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April 27, 2025Every carbon rim brand publishes an impact figure. Few say what it measures. Here is ours, next to the standard it comes from.
The standard: 40 J
The UCI approves wheels, not rims. Its wheel approval procedure (current version 23 February 2022) covers front wheels used in mass start road and cyclo-cross racing, except traditional wheels (alloy rims under 25 mm deep with at least 20 steel spokes). The test is one vertical drop:
- the wheel is tested without a tyre;
- a flat steel anvil faced with a 20 mm silicone rubber pad, with a mass of 6 to 10 kg, strikes at 40 J, plus or minus 5%;
- one hit, 90 degrees from the valve hole, between two spokes;
- the wheel fails if it shows any visible crack or delamination, or if its lateral or radial run-out changes by more than 1.0 mm.
The ISO standard for wheels and rims, ISO 4210-7:2023, asks for the same 40 J (plus or minus 2 J) in clause 4.6. Manufacturers run the test themselves and certify the result, and the UCI states that approval “shall not constitute certification” of the wheel: it only authorises its use in racing.
40 J is roughly a 10 kg mass dropped from 40 cm.
Our pass marks: 100, 130 and 150 J
We set our own pass marks well above that floor, by discipline:
| Discipline | Our pass mark | Against the 40 J standard |
|---|---|---|
| Road, gravel and XC | 100 J | 2.5 times |
| All-mountain | 130 J | 3.25 times |
| Downhill | 150 J | 3.75 times |
Every rim model goes through the same test programme. Only the impact pass mark changes with the riding it is made for. We drop on a built wheel with the tyre off, the way ISO 4210-7 and the UCI do, so a builder can compare like with like.
On camera, a Venn 520 rim built as a front wheel took a 110 J drop: 10 J over its pass mark, and nearly three times the UCI figure.
A pass mark is not a breaking point
A pass mark is the energy a rim must survive. It is not the energy at which it breaks. A rim that passes 100 J may well survive more; the pass test doesn’t tell us how much more, because it stops at the pass mark.
Some brands publish their pass marks as if they were failure points, or call a rim “UCI approved” when the UCI approves complete wheels, and only for racing use. When you compare impact figures between brands, ask two questions: is this a pass mark or a measured failure, and was it tested the ISO way, on a built wheel, or some other way?
What we test besides impact
Impact is one test of several. The same programme checks fatigue (596 km on the rig, about 1,000,000 cycles, with no cracks, bulges or spoke detachment), spoke hole strength (every hole above the 300 kgf pass level), lateral rigidity, tyre retention (120 psi held for 8 hours), heat, and the flatness and roundness of the built wheel. The full results for the Venn 520 TCD UL are on our technology page.
Read on and shop
- Venn technology: the filament wound carbon rim Venn pioneered (Eurobike Award 2015), and the test programme with its numbers
- Rims: road disc, gravel, mountain bike
