Brake Bridge on Fixed Gear

[Diablo Scott]

I'd like to remove the brake bridge on my fixie to fit a fatter tire.
Any advice with this?

It's a lugged steel frame but the brake bridge appears to be attached
with a simple braze. Am I right in thinking the bridge doesn't add any
structural strength? The bike's got sort of an urban bike messenger
look so hacksaw marks or burned paint would actually enhance the
appearance. The tire fits between the chainstays just fine; the only
clearance problem is at the brake bridge with the 32mm tire I got. I've
been riding it for two years with no rear brake.

[Qui si parla Campagnolo]

Post by Diablo Scott
I'd like to remove the brake bridge on my fixie to fit a fatter tire.
Any advice with this?
It's a lugged steel frame but the brake bridge appears to be attached
with a simple braze. Am I right in thinking the bridge doesn't add any
structural strength? The bike's got sort of an urban bike messenger
look so hacksaw marks or burned paint would actually enhance the
appearance. The tire fits between the chainstays just fine; the only
clearance problem is at the brake bridge with the 32mm tire I got. I've
been riding it for two years with no rear brake.
--
http://diabloscott.blogspot.com

The brake bridge DOES offer support for the seat stays..don't hack it
off...

[John Thompson]

Post by Qui si parla Campagnolo

Post by Diablo Scott
I'd like to remove the brake bridge on my fixie to fit a fatter tire.
Any advice with this?

The brake bridge DOES offer support for the seat stays..don't hack it
off...

Unless you braze a new one in higher up. FWIW, you *could* un-braze the
existing bridge, file it down so its a little shorter and therefore fits
higher on the stays, and rebraze the same bridge.

[o***@ozarkbicycleservice.com]

Post by Diablo Scott
I'd like to remove the brake bridge on my fixie to fit a fatter tire.
Any advice with this?
It's a lugged steel frame but the brake bridge appears to be attached
with a simple braze. Am I right in thinking the bridge doesn't add any
structural strength? The bike's got sort of an urban bike messenger
look so hacksaw marks or burned paint would actually enhance the
appearance.

If you don't mind (or actually like) the burned paint look, have a new
bridge brazed in a little higher up on the seat stays. I wouldn't risk
just removing it.

Post by Diablo Scott
The tire fits between the chainstays just fine; the only
clearance problem is at the brake bridge with the 32mm tire I got. I've
been riding it for two years with no rear brake.
--
http://diabloscott.blogspot.com

[Bob]

On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

[Diablo Scott]

Post by Bob
On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

Nope, got a good grade in engineering statics but it was a long time
ago. Got into this with Jobst a couple years back for a different
reason; he talked about the tetrahedron of the "rear triangle" and the
brake bridge wasn't part of the critical form. In fact now that I look
it up it was even more specific...

Jobst's quote: "The brake bridge plays no role in load
carrying. It is there to mount the brake and adds nothing to the
strength of the figure. The six elements are 2-chainstays,
2-seatstays, seat tube and rear axle. "

Now he wasn't talking about taking it out of an already built structure,
maybe he'll agree that it isn't a good idea, or maybe he'll bless my
hacksaw.

[Bob]

On Mon, 05 Jun 2006 07:16:43 -0700, Diablo Scott

Post by Diablo Scott
Now he wasn't talking about taking it out of an already built structure,
maybe he'll agree that it isn't a good idea, or maybe he'll bless my
hacksaw.

Maybe... and from an engineering viewpoint I think I'd disagree with
him if he does. The brake bridge has no effect on the vertical
movement of the upper stays. However, it does reduce horizontal
movement.

[c***@comcast.net]

On Mon, 05 Jun 2006 07:16:43 -0700, Diablo Scott

Post by Diablo Scott

Post by Bob
On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

Nope, got a good grade in engineering statics but it was a long time
ago. Got into this with Jobst a couple years back for a different
reason; he talked about the tetrahedron of the "rear triangle" and the
brake bridge wasn't part of the critical form. In fact now that I look
it up it was even more specific...
Jobst's quote: "The brake bridge plays no role in load
carrying. It is there to mount the brake and adds nothing to the
strength of the figure. The six elements are 2-chainstays,
2-seatstays, seat tube and rear axle. "
Now he wasn't talking about taking it out of an already built structure,
maybe he'll agree that it isn't a good idea, or maybe he'll bless my
hacksaw.

