Front Brake Scraping Noise - How to Correct? XM-3500Li

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stalcottsmith's picture
Last seen: 8 years 2 weeks ago
Joined: Thursday, January 22, 2009 - 21:24
Points: 6
Front Brake Scraping Noise - How to Correct? XM-3500Li

I've been driving my XM-3500Li now for 3 weeks and I have added about 400 kilometers on it.

I am very happy and love this machine despite its flaws.

My front brake makes a very slight scraping noise once per revolution. When I brake hard it is a louder noise and somewhat disturbing to me.

This weekend after leaving it sitting still overnight during which it was sprayed with a sprinkler, I noticed some oxidation possibly left by metal contacting the disc of the brake:


I am trying to determine whether I need to adjust the brake, replace the pad, or replace the mechanism.

I bought some tools but I am not sure how to approach the task and need to set aside a bright sunny day when I have nothing to do since this is my main mode of transit.

The rear break is quiet and works great although it is not as powerful. I hope this is not a safety issue.

Where would I obtain replacement parts for this if I wanted to replace or upgrade?


Last seen: 13 years 1 week ago
Joined: Thursday, October 30, 2008 - 09:10
Points: 63
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

This is a pretty common problem with this kind of disc brakes. If it is just barely brushing the disc once per revolution it probably isn't a big problem. If the brake is actually grabbing the disc it needs to be fixed.

The issue arises due to the fairly tight tolerances involved. In a perfectly aligned system there might be a single mm or less clearance between the brake pads and the disc. It can be devilishly difficult to diagnose the exact problem. It could be caused by a wheel misalignment, a bent rotor, an out-of-true hub, or probably 5 other things. I would start by checking wheel alignment.

Last seen: 12 years 11 months ago
Joined: Wednesday, June 4, 2008 - 16:13
Points: 125
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

Some assorted comments that I hope will help.

- Disk brakes do not have 'pad return springs' or any other way to open a gap between the pad and the rotor the way drum brakes do. Some contact will always happen until the pad material wears away. That slight gap 'goes away' next time one applies the brakes - and the cycle starts over. The pads are softer than the rotor and they'll wear first. (We'll assume you have brake pad material remaining on both pads - which you should after 400km.)

- Brake rotors (and drums) are untreated raw metal. They can't be oiled or coated to protect them from rust. By nature, the rotor and pads are designed to be 'worn away' each time the brake is applied. The rust you see is normal and will be 'cleaned away' next time you brake.

- Braking stops the bike by converting motion to heat. The rotors will get hot when they are used. If they are straight (not warped) in the morning, they'll generally be straight when the bike returns home -- unless... There are two main events that can warp rotors. The first is the initial 'stress balancing' in any piece of metal. The second is sudden cooling of part of a rotor when it is hot.

-- Warped rotor example one: A brand new rotor is cast or machined, then cut smooth and shipped (less expensive rotors/lower quality vendor). The first few times it's used and heated, the stresses in the metal will equalize. This can cause the rotor to warp. The only cure is to have the rotor re-cut/re-machined flat. It'll generally be straight for the rest of it's life.

-- Warped rotor example two: A new rotor is cast, heated to equalize stress, then machined and shipped. The rotor will generally be true for it's life. Some riders must ride thru water. Others like to hose the bike down after a ride. A straight/true rotor that is stress-relieved will generally cool straight if allowed to cool slowly. Splashing water on part of a hot rotor can cause unequal cooling - and that can lead to warping the rotor.

A warped rotor can be machined flat until it reaches it's minimum thickness (the 3mm you see in your picture). At that point, it must be replaced.

Ride your bike and use the front brake. Something like a couple of mostly-front-brake stops from 30mph should do it. If the rotor is warped, you'll feel the 'pushing back/pulsing' in your brake lever and parts of the rotor might not be 'rubbed' by the pad. Check the surface of the disk after you get a couple of stops - if all is well, the rust should be gone or significatly cleaner, the brake lever will give you 'smooth feedback' with no pulses.

I hope that helps,

Last seen: 6 years 7 months ago
Joined: Monday, March 10, 2008 - 08:43
Points: 340
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

on front (or rear) disc brakes the piston in the caliper has a gasket up front ,and may lock up if water or dirt gets into the space behind the gasket and between the piston and cylinder body !! take a flat blade screwdriver and pry the pads apart till the piston is compressed to the back of the cylinder then apply the brakes slowly . do this a few times and lubricate the cylinder a bit with a film of brake fluid and your problem may go away LaTeR

thank GOD I wake up above ground !!!!

Last seen: 6 years 10 months ago
Joined: Saturday, December 22, 2007 - 08:04
Points: 211
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

I have found three issues with the brakes on the Zapino scooters which may be applied
to your situation.
1) rotor bolt holes not tapped to the bottom, preventing one of the rotor bolts to not
be tight. Used a bottoming tap on all three holes to make sure I get a good seating of
the rotor on the rim.
2) caliper not really mounted properly. Loosened, then applied pressure in the applied
direction as I tightened the rotor bolts a little at a time.
3) the mounting pads on the rim are not machined very well. I used a dial indicator gage
to verify the runout and filed down a high spot until the rotor runout was true.

Robert Dudley
E-Scoot Tech

Johnny J
Johnny J's picture
Last seen: 1 year 2 months ago
Joined: Tuesday, July 8, 2008 - 17:02
Points: 377
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

Just want to meantion that this problem exists on most motordriven 2-wheelers,
the difference is that on a quiet electric scooter you can actually hear it...

Last seen: 12 years 11 months ago
Joined: Wednesday, June 4, 2008 - 16:13
Points: 125
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

Just want to meantion that this problem exists on most motordriven 2-wheelers,
the difference is that on a quiet electric scooter you can actually hear it...

Exactly! My '97 VW has disks in all four corners. I get rusty 'ovals' on the rear rotor when I leave the parking/emergency brake on overnight. I get rusty spots on the all four when I park the car for the weekend and it rains. The pads rub constantly after I install new rotors and pads then quiet after the pads break in.

It's the loud, squealing, grinding, metal-on-metal sound that lets you know you're out of pad that you need to worry about! ;)

Last seen: 12 years 3 weeks ago
Joined: Friday, December 19, 2008 - 23:25
Points: 441
Re: Front Brake Scraping Noise - How to Correct? XM-3500Li

The idea is (of course) to replace the pads before the pad is gone (or it breaks up because it is so thin).

To help the user detect the "time-to-change", most (many, some, ??) pads incorporate a metal "feeler" arm that will begin scraping against the brake disk when the pad is getting close to "worn-too-thin". This scraping is too high pitched for some (like me, with severe high frequency hearing loss) to hear, but those with normal hearing will hear the "scraping-screach" when the brakes are applied.

If the cylinder walls are smooth, the piston seals in good (flexible) shape, the new pads not too thick, and the pistons are compressed back far enough to insert the pads easily (about 1/16 clearance), the pistons should self-adjust and the brake pads, even brand new, should NOT drag.

If the pads are "forced" in without the clearance, the pistons will not be able to "travel" (move) enough when they first apply pressure. Without moving enough, the piston gasket does not deform enough to pull the piston (and pad) back (off the disk) when the brake pressure is reduced.

If the mechanic tells you that it is "normal" for the pad to drag (and thus overheat), I believe that is just a description of (or an excuse for) an improperly done pad-replacement job. I think that the mechanic who tells you that it is NOT normal is probably the one you want working on your brakes.

Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.

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