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The Ultimate Guide to Suspension and Handling Part I: Wheels and Tires

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The Ultimate Guide to Suspension and Handling Part I: Wheels and Tires

Post by DTechR on Tue Dec 01, 2015 9:28 am

The Ultimate Guide to Suspension and Handling Part I: Wheels and Tires

by Mike Kojima

Here is a Flashback Friday article to the beginning of our Ultimate Guide to Suspension and Handling. This long running (10 parts and counting) series has everything you need to make your car handle. Horsepower is sexy and chicks dig it, at least a lot of meatheads think that way. Making horsepower is relatively easy. A lot of competent people know how to make power, lots of it. Until drifting became the rage in this country, handling and cornering prowess was for geeks, the road racing elite and autocrossers. Handling was for dweebs that raced around cones in parking lots, or loners prowling canyon roads at night, not for cool people in the scene. How to tune a car's suspension was an unknown art in the world of mainstream performance, simply because most people didn’t care about handling. Most people emulated the world of road racing by making their cars low. Low was cool, low was handling.



Now that drifting, Time Attack, racetrack hotlapping and autocross are becoming popular, suspension tuning and handling are becoming important items for discussion, experimentation and debate. Building handling into a car is not necessarily any more complicated than building power; it’s just that the subject is somewhat more esoteric and definitive information is not as widely available.



Finding straight-line horsepower gurus for hire to help you is relatively easy, but finding an expert to make your car corner well is a tough deal, fraught with mystery and intrigue. The solution? Make yourself the guru. If your automotive interests are greater than the one-dimensional urge to blast straight down the 1320, brag about dyno sheets on internet forums or install neon and strobe lights to your interior and undercarriage, let’s get to work. There are 4 basic steps to achieving a well handling car and in this first part of our extensive series we'll discuss the first--wheels and tires.



Get Some Sticky Tires

Tires by far are the biggest contributor to finding more cornering force. By bolting on a set of gumball tires, you can make the biggest possible single net gain in cornering power in minutes. Generally, putting the widest tires and wheels that will fit inside your wheel wells without rubbing is the way to go. Specing out an ultra-high performance tire is also important. Most of these tires can perform decently during track days and are good for autocross in SCCA's ST street classes. If you are doing track events, autocrossing or are just plain nuts and simply have to have the most grip possible, you can try a set of DOT approved racing tires. Some of these tires are nearly useable as everyday driving tires while others grip almost like racing slicks and last about the same length of time. Below is a chart which categorizes these tires. Note that this is somewhat subjective and there is some overlap here.

Race compound DOT tires produce more road sucking grip than any suspension mod you can do but the drawbacks are many. First, these tires can be expensive; second, they wear quickly; third, the number of heat cycles that their rubber formulations can withstand before becoming oxidized and hard is limited; and fourth, many of them don’t do well in the wet and none work in the snow or ice. It is possible to end up with an expensive, fast wearing and not so grippy tire if these race tires are used on the street and subjected to many heat cycles. Most people who use these race tires have a second set of wheels and use them only for weekend warrior activities.



Within reason, put a wider wheel and tire on your car to put more rubber on the road. Generally you can stuff a tire two sizes larger than stock into most cars stock wheel wells. For instance a car that came with a 185/70-14 tire on a 5" inch wide wheel can usually easily accommodate a 205/50-15 on a 7" wide wheel with no modifications at all. On most cars you can roll the fender flange flat to get more tire clearance if needed. Do not accept any rubbing between the tire and any part of the car, be it the fender or the suspension, it can be very dangerous. When deciding which wider wheels for your car, try to stick to close to the stock proportions of offset to wheel width to maintain a reasonable scrub radius. We will explain why in later editions of our handling guide. Choose the size of your tire and wheel including the offset with this in mind for the best performance rather than going for the hella flush aesthetic unless that’s what floats your boat. Putting the tires on a wheel of the recommended width for the tire is important as well. Putting a wide tire on a narrow stock rim will cause the tread to tend to lift on the edges under side load. If you can’t get the exact wheel width recommended by the tire manufacturer for your size of tire, then always err to the wider side if you have the choice. A tire that is slightly stretched on a wide wheel will be more responsive and flex less under side load than one whose sidewalls are bubbled due to too narrow of a rim. Some autocrossers looking for the maximum responsiveness out of their tires will stretch a tire on a rim as much as two inches too wide. When doing this, the rim and tire are more prone to damage and debeading under impact so this is not advisable for street use or road racing where you sometimes put wheels off the track or hit FIA curbs.

Plus sizing, which is going to a larger diameter wheel and a low profile tire can help with faster response to steering input and less need for static negative camber for best grip due to the plus tires having a shorter and stiffer sidewall. For instance going from a 205/55-15 to a 205/40-17 on your typical FWD compact car will usually give crisper handling. Going overboard like several inches larger in your rim diameter and running ultra low profile tires generally is not always the hot tip.

Lastly, big heavy wheels and tires also add unsprung weight which reduces the effectiveness of the suspension. Unsprung weight is the weight of the components that are not suspended on a car. This includes the suspension arms, brakes, half of the shock absorber and the wheel and tire. Typically unsprung weight is 12-15% of the total weight of a car. For the suspension to work well, the ratio of sprung to unsprung weight must be kept as low as possible. For instance, have you ever noticed that the suspension of monster trucks hardly works? Even though they may have wheel travel measured in feet instead of inches, it’s hardly ever used and the trucks bounce and bang around like crazed Tonka toys. This is because Monster Trucks have a very high ratio of sprung to unsprung weight. Those huge earthmover tires possibly weigh more than the whole truck. When surface irregularities are hit, high unsprung weight generates significant force with an upward component which means a rough ride and difficulty keeping the rubber on the road. High unsprung weight means that the shocks have to work much harder to damp wheel movement as well. Reducing the unsprung weight and the suspension is worked less, the ride improves and the tire can be kept in contact with the ground better.

For these reasons, putting a 215/35-18 on the same compact FWD car that worked well with a 205/50-15 will probably reduce its performance. Doing something really dumb like putting a 245/25-22 on an EVO is very detrimental to performance. Generally for small bore naturally aspirated 4-cylinder like the older Honda Civics and Nisan Sentras, a lightweight 15x7" wheel is the best bet for performance. For larger cars about 18" is the usual practical maximum wheel diameter. Larger than this, you won’t find as many choices in sizes for true high performance tires. Wheels larger than 18" in diameter are mostly for show except in a few cases like the R35 GT-R which needs its big 20" wheels to clear its huge 15" brakes. Dub sizes don’t have ultra high performance applications either; the huge, oversized in diameter tires offered are mostly designed for the NBA player or gangsta rapper aesthetic.

Get Lightweight Wheels

The obvious way to make up for the disadvantages of increased wheel diameter and width is to use a light wheel. Light wheels are easier to accelerate and brake. They also reduce unsprung weight. Ever wonder why so many MotoIQ project cars run Volk wheels? Most Volks are forged. The forging process improves the grain structure and work hardens the metal (engineering terms for making it stronger!) It provides a superior strength-to-weight ratio. We like Enkei’s competition series wheels as well which have MAT formed rim sections. MAT helps improve the metal's mechanical properties much like forging for a fraction of the cost. Lower cost wheels are usually cast. The casting is a cheapest way to make wheels and nearly all low-priced wheels are cast aluminum.


DTechR
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