Dear Diablo,

What can the brake bridge do besides preventing the two seat-stays
from bulging outward and spreading apart, something that they rarely
seem inclined to do? There are very few reports of seat-stays
exploding to the sides.

If anything, a brake bridge weakens a frame in that it allows the
wheel to grab the frame through the brake seat at the seat-stays
between the axle and seat-tube and try to bend the seat-stays down
toward the ground.

Luckily, except in steel frames well-softened by age, the seat-stays
seem to be strong enough to resist the rear brake's efforts to buckle
them at the brake-bridge.

The same is true for the--uh, the kickstand bridge? I suppose that's
what we call those things lurking between some chain stays. If your
kickstand bridge has been removed from your bike (or, dear God, came
without one), you're already living dangerously and may as well throw
caution to the winds.

Notice that even in our marketing-driven little world, no one has yet
tried to sell a stronger frame with the obvious improvement of a
SECOND brake bridge, even though there's room for it.

("The first bridge lifts the stress out of the frame so that the
second bridge can eliminate it completely--try the new Gillette
dual-bridge frame today!")

("New! Improved! The Gillette Mach III frame with THREE brake bridges"
Stronger and less-stressed than ever!)

("It took a bigger frame, but we did it! Squeezing FOUR brake bridges
into the Gillette Mach IV frame gives the strongest, smoothest, least
stressed ride available today!")

("By using a smaller wheel, Wilkinson is now able to offer a
normal-size frame with FIVE brake bridges for superlative strength and
ride smoothness!")

("A lawsuit was filed today by Moulton against Wilkinson for small
wheel patent infringement . . .")

Of course, if you damage the seat stays while hacking your brake
bridge out, all bets are off.

Cheers,

Carl Fogel

[o***@ozarkbicycleservice.com]

Post by Diablo Scott

Post by Bob
On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

Nope, got a good grade in engineering statics but it was a long time
ago. Got into this with Jobst a couple years back for a different
reason; he talked about the tetrahedron of the "rear triangle" and the
brake bridge wasn't part of the critical form. In fact now that I look
it up it was even more specific...
Jobst's quote: "The brake bridge plays no role in load
carrying. It is there to mount the brake and adds nothing to the
strength of the figure. The six elements are 2-chainstays,
2-seatstays, seat tube and rear axle. "
Now he wasn't talking about taking it out of an already built structure,
maybe he'll agree that it isn't a good idea, or maybe he'll bless my
hacksaw.

FWIW, Surly see fit to put both a seatstay bridge and a chainstay
bridge (mistakenly referred to as a "kickstand bridge" by some) on
their proudly minimalist (i.e., if it ain't necessary, it ain't there)
fixie frame:

http://tinyurl.com/eagv7

[Diablo Scott]

Post by o***@ozarkbicycleservice.com
FWIW, Surly see fit to put both a seatstay bridge and a chainstay
bridge (mistakenly referred to as a "kickstand bridge" by some) on
their proudly minimalist (i.e., if it ain't necessary, it ain't there)
http://tinyurl.com/eagv7

The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

[G.T.]

Post by Diablo Scott

Post by o***@ozarkbicycleservice.com
FWIW, Surly see fit to put both a seatstay bridge and a chainstay
bridge (mistakenly referred to as a "kickstand bridge" by some) on
their proudly minimalist (i.e., if it ain't necessary, it ain't there)
http://tinyurl.com/eagv7

The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

I thought the chainstay bridge was used for fenders, too?

Greg

[o***@ozarkbicycleservice.com]

Post by G.T.

Post by Diablo Scott

Post by o***@ozarkbicycleservice.com
FWIW, Surly see fit to put both a seatstay bridge and a chainstay
bridge (mistakenly referred to as a "kickstand bridge" by some) on
their proudly minimalist (i.e., if it ain't necessary, it ain't there)
http://tinyurl.com/eagv7

The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

I thought the chainstay bridge was used for fenders, too?

