Archive for the 'Framebuilding' Category

Front Rack Design- Randonneur Sportif

Above is a conceptual sketch for a low-rider attachment for rando racks on bikes without low rider bosses.

I had been looking at different ideas for front racks for my Circle A which is really designed as a Sportif, because I couldn’t resist providing for the option of carrying a small front load in addition to the frontbag/handlebar bag that will go on the rando rack.

There have been a good number of variations over the years on the “low rider” attachments to french style rando racks. Look at this beautiful Boxer rack, which attaches to the dropout at the bottom and the small rando rack at the top:

or the Grand Bois, which is a little more contemporary as it bolts to separate low rider bosses in addition to the rando rack and the dropout:

These racks were traditionally designed for Sologne/Berthoud style panniers which differ from Ortlieb style bags in that the French bags have their attachment points at the top edge of the bag, whereas the Ortliebs mounting rail is about 3-4 inches lower. On the same rack, the Ortliebs will sit 3-4 inches higher.

Additionally, since most of the French bikes did not have low rider bosses, the bags were generally centered over the axle, rather than further back close to the steering axis as you see on modern low riders.

I like my Ortliebs and wanted to see if I could find a way to mount them lower and further back, without providing low rider bosses, partially because I think a lot of people might find the design useful, but mostly because I didn’t want to look at the empty low rider bosses for the 95% of the time that I would not have the low rack attached. Additionally, I thought there was a certain elegance to the lines of the rack that was lost when I drew it with a low rider boss.

The rack as drawn will result in the center of the Ortliebs being at the height of the axle, but pushed back so the steering axis passes through the center of the bag projected onto it (as the axis passes between the two bags, not through them).

I hope this will result in minimal wheel flop and good handling on a low trail bike.

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Fork Bender- first results

I went over to see the guys at Circle A as Brian had finished fabricating the fork bender and I wanted to see the test results. Brian had made some changes as he had an old part that he could modify to act as the fork tip restraint:

Overall, I’d say the results are excellent for the first bend, though the blade wasn’t bent all the way to the tip.

Since the restraint needed to be mounted to the table, Brian raised the block up off the table a little so he wasn’t grabbing the very tip of the blade. Additionally, he forgot to install the bracket that holds the end of the mandrel in place. Combined with the fact that the restraint is no longer tied to the block directly, this allowed the block to move a little relative to the restraint. As a result, you can see that while the blade is beautifully bent, with no rippling or bending in the upper portion of the blade, the tip of the blade is not bent as it raised a bit off the block.

Installing a base so that the block isn’t flying in the air and installing an angle iron at the end should fix things, but if there is still movement, he can install a strap that ties the block directly to the restraint, somewhat like the original drawing, but perhaps a bit lighter since he has the restraint bolted to the table and all he needs to do is prevent them from separating.

Here is the blade:

and another shot of the complete bender:

I suggested a couple of slight mods to fine tune the power of the arm, but Brian says he can easily bend the blade with one hand.

Hoohah!

Fork Blade Bender/Mandrel

I, too, am a fan of fork blades with a sharp low bend and read John Clay’s article in Bicycle Quarterly (Spring 2009) with interest. My friends at Circle A Cycles are building a frame  for me and I asked if they could make a fork with blades like the ones that John Clay’s bender produces.

Chris asked if I could draw the new bender and mandrel that would be required, so I studied the Clay bender and produced this drawing.

I spent a lot of time thinking about pivot and pulley placement and how they affect the mechanical advantage of the bender. It turns out that the ideal location(s) are off the mandrel, but knowing them can help find the best realistic location for the pivot. Additionally, in placing these two points, you need to be careful not to have the pulley run off the end of the fork blade before you’ve reached the proper rake.

With that said, I think that this bender’s pivot placement will be an improvement on the John Clay bender.

When choosing pivot and pulley placement, the idea is to maximize angular movement for the bending arm for the desired rake. That is to say, mechanical advantage comes not only from the length of the bending arm, but also from locating the pivot/pulley such that the arm moves through a greater angle than the blade.

For example, if by bending the blade so that the tip is effectively at a 70 degree angle, the bending arm moves through 105 degrees, you have a mechanical advantage of 1.5, just due the greater angular motion of the bending arm. This would make a 2 foot arm equivalent to a 3 foot arm.

It’s also possible to have a mechanical advantage less than 1 due to poor pivot/pulley placement. This would occur if in bending the blade through a given angle, the bending arm moved through a lesser angle. This would effectively make a bending arm behave as if it were shorter than it actually is.

Once Chris has this set up, I’ll post with a report as well as any modifications we make.


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