[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.

ive never consciously bedded in pads and regularly get very long lifetimes out of my hopes, mini and mono m4, with standard pads
ive never had the phenomenon of wearing a set out in a few hours of riding ???

Interesting stuff lentilshepherd

lentilshepherd - Member

.............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.

another theory blown 🙁

.......... 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?

Could be the heat thing. I think there is some continuous effects with teh heat effects on the pad

You get boundary layer effects and two types of friction adherent and abrasive and I guess the rubbing could destroy the boundary layer so you only get abrasive friction not adherent.

Ta - more data but another theory ruined. By my theories the pads from Spain should not have worn so quickly.

Were you using the brakes hard at all during the mam tor ride?

*scratches head and wanders off muttering*

Kimbers - hopes do seem to be far less affected by this - anyone had premature wear with hopes?

I've had hope sintered pads last ****in years. They wail like crazy in the wet so I went to organic ones and they are lasting pretty well. Will see what winter does to those. Hopes do have bigger clearance than shimangos though.

TJ, to further confound your theories how do you think the "holiness" of the rotor affects pad life? And I'm not talking about having them blessed..
More holes should clean out the gritty paste, but then could hold it on the edges of the holes to get mixed back in. Fewer holes would dry slower, but would offer a cleaner braking surface when dry. On the other hand more holes would equal less braking surface and higher pressure on the pad surface effectively shaving the top off the pad.
****.. I'm getting OCD about this now..

bikewhisperer

Its got you as well? I find puzzles like this fascinating - gawd knows how much time I have spent looking for data on it and checking engineering journals adn the like

All I would safely say about drillings is that they should help clean the surface - the first drilled discs on motorcycles was to remove the water film from the surface to reduce poor performance in the wet.

I am certain drilling pattern is a variable that affects pad life - but what is best - no data!

and there's material.. Softer steel makes for a better brake afaik. That's part of the reason why Hayes brakes actually work (that's work in the "just out of the box and not seen a winter yet" kind of way..) Their rotors are the fastest wearing I've ever come across.
I think half the time there's too many holes in rotors just to save weight. Look at any ashima rotors to see this. When the hole is half the width of the pad it's bound to offer uneven loading and excessive scraping

The seals pull the pistons back in!.....,WTF no they don't! A completely sealed hydro brake will "pull" the pistons back in mainly due to the spring in the brake lever sucking the fluid back when the lever is released.

Mountaincarrot:

Life is a delusion... god is a delusion... even atheism itself is a delusion.. we're all deluded. It's not just pad clearances. 😛

What about the newish Shimano ones with the big pad clearance? Good or not?

I'm 100% agreed with TJ here, about the bedding in. It does work.

However, to the OP, I know where you're coming from. I'm in Hampshire and when I had Hope brakes the pads lasted forever. But they have sintered pads....
I'm on Maguras now which can only be used with organic pads. I do bed them in, but they last 6-8 months. On CGs recent wet ride in Swinley there were LOTS of people getting through pads, myself included (Although mine were pretty worn and I was expecting them to go, so took spares) There certainly is something in the grit in Swinley that eats pads, for sure.

Shimano pads are generally organic ones. My tip would be to get some (Wait for it...) Superstar sintered pads. IMO when I had my XTs on a VERY VERY wet TransWales that they are better then the Shimano pads in every way for about 1/4 of the price.
Anyway, whatever you get, just get some sintered pads. 🙂

lookmanohands - Member

The seals pull the pistons back in!.....,WTF no they don't! A completely sealed hydro brake will "pull" the pistons back in mainly due to the spring in the brake lever sucking the fluid back when the lever is released.

Rubbish - it is the deformation and rebound in the seals. go read up on it, Have a look at the link I posted on this thread

Regarding the original issue of pad clearance, current XT and SLX have a two-stage actuation (that Shimano call 'servo-wave') that moves the pad quickly at the start of the lever stroke and slowly at the end. This supposedly gives greater pad clearance.

re the bedding in argument, I repeatedly bedded in some new disco sintered pads a couple of weeks ago.
Two more rides since and already the backing has started to come off one of them.
and they were bedded in comprehensively.

