nodnerb

I know I'll get alot of flack for this. BUT, from a physics standpoint, does anyone else think that v-brakes are inevitably more powerfull than disc? The closer the braking surface is to the hub, the more torque the brake has to overcome. Hence the reason that 8" rotors are more powerful than 6". Discs and v's both brake in the same manner, by opposing pads clamping a rotating mass. So wouldn't this v brakes provide more power overall since the braking surface is a 26" diameter rather than 8" or 6"? I know my xt v's on my older bike are far more powerful than the new hayes mx2s on my new bike. Feel totally different though. Of course you won't really hear anywhere that discs aren't as powerful, what bike shop or company would want that. Personally, I think discs are more just the new technology rather than being 'actually' superior in braking force. For feel, they rock. Never have ridden a hydraulic though but like I say, this is more a theory of physics question. Anyone else put any thought into this? Opinions?

Dannihilator

You haven't had a good disc brake yet. The Hayes hmx 2's suck. If you want a good mechanical disc, go with Avid BB7's, hydro:Avid Juicy 7/5's, Hayes Mags, Hope, Magura shimano xt.

d_D

Disks do have a mechanical advantage. The pads can be much closer to a disk than a rim. Disk pads may only need to move 0.5mm and the rim may pads need to move 2mm before hitting the rim.

Power is not the only characteristic a brake needs. For a lot of people disk brakes are the best fit for the characteristics they need in a brake even though a rim brake is more than powerful enough.

I don't need any more power than rim brakes provide but I use disks because,
Longer lasting pads.
Braking surface not as exposed to mud and water.
Better consistency when wet and muddy.
Need almost no maintenance.
Lower risk of damage to braking surface.

nodnerb

I hear ya and I believe you. And I know my mx2s suck. But like I said, regardless of brands, what shops say, and just simply the fact that they are the new technology and look cool so they MUST be better; from a physics standpoint it seems to me that v's would always be able to produce more stopping power, or at least require only a fraction of the force needed by a disc to stop the bike. I think discs do produce more clamping power, but only because they *need* 10X the power to stop the bike in that physical location. Just doesn't really seem "better" to me that they jsut *use* way more power to to produce the same result.......man, I think too much.

nodnerb

Yep, I agree with all that. The thing I like most about discs is the progressive feel. I'm not really bashing discs at all, jsut the theory that everyone says they are more efficient and powerful than the v's just becasue they are marketed that way and are the "new" thing. ANyway, both are jsut as good imo, just bored and something I've considered before.

john65stang

Hmmmm. Good question. There are a few other things you might want to put into the formula.

1) What is the surface area of the disc brake pads vs. the rim brakes ?
2) What is the coefficent of friction of the disc pad / rotor combonation ?
3) Is there any extra leverage based on how the discs are built ?

Jameson

physics i don't know. i do know that my lx v brakes were awesome. i had to purchase a new fork recently due to breakage and bought a disc only model which i attached an avid bb7 to. i have gotten over it but i was initialy surprised at the lack of power. i think the v brakes were better at stopping the bike with the exception of wet muddy conditions.

stapfam

A good V brake properly set up will be as good as most mechanical disc brakes- Some of them suck, but so does a badly set up V. Properly set up V brakes? Why did I change to hydraulic discs? Because they are better. I got Hope Mono M4's with 200 mm discs and these are the DGS BX. Mind you, at the price, I do wonder if it was worth it.

matheprat

If you think about it, both V brake and disc brake levers travel about the same distance (relative to the handlebars). However, disc brake pads move a fraction of the distance which V brake pads do, and so discs have a much bigger mechanical advantage in the first place, before you start to consider pad material etc.

phantomcow2

Raiyn will probably repost the explanation he has written about why the stopping power is more.
The HMX2 brakes are not that great, I think my V brakes have more braking power than they did. BUt hte avids were quite nice

valbowski1980

I duffed my rim up pretty bad when I was up in Boston, and would have been stranded with a set of Vs. That and I ride in very dusty/sandy conditions so Disks are the only way for me.

1Fast4500

I love my avids. (BB5's)

the organic disc brake pads and stainless rotors have significantly more friction than a rubber pad on an aluminum rim.

Also, Yes the V's have more leverage over the wheel as the wheel spins BUT the Disc's have more clamping power on the rotor, I'm sure if they could make V's clamp that hard alot of Rims would fail quick. Hydro's add even more clamping power and more solid lever feel. if that makes sense (no cables to strech)

I'm sure someone has a formula to prove what I have said.


seriously, test ride something with Avid BB7's or Hope M4's. I can bring the rear wheel up as far as I want and hold it there until I stop. I can probably stop (30-0 MPH) in around 30 ft. all day (asphalt). I had some XT V's that would do the same but would then need adjustments after a day of hard stops.

1Fast4500

oh yeha also I was down in sothern MD being stupid. LOL jumping my bike off a pier into a river. needles to say with my new (insert shameless plug here) Shimano clipless peadls, Michilen tires and Avid brakes and Oakly lifevest. I could ride out of the 5ft deep water get up on the beach and stop instantly, even with water pouring out of the frames weep holes onto the brake rotors. I never even though about the fact that the brakes where wet until my girlfriends little brother rode out and was amazed that he could stop he almost went over the bars.

