Re: suspension [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

mxlyons-at-cox.net wrote:
> As for RC tanks, do you care if your rig is bouncing around?  Maybe
> not.  Shocks might reduce side-to-side "rolling" and help aiming your
> gun, but for general travel the speed is probably low enough to keep
> the tracks on the ground.

Hate to spoil a good academic debate, but here are some real-world 
inputs based on my experience on the battlefield. (Of course, other 
veterans will have a different viewpoint ... but they're wrong :-)

1) Very few tanks actually shoot long range while on the run. 
Suspension or not, the accuracy required to hit moving targets at long 
range is not likely while moving yourself.  At close range, the accuracy 
required can be achieved by experience without suspension.  So, adding 
suspension for aiming reasons is not needed.

2) It is definitely true that independent suspension helps to keep 
wheels on the ground while moving over terrain and in turns (when the 
vehicle wants to lean).  So, in theory, a suspension will give our tanks 
more traction in those conditions then they would otherwise have. 
However, most battle-ready tanks currently have *too much* traction.  We 
have no problem climbing things (assuming the motors are strong enough) 
and no problems getting out of rough terrain.  (High-centering 
situations are the exception, but suspensions don't help there either.) 
  In fact, people have made changes to reduce the amount of traction 
provided by the tracks to help with turning.  So, adding a suspension to 
improve traction is not needed.  (This applies only to tracked vehicles, 
clearly suspensions are needed for wheeled vehicles in rough terrain.)

3) The most obvious benefit of a suspension is absorbing shocks, which 
increase significantly as the vehicle speed increases.  So, tanks 
running at slow speeds may see some jolting, but otherwise 
non-destructive shocks during the battle.  Those same obstacles could 
seriously damage a tank running at a faster speed.  Keep in mind, we 
routinely drive our tanks at battle speed over ruts, rocks and debris 
that could be 4-6 inches tall/deep.  Similarly, the stopping and 
starting of the tank generates dynamic shocks and those shocks can be 
severe depending on the type of speed control and the control of the 
operator. Our tanks see hundreds of such shocks in every battle and they 
add up to cause all kinds of problems throughout the tank.  Suspensions 
help absorb these shocks to the entire vehicle ... if designed properly.

4) Shock absorbing can be done in various ways, but you've got to be 
careful not to assume that one size fits all.   The bone crunching 
shocks (tank rolling over a tall object and slamming on its nose) can 
definitely break mechanical things and need to be avoided or absorbed. 
(BTW: A suspension doesn't help at all when a tank slams down on it's 
drive wheels, only the driver can prevent such shocks). But, the smaller 
high-frequency shocks (tank vibrating as it travels over the ground) are 
the ones that make nuts come loose and damage electrical components. 
Suspending the wheels will definitely help ease the bone-crunching 
shocks, but the springs will have to be relatively stiff to handle the 
weight of the vehicle and the heavy dynamic load without bottoming out. 
  Such stiff springs, therefore, won't help much at all with the 
smaller, high-frequency shocks.  A couple different approaches may be 
needed, close to the system to be protected.

5) You've always got to consider the "cost of complexity".  Adding a 
suspension can reduce the number of mechanical failures due to shocks, 
but can also increase the number of failures due to the suspension 
itself.  You can easily "decrease" overall vehicle reliability with a 
bad suspension design and implementation.  It only takes one small 
problem to take you out of the battle, which is always frustrating.

6) Finally, you always have to use a design that allows some "tuning" 
and "maintainability".  Although Dr. Sommer's grad students can 
certainly calculate dynamic loads and other wonderful stats for your 
design, they know (or should know) like all professional engineers that 
you've got to test it in the field to be sure.  So, make any suspension 
so that you can change the characteristics easily (moving a pivot point, 
changing a spring, lengthening a control arm, etc).  If it's easy to 
change, then you'll actually dial it in over time.  If it is hard to 
change, you'll simply "convince yourself" that it is good enough.  I've 
seen battlers in various combat hobbies use the same bad design for 
years because the effort required to change it was too high.  Nobody 
likes doing maintenance and tuning, so you've got to make it easier by 
building it into the design of the system.

Just a few observations from my experiences.  Your mileage may vary. 
But, remember one thing:  If you spend too much time trying to make 
things perfect up front, then you may never actually make it to your 
first battle ... which is *the* most important design step.  The 
battlefield and other battlers will always help find your faults :-)

        Frank P.