RKD in OKC

I recently posted about having to re-adjust the front rebound on my Koni reds to control understeer and rear tire spin after getting stickier tires so I thought I would post the differences and adjustment required after changing from the stock 5/18 to the 5/55 brake bias valve.

The brake bias valve regulates at what pressure in Barr the rear brakes receive less pressure than the front brakes. They come in 5/18, 5/33, 5/45, 5/55, and 5/65.

What is going on is when you first start braking the front and rear brakes are receiving the same pressure and the bias is controlled by disc sizes, pads, and tire sizes. As you brake harder the weight of the car transfers to the front. With less weight the rears cannot do as much braking and risk locking up so the bias valve reduces the pressure going to the rear. Ideally you want the system balanced so the fronts brake more as the weight transfers to the front.

So, why would you want to change it? Well, the stock bias valve was sized to match the weight transfer rates of a stock car. Increasing spring rates, firmer shocks, and stickier tires will all decrease the effective weight transfer rate. With these upgrades the stock bias valve transfers braking to the front before the weight has transferred actually reducing maximum braking. With no weight the fronts will lock up or kick on ABS.

With my firmer springs and Koni reds the car would go into ABS way to easily increasing braking zones significantly. Adjusting the rebound on the rear shocks adjusts the weight transfer timing under braking. Firmer rebound transfers slower and softer, faster. I tired full soft on the Reds and it was still difficult to modulate under heavy braking without going into front ABS.

Having used up all the adjustment available the next step was to go to a bigger bias valve. On my 90GT that had firmer Eibach springs I found the 5/33 was not enough so I installed a 5/55 on the GTS.

With the 5/55 bias valve and Reds set on full soft the rear would get light, a little loose, and walk back and forth under heavy braking in a straight line. Firming up the rear rebound 1 turn tweaked it just enough that the car stays stabile in straight line braking, goes into front ABS, but the rear is still light enough that the rear will start rotating if you trail brake on turn-in.

Man, I love it when a setup comes together!

brianrheffron

Rather than changing "fixed point" bias valves around has anyone gone
down the road of fitting adjustable bias valves such as the ones available
from Tilton and others. I have use a Tilton rotary valve mounted in the
cockpit of my track Triumph Dolomite Sprint and could dial in just enough
bias to ensure the rears didn't lock up under maximum braking and was
thinking of doing the same to my 928 S4. This is quite a common mod in
all classes of racing and rallying and the parts would probably be cheaper
than the Porsche bias valves.

123quattro

The stock bias does not provide nearly enough rear braking, period. The rear brakes essentially do nothing with it. I was working on an automatic car the other day and with the rear wheels in the air and in drive, it was all the rear circuit could do to stop the wheel from spinning at idle. I have the 33 in my 84. I wish I would have put the 55 in. Be forewarned though, with a higher pressure regulator the backend will get more tailhappy under braking (trail braking becomes much more effective).

dr bob

John--

Be extremely careful changing reare bias on street cars driven in the rain, particularly when the car does not have ABS. Porsche was way conservative on rear bias for non-ABS cars, then got even more conservative (why???) on 86.5+ cars with bigger brakes and ABS. My guess is that learnings in the back-motor cars had scared them into very 'safe' settings. All bets change when the road is slick and no ABS; the rear will come around in a heartbeat. There isn't enough steering lock available to coutersteer and power out of this situation. This is also a spot where having the inertail moments way out at the ends of the chassis can make recovery a bigger challenge. The rotation is slower than a mid-engine car but much harder to manage once you are spinning.

mark kibort

Weight transfer goes up with higher deceleration Gs, it doesnt change with rebound and tires, and rear brake bias changes, unless those items change wheel base, CG hight, and deceleration Gs.

Sounds like you were fine tuning for when ABS engages.

