Does The Weight Of A Wheel Really Matter - BMW Forum - BimmerWerkz.com
3-Series (E36) Chat relating to the BMW 3-Series from 1992-1999. Autodoodad Specific models include: BMW 316i, BMW 318i, BMW 318iS/ti, BMW 320, BMW 323, BMW 320, BMW 324, BMW 325, BMW 328.

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#1 (permalink) Old 08-10-2005, 06:06 PM
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Soooo, lots of people ask which wheel they should get, and many answers are always forthcoming, ususally backed up by something like "It's really lightweight, so your car will perform better, but you'll pay an arm and a leg". For example some wheels weigh 24+lbs each while others weigh more like 18lbs each - a 6 lbs per wheel, or 24lb per vehicle difference.

However, no-one ever seems to question the wisdom behind low-weight wheels and if any of us will really get any bang for our buck by forking out $1000 a wheel instead of $250, just to get something lightweight. And no, sorry, i'm not going to listen to some youth tell me he just spent $4k on wheels and he definately feels a difference - of course he does, he'd be admitting he was a moron otherwise! And i do understand that wheel weight, or in fact it's moment of inertia (raw wheel weight has little to do with it in fact) only affects acceleration, and not constant speed.

What i'm looking for is a good scientific analysis (cause i can't do it myself) of what affect heavier wheels (with a higher moment of inertia) will have on a typical 3000lb 3-series BMW. I've seen this done for road bikes and the resounding conclusion is that you're wasting your money on light weight wheels unless you're acceletaing all the time (velodrome racing) or climbing steep hills all the time. I'd like to see the same math done for car wheels. I'm of the opinion that the math will say the same thing - that you are way better off buying that low cost heavier wheel and spending the additional $750 per wheel on other mods.

Anyone want to volunteer for some math homework? Or know a link where this has been done already?
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#2 (permalink) Old 08-10-2005, 06:16 PM
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this is going to be interesting if anyone actually does the math. if you consider that the wheels you mentioned are at a marginal difference in weight (30% heavier at 24lbs)- but it is the difference between "lightweight" and "heavy" as the manufacturers tell us. i guess a good way to look at the weight issue is to take into account the bling bling 24" + rims on suv's. you can definitly feel the difference when you drive with those on vs. stock. acceleration and BRAKING.

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#3 (permalink) Old 08-10-2005, 06:53 PM
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Some added info i found:

1. Weight of tires on 3-series appears to be about 22lbs (for 205 up to 235 according to some notes on TireRack). So total wheel/tire weight of about 46lbs down to 40lbs, so you're lloking at a 13% reduction on rotational mass by going to our lighter 18lb wheel. But of course less of a reduction (who knows how much) in moment of inertia, since all the tire weight is on the outside, where it matters, and the wheel wiegth is further toward the hub.
2. Lighter wheels will require less of a suspension system to keep them on the ground as they ride over bumps, so for a given suspension system your ability for your tires to remain in contact with the road will improve as wheel weight decreases.

Still need to answer the "accleration" question tho....
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#4 (permalink) Old 08-10-2005, 06:57 PM
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I went from OE wheels to heavier 18" wheels (With tires I think they were at ~23lbs) and in all honesty, no I didn't feel a diff in acceleration....

I did feel a diff. in handeling, but that's a all diff. story


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#5 (permalink) Old 08-10-2005, 07:04 PM
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Yah - lots of comments o differences in handling but most (if not all) of that is from lower profile tires and wider tires. Not due to lower (or higher) weight).
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#6 (permalink) Old 08-10-2005, 09:44 PM
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But in all honesty, overall, the weight diff is too little to matter....

True if you think about it the rim does weight what 20lbs compared to a 12lbs rim. but the diff is only 8-10lbs per rim... 32-40...


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#7 (permalink) Old 08-11-2005, 12:06 AM
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having light wheels has to do with unsprung weight, theorithically you would like to have no weight in the wheel and brake area(unprung weight, weight that is not supported by the springs of the car), but that is impossible so the lighter the weight the less effort it is to control the wheels so better performance is achieched through lighter unsprung weight components, as for acceleration lighter things move faster than heavy things
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#8 (permalink) Old 08-11-2005, 02:39 AM
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32-40 pounds is a LOT of difference. These small addiions add up. Im not saying you should go out and buy lightweight wheel as your first mod, but if you're really up there in mods, why the hell not. Anyone ever held gram lights? Ridiculously light. If you spend th emoney, you will loose a lot of weight per wheel, more than 6 pounds.

When a person reaches the point where weight makes a lot of difference, their only options may be wheels. If you loko at it, wheels are a GREAT way to equally reduce weight around all four corners.

In your first post you basically said anything you need to answer your own qestion. Really, acceleration is everything. If you have tracked your car or have competed in races then you wil know that constant brakign and acceleration are key. Maintainign proper speed doesnt matter with weight, other than possibly fuel consumption.

Basically you dont need to applies to many complex laws of physics when you think about this, rather common sense. We could all post about inertias effect on any component of the car, but it's just a waste of sentences.

