M
Mark Hickey
Guest
Hey, whadaya know?
I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
movement... but the
r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to deflect a fork
that much.
Today I figured out a really easy (5 minute experiment), cheap ($0) way to figure out if what I was
seeing was an optical illusion or reality.
I'd noticed that by locking the front brake solidly I was able to oscillate the rest of the bike
back and forth easily with just a couple fingers by applying a little fore/aft pressure on the top
tube. It sure LOOKED like the fork was flexing, even when sighting down over the handlebars (using
them as a reference point and watching the hub move back and forth, just like that bumpy, wet
descent I mentioned). But how was I going to measure the apparent deflection?
I was thinking of taking a series of high-speed digital images against a background containing a
reference grid (nerdly idea, huh?). Suddenly it occured to me that If I want to know if the back of
the bike is moving, all I have to do is measure how much the REAR wheel rotates.
I rubber-banded a gold paint marker onto the rear triangle of my fixed gear bike, aligned so that
the marker's tip contacted the rear tire's tread just off-center. Then I locked up the front brake
and applied gentle force to the top tube, back and forth with two fingers.
The result was 14mm of movement (or plus / minus 7mm) of the rear wheel (I verified the front wheel
didn't move at all).
I held the top tube a bit more firmly and applied the same sort of motion to the top tube, but with
a little more force. This time I got 19mm of fore/aft movement at the rear tire.
I repeated both tests several times, and got nearly identical results each time.
My conclusion is that with only a few pounds of force applied intermittently AT THE RIGHT RATE, a
significant amount of fork deflection can be produced. It doesn't seem unlikely at all that similar
deflections can occur under certain braking conditions, lending credibility to those who claim to
have seen their front hub dancing back and forth in inch (2.54cm) or so.
Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
movement... but the
r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to deflect a fork
that much.
Today I figured out a really easy (5 minute experiment), cheap ($0) way to figure out if what I was
seeing was an optical illusion or reality.
I'd noticed that by locking the front brake solidly I was able to oscillate the rest of the bike
back and forth easily with just a couple fingers by applying a little fore/aft pressure on the top
tube. It sure LOOKED like the fork was flexing, even when sighting down over the handlebars (using
them as a reference point and watching the hub move back and forth, just like that bumpy, wet
descent I mentioned). But how was I going to measure the apparent deflection?
I was thinking of taking a series of high-speed digital images against a background containing a
reference grid (nerdly idea, huh?). Suddenly it occured to me that If I want to know if the back of
the bike is moving, all I have to do is measure how much the REAR wheel rotates.
I rubber-banded a gold paint marker onto the rear triangle of my fixed gear bike, aligned so that
the marker's tip contacted the rear tire's tread just off-center. Then I locked up the front brake
and applied gentle force to the top tube, back and forth with two fingers.
The result was 14mm of movement (or plus / minus 7mm) of the rear wheel (I verified the front wheel
didn't move at all).
I held the top tube a bit more firmly and applied the same sort of motion to the top tube, but with
a little more force. This time I got 19mm of fore/aft movement at the rear tire.
I repeated both tests several times, and got nearly identical results each time.
My conclusion is that with only a few pounds of force applied intermittently AT THE RIGHT RATE, a
significant amount of fork deflection can be produced. It doesn't seem unlikely at all that similar
deflections can occur under certain braking conditions, lending credibility to those who claim to
have seen their front hub dancing back and forth in inch (2.54cm) or so.
Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame