Observation about Avid BB7 Road cable operated disc brakes

[NHAero]

I had these brake calipers on my Anderson all-rounder and they just seemed weak and mushy. Swapped them for TRP hydros and haven't looked back. I've been converting my Litespeed Unicoi to a drop bar bike, and put the BB7 Road calipers on. The bars and brake levers came off the Anderson - the levers are Tektro R200 - and I'm using the same compressionless housing - literally most of the same pieces. So the only thing that is different is (obviously) the wheels - 26ers with knobby tires - and the rotors. On the Anderson the rotors are lightweight stainless steel KCNC Razors, 160mm front and 140mm rear. The Litespeed wheels have a mis-matched set of Avid steel rotors. The Litespeed stops way better than the Anderson did with these brakes. I'm scratching my head about why. As a mechanical engineer, I was taught that frictional force is F = u x N, where u is the friction coefficient and N is the applied force. Area doesn't enter this equation. The Avid rotors are definitely heavier and have more swept area. If this is the reason the brakes work better, I'd like to understand why.
Thanks!

[tuscanyswe]

I find rotor can make a difference. I had some bb7 a long time ago and got some fancy rotors (lightweight) and the braking was not as good but not much difference.

Can there have been contamination at play as an unknown factor? :)

[NHAero]

I thought about contamination of the rotor. Wouldn't it carry over to the pads, and therefore be transferred to the current set-up? And why would the rotors work well with the TRPs (maybe the answer is simply more force is being applied by the hydros).

I may clean both sets of rotors with some 90% isopropropyl alcohol to see if that makes a difference.

I find rotor can make a difference. I had some bb7 a long time ago and got some fancy rotors (lightweight) and the braking was not as good but not much difference.

Can there have been contamination at play as an unknown factor? :)

[Clancy]

If there are rotors that will make the BB7's work better, please let me know. My BB7's suck. I switched to Swissstop pads hoping to improve the braking. Nope!

So what rotors are recommended? Running 160's

[giverdada]

i'm terrible at math but i loved kinematics in high school and even ended up doing my independent project on the coefficient of friction of mountain bike tires. incidentally, this was in 1999 so it was a 26er. anyway, again, i am terrible at math, but what about the r value? wouldn't a shorter radius, r, and a lesser circumference, C, and the mass leveraged on both of these factors make for a greater 'overall leverage (???)' against the rotor when on a larger wheel? i don't really know, but i have a thought that maybe the distribution of force is somehow different on a different bike set up, particularly with 29er/700c as compared with 26er. sounds like you've got an experiment brewing!

[ultraman6970]

Ultra had a bike with those road BB7s, put the bike together as an experiment, easy to set up... but they never felt right. As the op says... mushy and no power at all. Put the crappy 10 bucks cantis back and happy again. Dont have the bike no more anyways.

[unterhausen]

I have no problems with my bb7's. I'm fat, I ride fast down big hills, and they stop me. My first ride with them was terrifying, because they weren't adjusted right. When I see these complaints, I wonder about that, or if a mtb brake got mixed in by mistake.

All these complaints about them, but it seems nobody ever sells them.

[NHAero]

No MTB brake mixed in by mistake - in fact, that's what was on the Litespeed, but swapping the bars with the road brake levers meant not enough pull for the MTB BB7s so I switched over to the Raod BB7s.

I have no problems with my bb7's. I'm fat, I ride fast down big hills, and they stop me. My first ride with them was terrifying, because they weren't adjusted right. When I see these complaints, I wonder about that, or if a mtb brake got mixed in by mistake.

All these complaints about them, but it seems nobody ever sells them.

[NHAero]

The outside diameter of the 26er with the big tires and the 700C with narrow tires is close to the same.


i'm terrible at math but i loved kinematics in high school and even ended up doing my independent project on the coefficient of friction of mountain bike tires. incidentally, this was in 1999 so it was a 26er. anyway, again, i am terrible at math, but what about the r value? wouldn't a shorter radius, r, and a lesser circumference, C, and the mass leveraged on both of these factors make for a greater 'overall leverage (???)' against the rotor when on a larger wheel? i don't really know, but i have a thought that maybe the distribution of force is somehow different on a different bike set up, particularly with 29er/700c as compared with 26er. sounds like you've got an experiment brewing!

