Why do people think 25km/h speed limitation is safe?

[peasjam]

While the second is certainly true, the first isn't. A disc brake's performance is primarily set by the disc diameter, faster vehicles have larger discs. A rim wall is the largest disc brake possible on a bike, and if the brake is of equal design quality and type, the rim brake will easily outperform the much smaller discs..

Except you're forgetting contact area, pad compound and the greater amount of force a hydraulic calliper can exert against the disc. It's not all about diameters; my old car had drum brakes on the rear which were larger diameter than the front discs. Guess which ones had more braking force.

Road bikes use rim brakes not because they are better but because the application doesn't require immediate and large braking forces to the same degree a downhill or cross country mountain biker does, thereby making the weight penalty an overall disadvantage and the rim brakes a better overall choice.

 

[flecc]

Except you're forgetting contact area, pad compound and the greater amount of force a hydraulic calliper can exert against the disc.

No, I said subject to equality of design and type, i.e. equal on the factors you mention.

Your comparison of drum and disc brakes is invalid for a whole host of reasons, in the same way that apples and oranges cannot be compared for superiority.

 

[Scottyf]

Hydro Calipers I believe are a bit overkill for a bicycle application. There more complicated and require more painful care than a cable operated system.


However in applications like Cross Country and downhill they have there need.


The Car brake one was a bit or a worrying comparison. Cars generally have a set Front / rear brake bias.
The rear drum may only have one piston lever plus there on the rear. If the same force was sent to the rear the wheel would lock.

Front brakes are usually 2 or more pistons in a floating caliper, while the disk itself ranges from solid, to pitted and groved, to vented. Heat is dispersed even quicker in the front wheels due to the air rushing in from moving forwards. Even more so if you have alloy wheels.

Anyway its a bit off topic. I just find that the feel of brakes and pads outweights how powerful they are from the inital bite.

The way bikes are setup with the center of gravity chances are you'll end up over the bars with most braking systems...

 

[peasjam]

No, I said subject to equality of design and type, i.e. equal on the factors you mention.

Your comparison of drum and disc brakes is invalid for a whole host of reasons, in the same way that apples and oranges cannot be compared for superiority.

Except it's not is it; both are mechanical systems designed to exert a braking force on a rotating axle. It seems odd that you feel it's not reasonable to directly compare them, parrticularly as you seemed perfectly happy to compare rim brakes with discs which, again, are different mechanical solutions to the same problem.

My disc brakes have less than 0.5mm of piston travel in the calliper for around a quarter distance pull of the lever. This compares to around 3mm for the brand new Avid V brakes on my wife's bike. The disc system has a much greater mechanical advantage than the V brake which more than compensates for the smaller diameter of the rotor compared to the rim. There is no way you could set up a rim brake with such small tolerances unless you stopped to true your wheel every few miles, they can never have anything like the braking force of a good disc system.

 

[flecc]

Except it's not is it; both are mechanical systems designed to exert a braking force on a rotating axle. It seems odd that you feel it's not reasonable to directly compare them, parrticularly as you seemed perfectly happy to compare rim brakes with discs which, again, are different mechanical solutions to the same problem.

Completely untrue. One's shoe pressed on the front tyre exerts a braking force on a rotating axle. Would you really compare that with other braking systems.

There are a large number of physical differences between a drum and disc brake which crucially affect their materials and relative performances.

Conversely, a rim brake is physically just a larger diameter disc brake, very directly comparable and calculated for in identical ways.

 

[Blew it]

A little snippet of info, Manufacturers of fairground ferris wheels quickly realised the ferris wheel should always be driven or braked at the rim, never the hub.

 

[Scottyf]

Due to the mechanical advantage of a larger rim. Ie a Lever...

 

[lemmy]

This is all typical *******s that cycling as a religion folk get into. European law dictates that the max assisted speed of an electric bike is 15mph. That is not going to change. You can pedal and go faster if you wish, of course. Brakes on most modern bikes can cope with whatever speed you can pedal at. Or if you are into selling dodgy goods while on the move, peddle at.

All the rest is debating how many angels can dance on the head of a pin.

Pull on the lever. Does it stop you ok? Well that's all right then.

 

[peasjam]

Completely untrue. One's shoe pressed on the front tyre exerts a braking force on a rotating axle. Would you really compare that with other braking systems.

Where the question is "which is the best method of braking, pressing ones shoe against the tyre or a rim/hub/disc brake" then of course it's reasonable to compare the two. In fact a comparison is necessary to answer the question.

Conversely, a rim brake is physically just a larger diameter disc brake, very directly comparable and calculated for in identical ways.

Yes it is calculated in identical ways, essentially it's all just levers, but obviously you haven't bothered to do that. I guess this is either (a) because just having a row is more important to you than the actual truth of the matter (b) you have by this point figured out that you're not actually correct in this case but are unwilling to man up and admit as much.

