Saturday, September 08, 2007

Path Racer Lives

My Path Racer was designed around some big honking tires. 700Cx35x38. That's not a designation that I've seen before, but I think the 35x38 refers to the width and height of the carcass respectively. So these are big for a road bike. The beads are also big, and when combined with a large tube, its hard to get the stem through the rim enough to start inflating the tire. Whew.

Anyhow, I'm using an old pair of sidepull brakes along with these big tires. While I've done my calculations about three times, its still concerning whether all will fit together correctly. So, before mounting the rear brake bridge, it seemed prudent to button everything up to see how fits were coming along.

There was a bit of prep work to be done first including:

  • Drilling the fork crown for the brake
  • Cutting the fork crown for the crown race of the headset
  • Reaming and facing the headtube for the headset
  • Reaming the seat tube and cutting the slot in the seat lug
  • Cleaning the BB threads.
The crown is of the full sloping type and had very little room (vertically) for the brake mounting hole. So, measure once and drill twice. In a heavy steel crown, the method to make the hole is to measure and mark the hole on each side, and use a center punch to leave a depression to start the drill. The bigger the hole that is being drilled, the more heat is generated and the more work is done. So, we start with a small bit to drill a pilot hole. Both the surface of the crown and the bit itself get covered in cutting oil to cut down on friction. Then we go through steps of enlarging the hole with a series of bigger and bigger bits. Also, the hole on the front of the crown needs to be a larger diameter than the one of the back. The threaded portion of the brake mounting shaft doesn't exend all the way to the front - so the front hole needs to be one size larger than the back one.

Notice we marked both sides of the crown. That's because we drill from both sides. If we were a little off of square drilling one hole, it might not make a difference. But if we continued to be off while drilling through the other side - we'd end up with a crooked hole for the brake mounting shaft. As a consequence, the brake would be crooked on the crown and mess everything up. By cutting two holes from two directions, we make sure that the holes are in a straight line and square to the crown so that the brake fits and works correctly.

When using a hand cutter (as opposed to a lathe) for the crown landing, it's easy to miss cut. One problem is undercutting the shaft, so that the crown race doesn't have a tight enough fit. The other is to leave the landing out of square. Both problems are caused the fact that the cutter body has a little slack in its fit around the steerer. This allows the builder to tip the cutter. Doing so narrows the effective opening making for too narrow a shaft. If the tipping is held consistently to one position when going around the steerer, then the land will be out of square. So this is a fussy operation. What I do is cut a little at the top to give me an approximate diameter. Then I take a file and take a little off at a time, first front and back, then left and right, and finally in between the preceding cuts, creating an octagon shape. A few light licks smooth over the remaining corners (so the cutter doesn't catch on them) and I'm ready cut.

Having less material to cut seems to speed the cutter through the work. In so doing, there is less opportunity to tip the cutter and mess things up.

Anyhow, I popped a nice threaded 1" stronglight headset in place and it fits perfectly! After mounting the wheels, the front brake is a great fit too! There's plenty of room for a fender and the pads are close to being centered vertically in their slots.

For a crankset, I fit up an old pair of Nervar arms with a Stronglight chainring. Both use the small (50mm?) BCD that works with the old TA Cyclotourist crankarms. The combination looks good - the Stonglight star-like pattern chainring is one of my all time favorites. The question is what makes for the best chainline? Currently its at 45mm, but flopping the chainring to the other side of the crank is apt to end up with a 39mm chainline. So, it looks like a different BB is in order.

Unfortunately, the Nervar cranks proved to be bent (hard to see, but easy to feel) and the pedal threads in the right arm gave out on the inaugural ride. That said, the bike rides easy, it's comfortable, stable (despite having low trail), and pedals easily (even with one foot). Apart from needing new crank arms, I'm very pleased with the result.

So, now I need to mount the rear brake bridge, finish clean-up and pack this off to Duane for some paint.

Oh, and I'll get some pics tomorrow to post here.


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