[marciero]

If they are mtb levers then that surely makes a difference. But you switched back to road... Still, if different levers that is one source. Also rotor size-whether the same on both bikes. Anyway, three things that come to mind that can affect brake force.

1. Levers (different pull ratios, bla bla bla)
2. rotor size (bigger yields more.)
3. Wheel diameter including tire (smaller yields more-giverdada has the right idea-is same lever-arm/torque effect as larger rotor. In fact the braking force is proportional to the ratio of the two radii)
4. Coeff. of friction. (pads and rotors)
5. Cable and housing; esp housing
6...

(Edit-continued)

6.. Pad surface area (Coeff of friction is temperature-dependent. Larger pad will have less pressure spread over larger area for the same force, so may heat up less. Probably mitigated somewhat by greater heat-dissipation of smaller pad. Am guessing negligible affect. )
7...

[NHAero]

In this case:
Same levers, rotor diameter on the front, same wheel diameter within 2%, same calipers and pads, same cables and housing - the items were moved from one bike to another, except for the actual rotors. Which is why I am questioning rotor type as the difference.
Thanks!

If they are mtb levers then that surely makes a difference. But you switched back to road... Still, if different levers that is one source. Also rotor size-whether the same on both bikes. Anyway, three things that come to mind that can affect brake force.

1. Levers (different pull ratios, bla bla bla)
2. rotor size (bigger yields more.)
3. Wheel diameter including tire (smaller yields more-giverdada has the right idea-is same lever-arm/torque effect as larger rotor. In fact the braking force is proportional to the ratio of the two radii)
4. Coeff. of friction. (pads and rotors)
5. Cable and housing; esp housing
6...

(Edit-continued)

6.. Pad surface area (Coeff of friction is temperature-dependent. Larger pad will have less pressure spread over larger area for the same force, so may heat up less. Probably mitigated somewhat by greater heat-dissipation of smaller pad. Am guessing negligible affect. )
7...

[vitaly66]

...
As a mechanical engineer, I was taught that frictional force is F = u x N, where u is the friction coefficient and N is the applied force. Area doesn't enter this equation. The Avid rotors are definitely heavier and have more swept area. If this is the reason the brakes work better, I'd like to understand why.
Thanks!

I believe your equation may not represent total stopping force, and that swept area is in fact a coeffecient within a complete equation of braking application.

In other words, you absolutely will get increased stopping power by applying friction over a greater area.

[spookyload]

Different rotor width would affect this. Wider rotor would contact sooner and change the leverage of the lever.

[Mark McM]

I'm scratching my head about why. As a mechanical engineer, I was taught that frictional force is F = u x N, where u is the friction coefficient and N is the applied force. Area doesn't enter this equation. The Avid rotors are definitely heavier and have more swept area. If this is the reason the brakes work better, I'd like to understand why.
Thanks!

The Coulomb formula (F = u x N) is a very simple approximation of a complicated system. Friction is due to interactions between two surfaces at a molecular level, and is affected by many, many factors. As a mechanical engineer, I'm surprised you haven't been introduced to the field of tribology (https://en.wikipedia.org/wiki/Tribology), the study of how moving surfaces interact.

While all the other components may be the same, it is very possible that the frictional forces can vary quite a bit between the different rotors, due to a large number of factors, including material (alloys), surface finishes and condi tions, contact area (cut outs), etc.

[Mark McM]

Different rotor width would affect this. Wider rotor would contact sooner and change the leverage of the lever.

While it is true that most levers have changes in leverage throughout their strokes, levers are usually designed to minimize these leverage changes, plus brakes/lever can often be adjusted for initial contact point so that the same portion of lever stroke is for any rotor width.

[Erik_A]

Larger rotor ie. 180 vs 160 will give more stopping power with slight loss of modulation.

BB7's are great brakes, try new pads and new rotors.

