Model Forum / Radio Controlled / Land Models / September 2003

hey james,
by 'bias' do you mean that the servo travels farther in 'forwards' than
'reverse'?
if thats the case, investigate what kind of trims your transmitter has.
some radios have Endpoint Adjustment dials (my futaba radios do) where i
can, for instance, dial back my throttle if im letting children or friends
drive my car. Then they only crash at 1/2 throttle... 8)

Additionally, on some better radios you have the ability to set up throttle
for 70/30 operation. If you have a gas car or forward-only electric car then
you need a nice long thottle travel but only a little bit for the brakes.
With a plain ol' mechanical speed control (MSC), you want 50/50
operation....see your manual or - if you dont have one, post the make &
model of yer radio and we'll help you look it up online.

Also, when you do that servo trick - where you remove the control horn and
rotate it one or two splines - its a good idea to zero out the trim dial on
the transmitter before you put the control horn back on.

g/l, mike

g'day Jay,

Ah hah!  I fink I know what the prob might be regarding trim / servo
"bias".  no probs for the reply, btw... sorry i didnt spot the
potential "bias" issue first up  :-/

there be three types of mechanical (resistive) speed controllers:

1) 3 forward speeds, 1 reverse - all "stepped" (ie not continuously
variable)
2) multi-stepped ... number of steps varies by model/manufacturer, but
the one constant is that you have the same number of steps in reverse
as you do forwards
3) continuously variable from full forward, through "neutral" (off) to
full reverse.  this one differs from the first two in that the first
two use fixed resistors (those white or grey ceramic blocks that get
stunningly hot - theyre rated to operate at 300 degrees celcius, btw)
... this third one uses a big wire-wound resistor - instead of having
a switch plate (operated by the servo) with wires from the switch
plate to the fixed resistor(s) (or, with earlier models, resistors
fitted under the switch plate), the servo operates an arm which
"wipes" along the resistive coil... its by moving that contactor arm
along the coil (thus varying the amount of resistance in the motor
circuit) that this type of controller operates.

numbers 2 and 3 both have equal steps/control both forward and
reverse, and thus require the servo to be at true neutral for the
motor to be switched off.  by "true neutral", i mean the centre of the
servo's possible throw (travel) - exactly the same as with the
steering servo.

number one, however, works differently - it has (usually) 5 distinct
steps, being:
* forward high speed
* forward medium speed
* forward low speed
* "neutral / off"
* reverse medium speed

which means that turning the servo to true neutral will actually
select "forward low speed", not "neutral/off".  its to accomodate this
type of controller (and to allow greater sensitivity of control in
forward mode using an ESC) that TX's often have a second "trim"
adjustment on the throttle (north-south) stick - presumably the same
extra trim is available on the triggers of wheel-type controllers if
thats what youre using.  this second trim adjustment may either be in
the form of a screw you loosen to allow adjustment, or simply a tab
you click from one end of its travel to another.  either way, what it
does is allow the control stick/trigger's "neutral" position (where it
springs to when you let go of it) to be offset from "true neutral".
this would be utterly useless for steering, but for throttle with a
number 1 type mechanical controller, its ideal - it allows the servo
to return to a position other than true neutral when you let go of the
stick.

my guess is that you either have a stick which is set to return to an
offset position, though you have a number 2-type controller which
requires the stick to spring back to true neutral (thatd be my guess,
having just grabbed the "came with my TL01" mechanical controller and
found it to be a #2 type, with 2 forward and two reverse speeds, plus
"off" in the true middle of the servo's travel)..... OR, you have a #1
controller (which requires your throttle stick/trigger to be offset)
and your throttle stick/trigger either isnt offset, or doesnt have an
offset adjustment.  determine if your stick/trigger's neutral matches
the neutral your controller requires, and i suspect you'll have
finally found your TL01's demon  ;)

phew, did any of that make sense?  id be more than happy to photograph
the various #1, #2 and #3 mechanical controllers i have laying around,
and also photograph a couple of control sticks set to true neutral and
offset neutral, and post the lot in, umm, the name of the newsgroup
escapes me now but i'll remember it soon as i send this post  ;)
really, i do understand how confusing the various controller types are
and how baffling my description was - a picture is worth a thousand
words, and speaks them more clearly.

re advice on an ESC, im not the one to ask, having only the one
high-current controller (all mine so far have been home-brew jobs for
scale boats... yes people, theres a heretic in your midst:).  that
said, these are the questions i asked when i decided to get a
bullet-proof ESC for my touring car:

* does it have braking or reverse, or is it a "forward only" model?
forward only is kinda limiting.

* will it cope with the current i'll be asking it to cope with?  ok, i
prefer to stick to "stock" (27-turn) motors, if only coz i question if
the TL01's geartrain is up to the kind of abuse an 8-turn modified
motor could hurl at it.  but... a stalled electric motor draws about 3
times its "peak effort" current.  so a motor that draws 25 amps when
its operating with its intended maximum load, will draw 75 amps (or
thereabouts) when youve got offroad tyres on your TL01 and have got
the thing bogged in deep grass in the back yard.  its moments like
those which determine whether that yum-cha brand ESC was a worthwhile
investment  ;-/

* is it a recognised, well-respected brand?  other hobbyists have
paved the way for us beginners - names like Novak, Futaba, etc, arent
popular for no reason.  stick with'em.  i did, without regret.

* what input voltage limitations does it have?  i wanted one that
could handle 12 volts (10 cells), so i could also use it in boats (12
volt gel batteries are about the standard power source for scale
boats).  alas, 6-7 cells (7.2-8.4 volts) is the norm.  you can get 10
cell (and MUCH higher) ESC's, but youll *really* pay for them.  its
your call.

*does it have BEC?  the answer for modern ESC's seems to be a
unanimous "yes", but better to be safe than sorry - make sure it does,
or make sure your RX has BEC, or you might find yourself having to
have a 4 cell RX pack as well as your main drive pack  :(

bottom line, i went looking for an ESC that would handle the current
loads i ask of it today, plus have the ability to handle more if i
should decide to switch to modified motors in the future.  i went
looking for an ESC with reverse (got lucky enough to get one with both
braking AND reverse - odd at first but you quickly get used to it, and
its an excellent feature) ... forward-only or forward & braking is
fine if you want to compete in events in which reverse is banned, and
ONLY want to compete in such events - if you want a general purpose
car though, you want reverse.  btw, the ESC i ended up with can have
reverse disabled for racing in events where reverse is banned
(ROAR-based rules, i think?  anyone?) ... i suspect this may be a
common feature of better ESC's.  i went looking for an ESC with BEC
(Battery Eliminator Circuitry - the ESC supplys the lower voltage
required by the RX, in the event the RX doesnt have the BEC feature
anyway) ... coz most of my RX's are older non-BEC jobs, and my days of
operating three or more distinctly seperate batteries to run one model
are OVER.  ;)

I ended up with a Futaba MC330CR.  13-turn limit, 6-7 cells, foward,
braking, reverse, programmable "no reverse" mode for competion, BEC
... umm, the damn thing even beeps when you turn the RX/ESC on before
you turn the TX on.  :)  Many will howl me down, say "no, you want
this or that, its better than the MC330", and in many respects and for
many different applications, theyre likely right... youd do well to
listen to their arguments, and consider the cases they make.  but, im
yet to regret my purchase.  *shrug*

hope this didnt take too long to read  ;)

cheers,

*DS*

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