Why do bigger rotors stop you quicker? (physics)

Anonymous

It would need to be like a drag racing funnycar for the front brakes to become ineffectual.
Which means that the front wheel would have a tiny amount of friction under normal circumstances, which makes it a terrible, and frankly stupid and dangerous design of bike.

Bringing something that is broken by design into this solves nothing. It's like someone defending their use of the rear brake by saying their front is broken. Well, sure, ok, but that does not make the rear brake a better thing to stop yourself with, it just means that in your particular case, something is wrong.

rhext

yeehaamcgee wrote:

It would need to be like a drag racing funnycar for the front brakes to become ineffectual.
Which means that the front wheel would have a tiny amount of friction under normal circumstances, which makes it a terrible, and frankly stupid and dangerous design of bike.

Bringing something that is broken by design into this solves nothing. It's like someone defending their use of the rear brake by saying their front is broken. Well, sure, ok, but that does not make the rear brake a better thing to stop yourself with, it just means that in your particular case, something is wrong.

Yep, that's right!

Anonymous

Gizmokev wrote:

A lot of that has been plagiarised from a physics forum

So. You have so little knowledge, that you resort to using posts from another forum. Yet, with your limited knowledge, you think you know better than Sonic?
And what, pray tell, happens, when this same argument happens on another forum somewhere, and they use YOUR posts to prove their (incorrect) point.
Are we to believe that verbatim too?

If you know that little, then stop arguing, and start learning. Or at least start asking questions instead of bringing in incorrect assumptions.

hobbitsharley

yeehaamcgee wrote:

hobbitsharley wrote:

yeehaamcgee wrote:

hobbitsharley, I have to say, I'm finding it very hard indeed to visualise a bike so odd in shape that the front wheel has so little weight over it that it cannot create sufficient traction to be an effective brake. Once the bike is decelerating, the weight transfer to the front wheel would just reinforce it. I think Rhext is right. I think it's psychology.

sent you pm from EBC

Yes, a page from EBC discussing crap design (more powerful brakes on rear on big custom bikes), and the rider's decision to use the rear brake.
Ultimately meaning nothing, except that it's expected that riders and designers of custom bikes don't really know what they're doing.

correct and having to change my riding style to cater for my bike choice - although changes are afoot to rectifie this design issue

hobbitsharley

rhext wrote:

yeehaamcgee wrote:

hobbitsharley, I have to say, I'm finding it very hard indeed to visualise a bike so odd in shape that the front wheel has so little weight over it that it cannot create sufficient traction to be an effective brake. Once the bike is decelerating, the weight transfer to the front wheel would just reinforce it. I think Rhext is right. I think it's psychology.

If you had a chopper, with the front wheel right out in front, heavy engine plus rider effectively over the rear wheel, that would rely on rear wheel braking.

check my Sig pic - heavily modified HD Fat Boy 725lbs of bad braking but an absolute riot to ride once you get used to it - i almost crashed on collection as i yanked front brake and almost hit the dam car in front

Anonymous

hobbitsharley wrote:

check my Sig pic

:?
I despise those things. All the downsides of a bike, with none of the upsides of a bike. And that wheelbase isn;t particularly long. I see no reason from a cursory glance why the front brake should be as bad as you make out. That's a LOT of weight, and a lot of it is going to want to go forwards, loading the front tyre, when decelerating.

But I can't imagine I'd even agree on the colour of the sky with a Harley fan. Something to do with those rose coloured glasses they look at the world through.

supersonic

A logical question does arise - if a smaller rotor needs more lever effort and pad pressure to provide the same torque about the hub axle, where does the extra force and energy go?!

Consider the diagram and text I posted above:

The relevant forces and dimensions on the wheel are indicated. We have the braking force B and the ground reaction force R acting at the contact patch. The disk calliper exerts a force D as indicated, tangential to the disc at the point of calliper contact. The radii of the disk and wheel are r1 and r2, and finally the angle of the dropout exit is 'a' in front of vertical. For this fork and calliper, the force D is virtually vertical. If it wasn't, there would be another angle 'b' for the angle the disk force makes behind the vertical.

Now for the simple sums. Let's assume we have a bike + rider weighing 90kg in total, braking hard and decelerating at 0.6g (6m/s^2) with the front brake alone (this is a reasonable estimate for maximal braking). The rearward force is 90 x 0.6 x g = 540N, and the vertical reaction force is 90 x g =900N (all the weight is on the front wheel). Taking moments around the axle, the force D exerted by the disk is given by D = 540 x r2 / r1. Here r2 = 13 x 25.4mm is the radius of the wheel and r1 = 72.5mm is the effective radius of the disk (ie to the centre of the force at the pads, rather than the outer edge = 10mm less than the full radius of 165mm / 2). So D = 2460N, acting vertically downwards. This acts entirely on the left hand side of the wheel, but the ground reaction force and braking forces are split equally between the two sides. So we are left with resultant forces of 2460 - 900/2 = 2010N vertically down and 540/2 = 270N rearward. The sum of those is equivalent to a single force of 2030N acting downwards at an angle of 8 degrees behind vertical. Resolving this parallel to the dropout opening angle (= 18 degrees ahead of vertical for a head tube angle of 72 degrees) leaves a force of 2030N x cos(18+8) = 1825N out of the dropout on the left hand side!

