garage sale GT

https://www.google.com/imgres?q=rod+b...1t:429,r:3,s:0

Does anyone have any authoritative, factory-printed information on whether rod brakes are supposed to be self-energizing as in the photo above?

I know it works the other way. I have ridden a bike with the brake pads out in front of the stirrup. Please don't bother telling me it works that way because I already know. I have also seen several old raleighs in photos and in person set up that way.

However, I don't see any reason not to attach the brake pad holder directly to the stirrup unless it is in order to make the brake self-energizing as shown above, or in other words, so the rotation of the rim would force the pad against the rim harder.

Mos6502

What do you mean "as in the photo above"? What in the photo leads you to believe they are self energizing?

ftwelder

I have not seen that set-up before, interesting. The mechanical action would create additional leverage but would not be effected by wheel rotation unless I am missing something.

garage sale GT

The friction causes the pad to press even harder into the rim, like the leading shoe of a drum brake.

The friction makes it want to rotate about the screw holding it to the stirrup, upward into the rim. I am not saying the screw is loose, I am just using it to illustrate the direction of the rotation I am referring to. It probably flexes the stirrup a bit.

If the pad were out in front, which is frankly like most rod brakes I've seen, the friction would tend to make it want to rotate away from the rim.

garage sale GT

I don't think there is additional leverage. I think that's a standard rod brake setup with those little extensions the pad bolts to in the back rather than the front.

garage sale GT

On a typical drum brake, the self-energizing shoe is the one whose trailing edge has the pivot and leading edge is on the piston.

Mos6502

Oh ok.
I have seen rod brakes set up backwards with the tabs facing forwards:


But usually I see them without any tabs at all:


I don't think any of the various ways would lead the brakes to be self energizing. When the tabs are mounted the right way around, with the pads between the forks, the pads contact the rim at a steeper angle which would seem to counteract any leverage created by moving the mounting point back. On the other hand, mount the tabs backwards as shown in the first picture would certainly lead the pads to "bite" on the forward corner hardest, and not much at all with the rest of the pad.

Schwinnsta

I had never seen brakes set this way but after further thought I this set up is correct. I will try it on the rod brake bike I am restoring.

I think what happens is that the brakes set up in front are pushed away by the rotation of the wheel; but by having them like your picture shows they can not be pushed away and are instead pushed inward against the rim so thus utilizing the momentum of the bike to apply additional force..

Neat thanks.

dddd

Self-energizing brakes are a lousy idea in that they exaggerate any variables that affect friction.

Add a stickier brake pad, and the braking power is then MUCH more intense. Even humidity in the air can cause this.

When rain causes a decrease in friction, the braking power is then MUCH lower. This is also problematic wrt heat fade.

Then there is the tendency of S.E. brakes to chatter and grab.

Mos6502

I think the tabs are probably mechanical nonsense and more than likely if they do anything they make the braking weaker regardless of how they're used because they introduce more play and flex into the system.

garage sale GT

Do your car's rear drums chatter and grab and go nuts when the air is humid? All right, then.

cudak888 

Neither are self energizing, for the upward angle of brake application isn't intensified by the movement of the wheel. If the wheel's movement somehow applied upward force to the stirrup, then the brakes would be self energizing - but that's impossible.

-Kurt

garage sale GT

It creates a torque, actually.

Schwinnsta

The set up that the OP shows is more rigid. In the usual pad forward design, the braking force pushes the brake pads outward. In the OP configuration, the pads the pads can not rotate outward, they are pushed inward, but there is nowhere for them to go accept into the rim.

In the usual front of for set up, you have the normal friction force on the rim. That force pushes the pads outward away from the forks.

When the pads are in the OP configuration the force still tries to rotate the pads in the same direction but now pads can not be rotated because the forced further against the rim. The additional force comes from the moment resisting that rotation.

To say they are self engergised is confusing.

Reynolds 

I don't think that setup is for self-energizing. Raleigh said it was to avoid brake chatter, IIRC.

dddd

They certainly do.
My truck always locks the rear wheels at the bottom of the first hill I descend if it sits for a few days.
Brake pad materials and the oxidized dust they produce are hygroscopic as well, making matters worse.

A single lengthy application of the brakes drives off the moisture for a decent period of time.
The same is often true of bicycle brake pads in humid weather, because the normal arm flexing produces a degree of self-energizing to a portion of the pads, even where the net braking force is not self-energizing.
Thus, brake squeeking is highly seasonal.
Deliberately introducing self-energization to a brake design always makes the brakes more sensitive to friction variables.
Even clutch discs on certain vehicles tend to chatter after the drivetrain has sat cool for a time in damp weather, again for the same reason that while a portion of the pad's contact surface may de-energize under load, a different portion of the clutch disc pad's will self-energize as the pads twist, rock and roll.

dddd

To the degree that Raleigh wanted to reduce chatter, they would design the caliper assembly to be slightly de-energizing.

"To say they are self engergised is confusing. "

This is very true. Since the components not only slide and pivot, but also flex, one has to analyze carefully what the net effect will be.
Cmplicated further by the simultaneous self-energization and de-energization of different parts of the friction surfaces, which undergo different temperature changes while all this is happening.
An initial squeel may go away as that part of the pad heats up to the point of fading and losing some of it's initially-high coefficient of friction.

I think in the case of the Avon setup that the travel path of the rim doesn't look quite perpendicular to the stirrup, but the angle of the slider post is just as critical, with even a slight deviation from perpendicular to the rim causing self-energizing or self-de-energizing effect.

garage sale GT

It's "Self-Energizing" and that's just what a leading shoe is conventionally called.

garage sale GT

So they want the tabs to be out in front, i.e. the brake should have trailing shoes?