Grassroots Motorsports
Issue link: http://grassrootsmotorsports.epubxp.com/i/397024
Grassroots Motorsports 114 PROJECT CAR: NISSAN 350Z curb weights, although cars in Street Touring trim can easily weigh 60 to 90 pounds less. Both the Evo and the STI use a 245mm tire. With a 9.1-inch tread width–at least for the BFGoodrich g-Force Rival, but other competitive brands measure out similarly–the 3219-pound Evo carries 354 pounds per inch of tread. The heavier, 3351-pound STI forces each inch of contact patch to deal with 368 pounds of glass, steel and plastic. On our 350Z, we'll be running 275/35R18 BFGoodrich Rivals (BFG doesn't offer a 285-width Rival), but we're still put- ting 10.2 inches of contact patch on the ground at each corner. That means our 3247-pound Z is only forcing its tires to deal with 318 pounds per inch. That's math we like. But that wide, effcient contact patch is wasted if it can't stay parallel to the ground. That's where we have to examine the Z's suspension design and assess its suitability for autocross. The 350Z uses a variation of a double A-arm suspension in the front and rear. On the surface, this is good news. One of the advantages of a double A-arm suspension is that its dynamic geometry lends itself to camber gain during body roll. This means the arc of the hub rotates upward as it travels, and as the car rolls, negative camber increases, basically compen- sating for the roll of the body. The result should be a tire con- tact patch that remains as fat on the ground during cornering as possible. In contrast, a MacPherson strut-type suspension will gain some negative camber on initial roll, but because of the fxed upper pivot point of the strut, the hub will begin to lose negative camber and possibly even gain positive camber as roll increases. Now, obviously plenty of manufacturers do just fne with struts–you sure don't see people complaining about Porsches or BMWs being bad-handling cars–but it's nice to go into the process knowing that some mechanically sound prin- ciples are working for us. So we know that the Z has a suspension design that should be benefcial for autocross. Now we can focus on its weaknesses and what we can do within our rule set to overcome them. The Z's most glaring fault is a lack of front camber adjust- ment. Remember, in a perfect world, we want our tire's contact patch to be parallel to the ground at maximum lateral g. Because we know the car will roll some, and even though our suspension is moving to counteract that roll, we'll probably want to build in plenty of static negative camber. But with no provision for front camber adjustment from the factory, the Z lets us make no such adjustments. Fortunately, the Street Touring rules allow us to substitute either the upper or lower arm of our suspension with an alter- native piece that allows for camber adjustment. In the case of our Z, there is no single lower arm. It's actually a two-piece lower setup with one arm handling lateral control and another one handling longitudinal loads. So we're really only able to change the uppers. This allows us to substitute some arms that provide for camber adjustment, so that's a big positive. What else can we do to keep those contact patches fat? Well, we can reduce body roll by installing heavier springs, and we can couple the roll from side to side with anti-roll bars, effectively using one side of the suspension to help out the other during lateral loading. Fortunately, we can freely substi- tute all these pieces under the Street Touring rules. We're also free to substitute bushings with non-metallic replacements. Our bushings were such a mess that we gave them their own sidebar. - ferent arcs during articulation. This can cr fects during cornering, like positive camber gain. W c point of the hub is much more similar. Theo- r esults in more precise control of the contact patch during hard cornering and articulation of the suspension. Illustrations courtesy carbibles.com