mark kibort

well, its really about HP to weight ratio.

mark kibort

I thought since there was a substantially amount of misconception about demands on the 928 braking system, i would run a simulation of something close to my race car from the final turn down the main straight to the first big slow turn.
45mph to 130ish mph down to 45mph.

There was talk of Brakes beng more "powerful" when you go to bigger calipers ro rotors. and this isnt allways true. that limit is on the Tire. however, during overheating, the larger brakes can keep the power near equal (at the limit) where smaller systemes might overheat where max tire grip cant be used. (commonly seen as "brake fade".)

this chart is two identical cars. one 500lbs heavier than the other. (3000lbs vs 3500lbs or 16% heavier). the speed reached by the two cars is different at certain distances or times. in the end, if we assum they can aceelerate to the same point and then start their braking , what is the thermal effect on the braking system during the decel?

Glen was right that there is the same mount of KE for the same time application of power. (i.e. WOT for the same amount of time, generally)
but because speed is the squared factor in the KE equation, the 16% difference in weight, is only equal to a 6-8% change (yes, increase ) in KE before braking .............But, because the rate of decel, if the force generated by the tires is the same, will be longer for the heavier car, evening out the KE dissipation rate . (actually, the rate goes down a little with the heavier car for other reasons too... less weighit transfer because of less g rate of decel, which further reduces the max potential of the grip up front)
So, in the end, the argument that adding weight requiring more brake capability is a poor one at best.

below are the charts for the light car on the right, and the heavier car on the right.
this was done by a very accurate simulator that takes a lot of factors into account. aero drag, inertia, hp torque curves, grip, rolling friction, etc etc.
its done for a 400rwhp type 928

mark kibort

check my math here, but if we have 15% more weight and the force to decel the car doesnt change , doesnt the rate of decel go down by the same amount?

A (decel)=F/m right? same force of 3000lbs decel. (13,363N/1363Kg)=9.8m/s (1g) if we assume 1g decel rate with 15% more mass the decel rate will be 85% of the decel rate of the original car. 3000lbs vs 3500lbs

This means that the power rate dissipated to the same final speed before turn in, could be higher for the lighter car especially since the heavier car at the lower decel rate wont have the same weight transfer, so probably couldn't achieve the same decl forces either.

mark kibort

So here are the calculations to show that by adding weight you actually increase the themal load on the brakes. (based on the simulation above)

KE start 307,247 light car

KE start 270,969 heavy car
both at near 43mph

KE end based on simulator at 1840ft down the track before braking at the same point:

KE end 1,991,956 Light car at 120mph
KE end 2,085,914 Heavy car at 114mph 4% more KE before braking

both cars have same force on the wheels and to the ground through the tires and slow to the same speed that they started out at :

Total KE to slow: 1,684,709 for light car
(1,991,956 start before slow - 307,247 J to orig speed)
Total KE to slow: 1,814,945 for heavy car
(2,085,914 start before slow- 270,969 J to the orig speed)


Heavy car with 15% more weight will slow at 15% slower rate and take 15% longer time (if at the same speed) but, its going 5% slower before its decel

we can call it 4 seconds for lighter car and then 4 seconds plus 15%-5% = 10% = 4.4 seconds.

this is 421,177 Joules for light car in 4 seconds or 564HP/sec.
this is 412,487 Joules for heavy car in 4.4 seconds or 552HP/sec

difference of 2% more power/second for the lighter car

In the end, i think at worst case, the effect on braking would be near the same with the extra weight.

If you have some comments on the calculations.. lets hear 'em

Ive made only a few minor approximations, that i think in the end , they would not be huge factors at all. the limits are easy to see.
( the decel distance for the heavy car is approx'ed at 4.4 seconds. ill plug it into the formula and see the exact time it would take to decel, assuming the decel rate is 1g for the light car and .85g for the heavy car)

James Bailey

So what was the point of all this verbage ?? Faster cars need bigger brakes ; light cars tend to be faster but in the end the tires are the weak link....even a 1978 can lock up the brakes anytime you wish.

mark kibort

Jim, the point is..... adding weight to your car doesnt require any additional braking capacity. the themal load on the brakes will be the same .

heavy cars going to the track with the same HP as their lighter cousins, dont need additioal braking capacity.

going lighter on your car, might require additional braking capability and might require larger brakes or rotors.

tires are one of the limiting factors, yes. the more your car weighs the slower the decel rate for a given grip force of the tires.... but the weight transfer increases this factor and thats not even taken into account in my simulation above . but yes, the grip potential of the tires is the limiting factor. it,and speed and weight of the car, determines the braking HP, not the size of the rotor or calipers. (it increases their capacity should the smaller brakes not be able to create the forces that go to the limit of the tires.

Lockup is not the best measure. .. ... example.. you slam on the brakes, the Mu goes down and the force is much lower than a slip % that is optimal for the tire. ABS is close to this factor. so , if you can get into ABS down to the final speed point, that is the max potential of the braking system and tires..
the other factor is weight transfer... you slam on the brakes before weight transfer and the weight on the tires isnt as high as if you progressively add brakes. HUGE difference. its the difference of beginner track drivers and those that have experience. over the edge in cornering or braking doesnt generate near the forces (laterally or linearly)

Imo000

Lighter cars need more brakes and heavier ones less? So a transport truck needs no brakes?

