Steering geometry

[Psychographic]

On my current build I added rake to the head, and am using a set of forks where the wheel mounts with no offset. The wheel wants to naturally sit at 90 degrees to the frame.

I'm wondering if I made a set of dropouts that mounted the wheel in front of the forks, rather than being centered on the tubes, would it cure the way it wants to track?

 

[MattiThundrrr]

What you are describing is known as "chopper flop". Negative caster (moving the axle forward of the steering axis) could be the answer. Think of a Sugarbear springer, supposed to be one of the most stable front ends ever.

If you look at the physics of it, you can see how gravity would cause the front end to fall into a straight position.

 

[Psychographic]

Thanks Matti. I might make some bolt on dropouts to see if I can make them look good and actually function before I do any grinding and welding.

 

[MattiThundrrr]

Hey, no probs. Thank you for sending me down a Sugar Bear rabbit hole! Guy is a freakin legend, still building his iconic front ends nearly fifty years later!

"If it ain't long, it's wrong!"

 

[Psychographic]

Your mention of the phrase "Chopper Flop" led me to this. I haven't read it all the way through, but it seems easy to understand. Maybe it will help other noobs like me.

http://chopperhandbook.com/rake.htm

 

[MattiThundrrr]

Haha was just browsing that exact site. Found some of that "math" stuff...

"Wheel Flop, or Fork Flop as it's sometimes known, isn't just something some builders decided to attach a silly name to. It is a very real, well-documented and studied, engineering principal that is a significant part of bicycle and motorcycle handling dynamics. There is even a mathematical equation that a person can use to determine what is called the "Flop Factor" for any particular bike or cycle geometry:

Flop Factor = trail in inches times the sine of the rake angle times the cosine of the rake angle."
So yeah, I was wrong when I told my trigonometry teacher I'd never even think about that stuff again.

 

[Psychographic]

Flop Factor = trail in inches times the sine of the rake angle times the cosine of the rake angle."

My eyes acknowledged words, unfortunately my brain acknowledged this.

https://instrumentalfx.co/charlie-brown-teacher-speaking-free-sound-effect-download/

I've got about 47 degrees of rake and a trail of 9". So I think I'm going to have to live with it, dropouts long enough to help are just plain ugly. I tried a Varsity fork on it, it helped a bit, but they ruin the look of the bike. I think in order to help I'd need dropouts about 3" forward of the forks.