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Author Various
Categories All Honda/Acura
Create Date January 18, 2002 17:52
Last Update April 06, 2002 12:33
Unsprung Weight

The Benefits of Low Calorie Donuts
(or Cutting Weight from Your Wheels and Tires)
By: Shawn Church

Yokohoma A008RSII (205-50ZR15) - 20 lbs
'94 GS-R wheels - 15 lbs
So you finally decided to take the plunge and modify your Acura (or Honda or just about any other car for that matter). You've looked at engine parts, you've looked at exhaust systems, you've even looked at body kits but nothing seems to offer the appearance and performance benefits of a good set of aftermarket wheels and performance tires. Congratulations chum, you've joined a pretty large group of car enthusiasts who have added uniqueness and performance to their cars...but do you know the whole story? There is no doubt that new wheels look great and going to bigger and better tires adds grip. There is more to choosing your running gear than just wheel size and offset (I know you already knew about those). "What else is there?" you say. What else indeed, it's the bane of all cars - added weight.

Before I start spouting off with a bunch of half-cocked (well hopefully closer to three-quarters) theories and partial proofs, let me say this. This is not intended to be a physics course. I've been out of college long enough to have forgotten a couple years of physics (took about 3 hours and a case of beer) and I have no desire to find, let alone read, my old textbooks. I'm going to try to show you the effects of weight using as much common sense as possible. If I've made errors, or if you'd like to work up some proofs, I will gladly listen.

Alright, down to business. Why is wheel/tire weight so important? Two concepts: Unsprung weight and rotational inertia. We'll address each one individually.

Unsprung Weight
How do you describe the job of a car's suspension? Well, there are a few different ways, but lets take the easy one. Your suspension is supposed to suspend your car above the road. If that was the only reason for the existence of springs and shocks though, why don't we just solidly mount the car to the axles? "That's obvious!" you say. Of course, you'd have a lot of trouble with bumps and corners with a solid suspension. This is because a suspension is supposed to allow your wheels and tires to follow the road, irregularities and all, while the body of the vehicle travels smoothly. Turning things around, the suspension should also keep the wheels and tires in maximum contact with the road for the best performance (this is more important than ride for us driving enthusiasts). So, to continue, for a suspension to be effective, it must allow the wheels and tires to accelerate and decelerate rapidly up and down while not allowing them to make excess motions (example - axle hop). The springs prevent the wheel assembly from traveling too far, while the dampers prevent oscillation by the spring.

O.K., time to tell you how wheel/tire weight plays in all this (in case you haven't figured it out). You see, every time you hit a bump, the wheel assembly is accelerated upwards, decelerates to a stop, then accelerates downward till it reaches equilibrium. If the wheel can't accelerate fast enough, shock is transmitted to the body, which may upset the balance of the car. As an example think of small, sharp edged speed bumps versus those gigantic, but wide, monsters in some lots. The sharp edged ones are much more annoying to traverse, aren't they? That's because they require the suspension to accelerate more rapidly. Now imagine going over some stutter bumps in a corner. You'll have a very rapid series of accelerations and decelerations. If the wheel is lighter, it will accelerate upwards and downwards faster (a=F/m). This means it will follow the road better and, even more importantly, allow the suspension to work better. The shock and spring will have to control less unsprung mass (weight will also suffice here), which means they can stop and start the motion of the assembly more easily. Unsprung mass (or weight) is commonly defined as mass not suspended (or sprung) from the suspension. This will typically include the wheel/tire, hub, brake, and some of the suspension pieces. On your typical FWD Integra, you'll probably see the following numbers (estimates).

Wheel/Tire:	30-36 lbs/each
Rotors:	        3-5 lbs/each
Calipers:	1-2 lbs/each
Susp Pieces:	10-15 lbs/corner
-------------------------------------
Total Weight:	44-57 lbs each corner

I think that I've probably over estimated some of the weights, especially the unsprung suspension pieces, but the wheels and tires are accurate according to reports. What happens if we increase the wheel weight by 10 lbs? Well, we just added between 15% and 20% to the unsprung weight. Guess we'd better add better springs and shocks as well. If we drop weight, we'd see the opposite of course and might benefit with better ride and handling.

In all reality, the effect of unsprung weight changes aren't very noticeable (unless the changes are large), but they are there. If you race, they are even more important, so be careful. If you're going to add 17" wheels to your Integra, you probably should think about upgrading your springs and shocks too.

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