Sunday, 6 December 2015

Front Brake again

As you all know I've been having trouble with my front brake on and off since my SQ4 went back on the road late last year. This year it has had a lot of attention, and is a lot more effective than it was.

Yet it still 'groans' particularly when reversing but also moving forward, which is probably down to the cable being a little tight and the shoes still bedding it in.

However, I wanted to check with members of the AOMCC forum if anybodies brake plates bend outward (in the direction of the arrow) by a few mm when the brake is applied. this is quite easy to see in the garage. I'm guessing that's not normal.

Something is maybe not as square as it should be. Maybe the brake shoes are able to move out of true, or the hub is slightly conical? Maybe there is some play that should not be available?

The brake shoe linings may have worn lopsided and when force applied the brake plate springs a bit. They are not actually that strong, and normally don't have to resist much force along the line of the axle axis. The shoes may have worn unevenly. The springs may be distorting the way they sit, or the plate may be too close to the wheel hub. Something wrong with the bearing installation.

So I stripped it down again. What I didn't expect was two cracked shoes...

Given that these shoes have a marginally smaller diameter than the drum, and have an adjuster at one end that allows the shoe to be brought into contact with the drum, it's easy to see how this would happen.

The story goes like this. If you do the adjuster up relatively tight, the slightest movement of the shoe brings the friction material into contact with the drum. If there is a gap between shoe and drum at the cam end of the shoe, but none at the other end operation of the cam will attempt to bring the cam end of the shoe into contact with the drum, bending the shoe about the contact point at the adjuster end; the resulting bending moment causes a tensile load on the inner diameter of the shoe, leading to brittle fracture due to the hardness in the casting.

While this brake design is used in every car drum brake I have ever seen, it is invariably with fabricated steel shoes which will be considerably tougher than this cast aluminium.

Splendid club the AOMCC. Not more than a few hours after posting the story of my plight on the forum, Paul Jameson had put these in the post!

Many thanks Paul.I also have this from Draganfly, in exchange for my previous, silver painted version:

So the next step is to fettle it all to fit and get her back on the road.

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