Question / observation - springs on front drum brakes (1967)

[MoPar~Man]

Something I find interesting is the discrepency between how the springs are arranged on the front brakes of my '67 Monaco vs what the service manual shows.

The manual identifies and shows the return springs on the shoes as being different, but I had mine set up (30 years ago, and again today) with the same springs on both shoes.

The brake hardware kits don't supply different return springs. I'm sure there's no real-world consequence in brake functionality. But I do wonder why they identify a "primary' and "secondary" return spring. What makes one "primary" and the other "secondary" ? Don't both shoes contact the drum during braking and both need to be retracted afterwards?

And a bonus question - why does the leading shoe have less friction material?

 

[CBODY67]

The brake shoes are designated "primary" and "secondary".

There is a reason the primary shoe has less friction material on it, but I don't recall what it is. It's been waay too long since I was told what the reason is. Possibly something to do with the "servo" self-aided apply effort?

CBODY67

 

[volksworld]

on a self energizing brake system (one where the adjuster floats with the shoes rather than being solidly mounted to the backing plate) when the brakes are applied the whole assembly rotates in the direction the drum is turning...this basically jams the rear shoe against the drum and causes it to do the most work, therefore it needs more material on it to wear at the same rate as the front one...the dynamics of this method results in needing less pedal pressure to get the vehicle to stop...it may also make them more prone to lock up but that"s another story

 

[MoPar~Man]

From either shoe's perspective, the wheel is turning the same relative to it, it's a symetrical system. Also, one shoe can't generate much friction by itself unless the other shoe is also contacting the drum. Or so you would think.

Look again at the diagram in the manual, and note the return springs. If the springs are different - if one is pulling stronger than the other, then you are garanteeing the shoe with the weaker spring will be the first to move and be the first to contact the drum. Once it does, the other shoe has no choice but to start moving outward and eventually contact the drum if enough brake force is applied. I would think the force (and hence the wear) the shoe with the weaker spring would have wouldn't be much until both shoes are contacting the drum, but they obviously gave both shoes their respective differential pad area to compensate for this effect to cause equal wear.

This differential pad area effect is thrown out the window if the return springs are of equal strength. But the springs in the hardware kits are the same, they've been the same for years.

Unless our C-body cars with front drums is set up with springs as shown in the manual (one stronger than the other) we are likely not experiencing braking as it was designed or came from the factory.

 

[Big_John]

From either shoe's perspective, the wheel is turning the same relative to it, it's a symetrical system. Also, one shoe can't generate much friction by itself unless the other shoe is also contacting the drum. Or so you would think.

Look again at the diagram in the manual, and note the return springs. If the springs are different - if one is pulling stronger than the other, then you are garanteeing the shoe with the weaker spring will be the first to move and be the first to contact the drum. Once it does, the other shoe has no choice but to start moving outward and eventually contact the drum if enough brake force is applied. I would think the force (and hence the wear) the shoe with the weaker spring would have wouldn't be much until both shoes are contacting the drum, but they obviously gave both shoes their respective differential pad area to compensate for this effect to cause equal wear.

This differential pad area effect is thrown out the window if the return springs are of equal strength. But the springs in the hardware kits are the same, they've been the same for years.

Unless our C-body cars with front drums is set up with springs as shown in the manual (one stronger than the other) we are likely not experiencing braking as it was designed or came from the factory.

You are assuming one spring is weaker and it's not.

The difference in the springs is mostly the length of the "hook". Wire diameter and number of coils is about the same between the two springs, so the spring rate should be the same.

Next, if you look at a later manual, this is from a '69 FSM. You'll see the brake springs are the same. Aftermarket parts are built to fit as many years, makes models etc. Just a couple years later and the springs are identical.

One of the things about the FSM, especially the 60's editions, is they use generic pics from earlier years. Things like brake springs may be different on the actual car. Possibly a change after the FSM was published, possibly a running change. What I'm saying is don't take the little details as gospel.

Let's go one step further and look at the '67 Parts book. Same brake spring for 11" brakes although they do list different springs for the front and rear shoes. for 10" brakes.

@volksworld explained how it all works quite well.

 

[Justin Plant]

my guess is the other side has the a pair of the opposite (shorter) springs on it.

 

[jollyjoker]

I remember (briefly) looking into the topic of self energizing brakes a few months ago, when my buddy asked me exactly what it meant. I did find a pretty concise and informative explanation in a MTSC booklet, somewhere in the 1963 time frame, IIRC. Sorry I can't tell you which one,as I'm out of town. But would be happy to dig it up again, if anybody needs the reference, when I get back . . .

