Pulsing disc brakes

[Crackle]

Now I've had this on and off. Each time I've changed pads it's gone away. So before I check everything out i.e. rotors are true and tight and other things I haven't yet thought of, I did some thinking. The fact that it goes away when I change pads is, I think, indicative of me not bedding the pads in properly, getting uneven transfer on the rotors which causes the pulsing. It only happens at high speed braking. Front and back but obviously you can feel it more in the front.

Anyone had this, any thoughts otherwise, etc?

 

[Cubist]

Checked all the obvious stuff like loose rotor bolts, sloppy headset, worn fork bushes (if a suspension unit) ?

Do the calipers line up properly.....can you see one part of the pad engage before another in a workstand, or even one side before than the other and the caliper flexing in its mounting to bite hard?

There could be uneven rotor wear, which you'll only discover with a micrometer, or slight warping, and even then it only needs to be a tiny bit out in a hotspot.

Does it affect braking performance, or just feel/confidence?

 

[Yellow Saddle]

Brake pulsation is usually the result of uneven brake material transfer from pad to disc and specifically, from "hot holding" your brakes at the end of a long hard run and continuous braking. On bicycles it is never due to disc warping because the disc is too flexible to have an effect on intermittent rotation speed.
Brake material and disc material continuously transfers between the two surfaces and each gets a microscopic coating of the other. That's how abrasive friction works. If however, you have very hot brakes and hold the brake lever after you've stopped, a section of the disc directly under the pad gets a larger coating than the rest of the disc. That's the section that becomes grabby and pulses.
The cure is aggressive sanding of the disc, new pads and a proper bed-in procedure. I've described that procedure here on a previous occasion.
Generally, new pads alone isn't the cure and if you are sure that's what happened previously, I can only speculate that the previous pads were resin and the replacement ones metal. That may have a slight effect on the pulsation, but certainly won't cure it or cure the disc's propensity to again pick up material onto the contaminated section.

Edit: I thought it prudent to define brake pulse. It isn't lever feedback felt as a pulse at the levers but the feeling of intermittent brake grab and slack at the frequency of wheel rotation. The bike feels like it is slowing down unevenly.

 

[Crackle]

Checked all the obvious stuff like loose rotor bolts, sloppy headset, worn fork bushes (if a suspension unit) ?

Do the calipers line up properly.....can you see one part of the pad engage before another in a workstand, or even one side before than the other and the caliper flexing in its mounting to bite hard?

There could be uneven rotor wear, which you'll only discover with a micrometer, or slight warping, and even then it only needs to be a tiny bit out in a hotspot.

Does it affect braking performance, or just feel/confidence?

It's having no effect other than the feedback is slightly annoying. I have recently checked the headset and forks but always worth checking again. I'll look at the bite as well. The original rotors were cheap ebay jobs so it's possible they are slightly suss but I tend to think not as the pulsing develops rather than being there from the off.

Brake pulsation is usually the result of uneven brake material transfer from pad to disc and specifically, from "hot holding" your brakes at the end of a long hard run and continuous braking. On bicycles it is never due to disc warping because the disc is too flexible to have an effect on intermittent rotation speed.
Brake material and disc material continuously transfers between the two surfaces and each gets a microscopic coating of the other. That's how abrasive friction works. If however, you have very hot brakes and hold the brake lever after you've stopped, a section of the disc directly under the pad gets a larger coating than the rest of the disc. That's the section that becomes grabby and pulses.
The cure is aggressive sanding of the disc, new pads and a proper bed-in procedure. I've described that procedure here on a previous occasion.
Generally, new pads alone isn't the cure and if you are sure that's what happened previously, I can only speculate that the previous pads were resin and the replacement ones metal. That may have a slight effect on the pulsation, but certainly won't cure it or cure the disc's propensity to again pick up material onto the contaminated section.

Indeed you have written about it before and it's one of the reasons I was thinking that way. It was Resin, sintered, resin. These resins are quite new so I was hoping that if I sanded the disc and possibly the pads and did a bed-in again I might get away with things. it's possible I've hot held but I don't know how hot it would need to be, it's largely been winter riding and short downhills. My other thought was to change the rotors from these cheap ones in the hope a better rotor might not be as susceptible to this.

