So in Part I of fork rake, I explained just what it is and how it affects a bike’s handling. In Part II, we looked at common fork rakes for different kinds of bikes and a rider’s practical experience of how fork rake affects a bike’s handling as well as another important dimension in fork design, axle-to-crown length. If you missed Part II, it’s important as details our first mention of a crucial part of a rider’s experience: threshold of perception.
In this installment we are going to look at issues to consider should you be thinking about purchasing a new fork for your bike.
That’s the first question, of course. Why would someone want to replace their fork? There are many reasons, but arguably the most common are: to replace a damaged fork, to improve a bike’s handling, to reduce a bike’s weight, to increase a bike’s functionality (more braze-ons to carry stuff while touring) or to increase a bike’s rowdiability (i.e., travel).
Rubber meets road
Back in the 1990s when Kestrel, Time and Look introduced the first carbon fiber forks, they were marketed as upgrades for road bikes. It was a quick way for a rider to take half a pound or more off of a 19-lb. bike. I recall many riders crediting a carbon fork with being the magic ingredient that allowed their bike to break the 20-lb. barrier.
Swapping that steel fork with 40mm of rake for a carbon fiber fork with 43mm of rake was not an issue-free swap. Initially, Kestrel and Time offered only one rake because a different rake required a whole new mold. The manufacturers reasoned that 43mm was close to both 40 and 45mm. Horseshoes, hand grenades and fork rake—that’s how the saying goes, right?
Don’t get me started.
Those carbon fiber forks weren’t as stiff as the steel ones they replaced, making the handling a little more unpredictable—the harder a rider turned into a corner, the less certain they could be about just what sort of line the bike would carve. Sounds crazy, right?
Axle to crown dissected
But wait, there’s more! (Yes, we’re going full Ginsu knife on this.) The axle-to-crown (A-C, and sometimes referred to as fork length) distance also changed. In a way A-C relates more to head tube angle than fork rake, but bringing it up in the first act would have been confusing as hell. I’ve now left this offstage for as long as possible. To recap: A-C is the distance from the axle to the top of the fork crown (where the headset crown race sits).
Axle-to-crown helps a builder or engineer establish what kind of brake will be used and how large a tire can be run. Standard caliper brakes, long-reach calipers and cantilever brakes all require forks with different A-C lengths, and the bigger the tire the bike is meant to run, the greater the A-C must be.
Because carbon fiber forks weren’t as stiff as their steel counterparts, one way that manufacturers made them stiffer was to decrease A-C. Why? A shorter fork blade is stiffer, no matter what it’s made from. If you were riding then, you may recall that many of those forks didn’t leave enough room at the crown to run a 25mm-wide tire. That’s why.
So why does that matter? Well, as A-C decreases, the headset crown race drops closer to the ground. That means that the head tube is now closer to the ground. Movement of the head tube is governed by an arc drawn from the rear axle. The upshot is that the head tube angle increases, and as you probably recall, a steeper head tube angle makes the bike’s handling quicker. Any rider who swapped a steel fork with 40mm of rake for a carbon fiber model with 43mm of rake and a shorter A-C ended up with essentially a different bike because the handling was so much quicker. Add to that less precise cornering because of the more flexible fork and riders ended up with a bike that handled like a drunken but running toddler, all to lose a half pound off their bike.
The other direction
Suppose for a moment you have a mountain bike and it can’t handle some of the riding you’re doing. We’re going to suppose the fork is set up correctly, but you’re going to fast, going off things too big or are just more Daniel Craig than your bike can handle.
Replacing the fork with one with more travel is a common solution. Funny thing, though: If your bike has been a bit too reactive at speed or doesn’t quite float over the rough stuff the way you’d like, a longer fork is going to help in not one but two ways. First, more travel will give the bike that extra impact flair. Second, a fork with more travel will have a longer axle-to-crown distance, because more travel. This will have the opposite effect that most aftermarket forks had on road bikes. A fork with more travel will raise the front end of the bike, slackening the head tube angle.
There is a rough rule of thumb here: for every 10mm increase in travel on a fork, the bike’s head tube angle slackens by about a half a degree (this is somewhat size dependent as small bikes will see a greater change and big bikes will see less change). This works out really well; if a 10mm change in fork travel resulted in a 5-degree change in HTA it wouldn’t be practical to change forks; you’d be locked into whatever the manufacturer had spec’d for the bike.
Bikepackers are also substituting forks on their bikes if their came with one with few (or no) braze-ons for bottle or gear cages. This one can go either way—up or down. The effect is even harder to gauge if part of what the rider wants is to run a larger width tire up front. What this means is that even if the A-C is identical to the old bike, running a bigger tire will raise the front end slightly, but more likely, the A-C has increased as well and the combination of the two could result in a 1-degree decrease in head tube angle.
Bikepackers would do well to consider the following if they are looking at a new fork: If the goal is to increase carrying capacity, that means the front end of the bike will weigh more. That will make the handling more sluggish because more weight on the front wheel makes it turn more slowly. Provided that the A-C doesn’t increase, someone looking to load more on the front end of the bike would do well to purchase a fork with 3-5mm more fork rake in order to keep the handling more sedan than school bus.
It wasn’t so long ago that if you looked at a bike’s geometry chart you wouldn’t see fork rake, axle-to-crown distance or trail. Increasingly, bike manufacturers and aftermarket makers of forks will give rake, A-C and hub/axle dimensions. Having so much information makes the aftermarket purchase of a fork a much less uncertain venture.
Next up, we’re going in for the big one: Trail. I promise you won’t need to go find your Texas Instruments calculator.
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