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Springer Fork Rocker Designs

Introduction

Since we first published our original CBH Springer Buildup article back in 2005 I’ve had numerous requests from site visitors and a few custom shops to do a follow-up article with more detail about Springer geometry and making custom rockers. Ironically I had already prepared a lot of material that addresses some of the more advanced areas of Springer building but simply didn’t have the time to finalize the work years ago.

What this article addresses is the specific design considerations for building custom Springer Rockers as a part of fine-tuning the handling characteristics of a typical Springer fork system. Keep in mind as you read along that what we're going to talk about here applies to Chopper front-ends. By that I mean forks that set on a 35-degree rake or more and forks that are extended at least 6-inches over stock. Don't try to apply what follows on stock or Bobber type front ends.

As most people know I’ve been building Springers, Girders and Leafers for almost 50 years now so I have a little experience with these fork systems but no matter how long you’ve worked with them you’ll always be learning something new. At least you should be. What I know about these systems I learned from other builders and then just added my own experiences into the mix that I pass on to others who are interested in building forks. I personally don’t think that there are any ‘secrets’ about building forks that most fabricators are not aware of but there is a huge amount of knowledge about forks that has never been published in the Chopper rags because most magazines and web sites aren’t really to serious about publishing technical information since it takes up to much space or to many pages.

This particular article is about Springer fork refinements and to be honest there is so much misinformation and mythology floating around out there that I really don’t know where to begin. In some ways I’ll be swimming upstream against a ton of mythology that is accepted as factual information by many people. To start off with then I need to address some of these false assumptions and myths about Springers in general.

First of all most Internet discussion board wisdom holds that Springers are kind of an inferior throwback type of fork system that is ineffective, heavy and obsolete by today’s standards.

In reality nothing could be further from the truth as a well-designed Springer is perhaps the best of all possible fork systems to be used on a typical Chopper motorcycle. I personally prefer Girder forks but Springers have several advantages for many builders.

First of all Springers are very easy fabricate in your own garage with a minimum of tools or special jigs and fixtures. This is the primary reason that so many shops and custom builders make Springers the specialty of the house. Springer building is simply a very high profit item of fabrication in most shops. Most people can build a very good custom made Springer for well under $500 at home in their own garage without needing a degree in engineering to figure out the geometry. These are cheap and easy forks to fabricate.

Springers will provide ‘real’ suspension travel and dampening on almost any type of bike, from a small Bobber to a massive Chopper with a 60-degree neck rake and 60-inch long fork tubes. You can’t say the same for hydraulic forks, which quit working on bikes with much over a 40-degree rake angle. You can adapt and tweak Springers far more easily than you can other fork types.

Springers, if properly tuned, will provide excellent road feel and superb handling characteristics on almost any type of bike compared to other systems. As I already mentioned, a well-designed Springer is almost infinitely ‘tunable’ and ‘tweakable’. There is no comparison that can be made between the road handling quality of a Springer as compared to hydraulic forks. It’s like rack and pinion compared to power steering.

Springers ‘look good’ on a Chopper. As Indian Larry used to say, “They look mechanical” which is what a Chopper should look like to begin with. Don’t discount the ‘Bling factor’ if you’re planning on building a bike that people want to look at. A set of Springer forks mounted on almost any bike will make it look lower, longer and leaner than any other fork system you could install. Even a stock raked bike with a Springer looks more raked than it really is.

A well designed Springer, if it’s properly tuned, will not ‘pogo’ which is a common misconception that a lot of people have about Springers in general. I’ve been riding for almost 50 years and I have yet to ride a well built bike of any kind that ‘pogo’s’ with a Springer so I’m not to sure how this myth got started to begin with. I’m sure bad spring tuning might cause problems but I’ve never experienced it myself so I don’t think it’s as common as many folks might imagine. Either that or a lot of people are riding around on really bad Springers.