Working through the maths it can be seen that a smaller rotor provides a LARGER downwards force into the axle/dropout (or to be precise a tangent from the rotor where the discs grap it).

cooldad

yeehaamcgee wrote:

Gizmokev wrote:

A lot of that has been plagiarised from a physics forum

So. You have so little knowledge, that you resort to using posts from another forum. Yet, with your limited forum, you think you know better than Sonic?
And what, pray tell, happens, when this same argument happens on another forum somewhere, and they use YOUR posts to prove their (incorrect) point.
Are we to believe that verbatim too?

If you know that little, then stop arguing, and start learning. Or at least start asking questions instead of bringing in incorrect assumptions.

Hate to resurrect the past but does fuel saving snake oil salesman ring a bell. A tenuous grasp of physics would appear to be common.
http://www.bikeradar.com/forums/viewtop ... &start=200

hobbitsharley

yeehaamcgee wrote:

hobbitsharley wrote:

check my Sig pic

:?
But I can't imagine I'd even agree on the colour of the sky with a Harley fan. Something to do with those rose coloured glasses they look at the world through.

good job im not your a typical HD fan then as i love winding them up by taking there beloved bikes and turning them into fire breathing monsters and riding the bejesus out of them - oh and getting them dirty and riding in the rain which im sure it said voided the warranty or something like in the manual

Anonymous

Oh but you are. You just haven't accepted it. You're even turned a thread about the physics of brakes into a harley thread. Must be all those torques going to your head.

The Rookie

supersonic wrote:

The same way a larger rotor has more rotor pass through the pads in a revolution. Each rev takes the same time... so the rotor is moving faster at the edge than a smaller one.

All irrelevant though to friction which is not dependant on speed or 'how far'.....

Simon

supersonic

It is relevant to kinetic energy being converted into heat though.

Gizmokev

@ Cooldad

WTF....I still stand by what I have wrote re HHO. I chose to apologise for spamming my company in the thread. Now I suggest you move on down the bus.

@ Yeehaa

You really try hard to be a wind up merchant but your powers of frustration will not work on me.

supersonic

Unless I have misread the equation, this indicates that distance x is important to energy turned into heat:

http://en.wikipedia.org/wiki/Friction

Coeff of friction, and force have not changed.

Anonymous

Gizmokev wrote:

@ Yeehaa

You really try hard to be a wind up merchant but your powers of frustration will not work on me.

What the fark is up with this. Any time I make a point these days, everyone says "oh, lol, good trolling".
Sorry, but that's not a defence for being a moron.

Gizmokev

yeehaamcgee wrote:

Sorry

Your apology is accepted

supersonic wrote:

Unless I have misread the equation, this indicates that distance x is important to energy turned into heat:

http://en.wikipedia.org/wiki/Friction

Coeff of friction, and force have not changed.

Going by the link that you have posted up it would suggest that coef of friction does change.

Wiki wrote:

While it is often stated that the COF is a "material property," it is better categorized as a "system property." Unlike true material properties (such as conductivity, dielectric constant, yield strength), the COF for any two materials depends on system variables like temperature, velocity, atmosphere and also what are now popularly described as aging and deaging times; as well as on geometric properties of the interface between the materials.

So with that in mind with the increase of tangential speed being in the region of 27% as well surely there will be a decrease in the coef of friction by the increase of velocity (being as the coef of friction is not a calculable value who knows by how much it is changed). Taking this further if everything in the system remains constant (as in rider, bike, brake calipers) surely there will be a reduced braking force or torque effect because this is reliant upon the coef of friction to find out the force being applied.

Hence I still stand by my original statement that a larger disc does not make for a larger braking force at a given lever force but it is merely aimed at the dissipation of heat to increase braking efficiency.

The Rookie

I think Harleys have no decent front brake as the forks and frame are so squdgy and the handling so bad its safer to allow for a locked up rear than a locked up front!

Simon

hobbitsharley

The Beginner wrote:

I think Harleys have no decent front brake as the forks and frame are so squdgy and the handling so bad its safer to allow for a locked up rear than a locked up front!

