Fork offset is a damn tricky topic and like many things in the mountain bike industry, there are options and with options you have varying opinions. We all know how opinions are, everyone has one and they all stink. Diving into some fork offset talk, I first want to say that I am nowhere near an engineer, just a bike nerd who wants to wrap his brain around the options we now have. After reading tons of articles, talking to people in the industry (some very talented engineers), and doing my own testing, I have begun to finally grasp this new topic. While different fork offsets have been a discussion between engineers and race teams for a few years, it has now trickled down to the consumer giving you another option to think about. We all remember when bikes had 3 chainrings in the front, brakes did not brake well, stems were long, and head tubes well...were not slack.
First things first, let's explain what fork offset is and how it affects your ride. Fork offset is the measured distance from the steering axis (straight down the headtube) to the mounting point of the front wheel. By finding the offset and headtube angle, you can figure out the next piece of this large puzzle, trail, which is the distance you get between the contact point of the front wheel and where the steering axis line hits the ground (see diagram below). Both headtube angle and fork offset can change the length of the trail number. By making the head tube angle slacker (decreasing the angle) you will make the trail longer. By reducing the fork offset, you will also make the trail longer. Below is a picture that should help put these definitions into use. We will get into how these affect handling in a bit.
Slacker Head Tube Angle = Longer Trail Number
Reduced Fork Offset = Longer Trail Number
This fork offset has gained recent traction by most notably Transition bikes, but other companies like GeoMetron, Whyte, and even Specialized have been spec’ing bikes with non standard fork offsets for a bit now (hate to use the term standard in this industry). In 2017, Transition fully redesigned their bike line up for their 2018 model year, with a new geometry concept called SBG, Speed Balanced Geometry. This consists of 5 parts: longer reach, steeper seat tube angle, shorter stem length, slacker head tube angle, and last, reduced fork offset. This creates a system that works together in making a bike handle better in more situations. Check out our blog with full on ride review of the SBG technology.
Before Transition tested, made popular, and eventually put into production, the last fork offset revolution was formed by Gary Fisher, and the G2 forks. This started when 29ers were new, and only for spandex racers on XC bikes. Once Gary Fisher experimented, the new longer 51mm G2 offset was set. By increasing the fork offset, Fisher was able to get a more nimble or fast feeling with the front wheel, helping the bike handle more like the 26” that people was used to. Then the 51mm offset forks was the new normal for 29” wheels until last year. From that time when G2 was created, 29er bikes changed dramatically. From XC bikes with high seats and long stems, to fun trail and enduro bikes, companies like Evil Bikes with their The Following started some new trends with low and slack 29” inch bikes. The Canfield Toir (Riot) also was an early 29” fun bike. Both of these bikes have short chainstays, and slacker head tubes with a low bottom bracket height, creating a feeling that was previously not possible with big 29” wagon wheels. Transition’s first generation Smuggler was a great 29” bike that also had some of these traits, and worked great as a light trail, heavy duty XC bike.
While Gary Fishers 51mm G2 offset forks are still spec’d on over half of 29” bikes, the reduced offset is gaining traction and for good reason. With the trend of bikes, especially 29” bikes, of getting longer and slacker, the front wheel gets farther and farther away from you. While a slack headtube increases stability, along with longer reach, you start to lose that front end feel and thus, losing traction. By reducing the forks offset, you are bringing the wheel back under the mass of the body, helping keep that traction, while still having a longer wheelbase and slack head tube angle.
As I had said before, slacking the head tube angle gives you more trail, as well as reducing the forks offset. This is giving bikes a much longer than previous trail numbers, and with a longer trail you will typically have a more stable or slow front end steering. There are far more variables to this as we do not ride on flat terrain, we ride on angles and go up and down, lean the bike over and bring it back up, creating many different scenarios and essentially not possible to test all. All of these can add up to a stable or nimble handling experience, which is why we are now having more options to choose from.
Once we started to test on our own, we found that bikes that are of the extreme geo actually perform better with the reduced fork offset. Timed runs helped determine a bit, but more than that was the rider feel. Most riders who were on a longer wheelbase and slacker bikes (about 66 degrees) felt more comfortable cornering up and down hill with the reduced offset forks.
It will not be a new riding aid - if you are not making the switchback on your normal ride, going with a reduced offset fork will not magically help you make it. However, it could help you get setup better, give you more confidence and smooth handling in that situation and eventually you can make it. It's not black and white, especially with bike geo. It is very much affected by rider style and experience. Some of the fastest riders I know can change fork offset and stem length and they will not notice. Other people I know can change front tire pressure by 2 psi or slightly move lever reach and they suddenly feel weird. Riders are sensitive to many variables and that is a reason why we are making more and more options.
Back to our testing, riding the forks on bike that were designed for them, of course feel great. Now we are putting them on bikes that originally were designed with the 51mm offset forks, bikes like the Yeti SB5.5, Evils The Wreckoning, new Devinci Troy and Santa Cruz Hightower LT, we found these bikes do have improvements with reduced offset forks. Does it handle like a new bike, no. But it does help with that cornering ability, straight line stability and also slow down the front end feel a bit. These bikes are slacker and longer than what we were riding 3 years ago let alone 8+ when G2 came out. Once we put them on bikes that were steeper than 66 degrees, some XC bikes or mid travel bikes, we ended up with a more nervous feeling front end. While the trail was increased, it did not have enough headtube angle to compensate for the front wheel being closer to the body mass. As Chris Porter from GeoMetron bikes said “There’s no substitute for slack head angles”.
Once we put back on 51mm offset forks to these bikes we were testing with 41-44mm offset forks, is when you really noticed where the reduced offset forks were helping out. Getting in and out of corners was benefited while still having the straight line stability these long 29” bikes are known for. We tested forks from Fox at 44mm offset, to Rockshox at 42mm offset, and MRP forks with 41mm offset. Swapping between the two the offset was hardly noticeable, as they are so close. You notice the difference feeling in the fork more than the 1-2mm offset difference. This could also be due to fork stiffness overall, and also where the bike sits in the travel with the different branded forks up front. MRP tends to sit a little more into the travel and while also having reduced offset felt the most different from the stock 51mm offset forks.
You can also calculate the bikes mechanical trail with this formula so kindly explained to us by Steven from MTB Podcast. This is different than what most trail numbers are found, as you can see in the picture below. If companies adopt listing this mechanical trail number, along with headtube angle and fork offset you could get an idea of how the bike will ride. To get the mechanical trail use this formula: Mechanical Trail = Tire Radius x COS (Head Angle) - Fork Offset
Should you go out right now and buy a reduced offset fork? Probably not. As said, it will not magically change your riding experience. There is a place for them, just as there is a place for all bike options. However, I think the bigger handling change is made up from the overall package of geometry. As Transition did with SBG, changing the bikes geo to accommodate slack head tubes and reduced offset forks also takes other changes, and by changing all of them to the full package you get an amazing well rounded mountain bike. If you’re riding a longer, slacker 29” bike, then you can definitely benefit from running an offset fork. Could also just be a placebo effect, and your faster because you think it's better. Either way, bikes will continue to get better and this is a step in the right direction. At the end of the day, a bad day on the bike is better than a good day at work right :)