Wanted to get the forum's take on going with either 19's or 20's. I'm getting different opinions from various track and street folks. Would appreciate your 2 cents. Thanks in advance.

 

19's are the best performance wise, but 20's are usually better looking, you need to sacrifice some power going with 20's.

I have 20's on my car

 

thanks for the input milou. I'm really torn. I'm also surprised that no one else has chimed in on this topic. Anyone else? Thanks again.

 

milou - can you pls give me some details about sacrificing power? what about ride/handling/comfort? did you change to aftermarket springs/coilovers?

 

The bigger the wheels the smaller the tire profile, that means less comfort, my car is a cab so its yours, if i was looking for the best handling car i would get a coupe and pay less

If you do a right alignment there is nothing to worry about, bigger equals heavier that's why is slower.

I have ordered the Techart noselift suspension, i'll be posting some info when i'm done

 

I agree with milou, 20" will look better. The lose of power is really not that significant.

 

20" = show
19" = go

20's look great but the wheels are heavier and the side-walls are narrower. You will get a less compliant ride and increase the risk of wheel damages from road imperfections. The extra mass is rotational mass, so 1 pound on the wheels is like adding between 4 to 8 pounds elsewhere, depending on which source you quote.

19's don't look as good but I cannot think of one performance disadvantage they offer.

18's offer even better performance as long as you don't have PCCB's.

Did you spend $140 large to look good or go fast? Well, whatever your answer, most of us mere mortals would be fine with either 19's or 20's but I think 19's are the way to go.

 

Every time wheels get bigger (remember when 17s were consider huge!)...these arguments rage...seems to me that you should just get what you like best.

I just bought 20s and they are being installed today...wait a few days and I'll let you know how they feel.

 

If you measure the size of the wheel & tire of 19" & 20" they are just about the same size just less tire more rim & if you pick a 20" wheel that is much lighter than stock which i believe the monobocks HRE are i don't see how your gonna lose power.But it's obviously a personal choice & probably the most important one you make as far as the looks of the car.

 

As always we're adding power and better handling to cars we work on so we never do 20s. 19s are our bread and butter.

 

Personally I fail to see how 20 looks "better," but no argument since this is a matter of taste. You might want to check the latest HRE forged monoblock offerings such as HRE P40, P43, P47, etc. They have a design "trick" that makes the 19 wheel looks larger than its actual size: Their spokes are carried all the way out to a thin outer rim.

With respect to performance & handling, ask any forum sponsor to the right and they will tell you 19 inch is better than 20, WITHOUT QUESTION. Any pro who sells wheel and tunes car, who tells you otherwise, fire him immediately. The 20 for sure will ride harsher and affects handling negatively; it's more than just power "loss." One may argue whether any given driver could feel the difference, but there is no question about the negative aspects of a larger wheel.

You might want to read the following article, not exactly a neutral voice but the info is correct until proven otherwise.
Remember that moment of inertia is proportional to the square of the wheel's radius, so a bigger wheel, even if it weighs the same as a 19, has disadvantages. In addition, you will have to question the rigidity of a 20 that weighs the same as a 19.
Bottom line: Unless you must absolutely have a 20 for the look, 19 is the way, whether or not you could feel the difference .



>>>>>>>>>>



Determination of moment of inertia for a wheel is very difficult because the “normal” calculations are for a thin wall cylinder or solid rod, the calculation being a function of the mass multiplied by the square of the radius. With the Dymag carbon / magnesium wheel being of different materials and different masses as you radiate out from the hub, then the calculation needs to take on some assumptions.

Therefore if we take the whole wheel, then the basic formula is:
MOI = ½ mr²

If the carbon barrel alone is measured versus a metal rim it would be:

MOI = mr²
So because of the radius being squared in each calculation, the greater the diameter of any wheel then the greater the effect of reducing the MOI.

As you can see if we use the generally accepted first formula then we are very conservative in our result, especially if we compare the Dymag wheel with an aluminium centred wheel where we could be as much as 50% greater on MOI than in reality! Hence the importance of measuring MOI of 2 wheels accurately on the same machine to make any comparison.
Porsche 997 rear wheel comparison

Comparing a 19x11.5” Porsche 997 Carrera S wheel with a Dymag carbon/magnesium 19x12” wheel.
Porsche: 30lb x19” M=30lb, R=9.5”
Therefore: ½ x 30 x 9.5² = 1,353.75 lb/in² MOI

Dymag: 19lb x 19” M=19lb, R=9.5”

Therefore: ½ x 19 x 9.5² = 857.375 lb/in² MOI which is 63% of the MOI of the Porsche wheel.


