Okay. I have been thinking about the potential benefit of getting lighter rims vs a lighter flywheel for a while now and I have come to the following conclusion (which I readily admit could be wrong!) : I don't think that the lighter rims would give much performance increase at all.

Using a simplified energy analysis, you can workout how much faster the light wheels would be spinning compared to the heavier wheels for a given combined energy of the car+wheels. This makes sense because the engine can only put out so much energy in a given time (horse power). So say two cars both start at rest and then accelerate for a given time. Since the powerplants are identical both engines will have put out the same sum of power over time (work). At the end of the race, both cars will have the same stored energy in the form of the combined translational and rotational energies. But, one car will be traveling a little faster because in order to have the same stored energy as the car with the heavier rims, the car with the lighter rims has to spin its rims a little faster. With me so far?

If you are a nerd like me you can write out the equations and solve for the cars' velocities. And since I am a nerd, I did exactly that! :D I came up with a result that says when the car with the light rims is going 60mph, the car with the heavy rims is only going 59.195.....not a very big difference at all. And I assumed that the lighter rims were almost 10lbs lighter than the heavy ones! (25lbs vs. 15.4lbs)

This makes sense because the translational energy of the vehicle is quite large compared to the combined translational and rotational energy of the wheels. For the numbers that I used, the energy stored in the wheels is only about 1.7% of the total energy in the system.

The analysis for the flywheel is complicated by the fact that we shift gears, but I have a sneaking suspision that the flywheel would be MUCH more significant because it spins at a MUCH higher rate - and it does so at a MUCH lower speed! So, while the vehicle is still going slow, the flywheel has stored a significant portion of the total kinetic energy in the system. As the vehicle moves at a faster rate, the flywheel has a diminishing effect because it represents a smaller proportion of the total energy.

So given the above flawed reasoning :p does anyone have any hard test numbers to prove the difference between light rims and stock rims, or light flywheels and stock flywheels? I hear lots of "butt-dyno" results, which I have to take with a grain of salt, but I haven't seen any numbers....

 

Sounds pretty sound.

reduced rotational inertia in everything spinning at the rate of the crankshaft (rotors, flywheel, shaft itself) should give a bigger "return" for the same percentage of RI reduction than on the rims (which, in addition to the reasons listed above, still doesn't change the RI of the tires, which will probably exceed that of the rims for light enough wheels).

Of course, there's a little bit of an intangible in there (at least in terms of energy equasions) since lighter rims will also reduce the unsprung weight in the suspension, and could be set up to give a little better grip on the road in the event of any little bumps. Also, you'll see some improvements in braking with lighter wheels since there's that much less energy that needs to convert to heat.

 

I worked out equation before and it comes to about 1.5 effective pounds for every pound of additional wheel weight in terms of acceleration worsecase*.

That means if you save 2.5 lbs per wheel or 10 lbs total on wheels. The effective weight reduction of the vehicle is about 15 lbs or .5% of the vehicle weight. This means the vehicle will accelerate .5% faster so a 0-60 of 6 secs would become something like 5.97 sec with lighter wheels. Noticeable improvement? Maybe; drastic? no. (BTW, your 1.7% improvement for a 9.6 pound lighter wheel is almost identical to my .5% for a 2.5 pound lighter wheel. So we are essentially in complete agreement.)

There is also the issue of reduction of unsprung weight which improves handling but by wildly speculated amounts. Some say it takes more weight than in our 2.5lb example to make a difference while some of the Miata folks probably fill their tires with helium to save picograms and swear they can tell the drastic difference.

Improvements in handling due to reduced unsprung weight are highly debatable. However, improvements in acceleration due to reduced rotational mass are very strictly determined by elementary physics and are not all that great.

Now moving onto the flywheel. The flywheel is a little trickier to analyze because the additional rotational energy of a heavy flywheel is stored and will be partially given back to the vehicle when shifting gears. So while a heavier flywheel takes more energy to spool up, it also give some of that additional energy back once you shift into the next gear (the rest is scrubbed as heat).

With that said I do think it is worth lightening a flywheel on a 9000 RPM car. I have a Mazdaspeed ready to be installed and it should really help in the lower gears making the car quicker.

