unsprung weight

totopo

Unsprung weight is SUPER important.

Basically, bumps apply an acceleration to the unsprung weight, not a force. So lighter unsprung weight means less acceleration, and less perturbation of the chassis on the other end.

The major way of measuring the end effect of springs on a suspension is the wheel frequency. Basically, you have 2 weights separated by a spring, so the wheel frequency is dependent on the weight of both, but more so on the lighter weight (the unsprung weight). The wheel frequency is almost linearly proportional to the minor unsprung weight. So a decrease of weight of 10% iirc is basically like having a 10% stiffer spring. This is where you get where people say xx lbs unsprung weight = xx lbs sprung weight. like shaving 15 or 20 lbs or whatever off a corner weight has the same effects on wheel frequency that shaving 1lb off unsprung weight will have.

but it isn't like exactly true, since shaving off unsprung or sprung weight has the same effect on the normal force on the tire and the lateral force needed to turn the car.

Other benefits of lessening unsprung weight is you usually decrease rotational mass as you do it, since most of the unsprung weight is wheels and rapidly rotating components. decreasing rotational mass effectively improves your braking and acceleration.

spacers are kind of interesting from a vehicle dynamics standpoint. They increase the wheelbase, which is super great for decreasing weight shift, but it changes your scrub angles in the front, and changes your overall suspension geometry (and i think kills bushings). I think the motoiq people actually like running modest spacers for cheap wheelbase increases for more lightly modded projects.

jz321

F = M * A

The acceleration of a mass is a force.

totopo

what I meant was that if you vary the mass (unsprung weight), it's the acceleration that stays more constant and the force that varies. so less unsprung weight means less force for same acceleration, and less momentum and less energy, and less perturbation of chassis.

cofo11

Except A = F/M. It's not really that the lighter wheels provide a lower force, the decrease in mass allows for an increase in acceleration (the same reason the car brakes and accelerates better). You will actually notice a rougher ride as the wheels' reactions to bumps will happen much faster. What also happens is a a reduction in momentum which, potentially, doesn't cause the wheel to travel quite as far initially or on the rebound stroke.

Slartibartfas

Change wheelbase to track and that's right. Track is lateral width while wheelbase is longitudinal length.

Most of you are beating around the bush. The unsprung weight works exactly like totopo wrote for handling and is a linear relationship. 1 lb reduction in unsprung weight is 1 lb in regards to handling and wheel frequency.

However, the rotational inertia is very much a non-linear effect. 1 lb of rotational inertia is the equivalent to 10 lbs (or your favorite number from your favorite source) in regards to vehicle acceleration, whether it be forward (mash the gas), backwards (slam the brakes), or turning. Take ten pounds off each corner and it feels as if you've removed 400 pounds out of the car when you measure at the drag strip or race track.

Lighter wheels and brakes will make the car feel livlier but, from direct experience, the ride isn't as good. The heavier wheels force the tires to absorb small imperfections so the imperfections aren't as strongly transferred into the springs. Also, if you have a wheelspin problem, heavier wheels can help.

Like everything else in life, there are compromises to wrangle.

Sorry to practice necromancy but felt it worthwhile.

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2011 Journey Sport sedan 7AT | SPL front, SPC rear camber arms | ARK Grip cat-back |