David Mercer – Mercer Bikes, Cape Town, South Africa

I met David Mercer at the Bespoke Bike Show in London in April 2014. I was impressed with the bicycle he had brought to the show, a touring bike he called the Monkey King. It could definitely be described as multipurpose, with a lot of hand made details including custom made stainless racks. David is a Vet by training and now splits his time between working as a Vet and building custom bikes, racks and tools. He kindly agreed to answer some questions about his workshop techniques which interested me:

Question: I understand you started out making racks which seem to be in stainless steel. Could you tell us what type of stainless tube you use and which tools you use to achieve the bends?

Answer: Mercer Bikes started with custom rack building. At the time I had no real experience of metal working or brazing and merely wanted to pick up an extra skill set. I’d built burglar guards, coat hooks and all manner of innocuous and useless metal bits and bobs as practice but wanted to put my skills to use in a bicycle oriented manner. I produced a chain tensioner for converting regular bikes to single speed and managed to sell a few of them but they were finicky and dropouts vary greatly in design – it was hard to make a universal tensioner that would fit everything.  I’d started using stainless steel and silver solder for the tensioners and one day decided to build a rack for my town bike – I bought a precision tube bender (made by Rigid) used for bending 10mm hydraulic tubing and found a supply of seamless stainless tubing in 10mm diameters from a dairy tubing supplier in Cape Town. Soon I was building racks for friends and before long decided to offer the racks to the general public.  The stainless tubing I sourced is available in 304 and 316 grades. I’ve used 316 predominantly – it has a finer grain and a higher level of corrosion resistance. It is a little harder to cut than 304 but the surface finish is generally much better and makes for quicker clean up afterwards.

Question: I also understand you use silver solder with a relatively lower percentage silver than 55% for your racks. can you tell us more about this. And do you use silver for any of your lug and fillet brazing, and if so which type?

Answer: When I started brazing stainless I had little idea how to go about it. I’d read that silver solder was required but gagged at the price. I found some stocks of 20 and 30% silver at  very reasonable prices and decided to try these out. I had tried some higher percentage silver before and was appalled at the way my amateur heat control made the filler disappear everywhere! The lower percentage silver didn’t seem to have quite the same quick flowing tendency and I stuck with it. I soon found that it was much easier to build fillets with the lower percentage silver. The joins seem just as strong as what I’d achieved with 55% silver but the rods were almost half the price. Also, at that cost building fillets no longer seemed like a waste of money. I’ve continued to use a 30% silver for most of my lug work. I find that with careful and slow heating it flows through joints very easily and yet has a little more filling ability that a 55% silver.

Question: You prefer to use Gussets instead of chain stay bridges to reinforce your chain stays. I can see the advantage of this, apart from having nowhere to clip a full length mudguard. Can you explain more about how you make them and whether you fit them to the inner or outer aspect of the chain stays or both. Would you add a chain stay bridge if you had used a lugged bottom bracket already?

Answer: I love gussets on the chainstays – they add a lot of clearance for tires (esp. on MTBs) and they look nice and clean. Its also a little area where the builder can add some detailing.  Keith Bontrager started using chainstay gussets on his original  Race Lite MTBs back in the early 90’s. He had analysed the forces acting through the BB and chainstay area and concluded that chainstay braces only served to concentrate the forces around the brace ends on the inner face of the chainstay. Without a brace many frames would fail on the outside face of the chainstay near the BB. He started adding gussets to the areas where frames failed without the brace in place.  My gussets are made from left over seat tube or down tube. I carve a shape out of the tube with an angle grinder and power file. The gusset is the slip brazed with brass to the mitred chainstay. I the use the angle grinder to cut the gusset material away from the mitre end, using the existing chainstay mitre as a template. The chainstay, now with attached gusset is then fillet brazed to the BB area. In the past I’d use silver to braze the chainstays but lately I’ve taken to doing it all in brass.
IMG_4846 IMG_4840

Question: I have seen the cyclo-cross bike you made with the twin top tube. This has obviously already been well tested and worked well. You made the long curved top tube merge into the seat stays by soldering 14mm seat stays into 16mm Gara tubing then curved the whole tube as a single piece on a home made chip board bender. I don’t suppose you have a photo of this bender or could give more detail on it’s construction?
IMG_3667-300x200 IMG_3673

