Project Update for June 6, 2014:
After the last update we worked another week on Brianne's Pikes Peak Subaru. In that week several key issues were built or completed, including a custom set of rear wing mounts for a new carbon 14x72" wing, new clutch and brake master cylinders, tome tow hook was added, and lots of other small items were buttoned up before she came to pick up her car. Let's cover this last installment before the 2014 PPIHC race now...
Firewall Mods + New Pedals + New Master Cylinders
This Subaru was purchased in 2011 by Brianne Corn Racing already semi-race-prepped. At that time the factory ABS system had been disabled, and they had very little time to prep the car for the 2011 PPIHC event (a handful of weeks). During the mad thrash to prep the car they looked into repairing the ABS, and replaced some sensors, but it looked like it had been disabled internally and Brianne decided it wasn't worth the effort to repair. Our name was on the fender back then because we sponsored camber plates. We never saw this car until May of 2012, a few weeks before that year's race.
She has since done multiple types of competition events in this car and even likes to use a handbrake for super tight turns, so having a non-ABS braking set-up has some advantages for her - so that ABS system was never revived. Also, with a heavily boosted turbocharged engine (600+ whp) and under racing conditions, the brake boost was erratic. So this Subaru has been used for the past several years with an OEM brake master cylinder, stock booster, and no ABS. Not the ideal situation, for sure, but we had a good plan to upgrade all of that plus give some real front to rear bias adjustment on the fly.
On a race car the weight of the entire driver/car package can change through a longer driving stint, mostly due to fuel load changes. And with the fuel cell normally located in the back of a car a brake bias adjustment is typically used through a driving stint to keep the 4 tires braking the same way, as weight is moved off of the rear tires due to fuel burn. Also, when bad weather hits (and for Pikes Peak it frequently does) you really need to alter the brake bias front to rear, to keep the rear tires from locking. And at Pikes Peak, a simple rear tire lock induced spin could send you tumbling down a 600 foot drop...
Normally the ABS can pretty much keep the brakes proportioned well enough to cover all of this. But when you don't have a computer proportioning the brakes for you "on the fly" hundreds of times a second, like a modern ABS system does, how do you alter the front to rear brake bias?
Tilton brake proportioning adjuster we're using on a different customer's race car to reduce rear brake line pressure
There are simple "knee point" brake proportioning valves that can somewhat adjust rear bias, and we're installing this Tilton unit above into a customers endurance car today (it was installed minutes before I wrote this). This type of proportioning valve is installed inline with the brake hydraulic line for the rear brake channel, just aft of the master cylinder but before it goes into an ABS block or any factory proportioning block or prop valve. The Tilton unit above is specified to adjust from no reduction in the rear pressure to as much as a 57% reduction.
But for true
front to rear adjustments nothing replaces a racing style dual master cylinder with a real adjustable balance bar
. This takes the brake pedal arm (with the right type of dual MC pedal assembly) and moves the pivot point before the dual MCs right to left, to alter how much stroke there is on the front or rear brake master cylinders. A cable operated adjuster (usually on a knob, sometimes a lever) remotely alters the pivot and the driver can make small changes throughout their driving stint to correct for fuel load or weather changes.
This is how most "true race cars" and formula car braking systems are set-up - with a racing pedal box set-up for dual master cylinders and often with a spot for a clutch pedal and clutch master cylinder as well. We forgot to get a photo of the brake bias adjuster knob installed, but it went in the center console right were the yellow "smiley face" decal is in the picture above. This knob adjusts a cable which slides the pivot point on the balance bar for the two brake Master Cylinders left to right, to adjust the front to rear brake bias. This will allow her to adjust for fuel load, weather changes, or even alter the handling - to allow for more trail braking or a more loose rear end, to suit her driving style as needed.
The stock pedal box next to the Tilton top hung style pedal box
We also picked up a 2 pedal, dual MC + clutch MC unit, shown above from Tilton (Tilton 72-601
), which is a "firewall mount" style that has additional mounting provisions along the top the pedal assembly. You can see the factory Subaru stamped steel welded pedal box next to the Tilton piece. In this image the Tilton unit already has a custom tubular steel welded structure bolted to the top, which Ryan carefully built to mimics the OEM mounting holes. This way the new pedals mount to both the OEM mounts as well as the newly fabricated firewall steel section for a much more rigid assembly and no flex when she mashes the brake pedal HARD.
Race Car Brake Bias Explained - http://www.stockcarracing.com/techar...ar_brake_bias/
Brianne had complained about the firewall flexing and the OEM pedal assembly moving when she stomped on the brake pedal, so we tested this. And she was right - the OEM firewall and brake master cylinder allowed 3/4" of movement upwards when you pressed on the pedal HARD, all due to firewall flex. This made for a wishy-washy pedal feel. The factory firewall is just a thin stamped steel sheet metal structure made of very thin metal that has a funky shape with lots of bends in it.
Ryan looked at this and decided to cut out a portion of the firewall where the OEM brake booster/MC and clutch MC passed through. He then took a piece of slightly thicker steel plate, marked out the Tilton firewall mounting pattern, then cut out the various holes with hole saws and drills. Once the new flat steel patch panel was built it was welded into the existing factory firewall and small patch panels were marked, cut and welded in place to fill in the gaps to the curvy firewall surface.