In this article we illustrate how to lower the center of gravity of a Scale Crawler to increase rollover resistance and climbing ability.
CG height and rollover resistance
The center of gravity, also referred to as CG (center of gravity) can be considered the midpoint in the distribution of a body's masses and also the point at which weight force is applied.
As shown in the drawing, the height of the CG plays a key role. The lower the CG, the greater the ability to cope with even side slopes.
CG dynamics
There are several experimental methods to find the CG position, but they have just static significance. In fact, during driving the suspension travel moves the position and the CG height.
However, from physics we know that the CG of a body made up of 2 parts, always lies on the line joining the CG of the single parts.
Unsprung masses and sprung masses
The 2 parts of the car we refer to are the unsprung masses (wheels and axles) and the suspended masses (chassis, electronics and body).
Take a look at the picture.
The CG of the unsprung masses (light blue) is located to a good approximation at the height of the wheel centers.
On the other hand, the center of gravity of the suspended masses (yellow) has a position that can vary from model to model, because it depends on many factors (chassis geometry, height and weight of the motor and battery, geometry and weight of the body, etc.). The only certainty is that it is located above the skid.
Lowering the CG
According to the physical principle in the previous paragraph, we know that the overall CG lies between the center of gravity of the unsprung masses and the center of gravity of the suspended masses, although we do not know the exact location.
It is becoming intuitive (physics confirms it) that the CG can be lowered following these 3 rules:
1 - reducing the suspended masses (above the CG)
2 - increasing the unsprung masses (below the CG)
3 - moving the masses downward
Reducing and shifting suspended masses
Elements of suspended masses and possible actions
Chassis rail, chassis hardware and suspension mounts | Replacement of parts with similar ones made of composite/plastic materials. Removal of superfluous parts. |
Motor and transmission | Replacement of brush motor with a lighter brushless motor. Plastic/nylon gears instead of metal ones. Lowering motor and transmission position, if allowed. |
Servo, battery, winch and electronics | Horizontal mounted servo. Small size battery. Electronics, battery and winch mounted as low as possible. Consider installing a servo winch if it can be mounted lower than the traditional winch. |
Body | Lexan when allowed by rules. Pickup body preferable to van. Opening windows and sunroof. Installation as low as possible. |
Ride height | As low as possible (according to surface conditions). |
Increasing unsprung masses
Axles | Replacement of plastic axles with metal ones. Replacement of axles components and steering hub carriers with metal or brass analogues. Differential covers made of brass or other metal. |
Wheels | Replacement of plastic wheels with metal wheels. Internal weights installation. |
Conclusions
We reported most of the useful tips to lower the center of gravity of a Scale Crawler. Although they are all technically possible, they may not be allowed by the applicable rules.
By the way, even though it is always needed to check the rules before joining the competitions, we are still confident that this guide will always be of valuable help.
Have a good work!
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