Mojoman Posted January 30, 2016 Posted January 30, 2016 Perhaps those Alligator pads are prone to glazing up and not being too effective on big downhill sections? I like to alternate between front and back braking when going down a long downhill so hopefully if one brake fails the other may save me....maybe....!
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 Could be a number of factors Heat buildup (fixed by using better rotors and pads)Contaminated pads and / or rotors (oil, dishwashing liquid, brake fluid etc)Air bubbles (not likely given that both failed) Had you recently washed your bike? Did you rinse it off properly? What did you use to clean it? Were the brakes recently bled, and if so did you remove pads before doing it? Many different things could have influenced it, but I'd go with contamination given that both failed...Yeah, I think its a process of elimination. I do however wash my bike with Prepsol, and then rinse off with water, and dry with a clean cloth, never touch the rotors tho. But then, never had this happen, after 10's of times with this setup. The brakes weren't bled recently, and according to my bike mech, did not need any bleeding either..?
Paul Ruinaard Posted January 30, 2016 Posted January 30, 2016 It did function back down Dorsberg down to Meerendal, although I did go down very slowSo they stop functioning - both of them, but after the accident are okay. When you say you had no braking ability do you mean the lever travelled to the bars without biting i.e. hydraulic failure, or that even though you had hydraulic pressure you had no stopping power. My logic says: No hydraulics= look at levers and bleeding. No power = look at discs and pads. IMHO How big are you and how serious is that descent. You say you feathered them a bit - but you weren't on them like clamps all the way down?
DJR Posted January 30, 2016 Posted January 30, 2016 It did function back down Dorsberg down to Meerendal, although I did go down very slowThen I'm starting to think that you just over cooked the brakes. Here's what The Torqued Wrench has to say about over-heating disc brakes: "It’s possible to glaze the pads, which would decrease brake power but not remove the ability to stop entirely (it’s called friction fade). The real danger is boiling the hydraulic fluid, vaporizing the fluid into a compressible gas. Since hydraulic designs are dependent on incompressible fluid, and gas compresses, vaporizing the hydraulic fluid would result in brake failure. Pull the levers all the way to the bar and nothing will happen.Thankfully, this process does not generally happen in an instant. The brakes will go a bit mushy before they fail completely — mountain bikers who were riding in the early days of disc brakes are likely familiar with the sensation. If this ever happens, stop and allow everything to cool."
Captain Fastbastard Mayhem Posted January 30, 2016 Posted January 30, 2016 You say you feathered them a bit - but you weren't on them like clamps all the way down?This. If you were constantly pulling, but varying the pressure, then it's heat buildup and resultant failure. Fix - brake less, and get better rotors! Bigger ones (180mm) and something with a bit of metal!
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 So they stop functioning - both of them, but after the accident are okay. When you say you had no braking ability do you mean the lever travelled to the bars without biting i.e. hydraulic failure, or that even though you had hydraulic pressure you had no stopping power. My logic says: No hydraulics= look at levers and bleeding. No power = look at discs and pads. IMHO How big are you and how serious is that descent. You say you feathered them a bit - but you weren't on them like clamps all the way down?There were hydraulic pressure, but zero braking power, the descent was long and quite steep, with riders close in front, looking quite skittish, hence the feathering, and at one point there was nothing, and the bike just picked up speed. I'm 78kg, and 9.2kg bike
Paul Ruinaard Posted January 30, 2016 Posted January 30, 2016 This. If you were constantly pulling, but varying the pressure, then it's heat buildup and resultant failure. Fix - brake less, and get better rotors! Bigger ones (180mm) and something with a bit of metal!From what I am lead to believe the mineral oil in Shimano is not prone to boiling like hydraulic fluids do from heat transfer from the pads in to the slave cylinders. But I agree with Myles here not knowing your weight and the type of descent and the amount of time before the incident that you were on the brakes. Heat and boiling leads to sponginess and lack of bite - but it doesn't present like a switch i.e. brakes then no brakes.So you may have had a couple of factors like the other posts say - glazed pads etc and you overcame the envelope of operation of those discs. Some ideas: 1.) 180mm discs - principle of levers and dissipation of heat over a larger surface area apply. Not sure if you are a weight weenie but/.....2.) Maybe look at Shimano Icetech rotors. I like them because of this and their cooling3.) New pads - definitely - possibly check what you had and see what is recommended. Resin bites nicely but fade and dont take them near mud. Sintered metal is what I rely on I dont think those KCNC rotors have much surface area. Just saying. Also how big are you and how hard are you on the brakes. The difference between and 80 kg guy and a 100 kg guy in terms of the amount of energy (cant remember it ifs kinetic - i think it is) is huge.
