Do It Yourself - Dick Allen Springer Build
At the close of page one we left off talking about the upper and lower trees and starting off here I'd like to go back for a moment and talk about what's typically called the 'Spring Bridge'. That's usually a piece of tubing, solid bar, round bar or even a casting that connects the two front 'sprung' legs together and also supports the bottom ends of the compression springs and the Spring Rods.
On this project we'll be building two different versions of the Bridge. For most home builders using a piece of square or solid steel stock is the easiest solution to use as the Bridge. In the first version we're building here I used a piece of bar stock 1.25-inches wide by 1-inch deep. This is a fairly typical design element seen on all kinds of Springer's including some of Dick's custom forks.
Almost every Springer made today is based upon a design developed by Harley-Davidson back in 1907 called the 'Bottom Link Design'. These forks went through several stylistic revisions up until 1948 when they were taken out of production. Harley spent a huge amount of time, money and effort in perfecting the Springer forks so when they did something to the system it was for a very good reason.
One thing Harley did was to invent the 'improved' Spring Rod that didn't pull itself out of the socket under the load of the compression springs nor push itself through the bottom of the Spring Bridge under suspension load. They way they did this was to machine a very slight taper into the lower end of the rod. Said taper matched the taper machined into the Bridge. It is a fool-proof and flawless design element and one I copy on all of my Springer's. Actually most builders do the same but you will see some builders, even famous ones, who try and take shortcuts by welding the Spring Rods into place or buy threading the ends and simply screwing them into threaded holes. Neither method is very durable but they are cheap to produce this way.
I'm assuming the home-builder will want to build something that's durable and similar to Harley's original design. It sounds complicated to cut the tapered hole in the Bridge but it's very easy to do using a little tool called a 'taper reamer'. The taper on the Harley Spring Rods is 1/4-inch per foot and in machinist lingo they call this a #8 standard taper pin taper.
The snapshot below shows the lower end of two different Spring Rods from two different manufacturers. There is minor dimensional variations between the two rods. For this reason I strongly suggest that you buy your Spring Rods prior to drilling any holes in the Bridge as every maker will be cutting their rods slightly differently but they'll all have the #8 taper.
You can just barely discern the slight taper on the ends. By the way both of these rods measure exactly .5-inches in diameter but the one on the left appears larger because it is Parkerized and the one on the right is Chrome plated. All Spring rods need grade 8 nuts and washers. Some rods come with special heat treated 4140 nuts and mil-spec washers. I usually but my parts from either the 45 restoration Company, Colony Machine or Paughco. These are all good outfits to deal with.
Cutting the taper in the Bridge is simple. You start by just drilling a hole completely through the bar that has a diameter very slightly smaller in diameter than the diameter measured at the very tip of the rod. In our case here the end of the rod measured .4375-inches in diameter so I drilled the holes to 27/64th.
After drilling the pilot holes you just chuck up your taper reamer and running the drill at a very low speed you re-drill the holes to the final tapered size.
Go slowly and stop often to check the fit of the rod in the hole. The final tapered hole has to be shallow enough so that no portion of the rod sticks out the bottom, except the threaded portion. You can do this by hand using a hand-held reamer but it takes a very long time. Sometimes for the sake of precision I will do the last few cuts in the hole using a hand-reamer to get a perfect fit, custom cut to the specific rods I happen to be using on a particular set of forks.
The picture above shows what the job looks like when it's nearing the final few cuts. This type of attachment of the Spring Rods will last for decades and as the rods start to show their age and take a set due to the constant tension they can easily be replaced without having to re-chrome the whole front leg assembly. When you see welded in rods you know that you're looking at substandard workmanship.
Installing the Spring Rods is actually the only 'hard' thing to do when building a set of Springer Forks but once you do the first job all the rest become easy and you won't even think about it since it's just become 'standard practice'.
I usually do numerous 'trial' fits to each and every part as they're being made, sanded, polished and /or otherwise worked on in any way. By doing a bunch of intermediate mock-ups you can quite often catch a problem while it's still just a nuisance instead of letting it become a bigger problem when time comes for final assembly and welding.
The 'Legs' I'm using here are just mock-up legs and still have the mill scale on them which is why they look dark. Once the final legs are polished they also be in the 'bright' as they say.
The thing I like the most about Allen's design is that it's extremely simple, and extremely strong. He didn't bother trying to make the design complicated by adding in 'offset' or drilling the trees at an angle or any of that other nonsense that only complicates matters and usually ends up making the forks handle poorer, not better.
