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Wrong Numbers: How Judging Bikes by Their Geometry Charts Doesn’t Tell the Full Story

Information overload has us missing the point

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The following story first appeared in the winter 2021 print issue of Beta. To get quarterly print magazines, sign up to be a Beta Pass or Outside+ member. Membership details HERE.


We (the mountain bike media) do our best to serve as interpreters between your next bike and you. But that sort of interpretation is subjective. You know what’s not subjective? Numbers. Riders are putting more faith in numbers than they ever have. Not just because numbers are concrete, but because we now understand far more of those numbers than we used to. Fork offset came out of nowhere to grab as much attention as head angles. The seat angle debate is suddenly a hill some of us are ready to die on. Even the almighty reach measurement has only been in our vocabulary for less than a decade.

At the same time, suspension performance has become intricately quantifiable. Most notably, leverage curves are getting a lot of press right now. They’re largely why Santa Cruz went to lower-link VPP, and why Evil’s DELTA link exists at all. There’s also anti-squat curves, anti-rise curves, pedal-kickback charts and axle-path plots.

It’s tempting to think that we can paint a nearly complete picture of every bike with just a series of equations. But of course, it’s not that simple. Not only do all these numbers share complex, dynamic links with one another, some of them are flawed from the start. So, let’s take a step back and discuss what some of these statistics do and don’t tell us.

Not all Seat Angles are Created Equal

Few numbers are more misleading than effective seat tube angles. The industry can’t even agree on how to properly measure them. Depending on your saddle height, frame size, and actual seat tube angle, you are probably significantly farther back than your bike’s geo chart claims. And even if you can calculate your way to the correct number, it’s still an illusion, especially on full-suspension bikes. We sag deeper into our travel when seated than standing, and deeper still when going uphill. For example, a 150mm-travel bike climbing a reasonable 7-percent slope will slacken by around 3 degrees, not including the slope itself which, in this case, is another 5 degrees. The shorter your travel and the flatter the ground, the more accurate the claimed angle will be, but in reality, most of our seat angles are nowhere near the number we’ve been shown.

Head Angles Aren’t Changing in a Vacuum

Even more nebulous is the headtube angle. Travel and topography play a role here as well, but so does wheel size, fork offset, and even reach and wheelbase. In today’s world, when it’s not uncommon to see a 64-degree head angle on a trail bike, 67.9 degrees on a bike like the Evil Following feels like something out of the past. But this is not the past. To keep using the Following as an example, its reduced-offset fork has a similar effect on steering stability as would a half-degree slacker head angle. The fact that it is built around a 29-inch wheel, not 27.5, does about the same. We’re also talking about a bike with a nearly 1,200mm wheelbase in a size large. That’s 50mm longer than the first-generation Following, and 100mm longer than the first-generation Santa Cruz Tallboy. When we look at head angle in a vacuum, we ignore how forgiving the rest of the bike is. And that’s not even considering how much more supportive our suspension has become, meaning we now spend less time with our front end at its lowest and steepest point. Though our suspension itself is often just as misunderstood.

Travel Confusion

Even measuring travel isn’t necessarily all that straightforward. We normally measure along the axle path which, in the rear, is mostly vertical. But up front, it’s measured along the steering axis. If a brand builds a bike around 20mm more front travel than rear, it has the effect of helping equalize the vertical travel between front and rear, so in that sense, a mismatched-travel platform may feel more balanced than a bike with matched travel. But the definition of ‘vertical’ changes once we start going downhill, and that balance is flipped on its head. Suddenly, the front axle-path travel begins to match its vertical travel, and the rear’s starts to fall behind. This is why the high-pivot concept is so disruptive to how we think about suspension. The rearward axle path forces us to imagine travel not vertically, and not necessarily along the axle path, but along the direction of impact. Because our bikes have wheels, an impact from any angle will compress the suspension, but it’s not that simple. The recent transparency around leverage curves helps us understand how a shock compresses in relation to forces along the axle path, but not forces along the direction of impact. But that will change with the size of every bump, so don’t expect to start seeing sepa- rate leverage curves for 3-inch, 6-inch and 9-inch rocks. However smart we feel when looking at leverage curves, they are guides, not gospel.

Nowhere is that more true than when it comes to suspension setup. Something has always seemed a little off about this process. When fork and frame manufacturers publish a suggested sag, we are intended to go through that ritual on flat ground. But again, we probably don’t spend most of our time riding on flat ground. Some of us live for long, arduous climbs. Others couldn’t care less about what happens between the descents. Yet, all of us are given the same baseline. Somewhere around 30-percent sag, and some number of clicks that correspond to the pressure it took to get us there. Most of us understand that it’s not that simple, though. We consider not just where we’ll be riding, but how. And we take into account everything that surrounds our suspension, not just the suspension itself. We should be approaching many aspects of frame geometry the same way. But it can get overwhelming, which is why we get caught up in the numbers, and regularly find them to be only part of the picture. It’s almost as if, to get a true sense of how a bike rides, you have to actually ride it.