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 Perhaps those Alligator pads are prone to glazing up and not being too effective on big downhill sections? I like to alternate between front and back braking when going down a long downhill so hopefully if one brake fails the other may save me....maybe....!I think you may be right,..I've not used Alligator Metal pads before, and certainly not down something this steep
Captain Fastbastard Mayhem Posted January 30, 2016 Posted January 30, 2016 There were hydraulic pressure, but zero braking power, the descent was long and quite steep, with riders close in front, looking quite skittish, hence the feathering, and at one point there was nothing, and the bike just picked up speed. I'm 78kg, and 9.2kg bikeby feathering, you mean constant pressure but at different levels? Assuming htat this is the case, it'd be gradual buildup of heat, resulting in fade and ultimately failure.
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 by feathering, you mean constant pressure but at different levels? Assuming htat this is the case, it'd be gradual buildup of heat, resulting in fade and ultimately failure.Correct, and could've possibly been avoided with, at least a 180mm front.
Rapunzel Posted January 30, 2016 Posted January 30, 2016 Greetings Hubbers, So this morning I've lined up for the Ashburton Meerendal Marathon, and 4km's into the event, I experienced something I have never experienced, never crossed my mind happening, and had no reference to the terrifying, helpless feeling it would be, should it happen... It was fairly hot already, and as we've summited the first climb, a long steep fairly rocky jeep track descend, with thick fynbos (thank God), on both side, followed. About 200m down, feathering here and there, my brakes suddenly had no braking ability at all, I started speeding down this jeep track, with nothing else to stop me, but sticking my leg out in the fynbos at 37km/h,..and take the "calculated" OTB,..luckily into fynbos.... My brake set-up is 2016 XT8000, two KCNC Razors 160mm rotors, and Alligator metal pads. I've used the KCNC Razors/Metal pad set-up for quite a while, and never had issues before... So I'm confused as to what could've caused this, and should I rather change my set-up, to avoid this happening again.. I came off light, considering the possibilities, with only a gash in my left hand, needing 3 stitches. ThxBrandon WP_20160130_07_39_00_Pro.jpgGlad you're ok - wasn't far behind you and it looked pretty spectacular when you went over the bars. If that was overheating then it's very worrying as you were only about half way down the steep part. Dust made visibility really bad so had to sit hard on the brakes down there.
PhilipV Posted January 30, 2016 Posted January 30, 2016 Ditch the rotors and the pads. Normal Shimano pads and rotors are slightly heavier, but the weight penalty is negligible compared to the performance drop you experienced. Bonus points if you use the Shimano icetec rotors and pads. And get a bigger rotor up front. 180mm. I'm sorry, but your weight weenieism cost you the race, and a couple of stitches. On some things you just don't skimp, meat on your rotors is one of those places.
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 Glad you're ok - wasn't far behind you and it looked pretty spectacular when you went over the bars. If that was overheating then it's very worrying as you were only about half way down the steep part. Dust made visibility really bad so had to sit hard on the brakes down there.Lol! Thx Rapunzel, glad I could put on a show for you ???? The other guy that walked back up, also had brake failure, but he lost complete pressure
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 Ditch the rotors and the pads. Normal Shimano pads and rotors are slightly heavier, but the weight penalty is negligible compared to the performance drop you experienced. Bonus points if you use the Shimano icetec rotors and pads. And get a bigger rotor up front. 180mm. I'm sorry, but your weight weenieism cost you the race, and a couple of stitches. On some things you just don't skimp, meat on your rotors is one of those places.Thx Philip,..yeah, lesson learned, school fees paid ????
V12man Posted January 30, 2016 Posted January 30, 2016 How old is the fluid in those brakes - sounds like it is water contaminated - water boils when hot, then brakes dissapear.. when they cool down, they mostly work again. Replace the fluid completely is a first step - should be done annually anyway.
BrandonF_ Posted January 30, 2016 Author Posted January 30, 2016 How old is the fluid in those brakes - sounds like it is water contaminated - water boils when hot, then brakes dissapear.. when they cool down, they mostly work again. Replace the fluid completely is a first step - should be done annually anyway.Thx V12man, these brakes cant be older than 8months (assumption as not 1st owner), seeing they are the latest XT M8000 Version. But I think your point is valid, and I should just get them bled again,..the hoses were shortened by previous owner,..maybe something went wrong then.
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