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[Cloud]

I've been trying to get 18" wheels because I care more about performance and ride quality. However, when thinking about why 18" perform better, I see some questions that need to be answered first before I can decide.

The lightest forged 19s weigh around 20 lbs on average.

The lightest forged 18s weight around 18 lbs on average.

Put a tire with 35 width aspect ratio to retain OEM OD on the 19s

Put a tire with 40 width aspect ratio to retain OEM OD on the 18s

We know that the tires are almost equal or heavier than the wheel itself

Assuming that the weight of the 40 width aspect ratio tire is lighter than the 35, the 19" might be a better choice (if the weight diff of the tires is greater than the weight diff of the wheels).

However, I also understand that the distance from the center of the wheel(moment arm) plays a part in this.

The 19" wheel actually pushes the mass of rubber 1" outwards and this increases the unsprung weight of the wheel/tire combo.

I hope someone (maybe an Mech engineer) can confirm whether 18s or 19s are superior performance wise.

[FruitCake]

you're talking about weight only?
its not only about unsprung performance but the hight of the sidewall. In the real world, the sidewall is more immediately apparent than weight. you are aware of what aspect ratio is? In this situation if you are keeping the sidewall size to keep the same rolling diameter, then it is no different because the heaviest part of the tire you are talking about, is the furtherest away from the center: the actual tread.

Of course this is only my opinion.

[GewoW]

It's all about sidewall. Weights are important, but they're both ~ equal depending on the setup you get...The fact that a 19'' wheel is 1'' bigger, even if the setup weighs the same, more weight is further out (radially) and less contained than an 18'' wheel...that's not that important, but also contributes to the matter.

This is my understanding.

[ersin]

Originally Posted by Cloud

However, I also understand that the distance from the center of the wheel(moment arm) plays a part in this.

The 19" wheel actually pushes the mass of rubber 1" outwards and this increases the unsprung weight of the wheel/tire combo.

I hope someone (maybe an Mech engineer) can confirm whether 18s or 19s are superior performance wise.

Not exactly. Assuming the tire+wheel weight are the same weight, the unsprung weight is also the same. However, the 19" would seem to have a greater moment of inertia than the 18". However, I have no idea what the weight distribution truly is so take that statement with a grain of salt. It might depend more on the brand and model of the wheel and tire thus negating generalities such as this. If someone can supply moments of inertia as well as weight I could calculate how much of a performance penalty there is.

A higher moment of inertia for the wheel/tire combo will make the car accelerate slower and will also have an impact (probably a negative impact) on handling. However, I doubt that the difference is enough to feel, and even for racing, how it impacts overall performance, considering that sidewall height as a good bit of impact itself, is hard to say.

Cheers.

[gixxer_kidd]

Subscribed. I've been wondering the same thing myself. Also I noticed some guys go crazy over the weights of wheels but when it comes to tires they don't seem to care as much. From the responses above it seems like tire choice should matter just as much.

[GewoW]

Originally Posted by gixxer_kidd

Subscribed. I've been wondering the same thing myself. Also I noticed some guys go crazy over the weights of wheels but when it comes to tires they don't seem to care as much. From the responses above it seems like tire choice should matter just as much.

True but the differences in tire weight are more justifiable than some differences in wheel weight. For example a heavier tire may have stiffer sidewall, but may perform better than one that is not as thick...

[swamp2]

Topic discussed in great detail in the past. Search the forum using google, just append site:m3post.com after your search terms.

[ersin]

Originally Posted by swamp2

Topic discussed in great detail in the past. Search the forum using google, just append site:m3post.com after your search terms.

Yes, it has, but I've not seen anything with real values or anything that is definitive (considering both wheel and tire). And yes, I've searched. I could approximate values but I don't know enough about wheel and tire design to be accurate. If numbers were supplied I could do some calculations and be able to give a performance difference. Maybe I'll just do this when I have time and see what we get. My gut feeling is that on a 3500+ lbs car you won't be able to detect a difference without sensitive measuring equipment, but I'll reserve judgement.

[zsapphire7]

just go with smallest to fit your brakes which is ~18. The 18" give u more tire choices, less unsprung weight, smaller rotational intertia. If you are tracking a lot and you are capable of driving at the limits, you could notice less tire rollover because of a stiffer sidewall with the 19" due to the weight of the car. This would probably give you better feel, turn in and more direct steering input but 18" should be more than enough.

[calintexas]

Pg 34 of the May Car & Driver has performance numbers on 15 - 19" wheels on a VW Golf which may or may not be relevant to our cars.

[AMPowerJ]

Funny I was going to post a similar argument as well .... people spend 5K+ on forged wheels, etc. and then go and put a 305/25 tire on it or whatever that weighs 30+lbs. Seems to defeat the purpose altogether. FWIW I have been checking into tire weights and Continental seems to have the lightest tires if weight alone is what you are looking for.

If you go back to good'ole BMW engineering they seem to have hit on a pretty good solution with the OEM forged 19 inch wheels. They are hard to beat except of course if you just don't like the way they look. For example, aftermarket wheels may be lighter or about the same weight but are also typically wider which require a wider, and here we go again, heavier tire.

[JCtx]

Originally Posted by calintexas

Pg 34 of the May Car & Driver has performance numbers on 15 - 19" wheels on a VW Golf which may of may not be relevant to our cars.

Originally Posted by Cloud

The 19" wheel actually pushes the mass of rubber 1" outwards and this increases the unsprung weight of the wheel/tire combo.

That's wrong; it's 0.5". Remember it's from the center of the wheel .

