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TR Rear Brake Drum Braking Physics/Logic?

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AllanJ71MGB Avatar
AllanJ71MGB Allan Johnson
Princeton, NJ, USA   USA
1971 MG MGB "Batmobile"
1971 Triumph TR6
I've been an MGB owner for 24 years (this weekend actually) and I have changed my fair share of wheel cylinders on my car and others over the years. The wheel cylinders on the MGB are dual piston forcing each shoe against the drum when the pedal is pressed.

When I disassembled my TR6 wheel cylinder/piston yesterday I got to thinking that there is only one piston yet there are two shoes. What are the physics that cause the second non-actuated shoe to press against the drum? Or is only one shoe ever pushed to cause friction? If so, why two shoes? I would think some other setup could be used to attach the return springs.

Looking at the setup I can't see how the rear shoe using the hydraulics or handbrake would ever be forced against the rearward face of the drum. I would think the springs would keep it pulled away as the forward shoe moves forward against the forward face of the drum. It seems that the rear shoe is always fixed in position as the top resting spot on the wheel cylinder doesn't move and the bottom resting spot is on the adjuster which doesn't move without a wrench and therefore not in contact with the drum.

Can anyone enlighten me on this question?

Thanks,

-Allan

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tirebiter Jeff Garber
Dighton, MA, USA   USA
The single piston rear wheel cylinder slides back once the piston moves the front/primary shoe into contact with the drum. A similar scheme is used in single piston front calipers used on many new cars.

The rear wheel cylinder in TR-6s is not bolted in place but has spring-steel clips that lock it to the slot in the backing plate. The cylinder slides in this slot.

AllanJ71MGB Avatar
AllanJ71MGB Allan Johnson
Princeton, NJ, USA   USA
1971 MG MGB "Batmobile"
1971 Triumph TR6
Hi Jeff,

First, Thank you responding. I wasn't going to post my question as I figured it was trivial but you just revealed something critical to me.

The wheel cylinder slides? Whoa. Didn't see that coming as the solid brake line is fixed to the swing arm. Wouldn't that stress the line?

I've installed this all backwards. When I installed my new cylinders I used the previous installation as a guide: Backplate, rubber, 2 steel clips. With that setup the rubber would not let the slide happen. I had an awful time getting the clips on with fresh rubber behind them. I just looked at the catalog and see the correct way to install. So, I will be reinstalling the correct way tonight with the rubber on last.

Jeff, thank you again!!!

-Allan

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ed.h Ed Hollingsworth
Omaha, NE, USA   USA
Likely more than you wanted to know, Allan, but you did ask about physics.

With a single slave cylinder, the forces on the two shoes are different. There is a "leading shoe", and a "trailing shoe". The leading shoe is the one that the drum will tend to drag into tighter contact with itself. This augments braking action to some degree. MGs, having two slave cylinders, have both shoes as leading, presumably offering enhanced braking, all other things equal.

A single leading shoe arrangement requires a self-centering cylinder, while the dual leading shoe brake fixes both cylinders.

Ed



Edited 1 time(s). Last edit at 2017-05-05 11:49 AM by ed.h.

AllanJ71MGB Avatar
AllanJ71MGB Allan Johnson
Princeton, NJ, USA   USA
1971 MG MGB "Batmobile"
1971 Triumph TR6
Thanks Ed! I appreciate the physics!

tirebiter Jeff Garber
Dighton, MA, USA   USA
The steel line is about 16" long. Ok maybe only 8" from where it clips onto the top edge of the trailing arm. The distance the wheel cylinder slides is about 1/16". Slightly more as the rear brakes go out of adjustment.

Some slight amount of bending equals a very small amount of stress but yes tle line is stressed when the brake cylinder moves. A better way would be for a longer rubber hose, directly to the brake cylinder. But a shorter rubber hose makes for a firmer pedal. That might be why the decision was made to do it the way they did when th car was made.