Sometimes I really think it is just the luck of the draw.

TJ, re your diagram showing the sliding square cut seal,
[IMG] [/IMG]

Is this not the case? - if not then why?

Mountaincarrot. the seal deforms first - then if more movement is possible the piston slides thru the seal but the deformation remains. Release the pressure and the seal rebounds to its original shape retracting the piston. That is how they work in all brakes I have seen. I don't know if the servo wave ones mentioned above are different but every caliper I have seen, and I have dismantled a few,(cycle, motorbike and car) work like that.

OK, so this relies on the surface sticktion (for want of a better word) and young's modulus of the seal being just right. Am I correct in thinking that if the seal "hardens", it would be more likely to first slide on the piston at initial braking, and provide no deformation, and then provide no return?

Bedding in defo works because the material needs to wear to the minute surface differences of the disc & the material needs to cure - my old C2s never worked so well as the first time I got them proper hot in the Alps, nice brown tinge to the disc as pad material was deposited on it, I could litterally feel them getting better through the decent.
TJ - I, like you have never got less than 1000's of miles out of pads (genuine Hope or EBC) & I'm sure some of this 'I got through a set of pads in 3 rides' is down to not bedding & hardening the friction material correctly leaving it soft & easily worn.

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.

Shimano pads are generally organic ones. My tip would be to get some (Wait for it...) Superstar sintered pads. IMO when I had my XTs on a VERY VERY wet TransWales that they are better then the Shimano pads in every way for about 1/4 of the price

I would also recomend these for winter. I found them to hard for summer/dry use as they set up vibrations throught the frame and forks but after a wet ride at afan recently the rear looked the same at hte end as it did at the start.
I am about to order some for my tektro brake on my HT before the hampshire grit eats them.

that seems plausible but I could not be sure. I[i] think[/i] its not just stiction but the shape of the seal edge - as it deforms it grips the piston. The defrmation is one by the fluid pressure as wella s the piston moving

remember the clearance is tiny - fractions of a mm

If you look at your brakes as you pull and release the lever does the piston not move away from the disc at all?

Oh and i have found that the pad cleance on my XT servowaves to be more than the Juicy's they replaced.

I'm one of those guys who can get through a brand new set of properly bedded in, sintered pads in about 10 or 15 miles.

I do think it's a heat issue.

Once the pads are hot, they work. The issue is getting them hot in the first place. If you're riding somewhere flattish, or with long fast descents, or with short sharp downs, and you're light, then in the cold and the wet, especially when the pads and rotors are sopping wet,and covered in grit its actually really quite difficult to get them hot - a typical descent for me may only involve 4 or 5 seconds worth of serious braking, and that's simply not enough to get things hot.

Here's a thought...
The bedding in process/getting properly hot. How much depth of the pad does it actually affect? I wonder if its simply a very thin layer. In the summer/heavy usage that hard, heat affected layer gets maintained as the pad wears down. In the wet & cold, the brakes never get that hot, so once the hard bit is worn away,it doesn't get replenished, leaving soft material that just falls to bits.

I solve the problem very easily to be honest.
I moved to the south of France 😉

Hi TJ [i]If you look at your brakes as you pull and release the lever does the piston not move away from the disc at all? [/i]
Yes, almost not at all. I can see some movement with springiness of the calipers relaxing, but I can't say there is any visible gap. This is with everything spotless and newly cleaned. There is however no rubbing contact (ie wheel spins with clearance), until the grit gets in.

I have only once not bothered to bed a set of pads in & I did go through them very quickly.
Now I always do 15-20 sprints up the road and then hard on the brake (front or rear). I do them individually, although it's hard to get enough heat into the rear so have to hang right back off the bike.

