Marc mentioned upsetting our cars' natural brake balance. This is true. "Threshold braking" involves backing off of the pedal just enough so that the fronts no longer lock up, but now your rears aren't working as hard as they could be. The result can be increased braking distances. If you find that this happens to you after making such an upgrade, the solution can be to install a brake proportioning valve. It isn't for everybody, however.
http://www.chevyhiperformance.com/techarticles/71398_install_brake_proportio ning_valve/ (I apologize in advance for the Chevy link) Of the "front disc only" Bugs that I've driven in, most of them have had a tendency to lock up the fronts first. This seems to be contrary to what an adjustable PV was meant to do -- prevent the rear brakes from locking up first on hot rods and modified muscle cars. If you are going to install only an aftermarket front disc brake kit, I recommend that you go abuse your new brakes someplace away from the general public before you put 100% faith in them while driving on the street. Find a road that is either gravel or wet -- anything except dry asphalt (you want road conditions that will allow your wheels to lock up easily without flat-spotting your tires). Practice a few "firm" slow speed stops, gradually increasing pedal pressure until either the fronts or rears lock up. Then increase vehicle speed a bit and try again. Let your brakes cool down for about thirty seconds to a minute between each test. The idea of starting at slow speeds first should be obvious. Once you are satisfied that you know which axle is locking up first (front or rear), you can plan to install the PV. Chances are it will be the front brakes that lock up first. Since an adjustable PV can only *reduce* hydraulic line pressure, you have to plumb it into the brakes that are locking up first. The idea is to lower the pressure to those brakes to bring your brake bias back into a more manageable ratio. Ideally, your front brakes should lock up first -- but only slightly ahead of your rears. While this does mean that you will no longer be able to steer your car under these circumstances, the result of a rear-only lock up can put you into a situation that no amount of steering wheel input can correct. We're talking about doing multiple 360's here. I knew I was going to need one of these for Sharkey before I ever took her onto the road for the first time. With "narrow" 205s up front and WIDE 255s out back, I knew that I'd have a larger contact patch on the rear tires (translation: more stopping grip in back). My rear tires are also about 1.0" larger in diameter than the fronts (translation: more braking leverage in back). Both of these factors meant that it would require *more* pressure to the rear brakes in order to make them lock up. The car is lowered more in the front, and the suspension is much stiffer than a stock car. These two factors meant that the front brakes would apply quicker. I ended up plumbing in a Wilwood PV into the FRONT brake system. First you have to cap off one of the two front brake outlets on the M/C, plumb in the valve, and install a "tee" to split the new regulated line pressure to the two front calipers. The PV uses replaceable inlet and outlet fittings to make it work on older SAE and newer metric cars, and the "tee" can be the stock ACVW tee that splits the rear brake line to L/R and R/R. Mounting the valve can be tricky. I mounted it to the belly pan under the gas tank with the knob of the valve within reach through the wheel well. You can also mount it inside the trunk, but this requires longer line lengths and involves drilling holes into the body to pass the lines through. Once installed and checked for air and/or fluid leaks, take the car someplace where you can test it away from public traffic (and try to avoid any panic stops while en route). Then repeat the lock up detection tests mentioned earlier, starting with value allowing 100% pressure to the front brakes (ie: valve turned in fully clockwise). Repeat as often as necessary (remember to allow about 1 minute cool down periods between times of severe heavy braking), adjusting the valve about 1 full turn counter-clockwise in between braking tests until the fronts no longer lock up first. You may be tempted to try and find the precise point at which braking is spread out 50%F/50%R but I find that erring on the side of caution (eg: 52%F/48%R) is usually safer. You DO NOT want the rears to lock up first -- especially during a panic stop in a corner! _______________________________________________ vintagvw site list [email protected] http://lists.sjsu.edu/mailman/listinfo/vintagvw
