Front/rear brake-balance on NSX question

[MvM]

In another thread here on NSXPrime about upgrading an earlier model NSX to the 97+ brakerotors & calipers there is a link to a Stoptech article. The article can be found here:
http://www.stoptech.com/whitepapers/rear_brake_upgrade_is_bigger_better120601.htm

The article mentions the fact that, quote, "the rear brakes only contribute about 15-20% of all the braking force the vehicle generates"

I was wondering if that is true for the NSX as well, given it's mid-engine layout and the fact there is more weight in the rear than in the front. Would the breaking force in the rear not be (a bit) higher than for a front-engine car?
And if so, would that create the possibility of reducing your actual stopping distance. I have always wonderd why some (newer) cars achieve shorter distances than the NSX when the use more or less the same tires and are of similar weight.

 

[DanO]

Would the breaking force in the rear not be (a bit) higher than for a front-engine car?

Yes and no; if a car’s distance from the front axle to its center of gravity is longer than another car’s, then there would be relatively more rearward weight distribution and (everything else being equal) more traction available at the rear wheels. So a mid-engine or rear engine could have a shorter stopping distance than a front-engine car, but there are so many other (lesser and greater) variables involved with the equation.

DanO

 

[ChopsJazz]

When the brakes are applied on any vehicle there is a dramatic shift in weight onto the front end. The resulting lightness in the rear end is a determining factor in the amount of braking force that can be applied on the rears. Too much rear brake bias and the rears lock up, too little and stopping force is diminished.

As a friend of mine said, "Brakes are rocket science".

 

[Taj]

The balance of a car is dynamic. It is way more complicated than rocket science.

 

[Andrie Hartanto]

Read this article. Scroll down to the section of
Front to Rear brake bias.

http://www.stoptech.com/whitepapers/brake_systems_and_upgrade_selections_122701.htm

 

[1BADNSX]

The balance of a car is dynamic. It is way more complicated than rocket science.

When did unsteady 3-D equations of motion become less complicated than braking?

To add to Dano's comments. Not only is the static weight distribution and wheel base a primary consideration in the dynamic loading, so is the height of the center-of-gravity. All three are better for the NSX compared to most cars and help allow the rear-end to contibute in the braking department.

Bob

 

[nsxtasy]

Like all cars, the NSX experiences weight transfer from the rear to the front during braking, as Dan points out – all the more so at the track. Does the front really do 80 percent of the braking…? Sounds like a math problem to me.

The swept area of the front brake pads on my ’91 NSX is 58.0 sq cm, and it’s 38.8 sq cm in the rear. The front pads come with 11 mm of pad material on them, and the rears with 9 mm. If you wear the brake pads down (at either end) until they have 2 mm, that means that you are using up 58.0 * 0.9 = 52.2 cubic cm of pad material on a front pad, and 38.8 * 0.7 = 27.16 cubic cm of pad material on a rear pad. I have used up 19 sets of front brake pads and 10 sets of rear brake pads, and I have generally used matching or similar pads on both ends of my NSX. Thus I have used up 991.8 cubic cm of pad material in front and 271.6 cubic cm in the rear. (Actually, it's double that, because the pads come in pairs - 991.8 and 271.6 for the outer pads in the front and rear calipers and another 991.8 and 271.6 for the inner pads - but the relationship between the two is the same.) This amounts to 78.5 percent in front, and 21.5 percent in the rear. Round it off, and...

 

[Briank]

Ken

That’s very clear, but you would not believe how many people will argue that fact.

 

[tunapie]

I have heard 55/45 from Shad at Comptech. Dan

 

[DanO]

Nobody can give exact numbers unless they know all the variables and do the math. There’s two things going on here, we can do the math and see what kind of work is possible, and then we can do the math and find out what kind of work is being done. The BIG variable here is how fast the car is stopping, the faster the car stops, the more the weight distribution approaches 100/0%. So theoretically (or abstractly) if you get yourself some unworldly phat, sticky tires and some correspondingly big phat brakes and slap them on the front, then stand on them, the car would do a rear-end wheelie and other than wind resistance you’d be hard pressed to get much rear braking work.

Potential
Vertical weight on tires determines grip, that is, more weight equals more grip available. Under deceleration, vertical weight at the wheels is a combination of STATIC and DYNAMIC weight.

Static
First we start with STATIC weight, which is simply the weight distribution fore/aft of the car
Rear weight = weight of car times horizontal distance from front axle to CG divided by wheelbase
Front weight = weight of car minus rear weight
“weight” here means the vertical weight on the tires

Don’t confuse STATIC weight with DYNAMIC weight

Dynamic
For cars in our world, under deceleration vertical weight transfers towards the front:
Potential Weight Transfer = weight of car times coefficient of friction times center of gravity divided by wheelbase
it does not matter what stops the car, whether the brakes, engine or a sand trap, weight will transfer the same amount.
“coefficient of friction” above is the variable that controls deceleration rate, as you can see, as this variable increases, so does weight transfer

Total Weight (under max potential braking)
Front weight = front static + dynamic transfer
Rear weight = rear static – dynamic transfer
Note: see how weight transfer is a zero-sum transfer (the rear weight goes 1-to-1 to the front)
If the coefficient of friction is low, so will be dynamic transfer, as the coefficient of friction gets lower the total weight will approach the static weight.
Rear vertical weight will be zero when dynamic transfer equals the rear vertical weight.

