Factory knowledge

Footwear Flex Testing: How We Stop Soles From Cracking

By DOING SHOES · 2026-06-30T00:00:00.000Z · 6 min read

Inside a real footwear flex test. See how DOING flexes finished shoes 60,000 times at room temperature and 40,000 times at −20 °C — and what counts as a pass — so outsoles and flex points do not crack in wear.

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A shoe spends its whole life bending. Every step flexes the sole at the ball of the foot — tens of thousands of times a year. If the outsole compound is too brittle, the bond is weak, or the construction is wrong, that flex point is exactly where a shoe fails: the sole cracks, or it quietly opens up between sole and upper. By the time it shows up, the shoes are already on your customer’s feet.

That is why a flex test is one of the most important physical tests we run on a finished shoe — and why we do it before mass production, not after a return.

See it: a real flex test on our factory floor

This is an actual finished-shoe flex resistance test at our factory — the shoe clamped into a Bally-type flex machine and bent repeatedly at the ball of the foot.

Real footage — finished-shoe flex resistance test on a Bally-type flex machine at DOING's factory in Quanzhou, China.

Our flex standard for finished shoes

These are the actual acceptance levels from our internal physical-testing standard. The numbers below are finished-shoe (whole-shoe) requirements — the kind of figure that decides whether a construction is fit to ship.

Test	DOING acceptance standard	Notes
Room-temperature flex (Bally flex)	≥ 60,000 cycles, no cracking	Hot-cut uppers ≥ 40,000 · PVC uppers ≥ 50,000
Cold flex resistance	≥ 40,000 cycles at −20 °C, no cracking	PVC tested at −10 °C / 20,000 cycles
Sports-lifestyle upper — flex	Room ≥ 60,000 · cold ≥ 25,000 (Bally flex)	Plus upper bond strength ≥ 2.5 kg/cm (stress areas)
Finished-shoe abrasion (national standard)	wear ≤ 10 mm	≤ 12 mm where the midsole is skived
Dynamic waterproof (waterproof styles)	20,000 flexes, no water ingress	General standard 15,000 flexes

What counts as a failure

Passing isn’t just “the shoe survived.” During and after the cycles, QC inspects the shoe and rejects it if any of the following appear:

• A new crack in the outsole or midsole
• The flex point opening up — bonding separating between sole and upper
• Leather cracking (龟裂) at the flex crease
• Mesh breaking, or obvious fuzzing / pilling of the upper
• Trims, overlays or accessories cracking, peeling or falling off

Any one of those is an automatic fail — the construction goes back to development before it can run as a bulk order.

Why we test cold as well as room temperature

Most factories that flex-test at all only test at room temperature. But rubber and many midsole compounds stiffen in the cold, and a sole that flexes happily at 23 °C can crack at −20 °C. For boots, outdoor and winter footwear especially, the cold-flex result is the one that matters — so we hold finished shoes to 40,000 cycles at −20 °C. It’s a harder bar, and it’s the difference between a shoe that survives a European winter and one that splits in January.

Where flex testing sits in our QC

Flex resistance is one item in a documented physical-testing programme that covers outsoles, uppers, components and finished shoes, aligned to Chinese national (GB) and international norms. Materials are confirmed and physically tested before mass production; only what passes is released. After production, a random, non-consecutive AQL inspection checks the batch before it ships.

• See the full scope of what we test → Footwear testing & quality standards
• How our 6-stage QC process works → Factory & QC
• For European buyers — chemical side → REACH & RSL compliance

What it means for you as a buyer

When you ask a supplier “do your shoes crack?”, the honest answer is a number and a method, not a promise. Ours is 60,000 room-temperature cycles, 40,000 at −20 °C, zero new cracks — and if you have your own flex standard or a specific target market, we test to that. That’s how you launch a line without the cracked-sole returns that quietly kill a footwear brand.