Published: November 3, 2026 | By YONGMAO Engineering Team
A thru-axle almost never fails where the drawing worries about it. It fails at the thread root inside the dropout, or at the fillet where the head meets the shank — and both locations look perfectly acceptable on a dimensional report.
Most drawings say only "pass ISO 4210-6", which hides the fact that the thru axle fatigue test 100000 cycles pass criteria are three separate checks: a crack check, a permanent-set check and a function check. A report stating "no fracture" has answered one of the three.
Here is what the laboratory actually does to a 100,000-cycle sample, where the crack begins, and why grain flow decides whether an axle finishes the run or quits at 40,000 cycles.
1. What Counts as a Pass in a 100,000-Cycle Thru-Axle Fatigue Test?
Three things, and a report that gives you one number is not a pass.
• Crack check. Dye penetrant or magnetic particle, read at 10× on the thread root, the head fillet and both bearing seats. The ISO frame and fork tests judge "no visible cracks and no loss of function", so a hairline at the thread root fails the test even when the axle never separates.
• Permanent set. Deformation measured across the bearing span after the run; brands normally write 0.1 mm or less. An axle that comes back bent but unbroken is recorded as a pass in a fracture-only report, and fails in service because the wheel no longer sits square in the fork.
• Thread function. The thread still engages the dropout plus 3 mm and the axle threads out by hand. Galling between steel dropout and aluminium thread is a function failure, and it is the one most often left out of a summary sheet.
Read any test report against the thru axle fatigue test 100000 cycles pass criteria above and the missing lines become obvious: a summary that reports only the final cycle count has told you the sample did not break, and nothing about whether it is still straight or still threaded.
The standard sets the load, not the interpretation: 100,000 cycles at 450 N for city and trekking, 50,000 at 1,200 N for mountain, 100,000 at 600 N for racing. At 1.5 Hz those runs take 9 to 18 hours of chamber time, which is why a lab rarely tests more than one sample per design revision — and why one good sample is weaker evidence than it looks.
One thing engineers outside the fastener trade usually miss: steel has an endurance limit, a stress below which it survives indefinitely. Aluminium does not. A 7075-T6 axle has no stress it can endure forever, so 100,000 cycles is a proof at one amplitude, not a promise of infinite life. Two suppliers can both show 100,000 cycles and quote completely different wall thicknesses, because the number means nothing without the load and the criterion attached to it.
That is the question YONGMAO puts on every drawing review: at what load, for how many cycles, and judged how? A supplier that cannot answer the third part is quoting a process, not a part.
✅ Bottom line: A 100,000-cycle pass is three checks at a stated load, so ask for crack, set and thread results as separate lines.
2. Where Does the Crack Start, and What Does Grain Flow Change?
At the thread root first, then the head-to-shank fillet. The thread root is the sharpest section change on the part, and under the dropout it sees full reverse bending once per wheel revolution. Fretting between the dropout face and the axle shoulder then breaks the oxide film, a pit forms, and the pit becomes a crack. The fillet is second: small radius, large moment, and the exact place where a machined part gives away its advantage. Bearing-seat shoulders come third, usually after a press-fit hub has been installed one time too many.
Numbers make the ranking concrete. A thru axle bending strength kgf requirement bicycle buyers write most often is 600 kgf for a 12 mm axle, roughly 5.9 kN, rising past 800 kgf for 15 mm mountain and e-bike use, which is where the YONGMAO YM15-AO-K is built. Those figures describe the static bending point, not the fatigue limit — a part that bends at 800 kgf can still crack at 450 N after 60,000 reversals if the fillet is wrong.
Grain flow is the rest of the story. Cold heading pushes the billet through a die so the fibres follow the outside contour, around the fillet and into the thread. Machining from bar cuts those fibres and leaves their cut ends sitting at the highest-stress point of the part. The grain flow cold heading vs machining fatigue life argument is settled by one sectioned sample and a macro etch: either the flow lines curve continuously around the radius, or they run straight into it and stop.
Threads repeat the effect. Rolled threads, formed after heat treatment, put the root in compression and typically add 30 to 50 percent fatigue strength over cut threads, where the root is a machined notch. On a hollow axle the wall adds a second variable: a 1.6 mm wall with ±0.1 mm eccentricity swings the local section by about 12 percent, and the thin side is where the crack starts.
So when you compare a quote for a custom axle, ask for the three items that decide fatigue life rather than the three that decide price: rolled threads, a stated fillet radius, and a grain-flow photo per lot. A cold-heading factory sends that photo within a day. A trading company sends a catalogue. Changzhou Yongmao Special Fastener Co., Ltd. has cold-headed since 1993 and still sections a sample from every batch, because a photo is cheaper than a field failure.
✅ Bottom line: Cracks start at the thread root and head fillet, so grain flow and rolled threads matter more than nominal wall thickness.
Frequently Asked Questions
Q: What are the thru axle fatigue test 100000 cycles pass criteria?
A: No visible crack at the root or fillet, permanent set within the brand's limit across the bearing span, and full thread function afterwards. Note the count itself is category-specific — mountain frames run 50,000 cycles at 1,200 N, a harder test than 100,000 at 450 N. Compare cycle counts only at equal load.
Q: Is 600 kgf of bending strength enough for an e-bike?
A: Usually not. Motor torque plus rider mass pushes the axle into a load case the 12 mm city figure was never written for, and the thru axle bending strength kgf requirement bicycle brands set for e-cargo and long-tail bikes starts at 800 kgf. Move to 15 mm and a heavier wall before you move to a higher price bracket.
Q: Can a machined axle pass the same fatigue test?
A: Sometimes, by adding wall thickness or opening the fillet until the stress drops — you pay for the missing grain flow cold heading vs machining fatigue life advantage in weight and material cost. YONGMAO quotes both routes when a drawing allows it, and ships the sectioned comparison so the choice is made on data. OEM programs start from sample quantities and include the grain-flow photo with the first article.
Next Step
Send the drawing with your frame category and target bending load, and YONGMAO (Changzhou Yongmao Special Fastener Co., Ltd.) returns a factory quote that states the fatigue plan in the same document: load amplitude, cycle count, the three pass criteria, and the fillet and thread process the part will be made with. Samples in about a week, sectioned photos with every lot.
See the bicycle front and rear thru-axle range for current diameters and lengths, or open the YM15-14-K hollow thru-axle for the 15 mm baseline.
www.yonmach.com
Changzhou Yongmao Special Fastener Co., Ltd.
