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Custom Forks

During WWII, the two great American motorcycle manufactures of the time, Harley Davidson and Indian supplied the military with bikes. The H-D was equipped with a springer, and the Indian sported a guider fork.
Now 50 to 55 years later, custom built versions of both styles are still considered vogue on the chopper.
Several manufacturers are hard pressed to meet the unrelenting demand for custom front ends, as their buying public eagerly consumes everything they produce.
Some of these manufactures are safety conscious and knowledgeable, producing a fork which is well designed and strong. Unfortunately, many other manufacturers turn out forks which range from marginally safe to downright junk. Motivation probably varies, but the margin of profit factor may be responsible for most of the junk flooding the market. Lack of engineering expertise more than likely accounts for the balance of otherwise well meaning makers of dangerous products.
Custom forks cost a lot of money to buy, and they cost a lot of money to build. The cost of manufacture rises sharply as conscientious builders satisfy requirements which produce a safe end-product. In many cases the retail cost of a dangerous fork is well below the manufacturing cost of a safer version.
That fact alone is a good rule of thumb for the prospective custom fork buyer. If it is cheap, stay away from it. It is virtually impossible to build a safe fork at a bargain price. While following this basic rule will eliminate many bad choices, it is not a fool proof plan, for even among the high priced forks, potential death traps are all too common.
In order to separate the good from the bad, the buyer must now resort to his critical eye and an above average knowledge of metallurgy, design, and engineering. He must also be aware of motorcycle handling problems, the pitfalls of chrome plating, and good welding practices.
It is my hope that this article will do much to acquaint the reader with the needed expertise when selecting his fork. If each fork sold was the result of a careful and knowledgeable choice, the builders of sub-standard equipment would soon be forced to either upgrade their product or find another means of livelihood, and it would go a long way toward improving the motorcycle accident statistics involving equipment failure.

DESIGN SAFETY

Basically, custom forks fall into three categories: springers, girders, and a sort of combination of the two. Advantages and disadvantages can be attributed to each design. In addition, poor manufacturing techniques can render any type unsafe, so the choice is wide open. It will be necessary to dig below the surface in order to choose wisely.
The springer basically consists of a rigid fork, affixed to the frame's steering head; and a sprung f9ork, equipped with a combination of compression and rebound springs. Both forks are connected to rocker arms which pivot at the rigid fork and connect to the front axle.
The girder consists of a pair of triangulated legs which are directly connected to the axle. A separate tree section bolts through the frame's steering head. Four leading link arms connect the two sections which are also joined by one or more springs or spring/shock combinations.
Forks comprising category number three are technically springers which resemble girders. Two girder-like legs,connected directly to the steering head, are equipped with pivoting rocker arms, to which mounts the axle and a plunger rod connected to either coil springs or torsion bars.

SPRINGER OPERATION AND DESIGN

The springer's rear, or rigid fork is constructed so that it is heavier and stronger than the front, or sprung fork, as most of the loads transmitted through the sprung fork are absorbed mainly by the four large coil springs.
The consistency with which the springer handles over bumps or pavement irregularities is governed by the rate ratio of the rebound springs to the compression springs. When the compression springs are overly favored the front wheel will constantly hop off the pavement, even when it is reasonably smooth. Some spring rate ratios favor the compression springs so much, that at highway speeds, the tire seems to contact the pavement only enough to keep the bike moving in a straight line, as the front wheel spends much of the time several inches in the air.
A rebound spring favored ratio will cause the front wheel to chop, which can be annoying. In addition, a very hard ride will result. It is sometimes difficult to tell what ratio, if any is favored, and since the ratio is determined by the springer's design, it is almost impossible to rectify.
If either pair of springs seems to be compressed more than is necessary, it could indicate an adversely favored ratio. The real test is to ride along side of a bike equipped with the springer in question, and observe its action, or to actually test ride one yourself.
The springs must also be positively located on each end, either in a machined well or over a close fitting spur. If the fork is to handle well, it is imperative that the springs stay in their proper location.
The spring rods are subject, not only to an up-and-down motion, but a fore-and-aft rocking as well. As with the springs, the spring rods must be positively located.