The velocity ratio between the lever and pad of a good hydraulic disc system is around 30/1 versus 10/3 for a decent V brake. These are actual measurements, not just figures pulled out of thin air. This means that the force applied by the disc is approx ten times that of the V brake.

I'm running a 203mm front disc on a 700c wheel which is 622mm diameter, meaning the braking area of the disc is approx 1/3 of the diameter. It only needs to exert three times the force to counter the effect of braking at 1/3 of the wheel diameter. Even on a more usual 160mm disc you still only need just under four times the force.
Clearly the extra force the disc system has more than makes up for the disadvantage it has with the lower diameter braking surface.

In reality you don't get such a large increase in braking power - the pads on a disc system are much, much harder than a rim block so you lose a bit of stopping power.

So there you go. Ignore it, disbelieve it, whatever. It's my last response on the subject, for after all if someone is adamant that 2+2=5 then there isn't anything that anyone can do about it.

 

[flecc]

Once again Peasjam, I said all things being equal.

That means hydraulic rim brakes like the Maguras. Of course cable operated V or caliper brakes wouldn't stand a chance of competing, no argument on that.

Why is it that so many cannot post disagreement without getting insulting and personal? Once again I get accused in abusive terms of looking for an argument when all I've done as ever is post my opinion on a subject in an entirely polite and reasonable manner.

So you disagree! Ok, just say so without the personal attack and let others judge for themselves.
.

 

[peasjam]

Once again Peasjam, I said all things being equal.

That means hydraulic rim brakes like the Maguras. Of course cable operated V or caliper brakes wouldn't stand a chance of competing, no argument on that.

.

I wasn't aware you could get hydraulic rim brakes so this is probably worth a reply;

It's not the hydraulics that makes the difference, it's the fact that you can set a pad in a disc calliper a quarter of a millimetre away from the disc and it not rub. There is no way you could do that with a rim brake, hydraulic or otherwise. This is what allows the 30/1 velocity ratio over the 10/3 VR of the rim brake, and that is what gives you more braking power. Unless the clamping force of the hydraulic rim brake is none linear then it will, to a high degree, suffer from the same lower VR as, for example, a V brake.

The rough calculations in my previous post already assume parity between systems - brake area, pad compound and both assume no energy lost in the system (i.e not taking into account the benefits of hydraulics at all). I could have written it about a cable-based disc system and it would still be entirely true.

You can keep moving the goalposts on this all day my friend, working out forces on basic lever systems has it covered.
Incidentally, I find the biggest problem with online specialist forums is people posting their opinions as fact when the reality is they don't really know.

 

[lectureral]

Personally, I have a game (both in car and on bike) where I try to use the brakes as little as possible - got about 350 km on the road between Paris and Geneva without touching them a couple of weeks ago. My anticipation beats your fancy brakes a lot of the time.

 

[Scottyf]

I think cycling makes you a better driver. I've always said that about motorcycling.

However cycling has taught me patience and using the brakes less and generally not being so much in a rush. I believe its made me a nicer kinder driver.

 

[tillson]

I wasn't aware you could get hydraulic rim brakes so this is probably worth a reply;

It's not the hydraulics that makes the difference, it's the fact that you can set a pad in a disc calliper a quarter of a millimetre away from the disc and it not rub. There is no way you could do that with a rim brake, hydraulic or otherwise. This is what allows the 30/1 velocity ratio over the 10/3 VR of the rim brake, and that is what gives you more braking power. Unless the clamping force of the hydraulic rim brake is none linear then it will, to a high degree, suffer from the same lower VR as, for example, a V brake.

The rough calculations in my previous post already assume parity between systems - brake area, pad compound and both assume no energy lost in the system (i.e not taking into account the benefits of hydraulics at all). I could have written it about a cable-based disc system and it would still be entirely true.

You can keep moving the goalposts on this all day my friend, working out forces on basic lever systems has it covered.
Incidentally, I find the biggest problem with online specialist forums is people posting their opinions as fact when the reality is they don't really know.

I'm aware of the principles of levers, coefficients of friction, force, area, torque and all that sort of thing. I am also aware that if I squeeze my front brake lever hard in dry weather, my rim brakes will lock the front wheel thus pitching me over the handlebars and I will land on my head. If I carry out the same procedure in wet, muddy or gravely conditions, this time my rim brakes will lock the front wheel, it will slide from underneath me and I will land on my face. If the procedure is then repeated with the rear rim brake, it will convert my expensive Marathon Plus tyre into an impressive black line on the road.

All of the above leads me to wonder why I would want to do any of these things more efficiently with a disk brake. Can't see the point personally.

 

[NRG]

...unless its about feel.

I had typed out a long reply but darn internet is still playing up.

I can't work out why the relationship between pad rim /disk clearance is critical. I have HS33's on my PCS and the distance is only a few mm but they can stop me on a penny and yet the Avid V's on my Alien can also do the same, locking the front wheel and lifting the rear....ok in the dry admittedly.

