Brake discs rubbing

[RJUK]

Hi folks,

I installed some slightly thicker discs the other day and now they rub pretty kuch the entire circumference.

Brakes are XT 4 pots.

I presume I just need to attach a bleed funnel to the lever and then push the pistons back into the caliper a bit?

Or won't that work?

 

[#lazy]

Should work but may aswell give the pistons a clean and lube with some min oil while you’re doing it .

 

[RJUK]

Should work but may aswell give the pistons a clean and lube with some min oil while you’re doing it .

The brakes are brand new! I just changed to a 2 piece thicker disc, as the stock ones weren't supposed to be very good.

But thanks for the advice.

 

[wenna]

Bleed the brakes with a thicker bleed block.

 

[VWsurfbum]

Bleed the brakes with a thicker bleed block.

No need for that. its a dumb system, no need to bleed.

 

[VWsurfbum]

The brakes are brand new! I just changed to a 2 piece thicker disc, as the stock ones weren't supposed to be very good.

But thanks for the advice.

I assume the thicker discs are not the hope ones? they might rub on the calipers?
Also i dont know if the pads will go all the way back far enough to allow the clearance if they are.

 

[RJUK]

I assume the thicker discs are not the hope ones? they might rub on the calipers?
Also i dont know if the pads will go all the way back far enough to allow the clearance if they are.

No, they're Swissstop discs. They're not much thicker than OEM. Enough to rub pretty much 360 degrees though. It just needs the pads retracting into the caliper a fraction.

I did a lever bleed when I first got them because they were poorly bled, but now the discs are thicker I think a little fluid needs to be removed to allow the pads to retract a tiny bit more.

 

[irie]

No, they're Swissstop discs. They're not much thicker than OEM. Enough to rub pretty much 360 degrees though. It just needs the pads retracting into the caliper a fraction.

I did a lever bleed when I first got them because they were poorly bled, but now the discs are thicker I think a little fluid needs to be removed to allow the pads to retract a tiny bit more.

Try this.

Remove the wheel and insert something a little thicker than the rotor such as a (clean) steel ruler between the pads. Squeeze brake and release several times. Refit wheel. Pads should now be slightly wider apart, but if not remove a very small amount of fluid (top bleed), and repeat.

Works for me with Swissstop 220mm rotors and Shimano M6120 brakes.

 

[wenna]

No need for that. its a dumb system, no need to bleed.

Bleeding with different thickness bleed blocks has always been my go to way when the brakes don´t have adjustments for bite point.

 

[VWsurfbum]

Bleeding with different thickness bleed blocks has always been my go to way when the brakes don´t have adjustments for bite point.

bite point will be determined by the type of seals around the pistons. how much pull back to resting position. (does that make sense)

 

[RJUK]

Thanks folks. There's a Lifeline tool on CRC that I was considering. It sorta fits either side of the disc and you loosen the caliper bolts, insert the tool between pads and disc, then pull the lever and torque the caliper bolts back down.

In this case though, I suspect I might need to remove a little fluid.

I've zip tied the brake levers down all week though, so help force air up into the lever, so that may even do the trick to some degree. Supposedly XT calipers can keep air in them, which is what causes the notorious wandering bite point. (Not that I've experienced this.)

 

[Bones]

Only guessing here but aren't you supposed to undo the bite point screw on the lever before the bleeding process?
I believe it lets the piston's fully retract and helps to stop the wandering bite point associated with Shimano brakes..

 

[RJUK]

Only guessing here but aren't you supposed to undo the bite point screw on the lever before the bleeding process?
I believe it lets the piston's fully retract and helps to stop the wandering bite point associated with Shimano brakes..

Not sure? My screw didn't want to budge when I tried to adjust it. Need to find a more suitable screwdriver head. It's currently in its standard position as it came from the factory.

 

[RustyIron]

In this case though, I suspect I might need to remove a little fluid.

You're on the right track. The pads originally pushed themselves out to meet the original rotor. You stuck a thicker rotor on, so the clearance was insufficient.

