'Fork-offset' or 'rake' is a complicated subject that involves many factors, including head angle, fork length, stem length, wheel size and the 'trail' number generated. If you don't understand offset, you can read more here about the basics
Until fairly recently, the general trend was to make offsets longer to speed up steering response and reduce steering flop as head-angles get slacker and wheels get bigger. Murray Washburn was known to be a fan of increasing offset on bikes like Cannondale's FSi (the new Lefty has 55mm offset for 29", a long stem and a 69º HA) for this reason, but slack for XC is a different kettle of fish than slack for downhill.
Adjustable Crowns Details
• Available for Rockshox Boxxer up to 2018 and Fox 40/49
• Up to -10mm / +8mm of adjustment
• 4 different sets of inserts
• Stem length location holes: -8mm, 0mm, +8mm
• Weight: 526g (Fox 49 version, uncut steerer)
• MSRP: $235 USD Rockshox / $285 USD Fox
Whoever decided what number the offset would be for their brand's 29" XC fork generally copied and pasted it across to all the forks in the range, partly because it was easy, and partly to meet OEM supply demands from bike brands. As far as my research goes, this was always aimed towards bikes that might want increase agility around an XC race track (or bike shop car park) rather than modern bikes looking for stability, speed and being leaned into corners rather than steering around them.
There are several other things to consider, including the fact that bikes of the past had high bottom brackets, slack seat angles, steep head angles, narrow bars and long stems. There has been a lot of change over the last five years, not only with bikes, but the riders and what type of terrain they want to attack. Some of the things that offset may have been used to correct may have been improved in other ways. For example, steepening seat angles can massively improve your front wheel steering precision as your center of gravity is moved forwards between the axles.
A few years ago, experimenting with shortening
fork offset to tune trail and handling was relatively unheard of outside the workshop of Chris Porter's Mojo Rising
(formerly Mojo Suspension, UK Fox Distributor), but his work has since led to Whyte Bikes and Transition working on the shorter fork concept simultaneously, with Whyte releasing their S-150 and Transition's range-wide Speed Balanced Geometry shorter offset and geometry combination. Nowadays, talk of fork offset is commonplace, with most companies' enduro and trail bikes using shorter offset forks than a year ago.
This Outsider Bikes adjustable offset crown for the Rockshox Boxxer (up to 2018 model) and Fox 40 is one of the first methods for adjusting fork offset to hit the market, and it promises -10mm / +8mm of adjustment in 2mm increments, using 4 different sets of inserts, at a weight similar to the stock crowns. The top crown also gives -8mm, 0mm, +8mm adjustment in stem length thanks to extra direct mount holes.
They retail for $235 USD for the Boxxer and $285 for the Fox 40 version. Installation
I don't want to give a full explanation of how to install this crown system, firstly because there is a great guide on the Outsider Bikes website
and more importantly, reducing the offset too much can
cause serious and dangerous problems like the fork's arch hitting the headtube of some bikes – this is explained on the frame compatibility guide here
But to be brief, it is simple to change the original crowns and preload and torque the headset and bolts in the usual way. The only extra job is to choose the inserts and orientate them correctly, then make sure the steerer is clamped to the lower crown securely before preloading the headset top cap.
Like mentioned above, offset and trail are part of a wide-ranging subject that has numerous factors that depend on head angle, wheel size, fork length, and the dynamic chassis angle of the bike. The test in this instance used a Saracen Myst 29" downhill bike, with a Fox 49 (standard offset = 58mm), and a head angle on paper of 63º, a 50mm stem, and 800m handlebar.
For the purposes of testing, we used the most extreme red inserts to test the outer limits, with the initial runs using the long position to give a 66mm offset. Secondly, we flipped these inserts to test with a 48mm offset, and then, for the sake of testing, rotated the crowns 180º and the inserts again, which reduced it to 28mm (interestingly, similar to an MX bike). The following are generalizations of what I found on the trail:
• The longest offset made the bike the most unstable and twitchy, and I could feel the contact patch moving forwards and backward along the tire when turning the handlebar. It was also harder to lean the bike over and keep it leaned over in the corners. In addition, the front wheel was easier to start understeering or washing out on flat corners.
• The medium 48mm offset was more stable, the contact patch of the tire seemed to stay in the same place and felt more 'normal'. I could lean the bike over more easily and keep it there, and while leaned over there was a smoother arc to the turn and less micro-adjusting of the steering. I could push harder into turns before starting to lose traction on the front wheel. This actually felt more similar to some of my enduro bikes with downhill angles (Nicolai Geometron / Pole / Raaw) with sub-50mm stems, and the 44mm / 46mm of offset that I find to offer a more confident front end than most of the current downhill bikes.
• The following is not recommended:
The silly reversed crown setup that gave a 28mm offset was interesting, but couldn't be tested to its full potential as my steering lock (the Myst has a chunky headtube area and frame bumpers to protect from the fork) was reduced to the point of danger and there was a risk of the crown hitting the headtube. We also had issues with the direct mount stem hitting the steerer tube spacers, and the bike was getting too short for me at this point, which made riding it more difficult. But, it was super stable at speed and seemed to deflect much less off angled wet rocks and roots; it also leaned into corners like a champion but slowed the steering response at lower speeds.
Running the crowns backward for a seriously short offset is not recommended, but, you can do some weird stuff with these crowns if you really want to...
Conclusion? Reducing the offset slightly on the current crop of 29" downhill forks seems to be good idea to increase stability, front wheel grip, and lean angle, and I think this is backed up by many manufacturers jumping on the bandwagon with plenty of aggressive trail bikes specced with a shorter offset for 2019. Going super short may not be a great idea either, as the bike starts to feel more sluggish – but I prefer my bikes to be more stable against incoming terrain, and if I want faster steering, I will just, err, turn the bars faster, or work on leaning into the corner instead of steering around it.
Do you need to get a set of offset adjusting crowns? Probably not, your bike will be great as normal, and adding this level of adjustment may not really make a difference to your ride. But, if you are a serial-fiddler who is in tune with your bike, then these are a great way to fine-tune your bike's handling. I'd also bet that Fox and RockShox will drop the 58/56mm offset 29" forks for something a little shorter very soon, I think we could start seeing 40-45mm offsets on downhill bikes with 29" wheels and matching stems soon, and probably a bit more added to the reach numbers of some brands to keep the sizing correct. 27.5" may go even shorter.Pinkbike's Take