Reduced-offset forks are all the rage at the moment, but what is fork offset? Wil explains in this two-part online feature about fork offset
Unless you’ve been living under a rock for the past two years, you’ll have noticed that fork offset has been a particularly hot topic in the world of mountain biking.
Of course for many frame designers, fork offset – and the role it plays in governing a bike’s handling – has been a consideration for a lot longer than that. But ever since Whyte launched its S-150 in early 2017, and Transition announced its range-wide ‘Speed Balanced Geometry‘ around the same time, the collective swarm of journalists, internet forum dwellers, racers, marketing managers and consumers, have been buzzing about it a whole lot more.
The main topic in this conversation has been around the handling advantages (and disadvantages) of using ‘reduced-offset forks’.
In this article, I’ll be discussing what fork offset is and how it interacts with modern mountain bike geometry. I’ll also be investigating why some brands are jumping on the reduced-offset bandwagon, why others are not, and whether any of that is actually that big of a deal.
Fork Offset – What The Heck Is It Anyway?
Put simply, fork offset is the measurement of how far the front hub axle sits in front of the bike’s steering axis.
And to clarify, ‘steering axis’ is just a different way of saying ‘head angle’. This can be visualised as a straight line pointing down through the head tube to the ground. Now imagine a secondary parallel line just in front of this, which runs through the front hub axle. The distance between these two parallel lines is the fork offset.
fork offset is the measurement of how far the front hub axle sits in front of the bike’s steering axis.
All modern mountain bike forks have a positive offset. Notice how the fork dropouts on your bike kick forward from the rest of the fork chassis? That’s part of what creates the positive offset. The other part is found in the crown of the fork, which also kicks forward from the steerer tube. Add these two together, and you have the total fork offset.
Fork offset varies from manufacturer to manufacturer, and it also varies between wheel sizes. For the sake of clarity though, this article is going to stick to 29er forks and offset figures from Fox and RockShox, since that’s what I’ve predominantly been testing lately.
In the early days of 29ers, most forks featured an offset of 44-46mm. Then Gary Fisher came along with his G2 geometry concept, which was based around a longer, custom 51mm fork offset. This evolved to become an open standard, and for a variety of reasons, the 51mm fork offset has since grown to become the most widely adopted for 29er mountain bikes. For this article, I’ll be referring to the 51mm offset as the ‘long’ or ‘standard’ offset.
Then in 2017, Whyte launched its S-150 29er enduro bike, while Transition revamped its full suspension range with ‘Speed Balanced Geometry‘. Both brands had designed bikes specifically around reduced-offset forks, which led to Fox and RockShox offering select fork models with two different offsets.
As well as the 51mm offset, Fox now offers its 29er forks in a 44mm offset, and RockShox has a 42mm option. It’s these latter two offsets (44 & 42mm), which I’ll be referring to in this article as ‘short’ or ‘reduced’ offset forks.
And What About Trail Then?
We can’t discuss fork offset without also discussing a key handling attribute that is directly affected by offset, and that’s trail.
Remember that steering axis we mentioned before? Well if you trace that line all the way to the ground, you’ll notice that this line sits forward of the front wheel. How much it sits in front of the tyre contact patch (or to really confuse you – how far the tyre contact patch trails behind the steering axis…) is called ‘ground trail’.
[Note: in terms of bike geometry, there are actually two different types of trail – ‘ground trail’ and ‘mechanical trail’. For the purpose of this article, I’ll be referencing ground trail, since that’s the measurement most manufacturers list in their geometry tables when they refer to ‘trail’.]
One of the biggest factors that influences how much trail a bike has is the head angle. A slacker head angle means the steering axis sticks out further ahead of the front wheel, which creates more trail. A steeper head angle will create less trail.
Fork offset also affects trail. Assuming the head angle stays exactly the same, a fork with a 51mm offset will produce less trail than a 44mm offset fork. This is because the steering axis remains the same, but the front hub (and tyre contact patch) shifts closer to the steering axis as offset is increased. Therefore, if you reduce the fork offset – as many bike brands are currently doing – you will increase the amount of trail. Less offset = more trail. Seems counter-intuitive doesn’t it?
