Weight Weenies

Which tire size would be the best on a rim with internal width 15mm en external width 21mm.

I doubt between the Specialized Turbo S-Works 24mm or 26mm.

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Either.

The narrower the better. Best performance would even have a 20mm tire, but comfortwise nobody likes to ride these anymore.

A wide tire on a narrow rim looks like a light bulb - the flanges are bent and a high pressure is needed to make it roll and be stable. If you run a low pressure for comfort it will feel squishy and have a higher rolling resistance.

But also 15mm inner width rims are hard to get these days, so this will adjust over time. 21mm inside will be more common in the future. Problem is that older frames won't fit these and narrower rims will have to stay availiable for a certain time.

Get a new wheelset with 19mm inner width rims, and use 23-24mm tyres.

please, read this:

https://intheknowcycling.com/2016/04/03 ... eel-sizes/

Best for what use? wider tires for comfort, narrow for aerodynamics. I would go wider for the comfort since if you care about aerodynamics you need to optimize the rest of the setup and tires would be at the end of the list for aero gains.

If you are a heavier guy I doubt you will be pleased with any tire on such narrow rims.

If you are a light guy then do 23/25.. or 25/28.. front/rear.

Just a suggestion. It helps to keep weight down and aero up while preserving some comfort.

So my rims are to narrow for tires +25mm.
I didn't know that they are so narrow, that the rims today are minimal 19mm internal width.

I don't need more comfort but I have read that everybody fits larger tires on the rims, because it benefits less resistance.
Now I have the 24mm sworks on them.

Wider tires come with an aero penalty.

If you are a lighter guy you might actually become slower by fitting wider tires. Then pehaps the aero penalty is greater than the rolling resistance benefit.

Optimal is to use a rim width that suits your body weight and tires that suit that rim width. 15mm internal is maybe optimal for 40kg riders with 19c tires on.

So in essense, only go as wide as you need. Wider is not always better. It's just a lazy interpretation of what some some people observe is happening.

your rims are fine with 30mm tyres it just wide tyres will ballon a bit. that has performance implications but its not unsafe. your question was is o.k with 26mm and yes it is.

Since both tyres will ballon i dont think there is going to be performance gain with the 24mm.

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The differences are not huge and people rode 23/25mm tires on 15C for years, it's just not the best performing combination.

And a wider tire has a lower rolling resistance on the same rim at the same high pressure. But because of the higher volume the tire gets less comfortable and people run lower pressures which compromises handling and rolling resistance. For 28mm comfort and 25mm speed you could run a 25mm tire on a 21C rim with low pressure. A 28mm tire on a 15C rim is of course possible and will be more comfortable, but performs worse - higher rolling resistance and a squishy feel.