Fork offset is basically how far the front axle sits ahead of an imaginary line that runs through the steering axis. The most visible evidence of fork offset are the short extensions for the dropouts that house the front axle. Less obvious is that most fork makers also angle the crown slightly to provide part of the offset as well. When all bikes had 26-inch wheels and similar head angles, fork makers and frame designers settled upon 40-millimeters or thereabouts and that's where it remained - until the 29er appeared.
Gary Fisher liked 29ers, but hated the heaviness of the steering. The Fisher folk experimented with different offsets and discovered that 51 millimeters gave his 29ers a similar feel up front as a 26-inch bike. Fisher's 'G2 Geometry' was born and with it came more widespread knowledge of how the relationships between head angle, wheel size and offset play in the bike's steering equation. Today, there are a number of offset options available for production forks. RockShox, for example, offers 40, 42, 46 and 51 millimeter offsets.
In this chapter of 'To the Point,' Pinkbike attempts to clarify why fork offset changes with each wheel diameter and touch on how wheel size and head angle factor into the steering equation.
What does fork offset do?Fork offset is used to adjust the amount of 'trail' in the steering geometry. More offset equals less trail.
What is trail?The distance that the center of the tire's contact patch falls behind the imaginary point that the steering axis (center of the fork's head tube) passes through the ground. This is what causes the bike's front wheel to self-straighten when it is moving forward. More trail firms up the steering and adds straight-line stability. Less lightens the steering pressure at the handlebar and gives the bike a more nimble feel. Too far in either direction makes the bike difficult to control.
How much trail is best?The correct number varies with head angle and other factors, but somewhere between 80 and 90 millimeters seems to be the sweet spot that produces a light feeling at the handlebar, with a stable feel at speed, and with predictable cornering.
How does wheel diameter affect trail?As wheel-diameter increases, the axle moves farther back along the steering axis, which increases trail. With a 68-degree head angle, the difference between a 26 and a 29-inch wheel is 15 millimeters (.61 inches).
How does the bike's head angle affect trail?Trail increases as the head angle becomes slacker.
How is fork offset used to adjust the trail to the correct amount?Adding more offset reduces trail by moving the center of the tire's contact patch forward, closer to the steering axis.
What is the relationship between fork offset and head angle?If one of the offset options offered by the fork makers does not provide the frame designer with the desired amount of trail, he or she can change the head angle by a half degree or so to correct it. For instance: the slow steering of the 29er can be quickened by adding a slightly steeper head angle, then the designer can bring back some high-speed stability by choosing a fork with less offset to add some trail. The reverse is true for slack head angles, where the designer can erase some of the heavy wheel flop by adding offset and thus reduce the trail.
Wouldn't an adjustable offset feature be the best solution for fork makers?Not in the real world. Experiments with various fork offsets show that getting it wrong is far worse than riding a bike that is nearly perfect. Now that fork makers are providing options, for the most part, bike makers are getting it right. That said, if you see CNC-machined fork crowns on World Cup DH bikes, or stanchions that are noticeably angled on single-crown forks at the EWS, they are almost certainly fitted to alter the fork offset.
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check this out: www.bicyclefilmfestival.com/wp-content/uploads/2013/06/Stayer21.jpg
this is a photo of motorpaced racing on the velodrome... these riders are commonly doing 45-50 mph average speed... the forks are so offset for stability that they look like they are mounted backwards... they dont like to turn, but the rider can go in the hurt locker pretty deep, have a lot of Gs pulling on him in the bankings and the bike is still stable...
If you wondering why steering angles don't get further slackened and trail further increased it is likely due to the fact that by doing so only small improvements can be gained. To a significant degree the impact of slackening the steering angle and increasing offset negate one another. The whole thing is a balancing act in which there are only small gains to be made. And as the steering gets slacker other problems like wheel flop become harder to manage. Still there are companies that do not appear to accept the current consensus on steering angles and fork offset. Marzocchi have always preferred a 44mm offset for their standard 26in forks (where the industry standard is 39mm more or less) making them suitable for bikes with slacker steering angles. And, Cannondale are now using a 60mm offset on their Lefty SuperMax 29er fork (a good bit greater than the common 51mm). These variations from the norm are not crazy, and may in the long term turn out to be the better geometry compromise.
At best it wont fit, at worst it will fit but be incompatible in some more subtle way that could lead to either a lack of damping (bad handling and possible crashes) or the lowers detaching from the uppers when you lift your front wheel (definate crashes, possible death)
Basically, you should be ok, but just make VERY sure there are no hidden issues before you do the job.
Wouldn't running these slack head angles make the bike handle like shit without additional offset?
