springer rule of thumb

[krymis]

so i have written down that i believe to be the rule for setting up a springer rocker mounting system. We have been working on and refining the rule and i will put out the inital writing and hope others can review comment and help make a solid design principle.

the 90* angle rule: this rule was set up to dictate the proper measuring and assembly of the springer rockers. First and foremost do not have any weight on the frontend. I block the front of bike frame with 2x4 and shimming stock. I set the front of the frame.75 to 1" above the final position of the weighted frame(containing motor trans and rider). For lighter riders i set closer to .75" for riders 200# or more i go a full 1". This rule dictates the position of the rear(rigid leg) rocker attachment hole. that in turn will tell you where to place your spring leg attachment hole.

rule 1: place the blade (the long end) of a carpenter's square along the centerline of the steering axis. If not using raked trees or raked cups you can simplify this and use the centerline of the rigid leg of the springer. Align the tongue (the shorter leg) of the carpenter's square with the centerline of the axle. If using the centerline of the rigid leg, the point of the square will be where you should place your rigid leg rocker attachment hole. With this hole made you can in turn make the spring leg rocker attachment hole by setting level at horizontal zero along the centerline of the rigid leg rocker attachment hole. follow the horizontal zero forward to the intersection of the centerline of the spring leg. the point of intersection is where the spring leg rocker attachment hole should be placed. Once the frame is taken off the cribbing and the weight is applied to the frame, the rockers will preload the spring leg and naturally set the rocker attachment holes 10-20* of horizontal zero.
rule 2. expanding upon rule one, if the 90* rule can not be utilized in a flowing rocker design you may have to go to the horizontal zero rule. and that is: if a 90* can not be acheived then set a horizontal zero line that intersects centerline of axle, centerline of spring leg, and centerline of rigid leg. this will place the unweighted rocker attachment hole in horizontal alignment with the axle. there by getting the rocker holes to be at a 10-20* off of horizontal zero along the centerline of the rigid leg attachment hole.


A good diagram from gary shows the 90* angle rule and how it will affect trail and design. An example of using rule 2 would be gary's rude crude radiused rocker. By setting the rocker holes to zero unweighted, when the weight is applied then the springs automatically preload and set the proper angular relationship.
gary's diagram and the point marked PR is what we are trying to acheive, realize this chart is of the frontend in the weighted position

here is garys radiused rocker in the weighted position

[railroad bob]

Am I missing something? I don't see any reference about how far from the steering neck you should place the carpenter's square to align the steering axis and the 90* angle. Variable, I suppose? Guess it depends on the rake angle and length of forks.
Do your rules account for those variables, or am I missing something?

Kinda thinking out loud here...

[triker_chewie]

Am I missing something? I don't see any reference about how far from the steering neck you should place the carpenter's square to align the steering axis and the 90* angle. Variable, I suppose? Guess it depends on the rake angle and length of forks.
Do your rules account for those variables, or am I missing something?

Kinda thinking out loud here...

it says on the rocker pivot

If using the centerline of the rigid leg, the point of the square will be where you should place your rigid leg rocker attachment hole.

[krymis]

Am I missing something? I don't see any reference about how far from the steering neck you should place the carpenter's square to align the steering axis and the 90* angle. Variable, I suppose? Guess it depends on the rake angle and length of forks.
Do your rules account for those variables, or am I missing something?

Kinda thinking out loud here...

you are using the steering axis and the centerline of the axle to set that point. there is no hard and fast rule for the length as there are variables of rake and trail #'s which will in turn change the position of the axle to set the 90*.

[gww25]

The 90-degree rule applies to 'dropped' rockers, even short ones like the one shown in the attached sketch set up for a bike with a shallow 35-degree rake. (If you're not using dropped rockers then the old original rule still applies which is to just have the axle about 1-inch higher than the rear leg pivot point when the weight is on the forks).

[krymis]

gary isn't that basically the second rule? or is there something i should add to state it is for the dropped rocker and another for the rest of the setups. would like to nail down some rules of thumb for setting rockers.

[gww25]

Yes it is but my first sketch showning the conventional rocker was a little confusing since it also showed a 90-degree line so I thought I would just clarify.

[rudog]

The 2nd rule is the orginal rule of thumb. I have no problem with you guys redefining the rules. In fact...I like it.

Would a discussion on rocker length be appropriate at this point?

[krymis]

i would say as long as the discussion is established in the first part of your post and direct questions are asked to keep on topic of the question then yeah why not. i just don't want to get bogged down in multidirection questions.

[rudog]

With respect to rocker angle...The reason for reason for a longer scimitar rocker angled 90* to the steering axis was touched on in another thread...to increase the distance to the point of rotation.

Why are horizontal rockers typically shorter?

Why horizontal and not some other angle?

[krymis]

if the rocker is angled so that the rigid attachment point is above the axle the ride tends to be harsher and heavier feel from what i have ridden. a straight horizontal line in the weighted position would not have a very good feel b/c of the lack of preload on the sprung leg. there are instances where the axle is at more of a 90 and at less than a 90 and the rocker still functions and well. the 90 just seems to be the magic spot at least in all my testing.

as far as the length deal that is totally set by where the axle is in relation to the rigid leg attachment point. The horizontals are usually shorter because the axle is closer to the rigid attachment point. if you refer to the photo a few posts up you will see these in a lineal view.

[gww25]

Springer design really has no limits and there are literally scores of variations on springers in general going way back to the earliest bicycle design days. In fact a lot of forks classified as girders today are actually springers just with the springs relocated. Anything that uses a 'link' which is what the engineers call rockers, can be made to function with springs of some kind. The design of the links themselves is open to wide interpretation as witnessed by the several dozen popular styles used by a variety of fork builders. The links can be made longer or shorter and this is the oldest and easiest way to change trail on a springer but there is a practical limit on how long they can be made until you have to start using custom made springs. Longer rockers reduce the effective spring rate, making the ride seem 'softer'. Also longer rockers will 'deflect' under hard cornering and they also put far more side-loading on the bushings and bolts so most builders try to keep rockers as short as is possible and still get trail properly set. One of the slickest springers ever made was the original Vincent design with used extremely long spring/shocks behind the main tube legs. The old original Harley springers were also really cool with the springs concealed within the tubes, kind of a variation of the old Thor forks which were the original design concept used by Harman for the Spirders. The springers we're most familiar with today evolved primarily because they are cheap to build by the factories but not necessarily the best performing. There is a lot that can be done to fine-tune the classic Chopper Springer and rocker design is just a small part of the process.

[rudog]

Having general rules for a garage builder to follow without fully understanding is a good thing. Would either of you say that these rules are based on the rockers interaction with the spring leg?

The specifics are better discussed in a suspension thread, but after this thread, where does the discussion go? And, how does it tie in?

[mvalenti11]

I thought the first rule of springer design was to not talk about springer design....

Very informative. I'm saving this one!

-Mark

[gww25]

The rocker interaction with the spring leg only impacts spring rates so the basic design of the rocker geometry with respect to handling characteristics has noting to do with the geometrical relationship between the unsprung and the sprung leg unless you want to look at spring rates at the same time. Spring rates for the most part are not a significant element in bike handling unless you need to evaluate braking dive or something like that and if you're into this kind of stuff you're very probably not building a chopper to begin with. Sport bike handling technology needs to be evaluated by itself and there is a huge wealth of information about sport bike forks on the Internet. A chopper is a chopper and even without any front suspension at all they work pretty well for their intended purposes.