When shopping for a springer, check carefully as to what method is provided to locate the spring rods while permitting full travel. The spring rod sliders should be replaceable as much of the springer's wear is encountered in these areas. If the system looks inadequate, it probably is, as the sliders cannot be too rugged.
Machined ends on the legs are always preferable to flattened ends. Flattening is maximum deformation of the tube, and fatigue cracks are sure to be present. In time, the cracks will enlarge causing an ultimate failure.
The rockers are usually the weakest design area. The rocker and its trunion pins must support about half the weight of your fully loaded bike. In my opinion, the wildest shaped rockers are directly indicative of a lousy springer. Perhaps not always, but you can pretty well bet that a set of cutesy Alladin-shoe rockers will be found on a piece of junk. And, ditto for twisted-stock legs.
Rockers must be provided with large diameter trunion, or knuckle pins if springer is to give long, maintenance free service. Too often small needle bearings, or the likes are the likes are fitted to bolt shanks, which are intended to pass as trunions.
Needle bearings are designed for relatively light loads and a rotating motion, in a generously lubricated environment. The plain bearing or bushing, is designed to withstand heavy loading, and it is much more suited to the oscillating motion encountered with fork rockers. As with other quality features, though, large diameter bushed trunions are too expensive for many manufacturers to contend with.
Look for precision machining in a quality springer. Things like reamed holes, blanched and centerless grinding, and close tolerances are more important than spike nuts and a low retail price.
The precision and safely made springer may lack octopus shaped rockers, flesh gouging dagger nuts, and curly-cue twisted-stock, but then, the quality will speak for itself, and it is unnecessary to distract the customer with flashy goo-gaws.

GIRDER OPERATION AND DESIGN

The girder is usually designed with a pair of triangulated legs made of equal diameter tubing. The triangulated section forms a truss, which is quite strong. It is hard to fault most girders from the standpoint of strength, but most are affected to a varying degree by adverse handling characteristics.
Even a girder which is geometrically correct [and few are] presents a large amount of unsprung weight, causing handling deficiencies. Granted, some of the solid steel springers contribute as much, or even more to unsprung weight, but at least those springers constructed of tubing do not add much unsprung weight.
Geometry i of utmost importance in girder design. An incorrect design can easily result in zero or negative trail. Under such a condition, high speed wobble becomes a major concern. The geometry of a girder should be carefully checked to be sure that at least three inches of positive trail will result on your bike.
I say your bike because the amount of rake in your steering head will contribute to the resultant trail of a girder. Generally it is safest not to rake your frame if you intend to install a girder.
Girders that are designed with forward swooping legs are even more prone to produce negative trail. The four leading links, connecting the girder to the trees form a pair of parallelograms, if the girder is correctly designed. All four links must be of equal length. Often a manufacturer lacking basic engineering knowledge will produce a girder with extra long bottom links. In this case, parallelograms are not formed by the links and you better believe some horrible handling will result.
In these cases, every time a bump is encountered, the girder will rise in relation to the steering head, causing the bike's wheelbase to increase. After passing the bump, the momentum of the frame, forced to rise in order to relieve the compression of the girder spring, will continue to rise as the girder's relative position lowers. This condition will cause the wheelbase to decrease. Once stabilized, the wheelbase will return to its original length, until the next bump is encountered. Imagine, if you will, what an elastic wheelbase does to the bike's handling.
Of course, the enterprising owner of a girder can shorten the lower links to match the length of the uppers, or build new ones, to correct the obvious handling deficiency. But consider this; if the original manufacturer did now possess such basic knowledge, one can imagine the integrity and craftsmanship in the rest of the girder.
Most girders possess yet another major design flaw. The make use of a single spring. And springs have been known to break, especially improperly chromed springs. If a single spring breaks on a springer the instant shift in rocker loading will cause some discomfort, to be sure, but not to the extent that the bike cannot be brought to a safe stop, under normal conditions. However, if that single spring on a girder snaps, you will probably find yourself in deep trouble.
The girder will instantly allow the frame to drop until it bottoms on the pavement. At the same time, its lack of loading will cause the front wheel to offer essentially no steering control. The hapless rider will be extremely lucky if he doesn't go down.
Alternatives to the single spring include dual spring and spring/shock combinations, both of which are on the market, and in either case the design deficiency offered by the single spring is eliminated.
Although, as stated, the girder is a very strong design, improper welding techniques, poor alignment, inadequate bushings, and undersize tubing can render it substandard.
If proper ratio between the overall length of the girder, and the diameter of its tubing is not maintained, elastic buckling can occur during normal riding. Be sure that the girder of your choice has relatively large diameter tubes in relationship to its length.