All you need to do is push the pistons all the way back into the caliper. Reinstall the caliper, center it up, and pump the brakes so the pads come out to meet the new rotor. Use a perfectly clean screwdriver to pry the pads apart. Don't be a hamfist. Be gentle. Don't goof up the delicate pads, and don't get them dirty.

It is unlikely that the system is overfilled to the point that you need to remove fluid.

 

[RJUK]

You're on the right track. The pads originally pushed themselves out to meet the original rotor. You stuck a thicker rotor on, so the clearance was insufficient.

All you need to do is push the pistons all the way back into the caliper. Reinstall the caliper, center it up, and pump the brakes so the pads come out to meet the new rotor. Use a perfectly clean screwdriver to pry the pads apart. Don't be a hamfist. Be gentle. Don't goof up the delicate pads, and don't get them dirty.

It is unlikely that the system is overfilled to the point that you need to remove fluid.

OK thanks. Question though. With the new rotor wouldn't the pads retract more after a few tough squeezes if that was the case? Surely if they're free to move in the caliper then they should be able to retract just as much as me pushing them back in?

My thought process being, when I did the lever bleeds, the mineral oil was filled right to the very top, to the point that when you put the bleed screw back in, a bit of oil weeps out as you tighten it down. (Instruction guides often tell you to hold a rag or some shop towel around the bleed screw for this reason.)

So logically, I'm not going to be able to push the pistons back in any further without "making space" in the system... Right?

The same is true in a car system. As the pads wear, the brake fluid level drops. You then add more to ensure a sufficient amount is in the system.

You later replace the pads with new ones (necessitating pushing the pistons back in) and find the fluid either above max in the reservoir, or overflowing from the top cap.

So I'm expecting to have to remove some fluid, unless there's already some air in the system that has made it's way to the top and can compress a little.

Regarding pushing the pads back in, I have a pad pusher/rotor straightener tool, so I'll probably use that.

 

[RustyIron]

With the new rotor wouldn't the pads retract more after a few tough squeezes if that was the case? Surely if they're free to move in the caliper then they should be able to retract just as much as me pushing them back in?

No. Envision the cross-section of the square o-ring that seals the piston. As you squeeze the lever and put pressure behind the piston, the piston moves outward and the square seal deforms into a parallelogram. When you release pressure, the parallelogram returns to its original square shape, pulling the piston back with it. The amount that the seal will pull the piston back will never exceed the amount of the original deformation. In other words, the piston can't be pulled back more than it is pushed out.

So logically, I'm not going to be able to push the pistons back in any further without "making space" in the system... Right?

It's not a "fixed volume" system. Consider the cap of the master cylinder reservoir on your car. It has a diaphragm and a vent to account for variation. If your brakes were initially bled with the bleed block, you'll be able to push the pistons all the way back into the caliper. However, sometimes people (myself included), will not use the bleed block and will end up with extra fluid in the system. In that case, the pistons won't be able to be pushed all the way back into the calipers. Since your brakes are new, this shouldn't be the case. But it's good that you're aware of a potential problem.

It sounds like you understand the system, so if you feel like bleeding out a few drops of fluid, it should be fine.

 

[RJUK]

No. Envision the cross-section of the square o-ring that seals the piston. As you squeeze the lever and put pressure behind the piston, the piston moves outward and the square seal deforms into a parallelogram. When you release pressure, the parallelogram returns to its original square shape, pulling the piston back with it. The amount that the seal will pull the piston back will never exceed the amount of the original deformation. In other words, the piston can't be pulled back more than it is pushed out.



It's not a "fixed volume" system. Consider the cap of the master cylinder reservoir on your car. It has a diaphragm and a vent to account for variation. If your brakes were initially bled with the bleed block, you'll be able to push the pistons all the way back into the caliper. However, sometimes people (myself included), will not use the bleed block and will end up with extra fluid in the system. In that case, the pistons won't be able to be pushed all the way back into the calipers. Since your brakes are new, this shouldn't be the case. But it's good that you're aware of a potential problem.