When I bed the pads in, the first few stops are generally rubbish and then I can feel the brake performance improve over the next few stops.

On my Juicy 7's, you can see the pads being pulled back very slightly as you release the lever.

I struggle understand how the grit/water mix that sits between your pads and rotor on a wet ride, when not braking can wear the pads significantly, as there is no pressure there to accelerate the wear. If there was sufficient wearing action on the pads, I would expect the brakes to be really draggy too.
But, quite a few people do experience very rapid pad wear in certain conditions, so I'm not saying there isn't something causing this.

So does the bed in temperature and pressure have more effect than that of the sintering process in manufacture?

Hi Jon. re "cold and wet and 4/5 seconds of braking". Yep that's me. And I'm using Superstar sintered.

BTW the pair I put on the woodburner last night DID change colour! I'll test the edges against a new pair with a metal file to see if they feel any harder!

JonEdwards - Member

.................
The bedding in process/getting properly hot. How much depth of the pad does it actually affect? I wonder if its simply a very thin layer. In the summer/heavy usage that hard, heat affected layer gets maintained as the pad wears down. In the wet & cold, the brakes never get that hot, so once the hard bit is worn away,it doesn't get replenished, leaving soft material that just falls to bits.

Seems logical but I have no data on this. There certainly are boundary effects but how deep the heat curing goes into the pad I don't know. Do you use big discs? They run cooler.

I'm not sure the woodburner idea is a good one, you've no idea of the temperature they are getting to, they might warp or crack & they'll wear at the high spots of disc to pad contact points more when you put them in the caliper (this will still happen when bedding in with the pads on the bike but the material will be softer so the wear will be more uniform)

What is the official line of the brake manufacturers regarding bedding in? E.g. Shimano, SRAM/Avid

Biggish.180 front on my XC bikes, 200s on my bigger ones.

I'm 10 stone dead, but do like my stoppies, and don't believe there's such a thing as being overbraked.

I think Jedward has it

Here's a thought...
The bedding in process/getting properly hot. How much depth of the pad does it actually affect? I wonder if its simply a very thin layer. In the summer/heavy usage that hard, heat affected layer gets maintained as the pad wears down. In the wet & cold, the brakes never get that hot, so once the hard bit is worn away,it doesn't get replenished, leaving soft material that just falls to bits.

Twice last winter I had to cut rides [i]really short[/i] due to almost complete loss of front brake - boths times it turned out that brake fluid had squeezed past the seals. Both days were sub-zero.
Googling showed this has happpened to other riders in cold weather.

I cleaned everything and replaced the pads both times - I'm still using the same seals with no further problems.

So I think shimano seals are adversley affected by the ambient temperature, perhaps they pull back the pistons better when its a warm day, but get more brittle/rigid when its cold, thus leaving our pads rub-rub-rubbing away at the disc.

Shimano seals will be a different material with them being mineral oil rather than dot fluid

JonEdwards - Member

Biggish.180 front on my XC bikes, 200s on my bigger ones.

I'm 10 stone dead, but do like my stoppies, and don't believe there's such a thing as being overbraked.

Jon - this could be an issue in that bigger discs run cooler - so might never be getting up to temperature. Brakes have a temp range in which they work best. Try smaller discs?

The reading I've done off the back of TJ's theory suggests that curing of a pad, and, more importantly, the transfer of pad material to the disc is something that needs to be done throughout the life of the pads/brake system under [u]normal use[/u].

A few microns of pad material on the disc is easily ground off in foul conditions, and if the brake isn't brought to a high enough temperature to transfer additional pad material to the disc, braking performance is diminished and the braking action changes from adhesive friction (low wear rates) to mechanical friction (high wear rates).

You therefore simultaneously lose braking power and increase pad wear rates.

Managing this in practice would seem to mean running a braking system that has more heat in it, allowing more braking events through a ride to get the heat up. The trade off of course, will be an increased propensity to over-cook the brakes on the more extreme braking events.