Patching running shoes' outsole

Most distance runners do not wear out their shoes' outsoles evenly. Typically, the lateral heel-strike area is first to be worn through [1].

Shoe wear pattern where the lateral heel-strike area wore out first

When this occurs, the exposed midsole will wear down quickly due to its softness. The shoe will then gradually lose shock absorbency as the midsole continues to thin. While it is easy to glue a piece of abrasion-resistant rubber over the worn out area, the graft can shear off when subjected to running's landing impact. Below are steps to achieve a reliable patch.


1. Using a ballpoint pen / permanent marker, outline the area where the outsole has thinned noticeably (a larger surface will increase the patch's bond strength).

Worn out area is marked for patching (top) and would-be graft (bottom)

2. Cut the donor material to fit the marked graft site (see appendix for sources of graft donor).
3. Remove thinned outsole inside the marked area. A heat gun / hair-dryer can ease the removal.

After removal of the thinned area

4. Clean / abrade the surfaces to be joined (I use a brass wire brush).
5. Apply contact adhesive to both midsole and graft
6. Leave contact adhesive to dry for ~15 minutes. Bonding before the solvent has evaporated can lead to a weak bond.
7. Align donor outsole with the marked area and bring together
8. Fit a G-clamp over the graft site and tighten until the midsole is compressed to two-third of its original thickness. Although the adhesive's instruction recommends using a hand-roller / nip-roller  [2], we did not have a good result with this tool as the patches came off later. We suspect when neither midsole nor outsole surfaces are perfectly flat, they can separate slightly after rolling; hence, the unreliable patch. Clothes pegs have been used to clamp the graft [3], but we suspect the reliability will be poor due to the low force. Not enough pressure is a prime cause of bond failure [4].

After grafting, the shoe is then G-clamped to the table

 

Another way to clamp the graft to the shoe

9. Remove the clamp after 12 hours, preferably 24 hours.
 

Donor grafted to shoe 

Appendix: sources of graft donor

The graft donor can either be harvested from car's floor-mat or old running shoes. In the latter case, a heat source (hot air gun or hair dryer) can ease the removal of the outsole.  

This type of floor mat (car) is a suitable graft donor. The flat section is ~ 2mm thick.

 

An old shoe donates its outsole with the help of a hot air gun

If no suitable graft donor is at hand, one alternative is buy a sheet of 3mm thick abrasion-resistant rubber. The below example was sourced online. The advert specified a durometer hardness of 75A [5].

A durometer chart for quantifying hardness. Rubber is usually quantified with the "Shore A" scale

Up to now, we have only considered rubber as the graft material. However, polyurethane (PU) is actually even durable [6]. Unfortunately, we could not find any vendor that is willing to sell one piece and has a reasonable shipping charge.

Polyurethane makes the most durable outsole

Caution

If you hope to extend your shoe life through patching, be mindful of the finding that the midsole (foam) lose s half of its shock absorbency after 500 km [7].

References

[1] T. D. Noakes, Lore of running, 4th ed., pp. 763
[2] "Bostik contact bond",  ver. 5, Mar. 2017, Available: https://www.bostik.com/globalassets/tdsdocuments/bostik-contact-bond-tds_australia_en/technical-data-sheet/bostik_contact_bond_tds_01032017.pdf
[3] "How to fix a pair of old running shoes with contact adhesive". Available: https://almost-a-technocrat.blogspot.com/2015/01/how-to-fix-pair-of-old-running-shoes.html
[4] "15 Frequently Asked Questions About Contact Cement", Available: http://www.cpadhesives.com/contact-cement-faq

[5] https://www.ebay.com.my/itm/3mm-1-9-Thickness-6-152mm-Square-Rubber-Sheet-Chemical-Resistance-High-Temp-/391668455110?hash=item5b314256c6:g:ygEAAOSwnHZYbM2g
[6] "Polyurethanes in footwear", Available: http://www.polyurethanes.org/en/where-is-it/footwear
[7] R. Verdejo, N. J. Mills, "Heel-shoe interactions and the durability of EVA foam running-shoe midsoles", J. Biomechanics, 37 (2004) 1379-1386