Yes, both seatstay bridges and chainstay bridges can be used to mount
fenders, but, IMO, that's a secondary function. I see chainstay bridges
on some bikes built so "tight" there is almost no hope of mounting a
rear fender.

[o***@ozarkbicycleservice.com]

Post by Diablo Scott

Post by o***@ozarkbicycleservice.com
FWIW, Surly see fit to put both a seatstay bridge and a chainstay
bridge (mistakenly referred to as a "kickstand bridge" by some) on
their proudly minimalist (i.e., if it ain't necessary, it ain't there)
http://tinyurl.com/eagv7

The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

IME, very few mount a rear brake on a fixie. As to fenders, if that's
the intent, why no eyelets on the dropouts or the forkends?

[Diablo Scott]

Post by o***@ozarkbicycleservice.com

Post by Diablo Scott
The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

IME, very few mount a rear brake on a fixie. As to fenders, if that's
the intent, why no eyelets on the dropouts or the forkends?

Apparently people build these as single speeds and flip-flops as well as
fixies:

http://www.bikecult.com/works/sales/steamrollerQ.html

The Surley site specifically says: "The seatstay bridge is drilled for
a standard/long-reach brake, as well... though there are no housing stops."

Also: "Tire clearance: Proprietary FFF™(Fatties Fit Fine) chainstays and
our custom fork give room for tires up to 700x38c with fender clearance
left over (P.S. -use clip-on fenders!!)"

[o***@ozarkbicycleservice.com]

Post by Diablo Scott

Post by o***@ozarkbicycleservice.com

Post by Diablo Scott
The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

IME, very few mount a rear brake on a fixie. As to fenders, if that's
the intent, why no eyelets on the dropouts or the forkends?

Apparently people build these as single speeds and flip-flops as well as
http://www.bikecult.com/works/sales/steamrollerQ.html
The Surley site specifically says: "The seatstay bridge is drilled for
a standard/long-reach brake, as well... though there are no housing stops."
Also: "Tire clearance: Proprietary FFF™(Fatties Fit Fine) chainstays and
our custom fork give room for tires up to 700x38c with fender clearance
left over (P.S. -use clip-on fenders!!)"

I missed all that....mea culpa.

[c***@comcast.net]

On Mon, 05 Jun 2006 10:51:10 -0700, Diablo Scott

Post by Diablo Scott
The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

Dear Diablo,

The fenders make sense.

I forgot them because fenders are rare where rainfall and snow-melt
amount to only 12-14 inches in a good year.

(So far in 2006, only 2.29". May be another drought year with only 4
inches of precipitation.)

But what does a chainstay bridge stop the tire from doing?

Getting wedged in between the narrowing tubes when mismounted?

I have a vague feeling that I've seen something about this tire stop
business before, but I can't remember what the idea was.

Intrigued,

Carl Fogel

[Diablo Scott]

Post by c***@comcast.net
But what does a chainstay bridge stop the tire from doing?
Getting wedged in between the narrowing tubes when mismounted?
I have a vague feeling that I've seen something about this tire stop
business before, but I can't remember what the idea was.
Intrigued,
Carl Fogel

Jobst's answer which should have been in the FAQ years ago:
http://tinyurl.com/zo8df

[c***@comcast.net]

On Mon, 05 Jun 2006 14:07:38 -0700, Diablo Scott

Post by Diablo Scott

Post by c***@comcast.net
But what does a chainstay bridge stop the tire from doing?
Getting wedged in between the narrowing tubes when mismounted?
I have a vague feeling that I've seen something about this tire stop
business before, but I can't remember what the idea was.
Intrigued,
Carl Fogel

http://tinyurl.com/zo8df

Dear Diablo,

Aha!

To stop a rear tire from wedging during frantic tire changes between
the narrowing tubes because of horizontal dropouts, unlike the more
common vertical dropouts.

Thanks,

Carl Fogel

[David L. Johnson]

Post by Diablo Scott
http://tinyurl.com/zo8df

His supposed point about wedging a tire in there is moot in this topic,
since we are talking about track frames (that is, fixed gear bikes that
have no need for a rear brake); one would presume that aside from
conversions we are dealing with frames designed for fixed gear riding,
with rearward-opening ends. (not dropouts, actually fork ends).

For such bikes there is no possibility of jamming the wheel in between the
stays. Even with one with horizontal road dropouts the problem is minimal.

[Diablo Scott]

Post by David L. Johnson

Post by Diablo Scott
http://tinyurl.com/zo8df

His supposed point about wedging a tire in there is moot in this topic,
since we are talking about track frames (that is, fixed gear bikes that
have no need for a rear brake); one would presume that aside from
conversions we are dealing with frames designed for fixed gear riding,
with rearward-opening ends. (not dropouts, actually fork ends).
For such bikes there is no possibility of jamming the wheel in between the
stays. Even with one with horizontal road dropouts the problem is minimal.

If you want to stay on topic, my OP is about a fixed-converted road bike
with horizontal drop outs and no brake; not a frame with track ends.
Any discussion of chainstay bridges is irrelevant to that, but
interesting for the same reasons.

Very few people have directly answered the implied question - "Is it
safe to remove the brake bridge?" Peter - NO. Carl - Yes (if you're
careful). Everyone else - <<analysis with no recommendation>>

[David L. Johnson]

Post by Diablo Scott
The seatstay bridge is both for brakes and fenders, the chainstay piece
looks like a tire stop.

Stop from what?

[b***@mambo.ucolick.org]

Post by Bob
On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

Do you have a engineering principle, calculation or FEA model
that supports the importance of either the chainstay or seatstay
bridge? I don't see how welding a 10-15mm piece of metal
between two 22mm chainstays, close to their attachent to the
BB shell, is going to add much to their stiffness. The seatstay
bridge may add a little strength for a bike that uses rear cantilever
brakes, which exert a force that tends to spread the seatstays.

This is close to a FAQ in r.b.t. Here's one of the more recent
threads:

<http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/a921f936901f790a/66042429a1ef464a#66042429a1ef464a>

[Michael Press]

In article

Post by b***@mambo.ucolick.org

Post by Bob
On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

Do you have a engineering principle, calculation or FEA model
that supports the importance of either the chainstay or seatstay
bridge? I don't see how welding a 10-15mm piece of metal
between two 22mm chainstays, close to their attachent to the
BB shell, is going to add much to their stiffness. The seatstay
bridge may add a little strength for a bike that uses rear cantilever
brakes, which exert a force that tends to spread the seatstays.
This is close to a FAQ in r.b.t. Here's one of the more recent

A structural tetrahedron only needs to be pinned, not
welded as a bicycle frame. A structural tetrahedron does
not work in bending, only compression and tension. It is
designed to for force only at the joints, and no force
couples. The rear tetrahedron of a bicycle is subject to
force couples: particularly at the bottom bracket. In
structures subject to bending the joints must be carefully
engineered.

I want to see a FEA of a bicycle frame, or hear from
someone skilled in the art. Sure, a frame can be built
without a brake bridge or chain stay bridge, but does the
frame maker redesign the seat lug joint and the bottom
bracket joint? I see the bridge as part of the joint.

[b***@mambo.ucolick.org]

Post by Michael Press

Post by b***@mambo.ucolick.org

Post by Bob
Skipped "Engineering and Structural Support 101", eh ?

Do you have a engineering principle, calculation or FEA model
that supports the importance of either the chainstay or seatstay
bridge? ...

A structural tetrahedron only needs to be pinned, not
welded as a bicycle frame. A structural tetrahedron does
not work in bending, only compression and tension. It is
designed to for force only at the joints, and no force
couples. The rear tetrahedron of a bicycle is subject to
force couples: particularly at the bottom bracket. In
structures subject to bending the joints must be carefully
engineered.
I want to see a FEA of a bicycle frame, or hear from
someone skilled in the art. Sure, a frame can be built
without a brake bridge or chain stay bridge, but does the
frame maker redesign the seat lug joint and the bottom
bracket joint? I see the bridge as part of the joint.

