Varaxis
Well-Known Member
If you ride a 29er, you have another option to consider when it comes time to get a new fork. If you have ever tried shopping for forks, there probably was an overload of choices and differences that you had to consider, from spring type, damper type, damper controls, travel adjust, stanchion diameter, dropout/axle type, steerer, stanchion coating, remote, dual crown, etc. To add to the confusion, you now can choose between a 51mm offset and a "standard" offset.
Offset, also known as rake, is the distance the wheel's axle is offset from the steering axis (shown as 1). The primary purpose of offset is to alter the "mechanical trail" (the measurement shown as 2). Mechanical trail is the perpendicular distance measuring how far behind the wheel contact point is compared to the steering axis. To understand what offset can do for you, you should understand what mechanical trail does. As a minor side effect, increased offset also increases front center, which increases the wheelbase and increases toe overlap clearance.
If a wheel trails the steering axle, it is pulled by the steering axis and straightens out behind it when you roll forward. This "auto-centering" effect is stronger the longer the mechanical trail is, which makes it easier to go forward in a straight line. With a weaker effect, from a shorter mechanical trail, steering is made lighter, as you do not have to counter (fight) the "auto-centering" effect to steer. To put this in another perspective, the rear wheel on a mtn bike also trails the steering axis, but has perhaps 1000mm of mechanical trail, instead of the ~85mm normally found the front. This makes its auto centering effect exceptionally strong, and is an effect top bike handlers take advantage of when they get sideways, knowing that it will straighten out if they keep the front rolling and the rear wheel on the ground, and is strong enough to pull your legs and hips along if they are sideways with it.
Understanding how it plays a role in your riding experience is the tricky part. Having straight line stability at high speed is good, especially on rutted descents, but having light and responsive steering is good at low speed, especially on climbs and tight switchbacks. For some riders, it's a preference based on familiarity, while to others, it may be a performance enhancement for the way they ride. With 29ers, you already have a relatively long wheelbase for stability at speed and you may desire more responsive steering for lower speed sections where you have to rotate your handlebars sharply and precisely, where the auto-centering effect might pull steering in one direction or wheel flop pulling steering in the other direction if you go to far. A rider that demands more offset might be one that was annoyed by the excessive force needed to fight auto-centering, which may had lead to over-correction in steering which led to a wheel flop, which was over-corrected into a wheel flop in the opposite direction, creating a loop of over-corrections, perhaps when going around a rider who decided to dismount on a steep climb just ahead and trying to go around and not steer into obstacles off the trail.
Bike makers making use of custom fork offset has made offset go from something the fork manufacturers set as a standard, to something customizable by consumers. Gary Fisher pioneered 29er geometry, calling his "holistic" system of considering fork offset, HA, wheelbase, longer front center, shorter head tube with internal headset to minimize stack, and shorter stem length the G2 system. G2 custom forks with 100mm travel or more have 51mm offset. The next progression is Cannondale's system, with the Trigger 29 with Lefty Supermax. Since Cannondale makes their own forks, they could take full control of offset, offering 60mm of offset in their new 29er Supermax, transferring some of the offset in the crown to the axle to allow for shorter stem lengths, compromising less with HA and other geometry figures. Ibis designed their Ripley around the use of a 51mm offset fork, which at the time was only available from the Fox F29 34 fork as standard. RockShox only now offers a widely available 51mm option with their Pike 29er fork.
One may wonder why 29er forks have more offset than 26" forks, or why some 29ers with "XC head angles" feel more like trail bikes rather than cyclocross bikes. That is because as the wheel diameter increases, the mechanical trail increases (cylcocross bikes have a smaller wheel diameter, despite using 700c rims like 29ers, due to smaller tires which may measure out to be 26-27" in total diameter). For a 29er with a 68.5" head angle to have the same steering feel (mechanical trail) as a 26" bike with a 68.5" head angle, the 29er fork would need to have about 13mm more offset than the 26" bike. Considering many 100-140mm 26" forks have 39-40mm of trail (considering a 26.4" diameter, estimated with a 26x2.0 tire), it would take 52-53mm offset to achieve that feel (considering a 29" wheel diameter). I presume fork makers increase offset with longer travel forks to offset the mechanical trail increase with slacker head angles.
All this info hopefully allows you to consider one more spec to make a better informed decision regarding a fork. It may be tough to find offset specs and it's not really clear on how to accurately calculate mechanical trail considering dynamics such as how wheel diameter changes according to tire size and tire pressure, and head angle changes with different axle to crown lengths (such as sag or travel use), if you discover that 80mm or 75mm mechanical trail is your sweet spot on one bike and want to achieve that on another. It's hard to find Fox and RockShox offsets, but they are generally conservative with offset with 40mm offset on the 26" Pike, 42mm with the 27.5" Pike, 39mm offset on 26" Fox 100-140mm forks, 44mm offset with 29er 32 forks. Marzocchi lists offset on their site. Manitou and X-Fusion tend to be more liberal with offset than Fox and RockShox, with Manitou Tower (29er) forks having 48mm offset and X-Fusion 27.5" forks with 46mm offset as examples. If you have ridden a G2 Trek/Gary Fisher do not characterize the steering on those bikes as what 51mm offset feels like; instead, characterize the steering of your bike right now and consider how it would feel with lighter steering, or perhaps you want to increase travel by 20mm and want to offset the effect the slacker HA would have on your steering, since you are happy with the steering feeling, you can opt to go from a 29 120mm fork with 44mm offset to a 140 fork with 51mm offset.
