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Old 10-09-2015, 08:03 AM   #601
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Originally Posted by codrus View Post
and yeah, stickier tires mean more grip means more weight transfer means less rear bias desired.

--Ian
From what i understood, stickier tires allows you to brake harder (with more pressure pedal and/or bigger brakes) and that increases weight transfer. If you have stickier tires at the rear, you also increase your ability to apply more brake torque on the rear, right?

When our car left the factory, the brake system were already front bias by some margin... Why would you wish to increase this bias and not try to use the rear braking "reserve". As long as you're not blocking the rear wheels before the font wheels (with a safety margin), i thought that you would apply as much rear bias as possible...

... but i'm not sure i "get it right" yet....
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Old 10-09-2015, 08:22 AM   #602
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I still can't wrap my head around this. I can barely get in the right gear for each corner.
All F1 drivers did that for decades until the new power plant rules which allow electronic adjustment of the rear brakes to account for the KERS.

You can go back and watch in car videos of the guys pulling their hand off the wheel on the straight, making an adjustment, going into the corner.

Although sometimes it was bias for the corner, the engineer telling them to control temps at one end, etc.
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Old 10-09-2015, 11:39 AM   #603
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From what i understood, stickier tires allows you to brake harder (with more pressure pedal and/or bigger brakes) and that increases weight transfer. If you have stickier tires at the rear, you also increase your ability to apply more brake torque on the rear, right?
No.

Imagine what happens when you brake hard on a bicycle or motorcycle. If you have enough grip, you can brake so hard that the rear wheel leaves the ground. (Look at sport/racing motorcycles and you'll notice that there are two front brake disks and only one in the rear, and even then, the front ones are much, much bigger.)

A car has a longer wheelbase and a lower centre of gravity than a bike, so of course you won't have the rear wheels in the air, but the same principle of forward weight transfer during braking applies. Get grippier tires and you get more forward transfer, meaning that the rear wheels will have less weight over them and lock earlier. The result is that more of the braking is done by the front wheels.

Go the other way and drive on snow or ice, where there isn't enough grip available to get lots of weight transfer, and you'll notice that an otherwise balanced setup will lock the fronts prematurely.
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Old 10-12-2015, 10:08 AM   #604
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Originally Posted by wannafbody View Post

It would be interesting to see some actual test data for stopping distances with stock systems vs the 11.75 kits.
if during your test, you keep the same tires you would see no difference in one stop. you would notice the 11.75" kit components would last longer, and be more consistent over multiple stops.
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Old 10-12-2015, 10:31 AM   #605
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Can someone explain to me why there's such a disparity over brake bias recommendations? On one hand, you've got people recommending to take out the stock prop valve altogether on an otherwise stock setup, and on the other you've got track gurus running more aggressive pads on the front than the rear even with big brake kits.

Is it as simple as that you track guys are running such grippy tires that they can handle and/or need that much front brake during forward weight transfer?

On my street car, on moderate street tires, I definitely feel like the fronts lock prematurely on my square-padded, stock-braked setup. Am I wrong?
think of it like this. your building a race car as a system,that racecar-system is built of other systems. the brakes (components of brakes: master cylinder, prop valve, pads compounds, rotor sizes) need to work in harmony with your suspension (Shocks, springs, sway bar, ride height, camber, roll center, movement center, anti squat/anti dive, pro dive/pro squat, caster and toe) both of those two systems need to control the body (weight, weight bias) and your tires (compound, size mass) so out of the 4 systems i listed all of that needs to work together. Changing any of the 20 components in those system will effect the others systems and over all performance of the racecar-system. now factor in the biggest variable the driver and it all get very confusing. so not that anyone is essentially "wrong" to recommend something like removing the prop valve, it's just that their car might have a suspension setting that allows 100% rear line pressure. I raise a red flag on those recommendations, because it can be a safety issue. if you don't have a similar set up it can cause a big crash.

it can be as simple as the grip of the tires. Me an Emilio have a tendency to disagree and that is human nature, but if you ask us both what the single most important component on a race car is, we will both say tires. now adding tires with a higher coefficient of friction will result in more body roll. this is simply from the tires are now overloading the springs.
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Old 10-12-2015, 10:48 AM   #606
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I'd counter by saying that brakes are more important. Sure, a sticky tire might allow .1 or .2 more G force in the corner but that 1 second lap time increase can be matched by being able to brake later with better brake pads.

