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The initial Fourbar mechanism was designed to have a quick return in order to get out of the way of the curb. It's grounded points were placed on the axles of the major and minor wheels of the wheelchair. This was done for simplicity in mounting and ease of attachment.
Kinematics
This device
Problems
This version's major flaws included a lack of curb clearance and the resultant angle of the wheelchair through travel. The footprint of the pushbar for this mechanism was made large in order to time the contact with the ground and to provide more stability for the CG. However, this meant that during the quick return travel, the foot intersected with the curb. Re-timing the system to avoid this meant that the wheel would not be the proper distance behind the curb. This version also took the user through around 20 degrees of deviation from the horizontal, providing too much deflection than the 6-10 degrees sought after.
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