There is a trend in the gravel bike industry to push bigger and bigger tires. Both 650b (Open Wi.De. with 2.4-inch tires) and 700c where bikes like new Pivot Vault can take up to 47c tires. The reasoning is very simple: the bigger tires the more comfort you get. Or is it?
To test this assumption I needed to push the limits of my benchmark bike (Jamis Renegade Exploit) which officially takes only 40c tires. Officially but using on everyday basis Panaracer GravelKing SK 43 I knew that my bike can take much bigger tires than 40c. The key to success was the Alexrims that are very narrow (only 17 mm inner width) which do not allow tires to get their true size. GravelKing SK 43 on my rim measures only 41 mm (tubed). Knowing that I asked Panaracer to send me 700x50c variant GravelKing SK and soon after I started testing. The front of the bike was an easy task. The tire inflated to 40 psi measured just below 47 mm and as you can see on the pictures below there is still some space between the tire and the fork. The back of the bike was a different story…
The first attempt was not easy and I needed to let go of all the air from the tire just to put the wheel in place. But when inflated I discovered another problem – the mounting point for fenders was blocking tire from any movement so I needed to file it off entirely (hey, it is all about the science! ?). After this not that easy task, I put the wheel back and inflated it. Success! But the space between tire and frame was VERY narrow so I sincerely do not recommend anyone using this big tire on steel Jamis on an everyday basis.
Having mounted huge tires on my bike I went for a test ride and soon I discovered that it handled differently. To keep the story short I would say that riding on 43c tire feels like you are on top of the front wheel and riding 50c tire makes you feel like you are behind the front wheel. The first setup is more agile and the second is more like running a huge tanker. I never thought that 2 mm of trial change will make that noticeable difference but to be honest I liked my bike with 50c tire more. It was just a little more engaging and the ride – dare to say – more rewarding because you have to work harder to move the bike (but the truth is that I ride a little too small frame size and a bigger bike with bigger tires could produce not so rewarding real tanker sensation). The other difference was with the handling when changing direction fast – due to the very narrow rim the tire, especially at 20 psi, sometimes behaved like it wanted to hold the wheel and then at the last moment it was somehow catapulting it. I believe that a wider rim would not produce that sensation.
When I adjusted to the handling of the bike I started measuring the vibration (using shockstop stem in the front and Ergon CF3 Pro seatpost in the back of the bike). First at 30 psi and then lowering it to 20 psi. Riding at 30 psi was (to my big surprise) not that comfortable. Even at 20 psi, I did not feel any magic carpet ride that I was expecting. And to be honest a quick scientific look explains this sensation – the bigger tire the more air it holds and the more air it needs to achieve the desired pressure. So when you riding on the bigger tire with the same pressure it has more air and thus is not so eager to flex and absorb bumps (the stiffness of a bigger tire with the same air pressure is simply bigger). You have to use lower pressure and this is the main advantage of using a bigger tire (you don’t need to worry about bottoming out and damaging the rim riding with 20 psi on 50c tire whereas on 43c tire it becomes really dangerous).
So what the numbers say? In my case at 30 psi, bigger tires are less comfortable in almost every situation (but in terms of the front of the bike the difference was so minor that we can simply say it is not relevant). When riding in the forest the bigger tire offered a very slight improvement in the rear (2,4%) but on the fast gravel route, it was less comfortable (-9,2%) than a smaller one.
The situation was even more interesting when riding at 20 psi. Less air means more flex and the bigger tire the bigger flex in can provide which also improves the performance of shockstop stem so no surprise that front of the bike was more comfortable when riding on a 50mm tire. But in the back happened something really wired. At 20 psi bigger 50mm tire created more vibration than 43mm tire both in the forest (2,3% more) and on a fast gravel route (16% more). Maybe a bigger tire with more air becomes simply more bouncy and flexing seatpost can’t compensate that (like shockstop stem does in the front).
To be honest I did not expect that kind of results. I thought that the bigger tire will be always more comfortable but the numbers create definitely a more complex story. For now, I can conclude that running a bigger tire can be beneficial in terms of the front of the bike but only when running low (like 20 psi low) air pressure and in the back, you can leave a 43mm tire (which will also be beneficial in terms of rolling resistance).
Of course, this all is valid for my case and my bike with a narrow rim and inner tubes with max tire width 47c (which can explain why inflated tires measured just below 47mm). Maybe with bigger inner tubes or running tubeless will produce different results and I will test the latter in the near future because I plan to experiment with a tubeless setup.
UPDATE:
Since my previous test, I managed to rerun them with a tubeless setup. Below you will find the results (first at 30 psi and then at 20 psi). In general, the trend remained the same: going with the bigger tire is beneficial in terms of front of the bike (but only at 20 psi) but the rear is much more complex story wherein the forest you gain little but on fast gravel routes you lose some of the comfort that the smaller tire provides. This is indeed a very interesting result but we have to remember that they apply to my testing setup and may be different tire (like Rene Herse supple tires) may produce different results. So in the future, I will try to get them and rerun those tests once again.
