Massive brake fade

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Dragster1
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Re: Massive brake fade

Postby Dragster1 » Thu Dec 11, 2014 1:01 am

jackthelad wrote:you do know that you shouldnt clean the discs?
maybe once a year, if that with isopropyl or spray can disc brake cleaner on a rag
i run avid elixers and bb7, all still on original pads, sintered ...
hardtail with panniers, commuting
and 2 duallies offroad, braking in pea gravel
all on original discs roundagon and G2's
a few issues i can see is non avid discs etc etc for avid brakes use...
the brakes are designed to run on the hardened steel of the manufacturers choice..
the only thing that is the different is the hydraluic versus cable
and pad choice
Shitmano i have no clue on

Yes agree your early model cable brakes ran softer disc material to get greater pad bite with less force on the lever. It even tells you on some rotors not to use with Hydro brakes. You can tell when your rotor is too hard in material for your hydros the braking is just crap and it glazes the rotor really fast and burns it out. You should roughen the disc rotor every time you fit new pads to get complete bedding so that you don't end up with hot spots on your rotor. Avids do seem to like the thick film from the pads that gets left behind on disks, have noticed that myself also but on the metallic pads it dosent seem to matter.
Last edited by Dragster1 on Thu Dec 11, 2014 9:21 am, edited 2 times in total.

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Re: Massive brake fade

Postby jackthelad » Thu Dec 11, 2014 1:26 am

the pad material left on the disc,
helps with the grab of the brake pads,
plus better not to waste time cleaning something that doesnt need it.
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Re: Massive brake fade

Postby MichaelB » Thu Dec 11, 2014 9:01 am

brand of disc vs calipers should be of such little consequence that it doesn't matter.

Yes, the manufacturers do their testing and development with their discs, pads, fluid, adaptors etc, but at the end of the day, it comes down to friction material and it's properties (friction coefficient, temperature range, materials and construction) that provides the braking. Yes, it's more complex than that, and I've oversimplified it, but it ain't far wrong.

For road bikes, discs are essentially discs made of stainless steel with patterns (marketing) that come in different diameters. Another forum member who works in testing of auto brakes will tell you with authority & experience that holes, patterns, grooves etc do not aid in braking.

What disc/caliper/pad combination work well for one person won't for another because of braking styles, patterns, riding terrain etc, etc, as well as personal preferences.

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Re: Massive brake fade

Postby Mulger bill » Fri Dec 12, 2014 12:06 am

Wish they still made Roundagon rotors, less noise than the CS :(
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Re: Massive brake fade

Postby MichaelB » Fri Dec 12, 2014 7:45 am

The new SRAM Centrelines (no, I won't spell it their way) are meant to be heaps better.

I just picked some up from CRC for $20 ea, so will soon find out when I swap the RS685 levers and calipers over

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Re: Massive brake fade

Postby Nate » Fri Dec 12, 2014 1:14 pm

Well not too good yesterday...

Ran the Kevlars with Avid HS1's...
fading & slight over heating on the rotor surface
And busted another spoke :\

Rebuilding the rear wheel with DT's.
Put in F1 organic pads & will see how they go.

All this "keeps heat out of the braking system"... i'm getting sceptical of... where does it go then?
It seems that any pad that insulates the system from heat is throwing it all back into the rotor! (where else can it go?)


Close to spending a weekend on the 2 descents with an IR temp gun & going through every combo for everyone... !

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Re: Massive brake fade

Postby MichaelB » Fri Dec 12, 2014 1:20 pm

Nate wrote: ..

Close to spending a weekend on the 2 descents with an IR temp gun & going through every combo for everyone... !


I thought about doing that a while ago, but never got round to it, as there is a whole pile of issues around accuracy of IR weapon, and surface finishes/materials that can affect temp reading of the gun.

Would still be interesting.

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Re: Massive brake fade

Postby mitchy_ » Fri Dec 12, 2014 1:24 pm

Nate wrote:commuting in all weather in Sydney traffic = Disc only.
Rim brakes = forget it.

looking at the forks:
Image

There appears to be a 180 to 160 adapter already on there?!?
So i should be good for 180's up front

If thats the case then i'll do a 180 upgrade for sure, i dont need to stop harder all the time - but just give a bit more headroom & reduce heat stresses on everything


whilst it is a +20mm adaptor, i'd say it's from 140 to 160mm. i'd also check with the manufacturer, as a lot of carbon forks are only rated up to 160mm rotors.

Nate wrote:Well not too good yesterday...

Ran the Kevlars with Avid HS1's...
fading & slight over heating on the rotor surface
And busted another spoke :\

Rebuilding the rear wheel with DT's.
Put in F1 organic pads & will see how they go.