 

[Big_John]

I remember (briefly) looking into the topic of self energizing brakes a few months ago, when my buddy asked me exactly what it meant. I did find a pretty concise and informative explanation in a MTSC booklet, somewhere in the 1963 time frame, IIRC. Sorry I can't tell you which one,as I'm out of town. But would be happy to dig it up again, if anybody needs the reference, when I get back . . .

Found it.

https://www.mymopar.com/downloads/mtsc/204.pdf

 

[MoPar~Man]

I think this spring difference is a "thing" - that there is something to it.

Doing some searching I came across this thread from 2018:

Drum Brake Return Springs ????

I have searched the internet for several days looking for information on the orientation of the two drum brake return springs on Bendix style brakes and have not found a definitive answer. My service manuals mention that it makes a difference but don't identify them and online tutorials just say...

www.binderplanet.com

From the above:

------------

When braking, the primary (front) shoe moves away from the anchor pin, while the secondary shoe does so only momentarily or not at all. Because of the 'servo' action, the lower section of the secondary shoe moves outward against the drum and its upper end may also slide on the pin as that part also moves outward. The primary shoe's spring is pulling the shoe directly towards the pin, the secondary spring is working at a greater angle, pulling the rear of the shoe towards the axle center and perhaps also sliding the top of the shoe (which is against the pin) towards the axle center.

As I see it, this requires the stronger spring to be in the rear, on the secondary shoe. The primary spring stretches more but has an easier job pulling inline.

-----------

Dana, your description agrees with the only online reference I have been able to find. I quoted it below. Got this from a website named what-when-how.

==============

http://what-when-how.com/automobile/drum-brakes-automobile/

Duo-servo Brake

This brake arrangement is also known as the self-energizing brake. Although this is a very powerful brake, its effectiveness reduces severely with the decrease in the friction value. A hydraulically operated duo-servo brake is shown in Fig. 28.18. The principle of operation is based on the utilization of drum energy to considerably boost the force applied on the brakes by the driver.

When the leading shoe is pushed to contact the forward-moving drum, it rotates partially with the drum due to the frictional force. This shoe movement, produced by this self-wrapping action, is conveyed through a floating adjuster to the trailing shoe so that the shoe is brought into contact with the drum. The force applied by the expander is supplemented by the self-energizing action of both shoes.

To minimize the delay in application of the self-energization action, the trailing shoe is held on the anchor pin by a stronger return spring so that the expander only moves the leadingshoe. In this arrangement, the leading shoe is called the primary shoe, because this shoe is made to contact the drum before the secondary shoe.
================

There are several comments on forums and discussion groups that say just the opposite though. Their reasoning is along the lines of the stronger spring should be toward the front since it has to pull the shoe further once the brakes are being released.

--------------

I was thinking that they did away with the dual-springs (especially in the aftermarket spring kits) to avoid installation confusion and possibly botched DIY brake jobs. Someone in the above thread also mentions this.

I think what you want in these servo drum brakes is that one shoe moves first and contacts the drum while the other shoe is still firmly locked against the anchor pin. In this way both shoes act as a single solid shoe that arcs against the drum, and then the back shoe can be forced out by the piston if there's enough pressure applied.

In this scheme, it must be the forward or leading shoe that moves first, meaning that it should have a weaker return spring.

 

[MoPar~Man]

Found it.

https://www.mymopar.com/downloads/mtsc/204.pdf

From that document:

a) image shows different return springs / different mounting locations
b) the "primary" or leading shoe moves first, it's job is to energize the secondary shoe, which must remain in contact with anchor pin
c) servo action causes the secondary shoe to do most of the braking (and we know it has larger pad surface)
d) the document claims the shoes are made with different material. The primary shoe has a higher-friction surface. The secondary shoe has a longer-lasting material (so harder material presumably).

Regardless if current aftermarket shoes are made with the same pad material, I think you do want the leading shoe to move first and hence have a weaker return spring.

 

[MoPar~Man]

Let's go one step further and look at the '67 Parts book. Same brake spring for 11" brakes although they do list different springs for the front and rear shoes. for 10" brakes.

I wonder why the different return springs for Taxi/Police service. Yes, I see they don't differentiate between the primary/secondary springs.