For practical purposes, the combined weight of wheel and tire should be considered as one unit for unsprung weight considerations (just like the C&D article did). I'm sure they're differences, as noted, but they're negligible, and nobody is going to bother measuring them.

Stock to stock, the 19" setup is lighter overall. But obviously, forged aftermarket 18" wheels/tires can be (depending on design) lighter than stock 19s. I'd never go above stock weight myself, but that's just me. A heavier wheel/tire set will alter suspension tuning, make ride harsher, wear wheel bearings sooner, and make acceleration slower. If you go wider than stock you'd alter other things as well, but many folks just care about the biggest wheels/tires they can fit... and many of them regret doing that after they find out their cars ride like crap . Good luck man.

[GewoW]

Originally Posted by elp_jc

That's wrong; it's 0.5". Remember it's from the center of the wheel .

For practical purposes, the combined weight of wheel and tire should be considered as one unit for unsprung weight considerations (just like the C&D article did). I'm sure they're differences, as noted, but they're negligible, and nobody is going to bother measuring them.

Stock to stock, the 19" setup is lighter overall. But obviously, forged aftermarket 18" wheels/tires can be (depending on design) lighter than stock 19s. I'd never go above stock weight myself, but that's just me. A heavier wheel/tire set will alter suspension tuning, make ride harsher, wear wheel bearings sooner, and make acceleration slower. If you go wider than stock you'd alter other things as well, but many folks just care about the biggest wheels/tires they can fit... and many of them regret doing that after they find out their cars ride like crap . Good luck man.

That's not necessarily true...

[swamp2]

Originally Posted by ersin

Yes, it has, but I've not seen anything with real values or anything that is definitive (considering both wheel and tire). And yes, I've searched. I could approximate values but I don't know enough about wheel and tire design to be accurate. If numbers were supplied I could do some calculations and be able to give a performance difference. Maybe I'll just do this when I have time and see what we get. My gut feeling is that on a 3500+ lbs car you won't be able to detect a difference without sensitive measuring equipment, but I'll reserve judgement.

Correct. I made some really rough estimates that I think would bear out in an apples to apples look at moments of inertia. The problem is that there is as much or more variation in a given size from manufacturer to manufacturer. That is really the best answer = lighter is always better. I agree about the overall effect in a 3500+ lb car. You might be able to feel the lighest 18s vs. heavy 20's but probably not much inbetween. Some track nuts have said they do believe in a 4:1 ratio of the effect of lbs unsprung vs. lbs sprung in overall performance. I'd really like to evaluate that number more precisely. Should be a quick calculation - knock yourself out.

[swamp2]

OK a couple of quick calculations, couldn't resist. One with actual numbers then a more approximate/theory type one:

1. The amount of rotational energy in a typical performance car wheel at 200 km/hr compared to the energy stored in the actual kinetic energy of the car itself is about 5%. In other words about 5% of your power is spent just spinning your wheels up. You can probably save about 20% in the moment of inertia of a light 18" wheel compared to a heavy 19" (see my prior very old post on this, this accounts for mass and distrubution). This means you are looking at about .2*.05*400 = 4 hp difference between the two (in our cars). Perhaps measureable but only with statistics, certainly not on the street. Of course at the drag strip or on the track when other unsprung vs. sprung effects come into play and cars are repeatedly accelerated and decelerated the effect can be amplified.

2. Another calculation shows where the famous "factor of 4" comes from but it is wrong. First you have to assume your wheels are perfectly annular (which is not really a very good approximation but not terrible). You also have to assume that the cars kinetic energy does not "contain" the wheels translational energy (a pretty good approximation, good to about 5% for our cars). You can then write the energy of the car and 4 spinning wheels more simply as:

E = Ecar + Ewheels
E= 1/2 mcar v^2 + 4 x 1/2 I w^2
using the annular approximation, I = mwheel r^2
then
E = r^2 w^2*(mcar/2 + 2mwheel)

Here you can see the total energy sensitivity on mwheel is 4x mcar, but each wheel is identical so it is not some magic amplification. There are 4 wheels, 1 lb saved on each is just about like 4 lb total saved of sprung weight from a moment of intertia effect perspective and then the additional 4 lb itself from lowering the total mcar. I think a factor of 2 is hence a more reasonable rule of thumb.

Therefore 1 lb saved in each of 4 wheels is like 8 lb total saved or double what the effect would be from simply saving 4 lb on sprung weight.

[Cloud]

Originally Posted by Cloud
The 19" wheel actually pushes the mass of rubber 1" outwards and this increases the unsprung weight of the wheel/tire combo.

lol got me there, musta been too tired.

[SenorFunkyPants]

Originally Posted by Cloud

The 19" wheel actually pushes the mass of rubber 1" outwards and this increases the unsprung weight of the wheel/tire combo.

I would have thought that with the same rolling diameter the majority of the mass of the tyre would be in the same place. Some of the sidewall is being replaced by alloy wheel and the tyre is slightly wider but I wouldn't have thought it would make a noticible difference if both the 18 and 19" tyres weighed the same in the first place.
As noted the weight of the tyre is going to make far bigger difference to the rotational kenetic energy required to spin up the wheel and to slow it down as the KE is a function of the mass of the item and its distribution from the centre of the spinning object such that losing 3lbs in the wheel weight is far less significant than losing 3lbs in tyre weight.
I know of one M3 tuner who replaced the standard OEM 19inchers with some lightweight Rays and claimed several 10ths benefit in back to back tests lapping in his E92 M3 racecar and I have no reason to disbelieve him, although I would think it hard to notice a difference in street driving.