Hey there!
I have a two sets of carbon wheels, one at 29mm and the other at 24mm, so I have some experience here.
With the wheels you are comparing, we’re talking about a 176g difference between the two. With any decision, there are gooing to be Pros and Cons.
Pros for 25mm:
- Light Weight
How it makes you faster:
- By reducing rotational mass, it allows for quicker accelerations due to the lower mass of the rims. This ultimately reduces your energy spent on the race course. Also, lighter wheels “feel” faster, which is a nice psychological benefit.
Pros for 30mm:
- The wider rim allows for better tire support
- The increased tire volume of a wider rim help improve traction/ ride quality
How it makes you faster:
-
In rough terrain, wider rims result in higher tire volume which can help smooth out bumpy surfaces. This can help to improve ride quality and increase traction. When you improve ride quality, less impact is transferred and you can spin more smoothly and waste less energy. When you improve traction, climbing steep slippery hills become easier, and you can carry more speed through corners.
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The additional tire support also helps improve your descending confidence by increasing tire support. This allows you to run lower pressures without tire squirm, which further improves traction and ride quality. This is especially important if you run tires wider than 2.2.
Recommendations:
25mm:
- XC Racer first
- On an XC type bike (~100-120mm of suspension)
- < 2.3in tires
30mm:
- Ride in rough/rocky/technical terrain
- Ride a trail bike ( > 120mm of travel)
- 2.3in tires or larger
- Technical riding is a race limiter
Hope this helps!