Internal Suspension Girder Forks - Page 4

To make it easier to follow this build article I've decided to start providing a 'label' to the various parts and bends in the tubes. I've already had a few phone calls about the project and it's kind of hard to communicate when we're talking about things like the "third bend, bender rotation angle". Referring to the 'labels will just make it easier.


The forks pictured in this sketch were drawn to scale and represent a 6-inch over length. It's pretty obvious these don't look very good so you can see why nobody bothers to put these on 'short' bikes. I personally don't think they look very good even at 12-over and in my opinion you probably might not want to make a set at anything less than 18-over to get the proportions to blend.

As mentioned earlier the bends in original forks all have fairly large radiuses and might have been done with a pipe or conduit bender. You'll often see a significant amount of 'flattening' on the inside of the bends and if you examine a set in detail you'll sometimes even be able to see that the bends on the left fork aren't always exactly the same as the bends on the right fork. This was just the nature of bending technology back in the sixties. 

Most of you will probably be using a JD2 or ProTools bender on your project. Thankfully the dies for 1" or 1.125" tubing for these benders aren't very expensive. I still use the JD2 dies in my benders. I still think they give the best bends and are easier to adapt to custom made benders so I've never switched to another brand. For this project I'll be using a 1" die having a 3" centerline radius and a 1.125" die having a 3.5" centerline radius. You can buy dies with different centerline radi if you want to build a set of forks that more closely adhere to the bends on the original forks. Of course if you go that route then you'll have to come up with adjustments in the dimensions shown on the plans we'll be publishing here. That's not hard to do but I want to make sure people understand that changes to the bend characteristics also change some lengths and a few bend angles. I'll eventually add a section about how to build a very cheap bender to use on these forks but the old original 2003 CBH 'bend-o-matic' can be adapted if you already built one.

(IMPORTANT NOTE: Over the past few days I've received some email and a couple of phone calls about the 'legality' of publishing this information so I guess I need to clarify that we are not, and will not, publish anything that is proprietary information belonging to any particular fork builder. The patent issued to John Harman expired in 1988 and nobody bothered to renew it or apply for a new patent on similar forks but we respect the rights of any company building from John's original work notes, like Executive Choppers for instance. Anything published at this site has been derived by taking 'average' measurements from a variety of Spirder type forks, not necessarily just those built by John Harman and his associates over the years. The CBH has no intention for produce this style of fork as a commercial product and the plans and drawings are provided free of charge for site visitors personal use and information. Any similarity to our drawings and products produced by other individuals or business endeavors is purely coincidental. Individuals may build products, whether they are patented or not so long as they do so for their own personal use and do not gain financially from the item produced).

I probably should have made this statement at the beginning of the article but didn't expect this short section would stir up so much interest. One visitor tells me he has already started building and will probably be finished before I'm finished with the write-up.

Anyway getting back to work again, as already mentioned,  I suggest that you buy some 3/4" o.d. electrical EMT conduit to practice with. This tube has an actual outside diameter of .97-inches. Alternatively you can buy some cheap 1"x.063 (16ga.) ERW tubing which bends a little better. You can usually find this at the bigger home improvement stores in 8-foot sections or order it from any one of the several online metal suppliers. 

For the 'real' tubing I suggest 1"o.d.x.125 wall 1020 DOM for a traditional fork set and 1.125"x.125 wall for the 'modern' version of the forks. The price for this tubing has almost doubled in just the past two years. For forks having an overall length beyond around 48-inches we used to use thicker wall tubing on the front legs going up to .134, .156 or even .188 wall. This extra thickness really stiffens them up and only adds about 5 pounds to the finished weight on long forks. Looking back I guess I should have mentioned that you can use almost any size tubing for the rear legs so long as you can find springs to fit. I use .125 wall material for everything since I usually only build long forks, 36-over and beyond. For the more traditional 12, 18 and 24 over forks you could use .083 or .095 tubing. 0.83 is much nicer to work with than .063 and it's significantly stiffer. The .095 is almost as stiff as .120/.125 and makes a good substitute if you're looking to save a little weight. I've been buying lately from Metal Supermarkets since they opened a store near my location so I don't have to wait on UPS. Their prices are competitive with more traditional steel suppliers if you're only buying a few sticks at a time.

I work from the bottom end of the forks towards the top. Other people work in the opposite direction. It doesn't make any difference in which order you do the bends. My first bend is usually the lower bend in the front tubes labeled as FB#1 in the diagram above and enlarged in the sketch below.

This bend and it's counterpart in the rear leg are actually the only bends in the forks that have to be accurate so it will pay to do several test bends in scraps of your material until you determine exactly how much spring-back you'll be getting with your material/bender combination. Once you know exactly what the spring-back will be you can put a 'stop' on your bender so that any subsequent bends for additional tubes will be identical. I usually cut the tubing long enough so that I have about 8-inches on the end that will eventually be coped back to 6-inches. Looking at the sketch you can see two offset dimensions. The center of the pivot sleeve needs to be 1.5625-inches from the centerline of the tube (which is also 1.0625-inches from the front edge). Those dimensions need to be exact so it is permissible to adjust either the 6-inch length or the 15-degree angle very slightly to hold those dimensions (for 1" dia. tubes). 

Once you get your hands on some good tubing make sure it's thoroughly cleaned, both inside and out. I use lacquer thinner and cotton rags. You'll be amazed at the dirt and grime you will get from what looks like 'clean' tubing. The interior bore of high quality DOM should look as shiny as a new gun bore after it's cleaned out.

I use a shaft collar to mark off the control points for the bends. I use a 'red' marker line to represent 'real' or mathematically  'calculated' control points and a 'blue' marker line to represent what most of us call 'bender' lines. There is usually a slight difference between the two sets of control points and you can determine this when you calibrate your bender. I had a good bender article we prepared years ago but it seems to have disappeared into the vast wasteland of the Internet somewhere. There is a good bender article on Pirate 4x4 and several other sites if you're new to tube bending.

I lay out the longitudinal surface centerline using a piece of 1"x.5" steel channel section. This works out far better than trying to use a piece of angle iron as some might recommend.

Both longitudinal and transverse control lines are shown on a piece of one-inch tubing in the snapshot that follows.



The tube in this shot is still resting in the channel iron. I use a lot of this material on my jigs and fixtures as it's the perfect size to 'cup' tubing from .75 to 1.25-inches in diameter.

The photo below shows the 15-degree bend FB#1 when it's finished.



We'll get to the next set of bends shortly.


More to follow ..................................

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