Bike tire rim compatibility: ETRTO calculator and safety guide

Ten years ago, it was simple: a road rim was 17 mm wide and 23 mm tires were mounted. Today is chaos. The modern rims reach 25 mm of internal width on the road (i.e. wider than the wheels of MTB 15 years ago!), and the standard « hookless » (without hooks) imposes strict safety rules.

One question comes up daily at the workshop: « Can I mount my 25 mm tires on my new wheels? » or « What maximum tire does my engravel bike accept? ».

Mounting a tire too narrow on a wide rim exposes you to a deadly risk of jogging. Mounting a tire too wide makes you lose in driving precision. To never doubt again, I encoded this calculator based on the latest standards ETRTO 2024 (European Tyre and Rim Technical Organisation) and ISO standards.

Your compatibility calculator (ETRTO)

First select your practice, then the inner width of your rim to know the compatible and secure tires.

Understanding internal width (Internal rim width)

That's the key measure. Forget the outer width of the rim (used for aerodynamics). What matters for safety is the distance between the two hooks (or vertical edges) inside the rim. It is she who determines how the tire will sit.

The 3 rules of compatibility

1. The light bulb rule

It's the « old » Standard. A wide tire was mounted on a narrow rim. The tire formed a bulb that far exceeded the rim. The problem: At low pressure, the tire floats, twists in the corners and gives a feeling of blur. This is bad for efficiency and accuracy.

2. The bell rule (Bell shape)

This is the modern trend « Aero » and performance. The sides of the tire are straight, aligned with the rim. The advantage: The air flows better (aerogain), the tire is better supported and you can run at lower pressure without blur. This is what brands like Zipp or Enve aim for.

3. The danger zone (too narrow)

If you mount a tire that is narrower than the rim (or barely wider), the tire rod will not be maintained enough by pressure. Risk: Immediate deflection at first turn or shock. This is why the new 23 mm or 25 mm rims often prohibit tires of 23 mm and sometimes even 25 mm.

The critical case of hookless (TSS)

The rims hookless (right wall without hooks, type Tubeless Straight Side) are increasingly common on road and gravel. They are lighter and stronger, but they forgive no mistakes.

🚨 Hookless Survival Rules

Which tire to choose for your rim?

Here are the most frequent combinations I recommend at the workshop for a perfect balance between performance and safety.

FAQ: your technical questions

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