Adventure Bike Wheel Sizes- 650b or 700c?

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.

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  1. Pingback: Comparing the fit and trail riding experience on the gravel and fat bikes | Beardy Star Stuff

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