My two cents,

-Mr. Wigggles

*do a search for "mr.wigggles" and "moment of inertia" and you should find the post where I did the calculations.

 

Yeah, you need to consider the unsprung weight factor of lighter wheels in addition to rotational weight (I do not have a formula, especially since there isn't going to be one for ride and steering feedback). Unsprung weight also improves the ability to keep a consistent optimum tire patch, which will not only improve grip for handling, but for both acceleration and braking.

Another thing that you have to consider when putting together a formula for the effect of rotational weight is how far out on the wheel the weight is. The farther out on the wheel the weight is, the more leverage it has, and the greater effect it has. Henceforth, lighter lug nuts will have very little overall effect, but lighter tires will have a significant effect. This is where alot of detriment is from increasing wheel size. The rim is going to be one of the heavier parts of a wheel, and the bigger the wheel, the farther out that weight is, and the more it affects rotational weight. Also, the bigger the wheel, the larger the circumference, so the more weight that is going to be in the rim section, and so a higher percentage of the wheel's weight will be near the circumference of the wheel.

Personally, I like lighter flywheels because of the control and throttle response they offer, not the meager performance increase they might offer. While it doesn't affect throttle response so much when accelerating under load, it does when you are blipping the throttle and trying to match revs. It simply gives the driver more control over the engine.

---jps

EDIT: In your formulas, you did incorporate the fact that there are four wheels, and only one flywheel, right?

 

Sputnik,
Yeah, without any more refined estimate of the rims' moments of inertia, I just assumed that they were a point mass with the same radius from the axis of rotation. That seemed like a reasonable way to make an estimate of the differences since they don't seem to publish the higher order information. Plus, the radii of the mass can't be that different from eachother since both rims are 9" radius!

The "unspung" mass only effects the ability of the suspension to keep the wheel on the road. It has to do with the natural frequency of the wheel/spring/shock system. In general, the higher the natural frequency of this system, the better it will be able to follow bumps in the road. The "unsprung" mass does not contibute any less to the total kinetic energy of the vehicle simply because it is attached at the end of a spring. The KE of the rim is still the translational + rotational energy. This would be true even if it were rigidly attached to the car and not at the end of a spring.

I did remember to account for all four wheels, its just that the energy is proportional to the angular velocity squared, and only linearly proportional to the moment of inertia. So since the flywheel spins at 6-9k RPM and the wheels spin a little less than 1k rpm, the effect of the flywheel is maybe 8^2/4=16 times that of the wheels?? (just for guestimation purposes - ROUGHLY saying that the wheels are the same moment of inertia as the flywheel-the four comes from the fact that there are 4 wheels)

 

You should also consider tire weight as well as rim weight.

Some tires are very very heavy compared to others. This can also kill a rim/tire package if you are looking for every extra bit of performance!

 

Yes, I know. It only has an effect on grip when accelerating and braking, and on handling. So if you are only considering acceleration performance, unsprung weight doesn't matter much. But if you are asking the question "Do lighter rims really enhance performance?" as far as which item to spend your money on, the other benefits of less unsprung weight should be considered for an RX8. That's why I mentioned it.

---jps

 

All of this analysis assumes that rolling radius is held constant. If smaller diameter wheels are used, you have to take into account the effective increase in final drive ratio, which increases acceleration capability.

 

I am dying for someone to do a correlation between tire weight and track times. One forum member found the lightest tire to be the Contiental Sportcontacts at 22.5 lbs versus 26 lbs for our RE040's or the Bridgestone S03's (the SO3's being some of the heaviest AND most highly regarded tires). The problem is the Sportcontacts are one of the worst rated tires out there in their category in terms of grip, wear, and ride quaility despite their weight. Tire weight certainly ain't the only thing I tend to look at track times and performance numbers that TireRack comes up with.

They usually use a 330Ci as their test car which is about as close as you are going to get to an RX-8.

I really think many make too big a deal of unsprung weight in general. A pound here and there will be difficult to feel. Whereas I don't think there are any negatives that come from lighter rims, lighter tires don't always come out on top in performance tests.