Offset, also known as rake, is the distance the wheel's axle is offset from the steering axis (shown as 1). The primary purpose of offset is to alter the "mechanical trail" (the measurement shown as 2). Mechanical trail is the perpendicular distance measuring how far behind the wheel contact point is compared to the steering axis. To understand what offset can do for you, you should understand what mechanical trail does. As a minor side effect, increased offset also increases front center, which increases the wheelbase and increases toe overlap clearance.
If a wheel trails the steering axle, it is pulled by the steering axis and straightens out behind it when you roll forward. This "auto-centering" effect is stronger the longer the mechanical trail is, which makes it easier to go forward in a straight line. With a weaker effect, from a shorter mechanical trail, steering is made lighter, as you do not have to counter (fight) the "auto-centering" effect to steer. To put this in another perspective, the rear wheel on a mtn bike also trails the steering axis, but has perhaps 1000mm of mechanical trail, instead of the ~85mm normally found the front. This makes its auto centering effect exceptionally strong, and is an effect top bike handlers take advantage of when they get sideways, knowing that it will straighten out if they keep the front rolling and the rear wheel on the ground, and is strong enough to pull your legs and hips along if they are sideways with it.
Understanding how it plays a role in your riding experience is the tricky part. Having straight line stability at high speed is good, especially on rutted descents, but having light and responsive steering is good at low speed, especially on climbs and tight switchbacks. For some riders, it's a preference based on familiarity, while to others, it may be a performance enhancement for the way they ride. With 29ers, you already have a relatively long wheelbase for stability at speed and you may desire more responsive steering for lower speed sections where you have to rotate your handlebars sharply and precisely, where the auto-centering effect might pull steering in one direction or wheel flop pulling steering in the other direction if you go to far. A rider that demands more offset might be one that was annoyed by the excessive force needed to fight auto-centering, which may had lead to over-correction in steering which led to a wheel flop, which was over-corrected into a wheel flop in the opposite direction, creating a loop of over-corrections, perhaps when going around a rider who decided to dismount on a steep climb just ahead and trying to go around and not steer into obstacles off the trail.
Bike makers making use of custom fork offset has made offset go from something the fork manufacturers set as a standard, to something customizable by consumers. Gary Fisher pioneered 29er geometry, calling his "holistic" system of considering fork offset, HA, wheelbase, longer front center, shorter head tube with internal headset to minimize stack, and shorter stem length the G2 system. G2 custom forks with 100mm travel or more have 51mm offset. The next progression is Cannondale's system, with the Trigger 29 with Lefty Supermax. Since Cannondale makes their own forks, they could take full control of offset, offering 60mm of offset in their new 29er Supermax, transferring some of the offset in the crown to the axle to allow for shorter stem lengths, compromising less with HA and other geometry figures. Ibis designed their Ripley around the use of a 51mm offset fork, which at the time was only available from the Fox F29 34 fork as standard. RockShox only now offers a widely available 51mm option with their Pike 29er fork.
One may wonder why 29er forks have more offset than 26" forks, or why some 29ers with "XC head angles" feel more like trail bikes rather than cyclocross bikes. That is because as the wheel diameter increases, the mechanical trail increases (cylcocross bikes have a smaller wheel diameter, despite using 700c rims like 29ers, due to smaller tires which may measure out to be 26-27" in total diameter). For a 29er with a 68.5" head angle to have the same steering feel (mechanical trail) as a 26" bike with a 68.5" head angle, the 29er fork would need to have about 13mm more offset than the 26" bike. Considering many 100-140mm 26" forks have 39-40mm of trail (considering a 26.4" diameter, estimated with a 26x2.0 tire), it would take 52-53mm offset to achieve that feel (considering a 29" wheel diameter). I presume fork makers increase offset with longer travel forks to offset the mechanical trail increase with slacker head angles.
All this info hopefully allows you to consider one more spec to make a better informed decision regarding a fork. It may be tough to find offset specs and it's not really clear on how to accurately calculate mechanical trail considering dynamics such as how wheel diameter changes according to tire size and tire pressure, and head angle changes with different axle to crown lengths (such as sag or travel use), if you discover that 80mm or 75mm mechanical trail is your sweet spot on one bike and want to achieve that on another. It's hard to find Fox and RockShox offsets, but they are generally conservative with offset with 40mm offset on the 26" Pike, 42mm with the 27.5" Pike, 39mm offset on 26" Fox 100-140mm forks, 44mm offset with 29er 32 forks. Marzocchi lists offset on their site. Manitou and X-Fusion tend to be more liberal with offset than Fox and RockShox, with Manitou Tower (29er) forks having 48mm offset and X-Fusion 27.5" forks with 46mm offset as examples. If you have ridden a G2 Trek/Gary Fisher do not characterize the steering on those bikes as what 51mm offset feels like; instead, characterize the steering of your bike right now and consider how it would feel with lighter steering, or perhaps you want to increase travel by 20mm and want to offset the effect the slacker HA would have on your steering, since you are happy with the steering feeling, you can opt to go from a 29 120mm fork with 44mm offset to a 140 fork with 51mm offset.
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