# HUMBER Duplex Fork - a reproduction



## Eric Malcolm (Dec 18, 2011)

Hi. I have put this fork together for a Gravel-Grinder that I am progressing on. The idea of the fork in this form is to get increased passive suspension over coarse irregularities common when riding on gravel. Hopefully, it will reduce vibration coming from the front end of the bike. Bench testing has shown a nice spring effect, and with hub in place, not much lateral movement. Being specific to gravel, I do not expect to be out of saddle throwing the bike around much, it is intended to be comfortable over the longer distance.









These forks have a long history being manufactured by Humber in England from 1900 to early 1960's. I first came across them when employed at the LBS as a kid. There was a pair hanging off the wall in the workshop. The bikes themselves were otherwise typical 26" English 3-speeds, but I was always impressed with the plush ride, and that pair of forks on the workshop wall were so light.

I have constructed these with Silvering in mind, so they have added spacers and flanges to give a large surface area for brazing. I used a MAPP torch and 45% silver. The plates of the crown are 3mm, but add 1mm for the spacer flange. 28.6mm steerer stem and 9.75 x .9mm (3/8 x .035") fork blades.






























Why Silver? Just to say that I have no home workshop. I do this work at my place of daily toil, and we have a lot of Dangerous Goods products here, and the building owner wants us to be thoughtful towards his insurance premium. Since the huge earthquake destroyed Christchurch here in New Zealand, everything is a bit touchy. A MAPP just does not seem threatening like O/A bottles do. Got to keep the peace.

Eric


----------



## BenCooper (Feb 25, 2013)

That's pretty. I had a 1914 Humber in my shop for a while, with the duplex forks - I didn't dare ride such an old bike hard enough to test whether they really worked, but they certainly looked elegant.


----------



## afwalker (Apr 26, 2012)

Nice!
Did the 2 ends on each side just fit into that Llewellyn front dropout?
Have to flatten them at all? Then just fill it up with silver right?
Your modified head tube does what? Stops for movement, or part of the production process?
cheers
andy walker


----------



## G-reg (Jan 12, 2004)

Woah, cool. How are brakes mounted?


----------



## mchimonas (Dec 19, 2008)

Have you done the brazing yet? I use and abuse the MAPP-air torch (and also oxy-propane) more than most on this forum, but even for silver brazing, I think it would be hard to get a 3mm plate and 1 1/8" steerer to silver brazing temperature with a MAPP-air flame. Perhaps if you are using BAg-1, which is the 45% silver rod with cadmium, and which also has a lower flow temp than BAg-5 which is the 45% silver rod without cadmium. (of course cadmium fumes are quite toxic). If your landlord doesn't let you have compressed oxygen cylinders, you might be able to find a used oxygen concentrator (a low end one that puts out 5L a min) to assemble a oxy-MAPP or oxy-propane torch. Of course, I imagine NZ has a more efficient health care distribution method from a monetary standpoint than in the US whereby oxygen concentrators are refurbished once patients are finished with them rather then allowing people to sell them on craigslist. An alternative is to get an assistant and use two MAPP-air torches, which can be effective but quite cumbersome, and you would have to be careful not to set each other on fire.

Your pics look great by the way.


----------



## Eric Malcolm (Dec 18, 2011)

Brakes are Caliper only, I don't think that Disc or canti studs will work. The loading would be too great and collapse the fork blades.

The blades are filed in the lower portion and it flows together. That's the Head tube with the stem. Internal gear cables, but that's for another day.

Yep, I can do 3mm and steerer tube, its done. Cadmium free silver.

I have been greatly enriched by Doug Fattic's welding processes but I really don't do enough to look at the concentrator at this stage. I find 2 MAPP bottles is all I use on a frame and fork. I am keeping an eye out for a concentrator though. I originally started frames on a coke stand, reticulated coal gas and an oxygen bottle years ago and am used to a low temp, big flame.

Eric


----------



## Eric Malcolm (Dec 18, 2011)

A couple of more pictures that I hope will answer any questions regarding the brazing.
Where a Tig joint is directly attached as is a bronze one, I re-think the process and break it down into maximizing surface area and re-inforcement. So the finished item has a more detailed effect, though once I got my thinking going, it has become easy to look at a joint and engineer it suit my unconventional way. Yes, it is time consuming, but this is a time filling hobby for me, that if it progressed too quickly, I'll have to take up knitting to kill a day, can't have that.

Enjoy

Eric


----------



## Clockwork Bikes (Jun 17, 2006)

Can't wait to see it on the rest of the bike and to hear how it rides.

-Joel


----------



## todwil (Feb 1, 2007)

Nice fork cant wait to here how it rides!!


----------



## TrailMaker (Sep 16, 2007)

Indeed;

The rest of the bike is equally well crafted, and quite unique. It's a stunner. I can't wait to see it all together!


----------



## Eric Malcolm (Dec 18, 2011)

This is the first plate crown fork that I have done. 

