Air Brakes Experts

[wireguy]

This is pictures of my brake shoes and my S cams right at the point where they lock up the drums. Looking at images of new air brakes shoes I can't see where mine are much different. Meaning they don't look excessively worn out.
To me the S cams look like they are right at the point where they are in danger of rolling over. Do S cams commonly wear out and is that likely what I am looking at here, meaning new S cams needed. When I took everything apart the rollers were frozen up so the S cams were probably scrubbing across them rather than rolling across them.

[wireguy]

Oh, the drums measure out as being in spec.

[MacC]

check your camshaft bushings. They fail first. You have bushings at both ends.

[family wagon]

Originally Posted by wireguy

To me the S cams look like they are right at the point where they are in danger of rolling over. Do S cams commonly wear out and is that likely what I am looking at here, meaning new S cams needed.

By 'rolling over' do you mean you're concerned that the cams have forced the shoes apart the maximum amount they're able? Then when the shoes wear down the cams wouldn't be able to compensate. Is that right?

I took the liberty of drawing a circle onto one of your photos. It looks like the photo was taken close enough to the axis of rotation to make this approximation work. The circle is centered on the dimple in the middle of the cam with the radius at the point where the lower roller is touching the cam. It was harder to notice in the original picture, but with the circle drawn on, we can see that the cam surface isn't a circle; it arcs outward away from the center of the cam. Seems to me that this cam has a lot of shoe-spreading ability remaining.

[wireguy]

A cam is an increasing radius, and to my eye the roller is sitting at the very top of the radius. Any further rotation is going to close in on where the radius begins to radically reduce. I wish I knew what the center to center at the cam high point measurement is supposed to be, and what the mathematical ratio is on wear reduction on the cam radius vs shoe spreading ability. In other words how much loss of cam radius results in a radical loss of shoe spreading. Is it possible these shoes are supposed to be twice as thick as what I have? That would certainly put the rollers back on the working face of the cam.

I replaced the bushings. New cams are pretty cheap and I think I will replace them but I will be shocked if the new cams resolves this.
I am reminded of the time I rebuilt the Toyota G-52 5 speed transmission in my truck. Because I didn't know what "normal" looked like on the shift forks I failed to realize the 1-2 shift fork was completely wiped out. When I put it back together I still had the "refusal to remain in first gear" problem that led to the rebuild. It wasn't until I did some further research I realized the beautifully machined faces on the shift fork were beautifully polished away by wear. This is a very similar issue. I don't know what a new S cam looks like for this application.
I have new cams located but in the picture of them I can't see much difference.
Rotation on the cam face here is clockwise. If you continue to rotate the cam clockwise you see the roller will quickly be on the "downhill" side of the cam.
Update: new cams on the way.

[family wagon]

The rollers rest in the indent/concave section of the cam when the brake is released; they climb up onto the increasing-radius face as the brake is applied. The cam in the second picture, the one labeled 035.jpg, rotates clockwise when the brake is applied. The third (037.jpg) rotates counter-clockwise when the brake is applied.

[wireguy]

I have become convinced the problem is the brake shoes are worn. Having never seen a new set of these huge - 10" across - shoes I thought the 1/2" to 5/8" of lining was plenty but now I'm thinking maybe I need to be seeing a solid 3/4 inch, possibly more. I have new S cams on the way but I don't think that is going to roll the contact area on the cams back any meaningful distance.

[TJones]

Your shoes are ok. The federal dot limit is 1/4 in for non steer axles and 3/16 in for steer axles. You s cams look fine to me. Measure from the indent in the center to the edge of the cam where the roller will contact it. The distance will get larger as you move toward the tail of the "s".

Ted

[wireguy]

Thank you Ted. It is useful to hear that the shoes look OK. I would just be more comfortable if the rollers weren't sitting up so high on the cam, but again, I don't have a standard to apply to this situation, having never dealt with them.

[sportyrick]

looks to me like you have the emergency brakes applied (air is down) and your S cams are rotated into a locking posture, your rollers should be tucked into their pockets when brakes are released, you had to do that to get the drums off right, charge your air up and release E brake and they should roll back home. Shoes look good, how much of a lip do you have on your drums?

[Booyah45828]

Originally Posted by wireguy

To me the S cams look like they are right at the point where they are in danger of rolling over.

They're not. They roll over when they're near 180 degrees of rotation. The s cam is nothing more then a ramp arranged around a center point. Here is a experiment you can do to prove it.

1. Start with the rollers at 0
2. Place a dial indicator near vertical on the roller.
3. Rotate the s cam from 0 to roll over while watching the linear movement of the roller on the indicator.

What you'll see, is that the shoes will continue to spread up until the roll over point.

You look to be around 75 degrees on s cam rotation. You have until 120 degrees until you need to service the brakes, whether it be shoes or drums or both.

Originally Posted by wireguy

Do S cams commonly wear out and is that likely what I am looking at here, meaning new S cams needed. When I took everything apart the rollers were frozen up so the S cams were probably scrubbing across them rather than rolling across them.

No, cam's don't commonly wear out. As long as everything is greased and functioning properly, they'll never wear out. The only time I've seen an s cam worn out is when the rollers were seized and were scrubbing across the surface of the cam instead of rolling. In that case it was doing that for a long time because the damage to the s cam and roller was obvious due to the flat spots and burrs that were present on both.

Originally Posted by family wagon

The rollers rest in the indent/concave section of the cam when the brake is released; they climb up onto the increasing-radius face as the brake is applied.

