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Professor Dingbat
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Discussion Starter #1
I just finished reading SCC's Ultimate Street Car Challengeand noticed that the second place car (a Supra) wore stock brakes fitted with Carbotech pads. This is interesting, as the next closest competitor was an almost identical, yet had 13 inch rotors, and required an extra 13 feet to come to a stop.

Let's discuss what it takes to have an effictive braking system.

First some physics. The purpose of the brakes is to convert the car's kinetic energy into heat at as fast a rate as possible. The formula used to describe the car's energy is:
KE=1/2MV^2
Well, as speed increases, the car's energy, and the work the brakes have to do skyrockets. Since we are interesed in speed, this is not something we are going to compromise on for increased braking performance (at least not in a track situation) So we can see that a reduction in weight will help reduce the amount of energy the brakes have to dissipate.

Pads and Rotors
A larger rotor allows for more brake torque, as torque is force about an axis and incresed distance allows for more torque. It also allows more mass and more surface area than a smaller rotor (given the same thickness) However, using a more agressive pad will also yield more torque in the form of more friction at the same distance from the axis. So theoretically, the same performance can be obtained by either using larger brakes and similar pads, or more agressive pads on the original size rotor. It would seem an obvious choice to use upgraded pads in favour of a big brake kit, since rotor size is only one part of the equation, and brake pad technology has the ability to compensate for this to a certain degree. The downsides of this are increased heat produced by the brakes that must be absorbed and dissipated. This is why we are lucky to have the NX brake upgrade availible to us, it's got decet availibility, and not very expensive, especially if you are already going to replace your pads and rotors.

Tires
When your brakes grip harder, there must be enough friction between the tires and the road to avoid lockup.The best brake setup in the world can't do much if the tires can't stop the car.
 

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A low Cg helps a lot at making the rear brakes more effective.

Getting the brake bias perfect and a great driver make a big difference.

Putting your tires at zero camber helps.

Brake feel helps the driver. Reducing compliance in all areas of the braking system from the pedal, lines, brake fluid, calipers, pads, rotor, etc helps brake feel.

You could probably shave a good distance off your own stopping distance by spending a day tweaking on all the compliances and bias of your braking system while practicing stopping from speed.
 

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Professor Dingbat
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Discussion Starter #3
That's a good point. I was thinking of getting a Wilwood adjustable proportioning valve and doing some tweaking.
 

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Captain Slow
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mtg said:
Brake feel helps the driver. Reducing compliance in all areas of the braking system from the pedal, lines, brake fluid, calipers, pads, rotor, etc helps brake feel.

You could probably shave a good distance off your own stopping distance by spending a day tweaking on all the compliances and bias of your braking system while practicing stopping from speed.
You have no idea how true that is. Every so often you run into a naive driver who just raves about the merits of stickier tyres and completely ignores the rest of the system or the human factor. They're worse than the people who think drilled B14 SE-R sized rotors help with cooling.
 

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Professor Dingbat
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Discussion Starter #5
What do you guys mean by tweaking the compliances?
 

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I run Mintex M1166 friction material on stock sized Brembo rotors and stock calipers with Elf XT3165 fluid. I run them on all corners of the car in the interests of brake balance as the car runs the factory bias arrangement. This setup will stop consistently on the track all day long. It works ok on the street too, apart from the copious black dust and a little brake squeal when the pads get low.

For those not familiar with heat speak, pay particular attention to the meaning and my use of the terms "radiate", "conduct" and "convect".

The one thing I might consider changing is the rotors. The one piece rotors are up to the task alright, but they conduct a lot of heat into the rims via the mating faces at the hub.

I think a two piece rotor would all but stop this - but then it would probably run hotter since the wheel is no longer acting as a big heat-sink, with added surface area for continual dissipation.

Last track day I took tire pressures immediately after coming off the track (yes I did a cooldown lap) and again a 1/2 hour later (during which time the car was in the pits) and the pressures hadn't changed. Therefore the wheel/tire temperature hadn't changed and were still hot enough to leave a burn after a few seconds contact.

I have a video clip of a rally car doing a stage with a magnesium wheel on fire. Eventually the car stopped and the fire gutted it. I always wonder whether this was caused by heat tranfered from the the rotors to the rims, or if something (like a caliper maybe?) came adrift and contacted the wheel.

With a smaller rotor, the inside diameter of a given wheel is further away from the disc. Knowing that with radiated heat, temperature is inversely proportional to the square (or was that cube?) of the distance from the source, a smaller diameter rotor will radiate less heat to the wheel than a large one. The smaller rotor also benefits by have more air around it to convect the heat away.

On the other hand, a one piece rotor will conduct more heat to the wheel than a two piece rotor. However, a large one piece rotor will both conduct heat to the wheel and radiate more heat to the wheel.

If wheels can be safely used as a heat sink, as with one piece rotors, then perhaps larger rotors are not necessarily the best option.

I guess it comes down to this:

1) Will the wheel get hot enough to ignite the magnesium alloy?

2) Will the wheel get hot enough to damage the tire?

3) Will the hub get hot enough to damage the wheel bearings.?