I like the Avid HS1 rotor's with Avid sintered pads

If you are willing to wait a few weeks, I have bought the HS1s from Malaysia for $5 each:

http://www.ebay.com/itm/Bicycle-AVID-Brake-Rotor-Metal-180mm-With-6-Blots-For-Bike-Cycling-MTB-BB5-BB7-/132004878179?var=431352181536&hash=item1ebc1a9763:m:mM8QFxhhzoVn5ToGBT5WzCQ

I run 180 front and 160 rear.

[David Kirk]

One thing to keep in mind about the BB7 brake.....the pad movement when coming off full-open is not linear.

The design is set up such that when properly adjusted the first very small movement of the brake lever will move the pads in a lot to take up the running clearance between the pads and the rotor. More movement of the lever moves the pads less relatively but allows for more force to be made against the rotor.

It's a clever design that can work very well if perfectly adjusted....and can leave you wondering why these things suck so much when not perfectly adjusted. I think there is a video on their site going over how it should be done.

dave

[Mark McM]

One thing to keep in mind about the BB7 brake.....the pad movement when coming off full-open is not linear.

I don't believe this is the case - at least I hope not.

The BB7 uses a ball bearing lead screw actuator, which is inherently linear. If they were to somehow use a varying pitch thread on the lead screw, it would result in inconsistent performance. It would mean that as the pads wore, the brake would shift into a different leverage realm. It would also mean that the leverage realm may also depend on the brake pad adjustment and the centering of the rotor in the caliper. I've used both road and MTB versions of the BB7, and I have not noticed any non-linearity in their pad travel.

On the other hand, many hand levers do have increasing leverage through their travel, depending n the locations of their pivot points. However, this is typically less than a 20% change, and periodic adjustment of the brake pads as they wear will keep the hand lever moving through the same travel realm.

[NHAero]

My understanding is that the KCNC rotors on my Anderson are stainless. I'm not sure what steel the Avid rotors are made of, does anyone know that? They do show some rust, whereas the KCNCs don't. Is SS more slippery than carbon steel?
And are you saying below that contact area matters?




While all the other components may be the same, it is very possible that the frictional forces can vary quite a bit between the different rotors, due to a large number of factors, including material (alloys), surface finishes and condi tions, contact area (cut outs), etc.

[gdw]

Has anyone tried using road levers designed for v brakes -Cane Creek,Tektronix rl520- with your bb7s? They should pull enough cable to work with both the road or mountain bike version of the Avids.

[Mark McM]

My understanding is that the KCNC rotors on my Anderson are stainless. I'm not sure what steel the Avid rotors are made of, does anyone know that? They do show some rust, whereas the KCNCs don't. Is SS more slippery than carbon steel?
And are you saying below that contact area matters?

The Coulomb formula (F = u x N) works as a good approximation for contact between rigid, uniform surfaces. But for deformable surfaces (such as rubber), the interactions between surfaces becomes more complicated. This is why wider tires generally give better traction than narrower tires (even on smooth, hard surfaces).

[Mark McM]

Has anyone tried using road levers designed for v brakes -Cane Creek,Tektronix rl520- with your bb7s? They should pull enough cable to work with both the road or mountain bike version of the Avids.

The trade off for longer cable travel is lower mechanical advantage (leverage). If you use a hand lever with a long cable pull (V brake levers) with a caliper meant for short cable travel (road disc brakes), the total mechanical advantage will be lower than with a matched lever/caliper. There will be plenty of pad clearance, but it will require more hand force for a given braking force.

[NHAero]

I'm asking specifically about the contact area between two hard surfaces, pad and rotor.

The Coulomb formula (F = u x N) works as a good approximation for contact between rigid, uniform surfaces. But for deformable surfaces (such as rubber), the interactions between surfaces becomes more complicated. This is why wider tires generally give better traction than narrower tires (even on smooth, hard surfaces).

