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Criteria | Weighting | 4-Wheel | Tadpole | Notes |
|---|---|---|---|---|
Suspension | 0.5 | 0 | This criteria is broken up into the design, manufacturability, and assembly of the front and rear suspension. Both will be factored into the final decision. | |
Front | 0 | To get better CG placement with a tadpole we need CG closer to the front “axle” than the rear wheel. With the small form factor we need to fit within, we’ll need to but the wheels at the side of the driver’s legs, thus making the car wider. To counteract this we can use a leading arm suspension. Whereas the four-wheel car would be able to have the front wheels, driver, battery pack, rear wheels in a line, similar to Top Dutch’s Green Lightning as can be seen here Top Dutch Steering Research. A 4-wheel car would use double a-arm. | ||
Design | 1 | 0 | -1 | Leading arm is more difficult to design due to the moments to counteract. Double a-arm doesn’t have this problem to the same extent. As well, the arm needs to be designed to leave room for the wheels to turn. |
Manufacturability | 2 | 0 | -1 | Due to the additional forces there is a greater likelihood that the parts from a leading arm suspension will need custom machining, whereas double a-arm suspensions would |
Assembly | 1.5 | 0 | 1 | A double a-arm would need some welding, which we need to outsource to ensure structure. A leading arm would have more OTS parts to assemble and press together, making it easier. |
Rear | 0 | Both designs would use trailing arm suspensions as the motor(s) would be mounted on the back wheels. A double a-arm would not be bale to mount a motor as easy. | ||
Design | 1 | 0 | -1 | If we were to reuse MSXIV’s rear suspension, we would need to do similar force analysis for weight reduction. However, there are some symmetry concerns with tadpole for the rear wheel, which MSXIV did not need to design around. The worry with tadpole is we might need to create a new trailing arm, whereas with 4-wheel it is practically guaranteed that it can be reused. |
Manufacturability | 2 | 0 | -1 | Weight reduction in either case would be complex to machine. But the tadpole risks creating a new trailing arm, which requires more effort in manufacturing for sourcing material specifically. |
Assembly | 1.5 | 0 | 0 | In both cases we can expect to see a lot of OTS parts for assembly which is relatively easy. |
Steering | 1 | 0 | This criteria is broken up into the design, manufacturability and assembly of front, rear, or all-wheel steering. In this case, we will use the best scoring implementation of the three steering types. Consider this a child decision matrix. | |
Front | 0 | These are virtually the same because they're both 2 wheels at the front - Malcolm | ||
Design | 1 | 0 | 0 | |
Manufacturability | 2 | 0 | 0 | |
Assembly | 1.5 | 0 | 0 | |
Rear | 0 | NOTE: I really struggled to find anything on rear-wheel exclusive steering, because most info is for all wheel so a lot if this info is based on intuition and my opinion. Here's a link for a couple things I could find about exclusive rear wheel steering Rear wheel steering Research -Malcolm | ||
Design | 1 | 0 | 0 | Tadpole benefits from only having one wheel, don't have to consider Ackermann steering or anything like that, however this is offset by the need to keep positive caster on the wheel -Malcolm |
Manufacturability | 2 | 0 | 1 | Most rear-wheel steering systems that I could find were controlled electronically and not manually (http://imperialjournal.com/wp-content/uploads/2019/06/Vol-3-Issue-1-5.pdf), so it seems like they’d have similar implementation, although the 4-wheel system would need to consider different wheel alignments for turning, so Tadpole wins here again due to its simplicity -Malcolm |
Assembly | 1.5 | 0 | 0 | Tadpole has an advantage due to its compact shape and simplicity, but loses points due to the overall surface area/interior area of the car being smaller than that of a 4 wheeler -Malcolm |
All-Wheel | 0 | |||
Design | 1 | 0 | 1 | The inclusion of a rear differential for the back as well as the front wheels would increase likelihood of failure, and more math required to be done to get the right ratios -Malcolm |
Manufacturability | 2 | 0 | -1 | Many previous iterations to work from, plus it'll be easier to maintain caster and stability on 4 wheels -Malcolm |
