Tyre and Wheel Size Versus Geometry

The bike industry has not done a good job of communicating this. The way some brands tell it, if you fit a different size tyre or you don’t flip their fork chip when switching from 650b to 700c then your bike won’t steer, it will handle terribly, you will hate riding it and probably crash into a tree. Instead of trying to explain it better, they simply said “do this”, which allows misunderstanding to creep in. In nearly every case they are aiming to maintain a chosen trail figure and the handling feeling that goes with it.

We gave the Assassin adjustable geometry to achieve the very opposite – to allow you to customise the handling to your preferences – because we think that’s a better way. We also think there’s a better way to explain tyre size and geometry. If you’re interested, please read on.

The point of 650b in gravel bikes is that you run a smaller wheel with a bigger tyre to achieve the same rolling circumference, gaining comfort and traction from the bigger air chamber. In the hypothetical scenario in which that rolling circumference is identical, geometry is unchanged. In other scenarios, when switching between set-ups with unequal rolling circumferences (or outer radius, axle to tyre surface, if you prefer to think of it that way), the geometry will change. Most road riders have experienced this when switching between, say, 23 and 25mm tyres on 700c rims.

Another source of confusion is the varying capacities of bikes to accommodate 700c and 650b wheels and tyres. Some bikes can fit 700×40 with 650×50, other bikes can fit 700×45 and 650×45. It depends on the limiting factor. If chainstay clearance is the limit, then the tyre width is what matters and maybe the maximum is 45mm in both rim sizes. If the stays are wide but bottom bracket and/or seat-tube clearance is the limit then the height of the inflated tyre becomes the issue rather than the width – the 650×50 fits because the stays are wide enough, but a 700×50 won’t fit because it would be much taller and not clear the BB.

To keep it simple, we gave the Assassin clearance for 47mm tyres in both 700c and 650b.

That clears up the different capacities, now what about the geo.

The way most of the industry tells it, you have 700c geometry and 650b geometry, and the two are mutually exclusive, as if your bike simply wouldn’t go around a corner otherwise. The reality, of course, is that a given geometry creates a certain steering feel, that tyre trail is a part of that equation, and that different use cases are best served by different geometry. It’s this last point that particularly defines the Assassin.

It simply is not the case that your 650×47 set-up just won’t work with geometry designed for 700×38, but the terrain on which you are likely to be riding that big tyre will be much rougher than the trails on which a 700×38 set-up would be at home. It’s like wearing running shoes and motorbike leathers. Sure, you can, but in what scenario does that make sense? 

It’s a similar story to the various classes of mountain bike, where geometry varies with suspension travel because travel is the defining parameter for each class. A bike doesn’t need slacker geometry because it has gained 20mm of travel, it needs it because of where and how you will be riding that longer travel bike. 

So, in gravel bikes, we shouldn’t talk about geometry in terms of tyre size as if one dictates the other. Rather, both are steered (but not dictated) by how and where you ride, and the feel that you like from your bike. That last point is crucial because maybe your preference doesn’t follow the convention.

This is why the Assassin has flip chips front and rear which make significant changes to the characteristics of the bike. Go long and slack to make the most of 47mm tyres on rough terrain; go short and tight to enjoy a fast, agile feeling suited to 38s. And, of course, play around with it to find what feels best for you.