RKD in OKC

If rear brakes do not contribute to overall braking, why bother to have them on the car.

That being said, you solution for brake fade on one corner on one track is higher temp pads.

Speedtoys

He has a higher Mu, broader temp range pad up front which was the original problem with not also upgrading the rears.

Now its that _plus_ a longer torque arm on the front brakes.

mark kibort

Welll, thats an easy answer. even what i say was true for all circmstances, you would need the rear brakes during heal toe, and you would need them in the trail brake area. also, in the initial dive before weight is transfered to the front.
Also, it depends on the car and the level of engine braking, but the most important point would be you would need it where the forces from the engine are very low. those forces go up as the speed goes down. (constant power of the engine braking) while braking power goes up with speed, as force is constant.

This is a good point but i think the pads i have currently , pagid blacks have a pretty comparable Mu and temp range.. after all, both pads were in pretty bad fade for that 1 second area of the final slow down into turn 2 at laguna. an easy fix would to put a set of the grippy rears on the car for the next race. maybe St41s on the rear might be worth a try. i wouldnt expect it to be a bad , and might help slightly with slowing distances from the inital bite of the rear pad where engine braking is at a minimum.

mark kibort

Bob???? I asked you to validate and elaboate on your claim of two major points.

1. that throttle set up position has an effect on engine braking ..
2. that racers dont want, need or use compression braking.

if you are going to make gross generalizations here, you better be able to back them up with a shred of evidence or proof. (or even logic).

tossing out insults, doesnt make your point any more valid.

Really now..... if im wrong on this , i certainly want to know!

mark kibort

Jim, you also made some claims that also dont make sense.

You know that the proportioning valve doesnt change the brake bias pressure that much. so, going to bigger rotors up front gives the same stopping force, with less pedal pressure, but the rears will theoretically have the same force on them. if you were threshold braking sucessfully before and had proper bias, nothing will change. the reason you want to go to bigger front rotors , is more force during the fade area, and to avoid fade to begin with, by having more thermal mass up front. proportioallly more mass . if before you changed the rotor, you couldnt brake up to the limit of the front tire, which is usually not the case, then you would be right.

in your analogy above, you mention stopping better by using the rear brake. thats also incorrect. if you are able to just lift the rear tire up on a bike and steer and control it, you are stopping "better" than the guy next to you that is using less front brake and more rear brake. its just simple physics there. braking decel has a byproduct of weight transfer. the quicker you stop, the more the weight transfer. the driver of the "bike" would need to modulate the fronts to be at the edge, just as a dirver needs to be at the edge of ABS, and not in ABS as GB insinuates. (at least with older ABS systems like the 928) this is because clearly, a skilled foot is much better than the ABS system for threshold braking.

littleball_s4

Mark,

Your calculations will conservatively imply your car will lock the rears in 2nd gear at anything below 0.74 gs of grip available just lifting the throttle. I don't think this is the case, is it.

I don't know if it's the way you use the maximum braking torque figure instead of a weighted average (your figure is like 230hp, but the cranking torque is no more than 10hp or the engine would not start), or the assumption than steady state and transient braking torques for the engine are in the same order of magnitude (1.5gs is 3150rpm per second in 2nd gear. How long does it take the engine to go from 7000 to idle in neutral?). Or plain measures, because 230hp of braking in the engine sounds a little too much.

But the result certainly sounds odd to me.

RKD in OKC

How about this one...

If you increase to rotor/pad area of the front brakes it makes less braking force with the same line or pedal pressure so the rear brakes producing the same braking force at a given pressure and making more rear brake bias. This may be enducing ABS sooner than the actual limit of the front brakes. So if you are braking at the ABS limit you are braking at the limit of the rear tires and NOT the front tires.

This is from and Audi racing forum on increasing front brake size to reduce brake fade. And it is more of a problem for them because the front and rear brake lines are cross linked, front right to left rear etc.

They recommend also upgrading the rear brakes to more area so the rear braking is matched to the fronts so the rears do not enduce ABS too soon. That or re-doing the brake system to separate the rears from the front so a bias valve can be used.

littleball_s4

I can't see at the moment how incresing pad or rotor area decreases braking force given same pedal force and materials.

Can you elaborate? (sorry if I'm thick lately).

RKD in OKC

Surface area of pad increases, pump cylinder area of Master Cylinder does not, Hydraulic leverage? Think they explained it as clamping force.


