Here's what it is and why you should care.
There’s lots of talk and chin-stroking about fork offset amongst the MTB illuminati at the mo. What is it? Should you know what your current suspension fork has?
What is fork offset?
Here’s a simple-as-possible illustration to explain things…
Key terms explained
- Head angle is an imaginary line through the steering axis to the floor.
- Offset is how far the front axle is ‘offset’ from the steering axis.
- Trail is the distance between the contact patch of the tyre and where the head angle line hits the floor.
The bigger the offset the shorter the trail figure. And vice versa, the shorter the offset the larger the trail figure.
Wheelsize affects trail too. The higher the axle, the more trail there’ll be. Bigger wheels result in larger trail figures.
It’s this reason that 29er forks often come with different offset figures than 27.5in forks. The most common offset for 29ers’ forks is 51mm (compared to 44-46mm for 27.5in forks).
But should they?
It’s the reason that some bikes can feel even shorter than their frame geometry numbers suggest. You need to have generous top tube lengths if running shorter offset forks.
You can buy 46mm RockShox 29er forks aftermarket if you want to by the way.
Watch: Transition Bikes introduces Speed Balanced Geometry
And now there’s Transition entering the game with their new Speed Balanced Geometry and its 42-44mm fork offset.
But let’s not forget – which we did do when we first wrote this article – that getting in there first was Whyte with the custom 42mm offset RockShox Pike forks on the new Whyte S-150 29er.
What’s wrong with having increased trail?
Not a lot in our opinion but the theory is that larger trail figures result in slower steering. Our instinct is – and marketing often yells at us – that slow steering is A Bad Thing. A lot of the bike industry is obsessed with “quick steering” just like they’re obsessed with things being “stiff”.
Now this is arguably an area where what words you choose have an effect on the debate. One person’s “slow” is another person’s “stable” is another person’s “lazy”.
It’s a bit – but not exactly – like having a steep or a slack head angle. Lots of trail results in more stable steering but it also results in the front axle being a bit closer to you. So for all-out straightish speed sections the bike will feel shorter. But on twisty hairpins in steep terrain the bike will carve calmer and hold its line better.
Overall there’s no substitute for a slack head angle and a proper amount of reach/front centre. But if your 29er bike feels a bit tucking-in and fold-y on tight, steep terrain, you may wish to check what your fork’s offset is and think about swapping to a shorter offset fork.
You’ll lose a bit of wheelbase but it may be a price worth paying if you’re all about the steep and twisty stuff. You’ll also need to think about how long your bike’s reach is. If your bike is on the short side, it’s arguably better to stick with the 51mm offset.
Fork offset is the new wheelsize
In other words, it’s yet another thing we can all needlessly argue about. Woo!
We’ve all got pretty used to hearing that slacker head angles make a bike handle better when descending, but really that’s only part of the equation.
Working out how your bike steers is more complex than a Countdown conundrum.
It’s that man again
Fork offset, combined with head angle, affects something called ‘trail’, and ultimately whether it’s a dog or a whippet on cdertain descents, thus explains bike fettler extraordinaire Chris Porter from Mojo.
A bike feels great when descending if it has steering that is stable at high speeds and when in the turns. You get that from having a large trail figure, something you can achieve either by running a shorter offset or making the head angle slacker.
Chris has been experimenting with head angles for years now, culminating with the 60° Geometron we featured in the March 2016 issue. But he’s also started messing with fork offset, bolting on different upper assemblies to change the position of the axle relative to the head angle, even going so far as running a crown and steerer backwards!
“There’s no substitute for slack head angles,” Chris says. “The painful truth is that single crown forks flex more than dual crown forks (a greater problem as you rake out the head angle) and we can compensate for this, and the subsequent nervous steering, by reducing offset, increasing trail and calming that steering down.”
“It’s something we learned years ago with mountain bikes and something we keep re-learning every time we watch a DH World Cup — slack head angles work,” Chris says.
“But you’ve heard of ‘finite element analysis’? Well, riding a bicycle has infinite variations and inputs [never mind riders!] so predicting how a bicycle will handle when you change one variable is even beyond the physicists… And if they can’t figure it out, then FFS don’t listen to me!”