However, when I first used a gravel setup on the Canyon Endurace, I found that depending on the tire width, the tire diameter will also be affected significantly, especially if the tire width differs a lot. This can be a problem, as a bike is designed around a nominal wheel size, with only a small variance allowed. If the wheel size differs too much, it affects the geometry of the bike.
Crank clearance with the ground will be reduced if the tire diameter is smaller. Also, the trail of the bike will be reduced with a smaller wheel size, which can cause instability.
If the tire diameter is larger, the usual problem is frame or fork clearance, but the bigger issue is a slower handling bike due to larger trail from the larger tire size. This may be good or bad depending on your desired purpose.
In this case, it is difficult for the bike frame designer to optimize for a single wheel size, if it needs to accommodate a wide range of tire diameters, from small 700C x 23 to large 700C x 43 mm, for example.
Which is why the Cervelo Aspero has a Trail Mixer on the fork which allows the trail value of the bike to be adjusted, depending on the wheel size and also your preference for a more agile or more stable bike.
I will not go through the explanation again on why I chose 650B wheels and tires, you can read about it in my previous posts.
DT Swiss G 1800 Spline DB 25 Wheelset
Panaracer GravelKing SK 700C x 32 Tires
Hunt 650B Adventure Carbon Disc Wheelset
Panaracer GravelKing SK 650B x 43 Tires
The Reynolds Assault wheelset is equipped with Continental GP4000 700C x 28 tires, but the actual tire width is almost 32 mm. This is due to the relatively wide internal rim width of 21 mm.
The DT Swiss G 1800 wheelset is equipped with Panaracer GravelKing SK 700C x 32 tires, with an actual tire width of 35 mm. This rim has an internal width of 24 mm.
Rim diameter looks almost similar, with the gravel wheelset being just a little big larger.
Close up look at the different tire tread patterns.
Tire diameter is very similar.
In this case, if the tire diameter is almost the same, swapping between the wheelsets will not cause significant differences in ride geometry. This was the case on the Canyon Endurace.
Moving on to the new Hunt 650B wheelset, the rim diameter is smaller, but the tire will add back some of the diameter by having a taller sidewall. A 700C rim has an outer diameter of 622 mm, while a 650B rim has an outer diameter of 584 mm. This is a difference of 38 mm, which means 19 mm on either side. If a 650B tire is to have the same outer diameter as a 700C tire, the side wall needs to be 19 mm taller, which is a huge difference.
650B x 43 mm tire on the left, 700C x 32C tire (actual 35 mm) on the right. The diameter difference is quite obvious.
The 650B wheelset on the right has a smaller diameter, as seen from the difference in axle height. This will also be the difference in bottom bracket or crank arm clearance.
Another view showing the difference in tire diameter.
Using a long thru axle to align the centre of the hubs, the difference in radius can be measured more accurately.
650B tire has a gap with the ground. Multiply this gap by 2 and you get the diameter difference.
Using a 10 mm Allen key as a gauge, there is still a tiny bit of clearance. I would say that the gap is about 11 mm.
Other than the diameter difference, there is a big tire width difference as well. The wider 650B tire on the right has 5 rows of small knobs instead of 3.
43 mm width on the left, versus 35 mm (actual) width on the right. This 8 mm width difference is quite obvious.
There is some difference in sidewall height, as the 650B tire on the left will have a taller side wall as it is wider. The height of the brown sidewall area is about the same, but the black sidewall area is much taller on the 650B tire.
Finally, let's also compare the 700C road wheelset with the new 650B gravel wheelset. These are the two wheelsets that I will use on the Cervelo Aspero. I will explain more about the road and gravel setup in a later post.
Hunt 650B wheelset on the left, for gravel riding. Reynolds Assault 700C wheelset on the right for road riding.
Significant difference in tire diameter between these two wheelsets.
Another view of the different tire diameter. Despite the wider tire width, the 650B tire does not add enough tire height to come close to the 700C tire diameter.
Difference in axle height, which is about 8 mm.
Big difference in tire width and tread pattern. 32 mm tire width vs 43 mm tire width.
Comparison of the tire sidewall height. The 650B tire is taller with much more volume, but not enough to make up for the smaller rim diameter.
In summary, here are the tire diameters as measured, and also the difference in BB to ground distance (which is calculated as half the difference in tire diameter).
Continental GP4000 700C x 28 (actual 32): 693 mm diameter
Panaracer GravelKing SK 700C x 32 (actual 35): 699 mm diameter
Panaracer GravelKing SK 650B x 43: 677 mm diameter
On Cervelo Aspero (Size 51)
BB to ground distance for Continental GP4000 (700C x 28): 268 mm
BB to ground distance for GravelKing SK (700C x 32): 271 mm
BB to ground distance for GravelKing SK (650B x 43): 260 mm
The differences between the 700C road and 700C gravel tires are not so big, while there is a big difference between the 700C road and 650B gravel tire. What does this mean?
Swapping from the 700C road wheelset to the 650B gravel wheelset will lower the bike by about 8 mm. That is 8 mm less crank arm clearance. Not sure if pedal strike will be an issue, as it has even lesser ground clearance compared to my other bikes.
On the plus side, a lower centre of gravity will help in cornering and descending at speed. A smaller tire diameter also means a lower trail value, which increases agility. Is increased agility good or bad for a gravel bike? This is up for debate, as increased agility can allow you to tackle twisty trails more easily, but also make a speedy descent less stable.
Regardless, the proof of the ride quality will require actual riding. Whether good or not, I will have to use the tires to see if it is suitable for my usage.
From left to right: GP4000 28 mm, GravelKing 43 mm, GravelKing 32 mm
The 650B wheelset is the junior here, having a smaller diameter than the other two.
This concludes the first part of this comparison, which is between these three tires. Later on, in Part 2, there will be an additional tire added to this comparison, stay tuned!