# Bicycle Mechanics - Why Rotor Flex?

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View Full Version : Why Rotor Flex?

vantassell
05-03-10, 08:35 PM
On my BB7 (road) brakes all the of setup/adjustments guides say to have 2:1 ratio of space on the inboard : outboard side of the rotor. This just seems to be setting up the rotor so it has to flex a ton in order to be fully engaged by both pads.

I'd think that this much flex is a bad thing, why am I wrong?

jgedwa
05-03-10, 09:01 PM
I have wondered about that too, and am curious to see if anyone has any informed ideas about it. I would guess that the flex would not hurt anything, and maybe the constant flexing will help bring any warping into line. Sort of like it gets a constant massage!?!

On the other hand, the "ton" of flex you are talking about must only be a mm or so.

jim

vantassell
05-03-10, 09:19 PM
On the other hand, the "ton" of flex you are talking about must only be a mm or so.

True, it might not be 'that' much flex. I have a ton of squealing right now (having equal space on both sides) and i'm wondering if having the 2:1 ratio holds the rotor in a way to stop the squeal.

5b00
05-03-10, 09:26 PM
Because there is more of a gradual application of the brakes, maybe it gives better modulation. Or maybe that's just the illusion of greater modulation. I'm not certain just how much braking force would result from only one pad touching the rotor.

Anecdotally, it reduces the incidence of howling, especially when my brakes get wet. This is probably because the force on the rotor from the outboard pad prevents the rotor from reaching a resonant frequency when bouncing between the two pads. When the rotor is flexing, there's always one pad to slightly damp the vibration.

All that being said, I don't really pay a whole lot of attention to the setup instructions. I just back the inboard pad out a notch if it's rainy and my brakes are howling.

vantassell
05-03-10, 11:20 PM
Alright, when I get home looks like i'm going to try setting these up with the 2:1 ratio. We'll see how it goes.

Anyone else have any experience/thoughts about the 2:1 gap ratio or 1:1 gap ratio?

Ingleside
05-04-10, 02:03 AM
I do the same thing when I install these types of brakes on customers bikes. The reason is so that you can then adjust the inboard pad in toward the rotor. By setting it up this way the static inboard pad is adjusted so it is pushed out farther than the outboard pad. This reduces rotor flex because the rotor does not need to move over as far to hit the inboard pad and it reduces the risk of the rotor rubbing on the caliper once the inboard pad wears down a bit.

149288

Mr IGH
05-04-10, 06:53 AM
If the pads aren't parallal to the rotor then all bets are off. Before I adjust pad distance I make sure the caliper is set correctly by loosening the caliper bolts and pulling the lever so the pads are tight against the rotor, then tighten the bolts. I don't measure the pad's space, I adjust the inner non-moving pad close but not rubbing. Then I adjust the outer pad to be close. The rotor should flex slightly when the brake is applied.

joejack951
05-04-10, 08:19 AM
Alright, when I get home looks like i'm going to try setting these up with the 2:1 ratio. We'll see how it goes.

Anyone else have any experience/thoughts about the 2:1 gap ratio or 1:1 gap ratio?

Look at Ingleside's illustration. That's what Avid is recommending, a pad to caliper gap ratio of 2:1, not a pad to rotor gap ratio of 2:1. I have about 4000 miles on my BB7 road brake (front only) and I can assure you that your braking will feel substantially better using as small of a 1:1 ratio gap from pads to rotor as possible. I just readjusted mine last night to tighten up the gaps and braking is so much smoother and the squeal under heavy braking is gone as well.

vantassell
05-04-10, 11:24 AM
If it's supposed to be like Ingleside's illustration, why does Park show it with 2:1 gap?

http://www.parktool.com/images_inc/repair_help/avidmech_101.jpg

I was reading a bunch of threads on mtbr and a bunch of guys there were saying to leave the pads adjusted well, but with light scraping. After a couple of weeks the pads won't scrap anymore, so it'll be perfect.

Glynis27
05-04-10, 11:58 AM
149288

This shows it perfectly. Do this.

bikinfool
05-04-10, 12:30 PM
The flexing rotor is simply part of the design due to the static pad, nothing to worry about. Been using BBDB/BB7 brakes for 10 years, too. You can use varying distance of the inboard pad to the rotor to tune the brake feel/modulation to your liking. The outboard pad recesses into the caliper, so you're not actually centering the rotor between the pads per se, the 2:1 ratio just optimizes the position of the rotor relative to the caliper body. Avid's recommendation, Park's instructions and Ingleside's illustration all suggest the same thing in any case.

joejack951
05-04-10, 01:06 PM
If it's supposed to be like Ingleside's illustration, why does Park show it with 2:1 gap?