-Mr. Wigggles

BTW, with 26" tires every pound save in weight is worth about 2 lbs of normal weight in the car in terms of acceleration.

 

Yes it's very hard to remove the variable of tire compound and construction when comparing tires. I don't think you'll ever be able to isolate tire weight in a performance test (wheel weight would be easier, and just as relavent, if you are examining the effects of unsprung weight).

And, I would argue that unsprung weight is more important on real-world roads than on a race track, at least for us track-dayers. Don't get me wrong - some professional race teams spend huge sums "minimzing tire force variations," but on the road, where constanly changing conditions means that you don't always know what's around the next bend, and anything you can do to keep your tires on the road is critical to both safety and performance. As Miliken and Miliken say, the tire forces are the only thing, from force-balance standpoint, that differentiate your car from an airplane.

One of my favorite local roads is full of pavement variations and undulating ripples - not harsh, square potholes but just constantly changing pavement. Much worse than you would ever see on a race track. Under these circumstances, keeping a constant tire patch is more important than just about anything else, including body role. Minimizing unsprung weight is the best way to do this. And yes, I'm one of those people who say that 4 lbs is a significant savings in a Miata (but would less significant on a heavier car like the RX-8). But 4 lbs isn't a deal-breaker, IMO, if it means getting the right tire. Others will disagree.

Some people do get carried away when counting lbs. One thing that never seems to come up, probably because it's hard to determine, is what the total unsprung weight of each corner is. This would include wheel/tire, brake rotor, caliper, hardware, a percentage of the control arms & shock, etc - everything that moves up and down. Once you know that you can at least judge what a 5 lb reduction would mean. Is it 5%? 10%? At least then you would have a context in which to make a decision.

 

Agreed.

 

Here's a well known and used formula to calculate the differences from various tire/wheel weight and sizes.

http://www.mazda6tech.com/files/rotational.xls

And a short article on the subject.
http://www.mazda6tech.com/index.php?...d=16&Itemid=32

Having gone from 46 lb (tire+wheel) stock tires/wheels on my 6, to 34.2 lb (tire+wheel) SSR comp's...I can definitely say the difference is utterly massive and all encompassing, especially when combined with aggressive rubber.

The difference is so huge, you can feel it just pulling out of a parking spot. The car feels like it just lost a few hundred pounds...the steering moves with much less effort, and the car responds much faster to driver input...almost like it wants to dive around the corner at breakneck speeds.

Btw those 34.2 SSR comp's (tire + wheel) will fit on the 8....

 

I've had a similar experience when i went to 13.5 lb wheels on my Neon ( now my daily driver ) from my 22 lb bling bling rims. It made a massive difference in every way the car handled as well as an improvement in ride quality.

 

Weight is bad, weight is the enemy...

 

I still haven't seen any hard numbers!!

 

Warning: Crotchety response!

This is one of those situations where numbers are absolutely meaningless. It’s all about how the car feels (the RX-8 isn’t a numbers car anyway). I have NEVER heard somebody come back from shedding 10 lbs or more of unsprung weight and NOT say “WOW – what a difference! I can’t believe I was driving around on those heavy wheels for so long!” They may choose to go back for the looks, but they always acknowledge the change in feel and handling.

The only way you’ll get meaningful numbers is to get a professional driver who is *very* consistent and let him drive the same car on a track, back-to-back, with the only difference being heavy wheels. Even then this will understate the real-world benefits, as roads tend to be, on average, in a greater state of disrepair than race tracks.

Sigh. I wish magazines would include two skid pad numbers – smooth and rough. They should have a skid pad with bumps, ripples, and camber changes built in. Only then could we see the downside to heavy wheels and stiff sway bars.

And, if we insist on numbers, ask any racer how important unsprung weight is. I’ve been told that top-tier race car teams (i.e., IRL, CART, etc) spend huge sums of money perfecting shock valving for each specific venue they race at. This is in an attempt to keep the tire force variation to a minimum. They do this for a reason – a tire that stays consistently planted will give shorter lap times. Lowering unsprung weight does this exact same thing – it minimizes tire force variation (i.e., minimizes the change in sidewall flex as the tire rolls over inconsistencies in the road). It’s just basic physics. Ok, perhaps it’s intermediate physics.

