IRS and unsprung weight question.

[gearbanger]

Why is IRS an advantage in unsprung weight? I see unsprung weight being a problem on the front of a car because that weight doesn't get transferred to the rear on launch, but the weight of a solid axle is always down on the rear tires so why would IRS help in that way?

[DonQuehotey]

unless you hit a bump or transfer weight. ONE of the arguments is more about controlling the suspension, the more weight, the more inertia it builds and the harder to control. With an IRS, the heaviest part of the rear axle is attached to the frame of the car and is therefore sprung weight.

[Steve1968LS2]

It's generaly lighter in unsprung weight but heavier in overall weight.. enough where it can sometimes cancel out the unsprung benefit..

Or so Ive heard.. lol

[Matt@RFR]

Put simply, less unsprung weight lets the coilovers control the suspension movements more precisely.

[PTAddict]

Why is IRS an advantage in unsprung weight? I see unsprung weight being a problem on the front of a car because that weight doesn't get transferred to the rear on launch, but the weight of a solid axle is always down on the rear tires so why would IRS help in that way?

Unsprung weight is almost entirely an issue of dealing with bumps. The simplest way to understand this is to think of the car as stationary, and the road flying by under the car, pressing upward on the tires (this is a simple application of Newton's "equal and opposite reaction"). The bumps, as they strike the tires, will "fire" the tires at the car like projectiles. The heavier the total mass associated with the tire, the more "impact" and disruption will occur on the car. If the tire is not tied to a 250 lb axle and differential, it won't make as big an impact on the car, or as big a disruption for the springs and shocks to handle. And the tire with less mass will likely "follow" the rushing road surface better, and stay in better contact with the road.

If you're talking about a very smooth surface, as on some race tracks, unsprung weight might not be very important because the bumps aren't significant. Some very fast race cars (trans am or scca gt-1, for instance) use solid axles and still obviously handle very well.

The unsprung weight issue is not the only reason to consider IRS. IRS, because all aspects of the rear wheel's motion are controlled by a system of linkages, often has greater potential for optimization of critical suspension parameters like roll center and camber gain.

Unfortunately, more potential for optimizing things also means more potential for screwing things up. In the hands of uneducated implementors, IRS can seriously damage the handling of a car. For example, ever worry about torque steer on a rear drive car? Just put deep dish wide rear wheels on an IRS not designed for it, and you might. Also, it is in general more difficult to get anti-squat with IRS while preserving other desirable geometric parameters. And as pointed out above, the total system is often heavier than a solid axle solution. So, no free lunch.

[gearbanger]

Cool, I love this site! I understand now the benefits for taking bumps in a curve or just keeping the tires on the ground better in general.

Do I understand correctly then, that there is no benefit for drag racing? I wondered about anti-squat as well. Since there is no twisting effect at the control arms, is there any squat or antisquat at all?

[PTAddict]

As a practical matter, an IRS will often have much less anti-squat. Here's a simple way to think about it: with a solid axle, you draw a line from the rear contact patch to the side view instant center. If this "lever" is angled upward, then applying an accelerating force at the contact patch will tend to "jack" the rear of the car upward - this jacking effect is anti-squat. With an IRS, because the axle torque reaction is not absorbed by the suspension links, you have to think of the acceleration force being applied at the wheel bearing, not the contact patch. Obviously, this means that the side view instant center will have to be much higher (by the radius of the wheel/tire) in order to achieve the same AS. Raising the side view instant center this high will often create other side effects, and hence many IRS setups have little or no AS.

[DonQuehotey]

In a racing application they are refering to reduced unsprung weight in the form of the tire, wheel and brake assembly. The lighter these components are, the faster they can be accelerated and decelerated by engine and braking force.