Abstract
Imagine a theoretical car with a center of gravity LOWER then where its tires contact the road. It might look like a regular car with a connected long heavy keel sticking below the surface of the road (use you imagination). In the case, when the car decelerates, the weight would transfer to the REAR, not the front!

Next, we can figure out how much work can actually be done with a specific hydraulic brake setup. This involves calculating the swept area of the front/rear pads, measuring the coefficient of friction of the pads, outside tire diameter, brake piston area and hydraulic pressure. Speaking of hydraulic pressure, don’t make the mistake to think that biasing valves deliver a linear reduction in pressure—they don’t. Most valves deliver a 1:1 ratio until a certain pressure threshold and then start to reduce the pressure to the rears at a selectable ratio. On some valves, both the threshold and the ratio are adjustable—this means that under light braking (like in the rain), both the front and rear do the same amount of work (because there is little weight transfer).

As far as I know, nobody (besides HONDA) has really calculated anything regarding brake bias and weight transfer regarding the NSX.

DanO

 

[nsxtasy]

As far as I know, nobody (besides HONDA) has really calculated anything regarding brake bias and weight transfer regarding the NSX.

Hey, Mr. Smarty-Pants! I calculated the volume of brake pad material that I've used in the front and the rear. That's something!!!

 

[MvM]

DanO,

Since you state that so much depend on the weight transfer to the front, might that be the reason that cars like the Mitsu Evo achieve shorter stopping distances than a NSX.
Also. does this mean that if you would like to have the NSX to get a shorter stopping distance you should (in theory at least) transfer some static weight from the rear to the front ?
Would mounting wider tires (for a dry road of course) in front help shortening your stopping distance ??

 

[NSX-Racer]

Hi Maarten,

from where do you have that info?
quote: "Since you state that so much depend on the weight transfer to the front, might that be the reason that cars like the Mitsu Evo achieve shorter stopping distances than a NSX."

As far as I can remember the NSX-R and the Evo VII had about the same stopping distance at the tests from the mag "sport auto" in Hockenheim (about 34 meters warm from 100 km/h to zero). The shortest distance in these tests ever measured for a "stock" car was a bit over 32 meters for the BMW M3 CSL with special designed Michelin sport tires (nearly slicks). And that's the main point: The best way to reduce stopping distance is more grip (and brakes that can withstand the stopping power).

Don't think too much about brake bias or weight transfer for the NSX. As long as you drive with ABS the system will give you nearly the maximum brake power front and rear. I have Porsche 993 turbo brakes front and Porsche 996 GT 3 brakes rear with no additional bias system and it works fine (at least on the track both with street and slick tires). If you want to play with some kind of bias you may use different pad compounds front and rear.

If you want to reduce absolute weight transfer, reduce weight overall ;-)

 

[nsxtasy]

Would mounting wider tires (for a dry road of course) in front help shortening your stopping distance ??

No. But mounting a tire with a softer compound that offers more grip would help.

 

[nsxtasy]

I can see how the question of front vs rear brake balance can have so many answers, because you can look at the question so many ways.

I looked at it as a math problem, and calculated that the front is 78.5 percent of the brake pad material used.

DanO then looked at it as a physics problem, with various equations and formulas.

We could also look at it as a financial problem, as follows.

The stock front pads generally cost 63-66 percent more than the stock rear pads. Applying my purchases of 19 front pads and 10 rear pads to this ratio shows that had I purchased only stock pads, I would have spent 76 percent of my brake pad cost on the front, and 24 percent on the rear.

A quick check of some aftermarket pads shows that the various types of Hawk pads at Racer Parts Wholesale cost 25 to 43 percent more for front pads than for rear pads, and the R4 and R4S pads at Porterfield cost 72 percent and 20 percent more for front pads than for rear pads. Applying the wider range of the Porterfield prices to my 19 and 10 brake pad purchases yields a financial expenditure of 70 to 77 percent for the front pads and 23 to 30 percent for the rear pads.

Then again, I know that I have spent 100 percent of my brake rotor cost on front rotors that needed replacement, and 0 percent on rear rotors that needed replacement. Hmmm.

Heck, we could even have some fun with this topic, by looking at it in other dimensions as well.

We could look at it as an ethical problem: "Bias, in any form, is wrong. All pads are created equal."

We could look at it as a political problem, such as in the recent Supreme Court rulings: "Bias is acceptable to achieve the goal of braking in our society, as long as you don't use numbers to calculate it."

We could look at it as a philosophical problem: "Front braking and rear braking are not different concepts, but rather, are the yin and the yang of braking. They work together in harmony to achieve the pleasure of slowing the car, making you at one with the world."

 

[DanO]

Hey, Mr. Smarty-Pants! I calculated the volume of brake pad material that I've used in the front and the rear. That's something!!!

Ha! That is something BTW, swept area in not the same as pad surface area. Swept area is the total area contacted by both the brake pads in one revolution, obviously pad width and rotor diameter (or distance to outside of pad) and pie come into play here.

Actually, I’d be fun to try do dome calculations. Here are the numbers that I know:
Overall car length 174.0 (in)
Wheelbase 99.6 (in)

Using the numbers above, I wonder if we can calculate the horizontal CG of the NSX if we know the weight distribution?

DanO

 

[nsxtasy]

BTW, swept area in not the same as pad surface area. Swept area is the total area contacted by both the brake pads in one revolution, obviously pad width and rotor diameter (or distance to outside of pad) and pie come into play here.

My bad. I was using the figure for pad surface area, as shown in the service manual - which is the appropriate one for calculating volume of brake pad material.