Push Rod Design Sprint
- Jens Dekker
Credits
I did not come up with this on my own, in fact I just copied and pasted the vast majority from the mech leads design sprint. So huge shoutout to Aidan Lehal, Min Qian Lu, Kevin Bui, and Emily Guo!!! They did an amazing job getting this together, I only made a couple changes to help make the process a bit smoother.
Sprint Goals
Get members accustomed to designing relevant components and critiquing each other’s work.
Making self-driven decisions throughout the entire process, starting from basic requirements + constraints, to selecting OTS components, all the way to sim validation.
This is very important as we anticipate to be designing with little direct supervision (i.e you can rely on leads so much; being able to think critically for ourselves is huge).
Getting comfortable with holding each other accountable to our own deadlines.
Documenting our progress and decisions.
Topics to be Covered
Force Analysis (hand calculations)
Geometric Analysis (more complicated hand calculations)
Off the Shelf Part Selection
Concept → Design
Material and Manufacturing Selection
Sprint Introduction
We are challenged with designing the bellcrank, bellcrank clevis, and pushrod for the inboard suspension pictured below. The given parameters are as follows. Requirements and constraints will be at the discretion of each person.
Purpose of Inboard Suspension: “The main advantages of a push/pull-rod suspension system on a track-focused race car have to do primarily with the ability to move suspension components closer to the ground, lower the chassis of the vehicle, and lower the centre of gravity to improve efficiency in cornering, body-roll, and high-speed stability”(Wikipedia). Additionally it allows for packaging of the suspension to be smaller than other systems.
The given dimensions and component specifications are in the table below:
Specification | Value |
|---|---|
Shocks from MSXIV | Spring Force: https://www.ridefox.com/family.php?m=bike&family=floatx2 |
Chassis and Suspension Geometry (assume the chassis and control arms can withstand the loads already) |
 Made in Solidworks 2019; hopefully there won’t be any issues with backwards compatibility. Shock Info: https://www.ridefox.com/dl/bike/my22/605-00-253-FLOAT-X2-Tuning-Guide-revA.pdf Shock CAD Model: https://grabcad.com/library/fox-float-x2-factory-2019-8-75-x2-75-mtb-bike-rear-shock-1 |
Mass of Vehicle | 192kg |
Center of Gravity Location | |
Loading Conditions | |
Assembly Max Height | Final bellcrank, clevis, and pushrod assembly should not exceed 100mm past the top or bottom chassis members. |
Timeline
Note that independent reviews with leads and other members throughout this entire period are encouraged! You are not limited to just the scheduled reviews!!!
Phase 1 [Feb 5 - 19; ~2 weeks] - Review Feb 12 & Feb 19 @ 3:00PM EST
Phase 1 is all about getting some of the thoughts on how to solve the design, along with some of the key constraints/considerations to get ready for detailed design.
Conceptual Design: Sketches of design concepts, with labels and descriptions where needed to effectively communicate design ideas.
Force analysis: Using the diagram of force locations on the model car above, find the center of mass/gravity (CG) for the vehicle, use the CG to determine the load distribution between axles, use the load distribution to find out how much load each independent suspension assembly needs to withstand, calculate how much load each component in your design undergoes based on the front left independent suspension load force.
Kinematic Geometric Analysis: Using different conditions, mathematically determine the optimal geometry of the parts to minimize forces experienced by the shock and chassis. Tie this into the force analysis to see what the shock needs to do in relation to the forces experienced by the tire
Off the Shelf Part Selection: Ensure that your sketches and designs include what types of off the shelf (OTS) parts you would like to use (e.g. You want to join two components together without welding? What type of fastener will it be? Will you use a pin? Shoulder Bolt? Zip tie?). In Phase 1, the selection process does not need to be specific (diameter, thread length, head type, etc.), but these factors should be considered when designing.
Phase 2 [Feb 19 - Mar 05; ~2 weeks] - Review Feb 26 & Mar 05 @ 3:00PM EST
Phase 2 gets into the nitty-gritty of the design. What dimensions do all the parts need to be? What materials? How are we going to make it? Is it cost effective? And plenty more questions. Hopefully by the end you have a fully designed suspension system that hits all the checkboxes.
Detailed Design: Concepts should be finalized at this point and moved into CAD. An accurate model containing all properly dimensioned components and OTS parts should be drafted (to be done in SolidWorks). Ensure proper modelling techniques are used, each component is a different part, proper naming of each component, properly mated assemblies, CAD files/mock replacements for OTS parts. The final CAD model should look exactly how it is expected to look in real life.
Material Selection: Each component should have a material assigned to it based on the requirements of your design. Use past experiences and intuition to select materials for a component (e.g. glass shouldn’t be used as the chassis material of a car because it will shatter upon impact) and refer to the material’s properties to make an accurate selection.
Resources to help with material selection: https://www.engineeringtoolbox.com/material-properties-t_24.html, http://www.matweb.com/
Manufacturing Method: How will your part be made? Consider the tools we have in the bay and in the machine shop. Will this part need to be outsourced for manufacture? Ensure you determine one or more methods of manufacture (waterjetting, CNC Mill, injection mould, lathe, etc.) for each component.
Refined OTS Part Selection: With your OTS parts selected, it is time to specify the exact parts you need for your design to work (with safety factors included). Use websites and catalogues to determine what exists out there and try to select a cost effective part for your design. Try to be consistent and use standard sizes where possible (e.g. A bolt size M6.789 is very specific and unlikely to find, try designing with M8 instead). For example, say you want to fasten two brackets together using a nut and bolt:
Refined Bolt Specifications: Bolt size, bolt material, tensile strength, shear strength, grade (needs to comply with ASC 2024 regs), etc.
Refined Nut Specifications: Nut size (should fit on bolt), nut material, grade (needs to comply with ASC 2024 regs), etc.
EXTRA: Some extra tasks that you can take on if you’ve finished and optimized your design of the pushrod, bellcrank, and bellcrank clevis.
Re-spec the Bearings Selected: The ones I selected are potentially over the stop, double check the force calculations and be sure that they’re within spec, and if they are the cheapest option.
Simulate the Pre-designed Parts: The wishbones and the clevis that hold them to the chassis were made just to give a rough sense of the geometry, but weren’t simulated to check if they would fail or not. Give it a go if you’ve got the time.
Resources to help with OTS parts: McMaster-Carr is a great tool to find OTS parts to fit your design needs (and most of them have CAD files too!), it can be a more expensive option though so once you have a part selected, try to find a cheaper one of equal quality.
In the middle and at the end of this phase, the participants should get together and review each others work, providing advice, feedback, and expressing issues they have experienced along the way.
Sprint Deliverables
The review sessions will be held to share ideas, get feedback, and inspire your teammates! The following deliverables should be presented in a Slide Deck**
TO BE COMPLETED FOR FEB 12: 3 Concepts sketches with details labelled and OTS parts (not specifics)
TO BE COMPLETED FOR FEB 19: Force and kinematic geometric calculations complete for one design.
TO BE COMPLETED FOR FEB 26: Detailed design (CAD) of one sketch with refined OTS parts and material selection.
TO BE COMPLETED FOR MAR 05: Manufacturing methods and some FEA on the Bellcrank and Bellcrank Clevis.
**it is recommended to keep a design log (in google slides format) throughout this project of all decisions/choices made, the log will make finding the content to present easier come review day!
Sample Design Logs:
Battery Box Thermal Analysis - Design Log
Kevin Bui FYDP FDR - Password is “midsun”
Appendix