Disc drag

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Zombie Master
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Disc drag

Post by Zombie Master » Tue May 30, 2017 2:19 am

84 R100S
So I'm moving on to the front end. I notice the twin discs mit Brembo Calipers has some drag. The wheel won't continue to spin after I roll it. How much drag is acceptable? The bike hasn't been ridden for 8 months.
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barryh
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Re: Disc drag

Post by barryh » Tue May 30, 2017 2:55 am

Some drag is common enough but with clean and properly lubricated caliper pistons the seals can withdraw the piston sufficiently for no drag to occur. I just went out to the garage to spin my front wheel by hand and it took 45 secs to stop which is drag from the bearing grease and seals.

The trick is to lubricate the pistons with brake grease on assembly, either red rubber grease or a silicone based brake grease but not any type of mineral grease which would swell the rubber and not normal brake fluid which attracts moisture from the air and corrodes the pistons. If the pistons are free to move and the pads free to slide the pistons seals will be allowed to do their job of pulling back the piston and pads when the brake is released.
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barry
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Zombie Master
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Re: Disc drag

Post by Zombie Master » Tue May 30, 2017 3:09 am

Thanx again Barry! This is excellent! I guess it's time for a caliper cleaning/lube. When the pound dropped, I order some parts from Motobins. Do you like them? I got a caliper rebuild kit, and two front rotors. Mine are fairly grooved, but braking is still good. I was hoping to get some more time out of my brakes as there is still lots of meat on the pads. I guess I could remove the calipers and see how the wheel spins to see if the bearings are dragging. It's amazing how my rear wheel will spin now that I've cleaned and lubed everything back there.
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Rob
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Re: Disc drag

Post by Rob » Tue May 30, 2017 3:24 am

Rob V

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Zombie Master
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Re: Disc drag

Post by Zombie Master » Tue May 30, 2017 4:13 am

Ouch! Mind you that was a single disc. Nonetheless something must be done! Would I have to remove the pistons entirely to lube them?
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barryh
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Re: Disc drag

Post by barryh » Tue May 30, 2017 4:56 am

Mine is a single disc too and spins the same. Twin discs could be made to produce the same result if the pads pull back properly.

If the calipers badly need an overhaul there is probably not much you can do before hand but if you want to try a shortcut here's one I've used in the past that might work which doesn't disturb the hydraulics. Access is difficult and it has potential for contamination if too much grease is used. Unbolt the caliper and suspend it with something so as not to strain the hose. Remove the pads and dust seals. If you see a lot of corrosion on the piston then this short cut probably won't work. If they don't look too bad, using an aerosol can of silicone grease fitted with a long narrow tube on the outlet, carefully spray a little grease around the edge of the piston. The grease is is very thin before the solvent carrier evaporates so it should seep down around the piston and provide some lubrication. Silicone grease sounds the wrong thing to use around brakes but It won't harm the seals or get past the seals into the brake fluid. The main thing is use the minimum amount and wipe off any excess before reassembly.
barry
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SteveD
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Re: Disc drag

Post by SteveD » Tue May 30, 2017 7:03 am

Might help. After 30 plus years, I'd replace those wiper seals.

http://www.bmbikes.org.uk/Forum/viewtop ... p.#p155143
Cheers, Steve
Victoria, S.E.Oz.


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Rob
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Re: Disc drag

Post by Rob » Tue May 30, 2017 12:30 pm

Mine were badly in need of overhaul.

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Pistons were badly corroded, too. New everything.
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Re: Disc drag

Post by Zombie Master » Tue May 30, 2017 5:08 pm

:) Rob! Somebody made a do do in your caliper! :D

Great link there Steve...thank you.

Barry: Will take off the calipers and have a look. When you say silicone "grease" spray, is that the same as light silicone spray? :ugeek:
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barryh
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Re: Disc drag

Post by barryh » Tue May 30, 2017 5:42 pm

Zombie Master wrote:
Tue May 30, 2017 5:08 pm
. When you say silicone "grease" spray, is that the same as light silicone spray? :ugeek:
It's real grease and much thicker than light silicone spray especially when the solvent carrier has flashed off.
barry
Cheshire
England

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