Understanding braking dynamics...

[VBNSX]

So i'm trying to understand a little bit more about this but I figured I'd just open this up to this forum just to see some of the responses.

It's very typical for many people to get different compound strengths in the front and rear. For example, I had a student with a track 350 and he said that Carbotech suggested a XP8 front and bobcat rear. Which in my opinion was like.. whoa.

But with such a strong front compound (versus the rear) wouldn't that cause the car to dive even harder? The whole purpose is to attempt to get the car to 'squat' evenly.

What am I missing?

x

 

[SugrueNSX]

So i'm trying to understand a little bit more about this but I figured I'd just open this up to this forum just to see some of the responses.

It's very typical for many people to get different compound strengths in the front and rear. For example, I had a student with a track 350 and he said that Carbotech suggested a XP8 front and bobcat rear. Which in my opinion was like.. whoa.

But with such a strong front compound (versus the rear) wouldn't that cause the car to dive even harder? The whole purpose is to attempt to get the car to 'squat' evenly.

What am I missing?

x

That’s a really interesting question!
So first of all let me preface this by saying I am going out on a limb here coz I have no idea what I’m talking about.
So I would guess that there are several factors to consider when selecting pads. 1. the hardness, which probably has a lot to do with how long they will last and how much they will tear up your rotors. 2. the coefficient of friction which is how much stopping power they will give for a given amount of pressure 3. how much they fade due to out gassing, how well they handle high temps, low temp performance, tons of other stuff I don’t know about.
Now I don’t know what you mean by a “strong compound” but if you mean a hard pad then I would not be surprised if a harder compound could give you less braking effect for a given pedal pressure than a soft one.

Waiting to here from an expert,

Patrick

 

[ncdogdoc]

Xavier,

I am no expert, but let me throw my 2 cents in here.

The compounds only play a role in grip strenght (mu) and resistance to heat buildup. Most people vary compounds to alter when an axle locks up, i.e. to prevent front lockup before rear or vice versa. Typically done this way once the rotors/calipers are changed to balance the system.

I am guessing Larry recommended this setup due to the front engine layout and increased temps on the front rotors. Probably the rears do not get nearly as hot, and therefore do not need as heavy duty a pad.

Front end dive is completely related to suspension design and shock/spring setup (and wt distribution, cg, instant roll centers, etc. etc. etc), not brake pad compounds. HTH.

 

[VBNSX]

That’s a really interesting question!
So first of all let me preface this by saying I am going out on a limb here coz I have no idea what I’m talking about.
So I would guess that there are several factors to consider when selecting pads. 1. the hardness, which probably has a lot to do with how long they will last and how much they will tear up your rotors. 2. the coefficient of friction which is how much stopping power they will give for a given amount of pressure 3. how much they fade due to out gassing, how well they handle high temps, low temp performance, tons of other stuff I don’t know about.
Now I don’t know what you mean by a “strong compound” but if you mean a hard pad then I would not be surprised if a harder compound could give you less braking effect for a given pedal pressure than a soft one.

Waiting to here from an expert,

Patrick

Strong meant higher coefficient of friction and bite... like right now i run on the panther plus' on the front and rear. I'm considering going to an XP8 front and PP in the rear... but don't really know if tha twould do anything for me as far as performance... cost?? eh.. just a bit.. but still..

x

 

[VBNSX]

Xavier,

I am no expert, but let me throw my 2 cents in here.

The compounds only play a role in grip strenght (mu) and resistance to heat buildup. Most people vary compounds to alter when an axle locks up, i.e. to prevent front lockup before rear or vice versa. Typically done this way once the rotors/calipers are changed to balance the system.

I am guessing Larry recommended this setup due to the front engine layout and increased temps on the front rotors. Probably the rears do not get nearly as hot, and therefore do not need as heavy duty a pad.

Front end dive is completely related to suspension design and shock/spring setup (and wt distribution, cg, instant roll centers, etc. etc. etc), not brake pad compounds. HTH.