All this "keeps heat out of the braking system"... i'm getting sceptical of... where does it go then?
It seems that any pad that insulates the system from heat is throwing it all back into the rotor! (where else can it go?)


Close to spending a weekend on the 2 descents with an IR temp gun & going through every combo for everyone... !


that's exactly where the heat goes, it stays in the rotor to dissipate. on a hydro brake, it's to stop the fluid from boiling.. however i'm not sure of it's purpose on a mechanical brake.

as for the anti-disc brigade... :roll:

with the wrong disc size or pad material, combined with the riders weight it can cause problems... it's no different to using the wrong pads on traditional rim brakes. OP said he had no issues with the kevlar pads and stock rotors, so it's clearly not a disc brake fault.
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Re: Massive brake fade

Postby eeksll » Fri Dec 12, 2014 1:46 pm

what the issue with running bigger rotors than specified of the fork?

The same force goes into the same spots on the fork. Maybe at maximal braking a larger rotor will provide greater force.

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Re: Massive brake fade

Postby Duck! » Fri Dec 12, 2014 1:53 pm

Bigger rotors move the caliper further from the axle. Greater braking radius exerts greater leverage on the fork, which can break the tips off if it exceeds the designed specification.
I had a thought, but it got run over as it crossed my mind.

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Re: Massive brake fade

Postby Nate » Fri Dec 12, 2014 2:15 pm

mitchy_ wrote:i'd also check with the manufacturer, as a lot of carbon forks are only rated up to 160mm rotors.

with the wrong disc size or pad material, combined with the riders weight it can cause problems... it's no different to using the wrong pads on traditional rim brakes. OP said he had no issues with the kevlar pads and stock rotors, so it's clearly not a disc brake fault.


Correct - manufacturer (local distributor) said no - dont go over 160's.

correct on the anti-disc stuff...
I had superb braking on a MUCH heavier bike - this is more a trial of new pads, which dont play well at higher outputs due to heat.

As for testing - how about we scrap surrogate outcomes & just use a phone's accelerometer!

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Re: Massive brake fade

Postby Dragster1 » Fri Dec 12, 2014 3:12 pm

eeksll wrote:what the issue with running bigger rotors than specified of the fork?

The same force goes into the same spots on the fork. Maybe at maximal braking a larger rotor will provide greater force.


Carbon forks flex like mad when you corner you may not see it with the naked eye but trust me you run a lot tighter pad clearances on a disc calliper brake and they rub really bad going to a larger diameter rotor will make the movement as you go out from the centre a lot more. Besides over loading the fork.

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Re: Massive brake fade

Postby barefoot » Fri Dec 12, 2014 3:24 pm

Drizt wrote:Are these issues mechanical disc related? Haven't had any such issues with r785 hydro. I'm using the stock pads. Done plenty of 80km/h decents with some hard braking


I haven't had any such issues with BB7-R either.

Coming up 2 years / 8000km on the stock pads. I don't know what the stock pads are, because they have never been out of the calipers. I sometimes think I should pull them out and check that they really do still have as much meat left on them as they appear to when I look through the window in the bridge of the caliper, but just haven't got around to it.

They're a bit noisy sometimes, then it goes, then it comes.

Rotors have some heat discolouration because I use them pretty hard when I need to.

I'm quite sure the BB7-R isn't as nice a caliper as the original BB7 (which I've had and loved on a couple of MTBs). I think the short cable travel / high cable tension is the culprit. Well, it has to be because that's the only difference in the calipers. Would be better if we could get V-brake compatible (long cable travel, low cable tension) brifters so we could use the MTB calipers.

Regardless, my BB7-R brakes just work, and work, and work. Sure, they could be even better, but no complaints. And no real interest in swapping to hydraulics.

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Re: Massive brake fade

Postby barefoot » Fri Dec 12, 2014 3:29 pm

Nate wrote:As for testing - how about we scrap surrogate outcomes & just use a phone's accelerometer!


You will (or at least you should) have the same limit of braking regardless of caliper/pad/rotor combination.

That is, your brake should be capable of skidding the tyre or sending you over the bars, whichever comes first. That's all about tyre/road friction and weight distribution, nothing about brake function. A stick in the spokes would be just as effective.

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Re: Massive brake fade

Postby trailgumby » Fri Dec 12, 2014 3:34 pm

No way I'd go 180m on a front rotor with roadie slicks. Massive overkill that will likely result in a busted collarbone. I've done it before with 32mm slicks and it was scary - especially in the wet. Much too sudden.

XT Ice tech rotors seem better in heat dissipation and have good surface area on the brake track to maximise braking power. Avid HS1's seem good with surface area too. Nether have the issues with brake pulsing that Avid rotor in the photo above is known for.

I agree you've cooked the pad. Stay with the OEM metallics, especially f they are Avids.

