Ira started the discussion in Philip's 308 brake thread... ABS is a great driver assist system, especially when properly tuned to the situation. Ferrari finally recognized this with the ABS ECU update for F355 Challenge in 1998. The F355 Challenge brakes increase available brake torque, torque bias and change weight transfer, especially combined with slick race rubber. The update takes this into consideration. [size=+1]Race Only ABS Systems[/size] A properly designed system in a racing environment can be relied upon by the driver to provide optimal threshold braking, but it still requires a sensitive and skilled driver to feel the tires griping at the maximum slip. He cannot just stand on the pedal and let the computer and pumps do the rest of the work. He can stand on the pedal, feel for grip/slip, and have confidence that if he exceeds available grip, he will not destroy a tire or have a huge push or loose condition. He may have a slight push or loose moment, which can be corrected. So in summation, driver skill is still imperative. ABS helps you go quicker with more confidence. [size=+1]Production Car ABS[/size] Now, for PRODUCTION CAR ABS modified by increasing available grip by installing sticky tires: The system is no longer optimized. You have much more weight transfer forward under braking than with road car tires. If the driver relies totally on ABS at each corner, he's overworking the system, is decelerating at less than maximum, is probably killing his brake pads and putting more heat into the system than if he didn't rely totally on ABS. Use ABS as insurance against lockup. Practice and learn threshold braking with ABS as feedback. With a production car system, changed with sticky race rubber, your goal is to just touch ABS slightly, as it will reduce line pressure and actually increase brake distance. Though it still keeps you from killing the tires with flat spots, and will help you keep control in passing maneuvers. The 348/355 especially should not be run without ABS unless you modify the factory bias valve and remove the delay valve. If you race without ABS on these cars, you'll soon find out there is way too much rear bias, which is there by design for the proper functioning of the excellent Bosch/Teves ABS system. One can argue that a poor ABS system is worse than no ABS, and I'd agree. But in this case of 348 on race rubber, you should use it. IME with 348 Challenge, 1993 - 1996, everyone was running ABS. I could tell if someone was not as it was all too easy to lock rear tires in passing or emergency situations. I tested disconnecting, ran a few races that way, and ultimately ran with ABS functional. Hope this helps, Rob Schermerhorn
This brings out an interesting point: ABS is optimized/tuned for the car delivered from the factory! 10-15-20 years down the road, with modern sticky rubber, it is no longer optimized, and may very well cause as many handling problems as it solves. For example, ABS, thinking that the car is decelerating faster than (15 year old tire technology) can manage, and interveins to prevent a skid when--bang--it causes a crash that would have been avoided it the system were not present.
Interestingly, if I turn off the ABS in my 355CH I get very severe FRONT lockup! I think part of the challenge kit includes a pressure reducing valve for the rear.
Different apple. You have Challenge brakes, not stock brakes. For you, front and rear brake torque and bias is vastly different than stock, hence front lockup. Rob
Interesting thoughts, but... In the real world, street driving, it may activate on a wheel sooner than grip allows for that situation, BUT, you really shouldn't be threshold braking in traffic (though fun). This analogy is perhaps no different than driving a car with less sophisticated, three-channel ABS that comes on with a seemingly hair-trigger, reducing braking ability slightly (Slightly!). In spite of this potential anomaly, it'd still be safer to run the ABS for emergency situations. Rob
You have a delay valve and proportioning valve. They are the original values, original components. Ferrari must feel that the system is fine (and I agree) with these components. The proportioning valve is sensitive to input pressure. At a specific threshold (stamped on the valve in units of bar), the output pressure is reduced at an increasing rate in proportion to the input pressure. This is different than a balance bar on a race car, where brake bias is set and is a function of leverage and not pressure dependent. So with a rear proportioning valve, you have more rear line pressure, as a percent of front, at low line pressures, and less, as a percent of front, as line pressure increases. At low pressure it's a 1:1 ratio, at higher, (I'm guessing actual values), it'll be 1:0.9, then as input pressure goes up, moves somewhat linearly to say 1:0.7 and so on. HTH, Rob Schermerhorn
and while we're on the subject, wouldn't it make sense to have one? The front and rear brakes on a 355CH are identical, so since the brake system is basically the same as the road car, wouldn't the rear therefore be overbraked? Am i missing something AGAIN?
