Wednesday, June 26, 2019

Disc or Drum Rear Brakes - Are Four Wheel Disc Brakes A Must?

    Most new RV's have Disc Brakes on all 4 Wheels. Mine doesn't. I have Discs on the front, but drums on the back. Recently I had some problems during a trip that triggered a set of brake repair woes. It's all fixed now, but I got to wondering if it wouldn't be better to have discs in the rear as well. There are kits available to make the swap (Around $400.00 for the parts plus labor) but is it really necessary? I mean I just replaced all the worn out parts in my rear drum brake system so they are working quite well now. Should I (or You!) make the swap? Is it beneficial...I mean is it?


My Front Disc Brakes
By way of background, during a trip I noticed a slight leak on my rear wheel while I was stopped for gas. I should have looked MUCH more closely. 12 hours later, my brakes failed. Well not all of them. Just one. The rear right drum's actuation cylinder ruptured and I lost most of my brake fluid while trying to get stopped. NOT FUN when towing a trailer. I did get safely stopped in a rest area and used Google to find local mobile repair guys. Called a few, one got back to me in the Early AM and promptly came out to look. We determined I could drive the RV, with a very low pedal and only front brakes the 1.2 miles to their shop. Long story short, I was stuck for 2 days waiting for parts to arrive. They did, and it was repaired. Of course, this could happen with disc brakes as well. A hose or steel line could rupture and the same thing would have happened. But....maybe the parts would have been easier to source?

My Old Drums
Let's take at look at drums versus disc brakes. In a drum based system, the friction pads (that's what stops you when you apply the pedal by converting kinetic energy into heat) push out onto the inner surface of the drum. Disc brakes squeeze a thin metal disc with friction pads. Drums are all internal, discs are external. Meaning you can see the pads and the surface they push against. The main enemy here is heat buildup. Drum brakes will work fine until they are so saturated by heat that they "fade" and lose their effectiveness. Once cooled down, they will work again. That being said, since they are heavy iron construction, the cool-down could take a while. Discs are out in the open air, so they cool quicker. They can still become overwhelmed by heat, but it's more difficult and they cool rapidly to restore effectiveness. So discs are better, right? Well not necessarily.

The Disc Itself
In most vehicles, the front brakes account for 60-90 percent of the overall stopping power. Current (beginning in the 1980's) drum brakes were more than up to the task of stopping the other 40 to 10 percent. In my case, 85% of stopping power is accounted for by the front brakes with only 15% delegated to the rear drum setup. So is it really a necessity to changeover? In addition, there is a device, called a proportioning valve that modulates the front/rear braking force (bias) and it's expensive if you need a new one, To swap to discs in the rear I may need to buy and adjust this as well. After much thought...No, it isn't. Perhaps when the rear shoes wear out or another failure occurs I will reconsider.

But, since I just spent a lot of money on an overhaul of the rear drum brakes...I'll stick with them. Besides, they have always stopped my RV without a problem...well, unless you count the day they failed!

Be Seeing You...Down The Road,

Rich"The Wanderman"

www.thewanderman.com

8 comments:

  1. I had a 1969 Volvo which had close to the same setup - disc brakes up front, and a combo disc/drum setup in the rear. The marketing material said that this was because the emergency brake worked better with drum brakes.

    It also had a dual brake fluid system beginning at the brake fluid reservoir; each brake had 2 lines feeding into it. This was to keep the brakes working after a brake line ruptured. I've always wondered if that system really works or whether one side back fed the other, so both sides lost brake fluid.

    There was a chamber fed by both systems. If one side had greater pressure, then a plunger would move, and set off an alarm or an alert light.

    Very clever these brake designers! That was decades ago, and I'm sure that brake systems are even more complicated today.

    ReplyDelete
    Replies
    1. Chris,
      Now THAT'S an odd setup...It would be interesting to know if, indeed, to know if the systems were truly separated.

      Rich "The Wanderman"

      Delete
  2. The automotive industry has been moving toward disc(k) brakes since the 60s. The big reason was fader resistance. This has become increasingly important with the decline in common sense. A well engineered disc-drum set will do a fine job unless you want to use the brakes to retard all the way down Grapevine or other long slope.

    There is a small disadvantage with all disc (Having had 6 passcars that were) and that is the parking brake is now a separate system that gets to be ignored to destruction.
    Our current coach is 40+yo with disc(k) and drum and the brake system serves quite well. One of the cables does need attention, but Hey, that is a 40+yo part.
    Matt

    ReplyDelete
    Replies
    1. Matt,
      You said it! Common sense is king. Sad that there is a current shortage. I pay close attention to my driving and the "feel." As long as you are aware of their condition and limitations....no issues with drums at all.

      Rich "The Wanderman"

      Delete
  3. One thing that has not been mentioned in the comments was the loss of braking when the brakes get wet. Drum brakes were notorious, you had to be extra careful to avoid water or you would have no brakes. Disc brakes are almost immune to the effects of water, get them wet, they still work.

    ReplyDelete
    Replies
    1. Unknown,
      If you work the brakes gently, they will be fine.

      Rich "The Wanderman"

      Delete
  4. We had disc brakes installed on our trailer and the difference is AMAZING! Now when I step on the brakes, WE STOP! I can't say enough about this somewhat costly switch that just gives SO much peace of mind on the road.

    ReplyDelete
    Replies
    1. MrTommy,
      yes Trailer brakes swapped to discs are amazing!

      Rich "The Wanderman"

      Delete

Thank you for your comment. Our moderator checks each one to make sure we keep the Spammers away. So the comment will likely not appear immediately.