How much fuel does brake pads dragging waste?
 

I was doing a brake job the other day and I noticed that the brake rotors are much easier to turn with the pads and calipers off than on. That started me thinking that if the pads were somehow spring loaded so they fully retract and do not rub on the rotor when not using them, that would reduce drag and extend pad life. Would it not? I must be missing something. If that would save fuel, why do car companies not do it from the factory?

I have heard of a system like that on drag race cars.
The system of springs and tensioners needs almost constant adjusting.
The OEMS use square O-rings on the pistons that are supposed to reduce brake drag by pulling the piston back a little when pressure in the brake system is released.

Discussed at length here:

http://ecomodder.com/forum/showthrea...ips-25907.html

Oh wow. I never heard of drag reduction clips. I could be wrong, but I do not think that the few ounces of pressure the drag reduction clips apply to try to pull the pads off the rotors would be effective since the pistons in the calipers are so hard to push back in anyway. But is the drag of the brake pads rubbing on the rotors enough to have a measurable effect on gas mileage anyway?

Drag race guys say it gives higher trap speed.
So it could be measurable in fuel economy.

Now take this one step further and measure the drag. Jack up a wheel, wrap a string around the tire, and pull with a spring scale. Then push back the brake pads and repeat. The difference is a direct measure of brake pad drag for that wheel.

Repeat for the other three wheels. Then report the total drag here, and we can estimate the effect on gas mileage.

I think the drag is noticeable at very slow speed on an unloaded wheel but does that drag stay constant or increase with speed and load? I'm betting it is always the same otherwise your brakes would be burning after a freeway run and they aren't. That little bit of drag you can feel with the wheel jacked up at slow speeds probably doesn't amount to anything measurable economy wise.

The one way to find out is to measure. The best way to measure drag is to coast to a stop without using the brakes, jack up a wheel, and measure. Then push back the piston and measure again. The drag after driving may be different from the drag after stopping because brake disk wobble can push the pistons back.

I can calculate the effect of the measured brake drag on mileage.