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Dear Dave - April 2008

Geek-At-Large

Apr 1, 2008
Sccp_0804_01_z+ask_dave+hammer Photo 1/1   |   Dear Dave - April 2008

Direct 'Dear Dave' tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.

Q More stopping or more going?
I have a base 350Z and I've been wanting to upgrade the brakes as my first modification. Since I read Sport Compact Car, not Sport Luxury Car, you can probably guess I'm on a budget. The planned set-up is a 13-inch, four-piston front with StopTechs and Racing Brake's 13-inch OE upgrade for the back.

The only problem is that the rears will still be the stock sliding calipers instead of a fixed, opposed-piston set-up like the front. Would this numb the feel on the front brakes because I didn't change the rear calipers?

My driving skill is good enough to feel and understand a vehicle's dynamics. I go to at least four HPDE events a year and do a lot of spirited driving. The guys I run with all say I should go opposed-piston all around, but they can afford cars that come with good brakes stock.

I guess I could try the Racing Brake upgrade and if I don't like it, eBay it. But skipping it altogether would save about $1,500 for that Quaife ATB differential I'm considering. I think I just answered my own question... but I would still appreciate your input.

Oh, and don't tell people to cut their springs, seriously.
Frank Monserrate
Alameda, CA

A.You should be very cautious mixing unrelated brake kits. Odds are good that not only will braking performance be hurt by your arbitrary rear upgrade, but the opposed-piston rear calipers might even make pedal feel worse.

First, braking performance. A properly engineered brake kit like StopTech's will be tuned to provide the correct brake bias. Most factory systems are a little too front-biased for maximum stopping power. In spite of their larger size, the front brakes in a StopTech Z kit actually exert less brake torque for a given line pressure than the stock brakes. This forces the rears to work proportionally harder and should get all four tires to the point of lock-up (or to the ABS threshold) at the same time.

Advantages of a big brake kit come not from increased brake torque, but from maximizing the stopping potential of the tires through brake bias optimization, improved modulation through more rigid calipers, and from better consistency through improved heat capacity and cooling.

Trusting that StopTech did its homework right on brake bias (and there are few companies who get gold stars on their papers more consistently), fitting bigger brakes on the rear is probably going to throw that bias off. While the exact brake torque available from a given brake kit is hard to calculate, it's reasonably easy to guess which way things will go when parts are changed.

Torque is a force times the lever arm it's applied with. When tightening a bolt, yanking on the end of a one-foot wrench with 50 pounds of grunt will get 50 ft-lbs. Same goes for brakes. The force is a function of the surface area and coefficient of friction of the pads, how hard they're clamped against the rotor, and how fast the rotor is moving. The lever arm is simply how far the caliper is from center of the rotor.

With the Racing Brake 13-inch OE upgrade, the OE calipers are used with larger-diameter rotors (this is made possible by Racing Brake's clever caliper bracket that re-aligns the factory caliper with its larger rotor). On the force side of the torque equation, the surface of a larger rotor will slide past the pads a little faster, slightly increasing the force applied by the pads. On the lever arm side, the caliper is further from the center of the rotor, so the arm is longer. More force times a longer lever arm equals more rear brake torque. To maintain proper brake balance, this stronger rear brake should only be used with a stronger front brake.

Does ABS sort things out? To a point, yes, but ABS doesn't simply watch wheel speed and react, it's programmed with some basic assumptions to speed up reaction times. Deviate too far from the line-pressure-versus-torque relationship it's expecting and ABS performance-and stopping distances-will suffer.

There's also the small matter of what happens before ABS intervenes. Imagine trail-braking into a corner with rear-biased brakes. Each tire only has a limited amount of grip-if that's used up with braking, there's less for cornering. Even before lock-up, heavily rear-biased brakes will tend to cause brake-induced oversteer. Have fun with that.