THE MECHANICS OF FORK STRENGTH

In order to positively assess the strength of a fork, a rather complex knowledge of metallurgy, engineering, and physics is necessary. However, a brief course of the basics and specifics will no doubt be an asset when buying a custom front end.
First, it must be understood that anything can break. Our planet could be shattered if it meets with powerful enough force. So we mush assume that all forks can be broken. A safe fork then, is a fork that will not break when it encounters ordinary riding forces.
If a safe custom fork were on your bike as you rode headlong off a 40 story building, chances are it would break, along with your impacted wisdom teeth. Yet such a force could not be considered a normal riding force.
Establishing the norm is difficult, if not impossible. For that reason, a margin of safety over and above the calculated norm is desirable, since normal service will always be exceeded.
Of course, in designing a safe fork, certain compromises must be made. A whittled solid block of steel, measuring four feet by one foot square would more than likely meet all normal riding forces with impunity, but aesthetically it would be hurting, and its weight would probably blow out both tires.
So then, the designer is faced with the problem of making the fork light in weight and looking good, and as a result, many forks have a safety problem.
In addition to building a fork so that it will not break, it must be able to withstand high loads that will just bend the part, for in a fork, bending will usually be enough to render it dangerous or useless.
Forks are subjected to locked up internal stresses and hydrogen embrittlement during manufacture, and several stresses, forces and vibrations when in use, all of which are damaging. If the fork is correctly built, the manufacturing molecular damage can be corrected, and the rigors of usage will be well within the fork's capabilities. If your fork is substandard, you will have to take your chances as the natural laws of physics ultimately and systematically destroy it.
The manufactured stresses will be covered later, but now let's take a look at what can be expected to happen as you ride.
Steel is not a rigid, unyielding substance. It can be likened to rubber and chewing gum in some instances to illustrate some of its properties.
Steel is elastic, like a rubber band it has the ability to return to its original shape after deformation. A rubber band has a limit, you can only stretch it so far before it breaks. That is it's limit of elasticity. So it is with steel, although to a far less degree. The degree varies with the proportions of alloying elements.
A glass rod is brittle, that is, it is the opposite of elastic. Even a slight deformation will cause it to snap. Applying the relationship to steel, one can see that the less elasticity possessed by a certain alloy, the more brittle it becomes.
Elasticity is a result of the molecular structure of the substance. A very old piece of glass or an old rubber band will break more easily than when they were first made. As they age, molecular changes take place that alters their limits of elasticity. Age brings about similar changes in steel, and age can be artificially induced by any number of manufacturing processes.
Steel is malleable, like chewing gum, it can be shaped and reshaped. Softer metals, such as lead and gold can be pounded into very thin sheets. Harder metals are less malleable and some steel alloys are less malleable than others.
When a fork is brand new, the consistency of the steel is not uniform. A safe fork will be more consistent than its poorly produced counterpart, but even the good ones will have hardened areas and softer portions. As noted the harder areas are more brittle and so are more prone to breakage. If a fork could be made that is of the same temper throughout [is is possible] localized high stress areas would be eliminated, greatly extending the life and integrity of the piece.
Vibration is constant when the bike is being ridden, and vibration attacks the hardened areas. Microscopic cracks appear after a time, which gradually widen and propagate until the area fails. The time involved depends upon the severity of the vibration, the brittleness of the area, and the extent of locked up stress induced during manufacture.
If much care, proper procedures, and high grade steel was used during manufacture, the area will probably resist vibration damage well beyond the life of the motorcycle. It should cause no concern if the fork is well made.
Road forces cause the metal in the fork to be deflected in a variety of ways. The types of deflection, or stresses most affecting a fork are tension, compression, sheer, and torsion.