Ed, I beg to differ but MGs have only one cylinder per wheel in the rear. Only the front drum brakes on front of the earlier MG-A cars have two cylinders per wheel. Unlike the TR-6 as you pointed out, the MG has rear has two pistons in the one rear cylinder. One piston acts on the the primary or leading shoe. The other piston acts on the secondary or trailing shoe

ed.h Ed Hollingsworth
Omaha, NE, USA   USA
Duly noted, Jeff. I was going from memory on my '57 MGA and should have specified such.

Ed

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
Some drum brake designs have the slider at the bottom and a single piston cyl at the top. This means that both shoes engage leading edges. (A leading edge on a brake shoe has a self servo effect, as the rotating drumtends topull the shoe into engagement )

tirebiter Jeff Garber
Dighton, MA, USA   USA
Tony, I've never seen a car with that brake setup. Not that I have seen every type of car that has ever been built but I'm curious. I guess that means my brain still works somewhat. Which one uses it ?

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
This side of the pond they are known as Dio Servo brakes

https://www.hemmings.com/magazine/hmn/2013/09/Duo-Servo-Brakes/3730001.html

My father had them on a 3/4 ton pick up truck, the rear wheels would lock at the meir thought of using the brakes!

AllanJ71MGB Avatar
AllanJ71MGB Allan Johnson
Princeton, NJ, USA   USA
1971 MG MGB "Batmobile"
1971 Triumph TR6
Thanks everyone. Got it all back together the right way today.

moneytime Avatar
moneytime Milt Patnor
denver, CO, USA   USA
1972 Triumph TR6 "Old Nellie"
Allan, since we are not having any snow in Denver and it's 60 degrees I decided to change out my drum brakes and cylinders. I had a leak in the cylinder and it was a mess getting the parts off, and now am putting the clips and dust cover (rubber) back on, and wonder what order you used to put it back and if you are putting the rubber on last then what holds the rubber on? Thanks for any responses..

poolboy Avatar
poolboy Ken D
Sandy Hook, MS, USA   USA
As far as the clips, the middle one goes into place last...after the other 2 which have no special order.

AllanJ71MGB Avatar
AllanJ71MGB Allan Johnson
Princeton, NJ, USA   USA
1971 MG MGB "Batmobile"
1971 Triumph TR6
Hi Milt,

Sorry on the delay. We had 5 inches of snow here yesterday. Wish I had 60 degree weather. I would for sure be out in my 6.

I slipped the rubber boot over the handbrake lever (65) before doing anything and just left it dangling until the clips are installed. Once you get the wheel cylinder in place (and the lever fitted correctly) get the clips (56-58) installed. The clip with the notches cut out goes on first pointed to the rear of the car. The big clip with the 90 degree bends will go in the middle pointing forward with the teeth that fit into the notches..... the small clip with the 90 degree bends goes on the outside facing backwards with the bent teeth facing out.

Now... I'll trust pool boy that the middle one goes on last. He's the man. (and I fought and fought getting these on so trust him)

OK, so now that you have it all together just pull the boot along the lever arm so it is close to the drum. Tuck the boot behind the bigger clips (I used a small flathead screwdriver) and you are all done.

This link shows the clips in order.



Hope this helps,

-Allan

moneytime Avatar
moneytime Milt Patnor
denver, CO, USA   USA
1972 Triumph TR6 "Old Nellie"
Thanks for responding to the cylinder install question on my 72' TR6 brake drum issues. I am having trouble with the clips because the hand brake bracket is in the way when I try to install the clips, If I don't put the hand brake bracket in the cylinder prior and then through the back plate it won't work. I understand how the setup works but obviously am doing something wrong, so any ideas would be appreciated. Thanks in advance
I figured the clips out, I changed direction the clips went on and it was a lot easier to finish it up, thanks to Allan and Ken..



Edited 1 time(s). Last edit at 2017-12-11 05:16 PM by moneytime.

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