Davis Steering System

The following project highlights my technical Solidworks skills as all of the designs and subsequent renditions were completed using the software. The Davis steering system project was for my university's concrete toboggan team, in which I was the steering lead for the year 2021-2022. The team had four subsections (steering, braking, superstructure and concrete mixing) in which we all had to collaborate with one another to merge the subsystems together and create a functioning toboggan. A toboggan is essentially a sled that is used on the snow to carry individuals down a hill. The "concrete" is what the skis are made out of in the toboggan. The team's concrete toboggan had five main functions:

• Carry five riders down a hill while providing protection in the case of a crash;

• Have a fully concrete running surface that supports dynamic and static loading;

• Navigate through slaloms by means of a steering mechanism; 

• Come to a complete stop by means of a braking mechanism. 

 

The steering design selected for the year was the Davis steering system. The selection process began by conducting research on possible steering mechanisms: Ackerman, crab, rack and pinion, articulated and Davis. A weighted decision matrix was used to determine the design that best met the objectives. The main objectives were that the steering system had to be durable and easy to use, which was best met by the Davis steering system. 

 

Davis steering works such that the front skis rotate while the rear skis are stationary; wherein there are two sliding pairs and two turning pairs. It is an exact steering gear mechanism, which means that the skis turn at the same instantaneous center. The traditional Davis steering mechanism causes friction within the sliding pairs and thus creates inaccuracies in the calculations. Hence, a scotch yoke was introduced into the steering mechanism in order to remove two of the sliding pairs (Figure 1). The scotch yoke was connected to the shafts that are in turn connected to the skis (Figure 2). The distance between the scotch yoke and the skis was 6’’, chosen due to the resulting steering angle. When stationary, the front skis formed a 45° angle between the shafts, resulting in a maximum turning angle of 25°. In the past years the team used a 30° turning angle, however it was determined that for successful maneuvering of the toboggan only a 25° turning angle was necessary; since Davis steering has a higher friction mechanism than those in previous years, the reduction helps reduce the force the driver must exert to turn.

 

The steering wheel was designed such that there were two openings cut in the wheel to allow the driver to hold with both hands, and also two bars attached if the driver prefers a different grip (Figure 3). Furthermore, the shaft connected to the steering wheel ran through the floor of the toboggan and into the slot of the scotch yoke to move it horizontally, hence turning the skis. A semi-circular cut was made into the floor of the toboggan as a back-up steering limiter to the U-channel which prevented the toboggan from either understeering/oversteering. If the U-channel did fail then the skis would still only be able to turn 25° due to the cut in the floor. An additional safety measure was that foam was added to the edges of the steering wheel and on the steering bars making it easier for the driver to hold and safe in the event of a crash (Figure 4). 

Unfortunately, due to COVID restrictions the toboggan was never fully built (Figure 5). Nonetheless, the reports were submitted to the GNCTR (Great Northern Concrete Toboggan Race) and the team successfully placed 3rd place out of 13 teams.