I think I could use more rear brake bias. Currently, at over 1g decel, the weight transfer is pretty high, with less 25% of the of the total weight, sitting on the rear wheels. a small fraction of decel is on the rear tires. In my race car at 3000lbs 24"Cg height and 100" wheel base, thats near 800lbs weight transfer, NO matter what you change on the shock settings. (weight transfer = gs of decel X (cg hight" x car weight/wheel base) )

Currently, my pad wear front to rear is pretty dramatic. 10 sets to 1 set wear . temp in the paddock after a race session is 500 degrees F vs 350F for the rears.

based on video accounts of race cars with all sorts of rear brake bias settings, even greater than what I have, i dont see me being at any real disadvantage. More of a fine tuning adjustment, or one to induce oversteer on trail braking for car rotation.

dr bob

Mark--

Weight transfer changes with rear rebound settings on the shocks. Stab the brakes and the rear lifts more with low rebound settings, shifts more weight to the fronts. Set rear rebound damping too high and you lift the inside rear on hard turns, so make small changes as you tune and test.

mark kibort

weight transfer is determined by 3 factors alone:

weight
Cg height
wheel base.

There is nothing you can do to increase or change weight transfer for a given g of deceleration.

You can slightly change the Cg height with anti squat, dive and shock settings.

mk

RKD in OKC

Mark...mostly. I did put "Fine Tuning" in the title of the thread. A common misconception, the bias valve on Porsches does not set the static bias between front and rear braking. It sets when the pressure is limited to the rear brakes. This setting is for the transition from acceleration into threshold braking so you can use more rear brakes later into the transition. And contrary to your static formula for weight transfer, the timing of that transition IS effected by springs, shocks, etc.

Rear shock rebound has the most effect on brake transition. Try this... Set your front rebound to full soft and go do a full throttle acceleration in first. Not a drop the clutch burn out, start it rolling then floor it. Then set the front rebound to full firm and do the same full throttle acceleration in first. Makes a BIG difference on my GTS, and is especially noticeable with sticky tires.

With the rebound on full firm the weight doesn't get to the rears soon enough and they spin. To accelerate quickly I have to lift to stop the wheel spin and get back in the throttle, or "Modulate" throttle application. Yes you can learn to modulate throttle application to match the transfer, but there is no mistaking with the softer setting more power gets to the ground. And not just sooner, but overall because of the smoother transition.

Same thing happens with rear rebound and braking. What happens if you are cruising along and you stab the brake pedal as hard as you can as quickly as you can? The front tires lock up immediately because the weight hadn't transferred to the front yet. You learn to modulate brake application to match your weight transfer just like you learn to modulate throttle application when accelerating. And yes, you can still brake well, just not quite as late as with more rear brake later into the braking transition.

I've learned that getting the braking transition fine tuned shortens my braking zones, takes a little wear ratio off the front brakes, and is one of the little tweaks that gets me FTD.

mark kibort

compression and rebound do effect body roll transision times. (longitudinally as well as laterally)

Even using rear brakes alone will transfer weight to the front. again, whatever your g rate of decel, there will be a fixed weight transfer. Timing is not dictated by spring and shocks, its dictated by G loading. there is some gain to have the body roll backward and have the center of Gravity height rise. It increases the weight transfer. if your compression in the rear and rebound in the front are too stiff, then you lose this added weight transfer and grip to the rear tires. It has nothing to do with the transisson of the weight to the rear. the rear suspension dropping down doesnt shift weight to the rear. the acceleration shifts weight to the rear.
If you are burning rubber at any suspension setting, you are applying the gas too quickly and not matchng the weight transfer on acceleration.

With my larger brakes and better pads, but mostly bigger front tires, i can put more pressure on my front wheels, get more weight transfer and started to notice more rear trail braking effect for a given brake pressure. Im using more than before, I have more weight transfer so the rear is lighter than before and for the increased pedal pressure, more braking force to the rear for better rotation of the car around those kind of turns.

Now, what you might be talking about is high speed rebound. you get on the gas quickly, but its like there is no weight attached to anything, the body rolls and then compresses as the springs bind in the rear. you just utilized the rockering back of the chassis, to have a momentary force on the rear. after that, the weight transfer is a factor of only those 3 elements.

you want to use 100% of the effective braking ability of the front brakes when every you are on the track trying to slow down. anytime you are not, you are leaving time on the table. the % slip of the front tires needs to be somewhere around 15-20% for max decel, just as it is on accel.

