Front brake surge

[MarMax]

Bought my 2001 about a year ago with 4600 miles on it. My issue is when applying the front brakes they surge. I suspect a warped rotor but was wondering if any one else has experienced this?

 

[02GF74]

Please describe what you mean by "surge"

 

[Fire-medic]

You might check the pads, maybe they cheaped-out on the install of the aftermarket rotors, and simply-re-used mostly-worn pads. I'd check the button-head screws for the rotors for tightness also. I suggest checking all the fasteners in the front end for torque, and look for Sean Morley's 'bounce-test' on the steering head either on here (advanced search) or go to youtube. There is a proper sequence for the front end fasteners, look it up in the factory service manual. VMX12- Service-Manual.pdf (vmoa.net) How-much air in your front end downtubes? What are your tire pressures?

 

[one2dmax]

You may need to clean and lubricate the buttons on the rotors.

 

[MarMax]

Please describe what you mean by "surge"

With even pressure they would feel like they grab and release as coming to a stop.

 

[02GF74]

Thanks in which case check disc runout = side to side movement of the discs.

Obviously a dial gauge would be the best but a quick way is to attach a piece of stiff wire to the fork so the end just barely touches the disc. Front wheel off the ground, turn the wheel. The gap between disc surface and end of wire should be constant.

The manual states maximum allowed run out, I'm gonna GUESS 0.5mm.

(I would probably have described it as pulsing but we got there in the end

 

[85vmax]

I've had similar rotors on and had the same surging/pulsing.
Rotors were warped on mine......visually also, never sat flat.
Cheaply made, after a couple hundred miles I guess the heat got to them.

 

[rfwilli]

I have had this happen a couple of times and cleaning the buttons did the trick for me:

 

[02GF74]

Fyi
Rear: 0.15mm
Front:0.15mm pre93
:0.3mm 93+

I would also grab the disc and see if there is any side to side play: should be none.

 

[MaxMidnight]

Have mentioned this before but will repeat it again.

You need to check thickness variation. If this is present then both the caliper pistons will move in and out together. As the pistons move out they will try to displace fluid back to the master cylinder but this is not possible as the brakes are being applied. You will feel pulsing through the lever or a brake on - off feeling.

With runout as the pistons on one side move out those on the other side move in. Very littl fluid is displaced.

The upshot of the above is that it take significantly more runout to cause a problem than thickness variation does.

 

[02GF74]

Yes worth checking, I didn't mention it as it shouldnt be that difficult to grind the discs to uniform thickness hence it should be less of an issue.

It may be worth an overhaul of the callipers because if piston(s) are sticky on one side, this would cause pulsing when an out of true disc tries to push the piston(s) in.

 

[MarMax]

First, thanks for all the help. I lubricated and spun all the buttons, no big difference. I then removed the caliners and cleaned and lubricated to pots on the calibers. I was trying to avoid dropping g the the wheel, but that next and removing the rotors to check for flatness. Trying to avoid all that so I can continue to ride.

 

[02GF74]

Have you checked run out ? Does not need front wheel to be removed.