They are showing the caliper to rotor gap, which is what I was confusingly referring to when I said pad to caliper gap. The premise is the same; you want the outer pad extended two times more than the inner pad which requires the caliper to be offset relative to the rotor. When you set the pads gaps to the rotor, you set them as close as possible and even so rotor flex is minimized.

bikinfool
05-04-10, 01:26 PM
When you set the pads gaps to the rotor, you set them as close as possible and even so rotor flex is minimized.

Not necessarily. I don't.

joejack951
05-04-10, 01:41 PM
Not necessarily. I don't.

Interesting. What have you found with regards to using offset pads, i.e. what works nicely and what doesn't?

bikinfool
05-04-10, 02:26 PM
Interesting. What have you found with regards to using offset pads, i.e. what works nicely and what doesn't?

What do you mean by offset pads? Do I have the inboard pad further from the rotor at rest than the outboard pad? Maybe by a bit (and had to go look at a few as I usually don't even think about it that way). I don't like my brakes to engage right away or have an off/on feel, either, so both pads are dialed back a bit from being as close as possible to the rotor and I don't know if I could measure how much flex I'm getting. I also use the lever's leverage adjustment to fine tune feel, and feel is what I use more than a particular number of clicks or a set distance for the pads. I'm going on about mountain versions rather than road, too.

Oh, and I never center my calipers on these brakes using Mr IGH's method (do on my Elixir CRs, though).

joejack951
05-04-10, 03:10 PM
What do you mean by offset pads? Do I have the inboard pad further from the rotor at rest than the outboard pad? Maybe by a bit (and had to go look at a few as I usually don't even think about it that way). I don't like my brakes to engage right away or have an off/on feel, either, so both pads are dialed back a bit from being as close as possible to the rotor and I don't know if I could measure how much flex I'm getting. I also use the lever's leverage adjustment to fine tune feel, and feel is what I use more than a particular number of clicks or a set distance for the pads. I'm going on about mountain versions rather than road, too.

I was apparently confused by your post. I understand now. With the road version of the BB7's, I find that unless I have the pads set as close as possible to the rotor that I get unreasonably long lever travel before the brake really engages. Also, because only the outboard pad moves, I find that a lot of clearance to the inner pads causes a pulsing feeling when braking and extra squeal especially when wet. I've heard other people comment on the lever travel issue when using the road BB7 so it likely is totally different when using the MTB version.

Oh, and I never center my calipers on these brakes using Mr IGH's method (do on my Elixir CRs, though).

That method is unnecessary with BB7s which allow you to dial in both pads with the adjusters. The cheaper Hayes brakes on my MTB only a dial adjustment for the inner pad so to align the rotor you are forced to use the brake lever to clamp the caliper to the rotor. The outer pad's gap to the rotor is adjusted by cable tension (unlike the Avid's where they specifically warn not to adjust the outer pad position using cable tension). Both systems work but the BB7's are a bit easier to adjust.

bikinfool
05-04-10, 03:34 PM
I haven't tried the road versions, but even then from what I've read on the forums the typical road levers don't seem to marry up all that well either. Curious, are there any road levers have adjustable leverage? I had to reread the OP before it sunk in about the road version, thus my mentioning it. The BB7 is the only mechanical disc I'd consider using, I actually favor them over the Elixirs for the most part due to their ability to be easily setup just the way I like.

Booger1
05-04-10, 03:41 PM
Unless you have dual piston calipers,you have a primary pad and a secondary pad with sliding calipers.Thus the different pad settings,to combat rotor flex.

bikinfool
05-04-10, 03:47 PM
What do sliding calipers have to do with any of this?

Mr IGH
05-04-10, 03:55 PM
Oh, and I never center my calipers on these brakes using Mr IGH's method (do on my Elixir CRs, though).

...That method is unnecessary with BB7s which allow you to dial in both pads with the adjusters.....

I do the technique when installing new pads and when the caliper gets knocked on a rock (when mountain biking). It is better way to get the pads parallel when the pads are out of line with the rotor, the other option is to use the brakes and grind the pads back to parallel (with all the squeaks).

vantassell
05-04-10, 04:09 PM
Just an update, I followed the park instructions for setting the pads and now have significantly less lever throw to engage the brakes (I like less throw). They are still super loud, but they seemed to get better today on the ride to work.

