I finished Sal’s rack over the weekend, and also added some 5/16″ cromoly hoops to the frame in order to help give a clean routing path for the cables that’ll run from his dynohub in the front to his tail-light in the rear.

Dyno Light-Mount

light mount detail

light mount detail

Light mount, polished and shiny...

Light mount, polished and shiny...

Underside, showing various bosses for attaching luggage, fender.

Underside, showing various bosses for attaching luggage, fender.

Here are some detail shots of the hoops, made from ~8mm slices of 58/8″ o.d. 4130 tubing, chamfered on the inside edges with a larger diameter drill bit, and brazed-on with silver.  The outer edges were slightly ‘eased-off’ by hand with a file to soften the sharp edges left from the tube-cutter.

Downtube routing for electrical wiring between Dynamo front-hub, and tail-light

Downtube routing for electrical wiring between Dynamo front-hub, and tail-light

Chainstay routing for electrical wiring between Dynamo front-hub, and tail-light

Chainstay routing for electrical wiring between Dynamo front-hub, and tail-light


Progress on the radial rack

December 22, 2010

My shipment of steel finally showed up last week, and I got down to work on D’s rack at last, and it’s coming together—with a bit of luck I’ll have it done this week!

After bending up the deck perimeter and the ‘hub’ circle, I pinned both joints with short lengths of tubing, split down one wall, and rolled into a diameter that would spring against the inside wall of the deck tubing, and self-jig the joints.

D's rack

Here you can see the deck and ‘hub’ compared to the drawing, and getting checked for squareness.  After a bit of alignment, I braze up the joint on the deck.

Once this is re-checked for squareness, I start brazing in the ‘spokes’, or radians, to connect the circle to the deck.

rack spoke

In this shot, I’m using a pair of stainless steel spokes and some duct tape as a makeshift jig to keep the short radian centered and in-plane with the rack deck during brazing. The spokes are resting on the outer deck, and are being tensioned by the tape, and acting against each other like a spring—the matched tension equalizes, and centers the radian tube perfectly.

Same routine again here with the circle. If you look closely, you can see that I’ve lined up one of the splices in the circle with the radian miter—I did the splices in the circle with some of the leftover silicon bronze that melts at a higher temperature, so that when I braze the radians, if I’m careful, the splice joints shouldn’t re-liquefy and flow out. Another way to tackle this problem would be to use silver for the radians, and bronze for the splices, the idea being the same—to have the higher-temperature material used on the first sub-assembly, and the lower-temp material on subsequent joints in the vicinity.

Brazed up and compared to the drawing again.

Here’s the second radian cut, mitered, and brazed in. Rather than working my way around the deck radian by radian, I’m adding every-second-radian first. This will allow me to check alignment at each stage and make sure everything’s lining up in-plane, and make corrections. Having a skeleton in place before adding the other radians should help keep everything aligned as I add the other pieces. If I added the radians one by one, the heat could have an incremental twisting effect, and things could end up veering out of plane pretty quickly.

Here’s another radian, mitered and ready to rock.

I’ve started fluxing and pre-loading mitered tubes with a bit of brass filler prior to brazing up the joint, which helps to both speed up the creation of an internal fillet, and acts as a good visual indicator of brass penetration around the perimeter of the miter. After the brass pre-load has all flowed around the joint, I build up a small external fillet.  Definitely getting much faster and cleaner.

And that’s it so far—time to get back into the shop!

Here’s a quick peek at the dimensional sketch for part of my current rack project. Click the image to see it full-size!

I’m waiting to take delivery on the steel for this one, ordered from Aircraft Spruce—should be here on friday!  This’ll give me the time to work out some details on paper and in the shop beforehand so that the build goes smoothly.

A few months back, before le Distraction Majeure (my wedding), and the subsequent trip out to Montreal, and the subsequent cold snap, I was spending a veritable heap of time in the workshop getting things outfitted, working on some prototype clamp designs for a handlebar support for Tom’s awesome Tauro-brand bike luggage, and generally getting acquainted with my torch setup, trying different brass fillers, and experimenting with different fluxes.

I was also building a custom front rack for Ariella’s Cross-Check. It’s a small-ish rack, measuring approximately 12cm wide x 23cm long across the top deck, and features a mildly ornamental backstay, as well as some pseudo-lugs (externally-sleeved construction joining the left & right halves of the top deck—necessary due to the limitations of my current crappy 5/16″ tubing bender).  It’s designed to sit nice and low to the front tire, both for a clean look and to minimize flex, and attaches to both the mid-fork lowrider mounts, and the fork crown, the latter by way of a stainless flat-tang.

The design of the rack was loosely based on the Nitto Mark’s Rack, but with a larger deck profile, and without the adjustable stays.  The nice thing about a custom rack design, is that it allows you to minimize the extraneous hardware needed to attach the rack to the bike, since the possible mounting scenarios are reduced.  The flip-side of this benefit is that the rack is primarily valuable on the bike it’s designed for, and its utility on other bikes may be somewhat reduced.  This rack splits the difference: the stays are a fixed-length which should work reasonably well on most 700-C wheeled bikes with mid-fork lowrider mounts, since the adjustable upper-tang is bendable to accommodate differences in the distance between the rack’s deck and the fork-crown of the bike.

The backstay of the rack was originally bent-up as a simple inverted ‘U’, but between a brazing error, and the legs of the backstay sitting wider than I liked relative to the rear sleeve, I decided to chop the ‘U’ in half, which brought the legs inwards, and straddle them with a brass-capped cross-piece. The fringe benefit is that you can wrap a bungee around the resultant ‘ears’ of the backstay.

Detail of the front sleeve.  This construction was necessary, since the discount 5/16″ outer-diameter tubing bender that I own is incapable of doing more than 180° of bending in one-direction, due to the fact that the bender’s arms are not offset, and consequently gets in the way of the workpiece.  The bender also has a bit of slip to it, and has no calibration marks on the mandrel, which makes it very difficult to accurately plan and measure compound bends. Watch out for a post looking more closely at the different benders I’ve got in the shop, and what differentiates a decent one from a poor one.

The sleeve was filed to resemble a pointed ‘lug’—I could have internally-sleeved the tubing for a clean, flush look that would be invisible after painting, but I didn’t have any 1/4″ tubing on-hand, and thought that this would make for a nice detail.

If you look closely at the front sleeve, you can see that the leftover brass on the outside of the sleeve is a different colour than the other braze joints in the rack. This is due to some confusion at the welding supplier; as it turns out, most local welding suppliers don’t deal with a lot of brazing supplies, and those that do are relatively unfamiliar with their stock.  Consequently I ended up with a pound of silicon bronze, rather than low-fuming bronze, the former of which has a liquidus (melting point) about 200° hotter than the latter, which was resulting in all kinds of charred flux and general brazing grief.  Luckily, they took it back and replaced it with the correct filler, which, in conjunction with some flux that the great folks at Gasflux mailed me, is resulting in way cleaner, more controllable joints.  I’ll be writing another post soon comparing a few different fluxes I’ve tried.