reducing drum brake drag

[MechEngVT]

Disc brakes typically have more drag than drums. The springs in drum brakes pull the shoes back away from the drum surface. Obviously if your adjusters have spread them a bit too far they still drag on the drum surface, but their tendency is to not contact the drum.

Calipers have no mechanism to spread the pads away from the disc after brake actuation. Many caliper designs have o-rings on the slides in an attempt to prevent knock-back where rotation and flexing of the disc (or wheel bearing) will push the pads away from the disc. A lot of drag racers use drum brakes because it is easier to set them so that they will have no drag and leave braking performance to the 'chute.

Making a parking brake meet safety standards with discs is a pain in the rear. The force required to clamp a rotor and hold it is so high that it is only user-friendly with power-assisted hydraulics. Parking brakes can't be hydraulic because of the risk of fluid leakdown causing a park brake failure, but the mechanical force required is rough. My truck has 4-wheel disc brakes but the rear discs have a park brake drum surface on the inside rear of the rotor with the mechanical cylinder and shoes behind the rotor. It's 2 brakes in one, which is more costly (not to mention heavier, and forces large diameter rotors, thus large heavier wheels).

[rbhollabaugh]

The cars I drive (old Citroens) have manual adjusters on the rear drake shoes. You only need to adjust them every 20k or 30k miles.
The brake shoes on our other car (Honda) have auto adjusters and, yes, they will drag a bit here and there. But the drag is sooo small I can't see it causing any measurable loss of MPG. If that's what you are after.

[ttoyoda]

It talks about the reverse braking type and the parking brake type of adjusters. I had never known of the parking brake type before reading your post.

And I had never known about the reversing type. Thanks for the tip.

[Concrete]

Diesel John,
do you have any pictures of your disk break mods
particularly the pull back springs?

[RP-CLMBR]

I found that when you put new soes on if you dont turn the drums to ruff sand the shoes to fit and then dont hit hand brake while rollig backward suddenly or this will actuate the aouto adjuster I may go back and eliminate the auto adjusters cause iIuse the hand brake a lot on hill take offs, like dsle john said let the road do your brakeing drive for every one way up in front of you,and the brakes will stay clean and free rolling
My dad had made some spring steel clips for a 81 starlet mod back in 89 or so and he would roll up to a light and time it to where gravity did the work and finish it with the hand brake, Oh try pushing a car that has sticky calipers a mile, I think disk brake drag cost as much as day time electrical recharge,if not more, If the speed limits were still 55 I would have four wheel drums on my crx hf, but for pulling a tree chipper and load of mulch, I am glad my mitz fuso has dual front calipers when you need them is when they pay you back for the drag

[RP-CLMBR]

Oh look at the honda insight it has rear drums,

[orange4boy]

The brake shoes on our other car (Honda) have auto adjusters and, yes, they will drag a bit here and there. But the drag is sooo small I can't see it causing any measurable loss of MPG. If that's what you are after.

My experience is that eliminating the drag from my rear drums made a very noticeable difference in my coast down. It now coasts for ever............... No quantification though.

Brakes are made for one thing: friction. and they are very good at their job. Friction is an ecomodder's #1 enemy. Don't misunderestimate it.

[Frank Lee]

Originally Posted by MechEngVT

Calipers have no mechanism to spread the pads away from the disc after brake actuation. Many caliper designs have o-rings on the slides in an attempt to prevent knock-back where rotation and flexing of the disc (or wheel bearing) will push the pads away from the disc.

I'd say the retraction accomplished via the deformation of the square-profile piston seals qualifies as a "mechanism". Yes in the past it wasn't much of a retraction but lately I've seen tidbits about lower-drag calipers on the new "high fuel economy" versions of domestic pickups. Are they really any better than what was standard, oh, 15 years ago? Here's a bit of info on what makes for a lower drag disc system: Classroom manual for automotive ... - Google Books

I've never run anything (car/truck/motorcycle/tractor/off-road toy) hard enough to experience "knock back". I would hope that's just a racing thing. I suppose those anti-knock back O-rings function the same way as the caliper piston seals?

[micondie]

I would suspect that the members of this forum are much more in tune with their vehicles than the average driver, and since self-adjusting brakes are just another step in making cars idiot-proof, removing them should be no problem.
As for the emergency brake drum within the rear disc, the one's in my 66 vette have been a headache since the car was new so I'm glad my Caravan has drum rears.

[brucepick]

Originally Posted by orange4boy

My experience is that eliminating the drag from my rear drums made a very noticeable difference in my coast down. It now coasts for ever............... No quantification though...

Did you already write that up? Can you post a link for us here?