My thought for the day.....
As long as your brakes can consistently produce more torque than your tires can hold, you have enough braking power.

Great topic - look forward to seeing other people's opinions. :)
 

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FastNX said:
What do you guys mean by tweaking the compliances?
See his first post.

Basically it is minimising all the wasted movemnt between pedal application and pad/disc contact.

e.g.

better brake hoses
adjusting pedal freeplay
using a good fluid (i.e less compressible and stable at elevated temps)
tossing out backing shims?

On another note.....

I've heard of a mod which involves drilling a small hole or cutting a small notch in the part of the caliper piston that contacts the disc. The theory is that air held captive between the piston and pad gets superheated and thus transfers heat to the piston and fluid. The small hole or notch allows the air to expand out of the hole as it heats instead of building up pressure and temperature.

Anyone have any experience or opinion on this?
 

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Professor Dingbat
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Discussion Starter #8
got it. I really could not think of anything besides stainless brake lines. I guess you're right about the shims too. I run shims on my street setup but not on the track setup so I guess i've already got that covered.

Having friction material also helps with feel. I was running these pads for about 10 laps not knowing how bad they were. No feel at all, i had to really push the pedal HARD to get the car to slow down
 

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Mate, that pic is scary - 10 laps :eek:

I had a similar experience with some cheap pads. They had plenty of material left, but just wouldn't bite because I WAAAAAY overheated them. I was pushing down with my foot and pulling on the steering wheel as well to get more weight on the pedal. When the car finally stopped smoke was billowing from all four corners.

Speaking of feel, which leads to the ability to modulate the brakes, the plot shape of the friction material plays a big part also.

There's an explanation of plot shape here. The page numbers in the top left of that link go to tables comparing the various attributes of different materials - some useful info there.

The plot shape on my pads isn't the most driver freindly - luckily I have ABS to save my butt.
 

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sleeper
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radiation is actually a function of temperature to the fourth. I'd think a bigger rotor would have better heat tranfer characteristics due to greater surface area. This should improve convection and radiation of heat from the disk. Since air is always flowing over/thru the wheels/brakes, I don't think the "smaller rotor has more air to flow over it" idea is valid. I've heard that the bigger brake kits are easier to modulate than smaller brakes. Part of it might be the ability to use a less agressive/organic pad. One thing for sure is that less brake pedal force will be required to obtain the same braking force as compared to a smaller rotor. This might be why it's easier to modulate.
 

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Thanks, I'm a bit rusty on the radiation formulae.

Perhaps I didn't explain what I meant so well. I'll try again.....

The mass of air at any given time between rotor and rim will be larger with the smaller rotor. Assuming that radiated heat transfer occurs almost instantaneously in a transparent medium like air - for the same quantity of heat radiated, this larger mass of air will not rise as much in temperature as a smaller mass of air. Thus we have lower temperature air surrounding the smaller rotor.

Now if we look at the rate of convective heat transfer - it's proportional to the temperature difference between the air and rotor. So while the larger rotor may have a larger surface area to transfer heat to the air, the smaller rotor benefits by transfering across a larger temperature difference.

Also, with a larger distance to the inside of the wheel, less of the radiated heat is reflected back to the rotor.

What this all means is that whatever size rotor you use, the rate of heat output will eventually balance with the heat input by virtue of arriving at a temperature difference that balances the equation.

Take F1 brakes for instance, they have a lousy co-efficient of friction which they overcome by using high operating forces to squeeze the pads on. So why do they use these carbon fibre materials then - because they can handle temperatures high enough to get the temperature difference (and hence rate of heat transfer) that they need with a package that has to fit inside a ridiculously small wheel.

Personally I think that a considerable amount of heat is transfered by conduction to the wheel at the hub mounting surfaces, when using a one piece rotor. All of the surface area of the wheel itself then becomes involved in the transfer of heat to air.

Therefore, when using a larger rotor, what percentage of a gain in surface area are you really getting when the heat transfering surface area of the wheel is taken into account.

Conversely, using a two piece rotor to minimise conductive heat transfer to the wheel, all of the heat rejecting surface area of the wheel has just been lost. In this case, once the rotor has heat soaked, is it really rejecting heat faster by virtue of more surface area - or by running at a higher temperature? I speculate that it is the latter.
 

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Captain Slow
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T4 Primera said:
The plot shape on my pads isn't the most driver freindly - luckily I have ABS to save my butt.
I donno if that plot shape bit is really that accurate. It is true that pads with a tendancy to grip more at lower speeds are hard to modulate, but then again, any pad which has a "speed sensitive" feel to it isn't very easy to use. I think the only reason people would find the pads with the #5 plot easier to work with is that people instinctively tend to increase the pedal pressure after they start braking. I'm used to making very few adjustments in pedal pressure after the initial application, so I would say the pad with the "flattest" plot would be the most user friendly. After learning how to use their brakes properly (at the limit), several of my friends now say the same thing as well.
 