Does 650B truly make me taller, more financially secure and attractive to the opposite sex?
so it did none of those things
So, what if you put more A2C on a 26 inch frame? First, what will happen to HA? It will slacken ever so slightly since there is 10mm more A2C. (see this from Cotic to help you understand.. change the fork stroke (thus A2C) and watch the HA change www.cotic.co.uk/geek ). This will add to stability at high speed, on the steep bits and in the rocks but will lose you some finesse during slow control on tight turns.
What will happen if you put more offset on a 26 inch frame? Well, it will push the wheel out front, and lengthen the wheelbase. This will add to stability at high speed, on the steep bits and in the rocks but will lose you some finesse during slow control on tight turns.
So, if you put a Pike 650B on a 26 inch frame it will be making at least two adjustments to the ride, and both will be ensuring greater control and speed and on the steeps at the cost of some cornering and climbing ability... well, that is what the theory that I understand suggests.
As I said, I am not a bike guru however, it is great to try and think about it....
More control going down, less going up is arbitrary though. How can that statement be measured? I bottled buying a 650b Pike and went for the 26" version as this detail is not available, this will cost me when I come to eventually upgrade to 650b, but I reckon I made the safe choice. I'd personally rather rely on expert opinion than base my own on bro science etc especially when the mistake could be an expensive one, but each to their own I suppose. Tight lines ;-)
Excuse the soapbox moment coming.... Sure, we could all just buy an off the shelf bike, based on reviews by the two PB Mike's or Steve Jones' of this world, however, and this for me is a HUGE point, there is so much meta-language used by bike and component manufacturers telling us about their "forward planted geometry" or "KS Gussetting" or "double-chutney-burning drop-link" that in the end, unless we try and educate ourselves as to what is happening with the geo of our bikes, we are at the mercy of the marketers. Thanks PB and all of you for helping me gain a better understanding of it all. Long may it continue.
Change the offset on a motorcycle, by as little as 1mm at the triple clamp, and you'll feel the difference. Now, the difference between a 40mm offset DH fork, and a 51 /54 - it's very, very pronounced. The only way such bad geometries on so many bicycles (of so many types) has been got away with, is lack of knowledge / feel by riders, their gullibility and "pride of ownership" in what they have bought, and, that even a relatively heavy, 40 + lb DH bike, is still a 'toy', that can be dealt with. That simple fact, allows dreadful designs, and geometry, to be accepted as the "bestest of things". Now, I favour less offset, for more trail, on both my Motorcycles and Bicycles, with less than chopperesque steering head angles, and with a good wheel base and chainstay / swingarm lengths. So, I'm 'different' to the seeming norm of kicked out front ends, with large (relative) offsets, short wheelbases, and short rear ends.
Replace the front fork and wheel with a 27.5 fork and wheel.
26 inch wheel in the back 27.5 up front will give you about one half of a degree of extra" slack" to make your bike more DH friendly.
Or if you want to mess with peoples heads put a 29 set-up up front with a 26 inch wheel in the back.
Manny possibilities.
Too bad you cant order custom off sets for your fork to dial in the amount of trail.
All I wanna do is ride-too hot though-f*ck 40 degrees tomorrow (centigrade)-wish it was wet and cold-lucky northern hemisphere dudes.Bring on winter.....
Does this make our enthusiastic geometry dabbling badly informed?
Reduce, re-use, recycle…
(yes, I get that they add wheel size to this one.)
Also I found that Fox varies by model...The 26" 36 has less offset than the 26" 32.
It would be nice if all fork makers consistently published the offset and AtoC with the fork specs.
On average yes a 26" will have the same trail as a 1 degree steeper 650B for the same catergory of fork which leads to a similar weight of handling which is what gives you a bike in the same part of the stability to turnability scale.
Trail is the thing that matters, but is adjusted by HA and offset is the short version.
@parallaxid: the traveled pike ist just 3mm higher compared with the revelation (Chris Mandell rides a pike in his 111....but i have no idea which one)
This does not seem correct. If offset is the distance from the center line of the stanchion, to a line parallel to the axis of the stanchion, but through the axle, then the farther you move that axle from the stanchion, the trail on the ground increases. So then increasing offset would increase trail, per the first figure in the article?
This seems to contradict with "trail increases with wheel size"???
Not sure what I am missing.
The fork crown off set also plays a role in how much trail a fork has.
The illustrations are inaccurate and misleading (as usual). It's more accurate to show the steering axis coming from the steerer tube as it goes through headset, not through the legs. The crown often has some offset built into it. Some forks, such as the german:A forks, actually adjust the angle that the stanchions connect to the crown, achieving the desired offset by making the fork's telescoping angle slacker than the steering axis.
Even with layman's terms explaining this, do you even understand offset enough to make proper use of the knowledge?
Do some research at the mountain bike hall of fame, RC was inducted almost twenty years ago.