mark kibort

yep.... sorry about that. got a little carried away

Ill buy!!

mark kibort

just came in.... they look great but a little nervous about these RB racing rotors.

the air ducting is subpar vs what ive seen from brembo, but there is a difference. holed vs flat with slots.

also concerned with the thickness of the flanges that mount the rotor. 7mm vs the brembo 8mm.

thoughts?

anyone use RB rotors before? Racingbrake.com

the good news is , these are not 15 year old rusted metal and the company has 20 years of experience in metallurgy, claiming many advantages over brembo and stoptech. also, their hardware has some advanages too, self locking, and no need for locktite.

J richard

If you want to shift the load to the front even while accellerating and drop the nose in the process....just turn the damn steering wheel....

just sayin....

mark kibort

that certainly helps!! im a big believer in that too!

mark kibort

go to 4:20

vs
go to 3:45

earlier on in the eclectic discussion of braking forces and performance, the KE discussion reigned. there was a lot of misunderstanding of why there could be a lot more heat in one particular point of a track, if the rate of speed change was high enough. some were saying that the fronts were doing too much work, others said that they had testimonial of cars reaching the same speeds with no braking issues, etc.... but the key point was that its not the speed change, its the rate of speed change. we worked out the rate of speed change for my car at the approach to turn 2 at laguna. Others (mike and a few others) chimed in about testmonials of others that have had no issues overheating brakes , and even running faster times there. But, they missed a couple important points. (besides the 911s having 2x the force on the rears and 20% less force on the fronts for the same rate of decel). they missed the point that its the rate of KE change, not just the KE change itself.

so, when I was watching a fell SCCA'ers professional video spot on youtube, I became interested in his decel rates, because this is a Transam Corvette, 1000hp, and same weight as me (3000lbs) and runs the times that have been quoted here. (1:30 to 1:33 at laguna). If you watch the video from 4:25 onward you can clearly see that he runs up the hill to a 138mph vs my 130mph and it takes a full 7 seconds before he turns in to turn 2 at a slightly higher turn in speed. if you watch my video, its 4 seconds.
almost half the time, to dissipate a slightly higher KE change. yes, he also has 14" rotors, but its no wonder that he has no temp issues and everything seems to wear much longer.

Ill spare the calculations for now, but the rate of KE change is greatly different to the demise of the smaller brake rotors.

333pg333

All I can see is that the guy in the Blue car coasts down the hill where you are still accelerating. He coasts a lot all over the track. Not saying that there aren't times where coasting isn't acceptable. The whole 'mantra' you're either flat out on the throttle or the brake is not necessarily true. So I can't see anything but the obvious in terms of him not having brake problems compared to you. Especially as he possibly has larger rotors than you?

mark kibort

Then you missed the point. when folks tried to chime in and explain they either are running faster laps with no brake issues or hitting the same speed (same KE release) with no brake issue, the point is that its all about the rate of change of KE, not the KE total.
Yes, he coasts a lot ... I do think its WOT or threshold braking and I haven't heard an argument against that yet that makes any sense. a lot of big HP cars will be on the brakes earlier and more gently. (VR was advocating that)
I don't buy it at all, and can easily prove that threshold braking is the fastest way to the entry point of a corner off a long straight. Im not talking about the finesse "S"s or sweepers that end up in a tight turn.
the obvious thing here is that if the rate of change of KE is the same as what I was seeing, anyone with 13" rotors will have issues in this particular section of track.

333pg333

I guess I am missing the point. If you're comparing the guy in the Vette's brakes and lack of issues to yours, all I can see on face value is that his are more than likely to be technically superior to yours and that from the evidence proffered, doesn't have to use them as hard as you. Ergo...?

mark kibort

he doesn't have to use them as hard, if he only wants to be a few seconds faster than my best time. but if he wants to be as fast as a top pro in that car, at laguna, he would have to step up his game to probably brake at the same point im braking, at 10mph faster, and run about 5-10 seconds faster a lap too.

again, the point is, the same KE is dissipated (generally) but it is done over a slower time. if he wants to do what I do, or beyond. (and he should do beyond if he wants to be at the potential of that car) those brake CAN handle it too. Ive been at that speed before with a howling tail wind at thunderhill.. (140mph) ..early braking can easly bring the speed down to turn in speed without much fuss! that doesn't cause brake issues. the change of KE vs time does.

mark kibort

not to bring up old discussion/ argument/ challenges... but .....

