LUW

Not on purpose, but I saw it happen to my daughter's 20" bike (steel rims). I had pumped up the tires, admittedly without paying attention to the gauge (just by squeezing the tire), and then she rode all morning. It was a really hot day (a little over 30ºC) and at lunch she parked the bike out in the sun. About an hour later the rear tire blew out.

The worse part was the looks of the whole family towards me, the avid cyclist, who caused it.

joejack951

Automobiles have brake bias valves that deliver 70-80% of the braking force to the front brakes, because a front wheel skid simply means continuing to go in a straight line whereas a rear wheel skid usually involves the rear end of the vehicle coming around. One pumps the brakes in an automobile to attempt to maintain steering control and some braking force while doing a front wheel skid. ABS does this automagically for us, now, unless you own relics like myself.

None of that is relevant on a bike because even a very short front wheel skid can take you down, and if you've ever activated the ABS on your car you know that the front wheels do briefly lock in a true panic stop. One tries to avoid skidding the front wheel entirely on a bike for this reason. Such little braking force comes from the rear wheel on a bike that ABS/pumping there would be mostly useless there as well. On my disc brake commuter bike, I barely touch the rear brake for this reason (and the fact that it only the front is a disc).

shelbyfv

Isn't that why they ban carbon wheels from some Gran Fondos or is it that the rims come apart when overheated? Can't remember.

ColonelSanders

Hah, cool story and all, but not really what I was getting at.

ColonelSanders

I am definitely not the right person to ask, coz although I love bicycling, I have literally zero interest in top level racing.

LUW

Not exactly come apart, but the resin used for the CF has a melting point of around 240ºC and the brake surface of the rim can get as hot as 220ºC, so heat buildup may be an issue. That's why Shimano is putting a layer of Al over the braking surface of their wheels.

Oh come on, I stoically endured their contemptuous looks and still my story doesn't count?

Bike Gremlin

ABS works very well on motorcycles.

Shimagnolo

They are called "proportioning valves".

See for yourself:

joejack951

I was not previously familiar with motorcycle ABS systems. There are some pretty significant differences between a motorcycle and a bicycle so I do wonder how effective a similar system could be for a very lightweight bike.

hsuehhwa

Koolstop Salmon, Swisstop +1


I did notice Salmon generates a lot of heat. Be careful if you're descending ~10% grade.

RChung

Rims *are* discs.

Modulation isn't a function of where the disc is located. It's almost entirely a function of how force gets transmitted from your brake levers to the pads, not whether those pads are acting on a 180mm disc or a 700mm disc. You can buy brakes operated mechanically (cable-actuated), or hydraulically in either format. In fact, if you really, really, were interested in improving the modulation of a common mechanically-connected rim brake, you needn't go to road hydraulic rim brakes; you could replace the brake cables with low-friction cables with compressionless housing, like discussed here.

That's modulation. If the tires are the same, the other issue is pad composition and disc rotor surface. Just as there are alloy and carbon wheel rims, there are alloy and carbon disc rotors. Carbon in either format has challenges as a braking surface -- they tend not to dissipate heat as well as alloy. The GCN video compared a hydraulic actuated alloy disc brake with a mechanical actuated carbon rim brake, and found no difference in dry weather -- but differences in wet weather. If you were concerned about braking in wet weather (and even though I live in a place that has been experiencing a multi-year-long drought, I'm in this category) the real lesson is that braking on carbon surfaces with current technology isn't ideal. That's true whether the braking surface has a 180mm diameter or a 700mm diameter.

Shimagnolo

Oh, high performance carbon brake technology does exist, (actually carbon/ceramic), but if you need to ask what it costs, you can't afford it:

TimothyH

This is not how Shimano C24, C35, C50 wheels are constructed.

The braking surface is not a layer over carbon but is part of the alloy rim. The alloy rim and braking surface are one. The rim is thinner and lighter than tradition alloy rims to save weight and carbon is applied over the alloy rim as reinforcement. Carbon fiber reinforcement is simply not applied to the braking surface.


-Tim-

Maelochs

This is a false equivalency because cars never used anything like rim brakes ... before discs they generally used internal-expansion brakes, with shoes pressing inside a found housing which was limited in size by the diameter of the wheel, which put more weight further from the center, and which didn't dissipate heat well.

Discs were easier to cool, easier to have more swept area, more convenient to package .... but the forces involved are also gigantic compared to the forces involved with bicycles.

Not saying discs are or are not better ... but the fact that they were and are used almost everywhere int he automotive world (except for on the rear axle of front-wheel-drive low-budget builds (like my '04 Civic) doesn't mean bicycle discs are superior to rim brakes ... it means automotive disc brakes were superior to internal-expansion drum brakes. (I guess some bikes also have drum brakes, but they are heavy and not particularly better than rim or disc brakes except they might last longer on long descents with a very heavy bike.)

As for the examples you posted of not crashing on your motorcycle (pretty much unrelated given proportional weight vs contact patch versus disc size) and once on your bike --- yeah, lots of people have done that stuff, myself included. That actually says more about rider ability than the specific brake system ... because every time I have made an emergency stop (or just an emergency braking event, which (as you note) often leads to an avoidance maneuver instead of a stop) .... i was on a bike using rim brakes.

Tell you what ... set me up with a pair of bikes, one with rim- and one with disc brakes, and I will do exhaustively documented comparison testing which you can use for your website.

EDIT:

Interesting. Again, I think tire coefficient of friction is the biggest issue. I remain open-minded, but so far am not sold that discs are measurably better in general use. I'd definitely get them for a commuter if I still commuted.

