OK, so all this talk about heating rims and exploding tires got me thinking. So I looked in my latest bike catalog and I see that disc calipers aren’t much heavier than standard calipers. Unless that weight is per pair instead of per caliper. In any event, the additional weight would only be about 300-800 grams for calipers. The added weight needed for hubs and rotor could easily be shaved out of the rim because the extra strength would not be needed to support braking.
So why aren’t road bike makers interested in making this next jump?n Am I just being naive?
I thought of putting Discs on the last frame i built. but with the ever changing calipers and pads i did not want to have replacement parts that were hard to find,also with all of the different replacement forks made from different materials it would be a nightmare,and i think it would not go over to well and the fork companys would not jump on this band wagan.I will someday build a road frame with discs as i think it is a great way to go,and would look so cool.Randall
I thought of putting Discs on the last frame i built. but with the ever changing calipers and pads i did not want to have replacement parts that were hard to find,also with all of the different replacement forks made from different materials it would be a nightmare,and i think it would not go over to well and the fork companys would not jump on this band wagan.I will someday build a road frame with discs as i think it is a great way to go,and would look so cool.Randall
Well, here is a twist to the whole disc brake situation. Due to at least one severe accident, information has been out in bike industry media that there may be a very serious problem with front disc brake design on bicycles with quick release wheels, especially on road bikes. The problem is this: when a rider applies heavy brake force on the front disc brake, the resultant force on the front hub is directly aligned with the open end of the fork dropouts and basically tries to do its best to shove the front wheel out of the dropouts, which will result in a potentially disastrous accident if the rider is at any speed at all.
This problem is especially prominent on road bikes and tandems because of their traditional fork designs and the angle of their fork dropouts. It also applies to conventional suspension forks on mountain bikes, but this may be somewhat resisted by the forks’ “safety” dropouts which will tend to catch the front wheel if a quick-release is loose. However, this may not resist braking force in all cases. Even though motorcycles have front disc brakes in the same position as on bicycles, they do not have this problem as there is one very important difference on their forks. Motor bikes (and now most ‘downhill’ mountain bikes) have a ‘through-axle’ design with the front hub. It is bolted securely in place with an encircling type of clamp.
Perhaps this may explain the slowness of the introduction of discs on road bikes? In any case, this design flaw appears to be a pretty major oversight…
The only application I see for disks on road bikes is for high mountain pass stages (with or without rain/sleet/snow) where braking is critical on the descents. Otherwise, they don’t appear aero, they will definitely be heavier, the hydraulic variety require more maintenance, you don’t need that much braking power, and imho they are ugly. Oh yeah…they will cost more too. … my 2 cents
I am not saying that you should take a mtb disc brake and put it on a road wheel. Certainly if the power is not necessary, then the hydraulic issue is moot. But how about this:
1 - I have seen some bad conditions in Paris-Roubaix and other classics. Do disc brakes become better then because of conditions. I recall some teams running canti brakes for clearance with the mud.
2 - What about a “lite disc brake” for road conditions?
1 - Conceivably, yes. But most pros don’t brake often (especially not on a flat course like P-R) and most don’t want the extra weight. They would also have to get the Mavic wheel trucks and motorcycles to carry additional sets of disk compatible wheels.
2 - Avid already makes road-oriented mechanical disk brakes.
“only 300-800 grams” !?! you realize 453 grams is a pound, right? a set of zipp 404’s (tubular 700) weigh less than 1300 grams, you really think you can shave enough weight out of the braking surface to counteract the added weight of the disk?
as someone else pointed out, the best use for road disc’s would be on steep downhills, but who’d want to haul that extra weight up the hill in the first place?
Ah…the UCI. Certainly not a thinking organization at the best of times. I’m suspicious they make decisions based on Bio-rythms, crystals and clelestial alignment. Reason apparently has nothing to do with their chioces.
Disk brakes on road bikes? I’ve looked into it with some interest and I would love a front disk on my bike, simply for safety reasons.
