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Can I buy a (105-based) road bike for a growing child, and when (from experience, after six months—sigh) it's too small for the child but just right for the parent, change its tires from 28mm to 35mm and turn it into a gravel bike?

This feels like a FAQ. Briefly, I'm asking:

Does installing (rugged) 35mm tires to a road bike make it a gravel bike, in the absence of mud and sand? The "gravel roads" in my area are mostly loose (big and small) gravel.

I can't read a geometry table very well yet, but if there is something in the geometry table I can look for to determine the answer to this question, do suggest it.

FWIW, the 35mm size is officially supported, which may mean that I might get away with slightly larger tires, if I'm willing to tolerate the risk of a stone getting stuck between fork and tire.

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  • Hmm, I see I may need to update my answer under what is the theory behind a gravel bike
    – Weiwen Ng
    Mar 12 at 22:16
  • Ah, sorry, I will stand by my answer for that question.
    – Weiwen Ng
    Mar 12 at 23:13

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Mostly no, I think, but there is actually one way to answer the question.

First, what is the definition of gravel where you ride. In Minnesota (US) but not the far northern parts, there are a lot of dirt roads and well-groomed gravel roads. Other places will have rougher gravel. The former is well suited for 35mm tires. If you expect a lot of mud, then your nominal 35mm clearance is going to run out. For large chunks of gravel, 35mm is probably in a grey zone.

To a lesser extent, you might also think of your preferences and off-road skills. Potentially, riders who are more skilled could put up with narrower tires on the same terrain.

Anyway, one answer is no, it's still a road bike, albeit with thicker tires. You can ride it on many gravel roads, though. Gravel bikes in 2024 tend to have nominal clearance for 40mm or more, usually more. They will probably be designed to accommodate the nominal clearance plus some mud. Not Unbound 2023 amount of mud, but a reasonable amount.

Another answer is that bikes with road-ish geometry and 35-40mm tire clearance are sometimes called all-road bikes or terms to that effect. That conveys that the use case is more towards roads, be they paved or gravel. Endurance road bikes, all-road bikes, and road-like gravel bikes tend to have similar geometry. Perhaps the bike wasn't marketed as one of those, but perhaps it functionally is an all-road bike. Hence, this is both a no and a yes.

If that were a performance road bike, then the handling might be faster than you'd like. Many race bikes have trail around 55mm (in the mainstream sizes; small bikes tend to have higher trail). It's not that this won't be suitable, it's that it may not be what you desire on a long gravel ride - but if you're comfortable with that trail figure on a 3-4 hour ride, which is entirely plausible, then it works for you. Remember that the first people to tackle Unbound gravel probably used a lot of cyclocross bikes; those would have 33-35mm ish clearance, possibly a high BB (<70mm BB drop), probably similar trail to performance road bikes.

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