Internal Suspension Girder Forks

The Harman Internal Suspension Girder, called a ‘Spirder’ or ‘Spurder’ nowadays was a revolutionary front fork suspension system in its day back in the early seventies.

There is still some historical uncertainty about exactly how the system came to be developed and who actually came up with the idea but by most accounts John Harman conceived the concept that still bears his name today.

The system was so unique for it’s time that it was patented in 1971 (approved in 1973) which in itself was somewhat unusual as almost no chopper builders were bothering to patent their ideas back in those days. I guess he thought competitors or his co-workers would rip him off if he didn’t protect his ideas. Paranoia did run deep in those times.  

 

For those folk who don’t know about Chopper history, John Harman was probably one of the most prolific builders this old planet has ever known and his legacy runs deep. He didn’t just build forks and frames but also put together some pretty radical power plants.

He is however best remembered for his Internal Suspension Girder (ISG) system.

At first glance these forks look incredibly simple but in my personal opinion they are perhaps the most complicated and complex forks a person can set out to build. This may be why so few independent builders have attempted to duplicate their design. As far as I know, no custom bike builder (not fork builders), anywhere, has ever successfully built a working copy of a Harman fork that can actually be ridden on the road. That should tell you something if you’re planning on trying one of these yourself.

It’s not that they are impossible to build. Bill Holland, who was Harman's partner after H&H still builds his version of the forks through a company called ‘Executive Choppers’. Their product is a ‘modernized’ version of the original Harman design. Jaxon Davies also builds his version of the forks and these are excellent front ends exhibiting outstanding machine work, perhaps closer to the original Harman’s than even the products provide by Bill Holland. Jaxon's fabrication system and fixtures look like something you'd expect to see being used to build aircraft components.

The problem is that compared to most fork systems the Harman has to be built with almost zero tolerance to work properly. Conventional Springers, Leafers, Girders and even Hydraulic fork systems can be build with extremely wide tolerances and still function relatively well. In fact some of these conventional systems can literally be ‘bashed’ together and still ‘work’. This isn’t the case with a Harman.

This need for precision may be why Harman himself never made a success of his own fork system. If you ever get a chance to see some of his work close-up and in person you can see that no two forks were ever quite the same. Every individual item was well made but you can tell that he was searching for some means to make these forks on an assembly line but it just didn’t happen. In fact some of his forks just didn’t ‘work’ to begin with and many buyers took them off their bikes after only a few days of riding. There were a lot of problems with ‘trail’ and ‘flop’ on some bikes, not to mention hardware failures.

There were also no end of problems with this early designs that used the ‘single’ slab type rockers since like a lot of ‘inverted’ springers there was no way to synchronize the movement of the individual rocker arms. Sometimes the right or left arm swung a little further than its opposite counterpart and that leads to some hairy-ass steering to say the least.

Basically his early forks were really good as a custom item on a totally custom bike but not so good as a bolt-on item to an existing bike; hence no market for the product because it’s a truly ‘custom’ fitment if you want it work properly. The later 'revisions' of the forks undertaken after 71 improved things considerably and another revision around 1974 represented another step towards perfection.

There is no doubt however that a ‘Spirder’ is about the most beautiful front end a person can put on bike. I don’t imagine that anybody will dispute this fact. How many have you seen? Probably very few, if any at all.  There is a reason for this and you need to think really hard on why this is before you try to build one of these fork systems.  

 

They need to be precise and this means spending time and money on jigs and fixtures and machine tools. Plus you’ll have to do some prototyping before the final product is fit to ride. These forks are not well suited to garage type building. I know for a personal fact that Jaxon spent years developing special tooling and techniques before he got his first ‘useable’ set of forks built. I imagine that Bill Holland has at least a couple of years of development work behind his first products under his new trade banner and he's been building them for almost 40-years. 

The photo below is one of Jaxon's steering stem/tree assemblies and there is a lot going on here that you don't notice until you start taking this thing apart. It's extremely complex but it works and eliminates a lot of the problems associated with the original designs.

 

One of the biggest problems with the original Harman forks was that they only fit on certain bikes that had a very specific neck length which of course included your bearings, grease covers, etc. If you happened to have a slightly longer or shorter neck length then you were basically S.O.L. with respect to using one of his products unless you cut it and believe me a lot of people cut the crap out his forks as seen in the snapshot below. The old original forks appear to have used three more or less standard dimensions between the tress and I suppose you needed to use shim washers to make adjustment in the stack height to get a perfect fit. There was a spacing for Sportsters, Hondas, and Big Twins.

 

 

Hacking up those bolt holes might look pretty horrible but keep in mind that back in the day we'd do just about anything to everything with whatever tools we had at hand. The original owner of these forks obviously had a neck that was a little taller than 'normal'.

Jaxon Davies has spent years in developing an 'adjustment' system incorporated into the trees for his Spirders so that this neck length problem isn't as critical as it once was but there are still limitations a person has to live with.

Before we talk about specific details it should be pointed out that these forks look best on 'long' bikes. Even though the ISG forks look like Girders you don't have the same visual impression. This is probably because Girders are typically mounted well out from the steering stem making them look longer, while some ISG's are mounted almost flush with the stem so they appear visually 'shorter' than they actually are. I once built an ISG that was 12-inches over to fit one of my 4-up frames having a 40-degree rake angle. The end result looked like crap in my personal opinion. As a result of that project I would not bother to build another one unless it was at least 15-inches over stock (35" from the lower edge of the bearing cup). Even that might look a little 'stubby' to a lot of people.

The forks in the photo below are 15-over on a 45-degree rake but the neck height is the factory stock 27-inches so the forks look a little short to my eye.

 

A nicer looking arrangement might be something like the setup shown below which 24 over on a frame having 6-inches up.

 

 

Or you might prefer something a little radical like this:

 

 

Or maybe even a bit more extended like this:

 

 

Or even more radical like this:

 

 

Trail on a Spirder can be managed much the same way it is on a Springer but most of these photos, taken from various magazines, were taken long before most folks worried about trail issues on a chopped bike.

If you’re prepared to spend the time and money to build this type of fork system then stick around and we’ll show you how to do it. Keep in mind however that this may be a slow-growing article as we expect the full development of a CBH version of the Spirder will take a lot of time.

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