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I’m upgrading my fork. Should I change the offset?

Posted on November 10 2018

Image - Bike Radar - 51mm, 44mm, 37mm offset Fox fork crowns

 

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Swathes of new bike models have been released lately and one of the key
features being thrown around is a reduced offset on the fork. What does this
mean and is it a good upgrade for other bikes?

When bike manufacturers tweak the fork offset they’re modifying the trail of the
bike. Trail is the distance that the tyre's contact patch trails behind the
imaginary line that the head tube angle would make with the ground (the
steering axis). By changing the trail measurement, two forces are influenced.
 
 
'Trail' explained. 
The first is the caster force, which is a stabilising force that results in the
wheel self-centring itself. The second is the wheel flop force, which is a de-
stabilising force that increases the tendency for the wheel to fall or flop into a
turn past a certain angle. It is important to note that increasing the trail
measurement increases both of these forces, however, the two forces are
most noticeable at different speeds. At high speed, the dominant force is the
stabilising caster force, where as at low speeds, the destabilising “wheel flop”
force is most influential.
 
With the current industry trend of making every new bike  longer, lower and
slacker than last years model, we now have some shorter travel trail and
enduro bikes that can tackle downhill tracks that would have scared Vouilloz
in his prime.  Slackening the head angle and increasing the reach has
resulted in bikes that are immensely stable at high speeds and through rough
sections but on more mellow, flatter trails getting appropriate levels of mass
over the front wheel to maintain fork sensitivity and front tyre grip can be
tricky. If you’ve ever struggled to keep the front wheel from wandering on a
climb, then you have experienced this. (check this)

Increasing the trail by reducing the fork offset allows for increased stability at
high speed and when combined with the typically steeper seat tube angles
found these days which puts the rider’s centre of mass further forward,
actually helps to improve the slow speed handling on climbs.
 
If you’ve read this far and are thinking “sign me up!” it might not be that
simple. Bicycle geometry is all about balance and the intended riding that the
bike is designed to do.  Designers and engineers spend inordinate amounts of
time pouring over tiny details in head tube angles, seat tube angles, stack and
reach measurements, chainstay lengths, bottom bracket heights, and fork
offsets.  Changing any one of these will have an effect on the handling and
therefore the performance of your bike. The increased wheelbase of newer
bikes thanks to their long reach and the slacker head angles result in slower
handling at low speed and less maneuverability in tight trails so the designers
feel the bike needs that little be extra from the reduced offset to get the
handling back where they’d like. If you ran a reduced offset fork on a bike
with geometry designed around less trail (say a 44mm offset fork on a bike
designed around a 51mm offset) you may end up affecting the handling at low
speed in such a manner that the bike becomes too “twitchy”, compromising
low-speed stability and maneuverability in tight trails for added high speed
stability. Improving one aspect of handling generally comes at the expense of
another aspect. Another interesting point to note is slackening the head angle
by a degree by increasing your fork travel will increase your trail measurement
by a similar amount as changing to the lower offset fork.
 

Hopefully this has given you the information you need to make your decision.
If it’s a compromise you’re willing to make, then we’ll happily get you sorted,
but our recommendation is to stick with the fork offset that the bike was
designed for.