impetigo

thought this article would be an interesting appendum to boilermaker's post about rims. the good part is that it gives a motortrend test of + sizing wheels on acceleration and handling/performance. the motortrend editor even comments at the end.

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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
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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.

Rotational Inertia (or Momentum)
How about ¡°Rotational inertia is a harder concept to deal with than...¡± Rotational inertia is a concept a bit more difficult to deal with than unsprung weight. Inertia can be thought of as why a car wants to keep rolling once moving, or remain in place once stopped (unless you forget to set the parking brake on that hill). I believe the terms momentum and inertia are interchangeable. The term ¡°flywheel effect¡± also refers to these concepts. In a car, there are a number of rotating masses which require energy to accelerate. Up front, ignoring the internal engine components like the crankshaft, we have to worry about the flywheel, clutch assembly, gears, axles, brake rotors and wheel/tire. Out back its a little simpler (for FWD) with just the brakes and wheel/tire contributing most of the mass.

Before continuing with our informal analysis here, I want to point out something very important about rotational inertia. We¡¯ve all seen the ice skating move where the skater starts spinning. She pulls her arms in and speeds up, then extends them again and slows down. Why is this? Well, the further a mass is from the center of rotation, the faster it must travel for a given angular speed (how many degrees of an arc it traverses per time unit). The faster anything moves, the more energy it has, so when the arms are pulled in, conservation of energy says that the rotation rate must increase. Applying this to wheels and tires, which have most of their mass spread as far as possible from the rotation center, I think you¡¯ll agree that it naturally takes more energy to accelerate them. Example: Take a two identical masses, but one is a solid disk of diameter D, the other is a ring of diameter 2D. The ring will require more force to accelerate it (in a rotational manner).

The point of this discussion is as follows: There is a great deal of rotational mass to deal with in a car and tires and wheels may only make up half of it.

Estimates for weight (o.k. for comparison since they¡¯re all in the same gravity field :-))

Front: Rear:
Wheel/tire: 30-35 lbs each 30-35 lbs
Flywheel: 15-20 lbs
Clutch: 15 lbs
Halfshafts: 7-10 lbs each
Gears: 5-7 lbs
Rotors: 3-5 lbs 3-5 lbs
Misc: 3-5 lbs 3-5 lbs
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Total: 115-148 lbs 76-90 lbs

So a couple pounds here and there on wheels and tires will make a difference, but that difference is magnified because that weight is placed further from the axis of rotation than any other mentioned (flywheel and clutch are closest) - remember the ice skater. All these masses must be accelerated, so any reduction is a good thing.

So, I¡¯ve given you my view of things, but can I prove it? When we talk about handling, I¡¯d be hard pressed to prove anything unless I went out and raced identical cars with differing wheel weights. Since larger wheels normally mean lower profile, stiffer tires, I¡¯d also be hard pressed to tell you about ride quality. You¡¯ll have to decide for yourself on that account.

When it comes to acceleration, however, I think I can provide at least a datapoint in support of my argument. Motor Trend in June, 1995 did a test where they evaluated the benefits of +1 and +2 wheel tire combinations on a BMW 540i (automatic transmission, I believe). Here are the key stats for the test.


Motor Trend Stats on BMW 540i Stock +1 +2
Tire 225-60/15 225-55/16 235-45/17
Wheel (in) 15x7 16x7.5 17x8
0-60mph (sec) 6.9 6.9 7.0
1/4 mile (sec/mph) 15.3/96.5 5.3/96.1 15.4/95.8
Rolling Diameter (mm) 651 654 643

Note the steadily decreasing acceleration times as the wheel size increases. Even with what should be superior traction from stickier compounds (I admit that a taller tire might launch better), the +1 and +2 combos don¡¯t perform. Especially the 17¡± setup which, with it¡¯s smaller diameter, should have a theoretical acceleration edge. Now this is only one datapoint where many more are needed, but it¡¯s important. It matches the seat of the pants reports from several Integra owners. It also pays to remember that despite the fact the BMW weighs around two tons (U.S.), it still has a superior torque to weight ratio over the latest generation Integra (~14:1 vs ~21:1). This will help reduce the effects of added weight.

¡°O.K.!¡±, you say, ¡±I am convinced, but handling is also important. It could be a worthwhile tradeoff.¡± This is undoubtedly true, but how much handling do you really get? I have never seen a tire test where +1 and +2 combos are compared with the exact same tire type and width. Here we are comparing a P600 vs. a PZero! The handling gains were fantastic, but I¡¯ll bet if they had run a 15¡± PZero (if they could find a 60 series version), the difference in handling would have been noticeably less.

Finally, what about appearance? Well, you¡¯ll get no argument from me on that. Without a doubt, big beefy wheels with O-rings for tires just plain look good. Hey, I went the same route and replaced 13¡± wheels on my old Civic with nice big 15¡± hunks. Now, however, I think there are better choices. Here are my recommendations.

Max Performance (accel and corner):
15x6.5 or 15x7 with 205/50 or 215/50 tires

Best Looks:
17x7.5 or 17x8 with 215/40 tires

Best Compromise:
16x7 or 16x7.5 with 205/45 tires


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Motor Trend Responds!
As the author of the Motor Trend article you quote in your wheel/tire article over at the Integra board, I thought I'd let you in on a few of the considerations made while performing that test.
First, there's really no way to go from a 15-inch wheel up to a 17 without increasing tread width (if you're attempting to maintain overall tire diameter). This is particularly true on the heavy BMW 540i (automatic) we used in our test. Load carrying is directly related to the volume of air in a tire. Of course as you reduce the aspect ratio of a tire, you reduce the volume of air available for load carrying. The solution, obviously, is to make the tire wider and therefore maintain a consistent volume of air (this is also why the 18-inch wheels used on the Porsche 911 Turbo have hollow spokes). Go back and re-read the article and you'll see that our trip up in wheel diameter was accompanied by considerations to maintain air volume.

One reason ride quality deteriorates so dramatically in Integras wearing large-diameter rubber is because the combination wasn't determined with an eye toward maintaining air volume. That's a consideration you may want to pass on to your readers as well.

We chose to use Pirelli tires consistently in our test so that we at least wouldn't be comparing between manufacturers. However, the reason we picked the P600 for the stock size, P700 for the 16-inchers and the PZero for the 17 was simply because that's what Pirelli sells and what they provided us. You're right, there aren't 60-series PZeroes around. However, when stepping up to larger diameter wheels, most buyers will likely also step up in the aggressiveness of tire tread and compound.

Again you're correct, the handling gains were tremendous. However, we did give up some ride comfort and probably poor weather traction (something we couldn't test here in sunny California).

All in all, I've enjoyed the Integra site quite a bit and found your article informative. You may just want to pass my thoughts along.

Sincerely,

John Pearley Huffman
Editor at Large
Motor Trend Magazine