spg993tt

two questions. on the cosworth dash, anyone have the equation for the brake bias ratio/percentage they show on the screen?
what brake bias ratio do most of you use in normal dry weather conditions? curious what you find is the tipping point for rear lock up or front lock up.

trader247

Funny you should ask. I run .7

My coach got in the car on Monday and it was at .2. He had not realized it got changed. We were installing a radio and I think one of the guys accidentally bumped in to the ****.

Anyways, he couldn't figure out what was up with the back end of the car. That and the tires were under inflated at the same time. He was having a hell of a time figuring out what was going on. We got the pressures adjusted correctly, but he still couldn't figure out what the deal was. Adjusted the bias to .5 and still was not satisfied. He told me either the track was slick or tires were off.

I pointed out that we have always run at a bias of .7 and now pressures where correct. But, I was a bit timid at first as he was sliding around all over the place.

Moral of the story. Set it at a .7

Car was great again after that, then I had my coach drive again and "son of a gun", it was all good at .7.

spg993tt

i asked someone i trust at pmna, they say factory spec is like .44, .45. i read the manual same thing. 50/55 ratio, so some rear bias.
i looked at an imsa friend who raced all last year. looked at his toolbox data file. he was like. .46 so thats where i put it.
.7 seems very rear biased. my concern with would be that you wont lock the front very often, but you might not also be getting full braking, wearing the rears too much and if you really jump on it, rear locking is a bad dealio. *** coming around and what not.
the manual actually suggested resetting the balance bar back to level. set the bias adapter at zoer and adjust from there. but their spec recommendation i found out earlier today was 50/55 or about .45 which jived with the data i looked at. we shall see. this is one of those areas where i likey abs. not idea to use abs to correct those mistakes. this is where turning off the abs, setting up bias nicely on how you drive, the track, tires. andthen flipping it back on.

trader247

I will look at the car and dash more closely when I go to the track tomorrow or Friday. But, I am pretty sure my bias is toward the front. I lock up the front way more than the rear. Will report back.

trader247

By the way, I am just turning the big Red **** that says F/R

spg993tt

if locking up the front more than the rear and bias is to the front, just turn a few clicks to the rear.
i think if you have a 991 and it shows brake info on the screens...hold pedal for 3 seconds the brake bias pops up. i think the equation is barF plus barR = total car. frontBar/total bar.
so if you're 19 front, 25 rear. 44 total. your front ratio is 19/44. 22/22 would be 50/50. they suggest 50/55 ora .46...a bit more rear bias.
.7 yes is heavy front. and you're not getting full braking out of the car even if you're not jumping on the brakes hard enough to lock them up more. harder braking at .7 i think probably results in more severe lock up. but mainly your fronts are doing 70% of the braking effort thus maybe not getting the most out of the rear, and leaving some braking power/distance on the table.
good luck. have fun.

trader247

I will check that our for sure. I just have driven on the settings I was given. I just got the car late last summer. But now am just starting to become comfortable. I've now driven the Cup about 120 sessions over 30 track days in the last 7 months.

I have only driven on take offs since I got the car to learn car feel a bit better and save money I pick up my first sets of stickers tomorrow

Here is a video from last month on 20 cycle year old take off tires at a bias of .7, Please let me know if the car looks unbalanced under braking, etc. Because, if so that would be amazing.

trader247

By the way, not me driving. That is my coach Brandon Chappell , he is just over 4 seconds quicker than me.

Matt Romanowski

As a note, make sure you approach your biss pressure with increasing pressure not decreasing. You will get different readings on application versus release of brake pressure.

93 FireHawk 968

CRex

The answer to the OP's question really is in his own data.

For all of last year I would've said that the 991 likes rear bias... So much that I ran most events at 45% front 55% rear (calculated values in Pi Toolbox, not the dial setting). Then I was given a pro's data and he's running 50/50 on the same tracks with same tires, and stopping shorter with less brake pressure...

I'd say you have to compare your own laps on different settings and see where that takes you... sorry if it's not the cut/dry answer you wanted.

spg993tt

thanks, i for sure will do some testing with the abs flipped off and try to dial in the bias i like the most. i just wanted to ensure iw as in the ball park. thanks everyone for the answers.

Viperbob1

We found .49-.47 to be the sweet spot. We had a pro driver get in and run a 1:50 at VIR after a single warm up lap. He also changed the bias way to the rear. When the normal driver got in and hit the brake coming down the front straight, locked the rears under braking and did a 720 off the track at 155 mph. Brand new set of slicks gone

Skibum1963

spg993tt

unless you're missing ready it, and its .37 or .4 then that 3.7, 4.0 maybe is the clicks of the bias adapter dial? mine goes o to 10. but that isn't a value relative to anyone else's car other than mine.
you should just have it show the pressures in psi or bar. front and rear. and just compare the two. you want that 45/55 ish kind ratio.
they have a formulat, which i think i had put below based on what i thought it was, which was the percentage of the front braking relative to the overall bars. Front_bar/(Fronn-Bar + Rear_bar)
and that would show that .45 ish kind of thing. on mine, when i step on my 991 brake pedal for 3 seconds, it goes to the brake value screen which shows Bars F&R, and the percentage.
and if i trn the bias adapter , it shows me the value. 5.2, 5.1, 5.0 if i turn it to increase the front bias.