"Last torsion" means that the heel seat is not horizontal when the toe part is properly grounded.
The height of the symmetrical edge is not equal to the centerline of the heel seat (parameter "i" is not zero), which means that the top lift of the heel does not lie flat on a level floor.
One might think that a torsional last is a mistake.
However, there are cases where the torsional angle (AngleRollHeelsheet: i) is necessary for the following reasons.
In the combined last and heel, the heel stands straight up in Figure 2a, viewed from the centerline of the heel, but in Figure 2b, viewed from the centerline of the shoe, the top lift is off-center.
The imbalance can be seen more clearly when viewed from the angle shown in Figure 3c.
If the top lift does not support the center of gravity of the heel during walking, the burden on the foot will increase during walking, and this will also lead to shoe deformation.
After all, I believe that shoes need to be balanced so that a person can stand on one foot.
This phenomenon is due to the swing angle at the heel seat, which becomes more pronounced the higher the heel and the greater the set-in.
To eliminate this problem, the heel seat must be rolled so that the top lift overlaps the center line of the shoe.
In the centerline view of the heel in Figure 4a, the heel is tilted, but as shown in Figure 4b and Figure 4c, the stability of the shoe is improved.
Since the model number of the heel to be combined is not decided in DDL's existing models, it has the minimum twist angle within the expected range.
However, not all the lasts used now are wrong.
When creating the last model, the modelist adjusts the heel to the last and adjusts it while looking at the angle shown in Fig. 2-b, so that the twist angle is naturally provided without being aware of it.
The heel curve is also more conformable to the changing shape of the foot if it is rolled with the heel up.
In DDL's existing models, "AngleRollHeelcurve" is set to the same value as "AngleRollHeelsheet".
It should be noted that these two are not directly related, so there is no need to set them to the same value.