First, a bit of backstory… I’ve had issues with my left knee since a junior high injury over 3 decades ago. It plays into my bike riding experience and is the reason I stopped riding twenty years ago. My hope is that by being careful my newfound ability ride bikes again will continue for many years but with that hope comes a caution regarding how I ride.

Fat Bike – look at those big, cushy tires!

I’ve been riding the Poseidon daily since the last week of July and have put on 2,700 miles of road and gravel riding. This past couple of weeks I decided to stay closer to home and return to some trail riding. Naturally I used this as an excuse to pull the fat bike out of the shed for a ride and what a blast! It’s such a soft, cushy ride! But after three or four days I started to get an occasional tension in my left knee. I’d felt this over the summer too when I rode but was careful and adjusted my seat, riding position and a few other things which reduced the problem somewhat. At least enough to keep riding. Then I started riding the Poseidon X in late July and the problem went away. I didn’t even think about it really. Then I started riding the fat bike and the discomfort returned. Hmmm.

Gravel Bike – can narrow tires handle the trails?

My first thought was to try to figure out if it was the fat bike or the nature of riding trails. So, to test I decided to try riding the trails with the Poseidon. It wasn’t too bad. Not as cushy as the fat bike which also meant taking it a bit slower. I figured if I could manage a few days I might see if the discomfort persisted. If it did I would chalk up the different kind of riding required by the trail to be the problem. Lots of standing then sitting and standing and sitting. And more torque and strain on the many little hills, twists and turns. It’s an over-all slower speed but I think more pressure on the knees. If the problem decreased or disappeared I figured the problem would point to some aspect of riding the fat bike specifically. After three days the discomfort had decreased and nearly disappeared. After an additional two days of riding there was no discomfort at all.

So it seems the problem may be the fat bike and I think I figured out that it is the width of the bike to accommodate the fat tires. Wider hubs, chain stays and, most importantly, a wider bottom bracket which means that the pedals stick out further too. The final result is that my feet are also out to each side a bit further and at somewhat of an angle. In the bike measuring world this is often called Q factor or “stance width” which is the better, more descriptive term. I noticed the difference when I got back on the fat bike after 2,700 miles on the narrower gravel bike. I actually felt it in my legs that my feet were farther apart but it didn’t occur to me that it would be a problem until it was a problem. Bummer because I do love riding that fat bike on these trails.

Okay then, to move onto the question, can a gravel bike, specifically the Poseidon X, be a trail bike? After riding the X on the trails for the past two weeks I’m feeling increasingly confident and comfortable using it as a trail bike. In fact, I have to say that I’m enjoying it more with each ride. Comparing it to the fat bike, it feels more agile and gets going more quickly. It feels faster. The fat bike with it’s heavier wheels requires more effort at the start and overall feels more comfortable but also less lively. In the first few days I rode the X with more caution and a bit slower. With less tire cushion and volume comes an overall bumpier feel especially at higher speed. I’d say that I feel a bit more focused when riding the X though as the days go on I’m increasingly more comfortable riding the trails at higher speeds.

I’m still experimenting with tire pressure, currently keeping it at 30 psi and will discuss further in the upcoming review of the Hutchinson Touareg tires and recently discussed the switch from 700c to 650b wheels. But for now I’ll conclude that trail riding (at least my trails) are very doable on the X with this wheel set and these tires.

Aside from tire volume and width of the tires, another notable difference is that the fat bike has a more upright position with higher handlebars which I prefer for comfort and feel. I was already thinking of changing to a shorter, steeper angled stem on the gravel bike to bring the bars closer and raise them up a bit. So, that’s next on the list.

The Poseidon X comes with 700c wheels with tires that have a width of 35mm. Those tires, Kenda Small Blocks, are pretty knobby so I never even rode them because I have a lot of pavement to get to my gravel rides. I put on WTB Nanos which have a pavement friendly center tread and are 40mm wide. After 1600 miles the tread on the Nanos had worn down quite a bit so I went ahead and ordered a 650b wheel set and some new Hutchinson Touareg 48mm tires that were on sale. I thought I’d offer a brief review of the Nanos and go over the difference in wheel/tire sizes for anyone interested in this kind of cycling.

