In Part I, we looked at how fork rake is determined and how it affects the way a bike steers. That’s most of what you need to know about fork rake, but not quite all of what’s useful, so in this installment of The Angle we will look at some other factors to be aware of should you be shopping for a bike or are considering aftermarket upgrades.
How much
No other dimension in a bicycle is as sensitive to change as fork rake. Most road forks have a rake of 40, 43 or 45mm. That’s not as universally true as it once was, thanks to carbon fiber (but we will get to that in a future piece). Were you to ride a bicycle with a rake of 43mm and install one with a rake of 40mm, you’d find the bike to be a bit calmer in handling, the sort of bike you could ride across town no-handed. Replace that 43mm rake fork with one of 45mm and you’ll find that the bike feels just a little more spirited, like it will allow you to slip through any gap between riders.
The range
For road bikes, mountain bikes and gravel bikes, rake is confined to a fairly narrow range—from as little as 36mm in the case of some mountain bikes to as much as 55mm in some gravel bikes. Road bikes tend to sit between 40 and 45mm of rake, while gravel bikes are frequently spec’d with forks of 45 and 51mm of rake. Many 29er mountain bike forks are built around 44 or 51mm of rake, while 27.5 wheeled-bikes will see rake more like 36 or 37mm.
The lead image of ENVE’s full range of forks will give you some sense of just how many designs a committed manufacturer will offer, and most of those models shown will come in multiple rakes to work with different bikes.
The threshold of perception
This brings us to a really interesting concept from psychology that will be useful as we explore just how changes in geometry translate to your riding experience. It’s called the threshold of perception. What this refers to is how much something must change before we perceive the change. It’s more than just different; the difference must be great enough that we can describe it.
In sound, the decibel is the standard measure of volume. A single decibel is the smallest change in volume that we can perceive. Any less than that and we won’t notice.
Granted, some folks are more sensitive to changes than others. As a former drummer, I’m very sensitive to changes in tempo, but I can’t look at a road bike and tell if the head tube angle is 73 or 73.5 degrees, the way some frame builders I know can. An artist understands the difference between a pink used for flesh tones and one used for bubble gum. A violinist can feel exactly where on the neck of a violin their fingers should fall, which is a feat given that in higher registers a few millimeters can be the difference between one note and another.
So how does this relate to bikes? Most experienced riders will notice the difference between a fork with 40mm of rake and one with 43mm, provided all other dimensions remain unchanged. Novice riders may not be able to detect that difference, but they’ll be able to tell something is different between forks with 40 and 45mm of rake, though they may struggle to articulate just what that difference is.
And to backtrack just a tad, a veteran racer will notice a 3mm change in BB drop. Most experienced riders will be able to detect a 5mm change in BB drop and even novice riders will notice the difference between a bike with 70mm of drop and one with 80mm. This is one expression of how thousands of hours of riding informs our sense of muscle memory.
Mediating factors
There are two other factors to consider with regard to fork rake. The first is axle-to-crown distance. This is the distance from the crown race to the axle. This will affect what size tire can be run on the bike as well as what sort of brake is necessary. The second is construction material. The greater the rake the more important it is that the fork be stiff. A flexible fork will undermine a rider’s confidence because it will make the steering less precise, so as fork rake and flexibility increase handling will become more vague. This has been a problem with carbon fiber forks in the past, though rarely an issue today, but it’s one reason almost no one tries to make a fork out of titanium—it’s simply too flexible.
Consider: the faster a rider is going, the more precise the steering needs to be because reaction time is cut. If a rider is only going 10 mph, they have twice as much time to react as a rider going 20mph. And if a bike isn’t doing exactly what its rider anticipates, there needs to be time to course correct.
Somewhat to my surprise, there’s going to be a Part III to our look at forks. In our final installment we will look at axle-to-crown distance in greater depth and factors to consider in an aftermarket fork purchase.
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Image: ENVE