Lastly, wheelsize also affects trail. A 29in wheel sits higher off the ground (axle height) than a 27.5in wheel, and – all things being equal – will result in more trail. That means if you wanted to maintain the same trail figure between two bikes of differing wheelsize, the larger wheeled bike would require a longer offset.
Fun Fact: This is actually the original theory behind Gary Fisher’s development of the longer 51mm offset. In order to replicate a similar steering feel on a 29er as a 26in mountain bike, the longer offset was developed to keep the trail figure the same between the two.
How Does Trail Affect Steering?
Put really simply; the more trail you have, the more stable the steering feels.
This is due to a mysterious force called the ‘caster effect’, which is the front wheel’s tendency to self-align when it gets bumped off course. The more trail you have, the stronger the caster effect is. And the stronger the caster effect, the more the front wheel wants to stay pointing straight ahead.
the more trail you have, the more stable the steering feels.
The easiest way to feel the caster effect is when you take your hands off the bars while coasting along a flat road. A bike with zero trail would feel unstable with a very wobbly front wheel. A bike with lots of trail (and a stronger caster effect) will feel steady and stable.
Real World Effects
To put it in an off-road context, imagine you’re riding down a hill, and the front tyre hits a rock. As the front wheel is deflected off line, you feel this force through your hands as the handlebars are twisted slightly. How quickly and easily the front wheel then self-corrects and returns to pointing straight ahead is influenced by the caster effect.
A bike with more trail (and a stronger caster effect) takes this kind of impact in its stride. Not only does the rider feel less of that twisting force at the grips, they’ll also find the bike holds its line with more stability.
However, too much trail can make the bike more difficult to steer, and less willing to change direction quickly.
Although the bike may be stable in a straight line, it will be less willing to make directional changes with a heavier and slower steering feel. The front wheel can still be turned, but the rider will need to apply more force at the grips to overcome the stronger caster effect.
What Are The Advantages Of A Reduced-Offset Fork?
As mentioned above, more trail creates a greater degree of self-correction to the bike’s steering. This makes everything feel calmer and more steady – particularly while descending at high speed.
Chris Porter of Mojo Rising and Geometron states that a shorter fork offset – and the increased trail that comes with it – is mostly about stability. “The wheel self centres more so is simply more stable”, explains Porter.
As a firm believer in the virtues of reduced-offset forks, Whyte Bikes’ Chief Designer, Ian Alexander, echoes a similar sentiment to Porter. “Well, the basic steering feel is just better, and better suited to fast off-road trail riding where the bike is prone to being knocked around and off line by disruptive forces introduced into the steering”, explains Alexander.
It isn’t just about steering though, as changing fork offset also alters weight distribution. When you change fork offset, you’re also changing the front centre length (distance from the BB to the front hub).
Pivot Cycles CEO, Chris Cocalis, reckons that a short fork offset can be particularly useful on bikes with long front centres. “When the reach starts to get super long on a bike, it’s more difficult to keep the front end weighted”, Cocalis explains. “The reduced offset helps compensate for this. The front tire gets more traction without having to ride quite as aggressively over the front end at all times.”
Andrew Juskaitis of Giant Bicycles shares a similar view. “We feel reduced-offset forks work best when we push our reach numbers out farther”, says Juskaitis. “By shortening the fork offset, this helps bring the front wheel back underneath the rider for snappier handling.”
From those responses, we can surmise that a reduced-offset fork is likely better paired to a longer reach. Both of which tend to increase front-end stability on a mountain bike. But is that always a good thing?
Santa Cruz Bicycles‘ Josh Kissner acknowledges the improved stability that comes with a reduced offset fork, but also recognises the limits. “More isn’t always better when it comes to stability, but it’s certainly where things are headed in the trail bike world these days”, says Kissner. “Longer, slacker, reduced offset – these all work together to make a bike that’s more confident in fast and/or steep terrain.”
And What About The Disadvantages?
As you increase trail with a reduced-offset fork, you’re increasing the strength of the caster effect. This can potentially create a heavier feel to front-end steering, which means more force is required to initiate a turn. And so rather than steering with the bars, the rider will need to lean the bike more aggressively in order to quickly change direction.
“A bike with more trail requires a more active riding style.”, states Vincenz Thoma, one of Canyon‘s Senior Product Engineers. While Canyon’s Strive enduro bike sports a reduced-offset fork, the Neuron CF trail bike has stuck with a 51mm offset.
