Improving the heel design

A big thank you to SFT’s beta customers because with their help, our semi-custom last design is improving quickly.

Because everyone’s feet are different, from a small pool of customers, we’ve been able to stretch the design to its limits. The same CAD model used for narrow, “Egyptian-toed” feet can be changed in 2 minutes to fit a person with wide feet and “Morton’s toes.” The toebox design has proved quite flexible.

However, fitting peoples’ heels has been more difficult. After taking measurement after measurement, making visual comparison’s of customers’ bone structures, and building many prototypes to test various hypotheses, we made progress in determining how much narrower the last should be than one’s foot tracing. However, each prototype seemed to fit a little differently around the Achilles tendon.

There must be some hidden weakness – sort of an Achilles heel, har har – in the last’s heel and ankle design. Since we’ve built up quite a volume of prototypes, all with different materials, rand designs, and many other facets, I decided to conduct a survey of heel comfort versus all the variables that might affect heel fit.

I selected 14 prototypes, 5 of which were adjustable, to try on and rate in terms of comfort. Then I collected data on heel angle, rand design, and six other variables and calculated correlation coefficients between these variables and the comfort ratings.

Heel comfort ratings

The orange bars indicate comfort ratings (each prototype was sampled three times), and the blue line indicates the average comfort rating.

When calculating correlation coefficients, prototypes with a comfort rating standard deviation of greater than 0.8 (graphically speaking: prototypes where the average rating differs greatly from the three samples) were not included.

The correlation coefficients indicated that heel stiffness was the variable with the greatest effect on heel comfort, followed by heel angle and whether or not the heel rand was continuous. The other variables measured were not significantly correlated to heel comfort.


SFT’s most recent shoes built for customers have drawn on the information learned from this prototype survey, and our customers have noted how much more comfortable the heels are. One customer said, “The heel fits like a glove and this shoe looks absolutely fantastic.”

SFT shoe with updated heel design.

SFT shoe with updated heel design.


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