girder-3

Girder Forks

Trees or Yokes

I personally prefer to call fork ‘Trees’ by the term ‘Yokes’ when speaking about Girders or Springers but I think everybody understands the particular pieces I’m talking about.

For Girder forks a person can build Yokes that range from the incredibly ‘crude’ up to incredibly ‘cool’ depending upon the amount of time and money they’re willing to invest.

Both extremes of design sophistication will get the job done so again it’s a matter of personal desire, machinery available and budget that determines what you’ll eventually come up with.

Stock Yokes on old bikes were extremely crude castings, many of which you can still buy from various restoration suppliers if you’re doing a ‘retro’ bike but the trend today is to use a ‘composite’ or ‘combination’ approach where the yokes are fabricated from both pieces of plate stock, and round or square tubing, or solid bar stock.

The images below illustrate just a very few of the common yoke designs that folks have sent in to the site.

If you access to a lath and a milling machine you can build Yokes in ‘billet’ fashion but for the cost of materials and the amount of work involved most people prefer to fabricate the Yokes using the ‘built-up’ technique.

Simply stated the ‘composite’ or ‘built-up’ method involves using pieces of cold-rolled plate or wide strap stock to construct the main segments of the Yoke and then welding-on tube spacers for thickness, if needed, and sections of DOM tubing or solid bar stock for the link pivot points. You can easily see examples of the composite fabrication method in the pictures above.

There is no cut and dried system, formula, technique or pattern for building Yokes out of separate pieces. Every builder will invent their own unique design based largely upon available materials and the equipment on hand. Here is a simplified exploded view of one possible scenario.

For instance you could simply weld a piece of thick plate stock, drilled for the steering stem, to a piece of DOM tubing or sold rod for the pivot carriers. Another method might involve building up plate thickness by using multiple pieces of thin sheet stock into a thick ‘stack’ cut every-other plate about one quarter inch undersized and you have a ‘ribbed’ looking Yoke to weld to the pivot shaft tube or rod. Another method uses a semi-billet approach, where the main flat portion of the yoke is milled from thick steel or aluminum plate and then welded to a pivot shaft or pivot tube. The possible fabrication combinations are numerous.

Illustrated below are three of the most popular basic Yoke configurations.

In the first illustration the links pivot on a machined shaft that is welded into a DOM tubing sleeve that is in turn welded to the main segment of the upper Yoke. In the second illustration the links are attached to long shafts that run through a piece of bushed DOM tubing and in the last illustration the links attach with shoulder bolts that screw into a threaded tube. in all cases the lower Yokes are similar. Keep in mind as mentioned elsewhere that it's not uncommon to mix and match various connections.

Remember that Girder Yokes are very different than Springer or Hydraulic fork trees in that on Springer and Hydraulic systems it is the lower tree that takes the vast majority of the structural loads and the top tree is, in a way, just going along for the ride from a strength standpoint. On Girders both yokes are usually loaded very nearly to the same levels and have to be identical structurally.

Copper or brass yokes and even forks are easy to do with Girders for those wanting something out of the ordinary.

Yoke Geometry

Older bikes usually adopted yoke geometry where the lower link pivot tube or pivot shaft ran at right angles to the neck and backbone immediately below and on the centerline of the steering stem. The upper link pivot tube on the top yoke was placed about 1 to 1.5 inches forward of the stem nut. This configuration is about as close as one can get, with a girder, to the equivalent ‘zero offset’ found on Springers or hydraulic fork systems. In effect this configuration is usually considered the baseline for girder fork yoke geometry. It is possible with a little creative machining and welding to get both link pivot tubes centered exactly on the steering stem but from a handling standpoint nothing is to be gained by doing this although it looks pretty good.

One advantage of a girder over other fork systems is that you can run a significant amount of offset to make a short bike look longer and you can offset either of the link pivot points on the yokes to make changes in trail or to make a bike with a shallow rake look like it has a long front-end. In fact with a girder you can build trees that actually have the link pivot points ‘behind’ the steering stem axis and it will handle just fine. Offset as we typically think of it applied to Springers and hydraulic forks just doesn't apply in the same ways on girders so you have to do some mental reconditioning and get rid of many preconceived notions about forks in general to fully appreciate the potential that girder forks offer the typical rider.

It is extremely hard to describe just how ‘open’ girder suspension really is to experimentation, artistic expression, easy of fabrication and low costs. Girder forks simply cannot be compared, at any level, with other suspension systems.

One of the ‘sticking’ areas however to building a true ‘low-cost’ girder system is that sooner or later you’re going to be faced with making a custom steering stem for your yokes and unless you have access to a lath you’re going to have to pay somebody to do the work on your behalf. This isn’t an item to cut corners on as your life depends upon the integrity of this small and seemingly insignificant piece of steel.

The actual dimensions of the stem will depend entirely upon the dimensions of your particular neck-bearing-yoke combination and it’s a very good idea to haul all of these pieces down to your local machine shop. You have to trust your machinist, not the Internet chopper ‘experts’ with respect to the material selected for the stem piece.

The Internet ‘experts’ will tell you to use threaded chromo tubing since it’ll save you 0.01 pounds of weight and it’s just the ‘trick’ thing to do on a custom chopper. Other experts will say that the stem has to be stainless steel or titanium with all kinds of special heat treating, etc, etc.

Nine chances out of ten your local machinist will have the right material on hand to make up a nice solid stem that has just the right amount of surface hardness and tensile strength to do the job without fracturing somewhere down the line. My guy likes 1040 mild steel and his partner likes 303 stainless. We haven’t tried the stainless yet. Your own guys will have their own ideas but if you don’t have 100% confidence in your machinists then shop around until you find a guy you trust more than your own doctor. Your life depends on it.

There are two ways to mount the stem on or into the lower yoke. One way is to simply drill a hole in yoke, cram the stem into it and then weld the stem in place from the bottom. The other way is to drill and tap the yoke to accept threads placed on the lower end of the stem, screw it all together and then tack weld the stem in place. It’s six one way and a half-dozen the other. The important thing is that when assembled, the stem is at exact right angles to the yoke, in other words perfectly perpendicular to the lower bearing surface of the yoke. There should not be any room for error whatsoever.

Continue to Part IV………………………………………

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