Answer: Hmmm. That bike! It certainly got a workout in Africa! It is hanging in the workshop at the moment awaiting a frame repair. After some 2500km the frame began failing around the back of the seat tube about 10 cm below the top tube/seat tube junction. The joins on either side of the seat tube had been acting as pivots causing the seat tube to flex and bow forward – in time this caused a crease and subsequent crack to for starting around the back of the seat tube. In order to repair it I will replace the broken seat tube and add a short brace from the top tube(s) to near the top of the seat tube – this will triangulate the seat tube junction and should prevent the bending moment generated around the top tube/seat tube join. This has been a valuable learning experience for me and was something I’d never have anticipated before. I’d initially thought that the frame would be likely to shear through those two top tube / seat tube joins, rather than pivot around them! I’ll send a photo of the bender. We started with a large sheet of chip board. The arc we required was plotted on the board and the arc’s centre point found. We pivoted a long aluminium bar on the centre point and used an angle grinder mounted on the bar to cut out the described arc. With the arc cut out the grinder was repositioned and the cutting disk changed for a wide grinding disk with a rounded profile. This was the run along the arc once again cutting a groove along the way. All that remained was to bolt a small sheet metal clip to one end of the arc to hold the tubing in place at the start of the bend.  The angle grinder was swopped out for a small grooved roller. The aluminium arm and roller bed the tubing along the groove on the outer edge of the chipboard arc and create the curved profiles used for the twin tubes. ( You can see this bike build at http://mercerbikes.co.za/zed-ps-zed-p/ ).
IMG_4837 IMG_4836 IMG_4835

Question: Lug cutting is something you seem to do a lot. Most builders don’t bother with it I suspect because it is tricky to get design symmetry when hand cutting and there must be concerns about weakening the lug. Do you have any tips on the use of simple tools, or do you simply drill a few holes and get to work with a file?

Answer: I do a little lug cutting – its mostly just accentuating the cast shapes and giving the edges a crisp, square profile. I make use of a lot of sleeves and gussets which are all made from scrap tubing.  When cutting sleeves and gussets I usually use an angle grinder and narrow cutting blade for doing the rough cuts. The final shapes are all done with hand files or sometimes a power file. I use needle files too but have modified some of the profiles to make the edges sharper – I find it can otherwise be tricky to get crisp corners on  cut-outs in lugs. When doing lug cut outs I begin with a few small holes that I join together with a small cutter attached to a Dremel. The bulk of the material is removed with needle files though. I also use some cut down hacksaw blades which have been ground along their length to make the blade much narrower. These can help sharpen inner corners and can be used for linking drilled holes.

Question: Finally, most novice framebuilders are wanting to use a framebuilding jig, as most of us were taught using one and it’s difficult to suddenly start building frames freehand. Do use a jig or fixtures in your framebuiding?

Answer: I have a monster of a jig which I built myself. Its not extremely accurate but I can use it to set all the required angles. I don’t braze  in the jig though – I merely use it to hold the tubing for tacking. Once the front triangle has been tacked then I take it out of the jig and lay it on parallel bars on my surface table – the seat tube and head tube should be parallel. I then tweak the frame as needed ad add a second tack to hold any adjustments before doing the final brazing in a stand. During the final brazing I frequently recheck the frame on the parallel bars and adjust my brazing sequence as needed to compensate for any minor deviations caused by heat distortion.  With the front triangle completed the frame goes back in the jig for attachment of the chain stays. Once these are attached I use a dummy axle – T piece to check the alignment of the chainstays – these are then given a final tweak if needed before the seat stays are added.
IMG_4833 IMG_4832

I hope this has been informative and adequate.

Please let me know if you’d like me to elaborate on anything.

All the best,  David

You can view David’s work on his website http://mercerbikes.co.za


Silver solder from Sif from the usual sources varies from 38 to 55% but lower percentage cadmium free solder is available in the UK from other manufacturers e.g. Silver-flo. Generally speaking the lower percentage of silver the higher the melting point making lower percentage silver more suitable for building fillets. I have not been able to find out the percentage silver in the widely used Fillet Brazage Pro from Cycle Design, mentioned elsewhere.

Cheap pipe benders are usually intended for bending copper pipes and are usually made of alloy. They are not really up to the job of bending steel, I have snapped one trying. The Rigida pipe bender mentioned is just the job. You can see one demonstrated here http://www.youtube.com/watch?v=ATrDYYPsXaw

SNH 2/6/14

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