Brake problems???

[Jenner]

chance you were on the throttle and brake at the same time. From experience and what I've read the drive by wire computer gets confused when you do this. If you read into the features of drive by wire, when you brake the ECU starts shutting down injectors to help slow the car down and conserve fuel. If you are on the gas and on the brake at the same time, these conflict.

Aside from that, I was at WOT on the main straight at Watkins Glen and so was LarryTT. I can't describe his experience, but mine was opposite yours. I over heated my brakes and instead of the pedal being hard, it just went straight to the floor with little or no affect.

The stock TT brakes are fine for the street, average daily driving, and have caused many TT owners to get rear ended because they are great before they over heat, but once you reach that point watch out!

[GWR]

A friend and I took my Audi 225 TTQ coupe to an auto-x yesterday, and experienced some issues with the brakes. The car's brake pedal seemed to go rock hard and there was very little (if any braking power). This seemed to happen in wide-open-throttle sections into a slow turn and even the shutdown area after the finish. I saw an earlier thread about some brake fading issues with the car, but this didn't seem like brake fade to us. It seemed like the car did not have any vaccuum left in the brake booster cansister. The reason it didn't seem like brake fade is that it wasn't consistent and the brakes seemed to work fine in later corners. Also, if we backed-off the throttle a little earlier, and weren't so abrupt between the gas and brake, the brakes seemed to work fine.

Any insight on this would be appreciated.

To remain legal in the SCCA A-stock class I can't change hoses, rotors, or calipers - about the only thing I can change are the pads

Thanks,
Gary

[AIRSTTRIKE]

[AIRSTTRIKE]

[smallTTs]

I doubt this is fade.

The function of the check valve is to maintain a supply of vacuum in the brake booster for just such an occasion as you describe. Full throttle is lowest vacuum condition, and if you lift and brake hard immediately without vacuum reserve in the booster, you effectively have zero boost, and a rock hard pedal. As the vacuum returns (due to closed throttle) braking begins.

Usually you can simulate this by dragging the brakes (left foot) while full throttle (in 4-6th) and depleating the vacuum...pedal should get very firm. Letting off the throttle will return the vacuum boost.

Another thing that occurs with (vacuum) boosted brakes is response lag. Many non-sporty cars have a lag from the time you hit the brake pedal until boost (assist) cuts in. You can test this if you wish, (closed throttle and clutch in preferred) by hitting the brake padal as hard and FAST as you can BUT release it almost immediately. It's sorta like stomping on a bug. If you get almost no braking, it's booster lag. I have not tried this on (her) TT yet, but I will.

The most responsive boosted brakes I've found is on the C5 Vette..if I "stomp the bug" it goes immediately into ABS. In most other cars nothing happens...unless you keep you foot on it!

BTW, if the check valve is TU, and you are coasting slowly and shut off the engine, you won't have boosted brakes. That's the other reason for the check valve. You should have a couple of stops of reserve vacuum.

My $.02

[Marius]

[TTop Gun]

the ABS kicking in because you're braking too late for the turn??? I've blown through many a cone wall because the ABS wouldn't let the car stop because I started braking 10 feet too late.

Professional conehead on closed course. Don't try this at home, kids..... :-)

[Marius]

Whatever we think of ABS, I find the car brakes better full-on (with the ABS doing its thing) than it does with a lighter pedal pressure, before the ABS sets in. So, if we're racing, it should be ABS braking all the time!

[TTop Gun]

My car can generate 1.1g's of braking force in a straight line. Feed in even the slightest touch of steering turn, and it drops to 0.6g as the ABS intervenes. When the ABS comes on, the pedal gets hard. I've got over a year's worth of GEEZ maps/graphs that illustrate this plain as day, doing *exactly* what the original poster was doing and getting the exact same results.

Now, the question is whether braking 25 feet earlier and getting 1.1g of force is faster than braking later and getting 0.6g of force. I'll take the 1.1g any time, especially in a TT. Besides allowing the suspension to settle for the turn after releasing the brakes, it lets anyone who right-foot brakes get their foot back to the throttle and start spooling up the turbo to power out of the turn. Getting the turbo spinning is extremely important on the 225 that the original poster is driving. Mat the throttle at the entry to the turn, and get tossed into the seat at just about the right spot on the exit.

To summarize, program a mental alarm. If you feel the ABS or the pedal gets hard, you're braking too late in the turn. If you do manage to get slowed down, your corner speed will suffer because the car is unsettled from the wild weight shift as you let off of the brakes. Then, you're sitting there with an unspooled turbo and can't dig your way out. Remember, brake on, brake off, settle, turn in, throttle, turn out, exit.

Of course, what do I know about this driving stuff. I only drove 20 tires down to the cord last year..... :-)

[Marius]