# measuring/fixing brake drag



## dtbaker (Jan 5, 2008)

I have noticed that my efficiency seems to have dropped significantly lately (from about 200watt-hr/mile to almost 400!), but have not found a specific source for it yet. I took the vehicle in for an alignment, and they did tweak it a little, but that has not returned eff to previous levels.

I recently noticed that the front wheels themselves were quite warm after doing some errands around town, even though I had not done much braking. I suspect the front brakes may be dragging a bit, or perhaps the wheel bearings are going bad, but don't know how freely front-wheels *should* spin in a front-wheel drive car when in neutral off the ground.

so.... any comparative 'spin test' info out there?

If I jack both front tires off the ground, in neutral, and give one a good spin by hand, what kind of spin down *should* I see? Then, if there IS drag, what are the most likely causes and ways to fix?


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## Salty9 (Jul 13, 2009)

Frozen caliper pins caused excessive wear on a Chevy I had. It had a single piston and depended on the caliper being free to move. If this is your problem you should be able to hear the brake dragging when you spin the wheel.

Chuck


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## piotrsko (Dec 9, 2007)

too many variables to help here, BUT should be free with "slight" drag after 2 full turns. Figure less than 20 inch pounds to get it to rotate on a smaller car, 50 to get the F250 turning then drop to about 15 to keep it turning. less is always better. I'll bet you either frozen slider pins or caliper slides, especially if it just snowed there. warm is always bad. If in doubt, it is time to change the bearings.


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## EVfun (Mar 14, 2010)

If you jack a FWD car up and spin the wheels there may be quite a bit of drag from the excessive half-shaft angle. Also, you are experiencing added drag from at least some of the transaxle parts.


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## Roy Von Rogers (Mar 21, 2009)

Pull your vehicle after a drive up where you can work on it. Lift it up and see how hard it is to turn the tires. Get to the calipers and retract them so the pads are loose. Try turning the tires again and see if you can feel the difference, that at least will tell where the drag is originating from.


Roy


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## dtbaker (Jan 5, 2008)

ok, turns out that pads seemed to be 'dragging' a little... there was audible scraping when spinning by hand. Not horrible, but there.

I removed calipers, wiped rust and grit off, cleaned the areas where the pad 'ears' touched and added a touch of brake caliper grease just to make sure everything could wiggle a little better and buttoned it all back up with my NEW carbotech ax6 pads ( I want to see if they are grippy enough to not need vacuum booster ).

system seemed to 'scrape' less after replacement.

I debated on whether to peel back rubber caliper boots and add grease, but I did not because I was afraid to rip the boots....

...will report results later.


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## Ziggythewiz (May 16, 2010)

On my bug the front wheels could do 20 revs, (with drum brakes adjusted for no dragging) now with disk brakes they do 2. The rears have never done more than about 5.


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## dtbaker (Jan 5, 2008)

Ziggythewiz said:


> On my bug the front wheels could do 20 revs, (with drum brakes adjusted for no dragging) now with disk brakes they do 2. The rears have never done more than about 5.


a bug would be completely different... with drums and tranny on the rear. But your info regarding maybe 2 revs with disc, versus maybe with drum NOT connected to a tranny sound about right.

My rear wheels, having drum brakes and no tranny, do spin LOTS better than the front. The front at least don't sound like they are dragging anymore, so we'll see. Other possibility that occured to me is that I had CV joints replaced, and perhaps the mechanic replaced transmission fluid with something too thick, or thicker than the 'royal purple' I had in there.


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## DavidDymaxion (Dec 1, 2008)

Don't forget the caliper guide pins. I have one seize once. I have also had guide pins with chewing gum consistency grease, and severely worn guide pins. You should be able to push the caliper back and forth by hand on the guide pins.

Can you push the piston back in with your thumbs? It takes a strong squeeze, but I have been able to do this on all my cars that didn't need a caliper replaced.

Also make sure there is no looseness in the front hub -- if the wheel clunks back that can induce brake drag.

Unlikely on a front wheel drive, but I did have a rear wheel drive where the rear wheels would telescope out on turns, and wear the rear brakes very quickly as a result. I had to put a spacer under the brake disk to fix that.


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## dtbaker (Jan 5, 2008)

caliper pins (bolts holding caliper on?) seem fine in that caliper can move back and forth once caliper piston was pressed back a little.

I could not press caliper by hand, and I have strong hands. I had to use a C clamp to retract the piston.

The 'ears' of the pads rest in these little spring steel clips which were pretty gritty and grimy. I cleaned and put a dab of grease so that pads have a better chance of being able to bump off contact w/ rotor.

seems to have helped, but I am thinking I need to pull them back apart and grease under the caliper boots?


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## Rational (Nov 26, 2011)

So, how many watts does 20 lb-in of drag torque gobble up? 

It may be 'down in the noise'. With a 2:1 ratio in energy consumption the mystery energy drain is not a subtle effect.


