RE: suspension [TANKS] A TyngTech Point of View

["Steve Tyng" <SteveT44-at-comcast.net>]

Title:

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