Springers do look ‘heavy’ but looks are often deceiving. I have to admit that I’ve had a few commercially made Springers in the shop that were indeed massively overweight but that is the exception rather than the rule. A good Springer will not weigh any more than a comparable set of hydraulic forks. In many cases a Springer will be lighter in weight. Some of my Springers weigh less than 35-pounds and even the last really long set I worked with from Sugar Bear weighed in at 62-pounds and these had solid steel legs 24-inches over stock. I had the unfortunate experience of working with another set of Springer forks from another famous builder that weighed 85 pounds. It took two of us to get them mounted on the frame. I have an old set of Dick Allen forks that weigh 42-pounds and that’s pretty much the weight that a good set of forks should weigh in at. I’ve seen literally dozens of Springer forks made by a variety of custom shops that are still using 1-inch thick trees and solid bar stock for the legs. In reality 5/8-inch trees and DOM tubing are more than strong enough for almost any conceivable set of forks. Some builders apparently think they need to sell stuff by the pound.

Anyway these are just a few of the reasons that I think Springer Forks work out to be a good deal for anybody building a custom Chopper on a budget. If a person chooses to believe in myths then so be it, as nothing I write will ever change their minds. One of the problems I’ve had for decades is trying to communicate the experience of riding a Chopper as opposed to a typical ‘sport’ bike. Most of the people I deal with at the site and in the garage have never ridden a Chopper so they have no basis on which to make comparisons. The result of that situation is that it creates a communication gap as we talk back and forth comparing apples to oranges. It’s really hard to describe what it’s like riding down the road at 80 miles an hour with your ass only 19-inches off the ground and having your legs stretched out in front of you to a person who has been riding all of their lives 30-inches above the pavement with their feet on the rear pegs. It’s kind of like a guy trying to describe an elephant to a blind man.

Sport bikes and hydraulic forks have their place. I’ve owned and enjoyed several such rides but if you’re going to build a Chopper then you really do need to look at a set of Springer forks. In fact when I see a so-called custom Chopper fitted with a set of hydraulic forks I pretty much know that the builder really doesn’t know a whole lot about making a Chopper handle. Choppers can be made far more nimble than most people imagine with some simple engineering and tuning.

 

Wheel Flop

We briefly talked about ‘Flop’ in the front fork section of the Handbook CD and here on the site but as we discuss Springer rockers we need to go back and take a second and more detailed look at this phenomena.

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.

The results of the exercise will give you a number (Flop Factor) that is the distance in inches the steering head of the cycle will lower as the wheel is rotated from its normal 90-degree upright angle when the forks are centered.

Take special note of the fact that the amount of flop is directly related to both rake and trail.

The Flop Factor for a variety of rakes is as follows (using 3-inches of trail for all cases):

30-degree rake: Flop = 1.30”

35-degree rake: Flop = 1.41”

40-degree rake: Flop = 1.48”

45-degree rake: Flop = 1.50”

50-degree rake: Flop = 1.48”

55-degree rake: Flop = 1.41”  

60-degree rake: Flop = 0.31"

(These numbers assume a complete 90-degree rotation of the wheel hub. In the real world the numbers can be easily reduced by 50% or more depending upon the amount of fork rotation).

The reason the steering head will lower as the forks are turned is because the wheel hub gets lower to the ground as the wheel is rotated. Wheel Flop is simply caused by gravity. The phenomena we’ve all experienced is a result of the wheel and forks wanting to find equilibrium once the wheel is moved from its normal upright position. When the forks are pointing straight ahead and the front wheel is perfectly vertical gravity is acting straight down on the assembly through the center of gravity so it is in a perfect state of balance. It’s easy to forget but keep in mind that as the forks are being turned the wheel is not being rotated about a vertical axis, as in a car, but is instead being ‘leaned’ over. It is this leaning-over and the resulting displacement of the center of gravity that cause the wheel to flop once it gets past a certain point. Wheel flop is progressive. The more the forks are turned the greater the weight and force act upon what the rider feels at the handlebars

If you have a bike handy you can see this phenomena in action just by slowly turning the forks using only slight pressure with one hand while you’re watching the up and down movement of the steering stem bolt as the wheel begins to lean further and further. If you don’t have a bike handy you can also see this process by just using a regular old Crescent wrench as an imaginary set of forks. Place the wrench on your workbench with the jaws acting as the wheel. Tilt the wrench to some imagined rake angle and slowly rotate the end of the wrench with your fingers as seen in the photographs below. You need to use a very light grip, just barely touching the wrench handle with your finger tips.