Tension is the stretching of the piece. Normally only a binding force produces critical tension in a fork, as a fork can easily support and lift its weight, as during a wheelie. When a load is applied at right angles so as to bend, or attempt to bend a piece, the side opposite the force will be in tension. The tensile strength of the steel relates to the amount force necessary before the part in tension will fail.
Compression is the opposite of tension. It is the compression of tension. It is the compressing, or squeezing of the piece. Again only a bending force causes critical compression in a fork, as it should easily support the weight of the bike. A bending force at right angles will place the piece in compression on the side nearest the force. When the force is strong enough to overcome the compression and tensile strength of the part, elastic buckling will take place and the part will either bend or break, depending on the properties of the steel and the severity of the force.
Shear is a force caused by two opposing forces or a single force upon an immovable or anchored part, such as the type of stress occurring during the operation of a paper hole-punch. Sheer is the most destructive stress encountered in forks, if the front axle snaps when hitting a chuck hole, shear is the culprit.
Torsion, or twisting, most often affects the springs. Although it is encountered elsewhere in the fork, its effects are seldom damaging.
The yield strength is that at which the piece can no longer resist a force without permanently deforming, or in some cases breaking. When a force is great enough to exceed the yield strength of a part, damage is sure to occur.
Ideally [but not usually] a fork is designed so that all the parts have a minimum yield point far in excess of calculated maximum normal road shocks. It is impossible to describe the maximum normal shock, so to be on the safe side, the fork is designed to withstand something like hitting a curb at a moderately high rate of speed and then multiplying the force by from three to seven times as a safety factor.
This does not mean that if you have a good safe fork, you should make daily demonstrations for your curbside cronies. It simply means that in an unusually high loading situation, you can probably depend on your springer or girder not to let you down.
A stress analysis for a certain girder once used a hypothetical situation in which a bike strikes a three inch [high] piece of lumber in the road at 60 mph. The front tire was figured to be 26 inches in diameter, and the rim to ground distance was 2-1/4 inches. The fork was raked to 38 degrees.
Upon impact the peak force generated would reach 5700 pounds. That much load would first compress the spring until it fully bottomed. The load would then be transmitted to the entire girder structure. The weak point in the analysis proved to be the lower spring bar mount, which would bend, but no future damage would occur.
Hitting the lumber at a much higher rate of speed, 120 mph, the linkage shafts would shear, although such a thing require4d a force of 17,600 psi. Shearing your girder's linkage shafts at 120 mph is one helluva way to start the day.
The fork in question is today one of the safest and respected girders in the industry. Most custom forks would not fair nearly as well, but then original factory equipment is sometimes not much better than even the flimsiest of custom forks. The NCCSI fork test machine made mincemeat of a popular original equipment fork designed for a so-called super bike. The test had to be terminated shortly after it began as the fork was too wasted to continue.
It would appear that although a small handful of custom fork builders are keenly interested in producing a safe product, some of the motorcycle factories are content to rest on their trademarks.
Countless inferior custom forks have broken. Often at the expense of the riders life or limb. I remember a dude who broke seven springers in one year. Each time it broke he would take it back to the builder, who would replace it [many will not even do that]. The disgruntled rider wanted his money back, but settled for an "improved" model each time. Finally after being lucky seven times in a row, he forgot his refund and bought a decent springer. As far as I know it hasn't given him any trouble since that day three years ago.
As I said before, some manufacturers want to make big profits fast and others simply do not know what they are doing. For your own protection then, it is good to know something about what goes into building forks, or at least, what should go into it.