Why is this important?
Consider 2 factors of the wheel in use, rotating and steering.
To rotate the wheel, the work energy required is calculated as force of net angular position change = ½ MOI x angular velocity², strictly speaking it is Force net θ = Δ( ½ MOI ω²)

Which in English means that the energy consumption goes up as a function of the moment of inertia x the square of the speed, or as you go quicker it takes much more energy! This equation also shows that both acceleration and braking are both effected significantly by a reduction in MOI

Steering changes are even easier to understand. The change in direction is governed by the momentum of the wheel which is calculated as the MOI x angular velocity, so in the above example, the Porsche on the Dymag wheel will use 47% less energy input to steer the car, either driver or power steering input, this is why the car feels “lighter” to drive and more responsive to steering input.

Rule of thumb calculations

This is a minefield of assumptions!! 2 of the old tuning rules of thumb were that 6lb weight saved on a car was equal to 1 bhp and that 1lb of rotating weight was worth 10lb of static weight, so in the Porsche example above, we are saving 11lb per wheel, x 10 = 110lb x 4 = 440lb ÷6 = 73.33 bhp we think this is probably excessive as the 10 factor does not take into account the diameter of the rotating part. A carbon propshaft would not have the same effect for example.
We have been stating that the rotating/static weight factor is about 6:1, this would give a result as above of 44bhp, which is roughly the gain effect that Parr Porsche said about the original tests of the carbon car wheels on the 996 GT3 RS!!

Power to weight calculation

Now if we take the Porsche wheels, the 997 Carrera S weighs 3131lbs and has 355 bhp, which gives it a power to weight ratio of 8.82:1 or, if you lose 8.82 lbs it is equivalent to 1bhp equivalent gained.

So if we take the Dymag 6x factor for the carbon wheels then saving11lb per wheel x 6, x 4 wheels ÷ 8.82 lb/bhp = 29.93 bhp an important 8.43% power gain simply by bolting on different wheels.
But if we take the sometimes accepted10x rotating to static weight factor this figure comes to 49.88 bhp

Obviously using this type of formula, the more power to weight ratio, the bigger the wheel effect on performance – not too surprising really!

Fuel saving and emissions reduction

The best estimates available for fuel and emissions savings by using carbon wheels are between 3.0% and 8.0% fuel savings. The increased % would be with stop – start driving for the obvious reasons of overcoming inertia and momentum. Also air drag factors mask the wheels performance at higher & more consistent speeds.

Emissions are much more difficult to calculate, especially when taking into account warm up and urban type cycles of car use. However as gasoline produces an average of 2.35kg per litre of fuel at the tailpipe, then it is safe to assume an initial saving of emissions to match fuel consumption. Again urban cycles will show a greater effect from the carbon wheel. So if the average car produces 165g of CO2 per km (at 7litres per 100km) the carbon wheels may reduce this to nearly 151g, a 9% improvement.

The fuel and emissions savings/reductions caused by carbon wheels need much more work. It is not possible to calculate these effects due to the other rotating components of the car which contribute to the overall moment of inertia, plus there are effects on non-rotating parts such as suspension where lighter wheels would give softer settings which require less energy to use the car in normal road conditions due to the reduction in energy losses with tyre deflections.

Conclusion

Carbon rims have a noticeable effect on a car’s performance and handling, the variations in moment of inertia of the complete system are not well enough researched to give absolute figures of the improvements that the wheels give. That the improvement is significant is beyond doubt, the independent testing and standards agencies now need to make these evaluations, equally condemning many of the very heavy aftermarket products.

 

wow! many thanks Can. Now I'm truly convinced to go with 19s. Much appreciated!

 

cannga
I see you compared the stock Porsche wheel to a Cabon Fiber wheel. The numbers will not work the same way on an aluminum 3 piece wheel.

Am I right ?

 

Well I just got my 20s today and now I'm wondering if half the people who are against them have actually ever had them? I was expecting a harsh ride (I have a back injury so I'm extra sensitive)...and frankly the car rides better in my opinion.

Now I don't track the car so perhaps in that environment it makes a difference however for street driving I think they are great and look fantastic.

 

I agree..my car rides better too...hard to believe!
I also DO NOT track my car but for the street...I love them!

As far as power loss.....lets face it 20's compared to 19's wouldn't be that big of a deal for the street.First of all there are so many variables to traction and putting down power on the street(dust,dirt,moisture,etc.).

Take a look at my album I have plenty of pics of my cab(non-turbo) to look at,you be the judge!

Do what makes you happy!

Stacy