Thoma explains this is because there was less of a priority on high-speed stability with the Neuron. “The focus here was to have a very easy ride, especially when you don’t have the most active riding style”, states Thoma. “It gives the rider a very predicable steering behaviour. You don’t have to extra lean or push the bike. It’s very easy to steer, also when you are tired.”
Kissner shares a similar sentiment. “What I’ve found is bikes with the shorter offset require a bit more aggression and leaning to get them to turn. It’s not quite as intuitive as the longer offset, and can take a bit of time to get used to. 51mm offset is more intuitive at first, or when riding less aggressively.”
Gary Fisher (owned by Trek) has been pushing the benefits of the longer 51mm offset for as long as it’s been making 29ers. Trek’s Senior MTB Engineer, Dylan Howes, explains that in the early days, the shorter trail figure (achieved by the 51mm offset) helped speed up the front end steering. “This provided a more nimble, sharper turning feel at a time when people were still adjusting to the bigger 29er wheels”, explains Howes. “This helped make the bike feel ‘more normal‘ – ie, like a 26in bike. It also moved the front wheel out a little further which reduced toe overlap.”
Pushing the limits
There are, of course, limits to how much trail is usable for a mountain bike. Exploring those limits is something that Chris Porter and Geometron has spent considerable time testing.
“I managed to get a set of offset adjustable crowns made one-off and I tested 30, 51 and 70mm offsets. The fork rode well at 51mm, but was completely un-rideable at 30 and 70mm offsets”, explains Porter. “So we did some more experimentation as part of the process of our MORC offset adjustable crowns testing and we settled on the 45-50mm offset as being the area where you could feel all the improvements, with no downsides.”
I ask Porter why the 30mm and 70mm offsets were unrideable.
“If you imagine that the trail is what creates the ‘self-centring’ effect of the bike, then braking transfers the rider weight to this area and heightens the effect of the self centring”, Porter explains. “The 30mm offset had a very large trail figure, which is very stable in a straight line and creates a lot of self centring effect. When I was trying to turn and decelerate on this set-up the self centring effect was so much that I couldn’t fight it to turn the handlebars! I literally had to be completely ‘off the brakes’ to get it to turn. If I touched the brakes the handlebars pulled straight! The 70mm offset was the opposite. It was so unstable that I could only ‘stabilise’ it by pulling the brakes a little. It was so unstable off the brakes that it was terrifying!”
Will It Work For Every Rider And Every Bike?
Depending on who you listen to, the handling behaviour of these short offset forks can suit a particular riding style, and therefore a particular type of rider, better than a long offset fork. And vice versa.
Dylan Howes of Trek provides a simple breakdown of where a certain fork offset is preferable. “Speaking in generals, 51mm offset would be great for XC applications where you value a quick handling bike in slow speed situations”, answers Howes. “44mm works well for current trail setups where stability at high speed is valued most.”
Santa Cruz’ Josh Kissner agrees with that generalisation. “I’d say that XC bikes are the bikes most likely to feel better with 51mm offset. If you don’t have a dropper post, it can take some practice before tight cornering feels good on a short-offset fork.”
Canyon’s Vincenz Thoma goes into more detail about why the handling attributes of a slack head angle and a reduced-offset fork aren’t necessarily a good thing for everyone. “For beginners or just average people who are not so into going super-fast down trails, bikes with more wheel flop are definitely harder to ride because they also tend to stick to any direction you give them.” says Thoma. “And if you counter steer, they dive more aggressively.”
Thoma raises a very good point about steering dynamics and how riders of differing skill and experience levels pilot their bikes. He suggests that often the people testing and reviewing bikes tend to be of a higher experience level than your average consumer. “We have the feeling that industry and media are quite far ahead these days compared to average people and average riding”, Thoma states.
He’s right too. Look at the riding style of Ratboy and Joe Barnes. What feels good to them, and what handling attributes they require is going to be very different to your average mountain biker.
Chris Cocalis and the Pivot team have been performing a lot of back-to-back testing with different fork offsets. “When we tested it (the Trail 429) with the 44mm offset, the additional front end bite did not compensate for the overly quick turn in off centre”, explains Cocalis.
