That short rear brake shoe.

[Cameron in UK]

We have been working on a car with replacement brake linings and all 8 are the same long length. On a quick test it stopped well and we saw no issues - we thought the need for the short lining on the rear leading shoe was that without it the brakes would lock up in reverse - but thinking now that doesn't make sense as the lining works the same both ways.
So it must be to do with balance and weight transfer under braking. We know that the front axle of a car has much more load under heavy braking and therefore on disk systems the rear disks are smaller to avoid the rear wheels locking up (which would not be good in any car - but particular Amphicar with short wheelbase, high centre of gravity and no ABS) so our thoughts are Amphicar made that lining shorter to reduce that risk. Although Amphicar front brake will always work better as they each have 2 pistons so maybe there was another reason !
Ideas anyone ?
And who is using the same length linings all round ? Any issues ?
Thanks

 

[mike_israel]

On most vehicles with drum brakes, short shoe is in the front. This from the ASE study guide for brake certification ...


The primary and secondary linings are often made from different materials and it's important not to mix things up. Notice where the lining is fastened to the shoe before installing. The primary shoe faces the front of the vehicle and usually has a shorter lining. The secondary shoe's lining faces the rear.

 

[Jon March]

Just had Gord re-line my brakes with his well regarded lining material. Havent installed as yet, so im interested in his comments and others regarding this engineering / physics question.

 

[Jon March]

some input from Gordon Imports -

Dear Jon,

I've always thought that the reason for the shorter lining in the rear was because of the weight transfer issue. It makes sense that you would have less braking power in the rear, because a majority of the weight transfers to the front.

Scott
Gordon Imports, Inc.

 

[mike_israel]

 

[Ken Chambers]

Amphicar brake design uses a twin leading system in the front and a leading/trailing system for the rear. The advantage here is the self applying effect where the piston side of the shoe increasingly magnifies the pressure towards the hinge side. In the front, both shoes utilize this effect for the forward braking direction. It's important the backing plates be installed on the correct sides or the shoes will operate in reverse and will not be able to take advantage of this characteristic, greatly reducing the forward stopping power.

The rear uses a leading/trailing design. The forward shoe uses the self applying effect for forward braking direction, while the rear shoe uses the self applying effect for reverse braking as well as for helping to hold the car using the parking brake when parked on an uphill slope. Amphicar engineers probably calculated the correct amount of friction to the rear wheels by reducing the area of the forward shoe friction material.

All Amphicar brake shoes are of the same friction material. But that is a topic of its own.

Amphicar does not use the primary/secondary shoe braking system. Disadvantages of this system include increased complexity, reduced reverse braking capability, as well as brakes that can grab or lock if wet or rusty.

 

[mike_israel]

Ken,

Great info.

I am always amazed at the little details the Amphi engineers thought of.

Mike

 

[Jon March]

Ken - heres a pic of the front backers that were removed from my car awhile ago -

what are the identifying marks to kelp me be sure to put them back on correctly (now that i painted over the RF & LF letters!)


PS- any preference as to whether the interconnect-lines should oriented be up-bent or down bent?

jon

 

[Ken Chambers]

Put them back on exactly as labeled in the photo, tilted 45 degrees as shown. Direction of the interconnect pipe shouldn't matter.

 

[jfriese]

Ken is absolutely right about that orientation. I spent years trying to figure out why one of my cars had much better braking than the other. Finally I realized that someone had switched the front backing plates and the braking action was being applied toward the piston. When correct, the braking action drives the shoe against the back side of the opposite cylinder. When I switched the backing plate location the braking was cured and equal to my other Amphicar. Just one of those little screw ups you find when you buy an old used car and it took me years to find it.

John Friese
67 White
67 Red

 

[mschlem]

Thanks for posting your question and photo on the front brakes, Jon. The discussion has been good for me to read.
My front backing plates apparently had been removed and replaced before I got my car 20 years ago. They are installed so that the two cylinders are directly at 12:00 and 6:00.....not sure which direction the cylinders are facing without pulling the hubs and looking. I had no idea they were wrong, although I suspected something was odd because the bleeder valve is directly behind end of the king pin making it almost impossible to get a hose on the end of it while bleeding the brakes.
Never had a braking problem, but I only have 1 car - maybe I have poor braking and don't even know it since I have nothing to compare to!
Looks like this will be a good Winter project!

Marc.

 

[okey0]

oh the things to ponder!

 

[Mark Richardson]

My brakes have always been a bit terrifying, and probably is the primary reason I do not drive it much. I am fairly sure even after not having hubs off for years that my backing plates & wheel cylinders are not at a 45 degree angle like the photo above. I think they are at 12 & 6 o'clock also.

Like others, mine probably were worked on before I got it and put together like a Ford should be rather than an Amphicar. I also am now questioning everything including short & long shoes being on front & rear. If I have to take it apart, might as well get everything right.

I now see how the front backing plates should be installed (again, thanks to the photo above), but how should the shoes be installed? Like the diagram Mike I posted above? Funny how that diagram above also has the wheel cylinders at 12 & 6!

How about the rears? Same way? Anyone have pictures of all 4 wheels how they are supposed to look with the shoes?

This could be one of the most enlightening threads I have ever read!

 

[Cameron in UK]

Rears are easy, cylinders at the bottom. Front as in pictures above. Take care with backing plate bolts, remember all braking force is transmitted through them. Don'y overtighten, the torque setting is low as they are only (we think) about 6mm. Also Loctite them as they can work loose.

 

[Ken Chambers]

The 45 degree tilt of the front backing plates have nothing to do braking action. If installed with the cylinders at 12 and 6 it's more difficult to access the bleed valve. The maintenance manual assembly drawing shows it this way but the photos show the correct 45 degrees installation.

Rear backing plates are installed with the cylinder at the bottom.

Refer to the manual for brake lining installation. The manual calls for riveting in new linings but brake shops today mostly bond linings to the shoes. They should also be able to arc grind the linings to match the drum ID.

Linings come with different coefficients of friction. However, the fairly large relining shop I dealt with didn't seem to know much about that. The response I got was, "That's the only lining we use here." While their bonding process and installation looked superb, I have not been very satisfied with the stopping power. I'm considering purchasing the linings elsewhere and having them reinstall them on the shoes.

How about a new thread on brake friction material?

 

[Cameron in UK]

We don't like bonded and always get them riveted as they were originally. There are many different types of lining material. Amphicar needs something soft that will not wear the drums which are now very hard to find and expensive and difficult to repair when worn.

 

[Mark Richardson]

Got it! I think. (Rear shoes are long in front short in rear?)

Thanks for the clarification.

 

[Jon March]

fronts
(note Gord Souters heavy duty linings)

 

[Jon March]

Rears - pls advise if anythings awry - (Graham makes a great point about loctite on the backer plates)

 

[Ken Chambers]

To help prevent water intrusion resulting in corrosion of the aluminum pistons and rusting of the cylinder bores you might want to pack red rubber grease under the dust caps and around the pistons.

http://www.redrubbergrease.com/