Re: [FWDLK] rear brake shoes and arching them

On Sun, Jun 30, 2013 at 12:56 PM, Dave Homstad <dhomstad@xxxxxxxxxxx> wrote:

If anybody locally here still has the capability to do arching, it is Brake & Equipment Warehouse.

Arching should give nearly 100% of the brake system's maximum braking capability immediately after a short break-in of the new linings.

Break-in should be done by short mild stops with some time to cool between. This is usually normal driving, without hard stops or stops from high speed. You want to avoid over heating the drum and making "hot spots" were the metalurgy is altered into hard spots, making the drum brittle in localized areas or distorting/warping it out-of-round.

If arching is not done, brake efficiency may be slightly lower for a time until enough material wears off the higher contact spots to allow the entire shoe area to make contact. It may take a few 100s or 1000s miles to happen. This is what arching does, machines off the high material contact points so the entire surface makes good contact.

I did not like one of the other fellow's description of how mechanics sometimes removed more material from the drum to match the shoes, instead of removing the material from the shoes to match the drum. 50 years ago, when new drums were cheap, that may have been OK. But today, there are no new drums and the ones we have need to last.

I suspect that arching was more critical on earlier cars that had smaller brake surface area to weight ratio, and probably lower hydraulic system psi levels. They needed all the help they could get.

The 56 Dodge Shop Manual has a description of a "Major brake adjustment" vs a "minor brake adjustment". With new shoes, the geometry will be different than the old shoes and may require adjustment of the anchor pin to compensate.

Dave Homstad
56 Dodge D500

On Sat, Jun 29, 2013 at 2:05 PM, debenson2 wrote:

Is it the same thing on parking brake shoes as well? And to do this do you need the drums with the new shoes to do the arcing?

From: Forward Look Mopar Discussion List [mailto:L-FORWARDLOOK@xxxxxxxxxxxxx] On Behalf Of John Hagen

Sent: Saturday, June 29, 2013 1:31 PM
To: L-FORWARDLOOK@xxxxxxxxxxxxx

Subject: Re: [FWDLK] rear brake shoes and arching them

Yes. And they must be arced to a slightly smaller diameter than the drum surface so that when the brakes are applied the pressure from the hydraulic system can push the ends out to the surface. Since getting the shoes arced to exactly mate with the drum surface (which will vary with temperature) is near impossible it is much better to be a tad under diameter than over, which would cause the center of the shoes to not meet the drums surface and pressure exerted on the shoes by the hydraulic system cannot make up for that situation.

I say this as back in the day arcing to a slightly smaller diameter was pretty much standard procedure but nowadays such information is not so well known.

For cars with Bendix type brakes later production shoes did not need arcing as they were made to be flexible enough and under size enough to allow for full contact with normal hydraulic system pressures. But he center planes always need to be arced.

John Hagen

From: Forward Look Mopar Discussion List [ mailto:L-FORWARDLOOK@xxxxxxxxxxxxx] On Behalf Of debenson2

Sent: Saturday, June 29, 2013 11:27 AM
To: L-FORWARDLOOK@xxxxxxxxxxxxx

Subject: [FWDLK] rear brake shoes and arching them

When replacing rear brake shoes on a 1956 must they be "arched"? or something I cannot recall the name but made to conform to the drums for better contact? Is this a true thing?

debenson2

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