I have some observations and queries regarding the plate crowned fork. If building a conventional single bladed fork, I can see that by varying plate thickness and blade width it could be tuned to give varying amounts of flex. That is, it would seem to be possible to create a 'soft' and 'hard' ride variables by using 3,4 and 5mm thickness plates. I also note that it appears that curved plates are desirable because they stiffen the crown. It would appear to me that this fork type is therefore 'tuneable' for varying end-uses. Have I understood this principle correctly and are there builders who do this very thing?

With the Duplex fork here, in bench testing, the fork behaves in an unusual way in that with a load applied, the middle 1/3 flexes like a spring - one side working against the other.
I know that this could be tuneable. The example that I have made uses a caliper brake so this dictates that I use a short axle to crown measurement - same as a road bike. My blades are short so this is the 'light' stiff version (670gr). But it would be possible to do quite a few variations, eg: lengthen the blades, increase wall thickness from .035 to .049, increase the diameter to 1/2" from 3/8". I don't know if plate thickness will contribute as the blades spring so much, it would be possible to spread the centres of the blades further apart on each side increasing effective triangulation. My fork crown width is determined by the use of CX tires, but the effect on say narrowing for 23mm or making it wider for 2.1 or so tires may influence the end result. If I used 1/2", perhaps disc mounting would be possible, but I would expect a stiffer ride quality.

So little is known, but the potential is interesting.

These are some of my musings. I won't able to give ride impressions for a while, the main frame is coming along nicely, but is a savoured project that I am quietly progressing on, no time frame is involved.

Eric


----------



## afwalker (Apr 26, 2012)

Question on the build process, I see washers for strengthening the plates. Did you braze these separately from the plates, or together? From the first pictures however, it looks like you did the lower plate, and no washer yet. When you did the washers/plates, did you have the fork pieces in to hold alignment, then do those last? It's a very intricate build, and rather unique. I wouldn't guess too many people have ever seen this, let alone contemplate building one But, it also shows I don't know what I don't know
cheers
andy walker


----------



## Eric Malcolm (Dec 18, 2011)

Andy

The Washer is actually a machined spacer, I can see how this is confusing as in the 2nd set of pictures - the middle one, appears to show it missing, don't know why - perhaps its an optical/angle thing.

This spacer has a 1mm wall thickness all around, very tight 28.8mm stem hole out to a 44mm O/D. I did it this way to have less mass for the low temp Silvering but it did double as an excellent alignment piece as it ran square in the lathe when it was machined for the lower head set bearing race.

The process I used was to braze the lower plate on first and check for square, it was, so on goes the spacer, top plate and bearing crown, heated as one piece. The blades held it all in place, so no alignment problems with plates. The blades were done separately in a second operation, basically because I needed to be sure I got step 1 right. If I did this again, I would go all out in one step now that I know the process is Ok.

Hope that clears up my poor photographing...

Eric


----------



## Velobike (Jun 23, 2007)

Good to see. I have 2 Humbers from the late 1940s. There is a difference in ride compared to other bikes of the era.

Nice workmanship.

BTW you can still buy those forks, but with 1" steerers.


----------



## Eric Malcolm (Dec 18, 2011)

Velobike

Great to hear from you. 

I followed my gut feeling on these forks and searched the internet for whatever info I could find. Mostly, I could only find Pictures. So I scaled what I could find as no examples exist here that I could measure.

I would really like to have the actual measured diameter of the fork blades.

Though I have made these out of CrMo tubing, the originals I would think are more likely to be manufactured out of a High Tensile tube. 

Thanks
Eric


----------



## TrailMaker (Sep 16, 2007)

(Whisper)...

Hey... buddy... Got some dope. This bike is going to be STUNNING.


----------



## Eric Malcolm (Dec 18, 2011)

O/k, I have now finished this over-all project in terms of construction, and will reveal it later, as I am testing and proving this bike at the present time.

I will report on the fork now that I can access it's performance. My bike has 700 x 35 Gravel tires. The ride quality is softer over irregular gravel and absorbs bumps with ease. Hopping over speed humps and landing are handled very smoothly. Under heavy front braking, I can lift the rear wheel, though I note the front axle tucks under the steering axis a little. This is O/K though. It just springs back. When riding out of saddle and throwing the bike side to side aggressively, the expected lateral weakness shows in the form of the wheel rim and brake pad touching (pads set 3mm off each side). This is really the only down side.

All good then.

Eric


----------



## Eric Malcolm (Dec 18, 2011)

Some further feedback on the forks performance.

I was out riding with my Sunday morning road ride group who all ride on 700 x 23, full Carbon and Alloy frame/carbon forked bikes. On a short, half mile, very poorly re-surfaced, coarse gravel chipped ashphalt stretch that we have to ride over, I noticed that I was not being 'roughed' up any where near as much as my road bike brethren were. It was quite noticeable. Having had this comparison, I am happy....

Eric


----------

