How much RWHP do those "heavy" wheels rob? Answers inside fresh from the dyno!
 

Now, I will preface all this by saying the following:

1. I realize that this test does not take into account the front wheels in any way, shape, or form. Obviously, there is a "rolling resistance" that must be overcome to accelerate. Regardless, this test was conducted to determine RWHP loss.

2. I realize that dynos are tuning tools. These tests are far from conclusive; however, every reasonable step possible was taken to make this test repeatable.

We have all heard over the years how important the weight of your wheels are. I've heard everything from "1 lb of unsprung weight = 2-4 lbs of sprung weight" to "each pound of unsprung weight robs .8 hp."

Many have theorized and some have tested, but I have never seen a test done such as what I did this evening.

I recently purchased 18" Chrome Z07 wheels from a private forum member. He got them from Ace Alloys. It is my understanding that Ace Alloys is the manufacturer and everyone else is just a distributor. I raced a fellow Z06 driver after mounting these wheels and lost serious ground from the previous week when I still had my stock wheels. Needless to say, I was concerned. I dynoed the next Wednesday and my numbers were much lower than expected.

I decided this week to redyno the car on a local, in-ground Dynojet first with the stock wheels and then with the Z07 wheels. I wanted to know how much RWHP the "heavy" wheels really stole.

First, I weighed each tire/wheel combination. I removed the drag radials from my stock wheels and mounted them on my Z07s, so my stock rear 18" Z06 wheels had no tires. I decided to use the stock 17" wheel with the stock Goodyear 265/40-17.

Combo 1: Stock 17" Z06 Wheel with stock 265/40-17 = 43 lbs
Combo 2: Chrome Z07 Wheel with Nitto 555R Drag Radials 305/35-18 = 59.5 lbs

That's a whopping 16.5 lbs difference for each wheel/tire. 33 lbs total.

I really didn't know what to expect. I determined before in my mind that the difference would have to be at least 15 rwhp for me to switch back to either my stock wheels or to an even lighter wheel/tire combo.

Each pull was done with ambient temps in the low 70s. The straps were equally tight (as close as can reasonably be). The car was right on the top of the rollers each time. The oil temps were also the same for each of the pulls. At least 2 pulls were done with each set of wheels. AFRs for each pull was dead-on 13:1.

We dynoed Combo 1 first (Stock 17"). The best pull was 421 rwhp and 395 rwtq.

Next we dynoed Combo 2 (18" Z07). The best pull was 409 rwhp and 386 rwtq.

So, the 33 lbs extra weight on the rear hubs robbed 12 rwhp and 9 rwtq. