It’s an interesting question for a few reasons. One reason it’s interesting is that unlike other changes in mountain bikes, like the switch from 26 to 29-in. wheels or the adoption of dropper posts, both of which happened over just a few years, this is an evolution that’s been going on since the 1990s, and in a way, since the 1970s, but I don’t want to get ahead of myself.
So the early production mountain bikes had head tube angles that ranged between 71 and 73 degrees. That really didn’t change for a long time. I spent some time going back and looking through my archives a bit, I could seriously have rabbit-holed this question, and what I found was that as recently as 20 years ago, some of the most popular cross country bikes on the market still had 71-degree head tube angles. Downhill bikes were just beginning to reach 67 degrees.
In the ensuing 20 years, they’ve gotten slacker and slacker. Consider that the Specialized Chisel, which is a hard tail cross country bike, has a head tube angle of 68 degrees, almost as slack as what a downhill bike was in 2004.
So why has this happened? Well, the simple answer, of course, is that slacker head tube angles make mountain bikes handle better. Okay, but that’s subjective. What I mean by that in a more objective sense is that it made the front wheel less reactive to bumps, rocks and other objects.
As head tube angles have gotten slacker, in order to make sure the bikes still turn, fork rake has increased. So we can visualize why a combination of a slack head tube angle and a fork with lots of rake would make a bike happy to go straight on bumpy terrain.
As head tube angle gets slacker and fork rake increases, an interesting thing happens. When you turn the bar, the way the fork rotates causes the head tube to rise slightly. This effect increases the slacker the head tube angle gets and as fork rake rises. Both features make it increase, too. The practical importance of this is that the more weight there is on the front of the bike, the more the front wheel wants to point straight. But it doesn’t take much strength to turn the bar to make the bike turn.
What I find so remarkable is that a single degree change in head tube angle is noticeable. As I mentioned recently about the difference between my Ripley AF and carbon fiber Ripley, the aluminum version has a head tube angle one degree slacker than the carbon one, 65.5 to 66.5, and there’s no mistaking that the Ripley AF is just a little calmer at speed.
Which brings us to how far will this go? Well 20 years ago I would have told you it would stop at 67 degrees, which is what the head tube angle was on the old Schwinn Excelsiors that Joe Breeze, Gary Fisher, Scot Nicol and others were riding in the 1970s.
I’m seeing stuff that’s in the 62.5 degree range. It sounds nuts, but maybe that will be where we are with trail bikes in ten years.
There is an end to this, a point that will be too far. There comes a point where the head tube angle is so slack that if you turn the bar far enough it reaches a tipping point and you get wheel flop. Put another way, the wheel dives into the turn, oversteering so badly the bike goes down. It’s the chopper effect. I don’t think we’re closing in on that point yet, so I expect we will continue to see numbers going down.
There is a point of diminishing returns when it comes to going real slack, but this depends on what you want out of your ride. First and foremost, I have not found slack head angles to have any meaningful effect on a bikes climbing abilities. Seat tube angle, fore/aft saddle position and bottom bracket height are way more important factors. IMHO/experience, head angle past a static 64 degrees are all about being able to drop your heels and plow while diminishing the potential that you’ll be thrown out the front door. However, the trade off is a loss in playfulness and the need for a steeper seat angle to make ascents manageable. I for one really love where we are geometry-wise. I am thrilled that mountain bikes have given road bikes the middle finger after years of that influence. I suspect that the further proliferation of eBikes will be the biggest driving force in the geometry wars in the coming years. Hang on as it will be a wild ride no matter what you dig.