1. Clean Sheet Design – Metallic Tubular Chassis
The Clean Snowmobile Challenge
Clean Sheet Design – Metallic Tubular Chassis
Matthew Wyatt, Otis Clapp, Darrell Flagg, Brian Wild
Advisor: Michael “Mick” Peterson, Ph.D.
Introduction
In 2003 the University of Maine’s Clean Snowmobile Team started work on a competition snowmobile based on a stroke Arctic Cat.  The Arctic Cat Stroke uses a modest engine to produce a good workhorse 4-stroke snowmobile.  There is a definite increase in performance associated with using a larger more powerful engine in the same chassis; however there are also corresponding drawbacks such as increased load on the chassis and increased fuel consumption. A different approach to achieve the same performance results would be to reduce the weight and number of components on the chassis. This is a well known design concept made famous by Lotus and various other companies as early as the 50’s that is used in racing as well as production vehicles to this day.
These sorts of cars are the inspiration for the clean sheet design of the University of Maine Clean Snowmobile. Chassis improvements, rather than engine modifications will be applied to increase performance. The design will be based on the concepts used in open wheel and sports car racing. The Redline snowmobile also demonstrated that these concepts can be applied with great success to a snowmobile design. This design will focus on using a mostly tubular metallic chassis to reduce total vehicle weight significantly over the stock chassis.
Objectives
Benchmark Current chassis
Basic Chassis Concept
Create Solid Works Model of Chassis
Finite Element Analysis
Refine Model
Determine Feasibility of Chassis Construction
Tunnel Benchmarking
The first step in designing a new tubular chassis is to benchmark the existing chassis. This requires determining the performance of the existing chassis. For our design purpose this consists of determining the stiffness and weight of the existing tunnel through FEA and real world testing.
The existing chassis was modeled in SolidWorks. We determined a realistic loading to be 1000 lbs distributed over the four mounting points. The leading edge was restrained. This load was applied in Cosmos Works, a Finite Element package integrated into SolidWorks. The following pictures show the deformation and stress in the tunnel.
The maximum deflection of the existing tunnel is .077 inches. For real world reinforcement of our numbers, we tested an existing tunnel, restraining it at the same point as in the FEA model, and applying load in the same manner. The test results were a bit different then the predicted results. The tunnel was significantly less stiff then expected. The maximum deflection was .393 inches. The inconsistency between these results and the calculated model results can be explained via the differences between the model and the actual tunnel. The actual tunnel was used, and most likely weakened through usage. Also, the tunnel could not be perfectly fixed in the testing. Overall the results are very reasonable, but it is to be noted that the FEA results are possibly too perfect to be reliable.
Basic Chassis Concept
The design goal of the chassis was determined to be lighter weight, and greater stiffness then the existing Arctic Cat chassis. The initial design is shown below.
Through FEA analysis of the chassis we then improved the design to the following design.
Chassis Comparison
The following graph shows the difference between the measured deflection and the predicted deflection.
The following graph shows the difference in deflection between the tubular chassis prediction and the aluminum Arctic Cat chassis deflection. The maximum deflection of the tubular chassis is in.
Chassis Feasibility
While the tubular chassis design is a significant improvement in regards to stiffness, the improvements in other areas are marginal. To comply with the rules of the CSC competition significant amounts of material must be added to pass the safety inspection. These additions would bring the chassis to a weight that is comparable to the factory chassis. Due to the nature of the CSC competition being primarily an exhaust emissions and sound level event, the gains made in the area of chassis stiffness are marginal at best. Removing weight from the factory chassis will net greater returns for the success of the CSC team then building a snowmobile chassis from scratch.
Initial Design
Final Design
Tunnel Deformation
Tunnel Stress