Unsprung Weight, Unlocking The Mystery

Lug Nuts latest blog on Aluminum Wheels was, as his usual product is, great. I have aluminum wheels on Rover, our 1990 Foretravel DP, and they are great. But there is one great advantage and that is that they are lighter as Lug Nut said. And as he said that reduces unsprung weight.

With Lug Nuts permission I would like to expand a bit on the mystery of unsprung weight other than the general weight reduction of the rig. Unsprung weight is the weight of the components between the springs and the ground. That includes the tire, wheel, bearings and hub, brake rotor or brake drum, caliper and pads, brake shoes and springs, wheel trim rings, lug nuts, and suspension members between the springs (or air bags), and the ground.

Now we can understand that as a wheel goes over a bump the laws of physics takes over. As the wheel, and all of the attached parts, are thrown in the air. Physics tell us that a body in motion tends to stay in motion and a body at rest tends to stay at rest.

So, we need the springs and shocks to hold the wheel down without transfering too much upwards motion to the chassis and the rig. The more the unsprung weight, the more the force is stored in the mass trying to keep the mass in motion (upwards), the stiffer the shock needs to be and the more resistance the springs need to stop the motion and push the wheel down to keep it in contact with the ground. This transfers to a stiffer ride. Reducing the unsprung weight will soften the ride.

That brings up another point. Shocks are safety devices. The shock is a hydraulic damper that absorbs the oscillations of the springs. Coil springs oscillate more than leaf springs and torsion bars offer less oscillation than all springs.

When a spring is allowed to oscillate the wheels can and will hop up and down on the ground when going over bumps and will leave the ground. This is what happens when the tail of your vehicle hops around a corner when the corner is full of wash board bumps.

The big problem here is that when the wheels are not on the ground you have no control of the vehicle. Again the body in motion law that says a body in motion will resist any and all efforts to change direction. This means that when traction between the tires and the ground fails you, the driver and passengers, are just along for the ride. Secondly it can cause uneven tire wear.

So, if we reduce the unsprung weight of a vehicle the safer that the engineer can make the vehicle and the better job the shocks and springs can do. Jaguar had this down pat for years on the rear of their cars. They ran an independent rear suspension and move the rear disc brakes in board to the sprung side of the chassis improving the rear suspension and handling.

The last point for today kiddies is the precision built into the aluminum wheels. They are machined on a lathe and as a result they are rounds and close to perfect circles, much more accurate than stamping steel wheels in a big press. That means smoother ride and less tire wear.

Thanks for starting this thought Lug Nut.

Brad