To be clear, I have never done an FEA of a frame and
am not qualified to. Since the poster I responded to
mentioned Engineering 101, I thought he might care
to offer a simple proof of the necessity of seat and
chainstay bridges. Chainstays that are heavily loaded
may require reinforcement at the stay-BB joint (for ex,
I have an aluminum frame that is gusseted on the
inside of the stays right at the BB joint, ahead of where
a stay bridge would go). I don't see an obvious reason
that the chainstay bridge itself is always at the right place
for a reinforcement, nor that conventional steel stay
bridges are big enough to make a difference.

You may find this FEA interesting:
http://www.sheldonbrown.com/rinard/fea.htm

It doesn't directly address the question of whether
stay bridges are necessary, although IMO, Figure 7
shows that stresses at the seatstay bridge (which is
what this thread started about) are very small. This is
not surprising, given that seatstays are commonly and
successfully built from quite small diameter tubing.
I think Figure 7 does also show frames with and
without chainstay bridges (Trek 770 and Bador/Vitus)
although it doesn't show the _same_ frame with and
without.

Google searching r.b.t. on e.g. chainstay bridge reveals
some testimony from builders who leave the bridges out
without ill effects.

[c***@comcast.net]

Post by b***@mambo.ucolick.org

Post by Michael Press

Post by b***@mambo.ucolick.org

Post by Bob
Skipped "Engineering and Structural Support 101", eh ?

Do you have a engineering principle, calculation or FEA model
that supports the importance of either the chainstay or seatstay
bridge? ...

A structural tetrahedron only needs to be pinned, not
welded as a bicycle frame. A structural tetrahedron does
not work in bending, only compression and tension. It is
designed to for force only at the joints, and no force
couples. The rear tetrahedron of a bicycle is subject to
force couples: particularly at the bottom bracket. In
structures subject to bending the joints must be carefully
engineered.
I want to see a FEA of a bicycle frame, or hear from
someone skilled in the art. Sure, a frame can be built
without a brake bridge or chain stay bridge, but does the
frame maker redesign the seat lug joint and the bottom
bracket joint? I see the bridge as part of the joint.

To be clear, I have never done an FEA of a frame and
am not qualified to. Since the poster I responded to
mentioned Engineering 101, I thought he might care
to offer a simple proof of the necessity of seat and
chainstay bridges. Chainstays that are heavily loaded
may require reinforcement at the stay-BB joint (for ex,
I have an aluminum frame that is gusseted on the
inside of the stays right at the BB joint, ahead of where
a stay bridge would go). I don't see an obvious reason
that the chainstay bridge itself is always at the right place
for a reinforcement, nor that conventional steel stay
bridges are big enough to make a difference.
http://www.sheldonbrown.com/rinard/fea.htm
It doesn't directly address the question of whether
stay bridges are necessary, although IMO, Figure 7
shows that stresses at the seatstay bridge (which is
what this thread started about) are very small. This is
not surprising, given that seatstays are commonly and
successfully built from quite small diameter tubing.
I think Figure 7 does also show frames with and
without chainstay bridges (Trek 770 and Bador/Vitus)
although it doesn't show the _same_ frame with and
without.
Google searching r.b.t. on e.g. chainstay bridge reveals
some testimony from builders who leave the bridges out
without ill effects.

Dear Michael & BJW,

Here's a post from Jobst in the middle of a chainstay bridge
discussion a few years ago, with Kinky replying that the bridge does
in fact help:

http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/7db5e62ef4a71dc1/7777444bf4cd4e3f?q=%22chainstay+bridge%22+%22two+purposes%22+jobst&rnum=1#7777444bf4cd4e3f

Kinky's point (I think) is that the practical details of construction
make the frame a less-than-perfect tetrahedron.

Cheers,

Carl Fogel

[Michael Press]

In article <***@4ax.com>,
***@comcast.net wrote:


[...]

