The Chopper Compendium

Gentlemen. I have a stupid question.
I'm going to attempt a springer build and at the moment, am just trying to make sure I understand everything.
In Gary's Article, he says: "Use a protractor or angle finder to set your rockers at the desired angle for the ‘no-load’ condition of the forks with the lower springs fully extended."
How do I arrive at that angle?
Thanks

After reading every springer post in this section, I think I've found the answer with the 90 ° rule.

dan,
let us know if you need help. Gary, myself and many others on here have built springers and are willing to help you through the project

When the springer is fully extended, the bike up on blocks, and the wheel just hanging in mid air the line between the rear pivot and the front pivot should be parallel with the ground. This is what's called the 'fully extended position'. The front pivot point can be a little lower than the rear if you can't get it just right but not by much more than .125-inch.

Without sounding condescending, why?

When I did mine I used the info I had based on designing suspensions, I never really looked into it, after getting the rake and trail where I wanted it, I was worried about wheel recession on travel. Your angle would be dependant on spring rate and travel would it not?

Just trying to learn a little more

The angle is actually determined by finding the pivot locations, full out and full in, that cause the least amount of deflection in the sprung leg (or spring rods) where the legs or rods run through the spring perch to minimize binding and to maximize the most direct push on the springs over their full range of compression. A good general rule of thumb for about 90% of springers is to allow for 3" total compression even if you might only be getting 1.5 to 2-inches in the real world. A big pot hole will compress the springs past their normal limits pretty easily because the springs will deform by trying to unwind but this is way better than having the sprung legs bend.

That last answer may be hard to follow so I made a simple sketch to illustrate the concept. Hope I can post it.

It worked. This is a simple way of doing the geometry for the pivot points that I was talking about. It has nothing to do with flop, trail, or axle placement. There are other ways of doing roughly the same thing but this method has stood the test of time, roughly 95 years and tens of thousands of springers since the first ones were introduced. It is probably not worth while trying to reinvent the wheel of fixing something that isn't broken. You can see that as the rocker moves thru it's full range of motion, from full compression to full extension, the sprung leg (shown in red) follows the arc of the rocker at the lower pivot point as it moves up and down. Ideally what you want is for the sprung leg to be perfectly parallel to the rear leg at two point in this up and down movement. One point is at full extension and the other is at full compression. Between these two points the sprung leg pivot point will arc out slightly which is normal. This is why the bushing for the spring rod at the perch is a very sloppy fit, sometimes a full .03" oversized. It allows room for the rod to move without binding for and aft.
This sketch is rocker at full extension. At rest the weight of the bike will cause the front end to settle somewhere between a half and three-quarters of an inch, maybe even a full inch on heavy cycles. When this occurs the angle between the rear pivot and front pivot will typically be around 20-degree from horizontal. This angle isn't magic but again history has shown that it works the best over the long term. I suspect the reason is because most forks spend about 80% of their time being compressed and only 20% of the time being fully extended. Therefore it seems logical for the spring preload created by that 20-degree change in angle at static load to be realistically needed.

Forgot to add. Keep in mind that the line between the front and rear pivot points is parallel to ground for a 'stock' springer. That is a fork having around a 30-degree rake angle. As the rake angle increases the line between the points will change proportionally to keep the pivot geometry correct. For instance for a 45-degree rake the angle between the front and rear points will be 15-degrees above horizontal (45-30) and so forth. You get the idea.

That last post makes much more sense.

It originally sounded like you where trying to say that there where hard and fast numbers no matter the situation, now that you've clarified what you meant it sounds much better.