BUT if you were to spend money on lightening your car, I would first recomend loosing weight where it really makes a lot of difference. Im talkign about lightening the roof, trunk, and bonnet. Anythign to reduce weight in the taller portions of the car would be a bit more helpful.


Here I would like to cross-link you to this topic by Autotechnica. Many weight issues are discussed, in lamens terms.

http://www.bimmerwerkz.com/forum/index.php...opic=10704&st=0


This topic was started by a member who has dropped almost exactly 200 pounds from his car. I'm sure he can easily feel the difference in acceleration, handling, and braking.


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#9 (permalink) Old 08-11-2005, 12:26 PM
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Hey MPower,

This sentence of yours "Basically you dont need to applies to many complex laws of physics when you think about this, rather common sense." is exactly what i was trying to avoid.

My whole point is that everyone "thinks" that losing weight in the wheels is going to provide a measurable benefit because they have heard it repeated so many times. Kind of like the advice by most health publications and so-called fitness experts to drink 8 glasses of water a day. They all say it and think its true and valid, but ask any of them where the science is and none of them will know - they're all just repeating something over and over again until it becomes "fact". Same with "walking a mile burns as many calories as running a mile", which is very well debunked in this months Runners World. I could go on ad nauseum with such examples.

I did see some math pertaining to some of the effects of wheel weight on acceleration on different cars with different gears and different rpms etc, and the basic premise there was that unless you are racing a very high revving, drag type car that needs to acclerate VERY quickly (i.e. much faster than you would on a track BMW) then lighter wheels are going to buy you nothing.

But it was not too clear on the exact benefits or lack thereof and what i'm really looking for (yes i'm anal) is for someone to do some math, or show me where it has been done already that says, on a 3000lb BMW 328 (for example!) reducing the weight of your wheels by 6lbs (or whatver weigtht they choose) will allow you to accelerate from 10mph to 60mph 0.01 seconds faster....
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#10 (permalink) Old 08-11-2005, 01:58 PM
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OK - looks like i am slowly but surely answering my own question...

I found this on the "SportCompactCar" web site. He's using some averages for how weight is distributed on a wheel, so numbers/ratios for a given wheel will be higher or lower, but the general jist of it is that 1lb on your tire (tire not wheel) will add 8lbs to your car (given you have 4 tires. In other words a total of 4lbs on tires=8lbs equivalent on the car. So the 6lb difference per wheel that i discussed prevoiously would add up to 48lbs of equivalent wieght on the car. He also noted that a lightweight flywheel (much cheaper than those expensive wheels) will make it seem like the car is 250lbs lighter in 1st gear (this is for a Ford Focus). In other words, spend th emoney on a lightweight flywheel (and probably 100 others updates) before spending it on really expensive wheels.

Rotational Inertia
The rotational inertia topic was so big, it took me two months (June and August '99) to get it sorted out. Here's the deal: There is this rule of thumb among racers that adding weight to something that rotates is far more detrimental to performance than if you add it to the body of the car. This is absolutely true, and by bumbling through some physics, and after slipping and falling on a radian, I managed to get a few formulas figured out that could tell you just how much worse.

Any moving object has kinetic energy, as does an object sitting in place and rotating. An object that is both rotating and moving (like a rolling wheel, for example), has kinetic energy from both, meaning that accelerating or decelerating that rolling object will take more power than one that is just sliding along. How much more power is the question.

The answer, it turns out, depends on how the weight is distributed on the wheel. An extra pound on the tread of a rolling tire has as much kinetic energy as 2 lbs on the floor of the car. As you move toward the center of the wheel, the rotational effect drops until, at the center, a pound is just a pound. The formula I derived to determine the exact relationship between weight on a wheel and weight in the car isn't worth repeating here for one simple reason. It requires that you know the moment of inertia of the wheel, and measuring that is virtually impossible. What you need to know is that changing to tires that are 1 lb heavier will effectively add 8 lbs to the car (four tires, remember) and that adding a pound to the wheels will effectively add somewhere around 6 lbs to the car.

That only considers acceleration and braking; handling is dramatically affected by unsprung weight as well, but no simple formula is going to tell you how big the effect is.


The one useful formula that did come out of my diatribe on inertia was the effect of a light flywheel. Because the flywheel's rotational kinetic energy is directly linked to the car's motion (through the gearbox), you can also calculate the effect of a lighter flywheel. Here's the formula: Next month, we will tell you all about the aluminum FocusSport flywheel we just put in our Project Focus. Until then, know this: The flywheel is 12.5 lbs lighter than stock and has a radius of 5.5-inches; the Focus' first gear is 3.667:1, the final drive is 3.82:1, and the tire radius is 12.1 inches. What's it all mean? In first gear, the car feels more than 250 lbs lighter!
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#11 (permalink) Old 08-11-2005, 04:18 PM
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I wasnt trying to restate what everyone else says, rather saying that you dont NEED the numbers to know a lighter car will help. What I also said was that wheels alone wouldnt be any big difference. The purpose of goign lightweight is for the overall loss of weight. Im saying that when you are seriously considering loosing weight on your car, thats when you have the option. I cant give you numbers or how much faster your car will pull out of a turn, or how much better your car might do on a skidpad, but ratehr generalize it for you. You already know this much. I'm not goign to sit here and do the AP Physics I took 4 years ago for you to figure out that a lighter car is better for many aspects. Sorry. What I will tell you is that if you are indeed considering lightweight wheels, try somethign else first. Like I said lightening the roof or body panels in order to lower your center of gravity.

