CSC rear drive

CSC rear drive is a project to modify the drive system of a Ski-Doo Snowmobile currently used by the University of Idaho Clean Snowmobile Challenge Project. The goal of the project is to relocate the drive wheels from the front of the snowmobile track to the back of the snowmobile track. It is believed that this configuration will increase efficiency and power output of the snowmobile; a portion of this project will be dedicated to testing the veracity of this theory.

Background
The SAE Clean Snowmobile Challenge is an "...engineering design competition for college and university student members that challenge engineering students to reengineer an existing snowmobile to reduce emissions and noise." The University of Idaho has had a CSC team participating in the challenge since 2001. An enterprising snowmobile drag-racer by the name of Shawn Watling designed a rear drive system for snowmobiles that is claimed to deliver a 72 percent increase in fuel economy. Watling's design requires substantial modification a snowmobile's chassis. Hoping to incorporate this substantial improvement in efficency into the University of Idaho CSC snowmobile, Team Powder Train has been charged with the design of a rear drive system that does not require any modification of the chassis, as any modification would violate the CSC competition rules.

Team Powder Train
Team Powder Train, who endeavor to design a working snowmobile rear drive, is composed of University of Idaho senior Mechanical Engineering students Zoltan Kiss, Josh Dalton, and Aaron Patterson.

Design Goals

 * System instillation
 * bolt-in design
 * OEM apperance
 * no chassis modification
 * Weight
 * no greater than 25% weight increase
 * Fuel economy
 * reduce the fuel consumed by the engine by increasing efficiency in the drive train
 * Efficency
 * increase percentage of engine power translated into movement
 * improve the effectiveness of the track system
 * Noise
 * do not increase noise
 * reduce noise if possible

Prior design
A prior team of senior University of Idaho ME students designed a snowmobile rear drive for the CSC team but the design failed to meet all the project goals. The design worked as a proof of concept but at over 90 pounds was more than twice the weight of the stock suspension system. The system utilized a drive chain to transfer power. The design incorporated redesigned suspension which turned out to be inadequate and prone to damaging itself during operation. The design did improve handling but its unknown exactly why.

RND Innovators design
As mentioned previously, the RND Innovators rear drive design is a fully developed, unique rear drive system although it doesn't appear to be currently commercially available. This design requires extensive modification of the chassis of the snowmobile, which is not an option for the University of Idaho CSC team as chassis modification violates the competition rules. The design does have purported advantages of shorter stopping distances, better handling, better acceleration, and significant improvements in fuel consumption over the stock configuration.

Design challenges
The design hurdles faced in developing a snowmobile rear drive are various. Challenges include where to put the rear drive wheels, what form those wheels should take, whether or not to use the stock track configuration, where to put the brake, what type of drive train to use, and how to fit that drive train into the snowmobile, among others.

Drivetrain
One of the primary considerations in the design challenge is the type of drive train to use to transfer engine power to the rear drive. Options considered were electrical, hydraulic, drive shaft, belt drive, and chain drive. Electrical was a violation of competition rules and was dropped early. Hydraulic was a favored concept but the necessary equipment turned out to be too expensive and too heavy to be practical. A drive shaft was considered impractical due to obstructions and flexibility in the suspension and the need for differentials. A drive belt was considered impractical due to the environmental considerations of snow and ice on the belt. The team ultimately decided that the previously attempted chain drive option was the best choice for the application. It was determined that a drive chain system could probably be fit in among the stock suspension system.

Rear drive axle and wheels
The stock configuration of the snowmobile has bogey wheels in the rear that are adjustable to apply tension to the track so replacing those wheels required a way of either adjusting the axle or moving the tension equipment elsewhere. Also important was the fact that the drive wheels, due to the design of the stock track, line up with the rails in the suspension and a way would have to be found to either work around the rails or modify the rails to work around the drive wheels. This brought up the question of whether the idea of using a custom track should be considered, since doing so would allow for the placement of the drive wheels elsewhere. The CSC competition rules include guidelines concerning modifying parts or using non-standard parts for nearly everything on the snowmobile and using a custom track would certainly not be OEM so that option was ultimately rejected. A multi-part drive wheel was considered but would have required a great deal of complexity so it was decided that altering the rails and using standard drive wheels was the best option. The previous U of I team also modified their rails but chose to move them outboard, apparently to accommodate the brake they installed on the rear axle. This outboard configuration presented the problems of a weakly supported drive axle, a robust and heavy bracket to support the axle and the width of this system was greater than the track width, making it vulnerable to strikes from external objects. The team decided to move the rails inboard to provide better stability and deal with the brake in other ways. The tension system decided upon is virtually the same as stock except the rear axle will incorporate more robust bearings than would be in simple bogey wheels.

Brake
The stock brake is a hydraulic caliper and disc brake attached directly to the end of the front drive axle and stops the snowmobile through the drive wheels, which engage directly with the track. In order to keep weight down the team decided to keep this brake in place but with the front drive axle disconnected from the drive train to that when the brake is applied it can still stop the snowmobile but isn't fighting with the drive train. however, power has to be transferred through the area of the front drive axle so the idea of a two part axle was put forward. The two parts would be connected and rotate on the same axis but would have bearings separating them so they can move independently and without interference.