Reduced offset forks - a hot topic in the ever-changing world of mountain bikes this past year. First they showed up on Transition’s Speed Balanced Geometry bikes, then on the Ibis Ripmo, the Gen 4 Ripley, and now a slew of other bikes. Fork offset has always been a component of bike design, but until last year it was as un-glamourous as the length of your head-tube. This begs the question: why the sudden change in fork offsets, and why does everyone seem to care about it now? To understand that, let’s first take a look at what the "trail" measurement is, and how it affects your bike.
What is Trail?
To better understand the effects of fork offset on your bike, we must first define the term “trail”:
Trail is the physical distance between the contact patch of the tire (sits directly below front axle) and the imaginary point where the bike's steering axis touches the ground.
It is not typically on geometry charts, and many of us don’t think about it, but trail is the primary dimension that changes when you alter a fork’s offset. Trail is also changed by two other traits you’ve certainly heard of: head tube angle and wheel size.
Trail can be increased in the following ways, each of which I will detail and elaborate on below.
- slacker head angle
- larger wheel
- shorter offset fork
1) A slacker head angle = more trail
We understand head tube angles, so let’s start by discussing how changing the head tube angle affects trail. Picture a downhill bike. These are bikes with very slack head angles, which therefore have significantly more trail than all-mountain or XC bikes. One effect of more trail is "wheel flop." If you've ever sat on a DH bike you've probably experienced this feeling. As you’re moving forward, any slight lean to either side will cause a substantial and dramatic “flop” of the wheel. The bike will reinforce your lean as it essentially falls over in that direction. It is pretty much the exact opposite of how a road bike's front end handles - a bike with very little trail. Because bikes with “high flop” (more trail) geometries strongly reinforce the steering input, the slightest input of the rider (simply leaning over) will initiate a turn. In effect, it is a sort of "power steering," or mechanically reinforced steering input.
This is somewhat of a negative during climbing, because if you accidentally lean a bit - which happens a lot when you're using your whole body to try and push up a hill - the bike will start pulling to the left or right. It takes effort to correct the bike's path and continue moving forward.
Bikes with “low flop” (less trail) geometries require a more deliberate input (ie. physically turning the handlebars) but they offer more precise handling and better adjustability of the bike mid-corner. This tends to be better for climbing. When you accidentally get off course, it's easier to make a quick steering correction and get back to center.
2) A larger wheel = more trail
Larger wheels are in fact why we arrived in this whole fork offset situation in the first place. Trail increased when manufacturers moved from 26” wheels to 29” wheels in the late 2000s. Higher offset forks were introduced to compensate and bring trail numbers back to where they were with 26” bikes. This is also why 29’ers have historically had steeper head tube angles than their shorter-wheeled counterparts - a bigger wheel increases trail, and a steeper head angle brings it back down.
3) A shorter offset fork = more trail
Different brands are taking slightly different approaches in their use of reduced offset forks. With their most aggressive trail bikes, the Sentinel and the Patrol, Transition has paired a lower offset fork with an extremely slack head-angle - 64° - resulting in very long trail numbers. Ibis, on the other hand, is using their low offset forks to be able to run a less extreme head-tube angle: 65.9°. Their reasoning is that “a 65.9° head angle feels like 64.5° without increasing the wheelbase. You get the stability of a slack head angle without giving up your ability to go around tighter corners.” This all sounds a bit convoluted, but what they mean is that a 65.9° head angle combined with a 44mm offset fork has the same amount of trail as a bike with a 64.5° head angle and a 51mm offset.
These various approaches work to make bikes with a different ride feel, and there’s no right answer. In fact, you may remember in the early 2010s when longer, 51mm offset forks were first advertised as the next best thing for 29’ers - the original Ripley being a prime example. Times change, as do priorities, and now we’ve gone back to where we started in regards to offset.
What of it?
I’ve spent a lot of time on the Transition Patrol - a high flop bike. That bike’s implementation of a very slack head tube angle and a 37mm offset fork creates a calmer feel in fast, long corners. This is because the bike’s geometry reinforces your steering input.
Especially during my first handful of rides on that bike, I found myself blowing corners more frequently - once it is set in its path, it takes more input to change routes.
It can also be harder to do the oft-photographed berm roost that is constantly gracing the pages of Pinkbike. It takes more effort to get that sharp direction change out of the bike. That’s not a bad thing though. Your Strava times may actually thank you for it, and the trail builders certainly will.
Another related effect of increased trail is that it helps the front wheel keep its path better at speed. Say you’re cruising along really fast, when your front wheel hits an unseen rock. The wheel will deflect and turn, but will come back forward more readily than a bike with less trail. That is the "power steering" effect I mentioned. If you've ever been on a motorcycle, you'll have experienced part of this. At speed, you can really throw your weight around on the bike and it'll continually re-direct itself forward, until you initiate and maintain a concerted lean.
Should you put a reduced offset fork on your bike?
Transition arrived at their geometry numbers through lots of trial and experimentation. Their Sentinel, with its big wheels, 64 degree head angle, and 44mm offset fork, has the same amount of trail as my downhill bike. It helps create a very distinctive ride feel.
But it’s not all that the guys at Transition changed. Front center (which is closely related to reach), seat tube angle and stem length come into play as well. Check out our Patrol review to see all the details, as there are a number of factors that make these bikes feel the way they do.
Other manufacturers have created bikes with similar geometries. The Knolly Fugitive, Evil Offering, and Forbidden Druid are a few that come to mind. Knolly claims you can run the Fugitive with either a 51mm or 44mm offset fork, but recommends a 44mm offset fork. Evil tested the Offering with a variety of fork offsets, and felt that a standard 51mm offset with the 29” wheels felt best on that particular bike. They found the reduced offset didn’t feel quite "right," and are of the opinion that the 51mm offset would best cater to their target user group. The Forbidden Druid is very similar to those two bikes with regards to geometry, but is only available with a reduced offset fork. There’s no right or wrong answer, but all these companies have arrived at their conclusions based on a lot of time experimenting with these forks on their bikes.
There are a number of misconceptions regarding these forks. A common one is the thought that a reduced offset fork will pull your front wheel towards you, significantly reducing the wheelbase length. In actuality, the change is negligible. In the case of a 29’er with a 65 degree head angle, going from a 51mm to a 44mm offset fork will reduce your wheelbase length by 5.78mm. This is a minuscule percentage of a bike’s wheelbase (0.5%) - so minuscule that it is insignificant to most of us. The effect on your bike’s trail will be more noticeable, as it will increase this number by roughly 5 - 6%.
My hope is that you now have a better understanding of how fork offset will affect your bike. If you’re ready to give a reduced offset fork a try, give us a holler and we’ll get you going! Don’t go into it expecting it to revolutionize your experience though. It will feel different, and you may like the change, but unless you’re replacing an old, junky fork, the change in ride quality won’t be enormous. There’s also no guarantee you’ll like the new setup. My advice? If you’re on a tight budget, stick with what you’ve got, and put that money into some components that will make more significant improvements to your ride, such as a high quality wheelset, or an upgraded rear shock. If you’ve got the coin to play, experiment away! Maybe hang onto that original fork for a while to make sure you like the new offset.