Ok.. i'm gettin it... so looking at the nsx. with the majority of weight in the center.. would it be adviseable to get the same compound all around? Or would just a one step deviation in pads (XP8 front, PP rear... versus the 350's XP8/Bobcat combo) be better?

Granted the front end dive is primarily related to the suspension design, wouldn't it be safe to talk about initial bite? The bigger (in relation to coefficient, not size) pad in the front should grip more in relation to the rear. Even though this could be countered with suspension changes, there's still a bit of front end load due to this....

x

 

[docjohn]

Most folks with stock calipers I know of use the same pads front to rear.Those of us with hybrid bbk's meaning bigger swept area on the front with the stock puny rear callipers will vary compounds to improve ballance.Why don't you talk to Andie At cobalt about it.

 

[pach]

So i'm trying to understand a little bit more about this but I figured I'd just open this up to this forum just to see some of the responses.

It's very typical for many people to get different compound strengths in the front and rear. For example, I had a student with a track 350 and he said that Carbotech suggested a XP8 front and bobcat rear. Which in my opinion was like.. whoa.

But with such a strong front compound (versus the rear) wouldn't that cause the car to dive even harder? The whole purpose is to attempt to get the car to 'squat' evenly.

What am I missing?

x

Carbotech doesnt recommend bobcats for track events, Im sure the reasons to recommend a more aggresive pad in the front was for temperature reasons, the XP8's can take alot more heat and a more aggressive compound in the front should add a front brake bias to make the car a bit "stabler" under heavy braking. You can only have extra brake dive if the front tires get extra traction under braking.

 

[OLDE GUY]

This is a very complicated area dealing with a car's weight distribution; the manufacture's built-in brake bias i.e. brake proportioning valve size; stock versus non-stock wheel/tire sizes; ABS; suspension spring/shock rates; etc. All of these factors have to work together properly not just for straight line 80 to 0 MPH braking times but for balance during trail braking, etc.

I would be VERY careful about taking second hand information from a person with a different car and trying to extrapolate this to the NSX. There are poorly informed drivers who will try anything and often end up with worse performance from their cars - sometimes they don't even realize it.

Only take your information from very experienced NSX drivers who have the lap times to prove what they are suggesting works.

 

[OLDE GUY]

Understanding brake bias

Xavier's question has prompted me to pull out my copy of Going Faster from Skip Barber Racing School and to review Chapter 14 on brake bias adjustment. My thinking is that using different brake compounds on the front versus the rear is similar to adjusting brake bias. Here are some excepts, not the full two pages.

"Load Transfer

First some basics about grip....More download delivers more grip....Under braking, for example, deceleration transfers load off the rear tires onto the fronts.....When the braking system is .....adjusted, a key consideration is the increased grip available to the front tires because of the increased download. Brake bias adjustments are an attempt to adapt the braking system of the car so that braking effort is proportioned correctly to use the different levels of grip available to the front versus rear tires........

Setting the Bias

....first run the car for a few laps, getting to the point where the tires and brakes are up to temperature. If you set the bias when the tires are cold and the grip is less than you'll experience under racing conditions, there will be less download on the fronts than there will be when everything is warm. Then you are likely to have too little braking at the front and too much at the rear when things finally heat up. This rear-bias situation can get overly exciting, so it's something you should avoid.

......after carefully checking the mirrors to avoid being run over. From high speed, squeeze the brake pedal on...until either the front wheel or rear wheel gets just to the edge of lockup.......

In an open wheel car, you should be able to notice the strobe-like effect of the front tires slipping across the road.....With rear bias, focus the mirrors on the rear tires while doing the brake testing to confirm the physical sensation of rear bias (the rear darts around during braking). In closed wheel cars, you have to trust your sensitivity and back it up with evidence of buffs of smoke out of the wheel wells.

