Quoted message said:I have no engineering knowledge but I decided to completely remove the
brake bridge from my Columbus Nivachrom Simoncini frame after I
accidently damaged it while trying to remove a thoroughly stuck rear
brake with a grinding tool. I've had no trouble since and I can't feel
any perceiveable difference in rigidity or ride feel.
I wasn't surprised at this because the rear brake bridge was so very
close to the seat tube in this frame and so I didn't reckon it could
add much rigidity. I think that the lower down the bridge is, the
bigger the role it would play in adding rigidity to the frame in a
side-to-side lateral sense.
I did notice that Thorn's heavy duty bikes have heavily reinforced
brake bridges :
http://www.sjscycles.com/thornwebsite/exp.html
Could this be a feature that dampens out the side to side motion of the
luggage rack when heavily loaded? If so, it could be that the brake
bridge plays a significant structural role only when the bike is loaded
at the rear and when this load is supported at the seat stays.
- Bernie Sluzalek
Dear Bernie,
You'd think that the cross-brace would be useful, but the actual frame
design means that the rear tubes are pretty much limited to straight
tension and compression.
Draw a diagram of a seat tube, the two pairs of stays, and the axle,
and you'll see that it's darned hard to make the stays do much of
anything.
A brace could stop both stays from bulging out (or in), but if
straight tubes in a diamond frame are doing that, you'd better get off
the bike and run before the whole thing flies apart under several tons
of load and the exploding frame threatens to kill you.
If the stays simply aren't welded very well to the seat tube, a bridge
could help, but that's mostly bad welding.
The double-triangular tetrahedron geometry makes the rear frame
ridiculously strong.
In the motorcycle world, add-on bridges are available for front forks.
They're just stout upside-down U-braces that bolt to the top of the
lower fork tubes and loop up over the tire. They show that a plain
two-dimensional ladder with thick tubes (front fork) is no match for a
a couple of triangular braces (rear bike frame).
The same is true of some motorcycle rear "forks", where the seat-stays
are missing. Massive intermediate bridges are often used on what's
really just a trailing ladder to try to brace it.
Here's a visual example of just how feeble a pair of forks are. Look
at how thin the old Honda front fork tubes were on an ancient 228
pound trials machine:
http://www.oldrice.com/TL250_Brochure_2.jpg
Now look at how thick the fork tubes are on a modern sub-180 pound
machine:
http://www.geocities.com/fn_owens/gasgas/0528.jpg
Since the pictures may not be clear to bicyclists (motorcyclists
sometimes can't tell whether a bike has 10 rear gears or only 7), I
slapped my calipers on the fork tubes in question.
The spindly fork tubes on the obese Honda are only 33mm thick.
Twenty years later, the ultra-lightweight GasGas that tries to save
weight wherever it can has 38mm fork tubes to try to stop the front
tire from twisting in rocks.
Ladders need bracing. Double-triangles in the rear frame pretty much
brace themselves.
Cheers,
Carl Fogel
