R1250GS calliper bolt length

[Daggerit]

For a bike being slowed by its brakes, the amount of energy (e) that is converted to heat can be expressed as:

e = (mV12) - (mV22)​

where m is the mass of the bike; V1 is the speed of the bike before braking; V2 is the speed of the bike after braking. Brake disc size doesn't enter the equation.

It should be noted that the function is not linear. The heat generated slowing from 120mph to 100mph is massively more than slowing from 30mph to 10mph.

I think that if stainless was best for brake components, they would use in OE in production.

I'm sorry but I'd say that logic is completely wrong on two levels.

1- The part that we're discussing factors into how much work the braking system has to do to get the bike from V1 to V2
2 - You're equating the kinetic energy loss to 100% heat from the front brake

For the first point, I'd hope you'd agree that if you could apply less pressure between the pads and the disc it would mean that there would be less heat i.e. for the same brake setup slowing at 5m.s^-2 would generate less heat than the same setup slowing at 15m.s^-2. The force required to be applied to the pad for a given torque (constant deceleration) would be F=T/(2.u.r) where T is torque, F is the force on the pads from the caliper, u is the coefficient of friction and r is the radius to the centreline of the pad. That comes from T=F.r with the coefficient of friction thrown in because the force applied between the pad and the disc is reliant on it. You can see there that if you make r larger then F will decrease, meaning less squeeze required on the pads and therefore less heat (hence my question about the different rates of deceleration previously). Unless you think I'm missing something?

For point two there's engine braking, the rear brake, friction and all sorts to take into account as well as the brakes not converting all the kinetic energy to heat so the formula isn't really all that accurate when attributing the amount of energy scrubbed by the front brakes for a given deceleration.

Lastly, for the fasteners I imagine that that stainless is probably a very good option but the cost versus coated carbon versions makes it the more appealing option when trying to make money. I'd say titanium fasteners for everything would be best for all fasteners on the bike but that would make things significantly more expensive and that's where the trade off comes in.

 

[Greg Masters]

The force required to be applied to the pad for a given torque (constant deceleration) would be F=T/(2.u.r) where T is torque,

That's the bit which is wrong - the 'torque' is not constant.

2 - You're equating the kinetic energy loss to 100% heat from the front brake

For point two there's engine braking, the rear brake, friction and all sorts to take into account as well as the brakes not converting all the kinetic energy to heat so the formula isn't really all that accurate when attributing the amount of energy scrubbed by the front brakes for a given deceleration.

I was careful to say:

For a bike being slowed by its brakes

(but drag, engine braking etc are constants in relation to changed front disc size).

Without doubt, bigger discs give more control over your braking and better heat dissipation. But you will need to ensure that the brake pads stay at a temperature within their good operating window.

Good luck with it all.

 

[Daggerit]

That's the bit which is wrong - the 'torque' is not constant.



I was careful to say:
(but drag, engine braking etc are constants in relation to changed front disc size).

Without doubt, bigger discs give more control over your braking and better heat dissipation. But you will need to ensure that the brake pads stay at a temperature within their good operating window.

Good luck with it all.

I was just assuming that there's a maximum torque that can be applied by the system before the tyre will lose traction and that will be relatively constant for a given situation, no? You could use less but then you're not slowing as fast as possible I guess.

I'm thinking that some fast road pads should be ideal for what I use the bike for and shouldn't need any real heat to be in them to work, so will hopefully be a good option.

Thanks for the discussion, enjoy things like this! Haha.

I'll post up the final results once I get things organised. I might do one track day or run a section of road quickly so that I have something to compare to and can actually give some feedback - hopefully it won't be 'it feels exactly the same and I've wasted my money'!

 

[fred_jb]

I'm sorry but I'd say that logic is completely wrong on two levels.
You ca
1- The part that we're discussing factors into how much work the braking system has to do to get the bike from V1 to V2
2 - You're equating the kinetic energy loss to 100% heat from the front brake

If, as already been pointed out, you ignore things which are the same between the situation with smaller and larger disks, which you should, then the different kinetic energy of the bike before and after braking needs to be accounted for in terms of where that energy has gone. You cannot just ignore the Laws of Thermodynamics - what happens must be in accordance with the law of conservation of energy which states that energy cannot be created or destroyed, but only converted from one form of energy to another.

If you don't believe that and think that with bigger brakes the bike can be slowed by the same amount in the same or shorter time period, yet with less heat energy being released in the brakes, then where does the "lost" kinetic energy which is not converted to heat go?

