To organize all thoughts in selecting 3- or 4-wheel, I will make a decision matrix and select all criteria that must be considered.
When discussing a 3-wheel design we will be considering both a tadpole/teardrop shape and a delta shape as shown below.
The 4-wheel datum is a vehicle rear-wheel drive vehicle, with Ackermann steering in the front, and brakes in all wheels.
- Delta off the menu, reasoning below
The only thing that delta does better than the other designs is that its steering would be simpler. You can see that even taking small corners it just falls over and damages the car. This video shows what I'm talking about: https://youtu.be/QQh56geU0X8 This effect is due to the cars difficult to balance weight transfer and it being very prone to oversteer. This huge disadvantage makes it nearly impossible to recommend over anything else.
More information on why tadpole is just a better 3 wheel option can be seen at this article summary: 3 vs 4 Wheel Task 2 , link to the original article is also included in that page.
Message me on slack if you think this is wrong - Malcolm Park
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 | |||
Design | 1 | 0 | ||
Manufacturability | 2 | 0 | ||
Assembly | 1.5 | 0 | ||
Rear | 0 | |||
Design | 1 | 0 | ||
Manufacturability | 2 | 0 | ||
Assembly | 1.5 | 0 | ||
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 | 1 | Tadpole benefits from only having one wheel, don't have to consider Ackermann steering or anything like that -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 | ||
Manufacturability | 2 | 0 | ||
Assembly | 1.5 | 0 | ||
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. | |
Design | 1 | 0 | ||
Manufacturability | 2 | 0 | ||
Assembly | 1.5 | 0 | ||
Stability | 2 | 0 | Min is researching the stability. 3 Wheel Vehicle | |
Handling | 2 | 0 | ||
Chassis | 1 | 0 | ||
Aerobody | 0.5 | 0 | Connor Hawkins left a comment on the 3- vs. 4-Wheel Car co-op task page which would be another consideration for this criteria. | |
Battery Box | 1 | 0 | ||
Motor Allocations | 0 | 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 |
Resources:
https://engineerdog.com/2015/09/09/engineering-a-3-wheel-vehicle-chassis/