Ok...I actually understand that...so what you're saying is i'm meant to have that device (it's in the brake line running to the rear of the car) but that it's not part of the challenge kit, but a standard 355 part?
Page 9 of the 1998 355ch tech reg book says "4.5-Braking System. It is permitted to use the rear brake regulator part no. 155808 as an alternative to the standard regulator part no. 157908"
Both standard production Ferrari components with different pressure reduction slope characteristics. We did this with 348, too.
Ok, so I understand the principle, but what is the difference btwn them as far as how the car is affected? does the optional one give more bias toward the front, and therefore better for trackwork and sticky tires?
Three things: The F355 has a lower center of gravity than most other road cars, a rearward weight distribution, and larger rear tires. The low center of gravity reduces the amount of weight transfer (from the rear wheels to the front wheels) under braking. The F355 starts with 57% of its weight on the rear tires, so after weight transfer to the front, there is still substantial weight on the rear tires. Finally, the large rear tires with significant weight can be used to absorbe lots of kinetic energy from the car under braking. Net effect: Around 55% of the energy transfer occur at the front, while the rest (45%) occurs at the rear. Since the fronts have brake ducts for cooling, the heat loading of the front and rear rotors is about the same. Last year when I cooked my original pads, the front rotors looked a little cooked, but the rear rotors look 'seriously' cooked--turing a battleship grey with streaks of blue.
Interesting information, and makes sense...but you cooked rear disks?? I've actually had to remove a cooling duct on the rear to get heat into them! You must be one hell of a demon braker!
REXRCR wrote:" Page 9 of the 1998 355ch tech reg book says "4.5-Braking System. It is permitted to use the rear brake regulator part no. 155808 as an alternative to the standard regulator part no. 157908" We did this with 348, too."" Great discussion! The 348 never had alternative valves and none mentioned in the challenge rule book. Have you been updating the 348 with this 355 valve or was there a valve specifically for the 348 that was a legal OEM part? How did the 348/355 part change the bias? Why would you change the bias? You have a lowered 355 with heavy springs so you do not get as much front weight transfer so you turn up the front bias to basically get the same kind of "dive"? And where is chasis rake in this discussion of relative brake bias?
I didn't write that, Peter did. It's in the update bulletins. I don't recall the optional part number, but I did install the alternate. I think it came from the 456. Longitudinal weight transfer, similar to lateral weight transfer, is only a function of CG location and wheelbase, as lateral is to CG and track width. So lowering affects the weight transfer, spring rate doesn't. Typically, ABS activates first on the rear brakes, both 348 and 355 Challenge (YMMV). Consequently, as I outlined in my first post, a driver who relies on technology to threshold brake will have a tendency to overheat components with the ABS on frequently. A possible tell-tale is smoked rear pads and/or rotors when the fronts look good. I highly recommend to the racers out there to start some sort of data collecting protocol with your self and any team members. Collecting brake component temperature and keeping a log of component wear will go a long way towards decreasing your lap times and decreasing your parts bills. Extend data collection to tire temperature and pressure, chassis alignment including rake. It's not a chore when it becomes routine. Back on topic. One would want the mechanical bias set as close to optimal without ABS first. So test without the system engaged, make bias adjustments, then always race with the system functional, but use it as an indication of maximum deceleration and for passing off-line as insurance. Don't let the computer brake for you on these cars every lap. Best regards, Rob Schermerhorn
Mitch, are you running a standard F355 or Challenge? I think Peter's post assumes we're talking about Challenge, as the Challenge 355 does have rear brake cooling ducts.
I am running a street F355 on street tires bone stock right down to the air cleaners. The only mod is the brake pads and fluid. The environment where I cooked the rear rotors was TWS in 105 degree heat (in the shade) after 15+/- laps. Front rotors were also cooked, just not quite to the same degree. I am planning on putting a NACA duct in the diffuser this winter to duct cool air to the rear caliper/rotor. When I have detected ABS stepping in, I feel the fronts go clickety click but not the rears.