With either type of car, you are aiming to get the brake bias adjusted so that the fronts begin to lock marginally sooner than the rears. You would rather have a little excess bias toward the front because it is the more stable of the two extremes. Going too far to the front, however, will compromise the overall braking ability of the car.

Bias and the Clutch

.....In most cases of threshold braking, the car will be decelerating in gear with the clutch engaged, except for the moments you dip the clutch for downshifts. In this case the brake bias should be set by braking the car with the gear engaged. In most cases the engine compression...is contributing to the braking of the rear pair of tires. If you set the brake bias with the clutch disengaged, you'll end up with too much rear bias if you slow the car with the help of the engine during the race.....

Brake-turn Bias

Once the bias is close in straight-line stops, it's time to try it under simultaneous braking and turning conditions (trail braking). If there is any possibility that the bias is too far to the rear, it will show up here. You may find that a bias that was perfect for straight-line braking will be biased a little too much to the rear for comfort under brake-turning. Under the combination of pitch and roll, which takes place while braking and turning, the inside rear tire is substantially unloaded and the drag of the rear brakes contributes to the rear losing cornering traction, allowing the rear of the car to slide slightly more than the front.

.....Anticipate that if there is a lot of aggressive braking and turning needed at a particular track, the bias may end up a turn or two more toward the front."

Obviously the author is writing about race cars without ABS but the desired result is the same. Similarly, I think that electing to use different brake compounds front versus rear (effecting a change in bias) should be approached in a similar fashion. It appears to me that the major reason for using a "stronger" compound in the front would be to correct for too much rear brake bias i.e. the rear darting around during braking; oversteer during trail braking.

 

[VBNSX]

good stuff... appreciate that Bill...

I called carbotech and b/c of the setup of our cars.. nearing the XP8 compound, most (he defined most as 90%) people get a similar compound all around.. (as stated by docjohn)...

Guess I'll just smoke through my Panther Plus' this Saturday and then switch to the XP8's for Sunday and Monday and see how it all turns out.



x

 

[Hrant]

Let us know how the XP8 performed comapred to the Pantherr Plus.

 

[OLDE GUY]

I think that part of the genius of the NSX design it that the ~ 42% front/58% rear static weight distribution changes under heavy braking toward 50%/50%. This combined with the wider rear tires (more grip for braking and turning) accounts for the beautiful balance of the NSX during brake-turning.

With the NSX mid-engine design, the rear brakes are doing a larger percentage of the braking than on a car with a front engine (which weights more in the front plus receives the additional download transfered forward with deceleration) . Under heavy braking, a front engine car may end up having close to 65% front/35% rear download distribution - think heavy, cast iron V-8 Mustang Cobra here. Thus the need for massive front brakes compared to the rather small rear brakes on such cars.

 

[Hrant]

Don't know what this means but sure looks like it belongs in this thread :wink:

http://www.daliracing.com/v666-5/catalog/stop_faster/racing-brake-kit/technical-details.cfm

 

[OLDE GUY]

Don't know what this means but sure looks like it belongs in this thread :wink:

I don't know exactly what it's implications are either. It could be that it is too much front caliper versus rear for the NSX. A little appreciated fact is that for 1997 and later NSXs as well as the NSX-type R, the rear rotors have a larger diameter than the front although they are thiner - less heat sink needed.


Rotors:
NSX Front: 282 x 28 mm
NSX-R and 97+ Front: 298 x 28 mm

NSX Rear: 282 x 23 mm
NSX-R and 97+ Rear: 303 x 23 mm

The larger rear rotor diameter means that the rear calipers are a greater distance from the axil center. So the rear calipers, because of their location, exert more torque on the rear rotors. This design further demonstrates the significant braking which occurs at the rear of NSXs. JMO: I would never put a "stronger" brake compound on the front versus rear of a stock NSX. I believe it would be too much front brake bias for optimal performance. If after-market brake systems were well designed, there would be no need in that case either.