Hydraulic brakes have certainly been a learning curve for me. From shortening cables, to contaminating pads, to trying different pad materials and now to this. I don't think there's anything left to go wrong or do wrong.

 

[Yellow Saddle]

It's having no effect other than the feedback is slightly annoying. I have recently checked the headset and forks but always worth checking again. I'll look at the bite as well. The original rotors were cheap ebay jobs so it's possible they are slightly suss but I tend to think not as the pulsing develops rather than being there from the off.



Indeed you have written about it before and it's one of the reasons I was thinking that way. It was Resin, sintered, resin. These resins are quite new so I was hoping that if I sanded the disc and possibly the pads and did a bed-in again I might get away with things. it's possible I've hot held but I don't know how hot it would need to be, it's largely been winter riding and short downhills. My other thought was to change the rotors from these cheap ones in the hope a better rotor might not be as susceptible to this.

Hydraulic brakes have certainly been a learning curve for me. From shortening cables, to contaminating pads, to trying different pad materials and now to this. I don't think there's anything left to go wrong or do wrong.

Resin, sintered, resin - looking for trouble unless you completely decontaminate the discs between swaps. It is hard work. The discs have to come off and sanded all over until the surface resembles fine satin. Then, bed the new pads in properly. It is a slowish, deliberate process. Also, don't use sintered pads on discs that are marked "Resin Only" .

Even just hot-holding after a gentle hill on unbedded pads will do the trick. New resin pads are not yet fully cured and outgases during bedding in. By hot-holding them you are definitely depositing material on the disc. Temperature at disc level is subjective and not really dependent on ambient temperatures. Humans are sensitive to variations that are very small compared to the variations in disc brakes. They go from ambient to 250 degrees C in just tens of meters of braking. The discs have a low heat mass and heat up quickly.

Disc brakes have many problems not foreseen by its advocates nor understood by the industry.

 

[Tom B]

Disc brakes have many problems not foreseen by its advocates nor understood by the industry.

"Hot Holding" has been causing warped disks on cars for decades. I agree a different issue but same root cause and similar feel. I've always avoided HH for the fear of warping bike disks and rock / crawling after a big stop to alow the rotors to cool evenly. My thinking was the thin disk would be more prone to warp. But I take your point of hem being smaller more flexible and having a lower mass they will be less prone.

Out of curiosity what braking system do you ride ys? You always seem a little skeptical / unliking of disk systems.

 

[Cubist]

Easy to overthink it though. For the average recreational rider ( if such a rider exists) current popular models of brakes function perfectly well. I can understand that a competitive downhiller or enduro rider is going to benefit from the trickle down technology of floating rotors and finned pads, all designed to dissipate heat, and these are available as standard on many bikes.

For riders like me, the weekend warrior mincing down the reds and blacks, or bimbling along bridleways and local trails, they are a dream. You fit them, you use them, they stop you and your bike even when you're pushing your own personal limits. When they're dirty they are easy to clean, and when the pads wear, you can swap the pads for a fiver an end. I use Kevlar pads since you ask.

I have shimano XT on both bikes because they offer incredible value for money, are nicely adjustable for my hands and setup. They are a sealed system and don't need bleeding, and despite originally being fitted with finned pads, my adventures have never caused heating issues.

 

[Crackle]

Easy to overthink it though. For the average recreational rider ( if such a rider exists) current popular models of brakes function perfectly well. I can understand that a competitive downhiller or enduro rider is going to benefit from the trickle down technology of floating rotors and finned pads, all designed to dissipate heat, and these are available as standard on many bikes.

For riders like me, the weekend warrior mincing down the reds and blacks, or bimbling along bridleways and local trails, they are a dream. You fit them, you use them, they stop you and your bike even when you're pushing your own personal limits. When they're dirty they are easy to clean, and when the pads wear, you can swap the pads for a fiver an end. I use Kevlar pads since you ask.

I have shimano XT on both bikes because they offer incredible value for money, are nicely adjustable for my hands and setup. They are a sealed system and don't need bleeding, and despite originally being fitted with finned pads, my adventures have never caused heating issues.

