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Hi, I've a GPZ900R that has been fairly well sorted, and have done several Euro tours on it recently so it's well used and looked after.
However, the front brakes are giving me cause for concern. The brakes are the originals but fully refurbed 10k miles back, braided hoses (single line to each side).
99% of the time they're working perfectly. But occasionally (1%), the brakes feel wooden, as though the pads are not really gripping the discs. Not enough to not stop, but enough to either release and re-squeeze the lever and then seem fine, or to just squeeze harder than normally.
For 99% they're great. I'm using aftermarket levers (good quality ones). Fluid is fresh. The calipers and master cylinder overhauled 10k back. Pads only 20% worn. All v clean.
Slightly wonder if I'm imagining it! But if not, any thoughts?
Sounds to me like they (the pads) may be getting knocked back in the caliper. I've had this due to rough road surfaces...
occasionally (1%), the brakes feel wooden, as though the pads are not really gripping the discs
That's probably friction variation. The brake pads will have a nominal co-efficient of friction (let's say 0.4) but the pads in your brakes can and will vary. I would expect it to vary by +/- 50% or more. So 0.2 to 0.6 and at 0.2 it will feel 'wooden' and at 0.6 it will feel grabby or powerful.
This is NORMAL for all brake pads. Bikes, Cars, Motorbikes etc... nowt to worry about.
Would that not just result in more lever travel? The feeling is more that the pads have hit the discs but there's either no real engagement or less grip (v wooden, or on-off, not progressive) ... it's momentary but noticeable
Would that not result in more lever travel? The feeling is more that the pads are meeting the discs but just not really engaging, lacking bite, not progressive - wooden feel.
Contaminated pads?
If there was just enough friction to generate some heat, whatever it is might flash off when the pressure is released? Might be worth swapping to some new pads and giving the disks a light going over with brake cleaner and wire wool?
Or badly fitting pads getting wedged in the caliper at an angle? Same could apply to pistons / sliders if they're worn?
JAG - that's interesting, thanks. Out of interest, how and why does the friction variation change?
Contaminated pads ... have given pads and discs a good clean - had assumed if contaminated then it would be a consistent issue?
Will check pad fitting and pistons ... they are 30+ yr old calipers so that might well be a thing!
Cold polishing of the pads. If you are a gentle braker they rarely get to operating temp and you get a polished pad surface that is less grippy.
I used to get this. Lent my bmw to a hard braking pal. It came back with much better brakes
The brakes come back once the polished surface is gone hence what you are feeling.
JAG - that's interesting, thanks. Out of interest, how and why does the friction variation change?
It’s caused by environmental changes. Temperature and humidity are the main influences but it depends on the pad material.
I'd start by rechecking everything. Are they opposed piston calipers or or single piston on sliding pins? When the brakes were refurbed were the calipers completely stripped & rebuilt using new parts, seals, pistons, sliders etc? Everything properly lubricated & red rubber grease in the seal recesses?
Pad knock back is a thing and is caused by the discs running out of true. This results in a wandering bite point & in extreme cases no brake on the first application of the lever.
Pad compound which has already been mentioned plays a big part in the feel & amount of initial bite. Some pads only work properly when hot. There are a number of things to check but from the sound of things you've already covered most of these.
I'd start with the easy stuff. Remove pads, clean & abrade pads & clean the discs thoroughly with scotchbrite & brake cleaner. Bed the brakes in carefully. This only take 5 minutes but deposits a layer of pad material onto the disc. This is important as this is what creates the adherent friction giving good bite and bottomless power to to the brake if everything else is working. No need to go mental with the bedding in & don't bring the bike to a complete standstill with the brakes applied whilst bedding in.
After that if your problem is still there you've got a mechanical issue & you'll have to get your spanners out.
Good luck. D.
Really helpful, thanks all so much.
Davesport - sounds like that is a good first step (refurb was a complete strip down, ultrasonic clean, then I replaced all parts, polished pistons and coincided with new discs etc so I'd hope knock back is unlikely as it's so occasional). Cold polishing is interesting but will clean the pads/discs as suggested and bed in and see how I go.
Have now read a few bits on friction variation, quite interesting!
Cheers all