 

[Greg Masters]

The issue with effective wheel torque - ie the force that is trying to keep the wheel turning against the brake - reduces as the speed comes down - it's the squared bit in e = mV2

There is, of course, a max when the tyre locks up - which depends on all sorts of things - tyre/surface coef friction; front rear weight shift; lean angle etc etc

 

[fred_jb]

The issue with effective wheel torque - ie the force that is trying to keep the turning against the brake - reduces as the speed comes down - it's the squared bit in e = mV2

There is, of course, a max when the tyre locks up - which depends on all sorts of things - tyre/surface coef friction; front rear weight shift; lean angle etc etc

If you just think about it in energy terms, then I think that none of that stuff really matters, though of course if the wheel does lock up, then more of the energy will go into heating up the tyre (and the road) and breaking the chemical bonds in the rubber by ripping rubber off the surface of the tyre!

https://en.wikipedia.org/wiki/Conservation_of_energy

 

[tanneman]

Flush the brake system and use dot 5.1 brake fluid. Not dot 5. This has a higher boiling point. Brake fade occurs because the fluid is boiling.

 

[Greg Masters]

If you just think about it in energy terms, then I think that none of that stuff really matters, though of course if the wheel does lock up, then more of the energy will go into heating up the tyre (and the road) and breaking the chemical bonds in the rubber by ripping rubber off the surface of the tyre!

https://en.wikipedia.org/wiki/Conservation_of_energy

That's absolutely right.

By doing mechanical stuff you can raise the threshold before the tyre slips - stuff like ensuring the dampers will cope with imperfections in the smoothness of the tarmac and transferring rider mass forwards during braking etc

 

[Daggerit]

If, as already been pointed out, you ignore things which are the same between the situation with smaller and larger disks, which you should, then the different kinetic energy of the bike before and after braking needs to be accounted for in terms of where that energy has gone. You cannot just ignore the Laws of Thermodynamics - what happens must be in accordance with the law of conservation of energy which states that energy cannot be created or destroyed, but only converted from one form of energy to another.

If you don't believe that and think that with bigger brakes the bike can be slowed by the same amount in the same or shorter time period, yet with less heat energy being released in the brakes, then where does the "lost" kinetic energy which is not converted to heat go?

I do agree that there's no energy 'lost', but I'm just trying to figure out how less friction being applied could generate the same amount of heat. If that was the case then why even bother with different size discs, unless it's purely to dissipate more heat (surely not the only reason)? I liken it to using your palm to slow down something spinning. If you do it close to the centre you're going to have to press harder and will feel more heat, further from the centre you won't have to press as hard and will therefore feel less heat. Is it something to do with the actual amount of work done by the pads on the disc rather than a simple 'heat energy generated == kinetic energy lost during braking'?

 

[tanneman]

The limiting factor to all this physics is that if the coefficient of friction between the tyre and road surface remains the same then the torque applied to the brake rotor will stay the same regardless of the size of the friction surface or distance from the centre of rotation. Hence the energy transfer by means of heat to the brake pads and brake fluid will be the same. What you will experience with a larger brake disc is that less pressure needs to be applied to achieve the same result but in essence nothing changes because the limiting factor has not changed ie the grip between tyre and road surface.

BTW switch off the abs to get the best performance from the brakes.

 

[tanneman]

I do agree that there's no energy 'lost', but I'm just trying to figure out how less friction being applied could generate the same amount of heat. If that was the case then why even bother with different size discs, unless it's purely to dissipate more heat (surely not the only reason)? I liken it to using your palm to slow down something spinning. If you do it close to the centre you're going to have to press harder and will feel more heat, further from the centre you won't have to press as hard and will therefore feel less heat. Is it something to do with the actual amount of work done by the pads on the disc rather than a simple 'heat energy generated == kinetic energy lost during braking'?

Do not confuse heat dissipation of the brake disc with heat dissipation in the caliper.

 

[fred_jb]

I do agree that there's no energy 'lost', but I'm just trying to figure out how less friction being applied could generate the same amount of heat. If that was the case then why even bother with different size discs, unless it's purely to dissipate more heat (surely not the only reason)? I liken it to using your palm to slow down something spinning. If you do it close to the centre you're going to have to press harder and will feel more heat, further from the centre you won't have to press as hard and will therefore feel less heat. Is it something to do with the actual amount of work done by the pads on the disc rather than a simple 'heat energy generated == kinetic energy lost during braking'?

I think it is something to do with the fact that with a larger diameter disk you are sweeping the pads across a bigger distance per revolution of the wheel - if you increase the radius of the disk you also increase the circumference.

Therefore, even if you effectively have a bigger lever allowing you to exert less force for the same amount of retardation, that lower force with its resulting lower friction between pads and disk, is exerted across a bigger swept area per revolution, so the same total amount of work is being done, which translates into the same amount of heat.

