1) Excessively wide wheels/tires. It goes without saying that "Donk" cars and their rubber-band tires are gaudily impractical, but there's also a trend to have wheels as "flush" with the fender as possible. To prevent the tires from scraping against the fender's inner lining, tuners often mount tires that are narrower than the wheel, which are then stretched to fit. While it may provide a Euro-aggressive stance, the tire's sidewalls aren't designed to handle such deformation, possibly leading to catastrophic tire failure. And although wheel weight can be offset with lighter forged wheels, the more plus-sized the wheel, the greater moment of inertia the car has to overcome.
2) Poorly designed aftermarket intakes. Intakes are a popular bolt-on modification because they provide power at low cost (typically around $200). But choose the wrong filter-on-a-stick and you can potentially wreck your engine. Cold-air intakes draw air near the bumper and away from the hot engine bay. The denser, cooler air may lead to more horsepower, but the filter is more susceptible to drawing in water, which can hydrolock the engine. Short-ram intakes draw air from within the engine bay, and make power by minimizing flow restriction; but in standing traffic the tubing warms up, leading to an initial drop in power known as heat soak.
3) Knock-off safety components. This week's federal warning about counterfeit air bags in thousands of cars highlights how the advent of eBay and flourishing, cheap overseas manufacturing have contributed to cheap replica parts. These companies can price parts at a fraction of cost because of the lower overhead; there's no need for engineers, stress testing, or marketing. No-name parts are harmless for cosmetic upgrades, but a safety liability when it comes to components such as suspension and brakes; they could be cutting corners by using inferior alloys, for example. And it may be a lesson you won't live to learn if a strut mount shears when rounding a mountain bend.
4) Bigger brakes don't improve braking. It's tempting to look at a Ferrari 458's massive brake rotors and calipers, and conclude that upsizing your own brakes will provide supercar stopping power. Truth is, even the most basic Toyota Corolla has ample brake power. Stomp the brake pedal on an older car without ABS, and eventually the tires screech as the wheels lock, showing that it's the rubber — not brakes — that's the main limiting factor. Plus, bigger rotors or multi-piston calipers may also throw off the front/rear brake bias of the car if not properly engineered.
Even stainless steel brake lines can be a problematic upgrade; OEM rubber lines provide an advance warning of failure because the line gets worn and stretched, which leads to diminished brake response. Stainless steel lines on the other hand can fail with little warning — and when they do, they can drain the brake master cylinder of brake fluid, leaving you with no brakes at all.
5) Pointlessly huge exhaust. We've all seen or heard import cars terrorizing the night with obnoxiously buzzy exhausts, and the truth is larger diameter cat-back systems (exhaust tubing after the catalytic converter) don't always provide a horsepower upgrade, especially for naturally aspirated cars. On those models, the exhaust pulses will tumble if the diameter is too big, leading to less power. Since the size of the pulse is dependent on engine speed, larger diameter exhaust systems can provide better top-end power, but at the expense of low or mid rpm output. Turbo cars benefit from a large exhaust diameter all around. Changing only the muffler does little for performance, and primarily changes the sound.
6) Rock-hard suspension. Sports cars may ride stiff, but spine-rattling dynamics don't necessarily equate to better handling. If the dampers and springs are too soft, the body motions aren't adequately controlled; but if they're too hard, then it prevents the shocks from quickly adjusting to imperfections, leading to a skittish ride over rough surfaces. Nor does a lower ride necessarily provide better performance; drop the car too low and it won't have enough suspension travel, and it'll continually ride on the bump stops.
Original Motoramic article can be found here: http://autos.yahoo.com/blogs/motoramic/six-ways-ruin-ride-terrible-add-ons-183625690.html