Disk brakes work better than single or double pivot caliper brakes. End of story.
Will the wheel fall out of the drop outs with repeated braking. Phhhhhttttt. Doubt it. I think that suggestion results from too much time in the sun and not enough hydration!
I ride in New Zealand (land of the long white cloud…among other translations) and it rains ALOT here. I had a ride to the West Coast of Auckland several years ago on my old QR. Good brakes in good condition etc. It started pissing with rain. I mean really bucketting down. I spied my computer (before the water caused it to crap out) and saw 41 kph…42 kph…43 kph etc on a moderate to steep downhill. I was pulling so hard on the brake levers I thought the cables would snap! 44 kph…45 kph. I was shitting myself on an unfamiliar road and water was splashing all over, particularly from the brakes and pouring all over me. Did I mention it was pissing down? The weak point was the rim/brake pad interface (obviously) since there was no wheel lock or skidding going on. Luckily the hill flattened out and my speed reduced, but nowhere as quickly as I would have liked.
Disk brakes are vastly superior in the wet. Most of braking effect on bicycles (particularly hard braking) goes on the front wheel, so a front disk would be extremely valuable. As for the wheel falling off…BS. The forces are pushing the bike ONTO the front wheel (and hub). How the wheel is going to fall off with all the extra weight pushing the fork onto the skewer is beyond me. The skewer is more likely to snap, NOT lift off!
I have tried to get an old MTB fork with lugs so I can put it on my road bike. I’d cut off the lower part of the barrel and remove the MTB dropout as well so a sleeve with two lugs is all that is left. I’d then slide the sleeve over my road fork and crush the barrel to approximately the size of the road fork. Then I’d take a couple of bolts drilled through the crushed barrel and loosely clamp it in position. Some decent quality epoxy to fill the gaps and tighten the bolts a couple of turns and Voila…Disk brake lugs on a road bike. Uglier than me in the morning but it’d work. Then add a disk compatible hub to a raod rim, a cable operated caliper and presto we’re in business!
I commute to work in all weather and would love a disk brake setup. It might help me stop before I go over a car hood.
This would of course prelcude me from entering UCI cyclocross events. Like I’ll lose sleep over that one. Phhhhtttt.
As for the wheel falling off…BS. The forces are pushing the bike
ONTO the front wheel (and hub).
Suppose you have a wheel of radius 34.1 cm. And suppose you and your
bike weigh 100 kg together: it’s reasonable to figure a maximum of 100
kg road force on the tire when breaking (beyond that, you’ll either
skid or endo). This constitutes a torque, which is normally measured
in foot-lbs, Newton-metres or the like: distance times force,
producing a twist.
To counteract a torque, you need the opposite torque. To counteract a
100 kg force at 34.1 cm radius requires 2 times the force at 1/2 the
radius, or 3 times the force at 1/3 the radius, or 4 times at 1/4 the
radius. Let’s say your disc pads are located at a radius of 10 cm from
the axle. They’ll exert up to 100 * 34.1 / 10 = 341 kg force on the
brake disc.
If the pads are located at 90 degrees relative to the direction of the
dropout, you will have a 341 kg force acting in the direction of your
dropout. If the pads are located in front of the fork, they will try
to drive the wheel up (more secure – no problem). If they are located
behind the fork, they will drive the wheel down and out. The fact that
all 100 kg of you and your bike have transferred to the front wheel
will not help you: there is still a net 241 kg of force trying to push
your wheel out of the dropout. I don’t know what the designs are, but
this is a bad place to put a disc brake.
To put this in perspective: normally your front quick-release feels
almost nothing in the way of a push-out force. A kg or two if you go
over a bump. A few kg at most in a hard corner. But you will come
close to this amount of release force on one side if you corner hard
with your body leaning way over and your bike dead vertical.
Which is to say: be careful out there. The forces are not always what
you think, and the unexpected is not outside the realm of possibility.
Even a slight change in the way you ride in an emergency manoeuvre can
make the difference between what was always good enough and an
accident. Make sure your quick-releases are done up well!
