'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
•
outsiderbikes.com 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.
InstallationI 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.
Ride ImpressionsLike 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 | Desperate to adjust your fork offset? You won't go wrong with these crowns from Outsider Bikes; they offer a level of fine tuning adjustment and some future-proofing protection against ever changing standards.— Paul Aston |
I’ve been running the same angleset for years tho and as long as you clean it once in a while and keep a good amount of preload on it the creaks will go away. @Flowcheckers:
I used a Cane Creak version for a while and couldnt ever get it right - I think headtube length plays a part in if you ever get one to be quiet - the Works one on the other hand has been trouble free.
I absolutely hate creaking noises, like Cane Creeks standard headsets though.
PS: Thanks Paul for the review!
Thanks Jason and Greg... ????
I have 64 degree head angle this made the contact patch feel like it was under my hands. I have constant traction on my front it almost feels auto steery at first because the traction.
For me I won't be flipping it back. I have a 160mm 27.5 Ribbon air BTW.
My take away from my experience with them was that there is no “one” offset that is always perfect. If you ride different DH tracks with really different characteristics like Sea Otter “DH” vs something more like a WC DH track then these let you optimize for the extremes. Or, if you always ride the exact same stuff these let you experiment to find optimal, then stick with it. Oh, and they never creaked.
Sold it not so long after. Had it for year, only 3 warrenty frames.
By my calculations, the Pole Machine with 51mm offset fork has 156mm ground trail...
If you only sketch it, you immediately see that doubling the ground trail implies doubling the turning radius of bicycle. From there comes my feeling of distant steering, in my opinion. I'm going to test this on Banshee Rune with stem correctly oriented forward.
If the longer offset is that much worse its surprising the new Fox 49 has 58 mm offset and nobody really complained about it. Could the increased offset be an advantage on some of the straighter world cup tracks?
"If you look at the SuperMax axle, you can see that there is a big offset of the dropout there. What that does is allows us to kick the head angle back to a relatively slack, 69 degrees, which if you kept that with a 45-50mmmm rake on a 29er, you would have the most sluggish handling bike in the world. It would be great at high speed, but the second it slowed down it would just be a pig. By kicking the head-angle out and kicking the fork rake out, it does the counter-intuitive thing of reducing the trail."
"Why don't you just eliminate the trail then to make slack bikes that handle incredibly quickly?"
"You need trail to offer some stability to the bike. Without it, the front wheel would behave like the wheels on a shopping cart. It is the balance of these subtle ingredients that separates great handling bikes from the rest."
Lots of different opinions out there, here is the most interesting article I've read about it:
nsmb.com/articles/short-fork-offset-whats-it-good
This isnt true, there is no 'inherant stability' from any given wheel-size, a combination of factors give a bikes apparent straight line stability - think about how stable a motorcycle is at much greater speeds and the size of the wheels.
If you want to understand trail, fork offset, head angles etc take a quick look at this yojimg.net/bike/web_tools/trailcalc.php
Essentially differnt wheel size = different trail figure and because of this the forks offset is altered to 'compensate' in an attempt to bring trail back to around the figure that we were all used to with smaller wheels.
The point here is that as head angles have got slacker, reach longer etc etc that we may need to re-think the point at which this lies and therefore you see this kind of crown - allowing reduced (or even increased) offset and therefore trail adjustment.
From my first comment, where I state wheelsize doesn't make much difference:
It's like fox wanted to do everything possible to help 29ers handle better, but in reality wheel size doesn't actually make that much difference to agility, it's more down to geometry.
And motorbikes wheels and tyres weigh more than an entire mountain bike, of course they'll be more stable, but it's gyroscopic stability, a bigger diameter or heavier flywheel is more stable, so 29 will be more stable, its just not noticeable to the average rider.
My point about motorcycles and yours about gyroscopic effects are irrelevant to my point about 'stability' of the bicycle - If the trail figure of a motorcycle is miles away from where it should be the bike could be unrideable, regardless of wheel weight or dynamic stability - This is also relative to the entire bikes weight, of course motorcycle wheels and tyres weigh more, so does the bike itself.
This discussion is about geometry changes from fork offset and therefore resultant trail, not gyroscopic aspects of wheel size, wheel weight etc.
That said, this isn't even a concept, just a rough idea. Not meant to replace the dual crown fork solution here but simply thinking of a way of how to realize this with a single crown fork as well.
Toptube or downtube I can understand, but I’m struggling with how the headtube can be contacted.
You can see what he means in the picture with the crowns reversed.
Small differences in crown offset is irrelevant in the real world of «modern» bike geometry where you are «lying» between the wheels and weighting/unweighting the front and throwing the whole bike around. The relation between front wheel contact point and bike contact point (handlebar position) is much more relevant for steering feel. That is, unless you are sitting upright turning circle on you 73 degree head angle yo eddy.
Well ... I like tinkering... with bikes
New bike technology continuously solving problems that don’t /shouldnt exist..