I’ll need to pick a wheel base and track width to draw my first lines in Fusion. But these two dimensions work together to create a personality of how the car will behave. The track width is the distance between the center lines of the tires when viewing from the front or rear of the car. The wheelbase is the length between the center of the circle drawn by the front and rear tire when viewing from the side of the car. The track ratio is the ratio created by dividing the track width by the wheel base. Text book theory says that a higher track ratio (wider width and shorter wheel base) as better cornering potential than a lower track ratio, and that a lower track ratio goes in a straight line better (like a long narrow dragster). This leaves us with an equation that has 3 variables.
For the sake of making this next step easy, I will oversimplify the concept of how TrackRatio affects drivability as a sliding scale between
A.) Maximum cornering ability, and minimum stability in straights
on one side, and
B.) Maximum stability in straights, and minimum cornering ability
on the other.
I want a reasonable mix of both, so I will be somewhere towards the middle of the sliding scale. however I will prioritize cornering ability slightly higher than straight line stability this time around. So my track ratio will be on the higher side of the window I choose from. Meaning the car will be wide in width but not very long in length.
If I knew the wheel base I could choose a track width that creates an appropriate track ratio for my goals. Or if I knew the track width, I could choose a wheelbase that creates an appropriate track ratio for my goals.
While I don’t have either yet, I do have an idea of what range they both need to be in. For example, on the maximum side, the car needs to fit between the lanes on a road which is about 11-12 feet. Past that, it needs to fit inside a race car trailer which is about 8.5 feet. And on the minimum side, it needs to be wide enough to fit a driver and passenger seat with room for a transmission to pass between them and doors to enclose them. That means the overall width needs to be at least about 6ft to be realistically comfortable. So our overall width will likely be somewhere between 6 feet and 8.5 feet. And it needs to be at least long enough to fit a transmission and engine between front and rear chassis mounted differentials
For the sake of reference I will list some widths and wheel bases of existing GT3 cars.
| Vehicle | Overall Width | Track Width | Wheelbase | Overall Width to Wheelbase Ratio | Track Ratio |
| BMW M4 GT3 | 2040 mm | 1710 mm | 2917 mm | 0.70 | 0.59 |
| Nissan GTR GT3 | 2036 mm | 1690 mm 1700 mm | 2817 mm | 0.72 | 0.60 |
| Lexus RCF GT3 | 2030 mm | 1700 mm | 2770 mm | 0.73 | 0.61 |
| Audi R8 GT3 | 1997 mm | 1667 mm | 2700 mm | 0.74 | 0.62 |
| Mercedes AMG GT EVO | 2050 mm | 1707 mm | 2635 mm | 0.78 | 0.65 |
| Porsche 992 GT3 R | 2050 mm | 1707 mm | 2507 mm | 0.82 | 0.68 |
Looking through GT3 class cars as a reference, it appears that most have track ratios between 0.59 and 0.68 with overall widths of 2000mm-2050mm. The Porsche stands out with a significantly shorter wheel base than the others, driving the track ratio up significantly.
I don’t want the car to be narrower than a GT3 car, but I want the car to be narrow enough to fit in a race car trailer. So for the sake of turning a variable into a constant to move forward with the design, I will move forward with an overall width of 2050mm or 80.71in (6.73ft)
To get the track width from the overall width I will need to know the tire width. So it’s time to pick a tire. Since this car is going to be AWD, and ideally the differentials would be mounted at a height that allows the axles to be parallel with the ground while parked at rest, the tire height will influence the center of gravity. A shorter tire will allow for a lower center of gravity. But too short of a tire might push me into needing an unusually small final drive in the differentials, and might also limit my options for brakes. Looking at GT3 cars for inspiration, most of them use 18″ diameter wheels with widths ranging from 12″ to 14″. Since I am prioritizing cornering over straight line performance, when a decision pops up where a good option must sacrifice straight line performance to gain cornering performance, I will choose that over an option that requires cornering performance to be sacrificed for straight line performance. So while picking a tire I am going to prioritize a large contact patch. Looking at 12″-14″ wide tires that can realistically be driven on the street and fit on an 18″ wheel, I have found the Toyo R888R 335/30R18. It has a 25.9″ height, 13.5″ cross section width, 12.9″ tread width, and fits on a 12″ wide wheel.
Bridgestone, BF Goodrich, Continental, Nankang, and Hoosier make race tires in this size as well.
If I factor in the 13.5″ tire width, that makes the Track Width come out to 1707.1mm (67.21″). Almost identical to the Porsche and right in the ball park with the other GT3 cars.
Then I can use a 0.59 to 0.68 track ratio window to find wheelbases that work with a 67.21″ track width.
Low:
Medium:
High:
The last constraint consideration for wheelbase and track width is if I were planning to use body panels from an existing car on the exterior of this frame. While I considered the idea of using an existing car body over the top of this frame, I think (for the sake of the challenge and making it my own) that I want to design my own body work for the exterior. I plan on taking inspiration from existing cars in the form of using existing headlights, tail lights, windshields, windows, etc. However the body lines of the car will be my own. So that being said, I can pick my own wheelbase and track width within the window of numbers listed above.
That being said, It would be nice to have a back-up plan if my body design attempts don’t look good enough. So I am going to pick the wheelbase of an existing front-mid-engine car that I like as my backup. I will use the wheelbase of the Mercedes AMG GT EVO which is 2635mm (103.7in). This makes it longer than the Porsche, but shorter than the other GT3 cars.
So now I have the following variables defined.
Overall Width: 2050mm
Track Width: 1707.1mm
Wheelbase: 2635mm
Track Ratio: 0.648
Wheel Specs: 18″ x 12″
Tire Specs: 335/30R18 (25.9″x13.5″)
The next step will be to put 4 tires on the ground plane of an empty cad file in the right location for the chosen track width and wheelbase.
Then I will scan the main components of the drivetrain using a 3d scanner and import them into the file between the 4 tires and make sure that the drivetrain can fit within the wheelbase.
Then I will start modeling frame tubes to bolt everything together, and then move on to double wishbone suspension design.
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