[dem]

KCNC Razor are known to be pretty cruddy compared to OEM, just look at the amount of swept area (actual material touching the pads) vs. Shimano or stock SRAM.

https://cdn6.bigcommerce.com/s-tqfdy/products/143/images/505/KCNC_Razor_master__67785.1447265132.190.285.jpg?c= 2

http://www.wheelies.co.uk/images/products/Avid-G2-Clean-Sweep-Disc-Brake-Rotor-19896-small.jpg

[bmeryman]

Something else to keep in mind is that the cone washers on a BB7 caliper will allow you to run the caliper out of parallel with the rotor if you're not careful. I'm not saying this is necessarily the case here, just something to think about. I've seen plenty of bikes put together with BB7s, plenty of the same model. One will feel great, one will feel squishy and imprecise. Double check how the pads are contacting the rotor. If the rotor is bending or flexing a bit perhaps consider adjusting the caliper.*

The ability of BB7s to accommodate frame manufacturer error is one of their strengths, but it's also a trait that lends itself to improper setup occasionally.

*Make sure you remove those tiny rubber bands holding the washers on the bolts when assembling the first time! Those add to the problem sometimes.

Sorry for the aside, back to friction!

[NHAero]

I understand that the swept area is lower on the KCNC Razor. What I haven't heard as yet is that in a friction loading between two hard surfaces whether swept area is important and if so why, according to the physics involved. In motor vehicles increases in swept area is important because of higher need for heat dissipation, which in most of our daily riding on bikes isn't a limiting factor.
Is there another reason beyond swept area that KCNC Razor is known to be cruddy?
Thank you

KCNC Razor are known to be pretty cruddy compared to OEM, just look at the amount of swept area (actual material touching the pads) vs. Shimano or stock SRAM.

https://cdn6.bigcommerce.com/s-tqfdy/products/143/images/505/KCNC_Razor_master__67785.1447265132.190.285.jpg?c= 2

http://www.wheelies.co.uk/images/products/Avid-G2-Clean-Sweep-Disc-Brake-Rotor-19896-small.jpg

[CMiller]

Has anyone tried using road levers designed for v brakes -Cane Creek,Tektronix rl520- with your bb7s? They should pull enough cable to work with both the road or mountain bike version of the Avids.

I have, using long-pull Cane Creek levers and mtn bb7s, worked better than a road bb7 to Shimano lever I had on another bike.

[Mark McM]

I'm asking specifically about the contact area between two hard surfaces, pad and rotor.

Relatively speaking, brake pads are soft. Braking with truly 'hard' surfaces is generally not very good: Softer materials have higher friction coefficients, softer materials conform better to contact surfaces, so there is more uniform pressure, friction and heat transfer, etc. Disc brake pads are harder than rim brake pads, because they must withstand much higher forces (and often higher temperatures), but compared to steel rotors, they are soft.

[NHAero]

So are you saying that in this case, swept area matters?

Relatively speaking, brake pads are soft. Braking with truly 'hard' surfaces is generally not very good: Softer materials have higher friction coefficients, softer materials conform better to contact surfaces, so there is more uniform pressure, friction and heat transfer, etc. Disc brake pads are harder than rim brake pads, because they must withstand much higher forces (and often higher temperatures), but compared to steel rotors, they are soft.

[Mark McM]

So are you saying that in this case, swept area matters?

A little bit. Anther factor that will vary with swept area is heat concentration (friction converts kinetic energy to heat, afterall), which will affect the friction coefficient.

But getting back to the main point - braking can be affected by the design of the rotors, so you may have an apples/oranges comparison. Even with rim brakes, I've found that different models of rims can have different braking, very possibly due to differences in the the aluminum alloys used.

[gdw]

"Has anyone tried using road levers designed for v brakes -Cane Creek,Tektronix rl520- with your bb7s? They should pull enough cable to work with both the road or mountain bike version of the Avids."

"I have, using long-pull Cane Creek levers and mtn bb7s, worked better than a road bb7 to Shimano lever I had on another bike."

Judging from your experience and that of other riders I've talked with it appears that the mtb version of BB7s paired with v brake specific road levers perform better than the road version does with most standard road levers. Has anyone else had the same results?