If you have a sensitive touch you’ll immediately notice that you can rotate the wrench easily and it stays relatively well balanced for a considerable range of rotational motion but as you’re slowly rotating the handle you’ll feel the wrench want to suddenly and rather forcefully want to just ‘flop’ over on its side. That point where the equilibrium suddenly changes is the ‘flop point’.

All bikes and cycles will have a slightly different ‘flop point’ since the angle where the flop occurs most powerfully is a combination of factors that involve the rake, trail, bike weight, wheel diameter, wheel weight and fork geometry. Most cycles have the fork stop angle set to a value just slightly before that point of rotation. No matter what anybody says, all bikes have a flop point, but that point, or rotational angle to be more specific is different for each and every bike. It is almost like a unique signature for a particular set of geometrical frame and fork factors.

It's very easy to forget that the front wheel of a cycle does not 'rotate' or 'turn' when steered as you see with automobile steering. You need to keep in mind as we move on that cycle wheels 'lean' when steered as mentioned earlier. This simple fact is critical to understanding how to make a chopper handle well. Most of us don't even notice this 'leaning' except when we see it as the bike is parked on its kickstand but even then its not very noticeable since the whole bike is leaning over as well. You can better appreciate how much a wheel leans when being steered by looking at a trike since the frame will always remain level.

This old scan of the 'Mailbox' trike is the best I could find that illustrates what I'm trying to describe with fork/wheel behavior when the forks are rotated about the steering axis. If you have a sharp eye you can also appreciate that the projected steering axis line intersects the ground a couple of inches ahead of the tire contact patch. I only point this out because you'll find a lot of people talking about how the front tire rotates about the contact patch which isn't at all true.

The flop formula above, like most engineering equations that people try to apply to Choppers provides only part of the story. There are several other factors that can be called into play to modify the effects of Flop. Most of these additional factors will also modify trail as these two elements are intimately connected.

It is possible, by incorporating a variety of design elements, to build a chopper that doesn’t exhibit any flop characteristics whatsoever. This isn’t necessarily a good thing.

So far we’ve only talked about flop as it relates to a cycle sitting still in the static condition. The effects of flop are actually more evident once a bike gets moving and we can see that the flop is also a force as much as it is a general characteristic of the front-end assembly.

 

Dynamic Wheel Flop

Wheel flop is not an altogether bad thing once a bike starts moving (dynamic flop) and a certain amount of flop is actually necessary to make a bike respond properly to the riders input at the handlebars. A bike with zero flop is extremely sluggish from a handling standpoint and the rider will have to exert a lot of force to put it into a curve. On the other hand such a bike is extremely stable moving straight ahead down the road and can easily be ridden ‘hands-free’. Many ‘exhibition’ bikes that stunt rider’s use are designed for a zero-flop condition. You can move around a lot on such a bike, even stand up on the seat and ride them down the parking lot without too much worry that they’ll veer off track. The ‘zero’ flop condition makes these bikes want to go in one direction only and that’s straight ahead. Riding such a bike on a typical twisting road is almost impossible unless you’ve got really strong arms and shoulders.  

Thankfully not to many bikes are actually designed to have a ‘zero-flop’ condition because the rotational inertia of a bikes front wheel at speed lessens the severity of wheel flop since it provides an opposing torque. Sometimes this is called the ‘gyroscopic’ factor but I think that’s getting a little to technical since real gyroscopes actually work in 3 dimensions while a wheel only works in one. It’s a cool way of describing what happens but that’s as far as it goes.

