What's just as important as the weight of a wheel is where the weight is located. You may go with 18" wheels and think you are making your car faster. Typically the car is actually slower, even if the wheels weighs-in lighter than 16's!

By going with a larger diameter wheel, you are moving much of the mass of the wheel away from the center of rotation, thereby making your vehicle more sluggish because of increased rotational inertia. Sluggish meaning your car accelerates slower and brakes longer, just as if you added a more weight to your vehicle, even if it tips the scale the same when sitting still.

If you think this sounds strange, think back to the last time you tried spinning, as in figure skating. Spin with your arms out, then pull them in. You spin faster. This is because you significantly changed your rotational inertia, even though your arms really don't weigh much compared to your body.

If you remember back to your physics days, I = 1/2*m*r^2. Which means when you move a piece of mass outward (r=radius) you increase rotational "inertia" by the distance's SQUARE.

So if you go with larger wheels, you are moving weight away from the center of rotation and increasing the effective weight your car sees when it tries to accelerate. Increasing your tread width without decreasing your profile can have the same effect.

If you don't believe me, do a search on the internet for people who tried swapping wheels back to back on dyno runs. During acceleration, the dyno shows less hp with larger wheels unless they are made of an extremely light material compared to the smaller wheel.

Note: This is something Tire Rack fails to explain...

http://www.tirerack.com/wheels/tech/perform.htm
http://www.tirerack.com/wheels/tech/plusone.htm

 

makes sense, but i think most people get larger wheels for the looks.

 

13s here I come!!

 

BLING is always better.

 

This is good information because a lot of people are not aware that a change in the distribution of mass will affect acceleration and deceleration.

I would argue however that an S2000 with lightweight 17" wheels and wide rubber is "faster" than one on stock wheels/tires. There may be a very small difference in acceleration time off the line but the speed you can carry through turns around the track will increase.

 

[QUOTE]Originally posted by VTEC4-2
What's just as important as the weight of a wheel is where the weight is located.

 

I would say it doesn't have to slow the car down. Yes what you post is correct but are you talking about just straight line acceleration or are you talking about handling as well? To figure out whether the larger wheels will slow the car is well beyond my math skills. You would need to figure the weight of the wheel and tires and where the weight lies for both options. With a lighter weight and wider wheel and tire you will get quicker turn in response and lateral stability.

Jim

 

IMO, the largest diameter wheel that still works well on the S2000 is a 17".

 

I'm talking about straight line only. Certainly handling can benefit from larger wheels. As you mention it's a bigger picture. Going to lightweight 17's will almost certainly result in shorter track times.

I was considering creating an Excel program to estimate rotational inertia based on tire size and wheel size. There is data available on recycling sites as to what and where mass in concentrated in tires... I could certainly use this along with overall weight from manufacturers to estimate the tire's effect.

Then I need to find a sample cross-section of a wheel to determine how much weight is concentrated where.

If you have any information along these lines let me know.

 

Another point to consider in this discussion is "Unsprung weight" (which is the weight of the tire/wheel). Fyi, "Sprung weight" is the weight of the car (which is supported by the springs).

Changing this weight will probably have a more noticeable effect on driving than anything else. Heavier combo's will "push back" on the car, and make the body move more than lighter combo's will. This is due to the shocks/springs having to control a heavier weight.

I would characterize the stock ride of an S2K as supple or compliant. This ride will tend to get harsher (IMO) if heavier combo's are put on the car. So....if someone upgrades tires/wheels, I would suggest carefully looking at the combined weight and trying to stay at or below the stock combination..

As a historical reference keeping unsprung weight to a minimum was one of Colin Chapman's (Lotus cares) best "secrets". You'll notice that he tended to use steel wheels on alot of his cars which were lighter than everything but Magnesium Minilites at the time.

I owned a '66 Elan S2 and the S2K is the closest vehicle I've driven since to have the same type of handling. It's the 21st version IMO.


Thanks.