ProCoach, (Kraus) Here is a mustang at laguna that not only uses 2nd gear for turn 2 and corkscrew, but 1st gear, as I said at turn 11.
stock street ratios. 4:10 or 4:30 rear end, and 1:1 in 4th gear.



https://www.youtube.com/watch?v=Pj-5...ature=youtu.be

mark kibort

As a side note... dug into my transmission after studying the spare in the back yard. (on why I was occasionaly getting 3rd gear lock up)... found out that the shift forks looked out of alignment moved them and its FIXED! BUT, equally important, I took it on a test drive and it felt so much better. went into gears easily AND NO MORE POP OUT OF 2nd gear!!!!!!!! I did a transmission dance in the street . going to reward the gear box with a nice redline oil change! no more one arm driving through 2nd gear turns!

atb

Pics of the out of alignment shift forks?

[Disregard. Just saw your post on the 928 Forum. Pics posted there.]

mark kibort

https://rennlist.com/forums/928-foru...l#post11720805

this shows the before and after change. it seemed obvious, but was very nervous to change the thing that seemed just plain wrong and out of adjustment. But, marked it with a magic marker just incase I screwed something up and move it. just drove it again. like "BUDDER" !

Im still so fired up!!! its like Christmas just came early!

brendon

Mark, sorry to bring this thread back to life... I only just managed to catch up.

I'm struggling with this engine braking thing. It seems like you are saying that you get more braking force from engine braking at higher speeds? That doesn't make sense to me, I'd expect more engine braking in second gear than in 4th or 3rd gear.

You said that you measured the engine braking force. What was the procedure for measuring that?

mark kibort

Now worries.... I had to read it a couple of times myself to remember the point. absolutely, there is more engine braking force from the engine in the lower gears to the tires. the point was that the engine braking force goes down with RPM. almost linear. I measured coast down and engine braking, on the dynojet 248e dyno. (20ftlbs down to 10ftlbs at the engine for coast down, but as measured at the wheels, and 180-100ft-lbs during compression for RPM range from 6500 to 4500)
This means through a 4.5:1 3th gear it would be near 800ft-lbs , or 400ft-lbs at each tire. (or 400lbs of force at the road at each tire). again, remember, under 1g braking, there can be near a 75% of the vehicle weight up on the front wheels, which leaves only 25% of the overall weight to be split between the rear two tires.

I measured rear braking force on jack stands , finding what gear I could go WOT and use the brakes to get constant RPM. since I know the torque of the engine at that RPM, I can deduce that that's the braking force at the wheels. (then divide by the gear ratio to get ft-lbs of force divided by 2 for each wheel...... and since the radius of the tire is near 12", that's the force at the rear tire acting on the pavement as well)

I also did an experiment where I totally removed the front brakes, and did an engine braking decel in 1st gear vs using just the rear brakes and compared the distance to 10mph from about 45mph. (6000rpm to 1300rpm) it was very close even with a very light bias valve. I even put in a higher rated bias valve and saw a slight difference in the stopping distance.
on the track at the runnoffs, that higher rating bias valve, made terrible tail wagging on trail braking turns, (turn 3 at laguna) that was quickly fixed by going back to the stock valve setting.

So, my comment regarding lifing off the throttle did point to 4th gear being 3.2 to 4:1 and that would mulply the compression braking by 4. the "50ft-lbs" was what the "even if" comment, showing that even a lower compression engine could produce this kind of force, (4x the engine braking force, or near 200ft-lbs) and that might be at turn in. but on my engine at near 200ft-lbs going down to 100ftlbs at turn in, there is a lot of engine braking force that equates to near 800ftlbs going down to 400ftlbs just off throttle and half of that at turn in when the engine RPMs are 75% of redline. both getting very close to the limit of what the tire can act on the ground. adding rear brakes at that point just creates heat as ABS fights lockup, the engine fights the rear brakes, and can create car handling instability.