Tension Springs Question

[Enzo Salas]

Hi,

My Tank is about 120 lbs and I'm having problems with my tension
springs, they are too weak for the weight of my batteries. My
suspension system is exactly:http://rctankcombat.com/tanks/
T014/03Suspension-large.jpg

What diameter and material should I use for a stronger tension spring?

Any form of help will be greatly appreciated
z0

[Steve Tyng]

Divide your fully loaded tank weight by the number of road wheels you
have and that's what each spring needs to be holding up. Start
searching on McMaster.com for extension springs in the length required
and look for a "spring rate" maybe a third higher than your figure and
that should give you something to start with. Some put heaver springs
on the front and rear roadwheels as these see the most abuse. Also,
if your tank is not balanced to the center point of your ground
contact you will need to adjust spring rates to compensate so your
tank "floats level". Some build-in an ability to adjust spring
mounting for this very reason. Of course, this can be mitigated with
a TyngTech captured suspension:

http://www.rctankcombat.com/archive/2007-02/msg00121.html

Steve Tyng


Warning: There's nothing scientific in my advise, just a gut feeling
backed by a modicum of experience.

[Frank Pittelli]

I multiply that figure by 2 to account for dynamic loads.

Frank P.

[Steve Tyng]

Frank wrote:

> I multiply that figure by 2 to account for dynamic loads.

Depends on the suspension design. In a captured suspension where the
bottom stops determine ride height, you can put in as much spring rate
as you want for higher dynamic loads. In a floating design (where the
suspension can go above and below the desired ride height), stronger
springs will reduce the suspension arm throw as the (extension)
springs will not stretch as far for a given weight and thus will not
offer as much “return” before the spring collapses back to its
original length. Basically, given the same length, the higher rate
spring will give a shorter suspension range than a lower rate spring.

I forgot to mention another issue which hadn't occurred to me
earlier. Force multiplication must be factored when determining
spring rate in a internal sprung design (such as used in tank T014).
Since most internal arms that the springs attach to are shorter than
the external suspension arm there is a significant force
multiplication from the force pushing up on the roadwheel and the
force being applied to the suspension spring. A simple example would
be where a tank with 2” long suspension arms and 1” long internal
spring arms will require a 24 pound force at the spring lever to
counteract a 12 pound force pushing on the roadwheel.

Here’s a handy lever force calculator.

http://www.engineersedge.com/calculators/levers/page_levers_1.htm

If using this calculator, the X variable will be the length of the
external arm (between the wheel axle and pivot point), L the length of
the spring arm (pivot point to spring attachment point), and F the
force pushing up on the roadwheel (tank weight divided by the number
of roadwheels).

Steve Tyng