The centre pulls on my Peugeot are on no way inferior to the Avids for stopping power but don't have the same feel and require quite a hefty pull but they still lock the wheels when required, so is this just down to lever / movement ratio?

So I'm a little puzzled...

 

[flecc]

I wasn't aware you could get hydraulic rim brakes so this is probably worth a reply;

It's not the hydraulics that makes the difference, it's the fact that you can set a pad in a disc calliper a quarter of a millimetre away from the disc and it not rub. There is no way you could do that with a rim brake, hydraulic or otherwise. This is what allows the 30/1 velocity ratio over the 10/3 VR of the rim brake, and that is what gives you more braking power. Unless the clamping force of the hydraulic rim brake is none linear then it will, to a high degree, suffer from the same lower VR as, for example, a V brake.

The rough calculations in my previous post already assume parity between systems - brake area, pad compound and both assume no energy lost in the system (i.e not taking into account the benefits of hydraulics at all). I could have written it about a cable-based disc system and it would still be entirely true.

You can keep moving the goalposts on this all day my friend, working out forces on basic lever systems has it covered.
Incidentally, I find the biggest problem with online specialist forums is people posting their opinions as fact when the reality is they don't really know.

At no point have I moved the goalposts Peasjam, in fact I've repeated my original qualifications of the conditions three times now, so your accusation is untrue, and followed by an uncalled for implied insult. As for "don't really know", it seems I do have a fuller experience than you. I've been through the cycle, motorcycle and motor trades and am a fully qualified motor engineer.

Early on you made a ludicrous comparison between a rear drum brake and a front disc brake, so it's little wonder that I doubted the credibility of your arguments. Now you've reinforced my doubts by saying you were unaware of the existence of hydraulic rim brakes, after earlier saying your judgement was from experience of both types. Clearly any rim rake experience you had was of cable brakes which I've already said could not possibly compete.

I wouldn't disagree on the VR of hydraulic disc brakes, but do on the rim brakes you were unaware of, them being much less different than you assume. There is still a small VR advantage for the disc, but the much smaller difference is in my opinion easily offset by the much greater grip of the rim pads which you've already acknowledged.

The webpage below will show you from the illustrations how the Magura and similar hydraulic rim brake piston calipers very closely emulate the caliper conditions and operation of disc brakes. In fact for most cycling applications they are a considerable overkill in performance terms, but people like them for their smooth and light operation:

Magura Hydraulic Rim Brakes

 

[indalo]

Have 4 bikes:

1) MTB equipped with discs front and rear

2) Ditto

3) Tourer 24spd derailleur and rim brakes front/rear

4) Ebike with hub brakes front & rear


Stopping ability, best to worst: 3 : 1&2 : 4

That's my experience on my current fleet. Perhaps interestingly, the best brakes are on the cheapest bike and the poorest stoppers are on my ebike.

Curiously, although my hub-braked ebike requires a greater squeeze than the other bikes to stop it, the brakes are very smooth and progressive in all weather and I've come to appreciate them. It should be noted that the ebike is a clear 12 Kgs heavier than my other bikes so that has some bearing on the performance.

Indalo

 

[flecc]

I can't work out why the relationship between pad rim /disk clearance is critical. I have HS33's on my PCS and the distance is only a few mm but they can stop me on a penny and yet the Avid V's on my Alien can also do the same, locking the front wheel and lifting the rear....ok in the dry admittedly.

so is this just down to lever / movement ratio?

Yes it basically is. The lever movement ratio means a loss of mechanical advantage in the form of pressure increase with greater distances between the two friction surfaces. With cable V and caliper cable operated brakes, this typically greater distance has the potential for lesser performance, though as you've said, some compensation can be given by exerting greater lever pressure if the lever to bar space and rigidity permit.

There is an academic element to all this of course, as you've observed. Either type can exceed safe operating levels in some circumstances

 

[Scimitar]

Interestingly, I found that by simply replacing my cantilever brakes with Avid V-brakes, whilst still using the original bar lever, I suddenly had a system that would throw me over the bars if I wanted. Honestly, I've never had such brakes on a bike before. Then I found there's a small attachment which I should have bought to reduce the mechanical advantage - I didn't bother with that, as I've got quite used to them now.

 

[amigafan2003]

I have a full sus mtb with Deore 2011 hydraulic disc brakes front and rear - 180mm disc front, 160mm discs rear - they are by far the best brakes I've ever had on a bike in all categories - power, feel, modulation and effectiveness in all weathers.

I have a Trek FX 7.5 with avid v-brakes and it's a close second - almost as good as the XT combo I had on the mtb before.

For me, disc brakes are a must on an mtb due to the adverse conditions they have to run in. For my commuter, good v-brakes are fine and worth the slight degredation in performance for the weight benefits.