I was just regurgitating what was on the Audi racing forum about upgrading brakes.

mark kibort

I like it.. you are actually calculating! Nice..... Ok then, first of all. the number was a peak number, but it is measured and valid. however, you are right, from 6000rpm (where i blip and release generally) and where i then are steady state or applying throttle, its about 1/2 that force.
( I still owe you the dyno run.. .. ill get it to you)

so, since the decel forces at near idle are very small. in the engine braking department... the HP goes down with speed going down, almost proportional. at 1500rpm, the HP for braking is only 10hp. so you can see at 5-700rpm or so, that would less than 5hp.

you also might be overlooking the fact that in the lower gears that HP produced for engine braking has the force multipled by a lot more in the lower gears. this is not a guess, this is a measured fact!
you are asking the right questions though, and yes, the range a racer will use the engine 3000 to 6000rpm can have 75 to 150hp for braking force on a weak 230hp engine, or near 100 to 200hp/ftlbs for braking on a higher compression or larger engine.

and, the rears dont lock, they will be slowed faster than the decel of the car and create more slip %. some is good, but with a slick, its a sharp edge. go over it and you get skidding.

Ive done and even can do even more tests! ive even driven withOUT the front rotors. i mount up the calipers and put a brake pad inbetween to simulate a rotor and rubber band it together. then all the brakes are the rears. interesting. you can see that the engine is very close to what is produced by the rear brakes in 2nd and 3rd, especially in the higher RPM ranges. I also can jack up the rears and go WOT and hit the brakes and keep the throttle pegged. to see the effectiveness of the rear brakes in 2, 3, or 4th. the rears cant control the speed in 2nd or 3rd, but 4th yes. I should video this, because this is actual proof of how things work in reality.


this is not true... i see where they are getting that , and its missing a key point. a larger caliper, yes...... larger brake rotor, no the caliper and brake system has no idea what it is clamping on........ but larger pad in an existing caliper... still no, but thats because you lower your PSI of pressure, but you have more area, so it is the same.

as far as ABS getting activated, the opposite is true. larger rotors up front mean, if you were at the limit of the front tires before, for less pedal pressure, you will get less rear brake effect. (but not much, due to the proportioning valve getting maxed out for rear pressure at only half or so of the overall pedal pressure. so, you will not get the rears to ABS, unless you have major fade, and then you push so hard that the cool rears get more and more pressure until they possibly lock the rears.

it doesnt. rotor area makes no change, only more leverage , so more stopping force for the same pedal pressure. more caliper size could do this...... more pad size doesnt do it either, even though the PSI goes down with the larger pad, there is more area, so the overall force on the rotor is equal to any pad you use.

They might be smokin something.

mark kibort

send me your email address or send me an email to
and ill send the decl engine braking curves.

mark kibort

ok folks, (for those still interested in this discussion)
I thought of how to actually measure rear braking forces and it suddenly became clear..... so I measured the force with moderate pedal force at the rear axles.

it was near 1700ft-lbs.

The interesting thing here is, that engine braking force is near 200ft-lbs at the engine, so through the transmission in 4th gear, that's over 600ft-lbs ( or 300ft-lbs at each of the rear tires)

So with threshold braking that 2000lbs being very real with the bias I have installed, and much higher than the rear tires would be able to handle. from a moderate pedal to pressing as hard as I felt I could without bending pedals, the force was pretty consistent and repeatable. that's 1000ftlbs on each tire that includes plus what engine braking provides, is well in the range of max capability of a tire with a Cf of .6-.8 and all the weight transfer.. even with a 75% weight transfer, that's only about 300lbs sitting on each of the rear tires. engine braking would be variable, in fact, when im in the fade area, the RPM is lower, so that engine braking range might only be around 50 to 100ftlbs. but pressing harder during a fade, the proportioning valve keeps the pressure on the rear bias near constant. (so even harder pressure on the pedal doesn't rise the rear brake pressures)

guess how I measured this??

mark kibort

jeff??

mark kibort

Did the tests at the track. the 33 bias bar made it impossible to approach turn 3 at laguna with any turning input while braking hard on the approach..... its one of those turns where you cant brake straight, you need to have some steering input on the approach. what happens, the back end steps out wildly and forces steering correction. this happens on all hard braking turning turns there, and had no positive effect on braking on the main straight. so, replace it back to the stock 18bar and its all fine again.
again, this made perfect sense. the rear brakes cannot contribute more than what is provided by engine braking at peak force. the physics don't add up ..... all the street tests with rear brakes only (front brakes disabled) proved this as well. there is just not much force created by the rear brakes, even with 335 rear DOT slicks. weight transfer is just too much in a 1.5g braking event, and that leaves very little weight on the rear tires. 300lbs a rear tire at best. so, that means at a mu of 1.0, you cant expect more than 300lbs of force to be applied to the rear tires. engine braking provides that and the rear brakes fill in the gaps when the decel RPM gets lower.
Great experiement. possibly, if the rear tires were new pirellie slicks, the results might have been different.

mark kibort

bought a very slightly used set of pagid Black pads (the pads that I started with when I was having the braking issues of fade with the additional HP of the stroker).

I went back to them, just a budget issue, and man, it was so different from the ST41s or the PFC01s. I ran two laguna events and they were junk. the disintegrated , and some of the fade came back .

got a new set of PFC11s (new model) and bedded them today. ready for laguna next weekend!