What The H*11 is Fork Trail? - Steering Geometry Sort of Explained

Head angle, Fork Offset, Trail, Confused? Maybe, curious as to what these terms mean and how they affect bike handling?

This time in the shed we explore MTB geometry and more specifically steering geometry. What are the variables that affect it and how they translate to a bikes handling on the trail.

If you have any questions about MTB terminology and geometry or have a suggestion for what you would like me to fix next in the shed, then leave your suggestions in the comments.

Thanks for watching!

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[nathanclair]

You mention that the slack head angle makes the bike take bumps better, which makes sense. but then you say to shorten the offset as well, to give the bike "lighter" handling. Slacker head angle and shorter offset would lead to larger and larger trail numbers, which leads to "floppy" and laborious turning, especially at low speeds, does it not?

[Jtheth]

Trail is affected by 4 factors
: Head Angle, Fork Offset, Rim Diameter, Tyre Width



Tyre width example
65deg head angle, 51mm fork offset, 29er rim with 2.3" tyre has a trail of 118mm
65deg head angle, 51mm fork offset, 29er rim with 2.6" tyre has a trail of 121mm
This is a 2.5% trail increase

Head angle example
65deg head angle, 51mm fork offset, 29er rim with 2.3" tyre has a trail of 118mm
66deg head angle, 51mm fork offset, 29er rim with 2.3" tyre has a trail of 114mm
This is a 3.3% trail decrease

Fork Offset example
65deg head angle, 51mm fork offset, 29er rim with 2.3" tyre has a trail of 118mm
65deg head angle, 44mm fork offset, 29er rim with 2.3" tyre has a trail of 126mm
This is a 6.7% trail increase

Rim diameter example
65deg head angle, 51mm fork offset, 29er rim with 2.3" tyre has a trail of 118mm
65deg head angle, 51mm fork offset, 27.5 rim with 2.3" tyre has a trail of 109mm
However since 51mm fork offset is not common on 27.5 forks the example below is realistic
65deg head angle, 42mm fork offset, 27.5 rim with 2.3" tyre has a trail of 119mm
This is effectively the same trail number between the 29" wheel and 27.5" wheel

Ride quality is seriously influenced by the fork setting and fork behavior
since it changes angle while compressing and rebounding, which is also changing trail

[hogdog]

There are three variables to trail, I'm not sure why you didn't mention wheel size.

[Fastfitnesstips]

Great video, do you mind me asking where did you get those bike vector animations? really helps!

[josephnubile]

so for my hardtail build i will be using a nukeproof scout with a 65 degree head angle and a fox 36 fork and the bike will be on maxxis minion 2.8 plus tires on a 35 mm internal rim. so if i understand your video i should run a 44mm offset instead of the 51mm offset for the best performace?

[JerryWDaviscom]

That was a damn good explanation. I took my Dback hard-tail w 67.5 degrees of head angle out yesterday for the first time in a month (lots of rain lately), and crashed twice. Rough crashes, too. The slack head angle was throwing me off, even though I ride that bike a lot. This last month, due to rain, I rode my trainer bike on streets and trails, and it has a steeper head angle and a similar offset. The handling is tight and crisp on my trainer, and I ride technical sections better on it.

I don't see a way to reduce the offset on my Dback unless I turn the fork around backwards, which will do as you say - bring my hub back towards me, tighten my steering and retaining the head angle. What do you think?

[richarddort]

Really nice video but, 68 or 69 dgree HTA are not old school XC numbers. 70-71 are, and many WC pros are riding bike with HTAs that steep. While GT have been running 69.5 on their FS XC bikes for the better part of a decade and Turner has had theirs at 68 for who knows how long, these are exceptions. 68.5 to 69.8 would be new school xc geometry, which was trail geometry 5 years ago.

[GameStaticOriginal]

I got giant glory dh 2008 and my head angle is 55, is that dangerois, what could happen? I have 190mm fork and 222mm rear shock