First, the Nanos. I’ve been running them at about 45psi. They’ve still got some tread after 1600 miles. I could probably ride them another 500 miles so they’ll stay on the wheels and be my back-ups. I don’t expect to put the 700 wheels again anytime soon but the Nanos will be there if I need them. They’re great tires. No flats, fairly low rolling resistance on the pavement, and decent traction on the terrain I’ve been riding. I’d buy them again if I were sticking to that wheel size.

So, what’s the deal with gravel bike wheel sizes? Coming from a road bike heritage gravel bikes have historically been based on road bike wheel and tire sizes but just a bit wider for off road use. So, instead of 25mm wide (and slick), gravel tires are wider, usually 35mm or wider and with much more tread. 700c wheels are also referred to as 29” and are the standard size for road bikes. And worth noting, they are run at very high pressure giving ride that is, in theory, faster and which feels fast given the tiny amount of contact with the pavement. With a road bike you feel the details of the pavement. The idea is that by minimizing the amount of rubber in contact with the pavement there is less rolling resistance to slow you down.

Enter gravel bikes. With gravel roads, dirt and surfaces other than pavement, suddenly more traction is needed and so, wider tires and grippier tread patterns including knobs were created for the 700c wheel size. But that’s just the beginning because riding on surfaces other than pavement means roughness, vibration, discomfort and less stability. A wider tire helps but even better is more tire volume and even more width. 650b wheels are 27.5” and also offer more width for even wider tires so more volume which means even more contact with the road.

Then there’s the recent change in thinking about tire pressure generally which is that less air pressure (even on road bikes) seems to generally allow for not only more comfort but the same speed. A lower pressure tire absorbs road vibration that would otherwise be energy transferred to the whole bike which, in theory, means a loss of energy and speed. Tests seem to back this up.

I’ve had the new 650b wheels and tires on for 1,000 miles now and without a doubt they offer a softer, more comfortable ride. It’s not a huge difference but it’s better. My ride times and average speed have stayed the same. Initially I felt as though I was going slower because it’s a cushier ride but my speed and calories burned are the same so my conclusion is that it’s just a change in how it feels. I’ve been riding the new tires with tubes and kept the pressure at around 40 to 42 psi to avoid pinch flats. Until last week.

Running lower pressure with tubes? In recent years many have switched to tubeless tires. Basically, a specially designed tubeless tire tightly seats into a specially designed tubeless rim and a sealant is added added. This set-up allows for running lower pressure and a more comfortable ride thanks to a more cushy tire. With tubes a certain minimum pressure is required to limit pinch flats which is caused when a large rock, hole or other obstacle causes the wheel and tire to compress together and pinch the tube causing punctures.

I’m not quite ready to go tubeless but since I’m riding closer to home now (always within 3 miles on my current gravel, pavement trail route) I figured I’d experiment with lower pressure. I’ve lowered the psi from 42 down to 30 and without a doubt it’s a huge improvement on the gravel and better on the trail too. I’ve only ridden 4 days at this pressure, no pinch flats yet (I did actually have a flat but it was due to a large thorn and not something caused by the lower pressure). My plan is to continue at about 30 psi for the next week. Then drop to 28 then 26. I’ll spend a week at each to test the comfort, perceived rolling resistance on the pavement and wait to see if I get any pinch flats.

About that thorn… another benefit of tubeless tires is that small punctures caused by thorns or similar objects are generally supposed to seal up on their own. Once the object is removed the sealant seals the hole (assuming it’s not too large a hole). Certainly worth thinking about. But there are also downsides (tubeless can be messy, costly and sometimes difficult to set-up and repair on the road). I’ve not ruled it out but for now will stick with tubes.