When a builder or engineer begins designing a bike, besides bottom bracket drop, one of the first dimensions of a bike they will set will be head tube angle. I’ve spoken with engineers who started there, even before BB drop. It’s a crucial dimension of a bike’s handling. That said, what many cyclists don’t know is how head tube angle does more than just inform how a bike will handle.
What it is
Head tube angle (HTA) is, unlike BB drop, really easy to measure and define. What isn’t so easy is to explain just how it affects handling. Because HTA is just that—an angle—it is measured in degrees. It is an acute angle, that is, less than 90 degrees. It is measured from a horizontal line that corresponds to flat ground and that line points toward the rider, rather than away.
If you look at photos and drawings of bikes through history, you can see that bicycle designers have played with HTA over an incredibly broad range with designs that have ranged from 90 degrees down to even less than 45 degrees. What you see on bikes today is the result of an entirely organic process based on experimentation. As a broad generalization we can say that for road bikes the average HTA is 73 degrees; for gravel bikes, that angle is more like 72 and for mountain bikes it’s more like 67. BMX bikes have HTAs that are steeper than road bikes and many cruisers are similar to mountain bikes, or even slacker—that is, a smaller number.
What it does
HTA determines (in conjunction with fork rake, which we’ll get to in a coming post) just how quickly the front wheel turns. The steeper, that is less acute, the angle, the faster the handling; the more acute the angle (a smaller number) the more relaxed the bike is in its handling. Put another way, the steeper the HTA the more the front wheel will turn in relation to the ground when the bar is turned.
What it doesn’t do
While HTA is usually the first word on steering geometry, it isn’t the final word on it. It’s only half the word at best. This is important to understand when you read a bike review. You’ll see many reviewers cite the bike’s HTA and then pronounce the bike quick handling or calm or sluggish or whatever. Um, yeah, maybe. It’s like driving in LA—you can’t calculate driving time to a destination on mileage alone. You absolutely need to know what time of day you’re making the trip because that will determine how much traffic you are likely to encounter.
Here’s an example of how this plays out: Reviewer Joe Bob Radman says the new Yoyo Cycles Prentiss has a 74-degree HTA. He pronounces the handling quick. In fact, what really defines how quick a bike handles is the interplay of HTA and fork rake in what’s known as trail. Trail will have to wait until after we get to fork rake because calculating it employs some reasonably fancy math.
Let’s pretend the Prentiss’ 74-degree HTA is paired with a fork with 20mm of rake; that bike would handle on the slow side for a road bike. It would have as much interest in turning as my boys have in bedtime—little to none.
Now, let’s swap out our make-believe 30mm fork for one with 60mm. That bike would indeed be quick handling, so quick handling that riding in a group would be an utterly nerve-wracking. A bike like that would do well to dispense Valium at the end of each ride.
So unless Joe Bob tells you the bike’s rake as well as the HTA, he hasn’t really told you bupkus about its steering geometry.
This would be a, Wait, we’re not finished yet! moment. HTA changes over a bicycle’s size run with small bikes have a shallower (smaller number) HTA and the HTA growing steeper (increasing number) as bikes increase in size. Bike designers do this for two reasons.
Reason one is that smaller bikes have an inherently shorter wheelbase because of their short top tube. To offset some of the quicker handling that comes with a shorter wheelbase builders will slacken (smaller) the HTA to calm the bike’s handling, making it less reactive to rider input. Similarly, big bikes will have a steeper HTA to help offset the effect of its longer wheelbase. This would be why engineers working for production brands begin with the 56cm size (road and gravel) and the medium (mountain) as the first size they design. It’s the middle of the range and is the ideal of how the bike should handle. That design is adapted into other sizes.
Reason two is that a slacker HTA gives the builder an extra tool for making the top tube shorter for the smaller rider and it also moves the front wheel out in front of the rider, decreasing the threat of toe overlap. In the bigger sizes a steeper HTA helps lengthen the top tube. It’s a pretty neat parlor trick.
In the rough
Mountain bikes have a notably slacker head tube angle because designers found that as they built bikes around increasingly slack HTAs (early production mountain bikes had HTAs not much slacker than road bikes), the bikes became more immune to the effect of hitting rocks and bumps when riding at speed. Nail a rock and the bar no longer gets twisted out of your hands sending you to the ground seemingly faster than gravity should allow. Today’s mountain bikes hit rocks and respond like someone punching the Terminator—with mild bemusement.
The slacker head tube angle plays an interesting role in gravel bikes. Because of the rough surfaces gravel bikes are meant to roll over, slowing the handling down some makes sense—again, less reactive to rocks and bumps. That slacker angle adds a second useful feature. The bigger tires of a gravel bike increase the likelihood of toe overlap, not just for smaller riders, but in all but the largest sizes, and sometimes even then. By building around a slacker HTA, that kicks the front wheel forward, increasing the clearance, though it’s still true that I encounter toe overlap on any bike in my size running a 40mm tire.
Engineers designing road bikes have one other constraint to worry about as they plot the geometry of a new road bike. Even if the bike comes in six sizes, accommodating everyone from someone 5-feet tall to a 6-foot-4 giant, all of those bikes need to possess a similar character when on the road. That’s because in a group ride everyone present needs to be able to take a turn at the same radius as the rider they are following. If they couldn’t, all the small riders would take turns sharper than the big riders and that would spray chaos all over everything and everyone. Think about the kind of turn a Mini Cooper can take and how much larger a turn radius a school bus has. For that reason, every size of a bike needs to enjoy the same sort of character in handling as the 56. They won’t handle exactly the same, but in the hands of an experienced rider they can all carve a turn when ensconced in the peloton.
So that’s the skinny on Head Tube Angle. We’re working our way around the bike as methodically as possible. You’ll see that the deeper we go, the more the various elements come together in dynamic interplay. Next, I’ll tackle fork rake, which will get us ready to talk about the single most interesting dimension of a bike’s geometry: trail.
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