James Bailey

I can only assume you consider yourself to be an " experienced " driver......yet you continue to rely upon the ABS as you trail brake into corners. ... sorry Mark but reality is reality. Repeating the same mistakes over and over just makes you consistent. ...thinking you are an exceptional driver is more a function of ego than fact.
This thread needs some chickens and fat mother jokes.......considering much of what you post is but a joke....

polecat702

Who cares? She looks great!!!!!!!!!

the flyin' scotsman

[QUOTE=CJ8 2 928;12769280]Mark in a nutshell can you tell me the advantage or disadvantage of an 89 rear end ? tks[/QUOTte]

thats some damn funny sh$$ Mike.......

Mark.......Ive no idea what you said as the function of the ignore doesnt permit but the retorts generated are truly amazing and yes cabin fever is setting up early this year.

James Bailey

trust me the snow will be deep this year !!!

mark kibort

your comprehension is amazing. I never use ABS. if you missed the debate with greg, he said that aBS is the fastest way to stop a 928.. he is dead wrong. ABS kicks in and alll hell brakes loose. (pun intended).. I used ABS to show the example of max grip of the tires during a decel for pointing out the tires are the limiting factor. way to distort the message.
I NEVER said i ever trail brake and use ABS.... thats funny!
Jim... my driving ability is shown by the results. its that simple..... anytime you want to compare times by anyone with the same HP/weight, be my guest. I know i swing the 928 around the track as good as anyone, and i have no problem proving that anywhere anytime.
Now, if you feel better insulting, rather than discussing.... have at it.

as far as this topic...... its all about misconceptions .... and the proof is in the simulation and the reality. adding weight is not harder on brakes on the track.
this is because, the rate of change of KE can be greater with a lighter car.
its that simple

Thats not what is being said.
lighter vs heavy of the same HP rating , at worst case , will need equal brakes.
this has to do with the rate of change of KE for the lighter car ...
this is on the race track where you are going full throttle to full decel every straight and turn approach.

street use is a different demand on the brakes. being able to keep a semi at a constant speed of 60mph, for 8 miles from 4000ft to sea level is a huge amount of heat to dissipate. its a force x a speed where heat can build up , so larger components are needed to do the job.

put 20,000lbs in a 928 down that front straight at laguna, and you will see the calculations/simulations pan out the same..... there can higher KE dissipated for the heavy car vs the light car before turn in, but because the light car will have a faster rate of change of KE for a shorter period of time, more heat can be put in the brake system . the braking force is limited by the tires. heavier the car, the slower the decel rate.

in my example (real life)

Total KE to slow: 1,684,709 for light car
(1,991,956 start before slow - 307,247 J to orig speed)
Total KE to slow: 1,814,945 for heavy car
(2,085,914 start before slow- 270,969 J to the orig speed)

for the same HP to get to a top speed before braking zones, there is more KE with the heavy car. but to stop in the same distance, it then all equals out.
16% more weight gives about 7% more KE 5% less speed. but if the rate of change of KE (decel) is greater for the lighter car, it can create more heat in the brakes.

truck brakes are sized for their worst case conditions, not for a race track

mark kibort

again...Jim.... i said "example". not that i use ABS... that would be silly, unless i had an 2010 cup car with real racing ABS! below also says, "ABS is close to this factor" I shouldnt have said it was "max potential" although greg brown says it is ( true for 2010 cup cars, not for 928s and old ABS systems). Hopefully you can see the point, rather than see an opportunity to distort the message and toss crazy insults.

I said:

"Lockup is not the best measure. .. ... example.. you slam on the brakes, the Mu goes down and the force is much lower than a slip % that is optimal for the tire. ABS is close to this factor. so , if you can get into ABS down to the final speed point, that is the max potential of the braking system and tires.

James Bailey

correct the braking force is limited by the tires, you got that part right ......and I challenge you to pull the ABS relay and try driving in your normal way....since you NEVER ignore a challenge You are going to find out just how much ABS has been doing for you ! It may be a quick lesson on under steer.

mark kibort

Jim, where have you been?? i run ABS about half the time...... All the World challenge GT races were run with no ABS... two of the those races were on real street tires the rain races i ran, 2 of them had no abs, had a broken sensor wire. if you want a lesson on how to use ABS on the 928, i can help you.
If you get a push by removing ABS... i can help you with your braking technique.

since the ABS is flakey, i sometimes remove the relay.... if you want... I'll pull the relay in the next video and show it and then go race. Not a big deal Jim.
Ive discussed ABS at great lengths here before... I also did a lot of comparisons with it in and out of the car.

what i have found, is that the ABS is a good safety net if you get on the brakes too quickly. it never has engaged under trail braking... remember there was enough decel forces on the rear brakes with the high bias to kick the rear end out? this happened well before any front lifting tire could go into ABS, even with the stock Bias setting.

anyway, if you want ill come down to willow springs and take you for a ride in my rig. heck, you can even drive it! (with me as a passenger)

And Jim... 15 years of wheel to wheel racing, near 200 races, is some pretty decent experience, so i think you might want to listen.... or is it you that might be speaking with a little too much ego?!