It sounds like you understand the system, so if you feel like bleeding out a few drops of fluid, it should be fine.

Thanks for explaining that. It puts to bed some things I had wondered about the system. TBH, I thought it was just the pad springs that pushed the pads away from the rotor, which always seemed odd, as they're really flimsy and don't seem like they could exert much outward pressure.

Out of interest though, what do you mean by it not being a fixed volume system? In a car reservoir there's a void above the brake fluid, you don't fill it right to the cap, but in bikes you do.

When I did the lever bleed after getting the bike I didn't use a pad spacer and just used the silly red shims they put in when they ship the bike, or maybe the wheels were on, so the disc would have been in there... Either way, perhaps that's why I'm expecting there to be no "space" in the system.

 

[RJUK]

All sorted. The pistons on the rear took some force to budge back in enough, but I didn't really need to open the bleed port I don't think.

There was a tiny rub both front and rear after, but nothing that the disc straightening tool couldn't fix.

Cheers chaps.

 

[e-mtbrider]

This is a handy tool to help set brakes.

Crest Tool - Bicycle Brake Adjustment Tools

The only universal tool for adjusting the contact point of bicycle hydraulic disc brakes.

crest-tool.com

 

[cresttool]

This is a handy tool to help set brakes.

Crest Tool - Bicycle Brake Adjustment Tools

The only universal tool for adjusting the contact point of bicycle hydraulic disc brakes.

crest-tool.com

Hi! I'm the owner of Crest Tool. Just wanted to say thanks for the recommendation! I read through this thread because I saw it drove some traffic to my website.

I've had similar problems to @RJUK with dragging after swapping pads or rotors. One thing that I like to do before trying to track down any problems is to just ride with them dragging a bit. I've found that getting some heat into the system can help work things into place. My sram codes were bad about this. They'd rub in the work stand but after a long descent they'd be fine.

I agree with @RustyIron that as long as the bleed block pushes the pistons all the way into the calipers, then you should be good. If the pistons don't go all the way in then your system may be too full.

If your brakes pull too far after resetting the pistons, that's when the tool I designed comes into play. I use it on my 4 piston XT brakes to work the pads in a bit closer to the rotor. I find shimano's bite point adjustment useless. I like my brakes to engage pretty early on in the lever stroke to keep the lever from hitting my knuckles. I've got a video on the website showing how to advance the brake pads closer to the rotor using my tool.

Hope this helps!

 

[RJUK]

Hi! I'm the owner of Crest Tool. Just wanted to say thanks for the recommendation! I read through this thread because I saw it drove some traffic to my website.

I've had similar problems to @RJUK with dragging after swapping pads or rotors. One thing that I like to do before trying to track down any problems is to just ride with them dragging a bit. I've found that getting some heat into the system can help work things into place. My sram codes were bad about this. They'd rub in the work stand but after a long descent they'd be fine.

I agree with @RustyIron that as long as the bleed block pushes the pistons all the way into the calipers, then you should be good. If the pistons don't go all the way in then your system may be too full.

If you're brakes pull too far after resetting the pistons, that's when the tool I designed comes into play. I use it on my 4 piston XT brakes to work the pads in a bit closer to the rotor. I find shimano's bite point adjustment useless. I like my brakes to engage pretty early on in the lever stroke to keep the lever from hitting my knuckles. I've got a video on the website showing how to advance the brake pads closer to the rotor using my tool.

Hope this helps!

I could probably do with this then. I'd like mine set a bit closer TBH. Though the brakes did work well on my first ride.

I suspect I need to do some repeated hard stops though, to bed them in.

 

[cresttool]

I could probably do with this then. I'd like mine set a bit closer TBH. Though the brakes did work well on my first ride.

I suspect I need to do some repeated hard stops though, to bed them in.

Bedding in always helps.

Saw you're in the UK. If you're interested in buying one send me a dm on instagram (@cresttool) and I can do a discount code. Shipping is pricey overseas.