Almost everybody in the world has experienced dynamic wheel flop if they have every ridden a bicycle. If you’re parked on the side of the road and then push off to gain some speed you’ll have noticed that the forks are ‘shaky’ or ‘wobbly’ until you finally get moving. There is a transition point between rather significant instability and eventual stability once the bike gets up to a minimum amount of speed. As the speed increase the bicycle becomes more and more stable.  Most people have noticed that as a bike or cycle is parked and set on the kickstand that the forks and front wheel automatically rotate towards the 'lower' side of the bike. That's flop in action. If a bike didn't have any flop at all the wheel and forks would remain pointing straight ahead as the biked leaned over on the stand. If you walk past a row of bikes parked at the curb you'll soon notice that each and every one has the front wheel pointed in a slightly different direction. That's because of the differences in basic wheel flop characteristics of individual bikes.

Without a certain amount of intrinsic wheel flop it is actually impossible to overcome this speed stability and make a cycle lean into a corner and negotiate a curve in the road. With a 'no-flop' bike you'll end up going straight off the road no matter how far you lean or how far you try to move the handlebars. Once a bike gets up to speed most of us never realize what is is that we're actually doing to get our bikes through tight curves. It is an instinctive process. We lean a little, changing the center of gravity, and then flop (and trail) come into play without us even realizing that it is happening. Cornering is steering fork geometry in action and we're almost completely unaware of the dozens of dynamics that are coming into play.

It is possible to design a rather radical Chopper with a wildly raked front end that can negotiate curves just as gracefully as any sport bike on the road. It's as simple as making sure that the Chopper and Rice Rocket have the same trail and flop numbers. Of course that Chopper won't be able to zip in and out of traffic like the sport bike but that's because of the Choppers longer wheelbase and wider turning radius, not because of any 'design flaws' with the fork system. Some fork builders claim that their bikes have zero trail and zero flop but if you run thru the equations above you'll easily see that even a true zero trail Chopper has about 1/2-inch of intrinsic flop. You simply can't ride a bike that has a 'zero' flop condition no matter how hard you try so beware of anything that you read on the Internet.

Anyway so much for Flop for the time being as we need to get back to looking at Springer rockers. Just keep in mind as we go forward that wheel flop and trail are intimately connected, almost inseparable, when we talk about fork geometry. In fact back in the old days most of us didn't even know what 'trail' was supposed to be and relied instead on looking at how we could control wheel flop. We didn't realize that flop and trail were just two different ways of describing the same basic geometrical relationships and fork performance concepts. Most of us had to learn the hard way about fork design and that was through trial and error and the errors sometimes hurt pretty bad not to mention that they became rather expensive. The text book most of used at the time was a huge masterpiece written by Phil Irving (of Vincent fame) simply entitled 'Motorcycle Engineering'. Even today this is still the best book for cycle designers to have  on their shelf. If you're seriously interested in bike design this is the book to buy as it goes into technicalities that we simply can't even begin to touch on in a short article like this one.

 

Weight and Center of Gravity

One of the things that has a tremendous bearing on Forks in general but especially with respect to Springer forks is the distribution of weight on a bike. This is the one variable that you may not have much control over as the rider can represent from as much as 35% to as little as 20% of the total weight of a typical bike. Even at the low range the weight of the rider has to be considered when you're looking at controlling wheel flop through the design of rockers. The more weight you have on the front wheel the more flop you will encounter no mater what measures you use to reduce the 'flop factor'.

Many riders, especially sport bike riders, are pretty surprised to find how well balanced Big-Twin Choppers actually are. Out of the scores of Chopper I've weighed and measured the average horizontal center of gravity is just a few inches aft of the wheelbase centerline. That figure is very nearly spot on with respect to a typical sport bike.

Based upon my personal experiences I use the following rule of thumb values in doing front wheel weight estimates for fork designs and the reader may find this information helpful when looking a spring rates.

Rake Angle (in degrees)

Percentage of weight on front wheel

30

42

35

40

40

38

45

36

50

34

Keep in mind that these values assume a percentage of weight on the front wheel with the bike weighed with the rider on board. Each bike will be slightly different but for most applications it is not necessary to do a detailed analysis using moments about the rear axle to find the location of the center of gravity and resultant weight distribution.

