Convertible Car Roof Mechanism

Designing a mechanized, unfolding soft-top car roof. The project involved creating a Product Design Specification and developing a Kinematic Analysis Model.

An individual project completed during 2nd year of University

Initial Design Concepts

Kinematic Analysis Model

Modelled Results Example

CAD Mock-ups

Evaluation

Initial Design Concepts

Created using mechanism design and simulation software (Linkage).

Key:

Attatchment Point

Ideal Cabin Space

Ideal Bounding Box for Folded Mechanism

Concept 1

An accordion-style design that comprises four scissor mechanisms. It minimizes the swept area and has a small footprint when folded. It has the drawback of intruding significantly into the cabin during unfolding.

Concept 2

A design where one large sweeping arm accounts for most of the roof's folding action. It reduces intrusion into the cabin relative to Concept 1 at the expense of a larger folded footprint and a far higher swept height.

Concept 3

Another sweeping arm design. When folded it has a small footprint and it unfolds with minimal cabin intrusion. The Centre of Mass also moves less during unfolding than the other designs. This comes at the expense of a large swept area, high swept height, and high relative mass.

Concept Choice

Through controlled convergence, it was found that Concept 1 best fulfilled the Product Design Specification.

Kinematic Analysis Model

An iterative mathematical model of the roof mechanism was created to calculate the torque requirements during opening and closing. For safety purposes, it is important to also calculate the velocity of the roof at all points during opening and closing. The mechanism also still has to be able to function while the vehicle is travelling at a speed of up to 20mph.

Simplified Mechanism

Simplified Analysis Model

To mathematically model the mechanism, simplifications had to be made.

The whole mechanism is represented as a point mass at the centre of mass of the mechanism. The position of the centre of mass changes as the roof opens. Its distance from the pivot point was calculated manually at seven discreet stages of movement. Between them, linear interpolation was used to get appropriate estimates.

Using this free-body diagram, an iterative mathematical model was created that gives the torque requirements as well as the velocity at all points along the roof's path.

Selecting a Gear Ratio and Motor

To proceed with the analysis, the mass of the mechanism must be estimated. This is done using the load-bearing requirements of the roof.

Loading requirements of the mechanism (approximated as two simply supported beams). Material choice was limited to steel or aluminium. To withstand these loads (multiplied by a safety factor of 1.5), beams of radius 13.2mm are needed when using aluminium, and beams of radius 8.48 mm are needed when using steel. Since the length of links needed is known, this allows for the mass to be calculated for each material. (10.1kg for aluminium and 25.5kg for steel). At this point it became clear that aluminum, was more efficient as the extra material costs would be saved by being lighter therefore allowing for a smaller motor.

Motor Curve for Chosen Motor

This motor (NSA-I) and a gearbox with a ratio of 60 provided enough torque to operate the mechanism in all conditions, and came at a low weight and cost. This torque curve was used in the kinematic model.

A damping effect was deemed necessary to prevent the mechanism from reaching dangerous speeds. It was also incorporated into the analytical model.

Initially, a single large rotational damper was proposed but a lack of cost-effective options made that unviable. Two linear dampers that produced the same effective damping coefficient were used instead.

Once this was all decided, the model was ready to be used for full analysis.

Modelled Results Example

Similiar data was obtained for the required torque and for a stationary vehicle.

CAD Mock-ups

Created using CAD software (Fusion360)

A comprehensive bill of materials table was written up based on the number of joints, length of beams, and the motor, gearbox, and damper requirements detailed above.

Fixed Joint

Involves 2 washers, 1 M5 bolt, one M5 nut and a ø10mm shaft. Used to connect the mechanism to the gearbox shaft.

Rotational Joint

Involves 4 washers, one M10 bolt, and one M10 nut. Used to connect links. There are 27 of these in the design.

Cad Model of Mechanism Midway Through Opening/Closing

Evaluation

A final comparison of the mechanism's modelled performance against the initial product design specification showed that it met all the requirements that could be theoretically tested. The design's biggest flaw was that it took up too much space over the rear seats when the roof is closed, though it does fall within the specified limits. Possible improvements include mounting the mechanism 15-20 degrees clockwise (and adjusting the motion paths so it still meets the windscreen at the correct point). This would reduce the area of cabin space taken up by the mechanism. Another issue is that the mechanism's centre of mass is further than ideal from the powered joint. This could be remedied with a counterweight. This would reduce the amount of damping required which would be a significant cost saving.

For access to the full design report, get in touch at Michaelsvanidze0@gmail.com.

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