rockville

Lighter wheels don't actually increase whp. The motor will for the most part generate the same hp no matter what wheels you have. However, as you mentioned, lighter wheels require less hp to spin up thus more of your power goes to accelerating the car rather than the wheel. The key difference being things like top speed which is very much affected by hp for the most part will no be affected by lighter wheels (all else being equal).
So chassis dynos will show a difference in whp based on wheel weight because they use an inertial mass. They measure how fast your tires accelerate a large drum. As this involved accelerating some of the engine power will be lost to accelerating the wheels. A dyno that takes power readings at a static speed would no see a difference in wheel weight.

cdelena

The rate you can accelerate a rotating mass with a given amount of power is more rapid as the mass is reduced. Lighter wheels and tires do make a difference.

I have not done much experimentation with my S2000 but I am reminded when I switch out wheels and tires on my bicycle. My puncture resistant wheels and tires weigh 20% more than my street setup, and it makes a huge difference. Since I am supplying the power it is easy tell the difference with the lighter wheels allowing much faster acceleration.

Fanman

On the Sportscar Revolution when they swaped to bigger brakes & wheels they wound up losing 12 hp. They explained it as weight moved farther out & it took more hp to turn the wheel. Also when I had my original tires (23 lbs. & 18") on my current car I dynoed at 232 whp, then I changed over to heavier 19" (33 lbs) recently & re-dynoed. I got 224 whp. Different conditions but you get the point.

Lighter wheels, should be lighter all the way around, unless you have a funky design where all the weight is on the outside of the wheel.

Recently import tuner magazine did an article on the Elise and dropped a substantial amount of weight off the car, and added power adding items as well. might be worth it to go look for a back issue.

rockville

To be clear, your engine is not loosing hp. What you are seeing is a direct measurement of the impact of the extra wheel inertia. So an extra 12hp is being used to spin up the wheels rather than accelerate the car but either way that 12hp is still being produced at the crank. However, that 12hp isn't always 12hp. It is related to how fast the car is accelerating (is that 12hp at .5g acceleration or .7 or .3?). If you doubled the inertia of the dyno it would take longer for the car to rev up and you would see a drop in hp lost to the wheels. If you reduced the inertia you would see the opposite. On the street you would loose progressively less power to spinning up the wheels as you got going faster. Like I said, top speed is completely affected by hp. All else being equal, more inertia in the wheels will not reduce your top speed. It will simply take longer to get there.

S2k03

Not really just put helium in your tires!

s2kpdx01

On a bicycle?? Yeah, maybe because a few pounds there is a much larger percentage of overall weight then a few pounds on a 2800lb car. Unless you have some very heavy wheel tire combo and go to a much, much lighter one, vice versa, or are a very experience road racer you aren't going to notice much if any difference at all.

ProV1

Heavy wheels DO MAKE A BIG DIFFERENCE.

When I got 18" M Parallel replica wheels for my BMW 540 sport, it made the car feel so sluggish off the line that I couldn't wait til I took them off. I had OEM 17" before. Braking suffered tremendously as well. NEVER AGAIN.

Keep in mind, this was for a car w/ 300 HP & 300 TQ. I can't even imagine how a 4banger would feel w/ some 30lb 19"s.


BTW, I just got a 04 BMW 325i sport. I think stock 17x8 rims are ~22lb each. I'm deciding 17" or 16" for my snow tires, and to see if I get added performance & fuel economy I'll go with 16x7.5 rims that weigh 19.5 lb instead of 17x7.5 (22.5lb). That's 12lb on wheels, and 4lb on tires (205/55/16 vs 205/50/17). 16lb total unsprung weight saved by downsizing from 17" to 16".

I will report back on the results of my "experiment". If I see as much difference as I saw when I got 18" pigs, I'm selling my stock 17"s and getting some SSR Comp 17x8.5 rims that weigh 15.3lb each.

S2k03

Do they make a difference? Yes

Does it matter to you? If you are just driving the car with some spirited driving, then no it probably doesn't make that much difference. If you are setting up the car for "TRACK" not street racing then yes I would say it greatly matters.

rockville

The difference isn't in the extra mass, it's in the hp required to spin it up from 0 RPM to say 1000 RPM. That can take a significant amount of HP. Especially when you have 4 to rev up.

Side note: All my previous stuff about wheels not reducing HP is true but you can think of it as a reduction in hp. But consider that on the dyno you only had to waist that 12hp to spin up two wheels. On the street you have to spin up 4. Depending on how fast that dyno was accelerating, you might loose the equivalent of MORE than 12hp (using the numbers above) to accelerate all four wheels.
All else being equal, light wheels will make a difference in acceleration and braking.

ProV1

did i say i was tracking my 540? i was only doing everyday commuting and the difference was HUGE. again i'm speaking from personal experience, why dont you tell us what you've experienced personally?