Sorry if I ranted.

George

 

Hey George! Nope. Not a crotchety response at all! Your point is well taken. The 8 isn't a "numbers" car after all. Agreed. The 8 is about the subtlties. But the numbers are still important for A to B comparisons for those of us trying to budget our upgrades - even if you don't want to compare them versus some other model car. You could get some performance stats even for an inconsistant driver. You'd just have to have them do more runs so that the statistical tests would have some way to differentiate the "before" from the "after" setups.

I also agree that people do seem to comment favorably on the effectiveness of reducing their wheel/tire weight. My comment about wanting to see some "hard numbers" just stems from the fact that people are natrually biased to believe that the upgrade that they just spent their whole paycheck on is actually working like they hoped it would! :p I have to take qualitative assesments with a grain of salt. Some people would admit that they made a mistake - but I think more would feel a difference even if it weren't really there! Besides, I'd rather spend my money elsewhere if I have to be a professional racer to enjoy the bennefits.

 

Ok, and looking back at your original questoin, I can see that we engaged in a bit of thread drift, moving from the effects of light wheels on acceleration to their affect on handling.

I have to agree with you and others - a light flywheel will have a greater impact on 0-60 times than light wheels.

George

 

Couldn't have said it better myself!

Subtleties matter! Feel matters! In fact, to me, feel matters more than the actual numbers. For example, my 3rd gen will probably post better numbers in the slalom than the 8. But I actually like the way the 8 feels in the turns better than the 7. So, to me, the 8 is the better handling car based solely on the way it feels!

 

Car & Driver did a test a year or so back comparing the normal wheels on an SUV with some dubs. I just searched their site a bit and couldn't find it, but I think mpg went down by 2 and the zero to sixty time went up somewhere between 1 and 2 seconds.

So, there are hard numbers out there....sadly, though, I don't have the link. I have a mere green belt in Google...any black belts out there that can find it?

(Oh..and there is a chance it was in Automobile or Road & Track...but I think it was Car & Driver. I get all three so sometimes I forget which articles were in which.)

~ Matt

 

I went from 16 to 18 inch wheels on my 94 Rx7. 16's weigh about 16 lbs and 18 weigh about 20 lbs. The car ran slow on the 18 and brake slower too. I felt my twin turbo drove like a Miata. I had to get a catback to offset the power loss.

I am telling you it does make a difference. Perhaps it's bc my rim size changed (bigger rim) instead of the weight. But the overall diameter of the wheel plus tire are the same.

 

FWIW, here are some "hard" LWF numbers for 2nd, 3rd & 4th gear

https://www.rx8club.com/showpost.php...9&postcount=31

Based only on SOP (seat of the pants) dyno, I would also endorse dropping as much wheel weight as possible. As a comparrison, for track events I've secured a set of SSR Competion semi-forged 18x8.5" wheel fitted with Dunlop DOT legal steet/track soft compount 255/35x18 tires. To the extent my bathroom scales can be trusted the SSR Comps +the (significantly IMO) larger Dunlops weigh in at @ 41 lbs per wheel tire vs. @ 46lbs for the stock OEM wheels and 255/45x18 Bridgestone RE040. The lighter weight helps a lot with braking and shock absorber demands as well.

HTH

 

I e-mailed BF Goodrich and was advised that the BFGoodrich g-Force T/A KDW 2 in 225/45/18 weigh 22.5 pounds each, which is 3.5 pounds lighter than the Potenza RE040. So 3.5 pounds lighter, with better traction, is a no-brainer for replacement if you keep the same wheels or wheel diameter. I'm getting mighty tempted, but can't decide if I should go the extra bucks and effort for a light weight 17 inch wheel. Keep thinking those wheel arches would look mighty lonely with 17's.

 

we have a high gear ratio just lowering the gear ratio with 17's will get you speed. the 6 lb's or so you can lose in each wheel. my bet is .2-.3 sec's off a 1/4 mile.

 

A related question is what tire width would you choose?

225/45R18 or 245/40R18. Presumably the wider tire is heavier, but is the wieght worth the extra contact area?