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Setup extras: Fork Offset and Swing Arm Length

Started by Asdrael, March 19, 2017, 01:52:01 pm

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Asdrael

March 19, 2017, 01:52:01 pm Last Edit: March 19, 2017, 09:22:39 pm by Asdrael
So we have a good suspension setup guide from Stonerider found here: http://forum.mx-bikes.com/index.php?topic=1463.0

... but why is noone talking about ForkOffset and Swing Arm Length?

Those two settings have, in my experience with OEM bikes, the most influence on the actual front and rear end behaviour of the bike - their combination resuting in drastic changing in feel and balance.

Those settings can be found in the Garage / Others tab / Geometry section.

Fork offset.

It's the distance between the fork and the steering axis. Most bikes have a default value between 22 and 25mm stock. Your offset value adds/removes from that. A 2mm change is already pretty big - 10%.

Decreasing this distance by putting a negative OffSet value generally:
  • Shifts the bike weight to the front, increasing front grip, decreasing rear grip.
  • Increases bike handling, giving a feeling of more controlled and sharper turning


If you find your chosen bike a big slugish with a fuzzy front end, go with -2 and that'll do wonders. Conversly, if you feel the rear end goes everywhere and only the front is reliable, +1 or 2 will change that.


Swing Arm Length.

This is the distance between the Swing Arm pivot point and the rear axle. A standard swingarm is around 590mm. The range of change is around 30mm. There are three positions: middle (1), short (0) and long (2).

A longer swingarm:
  • Shifts bike weight to the front
  • Tends to lessen any excessive wheelie issue
  • Increases turn radius
  • Gives a bit of a sluggish feeling when turning
  • Makes the rear less twitchy when powersliding - the rear wheel will not move as much
  • Makes the rear more mobile twitchy when it starts moving while breaking hard from the front


If you find you are losing the rear uncontrollably, try to go with a longer swingarm. Conversly, if you feel the bike is very rear heavy, a setting of 0 will help. Do not forget that changing the SwingArm Length changes how the rear shock is linked to the wheel! You will need to adjust your preload!

Setting interaction.

As you can see, the impact of some SwingArm settings overlap with the Fork Offset settings. But not all. So you can really impact the behaviour of the bike this way, and a chassis you don't gel with but love the engine off can be quite extensively modified to suit your style better.

Generally, a change of 1 step in swingarm length changes the weight distribution as much as 2 steps in fork offset.

As a rule of thumb, this is valid only when your suspension are properly tuned. A change in offset / length will never stop a fork from bottoming out, or a shock from being too hard and making you lose grip because it skims over terrain. Set you sag, get a good ballpark of Bump/Rebound values. THEN change fork offset and swingarm length. You will maybe fine tune the suspension again afterwards, but you'll be at least working in the right direction.


Hope that helps!
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StoneRider

thanks man :) thanks a lot ! we need you to redo all my tutorial !! xD

HornetMaX

I thought a longer swingarm gives a bigger turn radius. Typo or am I wrong ?

Asdrael

Quote from: HornetMaX on March 19, 2017, 08:52:32 pm
I thought a longer swingarm gives a bigger turn radius. Typo or am I wrong ?


You are correct, I did a typo - I was thinking that it lowers the performance in terms of turn radius and typed it too fast.

Longer swingarm - bigger radius.
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