On the issue of pedal feel, it's important to realize that the rear brakes don't clamp nearly as hard as the fronts, therefore don't flex nearly as much. There is relatively little pedal feel to be gained in the rear. On the other hand, a Quaife will make a huge difference on the track and, as you guessed, will be a much more effective use of funds.

Q.Who should you believe?
Years ago, while reading SCC, an article directed me to mount my EGT sensor about six inches downstream of the exhaust port exiting the head. However, the directions that came with my gauge want the sensor mounted after my turbocharger (old-school Subaru). Where should I mount the probe to get the most accurate reading for my EGTs?
Galen Helmgren
Bellingham, WA

A.When you think about the purpose of an Exhaust Gas Temperature gauge, the answer becomes pretty obvious. You aren't really interested in the temperature of the exhaust, you want the temperature inside the combustion chamber. Since in-chamber measurement is prohibitively expensive, the next best thing is to measure the exhaust right after it comes out.

Not only does a turbo soak up a lot of heat, but a Subaru turbo is several feet from the exhaust ports, so the exhaust entering the turbo has already cooled substantially.

If someone tells you to keep your pre-turbo EGTs under 900 degrees C and you're measuring after the turbo, you could miss that critical threshold by a couple hundred degrees. By the time you see trouble, pistons could already be melted.

Stick with the six-inch recommendation and, if possible, monitor the cylinder last in line on the fuel rail. On older Subarus, fuel flows into one fuel rail, crosses over to the other, then finally goes through the pressure regulator and back to the tank. In the event that you start outrunning your fuel pump, the last injector is likely to be the first to go lean. High EGTs will be your first warning.

Q.What would F1 do?
Please explain why a manufacturer would fit an anti-roll bar to a vehicle at the factory. I'm under the impression that well-thought-out suspension geometry at the design stage would eliminate the need for one-saving money in the manufacturing process and resulting in a more balanced ride. Granted, more time would have to be invested up front in designing the vehicle properly, and doing things the right way isn't always as important as the manufacturer's bottom line.

In some applications, I'm sure an anti-roll bar may be the result of sacrifices, such as on a grocery getter where trunk space is more important than corner speed and the bar is needed to keep the car upright while tooling around the mall parking lot. What are your thoughts?
Brian Beef
Geneva, IL

A.Whenever you have a theory about how a car would work if it were uncompromised by concerns over cost, packaging or civility, just look at a race car. Skip the production-based stuff and focus on the world of open-wheel cars. Trunk packaging and cost have little influence in Formula One, for example, where a simple brake rotor can cost as much as a cozy Midwestern ranch with a view.

You'll have to squint to find them, but even an F1 car has anti-roll bars. They're often difficult to recognize-as infinite budgets and exotic materials conspire to warp every part into some overpriced caricature of its Civic equivalent-but rest assured, that tiny, two-inch long, $800,000 piece of billet titanium is actually an anti-roll bar.

The fact is, every part of a car is a compromise, whether balancing trunk space against suspension packaging, crashworthiness against weight balance, or roll control against ride quality. In F1, the biggest compromise is the need to keep the aero bits in proper proximity to the ground, which forces the suspension to be unreasonably stiff.

Building a suspension that reduced body roll to a comfortable level without bars would mean raising the roll center extremely close to the center of gravity. That means orienting the control arms so the inboard side is much higher than the outboard side. From a packaging perspective, that puts the suspension mounts in the trunk or competing with the engine for real estate, and makes the outboard side fight with the wheel rim.

Even if the suspension is built, the resulting performance will stink. With such a steep angle on the control arms, the tires will move from side to side significantly as they travel through their stroke. In a straight line, this makes the car nervous and unstable. In a corner, the side-to-side scrubbing uses up their grip, lowering cornering potential.

Finally, the very forces that make this geometry reduce body roll can, in high-g situations, backfire and push the outboard side of the car up, instead of down. World-renowned suspension geometry expert, Ralph Nader, wrote a famous book about this phenomenon called Unsafe at Any Speed. Look it up.

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