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Old 03-12-2020, 03:08 PM   #1 (permalink)
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Brake pad drag reduction clips (theoretical)

I assume it might be worth it. Only expecting it to be say 10 to to 25 watts per wheel. I would be surprised if it saved more than 200 watts on all 4 wheels. In a perfect world with perfect brakes that never drag this would not be anything to consider. If they only save 100 watts definitely not worth it. Or they save 200 watts might be worth it over the life of a set of brakes.

According to Wikipedia the brake drag dynamic friction coefficient of brake pads is between 0.35 and 0.42u.
That means 1,000 newtons of brake force is countering 350 to 420 newtons of friction.

Math is a universal language and units are interchangeable so it come down to preference. So I self identify with units of imperial.

So if we know u, then we need force and distance so we can determine speed, once we know speed we figure power.

To get force simply take a measurement of how much the spring will be compressed, (hint: you may want to stretch the spring to a longer free length to get more force) then put the spring on a scale and measure the amount of force at target compression. I didn't actually do this last time but I have a set of springs in the mail, should be here today and I will measure them. So I will edit in an actual value later it's only going to be several pounds of force.
Worst case scenario I have to use the scales at work.

To get distance just measure the mean radius, or just measure from the center of the hub to as close as you can to the middle of the shinny area. You want the actual area of rub. This doesn't need to be high precision. For the rear of the leaf it's about 5 inches to the mean pad to the rotor rub area. So circumference is about 15.7 inches

Now we need speed. Go over to tirerack dot com, look up your tire make, model and size, go to specs and they have number of rotations per mile listed. For a leaf size energy saver A/S thats 836 revs per mile. I drive about 65mph so let's round that to 905 revs per minute, need it in units per second.
So the brake pad is covering about 19.73 feet of mean rotor circumference per second at 65mph.

If the brake pad drag reduction clips can reduce what seems lile a lot of drag lets go with what I think will be a high number of say 9 pounds off the pads, reducing let's say 3 pounds of force off the pads drag, that's 59.22 foot pounds of energy per second per wheel with that size brake rotor at that speed. Or 7,107 foot pounds per minute for the rear axle per minute at 65mph. That's substantially more than what I thought it would be.

Conversion of foot pound seconds to watts is
foot pound seconds /0.7376

So that comes out to 80 watts per back wheel. The fronts will be little more since they are bigger.

If we round all 4 wheels to 400 watts that's pretty substantial. At 65mph I run about 10kw so that's about a 4% reduction in drive power should directly translate to about 3% or 4% more range.
This is a higher estimate but I will know for sure when I compress springs on a scale. So it will still easily be 200 or 300 watts for all 4 wheels.

When you buy a pack of these clips you get 4. Put 2 on each front caliper when I put them on the leaf because the fronts were a little sticky, then I was going to order more and put them on the back. But I forgot. So they're only on the front.

Level of skill required:
If you can accidentally poke your self in the eye with a wire you have the necessary skills.
A c-clamp and some linemans pliers help.

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Old 03-13-2020, 07:15 PM   #2 (permalink)
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9 pounds of drag? How stuck are your calipers? That sounds too high to me when you exclude wheel bearing drag. I recall being able to keep the wheel spinning with not much effort from one finger once it got going.
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Old 03-13-2020, 09:09 PM   #3 (permalink)
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I measured the springs.
They relieve 3 pounds of force each, 6 pounds per caliper.
Seems to pretty much cancel out all the brake drag.
So about 53 watts per rear wheel.
A little more for the front.
So around 200 to 300 watts of reduction for a small car like the leaf on the highway.
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Old 03-14-2020, 11:05 PM   #4 (permalink)
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Your math is great, but just because the springs have 3 lbs of force doesn't mean the pads are pushing that hard on the rotors. It may only take .5 lbs to push the pads back and the extra 2.5 lbs is only creating more room after the initial clearance is made. It only takes a few thousandths of movement in the pads to get them off the rotors. Let us know what the results are.
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Old 03-15-2020, 08:14 AM   #5 (permalink)
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When I jack the car up and spin the tire with my foot it spins very freely now as if there is no brake drag.
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1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
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Old 03-17-2020, 03:45 AM   #6 (permalink)
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Thank you for this analysis.
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Old 03-17-2020, 12:23 PM   #7 (permalink)
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I'll have to jack mine up and look.

A few weeks ago my wife and I put new calipers on her Fit, and the calipers came with these clips. While I was putting the summer tires back on it last night I spun the front tires and (even with the CV axles connecting them to the transmission) her drag-reduction-clip'd front wheels spin with basically zero resistance, and keep spinning for way longer than I'd expected - they spin with the same ease as rear drums.
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Old 03-18-2020, 07:54 AM   #8 (permalink)
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When I purchased some aftermarket brake drag reduction clips for a prior vehicle (2005 Tacoma) I found that a single factory clip actually had more pushing force than two aftermarket clips. So when I got my 1999 4Runner, I ordered factory Toyota clips (not expensive if already purchasing other parts to save shipping) and put one on each side of each caliper. (I also put new calipers on... clips don't help when a cylinder won't retract) I tried stacking two on each side, but inevitably that never worked right and one would bind up and make it worse.
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Old 03-19-2020, 09:39 PM   #9 (permalink)
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Quote:
Originally Posted by Ecky View Post
I'll have to jack mine up and look.

A few weeks ago my wife and I put new calipers on her Fit, and the calipers came with these clips. While I was putting the summer tires back on it last night I spun the front tires and (even with the CV axles connecting them to the transmission) her drag-reduction-clip'd front wheels spin with basically zero resistance, and keep spinning for way longer than I'd expected - they spin with the same ease as rear drums.
You wouldn't happen to have any pics of what the clips look like do you?
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Old 03-19-2020, 09:55 PM   #10 (permalink)
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Quote:
Originally Posted by Fit_is_slo View Post
You wouldn't happen to have any pics of what the clips look like do you?
They looked exactly like these:

https://www.homedepot.com/p/Raybesto...RoC1zEQAvD_BwE

You slip them in the calipers, and they provide a very small force to help the pads pull back from the rotors after braking.

Normally disc brakes have no way to retract the pads, unlike drums.

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brake drag, drag, drag coefficient, drag is a drag

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