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## dtbaker (Jan 5, 2008)

well..... 20 in-# of torque is 2.26 watt-seconds


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## Rational (Nov 26, 2011)

dtbaker said:


> well..... 20 in-# of torque is 2.26 watt-seconds


So at 30 mph avg. speed at 120 seconds per mile = 271 w-s/mile = 0.08 w-h/mile.
Seems like something else is sucking energy out of this vehicle. 
400 w-h in 120 sec. is 12kw, 16 hp. The 8 hp of lost power has to show up as heat, light, noise or mechanical work. F'r sure this is no taillight stuck on, but it could be a faulty reading.


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## dtbaker (Jan 5, 2008)

Rational said:


> So at 30 mph avg. speed at 120 seconds per mile = 271 w-s/mile = 0.08 w-h/mile.
> Seems like something else is sucking energy out of this vehicle.
> 400 w-h in 120 sec. is 12kw, 16 hp. The 8 hp of lost power has to show up as heat, light, noise or mechanical work. F'r sure this is no taillight stuck on, but it could be a faulty reading.



....well, with the brake caliper cleanup so far, it seems almost back to what I remember.... averaged 275whr/mile for today with 'normal' stop and go suburban driving today. Thats about what I was getting with 'normal' errand trips.

I am thinking that next chance I get I will pull calipers again, and grease under piston boot to see if that helps more.


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## Salty9 (Jul 13, 2009)

Dan,

I used silicone grease on my caliper pins and had no further problems with brakes as long as I had the car. A remote sensing IR thermometer would be handy for diagnosing problems like these. They are quick and easy to use.

Chuck


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## tomofreno (Mar 3, 2009)

> well..... 20 in-# of torque is 2.26 watt-seconds


I guess you are converting to N-m then to W-sec? Doesn’t work that way. Work done by torque is the product of torque and angular displacement – how far you rotate the torque wrench. The length unit in your calculation is where you grip the wrench, and with the force you apply determines the torque. Power is work per unit time so the product of torque and angular velocity, or 2.26 * wheel rpm * 2 *pi/60 = 0.237 * wheel rpm, so about 840 * 0.237 = 200W for 12” dynamic tire radius and 60 mph.


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## DavidDymaxion (Dec 1, 2008)

Do not grease under the piston boot! If any of that grease works its way inside the caliper (like when you use a C-clamp to push the piston in) , it'll contaminate the brake fluid. You should pop out the piston, use a brake cylinder hone, and new seals. Rebuilt brake calipers are cheap enough if I get a stuck piston (can't push it in with my thumbs) I just get a rebuilt caliper. One caution: Some parking brake systems latch the piston in the out position, so be careful that's not the case. Very few cars have front parking brakes, so this is usually the rear brakes that might do that.

I read about an EV team that made sure the piston and cylinder were round and smooth to ensure the piston could move as freely as possible. BTW the seals are supposed to pull the piston back a bit to lessen friction after a stop.

It's also a really good idea to flush the brake fluid rather than just bleed it. That'll get all the corrosion inducing moisture out of it.

Anyway, great job on reducing the energy usage! Thanks for the report.


dtbaker said:


> ....well, with the brake caliper cleanup so far, it seems almost back to what I remember.... averaged 275whr/mile for today with 'normal' stop and go suburban driving today. Thats about what I was getting with 'normal' errand trips.
> 
> I am thinking that next chance I get I will pull calipers again, and grease under piston boot to see if that helps more.


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## dtbaker (Jan 5, 2008)

DavidDymaxion said:


> Do not grease under the piston boot! ....
> It's also a really good idea to flush the brake fluid rather than just bleed it. That'll get all the corrosion inducing moisture out of it.



ok, I won't mess under the boot. 

perhaps clean up the external area the pad ears rest in, and call it good. Then flush old brake fluid that I am sure has never been touched in 145k miles.

my car is the typical econo system, disc in front, and drum in back w/ parking brake (which actually works better with drum, right?) I have not pulled the drum yet so I dunno what shape the drums are in.


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## Rational (Nov 26, 2011)

You could just pull the car slowly on level ground by hand or by using a winch or block & tackle, and use a hardware store spring to measure the drag. 
Storm door snubber springs have a spring constant of about 45 lbs per inch, but measure it first. Valve springs are really stiff.


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## dtbaker (Jan 5, 2008)

Rational said:


> You could just pull the car slowly on level ground by hand or by using a winch or block & tackle, and use a hardware store spring to measure the drag.
> Storm door snubber springs have a spring constant of about 45 lbs per inch, but measure it first. Valve springs are really stiff.



are you serious?

I am all about scientific process, but this does not seem like an accurate or repeatable test for what I am after.

I think this is one of those times when a simple spin-by-hand and an audible test of 'do I hear brake drag on rotor anymore' is close enough.  Answer so far is that after preliminary cleaning of pad ear 'holders' things seem to be much better.


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## Rational (Nov 26, 2011)

dtbaker said:


> accurate or repeatable


Repeat the test several times and average the results. 

Accuracy depends on your measurement of the spring constant. I use a weight that I measured on scales at the Post Office or the supermarket to get really high precision.
You can also buy a spring balance, like the physics labs have.

Measuring the total force to slowly move a vehicle is also useful for acceleration calcs. You can always go into more detailed measurements later, if you need them.


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