By rhe way, harder compounds donlt necessarily generate less heat. SOfter compounds are more likely to grip better at the expense of faster pad wear and some of that heat energy is ablated into pad wear. But not organics. Organics dissolve too quickly in wet conditions, which is kind of problematic for road bikes as these are exactly the conditions in which you want to be using discs.

We had an issue with a squealing, ineffective metallic pad on my son's bike i Rotorua last week where I suspect the pad was partly contaminated with an oily fingerprint from the LBS owner (they were job-lot loose pads out of packaging). It bit OK-ish which is why I took so long to realise it was possibly contaminated, but it squealed like a banshee. Eventually we got sick of the noise, realising that it was never going to bed in and quieten down.

When I pulled it out, it was heavily discoloured, even the copper backing plates were heat discoloured black and blue. So was the rotor. But the pad had almost no wear!

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Re: Massive brake fade

Postby mitchy_ » Fri Dec 12, 2014 3:49 pm

Dragster1 wrote:
eeksll wrote:what the issue with running bigger rotors than specified of the fork?

The same force goes into the same spots on the fork. Maybe at maximal braking a larger rotor will provide greater force.


Carbon forks flex like mad when you corner you may not see it with the naked eye but trust me you run a lot tighter pad clearances on a disc calliper brake and they rub really bad going to a larger diameter rotor will make the movement as you go out from the centre a lot more. Besides over loading the fork.


my carbon forks with 15mm axle and very tight pad clearances don't rub one iota. ;) it's a wonder setup.
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Re: Massive brake fade

Postby eeksll » Fri Dec 12, 2014 3:52 pm

Hmmm I run 180 on the front of my commuter on surly 1x1 fork. It has a very on/off feel which I kinda like not so keen on sliding out the front.

However, judging by the articles and talk here, I doubt I ever brake that hard.

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Re: Massive brake fade

Postby Dragster1 » Fri Dec 12, 2014 4:02 pm

mitchy_ wrote:
Dragster1 wrote:
eeksll wrote:what the issue with running bigger rotors than specified of the fork?

The same force goes into the same spots on the fork. Maybe at maximal braking a larger rotor will provide greater force.


Carbon forks flex like mad when you corner you may not see it with the naked eye but trust me you run a lot tighter pad clearances on a disc calliper brake and they rub really bad going to a larger diameter rotor will make the movement as you go out from the centre a lot more. Besides over loading the fork.


my carbon forks with 15mm axle and very tight pad clearances don't rub one iota. ;) it's a wonder setup.

yeah I know but its a weight weenie to a lot of people and the OPs bike isn't a 15mm through axle but I dare say a lot of road bikes will need to go this way in the end.

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Re: Massive brake fade

Postby Thoglette » Fri Dec 12, 2014 4:11 pm

eeksll wrote:However, judging by the articles and talk here, I doubt I ever brake that hard.


It's not how hard, it's how long. Long slow descents where you're always on the brakes are the ones which kill them
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Re: Massive brake fade

Postby Nobody » Fri Dec 12, 2014 5:23 pm

Duck! wrote:Bigger rotors move the caliper further from the axle. Greater braking radius exerts greater leverage on the fork, which can break the tips off if it exceeds the designed specification.
I believe that the leverage of the tyre on the hub increases with decreasing rotor size. This is because the brake pads act as pivot point on the disk during braking.
So as the disc gets smaller the pivot gets closer to the hub and so the tyre's leverage increases. The clear test of the theory is the standard rim brake which is the biggest disc anyone can use.
You don't see any fork tips snapping off because people brake too hard with the huge disc that the rim is.
We've covered this in this thread years ago, complete with maths and links. Starts on page 39 and continues on page 40.

Anyway, one reason manufacturers of road bikes with discs will limit disc size is because the fork goes laterally inward from the hub to the crown, where the disc goes straight up. Below is a photo to illustrate.
Image
The 185mm disc barely fits in this Kaffenback 2 fork which has a recess for the disc.
Last edited by Nobody on Fri Dec 12, 2014 5:55 pm, edited 1 time in total.

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Massive brake fade

Postby RonK » Fri Dec 12, 2014 5:53 pm

mitchy_ wrote:as for the anti-disc brigade... :roll:

No anti-disc brigade here.

But a determinedly anti-bs brigade. :wink:
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Re: Massive brake fade

Postby Duck! » Sat Dec 13, 2014 9:35 pm

Nobody wrote:
Duck! wrote:Bigger rotors move the caliper further from the axle. Greater braking radius exerts greater leverage on the fork, which can break the tips off if it exceeds the designed specification.
I believe that the leverage of the tyre on the hub increases with decreasing rotor size. This is because the brake pads act as pivot point on the disk during braking.
So as the disc gets smaller the pivot gets closer to the hub and so the tyre's leverage increases. The clear test of the theory is the standard rim brake which is the biggest disc anyone can use.
You don't see any fork tips snapping off because people brake too hard with the huge disc that the rim is.
We've covered this in this thread years ago, complete with maths and links. Starts on page 39 and continues on page 40.

