What happens when you add fork travel?


Warning: For serious MTB nerds.

Lately I’ve been doing a lot of RideLogic™ bike setup consultations for members of the Lee Likes Bikes MTB School.

The school has a simple method and detailed calculators to help you dial in your bike for your body and riding style. One service I offer members is a $49 consultation. You tell me what you’re trying to achieve, and I do the calculations for you. This is a great way to dial in your current bike or choose your next bike — with total confidence. A lot of people spend thousands of dollars on bikes that don’t fit them. Not cool!

To run the RideLogic™ calculations I need accurate head angle and frame reach/stack numbers. When people install longer-than-stock forks, those numbers change, so I have to do some pre-calculation calculations.


Today I helped Shawn P. check the fit of his 2011 Transition TransAm 26″ with a FOX 36 160mm fork. The published geometry numbers use a shorter fork, so I had to model his new geometry.

When you add length to your fork, what happens to your bike’s head angle? And what happens to your frame’s reach and stack? What about your bottom bracket height? Your wheelbase?

Adding 27mm to the TransAm’s fork’s axle-to-crown height:

• Slackened the head angle by about 0.9 degree.

• Decreased the frame’s reach by about 15 mm.

• Increased the frame’s stack by about 10 mm.

• Raised the bottom bracket by about 10 mm.

• Lengthened the wheelbase by about 9 mm

Huh? A taller fork doesn’t just make the front of the bike higher?

It does, but it’s more complex than that. When you add height to your fork, the entire bike rotates backward around the rear wheel. This brings the front of your frame up and back, and it kicks your front hub forward. Like so:

Click for big.

Anyway, that’s today’s bike-nerd fix.

To choose and dial in your bikes (and learn to ride them!), check out the Lee Likes Bikes MTB School.

Have fun out there,

Lee


Know more. Have more fun!

Join the leelikesbikes mailing list:





4 replies
  1. Rob Lawrence says:

    Thanks heaps for the detail, Lee !
    Similar but different things happen when bikes have an adjustable rear where the BB drops and the fork slackens out. Would love to see those too 🙂 cheers

    Reply
  2. Jorge Corderi says:

    How may I contact you for an inquiry about changing a longer fork on a Trek Remedy 2020. I will like you to calculate the geo so I won’t risk a probable frame crack!

    Reply
  3. Jorge Cordero says:

    How may I contact you for an inquiry about changing a longer fork on a Trek Remedy 2020. I will like you to calculate the geo so I won’t risk a probable frame crack!

    Reply
    • leelikesbikes says:

      Hi Jorge,

      I’m not qualified to tell you whether a fork change will risk a frame crack. I’ve heard of companies voiding their warranty when you change fork length.

      In simple terms, each 20mm of increased fork height increases your head angle by about 1 degree.

      Lee

      Reply

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published.