P.S. I got .MOV quick time movies if anyone knows how to post them up

FrankJohns

It depends what you mean by "powerful."

Maximum braking power occurs when a wheel locks up. V's can lock a wheel just as well as discs can.

However, discs require less hand movement/strength for the same amount of braking force. That is the mechanical advantage that matheprat is talking about.

willtsmith_nwi

Yes regarding the friction ... AND ...

Disc brakes dissapate heat SOOO much better than rim brakes. They are also much less resilient to wetness and mud.

The fact that the rubber for v-brakes is near the edge of the wheel is the ONLY reason that these things work. If you tried a rubber on metal system near the hub it wouldn't have a chance of working.

Discs are a superior braking method. But for most, v-brakes are "good enough" (myself excluded ;-) )

MadMan2k

OK

Upload them here:
https://jonbuder.com/ftp_info.html


then post the link

snoopz666

ya i remember a while back there where hydro rim brakes. they were commomly known as rim crushers.

in regards to your other question, yes there is a bigger mechanical advantage on the outside of the rim, but the coeficent of friction is not high enough to make it worth it. if they were to use a pad compound with a higher coefficent of friction it would in turn create more heat, you would loose braking power and wear away at the rim surface. (i have also heard stories of people tubes popping because of the heat build-up)

the reason disk brakes are so popular is because they can use a very good pad compound and not have to worry about wearing away, or crushing, a delicate(in comparison too the disk) rim. more force can be transfered to a solid piece of metal as oposed to a semi-flexable rim. a disk also deals with heat better then a rim.

and i dont think there is any rim brake on the market that you could still one finger brake at the end of the day doing lift/shuttle runs. IMHO they are just plain better

wheelhot

K, Im living near a hillside and my house is uphill (the uphill is like 20degress - 40 degress) and I was wondering, should i use a V-Brake or a Disc?

em9ty

im goin for disc.better wheels no side walls

hihi...but for weight i would like to go with v's

harov3

First things first. the torque occurs at the centreline of the axle, what you are saying is that since the v brake is further from the torque the less braking force is required to overcome that torque and stop or slow the wheel. This is correct, Torque (T) is the sum of the force (F) multiplied by distance (d), T=F*d.
So to stop the wheel rotating the force multiplied by the distance must be equal but opposite to the torque at the axel, as the v brake has greater distance (d) the braking force (F) applied can be smaller.

However the reason 8" disks are more powerful than 6" disks has little to do with the distance of brake force application from the axle or hub, this has some effect but the increase in distance (d) is insignificant compared to the increase in distance (d) for a v brake. The more likely reason is that a small increase in diameter (2")approximately doubles the swept area of the disk, that is for each revolution of the rotor and wheel approximately twice the rotor surface area is gripped by the 8" disk pad than is gripped on a 6" disk and this is what makes 8" disks so much more powerful.

Some rough calculations give me swept areas of ~7000 mm^2, 14000 mm^2 and 10000 mm^2 for 6" disks, 8" disks and rim brakes respectively. So why are 6" disks more powerful than V's generally? There are three possible reasons. One: the coefficient of friction for a disk pad acting on a steel rotor is much higher than the coefficient of friction of a rubber block acting on an aluminium rim. Two: the disk caliper has a much greater mechanical advantage than the v brake and can apply much greater force than a v brake. Three: both the previous explanations could be correct. One of these reasons will be the answer.

Are disks more powerful? Of course, from personal experience a cheap disk will outperform a cheap v brake everytime and not just marginally but considerably so. Not being in a position to buy some top quality disks and some equally pricey V's my experiences at the bottom end of the market will have to do, but I've seen nothing to suggest that pricey disks wont out brake pricey V's every time.

OK rip it to shreds

1Fast4500

take the chalange I did and I still own my Avids and $79 each $180 for the set is a good price even for great V's

https://wheelworld.com/site/itemdetai...=39&sort=Price

DMN

Discs' also have one major advantage over Vs' that a mate of mine labled "Anti-go-over-the-handlebars". With Vs' the power is either on, or off, you can set them up to be more progressive, but its still pretty much a case of their either on or off. Disc brakes have a much more progressive feel, which allows you to bleed off speed as well as put on full anchor. Discs have a certain amount of "give" in them which means that when your flying down a hill side and put them full on, they will not lock the wheel straight up and you will not go flying over your handlebars. Of course well set up Vs' will be better than most cheap mechanical discs, but the ability to brake without fear is worth tens times the lost stopping power to me.

Raiyn

Most likely.

steveknight

well really good v's will do this too but they actually cost more then discs. so if the bike is not setup for disc's going with say pals compnent v brakes you have a very good brake with good modulation. but since they cost 115.00 per wheel they are pretty spendy. but I found with the same brake pads they were twice as powerful as the avid sd7's I had before.

GreenFix

Not true for riding. Maximum braking power occurs just before lock up. Once you lock the wheel, you have significantly less braking power.