[Closed] Is Pad Clearance a delusion?

Pads clearance is created by the seals pulling the pistons back. The seals deform as the brake is applied and as they relax the pads are pulled back from the disc.

Why not bed your pads in? I get thousands of miles oust of a set of pads 🙂 and it does seem to be one major factor affecting pad wear

our p[ads will not be wearing out during cruise - there is not enough friction to do this. They are wearing out because theya re not cured

IMO of course

Or use BB7s, then you can wind each pad out a notch for more clearance.

I love BB7s, I don't think I'd ever use hydros again.

Hi Jeremy, Yes I know about your thousands of miles..using Deore LX by any chance?

You suggest it's only the elasticity of the piston seals pulling the pads back. - Really? But on LX nothing much (not even the piston seals) obviously "pulls" them back, they need to be pushed back in.

TJ is right it is the seals that pull them back in.

As said, you need to bed them in...just a few hard stops down a hill needed to get some heat into the pads. If the seals have dried out the pistons may not retract properly, but they would be rubbing quite hard. If so, remove pads, pump out the pistons a bit until you can see the sides of them, and add some drops of brake fluid around the piston. Push back in and repeat a few times.

The seals will only pull the pistons back a fraction to clear the disc, if you pump the lever with no wheel they will move out quite a bit then retract a fraction, this is how they adjust for pad wear. Most brakes use the same system, including Hope.

What vintage of LX calipers are they? Mine have return springs, same as XT, (and Saint I think)

[url= http://www.chainreactioncycles.com/Models.aspx?ModelID=1907 ]Like this[/url]

[i]"They are wearing out because theya re not cured".. That old chestnut, sounds like rot to me. .[/i] I don't bed my summer pads in either, and they can last about 70 times as long on the same route.

Thats how it works - its the shape of the seals relaxing that pulls the pads back. Its a clearance in fractions of a mm

[url= http://www.autoshop101.com/forms/brake04.pdf ]a diagram halfway down the page explains this better than I can[/url]

Hopes for mebtw - they seem to have better pad life.

i was talking with friends about this just yesterday. There seems to be some factor that no one knows involved. I cannot find any real rhyme or reason behind this. Every theory I can come up with someone doesn't fit in with. I have collect a lot of data on this as it is a puzszle that is going to annoy me until I solve it 🙂 OCD strikes again!

I know that the pads in my hopes are far too hard to wear just from riding along with a bit of grit in them. I am convinced that getting the pads really really hot "cures" them and helps prevent the premature pad wear. I can believe conditions and grit type plays a role - bnut I cannot believe that it makes pads wear out hundreds of times quicker - there must be more than that. Poor self cleaning from the drilling pattern, riding style, bedding in, pad type

if I was wearing pads out in so few miles i would be returning them as " not fit for purpose"

The springs are purely to prevent rattle - they do not push the pads back

m

ountaincarrot - Member

"They are wearing out because theya re not cured".. That old chestnut, sounds like rot to me. . I don't bed my summer pads in either, and they can last about 70 times as long on the same route.

But in summer they get much hotter - you ride faster, brake harder and the air is hotter - and less water around to cool them

Do you run large discs?

TJ, if it's just heat that cures them, why don't they run them through a kiln as part of the production process? If it's works as well as you're convinced it does, wouldn't mfrs be selling pads as "oven baked for 20x durability" or something similar?

Ned - bedding in is 3 things - curing the pad under heat [i]and pressure[/i], coating the discs with a thin layer of pad material and conforming the pad to the disc.

All brake friction materials are like this - if they were prebaked then it would be much harder to get the conformity to the disc surface as the pad would be too hard to conform.

MTB disc brakes = shit design 😉

Bedding in works, [b]or does it[/b]? 😯

We do this thread every year, I love it!

Really like nedrapier's point though.

"oven baked for 20x durability"

yep agree with tj.
i have even noticed changes in wear rate if i just switch to another pad manufacturer.then that pad mates with the disc and then you start to see the performance.

Yes, tthew, those are not springs, they are too feeble to push in the pots, and yes, same as mine.

Jeremy, no I run 160 discs (I'm 67 Kg). I still don't really
buy this curing. If it was so important why not sell the damn things pre-cured?

In fact to prove a point, I'll go and put a pair on top of my woodburner, - yes I think I will.

As far as prssure and wearing out, it takes very little pressure to cut down disk pads, they are very easy to take down with a metal file for instance. I believe it's no coincidence that the scrape-scrape which accompanies my gritty wet rides coincides with such appalling pad wear. It's pissed down nearly every day over the last week, and it's very sandy soil here, the stuff jambs up the calipers instantly.

I'm baking some now on the woodburner! - If it works I'll go into production (I'm sure Fruit will do them at trade price) 😆

One of the reasons I am so convinced that its getting good heat into the pads that gives long life is the tandem a a lot of mass to stop and its ridden relatively slowly so brakes dragged a lot and both discs blued from the heat. Finished a descent the other day with smoke / steam pouring off the caliper and the disc well blued. Must have got to hundreds of degrees C to fdo that. I have never worn a set of pads out in 5 years of riding it - I have replaced part worn sets a few times as I was going on a tour or something similar. I estimate pad life at 2000+ miles - and we ride summer and winter and don't avoid mud

If the heat had no effect then surely we sould be wearing pads out quicker on the tandem than the solo - but the solo wears them quicker.

you have missed the point. you do it on the bike so it mkes buddies with the rotor.

Mountain carrot - you need the pressure as well.

used pads off the tandem can hardly be marked with a hacksaw - they have cured that hard - even organic ones

bedding in is 3 things - curing the pad under heat and pressure, coating the discs with a thin layer of pad material and conforming the pad to the disc.