What you are seeing is a timing difference of how you are applying the gas when rebound in the front is high or low. It gives you a little coushion as you get out of the hole, but its a technique thing. It has nothing to do with weight transfer time for a given acceleration rate. Its all about throttle modulation and tire slip.

under braking, you want as much front brake as possible, THEN, add as much rear brake as possible before you creat instability by rear lock up. the more front braking you have the more the weight transter. as I mentioned, my car under full threashold braking, is 2200lbs on the front and 800lbs in the rear (thats only 400lbs of weight on each tire. you can see, its not going to take much force to lock them up).

There are books written on the subject, and If I remember correctly, there is a section in Carrols book about it.

as a side note, the bmws have found a way to lower the rear ride height under braking, which lowers the Cg height and lowers the weight transfer for the same decel Gs. This allows more braking force to be applied to the rears. You get less forward squat, less camber change and toe out in the front and that futher increases longitudinal G loading capabilities. There is a reason you want some body movement into a turn. the wheels are unsprung. they can turn first andset the turn, the body can follow afterward, but there are limits. too much, and the body in motion, will act on the the wheels when it ends up running out of rope. There lies the rub. just enough to get the job done, in acceleration, braking, initiating turns and through the turns.

mk

slow motion launch video


http://www.youtube.com/watch?v=wL_6d...eature=related

IcemanG17

Interesting results with the 55 bias bar.....I have the 33 in the widow along with GTS big blacks with pagid orange pads...to say the brakes are powerful is a HUGE understatement....I also have fairly soft rear springs (300#) and the rebound damping of the rear IS too soft....I have to be careful in trailbraking...as the rear WILL rotate quite quickly in trail braking...so I typically don't do it...since the widow turns in pretty good as is....I'm sure as I push the car harder and harder I will uncover more weaknesses in the setup that will need to be addressed....but the car has to run 1st!! :>(

RKD in OKC

Go try the front rebound setting acceleration thing. And it's not a hole shot, get the car rolling then floor it. Unless your car is so underpowered it can't spin the tires at full firm, or so overpowered it can't not spin the tires at full soft you will be surprised.

I am absolutely certain it is NOT a difference in my throttle application. If anything I am very very consistent. I did some intercooler testing on a turbo at a test and tune night at the local drag strip. I did two runs with each of 3 different intercoolers. The goal was to be as consistent as possible to check the differences in the intercoolers. My two runs with each intercooler were within .003 seconds of each other. That is so consistent the local bracket or dial-in racers said I should be doing dial-in. The announcer at the dragstrip was even talking me up and telling the crowd what I was doing. The bracket winners usually have .020 to .030 seconds between their runs and that's what they are shooting for.

worf928

One thing to keep in mind is that Brian and MK are missing lots of weight in the rear of the car while RDK isn't (AFAIK.) When 928s are "lightened" for tracking, comparatively more weight comes out of the rear than the front.

RKD in OKC

Brian, I bet the widow is much lighter than my Street GTS and that would make a difference in the bias timing. The rear springs really don't effect braking. If you don't have adjustable rebound on the rear, for a quick fix you could go to a lesser pad on the back to reduce the rear braking.

mark kibort

what you could be experiencing is the rotation of the body, longitudinally, and it stopping on the rear springs. It adds a little force and can give you the initial bite for a drag race, but in road racing, or in most applications, there is some level of trade offs the movement we need is due to pavement consistancies and transitions around turns. you do NOT get weight transfer through acceleration via softening the front rebound and rear compression. again, that is determined by the g loading, Cg hight and wheel base.

You have to logically think what is going on. accel, turn it, the body roll and weight transfer is a double edged sword. Too much in a turn, you get thewheels to really bite and turn in, but then the body roll is too much and when it gets "the end of its rope" it pulls the car off the traction circle, raises the CG and lowers its gip holding capability. This is analogous to braking and accel as well.

Trust me, the reason my car does as well as it does, is that ive experimented with the too soft settings. They dont work and cause ALL sorts of problems in all aspects of road racing. they are fine tuning tools that make the car do what you want it to do.

Dennis K

Also they're running wider & stickier R-compound front tires. Greater deceleration = greater weight transfer & thus a need for greater front brake bias.