Grassroots Motorsports
Issue link: http://grassrootsmotorsports.epubxp.com/i/397024
Grassroots Motorsports 125 TECH: TRY THIS AT HOME DIY: Dyno Your Anti-Roll Bars. Skill Level: Easy to Moderate Tools You'll Need: • Jack and jackstands • Ruler • Wrench or two Parts Required: • Long bolt and matching nuts • Heavy barbell weight A nti-roll bars are a common and inexpensive way to tune the suspension of your car. Gener- ally speaking, adding a larger front bar means less oversteer, while adding a larger rear bar means less understeer. But that just tells us the direction of the change, not the amount. Sure, there are plenty of formulas and calculators foating around on the Internet, and they do a great job determining the theoretical rate of bars that are shaped like a C. However, the packaging constraints of today's cars, especially in the front, put the lever action of an anti-roll bar at all kinds of odd angles. Often, it is impossible to model such a "spaghetti" bar with simple formulas. Add in hollow bars of unknown wall thickness, and those calculations become more like ballpark guesses. How to accurately compare one bar against another? Measure them. In fact, if you measure the bars when they're mounted to the car, you can also determine the effect of frame mounting bushings and end links. For the sake of ride comfort, those bush- ings can often soften the bar for the frst half-inch or so of defection. We recently took some measurements of a variety of bars installed on our Honda CRX. We simply jacked up the front of the car so both wheels were in full droop, removed the wheel from one side, disconnected the end link, and hung a long through-bolt down below the control arm. We then hung a 50-pound barbell weight from that bolt. Then it was just a matter of measuring the bar's defection. Simple math will yield the bar rate in lbs./ in., just like a traditional coil spring. To get a sense of how much bar action is lost in bushing compliance, you can take more data points with additional weight added. That gives a curve that levels out once the bushing is fully compacted. Wondering how two dif- ferent anti-roll bars r compare? A spare bolt show the exact difference. story and photo by andy hollis