Suspension and Steering

Analysis

In designing all components, an analysis with engineering merit will need to be applied. Using a full body diagram and Statics a force can be calculated to be able to find the amount of resistance that is needed within the springs for the suspension. Mechanics of Material will allow us to calculate the proper diameter of the springs along with the correct material. Mechanics of Material will also help find the proper cross-sectional area needed for the wishbones, control arms, and any other smaller components that will be needed for stress testing. While also calculating the amount of deflection that the control arm and the wishbone will experience for axial load testing. Some components will need an analysis for column buckling such as the control arms, wishbone, and shock tower ensuring that they do not exceed their critical load. Smaller components like the link for the control arms to the chassis will need calculations for the amount of shear that will be acted upon them, finding the material and diameter needed to ensure stability in all testing. This will ensure that all components will have enough structure to not fail while in testing and competing in the RC Baja Competition.

Requirments

The requirements below are to prepare the suspension and steering for the RC Baja competition. For these requirements will be the basis for which the components will be designed to. These will be what the completed analyses are based on, more information can be found on the linked Project Report on the homepage.

Suspension must be able to absorb at least 300% of the vehicle’s weight before bottoming out.
From the bottom of the chassis to the ground there must be at least a 3-inch (75mm) clearance.
Turn 180 degrees within a turn radius of 10 inches (250 mm), while going at any throttle percentage.
The steering rods must not deflect more than 0.2 inches (5mm) upon a 20 lb. load.
Vehicle must not deviate more than 5 inches in a straight line for 50 feet (15m) while in 75% throttle.
Tires must be within 5 degrees of alignment from each other.
All 4 tires need to keep in contact with the ground from 0 to 75 percent immediate throttle.
Vehicle must have a flex of 2 inch (50mm) before more than one tire leaves the ground.
Suspension of vehicle must be able to experience a 3ft drop without compressing the suspension more than 1 inch.
The suspension will allow for a 0.5 inch down travel or 0.5-inch droop.
The wishbones are not to buckle under a 75lb axial load.
Steering rod is not to buckle under a 25lb axial load.

Completed Analyses

Steering Rod Deflection_edited_edited.jp

Steer Rod Deflection

Analysis 1

The Steering Rod may experience deflection from incoming impact forces. To ensure that no failure occurs. An analysis was done to find the dimensions needed for 1060 Aluminum alloys with no more of a deflection of 0.2 inches under 20 lb. based on Requirement 04. load. The length of the steering rod is to be assumed to be 7.0 inches and a safety factor of 1.5. A moment of inertia area of 0.00007 in4 was found. Allowing for the calculation of the diameter of a solid cylinder to be found. The minimal diameter of the cylinder came out to be 0.14 inches.

Suspension Resistance

Analysis 2

Suspension resistance involved finding the correct k-factor needed for a spring with 1-inch travel for a vertical impact force from a 3 ft drop. If done so this would satisfy Requirement 09. First, an impact force was needed, so that Hooke’s Law could be applied, and the k-factor could be found. A safety factor of 2 was applied, this being the go-to safety factor for suspensions in the automotive industry. A k-factor of at least 147 kg/s2 was found.

Wishbone Buckling

Analysis 3

With the wishbone, or the lower control arm, being most of the structure of the steering/suspension mechanism. It will need to take the majority of the force. So, a load critical was given to the wishbone of 75 lbs. with a safety factor of 2.0, based on Requirement 11. The member was given a base of 0.125 inches as this is similar to other parts that can be bought online, and this will allow for a height to be found. The wishbone will disburse the force across two different members dividing the load by two. With the member being double pinned a moment of inertia of 0.000012 in4 was found. Allowing a height of at least 0.105 inches.

Wishbone Deflection

Analysis 4

The deflection of the wishbone that is caused by the force of the suspension was needed to ensure no failure in the drop test. This will ensure that nothing breaks during testing Requirement 09. First, the force that was caused by the drop test was brought in from analysis 2, a force of 3.67 N. A safety factor of 2.0 was applied to the calculations due to this being the same safety factor applied to the suspension. With a length assumed to be 4 inches and a deflection of 0. The moment of inertia that is needed is 5.763 in4 spread across multiple members.

Material Analysis for Steering Rod

Analysis 5

Requirement 12 states that the steering rod must not buckle under a 25 lb axial load. This material will need to be able to support the axial load with the dimensions of 0.375 x 0.25 inch. This gave the component a moment of inertia of 0.000488 in4. Giving the analysis a 1.5 safety factor and a Pallowable of 25 lbs, a Pcr of 37.5 lbs was found. With an assumed length of 4 inches, the analysis gave an Elastic Modulus of 124,503 psi. When looking at the possible candidates for materials ABS, 6061 T6 Aluminum, and Low Carbon Steel were suitable materials for this application.

Turn Radius Analysis

Analysis 6

The vehicle must have a turn radius of 10 inches as stated in Requirement 03. In the analysis an assumed length of 10 inches was giving to the device. With the wheelbase know along with the turn radius. An angle of which that tires that will need turn will be 45 degrees for the vehicle to be able to have a turn radius of 10 inches.

Suspension Resistance With Applied Weight

Analysis 7

Requirement 01 states, that the suspension must not bottom out or have a travel more than 1.5 inches when 300% of vehicles weight is applied to the suspension. Shown in the analysis a safety factor of 2.0 was given to the analysis as this is a standard in the automotive industry for suspensions. The vehicles weight was assumed to be 2 kgs, once the safety factor and 300% applied the force came out to be 117.7 N was applied. Using Hooke’s Law, a needed k-factor of at least 3.1 KN/m was found.

Shock Tower Thickness

Analysis 8

In Requirement 09, the suspension must be able to support a 3-foot drop without causing more than 1 inch travel. First the impact force was brought from analysis 10, giving a force of 14.715 kN across 4 tires dividing the load by 4. Then giving an impact force of 3.679 N or 1.654 lbs. Using a full body diagram and equations of equilibrium a force acting on the spring came to be 7.277 lbs. Assuming the length of the wishbone to be 4.25 inches with the suspensions being 1 inch away from the chassis, giving a more vertical reaction compared to the front shock. A 2.0 safety factor was applied giving a force of 14.55 lbf. The stress equation was applied, with the base of .25 inches assumed a thickness of at least 0.01216 inches was found.