Simon

like your style, but they have got better since the days Hayes made there brakes

nozzac

Gizmokev wrote:

[

Hence I still stand by my original statement that a larger disc does not make for a larger braking force at a given lever force but it is merely aimed at the dissipation of heat to increase braking efficiency.

Do you also go around denying that it's harder to open a door by pushing it near the hinge?!

Anonymous

Gizmokev, now you've crossed the line into "being a prick" territory. Well done, good job.

Gizmokev

yeehaamcgee wrote:

Gizmokev, now you've crossed the line into "being a prick" territory. Well done, good job.

FFS lighten up kid this is only a forum...surely when we left the playground at 16 we stopped name calling.

Gizmokev

NozzaC wrote:

Do you also go around denying that it's harder to open a door by pushing it near the hinge?!

No because if you read my posts it clearly states that I accept that the torque is increased (although this is in theory only given my post about the coef of friction changing as a result of velocity) by virtue of the fact that it is further from the axis of rotation but that the amount in this case is not as everyone is saying and that there are many more factors in play than have been given credit.

cooldad

Gizmokev wrote:

@ Cooldad

WTF....I still stand by what I have wrote re HHO. I chose to apologise for spamming my company in the thread. Now I suggest you move on down the bus.

And I still maintain it is garbage. 100% of the worlds automotive engineers agree with me.
If I'm wrong find a manufacturer who is using the 'technology' to give their vehicles massive and cheap fuel saving.

cooldad

I think the thread derail has now been derailed more. My apologies.

supersonic

Coeff of friction with some pads does change as the system heats up very excessively (fade), that is true, and is a reason why people use larger rotors to aid heat dissipation.

Also all the brake manufacturers agree that larger rotors give more stopping power for a given lever force. The theory and experimentation add up.This has been proven on our Dynos.

Hence I still stand by my original statement that a larger disc does not make for a larger braking force

This is fundamentally wrong. The tangetial speed is what makes x larger in that equation. In the same unit time the extra speed has increased x. More energy converted to heat per unit time.

As before, extra heat dissipation is a second reason for larger rotors. Or a first for some!

nozzac

"No because if you read my posts it clearly states that I accept that the torque is increased "

How does that square with "a larger disc does not make for a larger braking force at a given lever force but it is merely aimed at the dissipation of heat to increase braking efficiency."

They appear to be opposite statements.

supersonic

Hayes:

http://www.hayesdiscbrake.com/hayes-u/

nozzac

supersonic wrote:

Coeff of friction with some pads does change as the system heats up very excessively (fade), that is true, and is a reason why people use larger rotors to aid heat dissipation.

Also all the brake manufacturers agree that larger rotors give more stopping power for a given lever force. The theory and experimentation add up.This has been proven on our Dynos.

Hence I still stand by my original statement that a larger disc does not make for a larger braking force

This is fundamentally wrong. The tangetial speed is what makes x larger in that equation. In the same unit time the extra speed has increased x. More energy converted to heat.

As before, extra heat dissipation is a second reason for larger rotors. Or a first for some!

Quite. We all accept that larger discs have an advantage in heat dissipation AS WELL AS the main reason...that being the undeniable decrease in clamping force required.

Gizmokev

supersonic wrote:

Hayes:

http://www.hayesdiscbrake.com/hayes-u/

Yep its clearly states in there that

Hayes wrote:

Rotor Diameter or effective radius of the rotor: Larger diameter rotors have longer torque arms and can generate more brake power with the same amount of clamp force than a smaller diameter rotor.

However it also states

Hayes wrote:

Coefficient of Friction (µ – pronounced “mew”) – A number measuring the “grip” of a material used in brake pads. Coefficient of friction can vary depending on the type of material used for the brake rotor. Typically service brakes are concerned with dynamic coefficient of friction, or the coefficient of friction measured while the vehicle is moving. The coefficient of friction may change as the brake system is required to perform through different applications. Below are a few of the main characteristics. Depending on the desired performance, the characteristics can be minimized or maximized.

Speed Sensitive – Coefficient of friction typically drops as the speed of the vehicle increases.
Pressure Sensitive – Coefficient of friction typically drops as more clamp force is generated.
Temperature Sensitive – Coefficient of friction typically drops as the temperature of the brake system increases.

So yes a larger disc should in theory give a larger braking force but as I have been trying to explain all along there are other factors in this which reduce the argument on the effectiveness of the larger disc increasing braking force, the 2 from above being temp and speed because as we have established the pressure will remain constant.

supersonic

Yes, we know the system can degrade, ie fade. In fact, with the increased heat dissipation AND less lever effort, the two help each other, not oppose!

Pressure Sensitive – Coefficient of friction typically drops as more clamp force is generated.

You need MORE clamp force with smaller rotors.