Of note is that Pivot has started spec’ing reduced-offset forks on certain models that were initially launched with a long fork offset, like the Mach 429 SL. “On the other end of the spectrum, when a bike gets a slightly steeper head angle (like an XC bike) the 44mm offset may in fact provide the best overall balance”, states Cocalis. “Which is the case for our Mach 429 SL XC racing builds that use a FOX 100mm Step-Cast fork with 44mm offset.”
This differs to Giant’s approach with the XTC and Anthem 29 – two XC race bikes that have a 51mm offset fork. “Both are XC racing-specific bicycle series. With a 69° headtube angle (Anthem 29), we wanted to keep the sharpness/acuity that XC racers demand for excellent climbing and reasonable descending traits”, explains Andrew Juskaitis. “A 51mm offset helps balance these characteristics (against a steep headtube angle and modest reach)”
Can I Slap A Reduced-Offset Fork On My Current Bike?
As reduced-offset forks become more widely available in the aftermarket, this is an increasingly common question. Riders who may be looking to fit a shiny new fork to their current bike are now being asked what fork offset they’d like at the point of purchase.
So could you just whack on a 42/44mm offset fork onto a bike that originally came with a 51mm offset?
If your bike is already on the short side in terms of reach, fitting a fork with a shorter offset is going to shorten the front centre length further. Bringing the front tyre in further underneath the rider may result in too much weight being placed on the front tyre.
“You have to have long reach numbers to offset the reduced offset. Get the equation wrong and the front wheel will tuck too much and create nasty oversteer“, states Juskaitis.
Having a sufficiently long reach is also necessary in being able to facilitate running a shorter stem. When moving to a reduced-offset fork, the steering will become more stable. Adding a shorter stem can help to bring the steering speed back up a touch.
Cocalis explains that Pivot was able to do this with the current Switchblade. “We updated the Switchblade to a 160mm fork with 44mm offset for this year”, says Cocalis. He also notes that many Switchblade customers have been up-sizing in order to get that larger reach measurement, and the ability to run a shorter stem in tandem with the reduced-offset fork. “That combined with the upgraded DPX2 shock ups its game for enduro racing and more aggressive (steeper) descending.”
However, Canyon’s Vincenz Thoma is more skeptical of this fork offset swapping business. “Of course, you could in theory also ride a short offset fork in the Neuron”, Thoma responds. “On one side it would give it more stability, but it would also load the front more. Maybe too much. I would recommend this person to also use a shorter stem so the balance is better. Hopefully the frame size is big enough then.”
Opinions amongst other brands seem to differ quite a lot though.
Geometron’s Chris Porter doesn’t believe there are any downsides to plugging in a reduced-offset fork, though he does come from a perspective where super long reach and slack head angles are preferable. “You can still have too steep a head angle though”, he says. “Using offset is not a head angle compensation device. You need head angle to create flop which creates the turning impetus. A bike with too steep a head angle doesn’t steer enough so it gets steered upright rather than being comfortable leaning over.”
While Porter is positive for offset retro-fitting, Ian Alexander of Whyte isn’t so sure. “I don’t think you will get anywhere near the same level of improvement by retro-fitting a 42mm offset fork to a bike designed with a 51mm offset fork”, says Alexander. “It’s of course possible to plug a reduced offset fork into any brand’s current bike, but bear in mind that the reduction in the offset also brings a reduction in the front centre of the bike and this isn’t insignificant.”
Dylan Howes is less polarised on the topic, suggesting the difference is less dramatic than some people make it out to be. “As most bikes these days have increased in length, and geometry differences are not necessarily that huge, differences in fork offset can simply be used to tune steering feel”, states Howes. “If the overall geometry and suspension set-up of the bike is dialled, the bike will still be just as good, it will just have a subtle difference in steering feel.”
And Part Two
As we’ve already seen here, there are many different opinions when it comes to fork offset and mountain bike geometry in general. Now that we’ve looked at the perspective of some key bike brands, it’s time for our own back-to-back testing, where I’ll delve into my experience of offset testing.
Just for you
Hey, thanks for being a Singletrack supporter. Because being a member is so important to us we’ve built automatic discounts into all our merchandise – like our new Singletrack Hoodie. Buying our merchandise really helps support us and enables us to create great content like this for you. Subscribing is also really great and being a subscriber gets you even bigger discounts.