Does anyone know of another car which comes from the factory with larger diameter rear rotors than fronts?

 

[NSXGOD]

I don't know what it's implications are either. It could even be that it is too much front caliper versus rear for the NSX. A little know fact is that for 1997 and later NSXs as well as the NSX-type R, the rear rotors have a larger diameter than the front although they are thiner - less heat sink needed.

I think that everyone that has one of the newer cars does. :biggrin: except maybe for Hrant who records 100F higher temps at the back than the front.

Rotors:
NSX Front: 282 x 28 mm
NSX-R and 97+ Front: 298 x 28 mm

NSX Rear: 282 x 23 mm
NSX-R and 97+ Rear: 303 x 23 mm

our brake kit uses the same size rotors since they are handy and pencil out favorably for balance.

The larger rear rotor diameter means that the rear calipers are a greater distance from the axil center. So the rear calipers, because of their location, can exert more torque on the rear rotors. This design further demonstrates the significant braking which occurs at the rear of NSXs.

actually what is shows is that "they" increased the braking capacity of the rear more than the front when they upgraded the rotors in 1997 since the pads are the same - JMO of course..

JMO: I would never put a stronger brake compound on the front of an NSX versus rear. I believe it would be too much front brake bias for optimal performance.

Brembo seems to feel that the ABS/TCS can handle a braking balance ratio of "0" (ie the front and rear have the same braking force) which always felt a bit odd to me without bias adjustment to tone the rears down. JMOYBDMV.

Biasing the braking a bit to the front allows for a more "stable" car. If you have stiffer front suspension with less dive then having more rear braking available should give shorter braking distances all other things being equal.

The idea behind the new kits is that less might be more - those that feel that "Bling is King" can always get something else.

 

[Hrant]

except maybe for Hrant who records 100F higher temps at the back than the front.


.......... Perhaps that's because the fronts are cooling better than the rears? Prior to removing the dust shield on the front, I was running similar temps, even with the Dali street (smaller) air deflectors. Last track event, I had the rear dust shield trimmed - not removed, and the temp difference between front and rear was still 100F to 150F, and I was really pushing the car.


our brake kit uses the same size rotors since they are handy and pencil out favorably for balance.

......... except as noted in a different thread, while both the front/rear floating two piece rotors weigh less than the OEM, the rear rotor has more mass relative to the fronts when compared to the OEM weights of the rear and the front. Presumably the weight is in the rotor and not the hat.


actually what is shows is that "they" increased the braking capacity of the rear more than the front when they upgraded the rotors in 1997 since the pads are the same - JMO of course..

............ I would "think" that the more mass of the rear rotor should perhaps help with more torque in the braking which may mitigate the benefits of more mass for cooling? Way too deep of physics and metallurgy for my brains .... all I know is that these rotors are working great after almost 10 track events with OEM calipers.


Brembo seems to feel that the ABS/TCS can handle a braking balance ratio of "0" (ie the front and rear have the same braking force) which always felt a bit odd to me without bias adjustment to tone the rears down. JMOYBDMV.

Biasing the braking a bit to the front allows for a more "stable" car. If you have stiffer front suspension with less dive then having more rear braking available should give shorter braking distances all other things being equal.

.......... Out of curiosity, aren't we all increasing the stiffness of the front suspension when we track the cars. I mean the Type R bars, the stiffer sway bars, the springs, shocks ....... all increase the ration of the front to the rear. That is what Honda did with the different versions (Type S, Type R, Zanardi .....) So presumably why they increased the braking capacity of the rears?

And if I recall, the hand brake can come handy in emergencies to put more braking balance on the rears :biggrin:

The idea behind the new kits is that less might be more - those that feel that "Bling is King" can always get something else.

........ True. But we all cheat to compensate our driving handicaps so intimidation is a key factor in psyching the other driver with what might be new and "better" goodies ........ :tongue:

 

[NSXGOD]

Originally Posted by NSXGOD
except maybe for Hrant who records 100F higher temps at the back than the front.

