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Center Gravity - 9.8 M/S2

Posted in How To: Suspension Brakes on August 1, 2004 Comment (0)
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Off-road driving is a constant battle against gravity. Every time you drive up, climb over, or slide down obstacles gravity is there to fight you every inch of the way. You could try building a burly vehicle with all the hard-core off-road equipment to even the odds, but the more weight you add, the harder gravity will fight. We know it's unfair, but those are the rules.

Just because there are rules doesn't mean you can't cheat a little. If you've ever seen a rockcrawling or hillclimb competition you've witnessed people use their vehicles to beat gravity, sometimes overcoming it altogether and getting airborne. They do it by exploiting gravity's one weakness-traction.

It's not really that hard if you know how to do it. The traction our tires give us is nothing more than adhesive friction magnified by the force of gravity pulling the tires into the terrain. Spread that traction evenly over four tires, add some throttle to the mix, and you'll send gravity back to Newtonian Physics 101. We'll spare you the vector diagrams and science lecture, but if you take some time to understand how gravity works against you, then you can find ways to make it work for you.

Common CG TricksAs with all motorsports, competitors lead the evolution of off-road chassis dynamics. Some of their tricks can apply to recreational 4x4s too. Others you should avoid like single-speed transfer cases.

1. Put water in the front tires-Avoid This trick adds weight low in the chassis, but puts unnecessary stress on the front axle components. Unsuitable for high-speed travel.

2. Center-mounted fuel tank-Do A center-mounted fuel tank means less weight transfer as the fuel level changes for more consistent chassis loading.

3. Compressing the front suspension with the winch-Do Running a winch cable to the front axle will let the driver limit the amount of uptravel the suspension has when climbing a waterfall. If left to fully extend, the front springs could literally push the 4x4 away from the wall and into a backflip.

4. Rear engine-Maybe A rear-engine vehicle can give a 50/50 weight distribution and make for more responsive handling. They certainly have visibility advantages when you don't have an engine blocking your view. Requires axles to be flipped upside-down to compensate for reversed engine position, engine cooling can be difficult, less cargo space.

5. Single seat vehicle-Avoid Unless you have no friends (and don't want to make any) make sure you have at least one seat for a good co-pilot.

Finding Your CGA 4x4's center of gravity is the three-dimensional point that you could suspend the vehicle from and it would balance perfectly in all three planes. Think of the CG as the vehicle's core where all accelerating, braking, and yes, even gravity's forces are concentrated. For best gravity-defying results you want your 4x4's CG as low as possible.

Calculating your vehicle's CG requires some easy math and some precision weighing equipment. We've used Intercomp digital scales, but they're not cheap ($1,750 from Pegasus Auto Racing Supplies, 800.688.6946, www.pegasusautoracing.com). If your vehicle weighs less than 4,480 pounds, you'd be better off with the $425 RuggleS'cales, also from Pegasus. Another option may be to contact the local Sports Car Club of America (www.scca.org) chapter and find out where you might get your vehicle weighed. Or it may even be possible to use the local CAT scale and a Hi-Lift. We have to try it out and let you know.

Assuming you've found a reliable means of measuring the weights of your vehicle at all four corners-here are the formulas you're going to need.

Front-to-rear bias To find the CG distance from front axle centerline:

rear wheel vehicle weight
overall vehicle weight
x wheelbase

Side-to-side bias To find the CG distance from vehicle centerline:

track width
2
weight of pass. side
overall vehicle weight
x track
width

Top-to-bottom bias (CG height) This measurement requires you to lift the rear of the vehicle 30 inches off the ground with a forklift or hoist while the front tires are on scales. The "raised wheelbase" is the effective distance from the rear-axle centerline (drop a plumb line from the rear axle and make a mark on the ground) to the front axle centerline.

level vehicle wheelbase x raised vehicle wheelbase x added vehicle weight on front distance vehicle raised x overall vehicle weight

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