For gravel riding, 650b wheels are becoming more popular. 650b wheels have a smaller diameter than traditional 700c road wheels, if the tire width is the same. The ETRTO rim diameter is also different.
700c wheels have a ETRTO rim diameter of 622 mm, which is actually the same as 29" MTB wheels. 650b wheels have a ETRTO rim diameter of 584 mm, which is the same as 27.5" MTB wheels. However, the point of smaller 650b wheels is not to have an overall smaller wheel size.
Rather, the idea of using smaller 650b rims is to allow a wider tire to be installed, such that the final tire circumference is similar to that of a 700c tire. This is due to the natural fact that when the tire width increases, the tire wall also becomes taller, resulting in a larger tire diameter.
Generally speaking, the tire diameter of a 700x28c tire is similar to that of a 650x47b tire. This is just a very rough estimate, as the exact size depends a lot on the tire manufacturer, width of rim used, and tire pressure.
Wheel size comparison. Picture from Salsa Cycles.
The final objective of this 650b wheel size is to allow a much wider tire to be used, without increasing the tire diameter too much. If you merely increased the tire width for a 700c tire, such as from 28 mm (typical road bike) to 43 mm, the diameter will also increase substantially, making the tire diameter really big and not compatible with many bike frames.
However, if you step down the rim size to 650b, you can install a much wider 650b tire (47 mm for example), and still maintain a similar tire outer diameter as a 700x28c tire. This makes it a bit easier to design the frame, as it is usually possible to widen the chain stays or fork for a wider tire clearance, than for a larger tire diameter, where the only option is to increase the chain stay length.
The Cervelo Aspero will match nicely with 650b or 700c wheels, as it has good tire clearance. Plus, it also has the Trail Mixer feature, which will allow you to adjust the trail of the bike to ensure good riding characteristics regardless of wheel size and diameter.
One downside of using wide 650b tires is that they are heavy, compared to 700c tires. Although 650b tires are smaller in diameter, they gain weight from the much larger width, plus the addition of knobs for grip on off-road surfaces. For example, a Continental GP4000 700x28c tire is about 260 grams, while the Panaracer GravelKing SK 650x43b tires weigh 470 grams, a massive 210 gram increase.
Not only does it make the overall bike heavier, the weight is added at the worst place possible, which is at the outermost part of the rotating wheels, which will increase the rotational inertia greatly. This will make acceleration more sluggish as you need to overcome the inertia when starting off.
Check out this detailed comparison between 650b and 700c wheels!
In order to cancel out some of the additional tire weight, I decided to get a lightweight set of 650b wheels, so as to minimize the weight increase. After much comparison, I decided that the Hunt 650b Adventure Carbon Disc wheelset is a good balance of weight, cost and quality.
You can get the wheelset directly from the Hunt website, with free shipping. Or, you can get it directly from the Singapore local dealer, which is OneBike Asia, as they work with Hunt to bring in the wheelsets. The prices are the same, either from the shop or online. In this case it is a better idea to get from the shop, as they can offer after sales service and also other advice.
Hunt 650b Adventure Carbon Disc wheelset. Plain graphics suit me just fine.
Spare spokes and spoke keys are included. So are the tubeless valves and Centerlock to 6 bolt rotor adapters.
External rim width is 30 mm. Tubeless rim tape is already pre-installed.
Internal rim width is 24 mm, just as advertised.
Rim height is just 22 mm, which helps to keep the wheelset weight low.
Carbon rim surface is smooth and looks good.
Hookless rims means that it is best suited for low pressure tires.
Maximum tire pressure allowed for different tire widths, to prevent the tires from blowing off the hookless rims.
Even then, these are the maximum tire pressures allowed. Actual tire pressure used will and should be lower.
Simple Hunt logo. Bladed spokes are used on this wheelset.
Standard J-bend spokes, with Centerlock disc rotor interface.
Hub axle end caps can be removed easily for maintenance. They can also be swapped to quick release axle types, but the adapters are not included.
Road 11 speed freehub body, with a ceramic coating on the freehub body to prevent damage by the cassette.
This freehub has an engagement angle of just 5 degrees, or 72 engagement per round. My guess is that there are 36 ratchet teeth in this hub, with 2 sets of 3 pawls. With the 2 sets of pawls offset by 5 degrees, it doubles the engagement from 36 (if all 6 pawls engage at the same time) to 72 (3 pawls engage at any one time).
Front wheel weighs 657 grams, which I think is really lightweight!
The rear wheel weighs 780 grams, which is also considered lightweight.
This gives a total wheelset weight of just 1437 grams, which is really lightweight! It is quite close to the claimed weight of 1425 gram. Hopefully this lightweight wheelset will help to offset some of the additional tire weight.
Curiously enough, the wheelset also comes with Centerlock rotor lock rings. Normally lock rings are included with the disc rotor, not the wheelset. Is there a reason for including these lock rings?
I noticed that these lock rings are of the external serration type, which means that they need to be assembled with a Hollowtech II bottom bracket tool. This is different from the more common internal serration type, which uses a cassette lock ring tool. Either way, there is no need for a special tool as it uses existing tools.
External serration lock rings are usually used on hubs with a larger axle diameter, such as the 15 mm thru axle type used on MTB front wheels. In those cases, the hub axle is too big for the cassette lock ring tool to be used. The lock ring thread on the hub is the same, regardless of internal or external type of lock ring.
Two external serration type lock rings are included with the wheelset.
For some strange reason, these two lock rings are different. The one on the left is a thinner steel lock ring, while the one of the right is a thicker but lighter aluminium lock ring.
As you can see, the thickness of the serration area is different between the aluminium and steel lock rings.
Just for fun, I found that the steel lock ring is 21 grams.
The aluminium lock ring is 11 grams, which is a lot lighter.
Both are aluminium lock rings, but the internal serration type on the left is just 8 grams, still lighter than the external serration type.
My initial thought was that the external serration lock rings are provided because the Hunt wheelset hubs have a larger diameter axle, which will prevent the internal serration type from being used. However, upon testing, I found that an internal serration type of lock ring works just fine.
Internal serration type lock ring used on the Hunt hubs.
No problem with the insertion of the cassette lock ring tool to tighten the internal serration lock ring.
Therefore, I could not understand the reason for Hunt to provide the lock rings, as they are not necessary at all.
However, after taking a look at the instructions for the Centerlock to 6 bolt disc rotor adapter, I realized the reason. The external serration lock ring is needed if the adapter is used.
Also, upon further research, I think the flatter steel lock ring is needed for the front hub, to prevent interference between the lock ring and the fork leg.
Hunt wheels instructions state that if the Centerlock to 6 bolt adapter is used, the Hunt-provided lock ring should be used.
Anyway, I will just be using the internal serration lock ring that comes with the Centerlock disc rotor.
With the new 650b wheelset ready, it is time to check out the new 650b tires as well! That will be a story for the next post.