Post by c***@comcast.net
Dear Michael & BJW,
Here's a post from Jobst in the middle of a chainstay bridge
discussion a few years ago, with Kinky replying that the bridge does
http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/7db5e62ef4a71dc1/7777444bf4cd4e3f?q=%22chainstay+bridge%22+%22two+purposes%22+jobst&rnum=1#7777444bf4cd4e3f
Kinky's point (I think) is that the practical details of construction
make the frame a less-than-perfect tetrahedron.

OK. That is what I conjectured: that it can be built
without bridges, but bottom bracket joint and seat lug
joint must be designed to take up the additional stress.

[Diablo Scott]

Post by c***@comcast.net
Kinky's point (I think) is that the practical details of construction
make the frame a less-than-perfect tetrahedron.
Cheers,
Carl Fogel

If there is no hub installed in the frame then there is no tetrahedron
at all. Another post I found in the archives said that the seatstay
bridge helps prevent accidental alignment modification while handling
bare frames; probably the reason most pure track bikes (of conventional
tube design) have them.

[c***@comcast.net]

Post by Michael Press
In article

Post by b***@mambo.ucolick.org

Post by Bob
On Mon, 05 Jun 2006 02:18:00 -0700, Diablo Scott

Post by Diablo Scott
Am I right in thinking the bridge doesn't add any
structural strength?

Skipped "Engineering and Structural Support 101", eh ?

Do you have a engineering principle, calculation or FEA model
that supports the importance of either the chainstay or seatstay
bridge? I don't see how welding a 10-15mm piece of metal
between two 22mm chainstays, close to their attachent to the
BB shell, is going to add much to their stiffness. The seatstay
bridge may add a little strength for a bike that uses rear cantilever
brakes, which exert a force that tends to spread the seatstays.
This is close to a FAQ in r.b.t. Here's one of the more recent

A structural tetrahedron only needs to be pinned, not
welded as a bicycle frame. A structural tetrahedron does
not work in bending, only compression and tension. It is
designed to for force only at the joints, and no force
couples. The rear tetrahedron of a bicycle is subject to
force couples: particularly at the bottom bracket. In
structures subject to bending the joints must be carefully
engineered.
I want to see a FEA of a bicycle frame, or hear from
someone skilled in the art. Sure, a frame can be built
without a brake bridge or chain stay bridge, but does the
frame maker redesign the seat lug joint and the bottom
bracket joint? I see the bridge as part of the joint.

Dear Michael,

Here's one test that addresses your question:

[Andrew Muzi wrote:]

"Regarding chainstay bridges- they probably won't hurt anything but
many years ago we did a series of sideways deflection test on frames
in progress before and after chainstay bridges but with a hub locked
in the frame and couldn't find any difference with 100lb hanging off
the axle, frame supported at seat and head tubes."

http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/8dd641a299db5b0c/97f03d8f5c2254f3?lnk=st&q=bridge+frame+test+muzi&rnum=2#97f03d8f5c2254f3

Cheers,

Carl Fogel

[Michael Press]

In article <***@4ax.com>,
***@comcast.net wrote:


[...]

Post by c***@comcast.net

Post by Michael Press
A structural tetrahedron only needs to be pinned, not
welded as a bicycle frame. A structural tetrahedron does
not work in bending, only compression and tension. It is
designed to for force only at the joints, and no force
couples. The rear tetrahedron of a bicycle is subject to
force couples: particularly at the bottom bracket. In
structures subject to bending the joints must be carefully
engineered.
I want to see a FEA of a bicycle frame, or hear from
someone skilled in the art. Sure, a frame can be built
without a brake bridge or chain stay bridge, but does the
frame maker redesign the seat lug joint and the bottom
bracket joint? I see the bridge as part of the joint.

Dear Michael,
[Andrew Muzi wrote:]
"Regarding chainstay bridges- they probably won't hurt anything but
many years ago we did a series of sideways deflection test on frames
in progress before and after chainstay bridges but with a hub locked
in the frame and couldn't find any difference with 100lb hanging off
the axle, frame supported at seat and head tubes."
http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/8dd641a299db5b0c/97f03d8f5c2254f3?lnk=st&q=bridge+frame+test+muzi&rnum=2#97f03d8f5c2254f3

I never thought the bridge makes the structure stronger. I
think the bridge makes the chain stay-bottom bracket joint
more reliable.

