This article is from the Misc Bicycles FAQs, by various authors.
Before the rear triangle is brazed on the main triangle is aligned. The
basic assumption with alignment is that the axle of the bottom bracket
is perpendicular to the plane of the bike. Since measurements are taken
off the tubes, it is assumed that the tubes are drawn accurately by the
The alignment table begins with a very precise piece of granite. The
bottom bracket platform is affixed to the table so that the axle of the
bottom bracket is perpendicular to the plane of the table, and parallel
to the plane of the frame. On the table are various other custom
fixtures to hold the various junctures in place as well as a device
which measures heights above the table to ten thousandths of an inch.
Once the bottom bracket is in place the seat tube is checked to make
sure it is perpendicular to the bottom bracket. The measuring device is
used at the end of the seat tube near the bottom bracket and then again
at the junction of the seat tube/top tube. To adjust the frame up or
down, brute force is the main force used. The difference on my bike was
about 20 ten thousandths of an inch low, so we pushed the frame up a
bit. If simple muscle doesn't work, they have a two by four about 6
feet long they use as a lever to move the frame.
With the seat tube correctly adjusted, it is fixed by place by one of
the custom fixtures on the table - a solid tube is slid into the seat
tube and it is held in place by the vice type object whose height is
adjust to match that of the bottom bracket fixture.
A similar process is done with the down tube. The height is checked at
the bottom bracket and then again near the head tube and pushed up or
down as necessary. It was off slightly down so it too was pushed up.
We went a little too far so we had to push it back down a little. It's
much easier to move back in the opposite direction of the first movement
since the metal has somewhat of a memory and wants to return to where it
Now the down tube and seat tube are perpendicular to the bottom bracket.
The next check is whether the head tube is parallel, or in the same
plane as the seat tube, which is fixed in place. Measurements are taken
on the lugs are the top and bottom of the head tube. An assumption here
is that the lugs are the same width. The head tube was low by about 20
ten thousandths of an inch. They have an acceptable range of 15 to 20
ten thousandths of an inch difference on the head tube for production
bikes, but less than 10 ten thousands for custom bikes. The bottom of
the head tube is affixed in place, since the bottom is matched up with
the bottom bracket already. It can't be held too tightly since it is
possible to dent the head tube. Then a solid metal rod is inserted in
the head tube and used as a lever to twist the head tube into the same
plane as the seat tube. While doing this, you have to watch the down
tube and top tube so that they don't start to twist or dent. With some
thinner tubing, it is possible to twist a tube, and they have done it
before. The tubing I had was thicker, but still you must be careful.
Once you think you have it close, the alignment of the down tube is
checked again. We ended up with the top of the head tube about 2 ten
thousands higher than the bottom.
With different types of tubing sets, different amounts of pressure need
to be applied. Thinner tubing needs much less pressure and more care.
Dave said he has stood and *bounced* on a titanium head tube to get it
to move 10 ten thousands of an inch!
Of lot of what we've done as far as alignment and movement involved
using plain muscle to do it and in some cases we're not applying that
much force. Isn't it easy then for a bike to become misaligned while
riding or moving it about? Not really. In each case that we've been
applying brute force, we've done it with part of the frame or fork
solidly held in place. With the seat tube, the bottom bracket wasn't
moving at all! With the fork blades, the head tube was held tightly in