Keep in mind that the physical weight of the front wheel assembly plays a huge role in how a bike reacts to flop so you want a front wheel/hub/tire combo that is as light as possible. A lot of people who own a very good Springer ruin it by mounting one of the fancy after-market 'mags' that weigh a ton. Many, if not most, aluminum/magnesium rims weight at least twice as much as a very high quality steel spoked rim assembly. Brakes are also heavy so take that into consideration as well. Even today I won't build a bike having a front brake unless it's registered in a state where this is mandatory or you're going to be riding in a place like San Francisco most of the time. On occasion I'll break down and use a spool hub fitted with a mini-drum which is about all that a chopper rider should need to meet most maneuvering situations. Choppers aren't intended to stop to fast so you just have to learn a new riding style if you want to ride one. I've personally used the old boot leather brake more than once in some touchy situations and it works a whole lot better than you might imagine. If for some reason you just absolutely need to run a mag rim and a brake setup then you'll also just have to get used to a little more flop. Everything will end up being some kind of compromise no matter what you do so just juggle things around until you're happy with the results.

 

Rocker Types

There are two basic 'types' of rockers. The first and most common are typically called 'Dropped' rockers. The second, and far more rare type is called a 'Reversed' rocker. A Dropped rocker gets it's name from the fact that the rocker body and pivot points are 'below' the wheel axle when viewed from the side. A Reversed rocker generally has the body and pivot points above the axle. The two types are shown below.

Type-A on the left in the sketch is a Reversed rocker while Type-B on the right is a Dropped rocker. There are some rockers that will fall right in the mid range of the two types. In this diagram both hypothetical front ends on set up with a 40-degree neck rake and 21-inch front wheel. The dimensions between axle and pivot holes are identical. The only difference is that the axle hole is .5-inches above the rear pivot on the dropped rocker and .5-inches below the rear pivot on the Reversed rocker. Notice that it is very difficult to get a tight Trail figure when using the Reverse rocker. There was once a common practice of simply 'flopping' or 'reversing' a standard dropped rocker to raise the front of a bike but as you can see such a procedure really plays havoc with a bikes trail figure. At least 95% of all rockers used on modern Springers today will be some variation on the 'Dropped' rocker design. There are a few makers however who still try and sell plans and finished front ends using Reversed rockers but I have a hard time understanding how they can make a good handling bike.

If you do an Internet image search for Springer Rockers you'll find literally hundreds of pictures of various rocker designs. At least 98% of those rocker images represent rockers that are designed and fabricated from a purely 'artistic' standpoint.

These are just a few samples I picked at random from a Google search. Some of these rockers were probably specifically designed for a specific bike but others are cut from 'standard' patterns that were popular back in the sixties and seventies.

At one time most custom springer builders actually engineered and designed their rockers to meet the particular requirements of the customers bike but today that practice is all but forgotten. Even Sugar Bear is now just using 4 basic rocker patterns for all of his custom forks. Most builders don't even offer the client any options at all except perhaps for some fancy pattern cutting on an otherwise 'standard' blank. Chris Croft (Krymis) and I will still engineer and custom make rockers if the bike in question needs something special to fine tune the fork set. I wish more builders would start doing the same.

No matter how wild or how mild rockers may look they all share some commonalities and the most significant of these is the distance between the rear (fixed) leg rocker pivot point and the front (sprung) leg rocker pivot point. I've measured literally hundreds of rockers and found that this dimension ranges from 2.375-inches which is the stock Harley specification up to 2.625-inches on really wild looking patterns. In general you can be pretty sure that this distance will be 2.375", 2.5" or 2.625" on about 99.9% of any rockers you will ever come across.

 

Some folks will try and tell you that this pivot to pivot dimension is super 'magical' and determines how 'soft' or how 'hard' the Springer will 'ride' and how well the bike will handle and how changing this dimension changes your effective spring rate and the list of 'claims' goes on and on. In reality this measurement has almost no effect whatsoever on anything that can be measured without resorting to extremely precise scientific tools. In fact you can bolt rockers having any of these dimensions onto anybodies forks and not see, perceive or feel any difference of any kind in the handling characteristics of the bike.

The difference in the amount of travel of the sprung leg between the stock rocker measurement and the larger 2.625" offset type is only 0.0123-inches through the entire range of spring compression. That's less than 1/64 of an inch. That changes your effective spring rate by about 1/1,000 of an ounce. This myth is one that needs to be set aside. On a further note a lot of people will try and tell you that you can't mount the 'longer' rockers an old stock Springer and this also isn't true. The difference in length between the stock rocker pivots and the longer rocker only changes the angle of the spring rods at the bridge by only 0.09 degrees at the most extreme angles of rotation at the rockers. That works out to being less than 0.042-inch at the spring rod bushing which is well within the tolerance range for these bushings. If you do such a conversion and are paranoid about binding the spring rod you can always clearance the bushing with a Dremel tool and a fine cone shaped bit. Such clearance modification isn't usually necessary.

The other thing that you'll notice about most rockers is that even on some of the wilder designs the axle holes are all more or less in the same position, geometrically speaking, relative to the pivot pin holes. There are exceptions and we'll cover these a little later. For the time being however the vast majority of custom shaped rockers are designed around a geometry that was first developed by Dick Allen or at least he's given credit for the design.

A very old Dick Allen dropped type rocker (the chrome one) is shown in the photo above. A little later on in the early seventies he offered these with three different axle holes so the buyer could do a little fine-tuning on their forks. Sugar Bear still offers these on his site. One of our own dropped rockers is shown to provide a size comparison.

During the early days of Springer development most of us did our 'engineering' the old fashioned way and that was through trial and error. It didn't take us long to realize that there was a well defined set of relationships between wheel diameter, axle location, pivot pin location, rake, trail, fork offset, flop and the bikes center of gravity. By mixing and matching these various items we could easily change the handling characteristics of even the most radical looking bike.

The first thing that became quite evident, way back in the early sixties, was that even extremely long, steeply raked forks started to handle better once you went past a 45-degree rake. This seems contrary to common sense but it is true. A bike raked out 55 degrees will often handle better than a bike having only a 35-degree neck, all other factors being equal. None of us knew why this was happening but if you take another glance at the little table of flop factors at the beginning of this article you will see why this occurs.

This phenomena is largely what lead a lot of us to build radically raked bikes. Later on we discovered that front wheels with a smaller diameter handled better than our old 21-inchers. This lead to the Fad of long bikes using tiny little 17-inch rims. Thankfully this was a short-lived trend. The smaller wheel diameter in effect was greatly reducing both flop and trail but we didn't know about the 'math' or the 'principals' that were going on. We  just knew that it 'worked'.

The third thing that a lot of us noticed was that the more 'drop' a rocker had the better the bike handled. This lead to the development of some pretty wild looking rockers. The most famous of these are of course those made by Sugar Bear (Al Myers) as shown below.

As I mentioned earlier most of us modifying old Springers and building new ones back in the early sixties knew instinctively that there was a very specific relationship between the axle location and the rear pivot pin hole with respect to fork rake. A lot of guys were making really radical rockers based upon nothing more than these instincts and they were working out really well on the road. I was no rocket scientist but I struggled through math and geometry and I drew up what I thought was a mathematical basis for rocker designs in 1967 based upon three different geometrical principals that hinged around various 'spirals'. These were the Fibonacci, Archimedes and standard involutes of regular polygons. At the time I was pretty sure that I had stumbled upon something almost 'magical' but in reality none of these 'curves' were exactly perfect. Never the less a lot of builders of the time used these concept sketches for making patterns for their rockers. The illustration bellow shows some of the spirals I was using to lay out the axle and pivot points. I used to spend hours and hours and weeks turning into months trying to find the 'perfect' solution to a problem that actually had no hope of ever being 'perfect'.

At that time I had no idea of why these designs actually worked out in practice because back then none of us had access to any technical data that specifically applied to long bikes. Once trail got over 4.5-inches the classic design manuals just didn't provide any helpful solutions. We were largely flying by the seat of our pants and trying to expand the edges of building wild looking bikes without getting killed in the process.  This sketch below shows how I envisioned what eventually came to be called 'Scimitar' rockers based upon my experimentations with curve formulas for any particular neck rake angle. This was a somewhat flawed concept but it 'worked' anyway so I knew that I was on the right track.