I believe there's a fairly major fundamental flaw in your logic. You're forgetting that a rim brake acts on the rim (i.e. directly on the ground contact part of the wheel; for the sake of simplicity the tyre can be considered integral with the rim), while a disc brake is acting on the hub, with its force then tranferred through the wheel via the spokes.

Secondly, the fork crown, where a rim brake is mounted, is a significantly beefier stucture than the lower fork legs. There may be virtually no stress applied at the fork tip, but a lot more is borne by the frame (particularly headset bearings). Given that a disc brake effectively transfers its load from the caliper to the hub (funnily enough mounted at the fork tip), the further that braking contact is from the hub/fork tip, the greater the stress on the fork.

Rotor diameter limits are not restricted to road forks (canting of the legs inward to the crown has stuff-all to do with it; they were built that way because until recently disc brakes didn't need to be accommodated); many XC MTB forks also have rotor size limits. If small rotors exert more leverage on the fork, why don't rotors get smaller the more aggressively a bike is intended to be ridden rather than larger as is standard everywhere?
I had a thought, but it got run over as it crossed my mind.

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Re: Massive brake fade

Postby Drizt » Sat Dec 13, 2014 11:26 pm

Heat!

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Re: Massive brake fade

Postby Nobody » Sun Dec 14, 2014 12:00 pm

Duck! wrote:You're forgetting that a rim brake acts on the rim (i.e. directly on the ground contact part of the wheel; for the sake of simplicity the tyre can be considered integral with the rim), while a disc brake is acting on the hub, with its force then tranferred through the wheel via the spokes.
For the purpose of this illustration, imagine wheel with a hugely deep rim with very small spokes of about 80mm. Then you made a rim brake which contacted this rim at the point where a disc pad would normally rub. So then the linear forces would be the same as a disc brake.

Now, if you were to do an endo/stoppie on this bike, then the linear force from braking on the fork at the hub would be the stopping force on the tyre, multiplied by the distance from the tyre patch on ground to the brake pad, divided by the distance from the pad to the hub. Or:
Force@hub = Force@tyre X Dist(t to p) / Dist(p to h).
To flesh it out a bit and using Kg instead of N:
= (0.5g X 85Kg) X 0.38m / 0.0925m
= 174.6Kg of force trying to push down on the hub, perpendicular to a line bisecting the hub and brake pad.
This force would be significantly higher for a 140mm disc, but I don't have one to measure the distance from the pad to contact patch. But likely over 220Kg.
That is why I believe on many modern disc forks with QR, the dropouts face forward.
0.5g come from Bicycle Dynamics
85Kg is average bike rider of 75Kg with 8Kg bike and 2Kg of clothes and extras.
Distance measurements came from my CX fork with 185mm disc and 23mm road tyre.


Duck! wrote:...many XC MTB forks also have rotor size limits.
I've heard about this, and the only explanation I've read that makes some sense is that the longer bolts can place more stress on the caliper mounting points.
I would argue if the fork caliper mounts were moveable to accommodate different disc sizes, there probably wouldn't be a limit.


Duck! wrote:If small rotors exert more leverage on the fork, why don't rotors get smaller the more aggressively a bike is intended to be ridden rather than larger as is standard everywhere?
As Drizt said, bigger rotors for better heat dissipation.
Last edited by Nobody on Sun Dec 14, 2014 2:37 pm, edited 1 time in total.

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Re: Massive brake fade

Postby trailgumby » Sun Dec 14, 2014 2:27 pm

It's a question of leverage. Bigger rotors provide more leverage, or, to put it another way, reduce the leverage ratio of the tyre contact patch against the brake pads.

Example:
Contact patch on a 29" tyre is 36.8cm from the axle.

Average distance of pad contact patch from axle of a 160mm rotor is 7.35 cm assuming a 13mm wide pad.
Average distance of pad contact patch from axle of a 180mm rotor is 8.35 cm assuming a 13mm wide pad.

Leverage ratio of tyre versus 160mm rotor is 36.83/7.35 = 5.0109
Leverage ratio of tyre versus 180mm rotor is 36.83/8.35 = 4.4108

4.4108/5.0109 = 0.8802 or a 11.2% reduction in leverage of the tyre against the brake pads by going to a 180mm rotor.

Putting it another way, it means 5.0109/4.4108 = 1.13605 or a 13.6% improvement in available braking force.

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