If you take your time in fabrication, a set of forks like this should probably have a useful working lifespan of at least 50-years, maybe even double that figure, so long as you don't let them get rusted away.
We're getting close to the time when we need to start talking about 'legs'. I've written several articles about Springer fork strength and don't want to rehash what I've already talked about elsewhere but the reader needs to understand that there are two schools of thought about building 'modern' Springer's in general.
One school, that followed by Jeri Exner, Freddie Hernandez (from Denvers), Mondo (also from Denvers), myself and a few custom builders is that the use of 'Tubing' for legs is a better way to go than using solid bar stock.
Another school, that followed by Dick Allen and Sugar Bear and a few other custom builders is that only solid bar stock should be used for the legs on Springer forks.
Who is right and who is wrong has yet to be determined since Springer's built over the past 40-years by everybody in the business, using both tubes and solid bar stock have about an equal number of 'failures'. There is also an equal number of 'successes' so we shouldn't only concern ourselves here with forks that fail. When a set of forks 'fail', even hydraulic forks, there have to be contributing factors that need to be looked at. With respect to Springer's of any type, made from any material, it appears the the biggest contributing factor appears to be placement and method of attaching a pivot point for the front brake caliper torque arm and/or 'fender' bracket mount points.
I'm sure that last statement surprised a lot of people who were looking for something more dramatic like fork tubes 'snapping' off at the lower tree or something akin to that like you see on stock and aftermarket 'Harley' styled forks, DNA in particular.
In all the years I've been building forks and in all the years I've been visiting the various discussion boards I have yet to find a single person who has reported their Springer forks, from any maker, or of any type, failing or even being 'bent' by normal road use. By 'normal' I mean some pretty tough use, potholes and wheelies included.
In fact you can search the Internet until you're blue in the face and I doubt that you'll ever run across a person talking about how their Springer forks bent or broke, with the exception of bad brake/fender mount designs unless the forks in question were cheap imports to begin with. This fact says two things. One being that solid bar forks aren't really 'superior' to tube leg forks and secondly that poor design leads to more failures than material selection.
For the average home/garage-based builder I still suggest that they use DOM tubing for the Legs as it just makes everything easier. For guys who have access to a large lathe than can accept round bar stock through the head having a diameter of 1.25-inches than I'd say go with solid round bar stock.
I've built with both solid stock and tube stock and can't really tell from 'riding' that's there's much difference. I have friends however who have far more experience than I have and they tell me that from a 'feel' standpoint they prefer forks built with tubing as opposed to solid bars. They say that forks built from solid bars are prone to 'oscillation' and 'bounce' or as they say 'reverberation' that they don't experience with forks made with tubing. Some day when I have time I plan on building two identical sets of forks, one with tubing and one with solid stock and see how they 'feel' so maybe I can better understand what they're talking about. Until then I'll keeping using Dom tubing legs.
A lot of makers use relatively thin-walled (.125-inch) 4130 chromoly for the rear legs on forks up to 48-inches long, that's about 20 to 24 over depending on how they're measured. I prefer to use thick-walled (.25-inch) regular 1026 DOM on most of my forks for the rear legs but for shorter forks I will use thinner walled material. For the beginner I suggest staying with either thick wall DOM or solid bar stock depending on what equipment you have at your disposal. 4130 is certainly good material but it's very expensive and also hard to weld unless you're an expert so I'd stay away from it for forks. Just as a side note all of Allen's early production forks were made from solid 4130 for the legs and solid 4130 for the 'flat' parts. He actually sold them for less than they cost to build just so folks would have a good set of forks for their bikes.
To get back on point, if you're working with tube legs you have to make, buy, or have made, some end fittings for the bottom of the legs where the rocker pivot bolts go. I use a relative simple 'lug' very similar to the design developed by Ed Roth to be used in the ends of old Ford radius rods for extended stock springers. I've been using this system for over 40-years and have never had an issue with it. You do need to grind in a very deep weld bevel on both the tube and the fitting to insure deep penetration as most of the weld 'crown' is ground away in the polishing process. The pieces seen here are 'in the raw' right off the milling machine. We have these made by a local machine shop and if you need them we can get you a set.
There are other options however. If you read the article I did about the 'Old School' Springer build project located HERE you can find some other less complicated methods of doing end pieces.
The 'ends' for the smaller diameter 'sprung legs' are identical just smaller and we'll get to those a little later.
If you going the 'solid leg' route the 'ends' are virtually identical except that they are machined into the 'raw' leg blanks you supply the machine shop or build at home.
To Be Continued ......................................................................
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