Assembly | 1.5 | 0 | -1 | the system required to keep the one wheel in caster and stable while also steering would be bigger on the tadpole car (I imagine, again difficult to find 3 wheel all wheel steering sources) -Malcolm |
Brakes | 1 | 0 | With the assumption that the implementation of brakes will be relatively equal between different wheel allocations (only front, only back, all wheels), we will only look at the over arching difficulty. Brake wheel allocations is being worked on by Ayush. Based on the findings of the matrix, we should use all wheel breaking Brake Allocations Decision Matrix | |
Design | 1 | 0 | 1 | for all three of these, the system is exactly the same as 4 wheel, but just accounting for 1 less wheel, so its better |
Manufacturability | 2 | 0 | 1 | see above |
Assembly | 1.5 | 0 | 1 | see above |
Stability | 2 | 0 | -1 | Min is researching the stability. 3 Wheel Vehicle Using Mins summary and working a bit with Jens, we determined that the 4 wheel car has better turning stability than the 3 wheel car. The equations used in the paper that Min summarized assumed that turning would occur on a flat ground at max speed, however this is not normally the case. Most roads have a slope in turns to help cars turn. This changes the moment arm and direction of lateral acceleration. Jens and I deduced a general trend that reducing the moment arm increases lateral acceleration, and since the moment arm on a 3 wheel car is less than that of a 4 wheel car, the 3 wheel begins to tip sooner than the 4 wheel, making it less stable -Malcolm |
Handling | 2 | 0 | -1 | Reading further into the article summarized in 3 Wheel Vehicle , It mentions that during the constant radius cornering test that 3 wheel vehicles have K>0 through the entire test, meaning that the vehicle understeers the whole time. While that means its still stable, it also signifies worse handling as understeer means you will be taking wider turns -Malcolm |
Chassis | 1 | 0 | 0 | From Mohamed, the wheel layout would not be too different between tadpole and 4 wheel designs, and the forces applied to the car would be the same although there would be a higher stress concentration at the back due to only having 1 wheel. In the grand scheme of things, this is not a big deal, as the chassis would be able to handle it just as easily -Malcolm |
Aerobody | 1 | 0 | 1 | Connor Hawkins left a comment on the 3- vs. 4-Wheel Car co-op task page which would be another consideration for this criteria. Smaller shape = more aerodynamic, and less holes in the car (spots for wheels) also makes it more aerodynamic, hence Tadpole -Malcolm |
Battery Box | 1 | 0 | -1 | The pack would need to occupy a smaller space, and thus be more dense, which makes cooling more difficult. |
Motor Allocations | 0 | -1 | Typically, for tadpole design the vehicle is rear wheel drive - Evan Dodd Consider the number of motors and if a differential would be required https://www.youtube.com/watch?v=gIGvhvOhLHU&t=282s After having discussion with other teams, when a single motor is used, it is common for teams to not use a differential and power only one wheel. Based on some preliminary calculation, one motor could suffice for the car. it needs to be verified by battery box for the battery pack requirements. Based on one motor, there aren’t many differences between tadpole and 4-wheel. To summarize, In the case of one motor, or two motors, the 4-wheel design can better accommodate one or two motors than the tadpole design. With 4-wheel we would power one wheel (no differential to be concerned about based on discussions with other teams), and for two motors we can power two wheels easily (likely the two rear wheels as MSXIV’s suspension would work well to hold the motors and no addition to the front steering). With tadpole, one motor is easy, as it would simply be mounted in the rear. For two motors, they would need to go to the front. This would add additional effort for the driver to steer the car, and with a leading arm suspension this becomes more cramped due to aero body constraints. As the motor quantity is still in question, the tadpole design has an inherent risk of a more difficult design and is thus penalized for it. | |
Weight Allocation Flexibility | 1 | 0 | -1 | From Jens’ CG bounding box cad files, 4-wheel has more flexibility with CG placement -Malcolm |
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