by Damon Rinard
When aftermarket forks became commonly accepted in the early 1990's, the differences in their lengths became apparent. Some people are concerned that the new, longer (or less commonly, shorter) fork lengths might adversely affect handling by changing the bike's steering geometry.
I've tried here to explain what exactly the length change is, how it affects a frame's head angle, and whether the change in handling is significant.
|Although some references differ, most agree fork length is measured parallel to the steerer, from the base of the crown race seat to the center of the axle.|
|As you can see from the numbers
(table, left) fork lengths vary by a little over 11mm
from the longest fork to the shortest fork in this group.
11mm is about the thickness of your little finger.
This collection of forks is probably a good representation of the total range of road fork lengths in production. The longest, the Tange CrMo TIG unicrown fork, can be found on many production road bikes made in the mid 1990's. The shortest, the Tange 7B CrMo, is brazed with a traditional semi-sloping crown, and represents what for many years was the standard brazed steel fork.
The frame pivots about the rear axle, so raising the front of the bike decreases the head angle. The change in head angle can easily be calculated if you know the lengths of the two forks. The formula is
[change in head angle] =
arcsin [(old length - new length)/(wheel base)]
But you don't have to use the formula. If you assume a wheel base of 1000 mm (most modern road bikes have a wheel base close to that), you can predict the change in head angle for a given fork swap on a typical road bike. The following table shows the numbers.
1. Look for your existing fork length in the left column
2. Find your new fork length in the top row
3. Find the change in head angle in the cell where the column and row intersect.
|CHANGE IN HEAD ANGLE, degrees|
|arcsin[(old length-new length)/1000]||NEW FORK LENGTH, mm|
|OLD FORK LENGTH, mm||374.7||372.5||371.3||369.3||367.3||366.3||363.5|
A negative number in the table means your head angle will decrease; you're installing a longer fork. A positive number means your head angle will increase; you're installing a shorter fork. The diagonal row of zeros shows, of course, that if you replace an old fork with a new fork of the same length, there will be no change in head angle. If all else remains the same, a steeper head angle is quicker handling, while a shallower head angle is slower handling.
I've changed the color of a few values in the table to red. These are the changes in head angle that are greater than half a degree in magnitude. I've made them red because, in my experience, any change over half a degree will probably be noticed. Some people say they can notice changes as small as a third of a degree.
Assuming you are installing a longer aftermarket fork, a shallower head angle is not necessarily bad, and in fact might be good. But a decrease of half a degree or more should be justified. For instance, the new fork may be stiffer than the old fork. A stiffer fork can bring a more positive feel to the steering, which can, in part, make up for the new, shallower (and thus slightly slower-handling) head angle. Or the new fork may have slightly more rake. More rake makes for quicker handling, which again will tend to cancel the slower handling caused by the shallower head angle. Or perhaps your bike is a little too quick handling already, and slacking off the head angle would slow the handling down to "just right".
Half a degree seems to be the borderline between two regions: the region less than half a degree where you can pretty much blindly change forks at will, and the region over half a degree where you should take a closer look to make sure the change will be beneficial.
To put the change in head angle in perspective, here are some changes besides swapping forks that also change the head angle:
- Variations among headset lower stack heights (from 11 to 15 mm: 0.25 degrees)
- Maybe frame building tolerances (quality frames up to +/-0.3 degrees from nominal)
- Running different tire widths front and rear (one 28 mm and one 20 mm tire: 0.48 degrees)
- Same model frame, but next size larger or smaller (typically 0.5 to 1 degree)
- Choosing a different model frame (as much as 10 degrees)
Take a look at a few manufacturer's frame geometries (they usually list geometries in their brochures). You can easily find head angles that vary from 72 to 74 degrees, and if you hunt a little among different maker's brochures you will find frames whose head angles differ by several degrees farther in both directions beyond those numbers. For example, a friend who is quite a good bike handler likes to pursuit on frames with 78 degree head angles, but he will just as happily pursuit on his bike with a 72 degree head angle! And in the other extreme, for many years bikes had head angles in the low seventies and high 60's. That's a total range of almost ten degrees. In comparison, the biggest change you can get by switching forks is just over half a degree. That's not much.
In the big picture, it is good to realize that there is a fairly large window of "safe" handling frame geometries. Just about any bike you can buy today has steering geometry in that window. Inside that window, individuals narrow down the range of "safe" geometries to a smaller region: their personal preferences. This small range is what we're talking about. When you change forks, the largest change you can make is on the order of personal preference. In other words, you cannot change the fork and suddenly make the bike handle so badly it's unsafe. In fact, it is entirely possible to change forks and find you like the new handling better. In fact, if you wish your bike handled a bit different, a fork change could make it just right.
You can probably tell that I think worrying about fork length when changing a road fork is usually not worth it. For information on what properties I think really are important in a new fork, see my article about the Rinard Fork Deflection Test. In it, I measure the stiffness of several aftermarket road forks and give my opinion on how you can choose a good fork for the kind of riding you do.
Go to the Rinard Fork Deflection Test
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