I'm still am very interested by what everyone is saying, i'm a disc noob. I'm not saying that the park method is right, but i liked the decreased throw it afforded me. I'll post here if the squeal goes away, but for now it's still a work in progress.

bikinfool
05-04-10, 04:42 PM
I do the technique when installing new pads and when the caliper gets knocked on a rock (when mountain biking). It is better way to get the pads parallel when the pads are out of line with the rotor, the other option is to use the brakes and grind the pads back to parallel (with all the squeaks).

Huh? Just dial the pads in like in the setup instructions. You knock your calipers on rocks a lot? The CPS washers don't always get everything nice and parallel alone, but that's what they're there for; I still wouldn't use the squeeze the lever method on the BB7s...how do you set your inboard pad for this "method"?

bikinfool
05-04-10, 04:50 PM
Just an update, I followed the park instructions for setting the pads and now have significantly less lever throw to engage the brakes (I like less throw). They are still super loud, but they seemed to get better today on the ride to work.

I'm still am very interested by what everyone is saying, i'm a disc noob. I'm not saying that the park method is right, but i liked the decreased throw it afforded me. I'll post here if the squeal goes away, but for now it's still a work in progress.

Squealing is more likely from contamination of the pads/rotor, an untrue rotor (easily trued btw) or you didn't get the pads setup as well (square to the rotor essentially) as you think...the CPS system doesn't solve all setup issues, either. Some bikes/forks also can accentuate harmonics from the brakes. Lots of people complain about Avid hydraulic brake noises particularly but personally I think it's mostly in their initial setup, and somewhat that they're a very popular brand (thus more complaints). You might venture over to the brake forum on mtbr, too; here's a link to their disc brake faq for those new to discs http://www.mtbr.com/discbrakesfaqcrx.aspx

DannoXYZ
05-04-10, 05:21 PM
I find it interesting to see how bicycle disc brakes are going through the same evolution as motorcycles, but 30-years later (same as cars 60-years earlier). We start out with fixed-rotor/fixed-caliper single-piston brakes that eventually evolve into floating-rotor/fixed-caliper dual-piston brakes in the end. The floating-rotor does away with a lot of the squealing, shuttering and uneven wear issues as the rotor will self-centre between the pads.

I kind of object to the Park tool site's procedure since it does make the rotor flex more than necessary. There is really no relationship 1:1, 1:2, 2:1 between the two sides that makes sense because they're really not related. What I like to do is:

1. set fixed/stationary pad 1-2 clicks out from where you can hear it rubbing the rotor (this setting is fixed regardless of spacing on other side)
2. set moving pad as may clicks out as necessary to give you the lever-throw before engagement you desire.

This minimizes the amount the rotor flexes and leads to flatter contact between the rotors and pads. I like this procedure here: http://www.twowheelblogs.com/avid-bb7-disc-brake-set-and-tuning

Mr IGH
05-04-10, 06:23 PM
Huh? Just dial the pads in like in the setup instructions. You knock your calipers on rocks a lot? The CPS washers don't always get everything nice and parallel alone, but that's what they're there for; I still wouldn't use the squeeze the lever method on the BB7s...how do you set your inboard pad for this "method"?

Gee, Dude, you make things hard. If the pads aren't parallel, they squeal like a pig until the rotors grind them parallel. While they're getting ground down, there's less sweep area and braking suffers. If you're riding home on a paved rode in the sun, it's not an issue, ignore what I posted and go on your happy way. Yes, between my two sons and myself, we do have rock strikes on our frt calipers, it does happen often enough to need a solution. We also use up pads, sorry, perhaps we're riding too hard for you...I make sure the pad's are not stuck at the limit, after making the pads parallel, then I tweak the clearence.

Squealing is more likely...you didn't get the pads setup as well (square to the rotor essentially) as you think...

:rolleyes:

joejack951
05-04-10, 06:23 PM
you didn't get the pads setup as well (square to the rotor essentially) as you think...the CPS system doesn't solve all setup issues, either.