Originally Posted by sportyrick

looks to me like you have the emergency brakes applied and your S cams are rotated into a locking posture, your rollers should be tucked into their pockets when brakes are released, charge your air up and release E brake and they should roll back home. Shoes look good, how much of a lip do you have on your drums?

Rollers don't rest in the pockets/concave section, they rest on the ramp where his picture shows them at now. The only time rollers will be in the concave section is when someone manually backs off the slack adjusters, or the cams have rolled over and flipped.

Originally Posted by wireguy

I have become convinced the problem is the brake shoes are worn. Having never seen a new set of these huge - 10" across - shoes I thought the 1/2" to 5/8" of lining was plenty but now I'm thinking maybe I need to be seeing a solid 3/4 inch, possibly more. I have new S cams on the way but I don't think that is going to roll the contact area on the cams back any meaningful distance.

New s cams aren't going to fix anything. There is nothing here that I can see needs fixed. Grease the rollers where they contact the shoe. Grease the pins on the other end where they contact the shoe and slide into the bracket. Reinstall the drum, adjust the s cam to achieve the proper brake chamber free stroke, and roll with it.

The new set of shoes sitting beside me now have roughly 3/4" at their thickest point. Like Ted said, yours are fine.

[wireguy]

The emergency brake is off. The drums are well in spec. I took this picture through one of the vent windows in the drum. The drum is on and bolted down, the air is off. I simply rotated the cam with the slack adjuster until it locked up the drum so I could get an idea of where the rollers are sitting on the cam at the point where the shoes are just locking up the drum. Is it normal for the rollers to be that far out toward the end of the cam at that point? Or as I see it, that far up on top of the cam. Just looking at it, and not being familiar with what I should be seeing, to me it looks like that roller is too far up the cam for hitting the point where it is just locking up the drum. I don't want to go over the top and lose my brakes.

[wireguy]

Thanks you guys. Your in depth explanations are a huge help. The way my mind works is I insist on understanding the why, not merely the what, and you guys have explained the why of it.

I have new S cams on the way. The rollers were seized up and the cams were scrubbing instead of rolling the rollers. The cams were 66 bucks delivered so why not. You guys saved me a ton on not having to buy new brake shoes for this monster. Still I think I will keep looking for a set. These 10" brakes are not commonly used any more. I found a place in Vegas that will reline them. Napa lists these drums at 850 each. I'm SO glad mine are in spec. They measure exactly 16.5 on a 16.7 max.

[wireguy]

Finally, all the new wheels and tires on, brakes gone through, tons of dried grease scraped off the underneath, a hundred zerks greased, it was time. I just got back from the maiden voyage of my 77 Gillig with 855 big cam. The brakes worked fine. I couldn't get it to go up through the gears past second. I know how to double clutch and that made no difference. Much to my surprise it suddenly snicked into 3rd gear without using the clutch at all. I took my cue from that and stopped using the clutch except at corners and such. I still have to figure out what the shift points are but when I hit one it just grabs it smooth as can be. I feel like I'm finally making some notable progress.
Thanks to everyone who helped me with the brakes learning curve.

[Brad_SwiftFur]

On my bus with a 5 speed manual, I often "rev match" shift (don't use the clutch except to start and stop) and for 10 speeds and other similar transmissions, they are designed to be able to do this.


As for brakes, the S-cams are *intended* to last the life of the vehicle but corrosion, seized rollers, rotation in dry bushings, etc. can cause them to wear out. It is completely normal for the rollers to "rest" at any point along the curves as the brakes wear and the slack adjusters compensate for this; it's the way the system is designed to function. When the rollers get close to the ends, it's time to measure brake shoes and drums, measure the shoes in the center (the ends don't wear much). Start planning on a brake job when it's around 1/4" (which is pretty much the wear limit). Shop around for drums, I believe they can be had much cheaper.

[wireguy]

Yea, I gotta learn the rev match points and that will come by driving it. It was a shocker to discover it doesn't even like the clutch.

[wireguy]

I have come to the conclusion there is a geometry between the S cam and rollers that is different than the geometry on a engine camshaft and rollers or lifters, which is what I am familiar with. With those once your roller goes over the top the lifter is headed downhill again. I think being familiar with that geometry has kept me from seeing that the S cam uses a different geometry with respect to the rollers and that even if the roller is over the top of the curve it is still pushing the rollers out, maybe because of the length of the cam face.
Booyah45828 started me thinking that direction, that I wasn't seeing the geometry correctly.

[okinawascuba]

Originally Posted by wireguy

Thank you Ted. It is useful to hear that the shoes look OK. I would just be more comfortable if the rollers weren't sitting up so high on the cam, but again, I don't have a standard to apply to this situation, having never dealt with them.

They look fine. You still have about 50% travel left on the cams, don't sweat it. Your shoes look fine. These aren't going to ever roll off the end of the cam, even with NO brake shoe left. Been a diesel mech for many years, I think everything looks fine.

[wireguy]

We drove it maybe 5 miles around the area yesterday and the brakes felt fine. Like my dad told me his dad taught him, before you make sure it will go make sure it will stop. These brakes had not been maintained for decades, the front brake rollers were seized, and the time and effort I put in to going through and understanding these brakes was time well spent. Thanks to all those whose feedback helped.

[okinawascuba]

Absolutely agree! Always better to check, inspect. and address potential problems before being on the side of the road wishing you had.