METAL SELECTION

Custom forks are build using either mild steel [usually SAE 1020], chromemoly [SAE 4130], or a combination of each. Chromemoly is the most expensive choice and probably the best, except for one single drawback. I don't know of a single manufacturer who welds it properly.
Mild steel is very easy to weld and it poses no special problems. On the other hand, 4130 is not easily welded and a multitude of problems are created. I am not saying that it is impossible to deal with, I only state that I know of no builders who are disposed to weld it correctly, and who fully correct the weakened metal afterwards.
When 4130 is produced, it is cold-rolled through dies which hardens it considerably. The pressure and friction causes the metal to move along slip planes until it conforms to the dictates of the rolling dies. The die forces are uneven and the resulting stresses are numerous and distributed randomly.
If left as is, minute cracks would result as the stresses attempted to relieve themselves. As the hardened areas are quite brittle, compared to the surrounding sections the cracks would rapidly enlarge until metal fatigued to the point of failure. Chromemoly has a marked tendency to harden as it is worked and the locked up stresses must be relived artificially.
They are relieved through a process known as normalization. The metal is uniformly heated to about 1650 degrees F, at which point the molecular structure is fully austentic [the layered components are dissolved into a homogenous substance]. The metal is then slowly cooled in still air to room temperature.
Subsequent working such as bending and flattening, and welding will again induce locked up stress which must be similarly normalized to preclude eventual failure. When a stressed section is welded, the cooling contraction which follows amplifies the stresses. Then when the part is loaded, the internal stresses spread until all of the metal that is available to carry the load has been stressed beyond its yield strength, at which time permanent deformation starts.
A welding jig can be designed and a welding sequence can be used which help eliminate internal stresses. If such measures were taken by fork builders, a giant step for safety would result. But, adding to the already difficult task of building a safe fork, some manufacturers compound the problem by using 4130 instead of mild steel.
One of 4130's biggest drawbacks is that it has the property of being white short, which means that is has extremely low strength at white heat, or above 2000 degrees F. Of course the temperature of a fork in use will never approach that, but when the fork is assembled by welding it does exceed white heat.
Chromemoly is so weak at white heat that every precaution against jarring or bumping the weldment must be taken to preclude cracks. Many fork manufacturers are blissfully unaware of this critical shortcoming of 4130. It is doubtful that even those in the know exercise proper care.
Another drawback of 4130 is that it becomes very brittle [lack of elasticity] when it is hardened excessively. It is hardened by heating and then quenching. The effect takes place when a weld is made on cold 4130. After welding, the base metal immediately adjacent to the weld becomes heated much more than the cooler metal, which provides the quench, and a zone about 1/4 inch from the weld on both sides of it, becomes quite brittle.
This unsavory situation can be eliminated by gently pre-heating a large area near the weld zone to a dull red. Pre-heating lessens the temperature gradient and permits the area to cool more slowly, eliminating the undesirable hardening.
Chromemoly is an air-hardening metal, while mild steel is not. So when building an identical fork using 1020 instead of 4130, the brittleness zones do not occur. After all welding is completed on a 4130 fork many locked up stresses are bound to occur, even after all of the rules have been followed.
The entire 4130 fork should be normalized at this time as it was when the tubing and plate material were first wrought. This would require rather large ovens to accomplish properly, simply passing the flame of a torch over the weldment does not accomplish normalization. You can bet your life that probably no one in the industry sees to this all important step.
Even the preliminary heating takes too much time for some, and as every manufacturer knows, time is money. I know first hand of at least one of the biggies in the custom fork field who doesn't waste time with the pre-heat procedure. According to him, it is not necessary when heli-arcing [TIG] relatively thin walled 4130 tubing. Maybe, he's getting by with it, but I wouldn't ride a bike with a 4130 fork welded the quickie way.
Chromemoly has several advantages, though, IF it is welded following all of the rules. In the normalized condition, commercial 4130 is a relatively high strength steel. Its 85,000 psi yield strength is far higher than that of about 51,000 psi for 1020 mild steel. However, assuming that sufficient wall thickness and diameter are employed, a 1020 fork's strength will far surpass its intended function, and leave a wide margin of safety.
When 4130's elongation is examined it does not fare so well. Elongation is the measure of brittleness. The figure is given in percentages, and the lower the number, the more brittle the metal is. Mild steel [1020] has an elongation of 25% compared to that of only 14% for normalized commercial grade 4130 [used by fork builders]. The elongation figures show that 4130 is considerably more brittle than 1020. When chromemoly's brittleness is coupled with micro-cracks and locked up stresses induced by improper welding techniques, it should be obvious that a welded 1020 fork has potential safety advantage.
An even better 1020 fork could and has been manufactured using low temperature silver brazing instead of welded joints. Upon completion the fork was virtually stress free and very safe. As a matter of fact, Harley's original springer was assembled by a similar low temperature method-furnace brazing. You have probably noticed that many are still in service and going strong after more than 55 years.
There is of course, an easy solution for 4130 forks. that is to eliminate welding altogether, if the metal's unflexible welding rules can't be followed. I do not know of any brazed 4130 forks on the market [brazing temperatures are far below critical] but there is at least one decent bolt-together model available. The trick in a bolted assembly is in the design. Properly executed it will be at least as strong as a welded fork, but unless considerable engineering has entered the picture, it could come apart like a tinker-toy front end when the going gets rough. It pays to check carefully before you buy.
Another thing to be wary of when buying a fork is in the machining. The custom forks looks really trick when beauty grooves are machined into the legs prior to assembling and plating. Be aware however that such grooves and even rapid changes in the thickness of the material causes an increase in the stress level at critical points during operation. Alternating loads can cause fatigue failures at these points. They should be avoided. If that is not possible, the transitions should be very smooth and gradual.

