Brake drums and "Hot Spot" Question

[Jim D]

A few years ago when I got my 66 Commander, I did a full brake job on it.
The brakes had worked "OK" but the linings were getting thin and the car
had sat for a few years, so I got new shoes and wheel cylinders. I had the
drums
turned at a local shop, and when I went to pick them up, the mechanic said
one
of the front drums had a "hot spot", which he pointed out as a small dark
spot
on the turned surface that was about 3/8 of an inch in size. He said it
might make
the brakes pull on that side and when I installed the drum, braking did get
that
warped disc brake rotor type of throbbing from that drum. I found another
drum
on ebay and replaced that drum (a low mileage drum with a lot of "meat"
left).

So....question 1): I have done a lot of brake jobs on drum braked cars and
never
ran into a "hot spot". What causes them? Is it a casting flaw?

question 2) Could I drill out the hot spot area and use the drum again? If
I could, would I have to chamfer the hole on the shoe contact side?

[Lee]

I'd never drill a drum and install it on a car. In my opinion, you
are asking for an exploding drum there.

The hot spot can be caused by many things.... There could have been
contamination (grease, oil, etc) on the drum causing that spot to heat
more than the rest of the drum's surface. I have also seen this
happen from hard braking, coming to a stop and keeping your foot on
the brake.

Years ago, my wife had a car that did the same thing thru 2 sets of
brakes. On the third go-round, I told her to stop a bit short of her
mark and then let the car creep forward while she sat out the light.
This allowed the hot mass of the brake shoes to come in contact with
the entire drum and was not localized in her stopped position. She
never had warped brakes again.

Post by Jim D
A few years ago when I got my 66 Commander, I did a full brake job on it.
The brakes had worked "OK" but the linings were getting thin and the car
had sat for a few years, so I got new shoes and wheel cylinders. I had the
drums
turned at a local shop, and when I went to pick them up, the mechanic said
one
of the front drums had a "hot spot", which he pointed out as a small dark
spot
on the turned surface that was about 3/8 of an inch in size. He said it
might make
the brakes pull on that side and when I installed the drum, braking did get
that
warped disc brake rotor type of throbbing from that drum. I found another
drum
on ebay and replaced that drum (a low mileage drum with a lot of "meat"
left).
So....question 1): I have done a lot of brake jobs on drum braked cars and
never
ran into a "hot spot". What causes them? Is it a casting flaw?
question 2) Could I drill out the hot spot area and use the drum again? If
I could, would I have to chamfer the hole on the shoe contact side?

Lee DeLaBarre
Daytona62

[Mike]

Hot spots are basically just that.
An area within the cast iron that has nonconsistent alloys within the
poured material. It can happen to most any rotor or drum material.
The braking heat will cause a "heat treatment" of sorts in small
localized ares.

Stopping hard and leaving the brakes applied do "not" cause hot spots.
They are caused by getting the rotor/drum "very" hot, normally for an
extended period of time. Like comming down a mountain and being on the
brake pedal constantly. That's what causes hot spots.
Actually...leaving the brakes on "lightly" can help disapate the heat
somewhat. "Lightly" being the key word!

It is actually a heat induced "spot" heat treatment. When the material
gets red hot, and stays that way for a period of time, the cast alloy
depending on the alloy mixture at that spot within the drum/rotor will
basically get heat treated to a higher hardness thAn the rest of the
drum/rotor. And when you have it turned, the normal shop
equipment...the cutter will just skip right over the hard spot and
continue to cut the more normal material.

Ever watch a NASCAR race on a short track? They're rotors are red hot
for most of the race. They get thrown away after that race rather than
resurfaced for the next race like on a track where the brakes aren't
used so hard.

I even hot spotted a set of "stainless" rotors on my motorcycle one day
playing with a coupla guys coming down Angeles Crest Highway (outside
Los Angeles) one day. and as in any other hot spotted brake
part...they were throw aways.

I do have some materials background, so when I did my own rotors, I
knew exactly what happened and why.

[Mike]

Forgot your last question -

Drilling out the hard/hot spots.......ABSOULTLY, DEFINATLY NOT.

1. If you try to drill just the hot spot, you'll have a really hard
time getting the drill to go thru.

2. If you somehow manage to get thru with a simillar size drill as the
dia. of the bad spot, it WILL crack.

3. IF some how you did drill thru larger than the bad spot, you'd have
a "big" hole and you'd throw the balance of the drum off.

Mike

[m***@earthlink.net]

The California DMV has a good section on mountain / hill driving in the
licence handbook.

From my observation, few people read (understood / remembered) it.

On one hill in SD to get on I8, I'm the only car with the brake lights
unlit - and this is for more than a minute going down.
When I use the brakes, it is for maybe ten seconds max before I let
them breathe.

Karl (Not always a flat-lander) Haas

Forgot your last question -
Drilling out the hard/hot spots.......ABSOULTLY, DEFINATLY NOT.
1. If you try to drill just the hot spot, you'll have a really hard
time getting the drill to go thru.
2. If you somehow manage to get thru with a simillar size drill as the
dia. of the bad spot, it WILL crack.
3. IF some how you did drill thru larger than the bad spot, you'd have
a "big" hole and you'd throw the balance of the drum off.
Mike

[Jim D]

Thanks for the info. Guess I'll be looking for another
spare used front drum.

Post by Mike
Forgot your last question -
Drilling out the hard/hot spots.......ABSOULTLY, DEFINATLY NOT.
1. If you try to drill just the hot spot, you'll have a really hard
time getting the drill to go thru.
2. If you somehow manage to get thru with a simillar size drill as the
dia. of the bad spot, it WILL crack.
3. IF some how you did drill thru larger than the bad spot, you'd have
a "big" hole and you'd throw the balance of the drum off.
Mike