Back in the old days if I had access to the simple little formula for calculating Flop shown at the beginning of this article I could have saved years of trial and error experimentation. As a matter of fact most of us punk-kid garage rats didn't pay any attention to 'Trail'.  Of course we all heard about it from older mechanics and shop owners but nobody would show us what was considered safe. That changed for the masses as far as us garage-based builders stood  when Ed Roth published the first tech article on rake and trail in early 1969. Up until this time a lot of chopper building information was closely guarded by the Guru's in the industry. Basically, at the time, if you had a 'secret' you could make money off of it. This is one reason that I still hold Ed Roth in such high esteem. He was a shit-stirrer, myth-buster and all around champion of the underdogs who always openly and unselfishly shared whatever information he came across. Many of us were building drag bikes and Choppers with trail numbers way over 5-inches so it was reassuring to find that numbers in the 8 to 12-inch  range were 'acceptable'

Armed with this 'revelation' of new information I revised my original curve formulas and ended up with a set of charts that I still use today.

The diagram below is a CAD copy of a chart I first drew up full-sized on cardboard back in 1970. At this scale it's pretty hard to read but I think that you can get the idea of what I was trying to accomplish. This particular chart was set up to determine the rocker geometry for forks where I wanted to maintain 3-inches of trail regardless of what the neck rake angle happened to be. On the full-scale chart it was easy to interpolate down to 1-degree of accuracy.

Those semi-circular arcs you see drawn between the axle and the rear pivot pin hole having nothing to do with the rocker shape but are there just to make it easier to understand what axle locations go with what pivot location. I eventually made charts for various rake/trail and flop conditions ranging from zero to 6-inches which we'll eventually post at the discussion board.

Ironically a guy who actually was really good at mathematics and geometry did the same exact thing but did it empirically by actually building a mockup in his shop consisting of steel bars drilled with dozens of holes. Through trial and error he tried scores of various rocker/rake relationships and physically measured the amount of flop for each and every possible arrangement of parts and hole locations. This guy was Sugar Bear and that's how he came up with his incredible rocker designs.

At this point it should be pointed out that one of the beauties of Chopper building is also an ugly fact of life and that is a person can get used to riding almost anything. The rider can adapt to the handling characteristics of almost any bike they happen to own. I've had bikes that I felt very confident in and felt comfortable riding that had over 12-inches of trail and fairly significant wheel flop. I've also had bikes with almost no flop and less than 2-inches of trail but in the real world riding either bike was pretty much the same experience and you really don't even consciously think about the differences even though they are significant. You just instinctively adapt to the machine, at least up to a certain point. Strangely enough a lot of people have more difficulty adapting to a more sensitive chopper than one that's a little hard to handle. I have had several friends and also heard about people never being able to adapt to a Sugar Bear Springer. I imagine this is simply because he has literally pushed the development of his designs to the cutting edge and perhaps they are a little to perfect for some people. I still think his designs are incredible and if I didn't build my own forks I would not hesitate to buy one of his sets.

My personal rocker patterns are more conservative but I do like a little more feel in my forks than what a Bear design provides. One of the great things about building your own forks and rockers however is that you can custom make something that specifically suits your own bike and your personal handling requirements. Nothing can be better than this.

 

Why Dropped Rockers Work

Dropped rockers will give the rider the impression (perceived feel) that the bike has less trail than it actually has in reality. In addition the rider will have the impression that it takes very little movement of the forks to produce a fairly significant amount of front wheel rotation. Some people will say that riding a bike with dropped rockers is like having 'power steering' in that it takes very little effort to move the front wheel. In some cases you can steer the bike using just your finger and a few ounces of pressure on the handlebars.

Basically what is happening with a set of dropped rockers is that you are 'tricking' the steering geometry into thinking that the bike is running a much smaller diameter front wheel that is set much further forward than the physical reality of the hardware. Of course there are several other factors involved but this is the easiest way to visualize what is happening.

For example in the simple sketch below we have two views, looking straight down the steering axis line towards the front hub of two different sets of forks.