 

[MoPar~Man]

See also:

Drum brake spring colors

Drum brake spring colors

www.concoursmustang.com

"The colors do make a difference with regard to the shoe return springs (up at the top, around the anchor pin). The colors indicate different spring strengths. The lighter _strength_ spring goes on the primary shoe. The primary shoe is the shoe that has a shorter band of braking material. The primary shoe is the leading shoe. The purpose for this is that with duo servo brakes, the primary shoe is pushed out first (because of the lighter strength spring), contacting the drum as the secondary shoe starts to move out. The primary shoe rotates with the drum and in turn pushes the secondary shoe as well. The secondary shoe is forced up against the anchor pin where then it all kinda acts like a wedge. The secondary shoe has more brake material because it is doing the majority of the braking."

 

[Big_John]

See also:

http://[URL]https://www.concoursmustang.com/forum/index.php?topic=13659.0

[/URL]

"The colors do make a difference with regard to the shoe return springs (up at the top, around the anchor pin). The colors indicate different spring strengths. The lighter _strength_ spring goes on the primary shoe. The primary shoe is the shoe that has a shorter band of braking material. The primary shoe is the leading shoe. The purpose for this is that with duo servo brakes, the primary shoe is pushed out first (because of the lighter strength spring), contacting the drum as the secondary shoe starts to move out. The primary shoe rotates with the drum and in turn pushes the secondary shoe as well. The secondary shoe is forced up against the anchor pin where then it all kinda acts like a wedge. The secondary shoe has more brake material because it is doing the majority of the braking."

I've been trying to figure out how to respond to that without sounding like a sarcastic ***... But then again, I've been called that before... LOL

That quote is from some random dude on a Mustang forum talking about Ford brakes. If you want to believe him, great... I disagree with what he says about the springs and quite frankly, just looking at the springs that your car came when new is one of the best arguments I could have against it without doing a lot of typing.

I can discuss and reply to a lot of subjects as long as the references are reliable. If I'm wrong about something, I'll admit it and try to learn from it. It's even possible that I'm wrong here or don't understand something. I've even been known to "agree to disagree". However, this thread just went into "it's on the internet, so it must be true" status and I won't go there.

I really hope that doesn't sound condescending, but I have to hold myself to my own rules.

I'm out.

 

[rags]

The secondary shoe is forced up against the anchor pin where then it all kinda acts like a wedge. The secondary shoe has more brake material because it is doing the majority of the braking."

i'd have to agree with that. if all the events in the braking function were to happen in slow motion over large areas i could see where different spring rates could matter. in reality it all happens so fast that i can't see how spring rates have any effect.

 

[65sporty]

I think your overthinking the springs, I have done a lot of drum brakes over the years. While I am not a expert on the theory of operation and the engineering into what springs need to be stronger, I use what is in the replacement spring kit. If it has 2 different return springs then they get put where they belong, same if they are the same. Between the big 3, I have seen a few different combos

 

[MoPar~Man]

I think your overthinking the springs, I have done a lot of drum brakes over the years. While I am not a expert on the theory of operation and the engineering into what springs need to be stronger, I use what is in the replacement spring kit. If it has 2 different return springs then they get put where they belong, same if they are the same. Between the big 3, I have seen a few different combos

I'm looking for a spring kit that has different return springs. For the '67 Monaco/Polara, Rock-auto shows a bunch of different spring kits, all of them with the same return springs. What car make/model/year do you have to put into Rock Auto for it to show you spring kits with different return springs?

 

[65sporty]

I looked on Rock auto, if you look at a 63 Fury with 10" brakes, it has different size return springs

 

[MoPar~Man]

Yes, for the '63 Fury with 10" brakes they show the DYNAMIC FRICTION 37043001 which has has different return springs - both are same length, different color (white / yellow).

CENTRIC 11863003 seems to be the same as DF 37043001, Application Charger '66-68 Coronet '65-69.

DORMAN HW7038 also seems to be for 10" drums, also has different colored springs.

DORMAN HW7116 is for 11" drums, same springs.

DORMAN HW7036 is for 9" drums, also same springs

RAYBESTOS H7105 might be for 11" drums, also same springs.

The kits with the yellow and white return springs, the springs seem to be the same length, but differ in number of coil turns. Sometimes the anchor-post hook length is much shorter on the white spring.

Hard to explain why they'd want different force return springs on the 10" drums but not 11". I'm going to buy a few sets of each of these and make some measurements.