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Some forks come in a variety of different fork offsets for the same model.

How does replacing the fork designed for the bike with:

  • A fork with smaller offset, everything else the same
  • A fork with bigger offset, everything else the same

Affect the handling of the bike, specifically for downhill and enduro mountain biking.

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  • This is a good question but it's got a lot of contentious points within. There are different eras of dogma about offset and they can collide in discussions about it. If you insist on framing the question in terms of what will it do if all else is equal, the answers to that aren't necessarily useful in figuring out what the best design is overall. Also, this question in practical terms harkens to the two very different areas in cycling where bigger offset numbers have been revisited in recent years; low trail rando bikes with handlebar bags and mountain bikes. – Nathan Knutson Nov 16 '20 at 4:22
  • @NathanKnutson As a matter of fact, MTBs are moving towards less offset. Early 29ers used 51mm offset forks, while the norm is around 44mm now, with forks available in the high 30s as well. Apparently, smaller offsets work well when combined with slack HTAs. – MaplePanda Nov 16 '20 at 5:09
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Offset primarily affects your mechanical trail - that is, the distance between the imaginary straight line going through your head tube down to the ground, and your tire’s contact patch. Normally, the contact patch is behind the steering axis, hence the nomenclature “trail”.

enter image description here Image from Wikipedia

Increasing the fork offset will bring the contact patch forwards, so trail is reduced.

Decreasing the fork offset will bring the contact patch rearwards, so trail is increased.

Trail affects the steering, as it is basically a measurement of the amount of influence your steering inputs actually have on the contact patch.

Also related are the effects of wheel flop generated by your head tube angle and fork. Since your HTA is (hopefully) not vertical (90°), your wheel will pivot side-to-side when you turn the bars, which adds stability. Among other reasons, this is why head tube angles get slacker as MTBs get more aggressive. Furthermore, fork offset is measured perpendicular to the steering axis. Since your steering axis is angled because of the head tube angle, the fork's offset actually has both a vertical and horizontal component referenced against the ground. The vertical component exaggerates the influence of wheel flop on the steering.

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    @leftaroundabout trail slows the steering down, in your case flipping the fork would have slowed steering response so much it becomes hard to balance. I agree that the statement “feels more stable” - is subjective and could be expanded. – Rider_X Nov 16 '20 at 1:52
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    I did a quick test on my MTB by spinning the bars around, and I found that it lifted the front end of the bike up. That should be a result of the non-vertical HTA combining with the offset (the offset is perpendicular to the steering axis, so there’s a vertical element to its effect relative to the ground). @leftaroundabout, you would be correct in experiencing extremely twitchy steering - the fork is constantly trying to swing around back to the lower, forwards position. – MaplePanda Nov 16 '20 at 4:27
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    There's a second effect called wheel flop that makes wheel with positive caster angle and high trail unstable at low speeds. If you look at the old derny racing bikes with reversed forks, they do also have very steep angles. – ojs Nov 16 '20 at 10:31
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    @MaplePanda I read that as a response to the question stating “the downside is obvious (it makes the bike harder to control)”. Anyway: my point is that “more trail will feel more stable” is an oversimplification that's just wrong as such, you should at least edit the answer to also mention the flopping effect. – leftaroundabout Nov 16 '20 at 17:00
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    @MaplePanda before this turns into universal truth I'd like to add that they're riding around velodrome, following a motorcycle. Velodrome events where you have competitors next to you require fast handling, and normal track bikes are famously twitchy because of that. – ojs Nov 17 '20 at 9:13
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All things being equal, a longer fork offset yields a net longer wheelbase whereas a shorter one lends itself to a shorter wheelbase.

Wheelbase is an important factor in determining the handling and ride characteristics of any bicycle. Broadly speaking, bikes with longer wheelbases lend themselves to stability (DH Bikes) and comfort (cruisers). On the other hand, bikes with shorter wheelbases are twitchier and more responsive at the expense of stability.

However, it is also important to consider that increasing the fork offset will inevitably affect your headtube angle making it slacker or steeper when the fork offset is longer or shorter respectively.

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    Offset will also impact trail, which is another important factor in handling. – Rider_X Nov 15 '20 at 18:42
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    The offset will not only impact the trail, it's the only effect that's really relevant. An offset that changes the trail enough to dramatically influence the steering behavior of the bike, will not influence the wheelbase by any significant amount. Same with the headtube angle. Wheel base and headtube angle are mostly determined by the frame and the length of the fork, not by the offset. – cmaster - reinstate monica Nov 16 '20 at 10:37

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