The fork set on the left is an illustration of a fairly typical set of rockers. The handlebars have been rotated 10-degrees and the wheel hub is displaced, rotated along the steering axis a distance of .6756-inches measured to the center of the wheel hub.

The fork set on the right has also been rotated 10-degrees but this set has a pair if 6-inch dropped rockers so the total displacement measured at the center of the wheel hub is 1.03-inches, almost twice as much for the same amount of handlebar movement. This is primarily why dropped rockers appear to have 'power-steering'. You only need to move the handlebars and forks a few degrees compared to the stock fork to achieve significant rotation. Since the movement is less, the force needed to rotate the handlebars is also significantly less than found on a stock springer.

That's one part of the equation. Another factor that comes into play with dropped rockers, as mentioned earlier, is that the bike thinks it is running a very small diameter front wheel, hence greatly reduced wheel flop.

The sketch above shows a fairly typical off-the-shelf Springer setup for a bike having a 45-degree rake angle and running a 21-inch front wheel while the sketch below shows the same hypothetical bike set up to run a Springer using dropped rockers.

Notice the 1.5-inch difference in the location of the wheel pivot point (PR) between the two fork sets and the 3-inch difference in trail. The effective length of the rockers is identical so there is no change in the spring rates for the two different forks even though the 'sword' type dropped rockers look far larger. The forks in the lower sketch will handle significantly better than the forks shown in the upper sketch.

 

Rocker Designs

The geometry and mathematics of a dropped rocker have nothing to do with what a rocker actually looks like. Once you lay out the desired geometrical and mathematical relationships between the axle and pivot pin locations the physical 'outline' of the rocker can take on almost any shape imaginable. The sketch below shows two very different looking dropped rockers but both are based upon identical measurements for the axle and pivot point locations.

When I was first starting out and working in a real Chopper Shop I spent weeks, maybe even months, just cutting out and dressing bullet shaped tube caps but after a while my boss put me in charge of designing, cutting and dressing Springer rockers. From that early experience blended with my later endeavors I can tell you that you can't really create nice looking rockers using a CAD package. This is something that really does need to be hand-drafted in order to look graceful and natural. I usually start out by locating the axis points and what I call the control lines and then just 'free-handing' a profile that I think looks good. I'll refine this rough sketch using several French curves and then cut the final pattern with a bandsaw or transfer it back to a CAD program if we're going to be doing any CNC cutting. The hand-drawn patterns, in my opinion, look about 100% better than anything I've ever done using a computer.

As you're developing a pattern you have to remember that a typical rocker revolves thru a full 65-degrees of rotation from full spring extension to full spring compression so you have to watch for potential interference with the rocker and the leg end caps and also potential interference with the sprung (front) legs of the forks and axle itself. I've seen some really cool designs that ended up being a disaster when they tried to actually use them on a bike. I learned this lesson the hard way so I want you to avoid that experience if at all possible.

Summation

In summation all I can say is that if your considering buying or building a Springer then you also need to consider looking into using some type of dropped rockers to optimize those forks for your particular bike. The days of the old 'off-the-shelf' so-called custom Springers are coming to an end thanks largely to innovators like Sugar Bear.

Thankfully more and more Springer builders are finally catching on to the advantages of dropped rockers and providing some new products for their customers. I've seen several small shops now using dropped rockers on their semi-custom Springers and I hope the trend continues. Keep in mind however that you can't install dropped rockers on an existing front end as they require longer legs so this style of rocker is not a 'retrofit' type of deal.

Always keep in mind that mathematics and geometry are great tools but please don't get so embedded with technical perfection that you loose track of what building a Chopper is all about. This craft is largely a means of artistic expression and as such you will often find yourself pushing the envelope far past what the 'experts' have to say about almost everything with respect to motorcycle engineering. Nothing is chiseled in concrete when it comes to building bikes of any kind. You cannot build Choppers from 'textbook' formulas or equations and even the stuff we publish here is just 'guideline' type material.

We will eventually post full-size patterns for the most popular sizes of dropped rockers but I think that most readers have enough information now to design their own creations without to much trouble.

This article is still being expanded on so check back periodically to make sure you have the latest version.

 

tO

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