HYDROGEN EMBRITTLEMENT

The first thing that catches the eye about a custom fork is the shiny chrome. Unfortunately, at the last stage of fork manufacture, the plating process could damage an otherwise safe front end, unless more rules are followed. And, just as unfortunate is the fact that following the rules will cost the builder more money.
During the cleaning process and while the nickel plating is being deposited, the steel will absorb atomic hydrogen. The hydrogen penetrates the steel's molecular structure where it forms hydrides and hydrated ions, causing changes in the molecular volume. The reaction ruptures the surface layers of the steel, accumulating high stresses in the process. Unless the embrittlement is relieved the steel will snap like a dry twig.
Relieving is accomplished by baking the piece, after plating, usually at about 375 degrees F for about three or more hours. Hydrogen embrittlement is especially destructive in steels with a tensile strength of 160,000 psi and over. So plating is an particularly dangerous process for spring steel and more critical for chromemoly than with mild steel. As I mentioned before, a potentially disastrous situation can arise when a fork is equipped with a single spring.

SELECTING A FORK

While taking photographs for this article, I ran into a chopper shop owner, and we discussed forks for some time. He mentioned that he was not in favor of an article of this nature, as he felt it would adversely affect his business. He stated the "less they now out there," the more forks he would be likely to sell. Questioned further, he revealed that he stocked quality forks as well as some cheap junk. Even though his profit was somewhat higher when he sold a good fork, he felt that since they cost almost double the price of a cheapy, he would, in the long run, bring in more cash selling cheapies. "Besides," he said. "the dudes who make the cheap fork are real good about taking them back when they break." To me his philosophy makes no sense. Surely, when a customer is happy with a product, he is more likely to return than when he discovers he was stuck with a piece of junk. Even a cheap fork sells for a tall stack of change.
The more knowledgeable buyer would save a little longer, or scratch a little deeper, but he would eventually end up with the fork of his choice. It is unreasonable to assume that his desire for a girder or springer would diminish in the least because of a bill or so in difference in price. The proof of this lies in the fact many quality forks are being sold, in spite of the cheapies.
I have no doubt that the custom shop owner encourages the purchase of junk forks to all who are confused enough to listen. A custom fork is a major purchase, one upon which your life may very well depend. Ask pertinent questions before you buy. If the counter man doesn't know, call or write the manufacturer, but be advised that you might get a snow job. If the builder has a good reputation you should be able to rely on his word. If the place is questionable, it got that way for a reason.
After all, all you really want is a fork that will lead you on your merry way, without breaking your neck.

Custom Chopper 10/75...Author unknown

Donnie Smith makes a quality girder front end among other as well as Denver's Choppers for a well build springer front end. You can find Smith's front ends are available from Chrome Specialties @ 800.811.8904 for a local dealer and Denver's web site is www.denverschoppers.com.

****MURPHY'S LAW****