.......... Perhaps that's because the fronts are cooling better than the rears?

of course - they get better airflow in the front.

.......... Prior to removing the dust shield on the front, I was running similar temps, even with the Dali street (smaller) air deflectors.

as is mentioned several times on the website - they don't do shit unless you remove the dust shields.

.......... Last track event, I had the rear dust shield trimmed - not removed, and the temp difference between front and rear was still 100F to 150F, and I was really pushing the car.:

then you need more air on the rear brakes :biggrin:

Originally Posted by NSXGOD:
our brake kit uses the same size rotors since they are handy and pencil out favorably for balance.

.......... except as noted in a different thread, while both the front/rear floating two piece rotors weigh less than the OEM, the rear rotor has more mass relative to the fronts when compared to the OEM weights of the rear and the front. Presumably the weight is in the rotor and not the hat.

One might "presume" that. :tongue: ever heard the term "anal retentive" as applied to having too much time on your hands to endlessly dissect the equivalent of bellybutton lint? The "balance" referred to on our napkin is not how much heat you can process, (related to rotor mass and design and external airflow) it is referring to braking "torque" or leverage or stopping power whatever you want to call it - don't confuse yourself any more than you already are. :biggrin:

Originally Posted by NSXGOD:
actually what is shows is that "they" increased the braking capacity of the rear more than the front when they upgraded the rotors in 1997 since the pads are the same - JMO of course..

.......... I would "think" that the more mass of the rear rotor should perhaps help with more torque in the braking which may mitigate the benefits of more mass for cooling? Way too deep of physics and metallurgy for my brains ....

clearly. :biggrin:

..........all I know is that these rotors are working great after almost 10 track events with OEM calipers.

cool - I'm happy that they are working out for you.

Originally Posted by NSXGOD: Brembo seems to feel that the ABS/TCS can handle a braking balance ratio of "0" (ie the front and rear have the same braking force) which always felt a bit odd to me without bias adjustment to tone the rears down. JMOYBDMV.
Biasing the braking a bit to the front allows for a more "stable" car. If you have stiffer front suspension with less dive then having more rear braking available should give shorter braking distances all other things being equal.

.......... Out of curiosity, aren't we all increasing the stiffness of the front suspension when we track the cars. I mean the Type R bars, the stiffer sway bars, the springs, shocks ....... all increase the ration of the front to the rear.

only the springs & shock valving affect brake "dive" - the "bars" you mention are for increasing chassis and/or roll stiffness - that is not part of the equation that affects what you are discussing.

.......... That is what Honda did with the different versions (Type S, Type R, Zanardi .....) So presumably why they increased the braking capacity of the rears?:

like your regular old flabbyass 1999T or whatever year it is with the OEM springs does not have the same brake system as the Type S, Type R, or Zanardi he asks rhetorically? :biggrin: (yes I know the Type-r has nice slotted rotors and pads - big deal - they are the same size and it is the same caliper)

And if I recall, the hand brake can come handy in emergencies to put more braking balance on the rears.

I bet El Presidente likes that maneuver. :biggrin:

Originally Posted by NSXGOD: The idea behind the new kits is that less might be more - those that feel that "Bling is King" can always get something else.

........ True. But we all cheat to compensate our driving handicaps so intimidation is a key factor in psyching the other driver with what might be new and "better" goodies ........ :tongue:

whatever it takes. :biggrin:

 

[VBNSX]

Let us know how the XP8 performed comapred to the Pantherr Plus.

Amazing! They hold through the whole zone without any major fade. They just keep grabbing. Definitely much better than the PP's. Initial bite is very stiff.. If you're too agressive you can kick in the abs. (I'm still on street tires).

On a side note... I hit 2:19.47 at VIR full today!!! woot!!! :biggrin:

cheers,
x