coaches is much improved
and whilst the cost of the 14mm disc wheel has significantly increased to £2.50
/axle in 2012 (around a decade ago Romford wheels were 84p) for many the
expense remains worthwhile.
Back-back gauges
Besides using a Vernier gauge several
manufacturers make back-to-back gauges.
GW Models of Lancing, West Sussex make an excellent
adjustable back-to-back gauge primarily intended for steam locomotives
(mail-order but not on-line)
DCCconcepts produce a number of gauges including
both the standard 14.5mm, ‘fine 14.75 and ‘special fine’ 14.85mm gauges.
Both C&L and Markits produce standard 14.5mm back to back
gauges.
The standard ‘00’ back-to-back gauge
is 14.5mm. However, although finescale modellers may use 14.75mm /14.85mm
gauges these are not normally recommended for proprietary rolling stock unless
you are sure their use is compatible with the wheel’s profile.
Checking
the Back-to-Backs
by LC&DR
Modern ‘00’ wheels should normally be 14.5mm between inside
faces and if narrower that that will cause derailments, especially at turnouts
(points). It is possible to buy a special gauge to measure this, but it is
perfectly possible to use a cheap Vernier gauge instead. Wheels found to be out
of adjustment can usually be adjusted by gently sliding them on the axle.
Locomotive driving wheels should only be tackled by experienced modellers.
©Image
LC&DR
Further
reading
There are defined wheel contours for RP25
and NMRA recommended
rail practices.
The Double O Gauge Association also
publish their standards.
NMRA also has a recommended practice for wagon weight.
The Scalefour
Society publish on springing for
railway models (very specialist).
©BloodandCustard