low mpg
December 6, 2006 8:46 AM   Subscribe

I seem to be getting horrible mpg after changing my own disk brakes.

After I replaced the front brake pads on my 1999 Honda motorcycle, my mpg dropped by 25% and sometimes after braking I’ll hear a mild “whizzz” coming from the front wheel, which I’m guessing is the sound of the pad still making slight contact with the rotor. After a couple of months (I waited this long because I’ve heard that new brakes will sometimes make noise until they’re broken-in) I took a look at the pads and they didn’t seem to be wearing funny. I’m a city rider, so I’m not too worried about my brakes over-heating during my short trips.

Is there an obvious reason for this? Is there a mechanism that should be spreading the calipers apart after breaking? I’m planning on taking it in unless someone points me to the solution.
posted by spork to Travel & Transportation (10 answers total)
tilt your bike back on its center stand and try to spin the front wheel. It should spin freely, just like a bicycle wheel.
posted by M.C. Lo-Carb! at 8:47 AM on December 6, 2006

How did you break the pads in? Did you follow the package instructions to break them in properly, or did you just take off and start riding on them?
posted by SpecialK at 8:50 AM on December 6, 2006

Response by poster: There were no brake-in instructions. I did take it very slow at first.
posted by spork at 8:59 AM on December 6, 2006

Is it a hydraulic brake system like on cars? If the calipers aren't retracting the pads when you release the brake pedal (or hand lever), it could be that there's excess pressure in the system. On cars, there's a brake fluid reservoir and if it has lots of air in it and that air gets hot, it's possible that that small amount of pressure could cause the caliper's pistons to partially eject. When the bike's warm, open the reservoir, release the brakes, and top off the fluid.

But if there's excess air in the system, you'd probably notice that as "squishy" brakes. And if it's just a mechanical cable mechanism, then could be a tension problem.
posted by cyclopticgaze at 9:26 AM on December 6, 2006

True. Did you bleed the brakes, or just swap the pads? How old is the bike -- is it possible that something's wrong with the pistons or seals? How does the brake lever feel -- really tight, like you apply and BOOM you're stopped, or is there a lot of play in it?

(And yes, motorcycles with disc brakes all use hydraulic systems.)
posted by SpecialK at 9:34 AM on December 6, 2006

Asked my buddy the mechanic:

"I just replaced the pads on the bird a couple weeks ago and teh first day or two I heard a sound. Its normal tho as the pads get worn to the rotor. It should stop after a couple hundred miles (assuming there is a decent amt of braking being done). If you can get the pads in, then the pistons retracted far enough -- assuming they are the correct pads. If its not better after a couple hundred miles, bleed the brakes. I generally bleed them when I replace pads regardless"
posted by SpecialK at 9:40 AM on December 6, 2006

Make sure you didn't twist or kink the fluid lines going to the brake calipers, which can prevent the pressure from being released when you release the brake lever.
posted by knave at 11:22 AM on December 6, 2006

Response by poster: I didn't bleed the brakes but did top off the reservoir after I was done. They feel neither squishy nor tight -just normal.

I only have a side stand so I'm not able to spin the wheel.

Also, I was careful not to damage the hydraulic hoses.

Thanks for the help.
posted by spork at 12:24 PM on December 6, 2006

Debris in the piston can sometime work it's way into the line and there act as a partial one way valve. This can cause the caliper to drag for a few minutes after each brake application resulting in reduced performance and mileage.

I'd bleed the system and make sure the piston and pads are free to travel. I've once seen rust build up on the caliper/pad interface jam a pad causing it to drag.
posted by Mitheral at 2:19 PM on December 6, 2006

The way you describe it, it sounds like your calipers aren't sliding open freely. Perhaps lubing the rails will help this.
posted by kc0dxh at 8:13 AM on December 7, 2006

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