I'm amazed at the number of guys who take their 400HP car to a track day with street pads. One track day I was at, a guy with a new BMW had cooked his brake pads so badly that they were hissing from the resins melting, and he came in before the first 20 minute session was over.
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Old 10-12-2015, 10:52 AM   #607
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Originally Posted by wannafbody View Post
I'd counter by saying that brakes are more important. Sure, a sticky tire might allow .1 or .2 more G force in the corner but that 1 second lap time increase can be matched by being able to brake later with better brake pads.

I'm amazed at the number of guys who take their 400HP car to a track day with street pads. One track day I was at, a guy with a new BMW had cooked his brake pads so badly that they were hissing from the resins melting, and he came in before the first 20 minute session was over.
But you can't use good pads without good tires. It's discussed earlier in this thread. Doesn't matter if you have 20 inch brakes and PFC01s. If you are on snow tires you won't brake for ****.
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Old 10-12-2015, 11:23 AM   #608
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Originally Posted by aidandj View Post
But you can't use good pads without good tires. It's discussed earlier in this thread. Doesn't matter if you have 20 inch brakes and PFC01s. If you are on snow tires you won't brake for ****.
After cording a tire I swapped to a spare set of wheels once that had all-seasons on them and they turned to grease by the end of the warmup lap. Even with careful modulation and no lockup they just went to slime in the braking zones. I felt they were unsafe and came in.

Let's face it, all of the components are important and need to be balanced to work well together.
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Old 10-12-2015, 12:08 PM   #609
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Originally Posted by aidandj View Post
But you can't use good pads without good tires. It's discussed earlier in this thread. Doesn't matter if you have 20 inch brakes and PFC01s. If you are on snow tires you won't brake for ****.
I understand that within reason. I'm talking about the difference between running Hoosiers and NT01's. I felt more comfortable running NT01 with 1.8 Cobalt pads than Hoosiers with 1.6 EBC Yellows. It's about having a balanced set of parts that work well together.
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Old 10-12-2015, 12:51 PM   #610
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Originally Posted by wannafbody View Post
I understand that within reason. I'm talking about the difference between running Hoosiers and NT01's. I felt more comfortable running NT01 with 1.8 Cobalt pads than Hoosiers with 1.6 EBC Yellows. It's about having a balanced set of parts that work well together.
your not allowed to bring up ebc.. nope not allowed.
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Old 10-12-2015, 12:58 PM   #611
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Complains about 400hp bimmer guys with Street setup, matches 1.6 yellows with Hoosiers.... OK.

Yes, the whole system needs to work. But you choose the tires based on lap times you're shooting for, and then match the pads (and suspension) to the tires, not the other way around.
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Old 10-12-2015, 07:00 PM   #612
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Complains about 400hp bimmer guys with Street setup, matches 1.6 yellows with Hoosiers.... OK.

Yes, the whole system needs to work. But you choose the tires based on lap times you're shooting for, and then match the pads (and suspension) to the tires, not the other way around.
I ran the car at Nelson Ledges the last couple years and 1.6 brakes were fine there. There's only a couple aggressive braking zones.

This year I ran on the full course at Pittrace and it's a definitely harder on brakes. There's probably 5 aggressive braking zones and a couple other places where you'd use the brakes to set the car depending on the line you take.

The full course at Pittrace is now longer than Mid Ohio.
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Old 10-15-2015, 02:59 PM   #613
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Old 10-19-2015, 10:36 AM   #614
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I have a question on increasing the rotor size. With some upgrades going to a larger rotor also increases the rotational mass. Yes you get a larger, better heat sink to dissipate the heat, but at what point is there a diminishing returns problem. Let's say if the rotor weight increases by 1.5-2 lbs for front right and left (total 4 lbs). Let's say one car is driven/trailered to the track and the other is a fun street car with 1-2 HPDE a year. Both have 300 whp.
I guess another way to see it is sport rotors and calipers on the front and then extending the caliper bracket to fit the Mini cooper 11.5" rotors.
Thanks for your help
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Old 10-19-2015, 11:04 AM   #615
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For further detail, this is a sport miata front rotor.