I largely agree. Three bikes in the fleet with discs and only mine has been problematical and most of that was of my own making. Whilst I appreciate that bedding in may well be wise, it's never been done on the other two bikes nor by any friends I know. I doubt very much it was done on the hire fleets of bikes I hired before I built mine. So I'm left wondering what I've done to mtn bikes in the past to deserve this. Of course it could be my old Marin getting revenge after being relegated to touring, after all they are next to each other. My money is on the cheap rotors and if everything else looks good, I may well buy some new rotors with the pads, cross my fingers, say a prayer, bed them in and hope to never think about disc brake problems again.

I also have to cast my mind back to when I did use the Marin and found out that I'd worn down a new set of V-pads in a few hours on a particularly gritty trail. I even had to adjust them mid ride once. Not too mention grabbing handfuls of brake and hoping to stop somewhere before the big tree I was heading for.

 

[Cubist]

Check out Superstar. I use their rotors without problems. They've probably got a special offer running, they usually do.

 

[Crackle]

Check out Superstar. I use their rotors without problems. They've probably got a special offer running, they usually do.

Everything in 160 out of stock. I noticed recently that they are out of stock a lot. It's not the first time I've been back to check their rotors as they had some very nice floating ones some time ago but they too have never been back in stock.

 

[Cubist]

Shame. I quite like theirs. What's the eBay equivalent, über or something like that?

Edit. They were replacement pads. There are unbranded rotors on the bay for disposable money. Clarks floating carrier rotors are certainly bargain money, but their kit is viewed with suspicion re quality

 

[Yellow Saddle]

"Hot Holding" has been causing warped disks on cars for decades. I agree a different issue but same root cause and similar feel. I've always avoided HH for the fear of warping bike disks and rock / crawling after a big stop to alow the rotors to cool evenly. My thinking was the thin disk would be more prone to warp. But I take your point of hem being smaller more flexible and having a lower mass they will be less prone.

Out of curiosity what braking system do you ride ys? You always seem a little skeptical / unliking of disk systems.

Contrary to popular belief, hot holding doesn't cause warped discs. It causes a grabby spot on the disc from the deposits. As I said, a warped disc cannot be felt in your body or through the lever. The disc simply follows the line dictated by the pads. Even if an infinitely stiff warped disc was to cause pad movement, the movement between the left pad and right pad would cancel out any fluid pulsing up the line. Only a disc with a bulge/dent causes pedal or lever pulses. These bulges are not from hot holding but by imperfections in the disc from poor sprew management at the casting stage that are revealed as the disc wears down.

Although I currently don't have a disc-braked bike, it isn't because of a dislike, just current circumstances. My next bike will be disc braked and I'm buying it this summer. I'm certainly not skeptical of (hydraulic) disc brakes but I do think they have some problems which are misunderstood. I have however nothing good to say for mechanical disc brakes.

 

[Crackle]

Took myself to the shed over lunch and checked a few things. Headset and stanchons, tight and good. Pad bite looks even to my naked eye, rotors seem straight, no obvious imperfections that you can see or feel and look straight, so all good. I took a couple of pics for posterity but honestly I don't know what I might be looking for as the rotors look the same on the other two bikes, though the Cube has finer wear lines and a shinier not so dirty surface.

 

[Yellow Saddle]

Took myself to the shed over lunch and checked a few things. Headset and stanchons, tight and good. Pad bite looks even to my naked eye, rotors seem straight, no obvious imperfections that you can see or feel and look straight, so all good. I took a couple of pics for posterity but honestly I don't know what I might be looking for as the rotors look the same on the other two bikes, though the Cube has finer wear lines and a shinier not so dirty surface.

View attachment 338807 View attachment 338808

Plenty of unwanted pad deposits on those discs. Disc alignment, straightness and pad contact is all irrelevant wrt your problem.

 

[Crackle]

Plenty of unwanted pad deposits on those discs

But why? I took another look at the Cube discs and there are nowhere near the same level of deposits (those black lines I assume). The rotors are a finer more polished item than mine, so again, I'm aiming this problem at cheaply finished rotors?