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Steve,
Excellent post. Thanks
very much for divulging that little secret of the Cromwell's suspension.
I'm building suspension into my first tank (M1A1) for several reasons: the
tank is large and is sure to be quite heavy, it's running at 24V and despite the
weight should still be pretty quick, and control while at speed is important to
me as I think attacking and moving at high speed will fit my style of driving
very well.
While I've given thought to
having the tank balanced, I never thought to just pull the suspensions arms all
the way to their stops and let it ride like that. I will definitely give
this setup a try and see how it works for my driving style and my
tank.
Of course, none of this matters
if I don't get the tank built and moving under it's own power... Back to
the workshop!!!!
Jeremy
----- Original Message -----
Sent: Friday, February 09, 2007 4:55
PM
Subject: RE: suspension [TANKS] A
TyngTech Point of View
Peter wrote:
> Most tanks only have shocks on
the first one or two wheels and the last
> wheel on each side. The other
road wheels are sprung but not damped. That
> blisteringly fast Cromwell
that we all know and love is well
> suspended but I don't think any of
the wheels are damped.
I've been following this thread on suspensions
with much interest. It started with a builder wanting to know if a
suspension was really required for a decent performing tank. Let me say
right now that a suspension is not required for a competitive vehicle.
There are many solid axle tanks that get the better of the Cromwell (and me
;-) all the time! Amir recommended first time builders go the solid axle
route for their first tank and I whole heartily agree. Particularly if
the first time builder is not yet sure of their fabrication skills.
I'm
feeling in a generous mood right now so here are a couple of TyngTech trade
secrets on suspension and damping.
When I first envisioned a tank with
suspension I put some thought into the problem of damping because who wants a
tank that looks like a bobble-head while moving across the field. I
thought about r/c truck shock absorbers and other ways to induce friction in
the suspension but it all seemed overly complicated for what I wanted to
do. I finally came up with an idea that looked like it would solve the
problem of excessive bounce and it was simple in its execution. I
decided to side step the issue altogether by PUTTING ENOUGH SPRING STRENGTH IN
THE SUSPENSION SO THAT THE SUSPENSION ARMS ARE PUSHED SOLIDLY AGAINST THEIR
BOTTOM STOPS WHEN THE TANK IS AT REST. That's right, the Cromwell does
not "float" on its suspension springs somewhere in the middle of the
suspension travel. I figured if the suspension arms were pushed to their
bottom stops with strong springs they would not be oscillating up and down all
the time. You may think that forcing the suspension arms to the bottom
of their travel negates the advantage of a suspension but in practice it does
not. The secret is to use springs just strong enough to keep the
suspension arms at the bottom of their travel while the vehicle is at rest or
when traveling along on a smooth surface. The springs should only give
if that roadwheel encounters a "bump in the road". With this setup, when
the Cromwell encounters that bump in the road, the roadwheels lift up to clear
the obstacle and immediately snap back into place after going over it.
This system has worked out very well; it provides a very stable hull with
little or no bounce while traversing rough terrain. You
scale/low-speed/maximum-traction/etc guys may ask "but how does a roadwheel
get pushed down into a hole or dip in the ground if it is already at the
bottom of its travel", or "Don't you loose traction if the track is not
engaging the dirt". My answer is I don't care if my roadwheels and track
can't depress into a negative elevation. In practice it doesn't affect
my tanks ability to move from point A to point B whatsoever. Besides, if
the dip is large enough the roadwheels to the front and rear of the depression
will compress allowing the wheels in the depression to engage dirt.
Also, if the Cromwell is going fast enough it tends to ignore potholes in the
road, basically "catching air" as it goes over them. ;-)
Another
item regarding suspensions that I've seen no discussion on is balance. A
suspended tank is basically floating on its springs, even the Cromwell with
it's "Captured Suspension" (copyright, TyngTech ;-). You need to take
into account when designing a suspended tank that THE VEHICLE SHOULD BALANCE
AT THE MIDPOINT BETWEEN THE FRONT AND REAR ROADWHEELS. This ensures that
an equal amount of weight is being transferred to each suspension
spring. If the vehicle is not balanced you will need to fiddle with
different spring rates for each of the roadwheels and that sounded like a pain
in the arse to me. When designing the Cromwell I had little idea how
much the thing would weigh or where the weight would be distributed (I'm an
intuitive designer, not a trained one). I did know that I wanted the
tank to balance so I left a fudge factor as to where the batteries would
ultimately be mounted. Like ballasting a boat to sit level on its
waterline, I balanced the tank on its midpoint (after the Cromwell was mostly
complete) by positioning the batteries to their current positions.
And
that's all I have to say about that!
Steve "Damn that Depression,
Full Speed Ahead" Tyng
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