All brake friction materials are like this - if they were prebaked then it would be much harder to get the conformity to the disc surface as the pad would be too hard to conform.

Thanks nonk,

The point being, what pulls in the pistons if the seals are (demonstrably) ineffective at doing so.

sorry man i think i am guilty of trying to help.
bye.

Nothing. its only the seals that do this. its only a tiny movement - clearance on mine is a couple of tenths of a mm.

They cannot not do it. Physically impossible. It is not movement between the seal and the piston - it is elastic deformation and rebound in the rubber.

Did you look at that diagram - near the bottom of the page that I posted a link to

All LX hydros that I know off run really really tight clearance, much closer than any other Shimano brake I've ever used, got so hacked off with it myself that I traded down to old style 525 deore, much betterer

TJ- Yes, well [i]that[/i] bit's obvious enough.

Thing is, if there were a return spring of some sort in the master cylinder, that should enable atmospheric prerssure to push the pads back in somewhat. I don't know the arrangement of the master cylinder valve and how that connects to the reservoir.

I've just finished sorting out my rear LX calliper which had exactly the same problem. Slightly sticky piston one side and it chewed right through one side after a couple of wet Swinley Forest rides. Have to borrow some pads from another bike so I can ride tomorrow.
I think the Shimango resin pads are relatively soft, and the pistons do tend to be a little sticky until the seals have aged a bit.
Got 4 sets of cheap sintered pads for £23 on ebay this eve. Problem hopefully solved..

I was using "cheap sinterd pads", and mine are 2 years old, and have been cleaned out countless times..

cheap sinterd pads

Could be part of your problem? Buy cheap buy twice? I use hope original mainly

.. but exactly my problem bikewhisper. At least I'm not alone..

but the same pads (same batch even) last >4 months in the dry, (cool, we aren't talking bluing disks here), we are talking wearing pads out even when the brakes aren't being used.

OK Jeremy. Do your discs go "scrape scrape..." after you ride through a puddle. - And if not why not?

TandemJeremy - Member
Ned - bedding in is 3 things - curing the pad under heat and pressure, coating the discs with a thin layer of pad material and conforming the pad to the disc.

All brake friction materials are like this - if they were prebaked then it would be much harder to get the conformity to the disc surface as the pad would be too hard to conform.

Fair enough, makes sense!

You are not alone - quite a few folk get this premature pad wear and there appears to be no real pattern to it.

With your next set try getting them rally really hot - drag he brake while pedalling down a big descent - just to see if that helps.

My [i]guess[/i] is that is summer they get hot enough to cure and in winter they do not in normal riding

Mountain carrot - yes they do.

Cheap sintered should still be hard enough.... Mind you, I lent that bike to my girlfriend for the first few months after I'd replaced the callipers (one m465 and one 9 year old deore lever, with LX callipers that cost me £9.99 each with pads! bargain upgrade!). I don't think she's heavy enough to properly bed in the pads..
If the pistons keep getting out of shape and sticky I'd get a can of Stendec Superglide. It's a silicon spray that dries on thin. It's good for making fork seals uber slippery too. Pump the pistons out some, get them really clean with disc brake cleaner, and then squirt some stendec on and let it dry. Then work each piston in turn by holding the other one back with a screwdriver and a cloth. Repeat until they don't feel like they're "popping" out and move smoothly and then reset them and wipe off any excess gunk. You may have to do this every few months or so until they settle down.

I'll look for Stendec. I've tried all sorts to lube them. Last week (before they went sticky again) they were spotless with WD40 and Finish Line.

You're putting WD40 on your piston seals????

what he said ^

Might be OK with mineral oil systems? But might well make the seals swell and be causing your issues.

WD will ruin seals in dot fluid systems IIRC

Here we go - WD40 the bane of rubber seals.
It did for my old Honda's caliper seals.
I remember the memo doing the rounds about the effects of WD on rubber.

I used shimano mineral oil today in lieu of superglide. Would be the obvious choice..
I'm sure the volatiles in WD are bound to mess it up in some way or other.

I think the OP has a point... a wet gritty ride can eat a set of pads, bedded in or not. I put in a set of pads just before Easter and rode them for a hot dry week in Spain. There must be some truth in the bedding-in thing, because they were still only half worn six months later. This is a pretty good pad life for weekly riding in the South Pennine grinding paste. They were bedded in, without a doubt.

Then a wet ride round Mam Tor destroyed the rest of the pads and half the backing plate in two hours. And for most of that ride they were rubbing like cuss.

Some things I've noticed:
- In cold wet conditions the pistons do not retract as far as they do in warm dry conditions. I tried to reposition the front caliper mid-ride yesterday to stop it rubbing, but couldn't do it. This only happens in the wet.
- A pad that is rubbing constantly but lightly on the disc wears at an inordinately fast rate. It seems to me that light rubbing wears the pad faster than hard braking. Is it a heat thing? Is bedding-in a continuous process that only hardens the top layer of the pad? Or does the pressure of hard braking somehow hold the pad together and make it more resistant to wear?

OK, good point. But I had taken the same view on the mineral oil being OK then..

Memo to self to try always use the silicon instead.

Not bedding in your pads definitely causes premature wear. This was beautifully illustrated at the 'Puffer 3 years ago when some guys got through several sets of pads. It's a bit difficult to bed them in when the temperature is around zero and it's piddling down. Spare pads were in such short supply that Square Wheels stripped them off the new bikes in the shop. A lot of racing time was lost to pad replacement.

We haven't had a repeat, but that's maybe because people have learned to take spare bedded pads with them.

I run drum brakes, so no problem 🙂

OK, I'll slap in some new seals, and promise to use only silicon.