All what I said may also be implied formt his line in the article you just posted,

"That only considers acceleration and braking; handling is dramatically affected by unsprung weight as well, but no simple formula is going to tell you how big the effect is."


Good Luck and keep discussing!


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#12 (permalink) Old 08-12-2005, 12:30 PM
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Yah yah - understand what you're saying and no disagreement. I think i got where i wanted to go, and that is that, as you say, spend your money elsewhere first, but that in the end if you keep at it the wheels will make a noticable difference. Interestingly i now am interested in lighter wheels not due to any effects on acceleration but instead on abilityto keep the wheel onthe ground. I have a convertible, which as you know flexes more than most, and i already have issues with whell hop on the inside wheel when hitting a bump on a corner. I definately need new shocks (originals as far as i can tell on 100k) and a strut bar, but light weight wheels are probably going to help this more on a vert than on a coupe or sedan.
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#13 (permalink) Old 08-12-2005, 03:56 PM
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Yep, it doesn't do much for a stock car. However, if you have dropped money everywhere else then you will notice and appreciate the differnce between stock and lightweight wheels/tires. But as you can tell, there is not a huge difference. Lots of people go with "bling" on cars that are supposed to be performance cars. So going from a 15/16" wheel to a 19" doesn't kill any aspect of the car, however it does not allow the car to make as much use out of the other parts that the car has, such as the brakes, engine and suspension. If you're serious about performance, go out and do it, if not, stick with what you have. Love wheels? Go for some 18's or 19's, they aren't going to ruin the car.
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#14 (permalink) Old 08-12-2005, 05:45 PM
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lets put it this way, baer brakes, a company i work one on one with frequently, did alot of braking tests with their brake systems when impementing them.. one thing they found out was on the escalade, when you upgraded from factory 17"'s to 20" wheels, it took you an 20-40 feet to stop from 60mph with the larger wheels and everything else the same.

rotational enertia was fighting against the wheels comming to a stop..
the same can be said about tryin to accelerate. the more mass there is, and the further out it is, the more force it will take to start or stop.

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#15 (permalink) Old 08-12-2005, 06:15 PM
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Here ya go.... I wrote this a long time ago... the topic is below

http://www.bimmerwerkz.com/forum/index.php...76&#entry166576

This is only showing how much HP differs with differen rotational mass...

Moment of Inertia - The property of an object associated with its resistance to rotation. It depends on the objects mass and the distribution of mass with respect to the axis of rotation.


Basic Rotational Inertia formula. I= ½ MR2 (2 = squared)
I= Moment of Inertia... M = mass and R = radius of the wheel.

So... lets take a 20 pound wheel... 17 inch.. (I'll work in SI units)

I= .5x(9.0718474)x(0.4318/2)>2 (>2 means squared)
I= .959040098 kg/m>2

The same equation for a 15 pound wheel that is 17inches...

I=.5x(6.8038856)x(.4318/2)>2
I= .539460063 kg/m>2

Just for clarity.. the next exp. is the same two wheels being pushed by 200 HP. ( Its not exactly right because I'm not taking in acount for the weight of the car.. but this should demostrate how much the weight of the wheel actually plays in acceleration of the car..)

Ok.. still speaking of a 17 in. wheel here...
Equation for torque being applied to both wheels is: Force times the Radius:
BTW 200HP is equal to 529559102.065818 Newtons

t = Fr = (529559102.065818N) x (.4318/2)= 114331810.1
This torque value is a constant... because it is just showing how much force is applied.

AND finally... when you take the Torque.. or Force being applied to each wheel.. and DIVIDE it by the moment of inertia.. which I remind you is the resistance to rotation. You get.....

20lb Wheel : I/t = (114331810.1)/(114331810.1) = 119214838.2 rad/sec>2
15lb Wheel : I/t = (.539460063)/(114331810.1 ) = 562500008.2 rad/sec>2

Ok... So.. now your wondering what the hell I've found with these numbers... well after another converstion from radians per second squared.. to RPMs... I get..

20lb Wheel : 5270.45301489 RPMs
15lb Wheel : 24867.9603047 RPMs

Both of those nubers clearly demonstrat... that with less weight you can speed that wheel up faster... an almost 2000 RPM increase due to 5 lbs of weight.

So... in short... lol... it takes

This is how much HP it takes to move each wheel....

20lb = 0.13039 horsepower
15lb = 0.07335 horsepower

As you can see... the 15lb wheel... takes less horse power to move..

All in all.. this is a sick conclusion.. to... who cares about 5 pounds.. when it comes to a cylindrical object being pushed by your engine... unless you want to gain... 0.5704 HP. Your choice...


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