I think the question I am about to pose is a serious one for anybody that cares about straight-line performance.

If anybody knows the answer to the following question, or knows where to find it, let us know.

Let's take a car (lets say a 350z for the sake of argument) and swap the 17" rims/tires for 20" rims/tires that weigh roughly the same weight.

Now take the 0-60mph time for the 350z (roughly 5.8seconds ??).

What would the new 0-60mph time be for the same car under the same conditions with the same driver that CONSISTENTLY drives the car the same way.

By the way, this is pretty much a math question. I'm estimating the wheel diameter of the 17" wheel/tire to be 21" and the 20" wheel/tire to be 25".

If somebody knows a website that calculates this with based on formulas, or if anybody knows how to figure this out on paper, let us know.

Figure this out and you'll know if getting 19"s or 20"s TRULY does something to straight-line acceleration.

Thanks

 

Rotational Inertia. . . Hmmm... Physics book buried in box...

 

a few notes..

1. There will be a difference. A larger wheel will induce a higher moment of inertia, even if it has the same overall weight as the smaller wheel.

2. The overall diameter of any proper wheel and tire combination will be equal. The things that change are wheel diameter and the size of the tire's sidewall.

3. The rotational inertia is proportional to the integral of mass across the radius of the wheel/tire.

4. The only way to actually calculate the rotational inertia would be to know what the mass of the wheel is at every point across the radius, and to know the same information about the tire. The results will vary from wheel to wheel depending on the wheel design.

5. Doing the math is nice, but in the real world, other factors come into play, like friction, wind, etc, etc. The only real answer is to do real-world testing on a track.

 

I agree, there are way too many variables and assumptions to consider, but just for fun and to prove that I’m a nerd, here it goes:

The acceleration of the system, Z, is 5.8 second from 0 to 60 mph. Since Acceleration = change in speed / time the acceleration of the Z at this point is 15.17 ft/sec2 .

The torque for the car will be constant for both sets of tire diameters since it is coming from the drive shaft. We’ll say that’s it 100 ft-lbs*[banana], where banana is the variable that changes as the rpm changes.

Given the equation that momentum = force * distance. Where momentum = 100 ft-bs * [banana] and distance is the radius of the 21” diameter proposed tire. Force = 114.17 pounds of force * [banana].

Since everything is constant and consistent, a 25” proposed tire requires 96 pounds of force * [banana].

So, with a 21” diameter it takes 114.17 pounds of force * [banana] to move to Z and with the 25” diameter it takes 96 pounds of force*[banana] to move. I included this, to show the difference in required force that the engine has to supply.

Using the law of similar triangles, we can use that 114.17 pounds of force*[banana]yields an acceleration of 15.17 ft/sec^s and 96 pounds of force*[banana] will yield a constant we’ll call [turkey]. Solving for [turkey], the acceleration with the 25” tires is 12.77 ft/sec^2. Torque and [banana] cancel each other out. So we actually could have used that 21” tires yield an acceleration of 5.8 and 25” tires yield [turkey].

Using the acceleration equation again and using the change in speed as 0 to 60 mph: The acceleration is 6.9 seconds with 25” diameter tire. And again that is if nothing changes in the Z, the wheels and tires, the environment, and the driver, but the change in outer tire diameter. I wasted about a half hour doing that .

 

Wow that was crazy :P

So thats 1.1 second difference.

THATS HUGE.

Thanks for answering that question because it might shed some light on what your losing in straight out acceleration.

Thanks for doing that man

 

17" wheel is about a 26.5" diameter though.

 

I don't think bananas and turkeys get along very well.

That probably contributed to the reduction in speed.

-Doh!

 

Ok in which case a 20" wheel with, for example, a 255/40 tire on it would make it rougly 26" too?

 

I started a thread in the engine/drivetrain performance forum since this is performance related as well....

https://my350z.com/forum/showthread....246#post733246

 

one HUGE fact you guys are forgetting.. i mentioned this in my post above...

It doesn't matter if you have a 15, 16, 17, 18, 19, or 20 inch wheel. The overall diameter of a proper wheel and tire combination is THE SAME. ALWAYS.

When you buy a larger wheel, you choose a tire with a smaller sidewall to compensate.

Bottom's calculations above assume that there is a different rolling diameter, which isn't the case. The difference in acceleration comes from the rotational inertia component of the unsprung wheel/tire mass, which increases with a larger wheel due to weight distribution closer to the edge of the wheel and tire.

 

I feel really stupid right now

 

I still agree with you, we could make a huge list of factors that would go in to determining a change in wheels. But I'm too lazy, and I thought that the main question was to figure out if there is a difference in just the outer tire diameter alone. If we want to figure out the actual difference, we cant quit our jobs and start a research company since we're both in Colorado. If anyone out there wants to fund this company, you can come up with the name.

David

 

I definitely agree with you on the tire sizes outer diameter staying the same with a new wheel size, wheel weight, tortional ridigity of the tire's wall, and weight distribution from the center.
David

 

What is the tire sidewall for a 17" stock 350z wheel?

 

If you look on your tire it should say something like:
P245/35 R17 90 Z

For the side wall you will use the 245 number and 35 number. The 245 is the width of the tire in milimeters and the 35 is the aspect ratio. So multiple .35 by 245 mm to get the side wall to get 85.75 mm.

These numbers are just examples, I don't know what's even on my tires off hand. But you can see how it works.

bknezevic, check out this website:
http://www.performancewheel.com/KnowledgeBase.htm
It has some stuff on wheels and tires.
David

 

Ok, lets state the real facts....

17" stock wheel ~ 28lbs
20" aftermarket wheel ~30lbs

Depending on the distribution of weight of the wheels, the 2lbs may make no difference whatsoever.

There can potentially be no real difference in accelleration if you choose lightweight tires, such as Toyo T1-S's and a lightweight 20" wheel. Remember tires are heavy, and the stock tire puts a lot of mass on the outer edge of the rim, which makes it hard to accelerate, a lightweight tire would essentially do a good bit to increase acceleration.

A good rule of thumb: each lb of unsprung weight = 3lbs of sprung weight.

All in all, no real way to determine results except for a track.

Enjoy.

 

You'd have to be a real idiot to increase your tire OD (overall diameter) by 4"

In that case your 0-60 time would be something like 20 seconds because your tire would rubbing on your fender creating all kinds of stupid smoke.

Having said that, Bottom's explanation would be technically correct. But one correction, momentum <> force * distance. Force * distance is torque. Maybe you meant moment arm (closer).

Momentum would be mass * velocity and is not particularly relevant to this discussion

But i get what you're talking about.

Jason

 

I won't get any points marked off, will I? If I don't get a good grade on this I'll lose my scholarship. And I'll be working at Costco forever

Thanks for finding my mistake, but don't blame me for the question, I just tried to answer it.

 

Dosent anyone here with 20's ever run at the track of use a G-Tec 1/4 mi timer!?

 

What do you guys think just chroming stock 17" wheels would do to hinder performance.