If instead of using less lever pressure with the larger brakes you use the same as before, then you will get more retardation, assuming the tyres are up to it, and so more heat generated.

 

[Greg Masters]

I do agree that there's no energy 'lost', but I'm just trying to figure out how less friction being applied could generate the same amount of heat.

The rubbing distance/area is greater if you have a bigger disc.

 

[fred_jb]

Overall I think fitting larger diameter disks to a machine which already has enough braking power to overwhelm the tyre's ability to grip, and so activate the ABS, is just an expensive way to reduce brake lever effort, which is by no means excessive as standard anyway.

 

[Deleted account 191119001]

I've found that they fade under heavy road use and since I want to take it on some track days with friends I want to make sure it can stop.



It just works on the torque principal in that if you move where you're applying a given clamping force further away from the wheel centre you get a larger braking force overall. The same as using a breaker bar versus a little ratchet for torquing a bolt. It means that you can get the same braking effect with less overall heat being generated in the system basically, which should keep my brakes working more consistently for longer. That's the theory at least!

Perfect thanks for that simple explanation, now makes a load more sense to me..........

 

[tanneman]

Are you fookin thick? Have you examined the braking system on your bike and did you analyse the result of your repeated braking ie the brakes fade to determine the root cause? Regardless of what you do to the size of the brake disc it is still the same amount of energy you need to get rid of. Where does at energy go? In what form is it converted since we can't destroy it? Etc, etc.

Why does brakes fade? Because the brake fluid boils.
How does one stop this boiling of the brake fluid? A few solutions but in your application the answers are a bit limited,
1. Fresh brake fluid.
2. Different brake pad compound.
3. Different brake disc design or material.
4. Use a brake fluid with a higher boiling point.

Let us know how you get on. Would like to see how much bigger discs you can fit in that 19'' front wheel. As you have been informed previously.

Maybe I shouldn't rant but fuck me, this is the technical section. Use your brains.

 

[jetjock]

Don’t forget larger discs will increase the rotational kinetic energy so more braking effort required . I’d just put Brembo track pads in .

 

[Blackal]

Brakes fade due to heat-induced impairment of the pad/disc friction. Brake fluid will heat up locally, and may change phase (or included water will) and cause an increase in the volume of the now 2-phase fluid. This can push fluid out of the reservoir.

But - when brake pressure is re-applied, it returns to liquid phase due to the elevated boiling temperature of the increase in pressure.

Two-phase brake fluid only causes sponginess (and loss of fluid from the reservoir) - not brake fade. IMO

 

[Cliffbase]

Are you fookin thick? Have you examined the braking system on your bike and did you analyse the result of your repeated braking ie the brakes fade to determine the root cause? Regardless of what you do to the size of the brake disc it is still the same amount of energy you need to get rid of. Where does at energy go? In what form is it converted since we can't destroy it? Etc, etc.

Why does brakes fade? Because the brake fluid boils.
How does one stop this boiling of the brake fluid? A few solutions but in your application the answers are a bit limited,
1. Fresh brake fluid.
2. Different brake pad compound.
3. Different brake disc design or material.
4. Use a brake fluid with a higher boiling point.

Let us know how you get on. Would like to see how much bigger discs you can fit in that 19'' front wheel. As you have been informed previously.

Maybe I shouldn't rant but fuck me, this is the technical section. Use your brains.

An interesting topic that you have managed to drag down to the usual levels of UKGSER. Well played sir.
Do you find it so hard to communicate without swearing & ranting? You let yourself down.

 

[tanneman]

Brakes fade due to heat-induced impairment of the pad/disc friction. Brake fluid will heat up locally, and may change phase (or included water will) and cause an increase in the volume of the now 2-phase fluid. This can push fluid out of the reservoir.

But - when brake pressure is re-applied, it returns to liquid phase due to the elevated boiling temperature of the increase in pressure.

Two-phase brake fluid only causes sponginess (and loss of fluid from the reservoir) - not brake fade. IMO

Ah, but the sponginess at the lever is usually described as brake fade. The ABS pump would try to keep the pressure at optimum for the required effort. So more travel at the lever to get the same feel from the brakes I guess is the experience of the OP. No excuse for the ignorance that brake fade is the reduction in friction. Does it mean the OP has 2 problems or just 1? A change of brake pads and/or a change of brake fluid?

An interesting topic that you have managed to drag down to the usual levels of UKGSER. Well played sir.
Do you find it so hard to communicate without swearing & ranting? You let yourself down.

My apologies for such a shit post. Looking at my post written last night I would remove the first and last 3 sentences. If you read it as such then it is elevated above the usual UKGSer standard. But none the less, no excuse can be offered and I can't turn back time. I'm a slave to the words and will be judged accordingly.