[a***@fsmail.net]

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

[c***@comcast.net]

Post by a***@fsmail.net
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
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:

Loading Image...

Now look at how thick the fork tubes are on a modern sub-180 pound
machine:

Loading Image...

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

[David L. Johnson]

Post by a***@fsmail.net
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.

How exactly do you think that bridge could affect rigidity in the frame?
Sure, the effect is increased as you move it down, but it is still
infinitesimal.

Post by a***@fsmail.net
I did notice that Thorn's heavy duty bikes have heavily reinforced brake
http://www.sjscycles.com/thornwebsite/exp.html

A brake may exert lots of force (torsion, predominately) on a brake
bridge, but we were talking about the practically non-existent forces
when there is no brake on the bridge.

Post by a***@fsmail.net
Could this be a feature that dampens out the side to side motion of the
luggage rack when heavily loaded?

No more so than the rest of the frame. Think about how most brake bridges
are attached. Even in the good old days, only the fanciest bikes had
separate lugs for the brake bridge. Most just brazed it on. On road
bikes, it is there to hang the brake. On track bikes, it is hard to
imagine a reason for its existence. Same can be said (even more so) for
the chainstay bridge.

What forces could a butt-joined brazed cross-tube carry that the bottom
bracket and the seat lug would not do better?

[Michael Press]

Post by David L. Johnson

Post by a***@fsmail.net
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.

How exactly do you think that bridge could affect rigidity in the frame?
Sure, the effect is increased as you move it down, but it is still
infinitesimal.

Post by a***@fsmail.net
I did notice that Thorn's heavy duty bikes have heavily reinforced brake
http://www.sjscycles.com/thornwebsite/exp.html

A brake may exert lots of force (torsion, predominately) on a brake
bridge, but we were talking about the practically non-existent forces
when there is no brake on the bridge.

Post by a***@fsmail.net
Could this be a feature that dampens out the side to side motion of the
luggage rack when heavily loaded?

No more so than the rest of the frame. Think about how most brake bridges
are attached. Even in the good old days, only the fanciest bikes had
separate lugs for the brake bridge. Most just brazed it on. On road
bikes, it is there to hang the brake. On track bikes, it is hard to
imagine a reason for its existence. Same can be said (even more so) for
the chainstay bridge.
What forces could a butt-joined brazed cross-tube carry that the bottom
bracket and the seat lug would not do better?

Preserve the chain stay-bottom bracket welds from
torsional and sheer strains.

[Bob]

Post by Michael Press

Post by David L. Johnson
No more so than the rest of the frame. Think about how most brake bridges
are attached. Even in the good old days, only the fanciest bikes had
separate lugs for the brake bridge. Most just brazed it on. On road
bikes, it is there to hang the brake. On track bikes, it is hard to
imagine a reason for its existence. Same can be said (even more so) for
the chainstay bridge.
What forces could a butt-joined brazed cross-tube carry that the bottom
bracket and the seat lug would not do better?

Preserve the chain stay-bottom bracket welds from
torsional and sheer strains.

The seat stay brazes at the seat lug are the most delicate part of the
bicycle's structure. On a lugged frame, they are typically the only
joint that is just a tube to tube braze - no lug, sleeve, etc. LIkely
plenty strong when the rear wheel is holding the structure in place
but easily abused when the rear wheel is not mounted.

[Michael Press]

Post by Diablo Scott
I'd like to remove the brake bridge on my fixie to fit a fatter tire.
Any advice with this?
It's a lugged steel frame but the brake bridge appears to be attached
with a simple braze. Am I right in thinking the bridge doesn't add any
structural strength? The bike's got sort of an urban bike messenger
look so hacksaw marks or burned paint would actually enhance the
appearance. The tire fits between the chainstays just fine; the only
clearance problem is at the brake bridge with the 32mm tire I got. I've
been riding it for two years with no rear brake.

Do not remove it. If you do, then keep a close eye on the
seat stay-seat tube joints for signs of cracking.