Cervelo Aspero: 700C vs 650B Wheel Size Comparison - Part 2

In the previous post about the Cervelo Aspero, I introduced my new gravel tire, the WTB Venture 47 650B Road Plus. Being a wider tire than the previous Panaracer Gravelking SK 650B x 43 mm tire, there are some differences in size and dimensions.

Let's start with the wheel size comparison between the Venture 47 tire and the GravelKing SK tire. A wider tire is usually taller in profile as well, so I would expect the Venture 47 to have an overall tire diameter that is slightly larger than the 43 mm wide GravelKing SK.

Both tires will be mounted on the Hunt 650B Adventure Carbon Disc wheelset, which has an internal rim width of 24 mm. This is pretty much the standard rim width (in first half of year 2020) for gravel wheelsets.

WTB Venture 47 on the left, GravelKing SK 43 on the right. The shade of brown on the sidewall is different.

The Venture 47 which is at the back is slightly larger in diameter.

This is due to the taller tire profile of the Venture 47, which can be seen by comparing the axle height difference as well.

Close up look at the different tire tread profile. Venture 47 on the left has nice big lugs at the side for good cornering traction.

Width difference of 47 mm vs 43 mm is noticeable as well.

In short, the wider Venture 47 tire is also larger in diameter as well. This is actually a good thing, we I found that the 43 mm wide GravelKing SK was a bit undersized in terms of tire diameter, compared to standard 700C tires.

Another comparison to be done is with the standard road setup wheelset, which has Continental GP4000 28 mm tires mounted on the Reynolds Assault Limited Edition carbon wheelset. Apparently a 700C x 28 mm tire should have roughly the same tire diameter as a 650B x 47 mm tire.

I think the calculations may have been something like this, where the rim diameter is added to the tire width/height to get the overall diameter. Not sure how accurate or true this is.

700C x 28 = 622 + 28 + 28 = 678 mm

650B x 47 = 584 + 47 + 47 = 678 mm

584 mm and 622 mm are the ETRTO rim diameter of 650B and 700C rims respectively.

Road wheelset vs new gravel wheelset

Road wheelset shown at the back is still larger than the 650B gravel wheelset.

Tire tread pattern is totally different!

Huge difference in tire width as well. Actual tire width of the road tire on the left is 32 mm, vs 47 mm on the right.

Here are some numerical data of the actual measured tire diameters, with the different tires and different wheelsets.

Continental GP4000 700C x 28 (actual 32 mm): 695 mm diameter
Panaracer GravelKing SK 650B x 43: 677 mm diameter
WTB Venture 47 650B x 47: 681 mm diameter

There is still a 14 mm diameter difference (7 mm radial difference) between the road wheelset and the gravel wheelset (with Venture 47 tires), which is a noticeable amount. I would say that instead of the gravel tires being undersized, it is the GP4000 tires being oversized, as the actual width is 32 mm instead of the rated 28 mm.

I suspect that this older generation of GP4000 tires were designed on narrower 15-17 mm wide road rims, so when installed on the wider 21 mm Reynolds wheelset, the actual tire width balloons out from 28 mm to 32 mm.

As a result, the BB to ground clearance is also affected by the different tire diameters.

BB to ground distance for GP4000 700C x 28 (actual 32 mm): 268 mm
BB to ground distance for GravelKing SK (650B x 43): 259 mm
BB to ground distance for Venture 47 (650B x 47): 261 mm

7 mm less ground clearance for the Venture 47 gravel wheelset, compared to the GP4000 road wheelset.

No wonder it is confusing for consumers to select the correct tire and wheels, as the actual width and actual diameter can vary so much, depending on your combination of tire and rim which is practically infinite.