LUW

I stand corrected. So the Al is just there to help dissipate heat? Or for structure too?
I know that tubular wheels are less prone to overheating the brake surface because there's more material there compared to a clincher rim.

Bike Gremlin

For warm weather, there are other, perhaps even better choices, but for rain/winter - Salmon all the way.

Though I use them all year long. On long descends, overheating can be avoided with hard (as hard as safe and comfortable), short braking, instead of feathering the brakes for longer. Also, good thing is allowing the speed to reach maximum of your safety and comfort zone, while staying more upright - letting the wind do your braking as much as possible.

Bike Gremlin

Bicycle is light weight. ABS would add more weight, cost a lot, while speeds are not nearly as great as reached with cars/motorcycles. However, loosening grip on brakes (not letting go completely) when wheel starts to skid goes a long way - both with motorcycles (with, or without ABS) and with bicycles. If riding upright (not leaned in a turn), even a front wheel skid can be managed.

Bike Gremlin

Drum brakes on a motorcycle are the hardest to modulate. Very dangerous and notorious for their ability to lock the wheel (increase the braking force with the same lever pull strength) without a warning. They are, on the other hand, good for winter riding, contamination from road salt doesn't affect them as much as disk brake callipers. Even "enduro" motorcycles with rubber cover over the brake callipers (making them not directly exposed) are more sensitive to road salt than older drum brake models.

On bicycles, disks do provide a bit better modulation. More predictable. Especially when wet. In dry condition, especially on pavement, they aren't noticeably better than rim brakes. Except they require a bit more force on the lever - for me that's not an issue, even though I never use more than 2 fingers on the brakes.

However, compared to rim brakes, disk brakes:
cost more
weigh more
add more stress to the front fork and it's bearings, calling for stronger (and heavier) fork system
make the wheels more bulky - when trashing bikes in a crowded bicycle parking, disks can be bent, damaged more easily than rim brake calipers

Maelochs

This is where I am coming from.

I think discs will ---one day---be on every bike---and will be lighter than rim brakes. And something will be done about disc alignment--from what I hear nowadays, people like through-axles because a QR never quite sits in exactly the same place each time, and the disc rubs ... but for a regular road rider, having to carry an extra wrench to take off the wheel is just one more thing to forget, one more bulky, heavy piece of metal to tote along ... but if you ride discs, better to tote a wrench than endure constant screeching, I think.

Right now, for most of my riding (mostly in the dry) discs simply don't make sense ... and I suspect that is the case for most riders. Five years from now ... we might see both discs and electronic shifting on the majority of anything-above-entry-level bikes ... and five years later, even Wal-mart will sell all their bikes with discs and electronic shifting

I tentatively plan to still be riding in ten years ... and that might be when I can afford a new bike.

Bike Gremlin

Mostly agree - I went back to rim brakes after having tried disks. Even for winter riding (i ride all year long). When I hear some people talking about disk vs rim brake, it sounds as if rim brakes don't work at all, when they are just slightly worse than good disk brakes - by a small margin, for most applications (not counting extreme off road, -20 C winters with lots of snow and hilly terrain, or very long descents that are such one needs to brake a lot).

However, alignment is not much of a problem for mechanical disks at least.
Carrying a wrench - my commuter has old system with nuts holding the wheels in place - bit more hassle for the thieves IMO. I carry this tool in my patch kit pack:



Not much weight and size - no problem.

RChung

In that video they said they were using the same tires. I think the biggest issue is that the rim brakes were being used on a carbon rim surface.

Maelochs

LOL, a dog-bone wrench ... I cannot remember the last time I had one of them .... I can't even remember the last time I remembered that such things existed.

RChung

Heat dissipation. Really old riders will remember wooden rims (I'm not that old; I've only seen them hanging on walls). There's a lot of mass in those wooden rims but wood isn't great at heat transfer so wooden rims had terrible braking performance. The two desirable characteristics of a brake are that (1) they be able to convert a lot of kinetic energy to heat; and (2) that they be able to dissipate that heat as quickly as possible. Wooden rims failed on (2). Although many modern passenger cars have switched to discs, heavy trucks still use drums. That's because drum brakes can have a *lot* of swept area if you make them big. You'd have to make a disc rotor the size of the wheel to get equivalent swept area. So truck drum brakes can convert kinetic to heat really fast. The issue with drums is that they're not as good at dissipating heat as discs. As I mentioned, the tire rim is actually a pretty big disc, so it not only has a fair amount of heat capacity but, if it's made from alloy, can get rid of that heat pretty quickly.

Modern bicycle disc brakes are pretty impressive but there's nothing intrinsic about discs themselves that make them brake better or with better modulation. You can design a disc brake with inappropriate materials that will brake like crap (*cough* Avid BB5 *cough*). You can design a rim brake that will brake like crap. You can design a disc brake system that will perform great. You can design a rim brake/wheel rim combo with appropriate materials that will perform great. The disc itself is just a small part of a larger braking system.

joejack951

What type of thru-axles are you talking about? I just purchased a set of thru-axles for my new build and there were two options:

1. standard lever cam style
2. low-profile style that use a hex key for removal, or a removable lever (a glorified hex key)

Are you referring to the weight of a hex key above? I always ride with some sort of multi-tool so there will be no extra weight to carry to remove the low-profile axles I purchased.

wphamilton

What potential energy are you thinking about? Normally you'd think of brakes as converting kinetic (from the vehicle's mass and velocity) into heat and, perhaps, into a bit of mechanical potential energy (from the momentary mechanical compression of spokes, suspension etc). It's unclear to me what you're talking about.