<VV> Re: brake bias/balance

[djtcz@comcast.net]

> Ken questions front-skid versus rear-skid brake balance. the answer, quite 
> simply, is that occupant safety is optimized for straight-on impacts. with 
> the fronts locked, your vehicle will point in the direction of travel of the 
> CG of the vehicle, and if you hit something at least you'll hit something 
> head-on. 
> 
> oversteer, whether induced by throttle or by brakes, is an unstable 
> condition, and sometimes full opposite-lock steering input is not enough to get 
> things back in line. 
> 

Fronts-should-lock-first is about all I've read since 196?.  Real world Experiences suggest that is indeed the standard design philosophy. 

The notable and maybe only exception is from the un-numbered SAE paper "The Chevrolet Corvair - origin and development" by K.H. Hansen. In my faded pink copy of the 1979 Motorbooks collection it says on page 11.  "In actual practice it is desireable to calibrate the braking effort distribution so the rear wheels slide first in a panic stop.  As known from experience, a sliding wheel can take no direction, but if only the rear wheels are sliding a car can still be directionally controlled by the front wheels which is especially important for emergency conditions."

This curiously unique statement makes me  wonder what OTHER assumptions that GM design team (or was it one iron willed chief engineer's assumptions?) made about 1950s driver skills.

There is another explanation for choosing "FLF" that means more to me than the stable sliding behaviour.  That is the lost front braking capacity.  That is, if a driver is braking hard, and successfully maintaining the situation on the verge of wheel locking, then RLF must always limit or hide the amount of "excess" front capacity. On the other hand, FLF means ALL of the potentially major brake capabilities are available, all the time. 

In the absence of some active rear brake sensing and control (like a height sensing proportioning valve) the rear wheels (RLF) would have to be naturally "proportioned" to close to 50% to avoid locking the fronts too easily on slick surfaces.  That same SAE paper shows the 1960 Corvair brake balance at 46/54% and the 'weight transfer" at nearly 50/50 during a (16 fps2) 0.5 G stop, when the equivalent big Chevy is more like 60/40.  The same paper has the Corvair stopping at 28 fps2 at (rear?) wheel lock-up.  Braking real hard the Corvair would be at ~60/40, and the big Chevy would be ~ 70/30

Modern FWD cars start at 60/40. Hard braking has to shift that to near 80/20. It looks to me like Even dinky little rear drums are probably not a real hindrance on some cars, and rear disks may have to be turned way down.

Certainly on vehicles with sticky tires, powerful brakes, and high center of gravity/short wheel base, like modern motorcycles the rear wheel brake might as well be a shriveled appendage when braking at the "limit."
http://www.frankseidel.de/Pics-YZF/stoppy1.jpg

The highly regarded Motorcycle Safety Foundation teaches "riding out" a rear wheel brake skid, but for reasons unrelated to car craft.