650b wheels and tires

In part one of my 650b vs 700c shootout I concluded that 700c is a better choice for someone looking for a comfy ride. But then I was doing all the tests on my benchmark bike fully equipped with comfort improving parts like Lauf Grit SL for and Redshift suspension stem. This time I did all the testing on a rigid bike without any comfort improving parts. And this time the results were quite different…

650b vs 700c is a big topic on a gravel bike scene, especially because nowadays almost every gravel bike gives you that option. And it is you that should decide which wheel standard you will use. 700c is a bigger wheel that feels faster on-road and rolls slightly better on bigger obstacles (due to the bigger wheel radius). But most of the current gravel bikes can only take 45 mm 700c tire so if you are really after the biggest possible tire, then the only option is to use 650b tires and go for 50mm of an even more. The smaller wheel makes your bike feel much more nimble (yet sometimes it can become too nimble so you have to check the trial of your bike first and compare it to 650b wheel/tire combination: you can use online calculators like this to calculate your bike trial). If your trial is already quite low (like 60 mm low) then going for 650b wheels and something like 50mm tire will result in twitchy and nervous handling.

Then there is also a matter of a Bottom Bracket drop. Usually, gravel bikes have BB drop around 70 mm but there are also bikes like Specialized Diverge with 85 mm BB drop. From my experience drop bigger than 70 mm will result in frequent pedal strikes because changing the wheels from 700c to 650b will lower you BB height quite significantly. But if your trial will be OK after the 650b switch, and your bike BB drop is quite modest, should you go for 650b wheels?

To find out this time I tested and measured only front end vibrations and compared the results between 700×42 tire, 700×50 tire and 650b x 48 tire. In all of those cases the tire was Panaracer GravelKing SK tire (unfortunately I did not manage to get much more supple Soma Cazadero 650×50 mm tire in time for testing, but for making this comparison interesting, I will also show the results I got from 700x42c and 700x50c Cazadero tire). Each and every time I run the tire tubeless just to be sure that the inner tube is not influencing the results in any way. Of course, we have to keep in mind that the wheel itself can somehow influence the results and I was comparing tires on a different wheels (AlexRims ATD470 for 700c and DT Swiss m1900), but according to this Cervelo chart, the influence of front wheel is rather small when compliance is considered. So for the sake of this comparison, let’s not take the wheel impact into consideration.

To get the most comparable results I used different air tire pressure in each size of the tire. Why? There is a Laplace’s law which says that the tire casing tension = internal pressure x tire’s radius. What this means is that to get the same tire tension you have to lower the tire air pressure when you go for a wider tire. The explanation is very simple – the bigger the tire, the more air will be in it at the same air tire pressure and thus the casing tension will be bigger. You can verify this experimentally. Just get two tires with different width and inflate them to something like 30 psi air pressure. Then try to squeeze the tires. You will quickly find out that the wider the tire the more force you have to use to make the same dent. The Laplace’s law makes the comparison between different sized tires run at the same air pressure simply pointless.

So what is the correct air pressure? To calculate that you first need to establish the starting point. Which means choosing the narrowest tire first and experiment with air pressure to find the best one – which is the one that offers the most amount of comfort without losing the steerability and cornering ability (if the tire air pressure will be too low, the tire casing will move on the rim causing a lot of unwanted traction problems). After a lot of experimenting, I found out that 25 psi is the lowest possible air tire pressure for 700x42c tire. Going lower simply affects the handling too much in my opinion. So I had the starting point. To calculate the proper air pressure for 700×50 tire I had to make a simple calculation: 42/50 (narrower tire width divided by a wider tire width) * 25 (air pressure of the narrower tire). This resulted in 21 PSI. To calculate the 650b 48mm tire the air pressure I needed first take into account the different wheel radius. I used this diagram. It shows that 650b tire with a 48 mm width will have roughly the same amount of air as a 700c 47 mm tire.  So (42/47)*25 resulted in 22 psi air pressure for 650b 48 mm tire. Now, I had everything set up for the final 700c vs 650b shootout!

And what are the results?

650b wheels are comfier when compared to 700c and using the same tire (13% more comfy on bumpy forest route and 13,5% more on a fast gravel route). But this is only true when comparing 48 mm 650b tire to 42 mm 700c tire. When you put into the comparison 700×50 mm tire and 650b x 48 mm tire then the bigger wheel is 3,7% more comfy on forest route and 2,6% on a fast gravel road. As you can see the difference is not that big and are probably the result of a bigger wheel rolling slightly better, especially on bigger obstacles (which I already mentioned).

Like I said, it is a pity that I did not get the 650b Soma Cazadero in time for this test (I will make a separate comparison between 700×42, 700×50 and 650×50 mm Cazadero in part 3 of this 650b vs 700c shootout), but my test shows that as much as tire width, the tire suppleness is also very important in terms of comfort. Just see the results: 700×42 mm Soma Cazadero was even slightly better than 650×48 mm Panaracer GravelKing on a bumpy forest route. Yet on a fast gravel road (where are different, higher frequency vibrations) 650b wheel and tire combination were better (they were reducing the vibrations 8% more effectively than 700×42 Soma Cazadero).  On the other hand, Soma Cazadero 700x50c was the comfiest one no matter the situation. So, if your bike can handle that big tire, maybe you should not look for 650b wheels at all…

Am I switching to 650b this time?

It is a really tough question to answer. And I am trying to decide this for some time now. On one hand, there is certainly a tangible improvement in comfort when switching from 700×42 to 650×48 tire. But on the other hand, especially with my current bike, a 650b setup results in a quite nervous handling and a frequent pedal strikes. Not an ideal situation. But if I am going to change my benchmark bike, will I go with 650b setup? I might, especially because I do not want to go as big as 700x50c for the best comfort. The only thing that would still bother me is the odd feeling of being on a smaller bike and, at least subjectively, being slightly slower on a road (which is a combination of a wider tire and a smaller wheel). The smaller wheel also rolls slightly less effectively on bigger bumps resulting in more bike movement but you can also describe this as a more agile and fun ride. So, it depends on your opinion. And that, I think is the best summary of my comparison. It all boils down to the question: do you like your bike with 650b wheels or not? Comfort-wise for sure, you will gain much. But maybe, the cost of the improved comfort will be too big for you and you will settle with 700c wheels and 42 mm supple tires. After all of my testing, this combination still looks like the best compromise between traction and comfort on the one hand and speed both on-road and off-road on the other hand. And, at least for now, I think I will still use it as my main choice when it comes to my gravel rides…