One other point to make about disc wheels and weight that seems to be often overlooked. Yes, the rim weight can be reduced since it doesn’t need to support the calipers.
However, traditional brakes exert all of the force at the edge of the wheel (rim); roughly the same place where the force is transferred to the pavement (tire), so the rim only needs to be strong enough to transfer this along the edge.
With disc brakes, the rotor handles all of the stopping force and transfers it completely to the hub. Only through the spokes does this make it to the rim and ultimately the road. Think about how much beefier a rear wheel is to handle the torque from the drivetrain, and obviously you can generate much greater deceleration forces with braking than you can acceleration with pedalling. There are more spokes, and they are usually laced differently.
The funny thing is, stop by your local MTB shop, and you find hoards of bikes of all kinds with front disc brakes, and many of the front wheels are the same as the non-disc variety (even having radial and not cross-laced spokes!). Perhaps mtb wheels are over-engineered, but I doubt that’s true for the road variety.
It’s been many moons since newtonian physics for me, so there’s my disclaimer if I have it all wrong. But if I have it right, a vector diagram of the forces you describe would show an arrow of length “x” directly into the road to represent rider+bike weight, and a second arrow of length 3.41x in the same direction representing the torque of the disc brake on the hub. Barring a road failure, how does this tend to remove the hub from the dropouts?
I do not know a thing about disk brakes, but I am about to learn. I just did a multiday tour (cyclenc) and decided that I would love to do it next year with my son and given his age (now 9) a tandem looked like the way to go, even though I had never been on one. Anyway-I just ordered a Cannondale. I selected it due to the combination of $$$ (they are all pricey) and the fact that it has disk brakes. On our trip a family on an admittedly old tandem pulling a tag along with a kid on it, tacoed the rear wheel on a mountain descent. It was amazing that no one was seriously hurt. There were a number of contributing factors but it sold me on at least trying disk wheels on the tandem. I have never had or witnessed an experience in any of my triathlons that would tempt me on a single person road bike.
an arrow of length “x” directly into the road to represent
rider+bike weight, and a second arrow of length 3.41x in the same
direction representing the torque of the disc brake on the hub.
To keep it simple, I would put it like this. For now, ignore the rider
weight and look at braking-related forces only. Consider (also for
simplicity) a vertical dropout, and the disc pads mounted horizontally
straight back behind the axle. We begin ‘where the rubber meets the
road’. We are ignoring rider weight, and only looking at the braking
force: this is straight backwards, of length “x”. This is our
deceleration force: we need it to slow down. In our scenario, since
the disc brake only acts vertically (the disc is moving straight up
when it is under the pads), the only way for this horizontal braking
force to be balanced is by a forward horizontal force on the axle:
straight forward, of length “x”. (Note, this is force on the axle; the
axle’s force on the fork will be opposite, namely, backward – thus
slowing the rest of the bike.) This forms a pair of opposite forces
with a distance in between. It’s like pushing with one hand on one
side of a steering wheel and pulling with the other on the opposite
side: it produces a rotational force. It is this rotational force that
causes riders to go over the bars if they are too high.
Now, in a proper physics diagram, the forces and the moments (torques,
or rotational forces) need to balance. We’ve balanced the forces, but
where how are the torques balanced? We have a force downward on the
wheel from the brake: 10 cm behind the axle, there is a vector acting
on the wheel straight down, of length 3.41x. (Note, we are talking
about forces on the wheel only.) Again, the only way this downward
vertical force can be balanced is by an upward vertical force on the
axle, so we draw a vector straight up at the axle, of length 3.41x.
Notice that this is the balancing torque that we were seeking: we
torque on the wheel via the disk, so that the wheel can torque on the
road. Again, note carefully that the upward 3.41x force is on the
axle. This means that the quick release skewer, by means of friction
through the dropouts, needs to provide 3.41x of upward force, or the
wheel will come out.
