Automotive Interior Design: An Occupant Package Case Study

The vehicle segment selected for this example is the two-seater luxury sports car. This vehicle is usually purchased by people who are well off financially and are over the age of 50. On average the household income of an individual who is purchasing a luxury sports car is over $145,000 a year. The majority of customers are male with a college education. They are proud of the fact that they have earned the right to purchase a luxury sports car and like to buy a vehicle that shows off their success.

At this point in their career, their kids have already graduated from college or are about to graduate so they have some extra money to spend on their toys. They usually have another vehicle, which is considered their daily driver, and are buying a sports car to have some extra fun on the weekends at the track. This kind of persona-driven target-setting is a standard part of vehicle benchmarking.

The competitive vehicles benchmarked for this example were:

1. 2022 Chevrolet Corvette 3LT
2. 2022 Porsche 911 Carrera
3. 2022 Audi R8 V10

Overall, in size these vehicles are all about the same size, the Corvette is a few inches longer than the Porsche and the Audi. The Porsche is the only rear-engine vehicle both the Audi and the Corvette are mid-engine vehicles. Of all the three vehicles the Porsche is the most fuel-efficient vehicle, while the Audi has the greatest horsepower. However, the Corvette is the overall best deal as it provides comparable if not better performance at a fraction of the cost. An overview of these three vehicles can be seen in the table below.

Vehicle	2022 Chevrolet Corvette 3LT	2022 Porsche 911 Carrera	2022 Audi R8 V10
Vehicle Dimensions	182″ L x 76″ W x 49″H	178″ L x 73″ W x 50″H	174″ L x 76″ W x 49″ H
0-60	2.8	3.5	3.2
MSRP	$79,850	$106,100	$148,700
Max Speed	205 mph	182 mph	204 mph
Fuel Economy	16 City / 24 Highway	18 City / 25 Highway	14 City / 23 highway
Horsepower	495 hp	397hp	602 hp

Exterior and Interior dimensions:

For all measurements taken in this example the standard SAE coordinate system was used, where z=0 at the ground, x = 0 at the front of the car, and y = 0 at the centerline of the vehicle. See the standard SAE coordinate system below.

Several of the basic exterior dimensions for these three vehicles were readily available online, however, information such as Ramp Breakover Angle (SAE A147), Tumblehome Angle (SAE A122), and Windshield Angle (SAE 121-1) required calculations to find. To find these dimensions, the vehicle blueprints were used, and measurements were taken to calculate any missing dimension. Any description field which has an asterisk is estimated based on the measurements and calculations performed. Once all the SAE dimensions were calculated for the competitive set, targets were set for the new vehicle to ensure every dimension selected fell within the desired vehicle segment. This process follows the same standardized measurement conventions defined in SAE J1100. See the table and blueprints below, which show how the new vehicle compares to the competition in its segment.

Row No.	SAE Dimension code	Description	Units	2022 Corvette 3LT	2022 Porsche 911	2022 Audi R8	New Vehicle
1	L103	Length	mm	4622.8	4562	4429	4500
2	W103	Width	mm	1930.4	1852	1940	1925
3	H100	Height	mm	1244.6	1270	1244	1250
4	L101	Wheelbase	mm	2723	2456	2650	2700
5	L106	Front Overhang	mm	900	1061	994	950
6	L107	Rear Overhang	mm	1008	1044	782	850
7	H143-1	Cowl point*	mm	630	860	896	800
8	H143-2	Deck Point*	mm	730	975	1005	900
9	H25-1	Belt Height*	mm	457	457	457	457
10	H156	Ground Clearance	mm	135	109	110	125
11	A106-1	Approach Angle	degrees	11.5	9.8	16.6	12
12	A106-2	Departure Angle*	degrees	10.3	13.8	14.3	15
13	A147	Ramp Breakover Angle*	degrees	11.3	12.7	12.5	12
14	A121-1	Windshield Angle*	degrees	64.8	62	67	65
15	A121-2	Backlight Angle*	degrees	79	77	72.3	79
16	A122	Tumblehome Angle*	degrees	28	25.7	25	25
17	H61-1	Effective Head Room – Front	mm	965.2	965.2	978	960
18	W3-1	Front Shoulder Room	mm	1371.6	1372	1400	1380
19	W5-1	Front Hip Room	mm	1320.8	1405	1452	1400
20	L34	Front Leg Room	mm	1087.12	1057	1039	1080

Exterior Vehicle layout:

Based on the exterior dimension calculated in the previous section, the exterior of the vehicle was laid out. Aesthetically the vehicle has exterior styling inspired by both the Bugatti and McLaren supercars. This vehicle is intended to have the mid-engine design which is found in both the Corvette and Audi R8 to ensure better weight distribution and a smoother ride. The exterior envelope of the vehicle is 4500mm L x 1925mm W x 1250mm H. To see a full detailed view of the vehicle please see the blueprint drawing below.