I complained a lot about my front disc brake during my first attempts at set up. I found through trial and error that you need to be quite careful when torquing down the caliper so as not to undo the CPS system adjustment. It's very easy to flex the rotor while torquing the caliper bolts and in doing so, you ruin the parallel adjustment. The trick I found was to slowly torque the bolts, alternating back and forth between the two tightening maybe 1/4 turn at a time at least until they are considerably snug. Only then apply the real tightening torque. I went from having an always noisy brake to a brake that only squeals like a pig when wet (apparently like all Avid brakes squeal).

bikinfool
05-04-10, 08:18 PM
I find it interesting to see how bicycle disc brakes are going through the same evolution as motorcycles, but 30-years later (same as cars 60-years earlier). We start out with fixed-rotor/fixed-caliper single-piston brakes that eventually evolve into floating-rotor/fixed-caliper dual-piston brakes in the end. The floating-rotor does away with a lot of the squealing, shuttering and uneven wear issues as the rotor will self-centre between the pads.

I kind of object to the Park tool site's procedure since it does make the rotor flex more than necessary. There is really no relationship 1:1, 1:2, 2:1 between the two sides that makes sense because they're really not related. What I like to do is:

1. set fixed/stationary pad 1-2 clicks out from where you can hear it rubbing the rotor (this setting is fixed regardless of spacing on other side)
2. set moving pad as may clicks out as necessary to give you the lever-throw before engagement you desire.

This minimizes the amount the rotor flexes and leads to flatter contact between the rotors and pads. I like this procedure here: http://www.twowheelblogs.com/avid-bb7-disc-brake-set-and-tuning

There is a downside to the floating rotors somewhat as not all forks play well with the extra width on a floating rotor. The slight flex involved with my setups have never caused an issue. Flatter contact just isn't that important IMHO. Is the Park tutorial that different from Avid's? I'll have to go look harder perhaps at their version...

I complained a lot about my front disc brake during my first attempts at set up. I found through trial and error that you need to be quite careful when torquing down the caliper so as not to undo the CPS system adjustment. It's very easy to flex the rotor while torquing the caliper bolts and in doing so, you ruin the parallel adjustment. The trick I found was to slowly torque the bolts, alternating back and forth between the two tightening maybe 1/4 turn at a time at least until they are considerably snug. Only then apply the real tightening torque. I went from having an always noisy brake to a brake that only squeals like a pig when wet (apparently like all Avid brakes squeal).

I disagree all Avid brakes squeal, and that other brands don't when wet (have heard just about all brands squeal when wet). I do agree with any pair of bolts like that need to be tightened as you describe, though.

Gee, Dude, you make things hard. If the pads aren't parallel, they squeal like a pig until the rotors grind them parallel. While they're getting ground down, there's less sweep area and braking suffers. If you're riding home on a paved rode in the sun, it's not an issue, ignore what I posted and go on your happy way. Yes, between my two sons and myself, we do have rock strikes on our frt calipers, it does happen often enough to need a solution. We also use up pads, sorry, perhaps we're riding too hard for you...I make sure the pad's are not stuck at the limit, after making the pads parallel, then I tweak the clearence.

:rolleyes:

Mr IGH
05-04-10, 08:30 PM
...Rolleyes indeed, you don't even have mountains in your locale. ....

LOL, I get out sometimes :)
http://images2e.snapfish.com/232323232%7Ffp53838%3Enu%3D3275%3E4%3B3%3E733%3EWSNRCG%3D33%3C9%3B559%3C932%3Anu0mrj

http://images2e.snapfish.com/232323232%7Ffp537%3C7%3Enu%3D3275%3E4%3B3%3E733%3EWSNRCG%3D3459%3C5543%3B32%3Anu0mrj

http://images2e.snapfish.com/232323232%7Ffp537%3C8%3Enu%3D3275%3E4%3B3%3E733%3EWSNRCG%3D3459%3C5544532%3Anu0mrj

Mr IGH
05-04-10, 08:33 PM
...My house sits higher than anything in your entire state and my local trails, let alone all the places I ride, am sure offer more challenge than yours....
and my kid can kick your kid's butt :p

bikinfool
05-04-10, 08:46 PM
and my kid can kick your kid's butt :p

That's too easy, don't have any kids...and I doubt that in any case (i.e.that he can keep up).

mkane77g
05-05-10, 08:44 AM
We use BB-7's on a tandem, have since they came out. We use the squeeze the lever method to get the pads true. I run the pads as close as possible to the rotor, both sides. No squeels here, brake is awesome. Lot's of hands on experience with these, and they are good.