As far as rider+bike weight goes, they never gain weight during a
deceleration, so the vertical component is always x. (The horizontal
component changes due to acceleration.) This is a downward vector on
the axle, which means you subtract it from the 3.41x upward that is
required. Thus 2.41x.
On a more practical and almost related note, for those who remove
pedals: put the crank arm forward, and let the wrench come back from
the pedal. Adjust your grip so you apply force in the direction of the
bottom bracket will prevent the cranks from turning on you. If you use
a really short wrench, the cranks will want to turn forward. If you
use a long enough wrench (and handle it at the end :-), the cranks
will want to turn backward, lifting the pedal despite the fact that
you pushing down. This is only related in an oblique sort of way, but
maybe it helps get the concept of how a torque can produce a seemingly
opposite force.
More physics tips (I assume you already know about weight transfer and
reduced capability to stop on a slope):
A trailer gives you more momentum without more force on stopping
wheels (stopping ability increases with weight on the wheel). Minimise
the weight in the trailer for safety.
Trailer mount point can make a difference for safety. If higher up
(eg seatpost), it will tend to lighten your back wheel during braking
and transfer your weight forward. It’s safer to have it mounted low,
even axle-level if possible.
Tandem braking is different from regular: more use of the back brake
to get optimal stopping power. Therefore beware of skidding the back
wheel with a trailer pushing you (not that skidding the front is much
better…).
Fair enough. Reasonably thorough explanation, (but apparently not taking in to account all forces) but that may be a moot point.
How come MTB bikes with disc brakes (and normal dropouts) don’t have wheels leaping off the bike whenever braking exceeds bike and riders combined weight?
How is a disc brake on a road bike gonna behave fundamentally any differently from a disc brake of a MTB? They travel at similar speeds as road bikes, with similar weight riders.
I want a disc brake becasue at times (ie in the wet) the braking force applied by a normal road bike caliper system is inadequate to do the job. A disc brake does this better. I’d rather risk a skid because the brakes worked and the tire/road interface failed, rather than hit an object in my path because my brakes underperformed (partially failed). I wanna stop reliably regardless of the weather and disc brakes on my road bike would help me do that.
Oh, yeah, yeah, yeah, a little more of this and I’ll be ready to start cutting class again. When’s the final? Actually everything you say makes perfect sense, The idea I think I was struggling with is how the wheel/axle can leave the dropouts during the actual braking when most of my weight is on the fork/axle. If I’ve got you now, when the QR fails, the torque (moment, whatever) will actually lift the fork off the axle (relative to the road), bringing the ride to an abrupt conclusion and needlessly muddling a well balanced force diagram. qed. Thanks, I promise to keep checking the QR on my disc brake commuter (mountain) bike.
1 - Conceivably, yes. But most pros don’t brake often (especially not on a flat course like P-R) and most don’t want the extra weight.
I believe it was Frankie Andreu (or one of the other pro diarists) who wrote of wearing a new set of brake pads down to the metal in one flat Tour stage when it was raining.
“How come MTB bikes with disc brakes (and normal dropouts) don’t have wheels leaping off the bike whenever braking exceeds bike and riders combined weight? How is a disc brake on a road bike gonna behave fundamentally any differently from a disc brake of a MTB?”
—The answer is they DON’T behave fundamentally different. Yes, the front disc brake safety issue is especially prominent on road bikes because of their traditional fork designs and the angle of their fork dropouts. This is not theory–it has already happened to folks. I have read of a case on a road tandem with a front disc.
HOWEVER, all of this also applies to conventional suspension forks on mountain bikes, but this MAY be somewhat resisted by all suspension forks’ “safety” dropouts which tend to catch the front wheel if a quick-release is loose, but I wouldn’t bet that this will resist braking force in all cases. And even though motorcycles have front disc brakes in the same position as on bicycles, they do not have this problem as there is one very important difference on their forks–motor bikes (and most ‘downhill’ mountain bikes) have a ‘through-axle’ design with the front hub which is bolted securely in place with an encircling type of clamp…