Grassroots Motorsports

DEC 2014

Issue link: http://grassrootsmotorsports.epubxp.com/i/397024

Contents of this Issue

Navigation

Page 51 of 187

Grassroots Motorsports 52 Step 2: Spread the Load On the way to packaging all the important parts, a keen eye has to be applied to keeping the mass as low as possible in the chassis. The center of gravity acts as a lever against the suspension geometry–through the roll centers, which are not static–so the lower the mass, the smaller the lever. This reduces the torque acting to lean the chassis in a turn, making the tires' camber less upset and resulting in a higher cornering limit. It's intuitive: A lower center of gravity makes for a better-cornering car. This leads us to the important decision of weight balance. Since we already have four wheels to hold up our chassis, it's best if each one carries the same load. Four people, equally strong, can run fastest with a pallet of bricks if each one carries the same load. If two are carrying too much of a load, they will suffer and the other two will be coasting. Peak carrying capacity will be less. The ideal sports car would have a 50/50 weight distribution on both the lateral and longitudinal axes. In my frst layout for the third-generation RX-7, we put the radiators in the rear of the car as a way to balance out the mass of our front-engined package. In the end, that wasn't necessary for the car to achieve the optimal 50/50 front-to-rear weight distribution–good thing, because rear radiators would have been expensive and problematic. When the car hurls into a corner and its weight transfers to the outside tires, it gets complicated quickly. But in a steady-state turn–like on a skidpad–you want the two outer tires to be equally loaded. This takes balanced front/rear weight distribution. Weight distribution affects more than just whether the car oversteers or understeers, as it raises the discussion of how fully loaded each tire is to its capability. Overloaded tires run at higher slip angles, and the discussion goes deep into tire dynamics pretty quickly. There's the quick fx of larger tires that can be used to offset poor weight balance. (For an example, see the Porsche 911.) Suffce it to say that having each tire carry the same weight keeps the dynamics pleasant and makes our sports car handle and corner better–handling equals feel, cornering equals ultimate g-force. This applies to front-to-rear and side-to-side weight balance. It's rare that a car reaches the optimal 50/50 goal, but it is an important goal. For the Miata project, we put all the heavy parts between the front- and rear-wheel centerlines, as do most sports car engineers. One item we lost out on was the battery: It started behind the passenger seat, an homage to my MGA which brilliantly used two 6-volts wired in series, one behind each seat. In the end, the battery had to go to the trunk, behind the rear- wheel centerline (ouch), for structural reasons. To take the pain out of the compromise, we specifed a special lightweight battery. I take particular pleasure in what we discovered one day when we measured the corner weights on my own 1.6-liter Miata with hardtop installed, half a tank of gas, and 160 pounds of me in the seat. The Miata's front/rear balance was 50.1/49.9, and its left/right balance was 51.0/49.0. The 20-pound supercharger on the left side of the engine threw us off of perfection on each axis, but it more than carried its own weight. (You can never be too light or have too much power, as they say in racing.) I'd be remiss if I did not mention one of the best (and most underappreciated) vehicle packages ever accomplished: the Porsche 914. All signifcant weight is between the wheel's center- lines and low, low, low to the ground. There's a spacious interior and two trunks, with room for a rigid Targa top in the rear trunk. Study the packaging of this car to see what some of the best engineers in the world at the time came up with. It is a thing of engineering elegance on many levels. The Lotus Europa and Esprit are probably the two best- handling cars ever built, both being lightweight with superb suspension designs. All three of these examples are mid-engined cars, something to discuss another day. It's a pretty basic concept to grasp: The tires have to stay in contact with the road to achieve any traction in a corner. The gremlins that spoil keeping the contact patches solidly planted reside in the suspension design. A sports car engineer gets all sorts of coupons with the other design teams when it comes to suspension design: Mess up this area, and the car will be laughed off the pages by journalists. If you're calling a car a "sports" car, it'd better perform well against the competition in the twisties. We talked a bit about how the center of gravity works against the suspension's roll centers to force the car's chassis/body to lean in a turn. Some lean is inevitable, and each degree of lean

Articles in this issue

Archives of this issue

view archives of Grassroots Motorsports - DEC 2014
loading...
Grassroots Motorsports
All Grassroots Motorsports subscribers must register to receive access to the digital editions. Click button at bottom for a free 11-page preview (not available on mobile devices).

If you need to register click here: Create Account
Subscribe to Grassroots Motorsports: Subscribe
Renew your Grassroots Motorsports subscription: Renew
Retrieve Password click here

 or  free preview remember me