Interior package layout: how automotive interior design is built around the occupant

The interior of the vehicle was laid out around the occupant — the core principle behind all automotive interior design. The first thing set was the seating reference point (SgRP) and the ball of the foot (BOF) of the driver, the same reference points defined in SAE J826. Knowing these two key points, in addition to the exterior dimensions, allowed the SAE manikin to be placed in roughly the correct seating position to calculate and derive all the interior dimensions. This same occupant-first approach is central to vehicle package engineering more broadly. An in-depth look at the calculations is reviewed in a later section. Below is a pictorial representation of the interior vehicle package. The table that follows lists each dimension along with its SAE dimension code and value.

Maximum comfortable reach cross sections:

To find the cross sections of maximum comfortable reach in vertical planes parallel to XZ planes at 300mm to the left and 300mm to the right of the driver’s SgRP, the H30 and H17 values were assumed to be 350mm and 511mm respectively (these were calculated in the calculations section below).

Calculations:

1. G=.00327*H30(350)+.00285(511)-3.21=-.671
2. HR plane(rearward of AHP)=786-99*G(-.671)=852.4mm
3. AHP to SgRP-HR=868.7-852.4=16.3mm

Assumptions required to find the appropriate table to use in SAE J287

• 50/50 male to female population mix
• Restrained Hand Reach (occupant was restrained by a lap and shoulder belt)
• SgRPz=450.7mm
• Driver is utilizing a 3-finger grasp

Table 7:

As can be seen from the table above, the horizontal reach forward of the HR reference plane for maximum comfortable reach for the left hand is 600mm and the right hand is 649mm. To find the value being asked for “cross sections of the maximum comfortable reach in vertical planes parallel to XZ planes at 300 mm to the left and 300 mm to the right of the driver’s SgRP,” 16.3mm is added to each of the values from the table — the distance HR is forward of the driver’s SgRP. The final answers for maximum hand reach in vertical planes parallel to XZ planes at 300mm to the left and 300mm to the right of the driver’s SgRP are 616.3mm for the left hand and 665.3mm for the right hand.

Calculations:

To determine many of the vehicle’s dimensions, blueprints of the appropriate vehicles were used, along with the website imagemeasurement.com for other dimensions not readily available. This website utilizes existing dimensions to understand the scale of the image and accurately determine any dimensions not included in the blueprint. An “Occupant Package Dimensions Computations” worksheet was used to determine many of the dimensions for the vehicle. This worksheet has many of the calculations already built in, with the required values measured or assumed in the inputs section.

Trig was used to calculate the degrees for approach angle, departure, ramp break over angle, backlite angle, and tumblehome angle. Sample calculations for the 2022 Porsche 911 can be seen below:

The remainder of the interior calculations for the new vehicle can be found below, including the SAE reference documents and equations:

Conclusion:

This example illustrates which calculations and assumptions are needed to determine all of a vehicle’s important dimensions (interior and exterior). These dimensions were calculated on multiple different sports cars, for comparison purposes, and some of those dimensions from the sports car segment were used to create the new vehicle’s own dimensions.

The new vehicle’s dimensions were compared to body dimensions of US adults at the 95th percentile male/female to understand whether they could fit in the vehicle (from anthropometric reference data, table 13-1). The results are below:

After reviewing the table above, it can be concluded that the new vehicle is not tall enough for a 95% male — the 95% male eye height is below the vehicle’s eye height. The ability to move the seat up might make up for this height difference. The width of the vehicle is appropriate to accommodate a 95% female or male at both the hip and elbow height. When comparing the entrance height to the vehicles in the competitive set (2022 Chevrolet Corvette 3LT, 2022 Porsche 911, and Audi R8 V10), they are very similar, with the new vehicle fitting in the middle of the set. With this information it can be concluded that these sports cars aren’t necessarily designed to comfortably accommodate a 95% male/female.

Reference:

1. https://www.carsguide.com.au/audi/r8/car-dimensions/2022
2. https://www.caranddriver.com/chevrolet/corvette/specs
3. https://www.chevrolet.com/performance/corvette
4. SAE International (SAE J1100) Motor Vehicle Dimensions
5. SAE International (SAE J1050) Motor Vehicle Dimensions
6. SAE International (SAE J287) Motor Vehicle Dimensions
7. https://www.imagemeasurement.com/en/
8. http://hyperphysics.phy-astr.gsu.edu/hbase/ttrig.html

Appendix: