But I know that the two bikes have different tire sizes, but I don't know why they are designed this way
3 Answers
Sometimes there is no difference whatsoever. My gravel rim is 17 mm and can also be used for the road.
The main difference is that road wheels can and strive to be lighter and use smaller number of spokes. Gravel wheels must count with larger loads and potentially some more jumps.
And yes, due to the wider tyres the rim will typically also be wider, more than those 17 mm I have, that is now considered very narrow. The range of tyre widths that can be used with a particular rim is usually very wide but if one uses a tyre that is on the border of the acceptable range, there may be compromises in the riding qualities. Check the table from Schwalbe for those ranges https://www.schwalbe.com/files/schwalbe/userupload/Images/FAQ/reifen_felgen_2020/Reifen_Felgenkombinationen_2020_EN.jpg
Basically, the wider the rim is, the wider tyres can be used on the rim. E.g., according to the chart, with 19 mm rims you can use tyres in the range (22-)25-63 mm. Gravel tyres typically fit into this range with some margin.
If the rim is too wide for the tyre, the tyre may not be connected well enough. If the rim is too narrow, the tyre is squeezed at the rim and may have a tendency to a sideways movement or instability, especially when cornering.
answered Feb 19, 2022 at 10:08
Until a few years ago road bike rims and tyres tended to be quite narrow. 13 – 15mm inner width for rims used to be common. 23mm wide tyres was normal. Rims like that are only really good for less than 30mm tyre width.
These days road bike rims and tyres have gotten wider. 17 – 21mm rim inner width and 25 – 32mm tyre width are quite common. You can use these rims all the way up to ~50mm tyre width. Gravel bikes usually use 35 – 48mm tyre width.
So depending on which bikes/wheels you are comparing there might not be any difference regarding rim width.
As to why we see/saw rims getting wider: Using wide tyres on narrow rims can lead to “collapsing” tyres in tight turns. Wide tyres can bend and damage the sidewalls of narrow rims. Wider rims improve aerodynamics since the transition from the tyre to the rim sidewall is smoother (hence why e.g. the famous Zipp 404 wheels now have a 23mm inner width).
If you look at the specs of the current range of some manufacturers, you can see that gravel rims are wider than road ones (and are less deep/aero for vertical compliance):
- DT-Swiss: road (endurance) 20mm, gravel 24mm
- Fulcrum: road: 19mm, gravel 23mm
- Mavic: road: 17-21mm, gravel 23mm
It's somehow logical: gravel bikes have larger tires at lower pressure (recommended pressure for the 40mm gravel tires I have now is 3-5 bars, values that you will probably avoid with road tires), a wider rim provides more stability when turning for that kind of combination.
That being said, gravel is a wide category, so it's difficult to generalize. It goes from bikes closer to endurance road bikes to the bikepacking that fit MTB tires. So if you are on the road side of gravel, you can be perfectly fine with 17mm rim (inside) and 35mm tires at higher pressure. But if you fit 45mm at 3 bars, it will certainly feel wobbly with 17mm rims.