And this is the 11.5" mini cooper rotor, a pretty porky rotor.



Nearly an extra inch in disc diameter, if I've got the right "sport" rotor #.
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Anyone have any brake questions?-51yxxmi4e0l._sy300_.jpg   Anyone have any brake questions?-51fgvsqyral.jpg  
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Old 10-19-2015, 11:35 AM   #616
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Porky.....your being kind. Yup, the math says 1.03 lbs heaver. Actually not as bad as I thought. So, might not be a big deal as far as rotating mass? Remember example is almost race car and one is fun street.
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Old 10-19-2015, 11:42 AM   #617
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Originally Posted by 2manyhobyz View Post
I have a question on increasing the rotor size. With some upgrades going to a larger rotor also increases the rotational mass. Yes you get a larger, better heat sink to dissipate the heat, but at what point is there a diminishing returns problem. Let's say if the rotor weight increases by 1.5-2 lbs for front right and left (total 4 lbs). Let's say one car is driven/trailered to the track and the other is a fun street car with 1-2 HPDE a year. Both have 300 whp.
I guess another way to see it is sport rotors and calipers on the front and then extending the caliper bracket to fit the Mini cooper 11.5" rotors.
Thanks for your help

Rotational mass is 4x as important as sprung weight. it is by far the most critical area on the car to add or subtract weight from. for example lets say you add 4lbs per corner. 4lbs X 4 wheels = +16lbs of rotational mass that would feel the same as adding 64lbs into the chassis.

so to answer your question. it's always bad to add more rotational mass. when i'm building a kit i use a rotor with an aluminum hat. That way i can increase rotor size AND lose a little rotational mass.

On the 350z with brembos the PFC two piece rotor is 5lbs lighter then the oem piece. if you can get that on all 4 corners it would feel like you where pulling 80lbs out of the chassis.
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Old 10-19-2015, 11:59 AM   #618
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All the extra mass of the larger mini rotor also has a larger moment of inertia than the sport rotor, simply because it's a bigger rotor.

If you were to change one 15" for another heavier but wider 15", the moment stays the same, mostly. If you were to step up to 17's, the rotational weight has a larger moment again due to the larger diameter, and any extra weight is extra taxing.


Have you tested when using a smaller, lighter rotor is beneficial, since it's such a massive 4/1 ratio?
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Old 10-19-2015, 01:06 PM   #619
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Originally Posted by deezums View Post
All the extra mass of the larger mini rotor also has a larger moment of inertia than the sport rotor, simply because it's a bigger rotor.

If you were to change one 15" for another heavier but wider 15", the moment stays the same, mostly. If you were to step up to 17's, the rotational weight has a larger moment again due to the larger diameter, and any extra weight is extra taxing.


Have you tested when using a smaller, lighter rotor is beneficial, since it's such a massive 4/1 ratio?
it really depends on the racing and the car. In time trials dedicated car yes we'll use a narrow (lighter) rotor of not carbon. Track day and Club racers use an endurance style brake kit (as big as it can be), just for cost reasons. most of the time we have wheel size limits. the car was built around running a cretin tire compound and size. to that we will try and fit as large of a rotor as we can. the larger rotors allow us to run less compound and provides better feel.
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Old 10-19-2015, 02:02 PM   #620
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it really depends on the racing and the car. In time trials dedicated car yes we'll use a narrow (lighter) rotor of not carbon. Track day and Club racers use an endurance style brake kit (as big as it can be), just for cost reasons. most of the time we have wheel size limits. the car was built around running a certain tire compound and size. to that we will try and fit as large of a rotor as we can. the larger rotors allow us to run less compound and provides better feel.
"so to answer your question. it's always bad to add more rotational mass. when i'm building a kit i use a rotor with an aluminum hat. That way i can increase rotor size AND lose a little rotational mass. "

Right, I get it, but it seems to be a compromise to fit the situation.
You are trying to balance more heat dissipating mass, more rotational mass, and (possible) increased cost of running a two piece rotor.
It seems like if you do just a few HPDE events, could a case be made for taking the rotational weight hit for increased braking performance?
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