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Captain Slow
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spdracerUT said:
I've heard that the bigger brake kits are easier to modulate than smaller brakes. Part of it might be the ability to use a less agressive/organic pad. One thing for sure is that less brake pedal force will be required to obtain the same braking force as compared to a smaller rotor. This might be why it's easier to modulate.
Part of it could also be attributed to the fact that big brake kits usually come with different calipers as well. I'm not disagreeing in any way (in fact, I agree with you), I'm just saying that that's another factor that needs to be taken into consideration if you use the case of brake kits.
 

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reverm said:
I donno if that plot shape bit is really that accurate. It is true that pads with a tendancy to grip more at lower speeds are hard to modulate, but then again, any pad which has a "speed sensitive" feel to it isn't very easy to use. I think the only reason people would find the pads with the #5 plot easier to work with is that people instinctively tend to increase the pedal pressure after they start braking. I'm used to making very few adjustments in pedal pressure after the initial application, so I would say the pad with the "flattest" plot would be the most user friendly. After learning how to use their brakes properly (at the limit), several of my friends now say the same thing as well.
According to the link I provided, the plot shape for my pads is a 2 which rises slightly. I find that I need to lift off a bit as I plunge into the braking zone - entirely unnatural. I use the ABS activating as my cue to back off.

You could be right though, maybe I actually am applying more pedal as I go without being concious of it. Then when the ABS tells me to let up, maybe I'm putting it down to plot shape rather than unconciously applying more pedal.

I have another track day coming up soon - I'll try to watch out for unconcious pedal pushing :)

Please don't take my dissertations on thermal dynamics above too seriously. I was just speculating at 4am on a nightshift and am more than happy to be corrected. :cool:
 

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reverm said:
Part of it could also be attributed to the fact that big brake kits usually come with different calipers as well. I'm not disagreeing in any way (in fact, I agree with you), I'm just saying that that's another factor that needs to be taken into consideration if you use the case of brake kits.
Good point. How many people make changes to the master cylinder along with a big rotor/caliper upgrade.

Temperature isn't a problem with my setup. The temperature at which heat input balances heat rejection is well within the capabilities of my rotors, calipers and fluid.

What I am concerned about is how hot my wheels and possibly wheel bearings and bearing seals are getting from heat conduction from the rotor at the hub faces. :confused:

I'm tempted to fit some ducting but I don't want the pads to run too cold either because they wear faster below a certain temperature.

Maybe some kind of thermal barrier between the rotor and hub/wheel mating faces? Anyone seen or heard of anything like that?
 

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Captain Slow
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T4 Primera said:
According to the link I provided, the plot shape for my pads is a 2 which rises slightly. I find that I need to lift off a bit as I plunge into the braking zone - entirely unnatural. I use the ABS activating as my cue to back off.

You could be right though, maybe I actually am applying more pedal as I go without being concious of it. Then when the ABS tells me to let up, maybe I'm putting it down to plot shape rather than unconciously applying more pedal.

Please don't take my dissertations on thermal dynamics above too seriously. I was just speculating at 4am on a nightshift and am more than happy to be corrected. :cool:
Haha. Don't worry, I'm not arguing with you. I'm just enjoying the conversation/speculation. I wish there were more of this kind of intelligent speculation by knowledgable people around here.

In your case, you might not be applying more pedal pressure. It could very well just be the pad (I know of some cheap metallic pads which would probably be 1 on that scale. They were impossible to work with, even on the street). What I was really trying to say is that where pads with 1 or 2 are pretty hard to work with, pads with plots 4 or 5 probably aren't really very user friendly either. That's really just an opinion though. The only thing I can back it up with is more opinion.

Basically, don't take anything I say too seriously either. :tongue:
 

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sleeper
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I'm kinda wondering about the wheel as a thermal mass. I know they get hot as I think some guys with Kosei K1 wheels have had the paint melt, but I think that's localized to the hub area. It's my guess that the heat transfer rate between the wheel and rotor is low relative to the rotor to the air based on low contact area. Also, wheel's are generally coated which would act as a thermal barrier. And then you have conduction through the wheel. Eh, it's my guess that the wheel doesn't factor in too much as far as cooling down the rotor. Hard to say without actually doing some calculations and knowing a couple things like the conduction coeff. between the wheel and hub.

I know wheels designed to aid airflow help a lot; I'd guess more than acting like a heat sink.

F1 brake designs are nuts. They have some insane cooling devices.
 

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spdracerUT said:
I'm kinda wondering about the wheel as a thermal mass. I know they get hot as I think some guys with Kosei K1 wheels have had the paint melt, but I think that's localized to the hub area. It's my guess that the heat transfer rate between the wheel and rotor is low relative to the rotor to the air based on low contact area. Also, wheel's are generally coated which would act as a thermal barrier. And then you have conduction through the wheel. Eh, it's my guess that the wheel doesn't factor in too much as far as cooling down the rotor. Hard to say without actually doing some calculations and knowing a couple things like the conduction coeff. between the wheel and hub.

I know wheels designed to aid airflow help a lot; I'd guess more than acting like a heat sink.

F1 brake designs are nuts. They have some insane cooling devices.
Yep, I agree it's just supposition without doing the calcs. But then again, air isn't such a good conductor of heat either.

I've never seen a wheel with an obvious design for airflow. What do they look like - impellors or something?
 
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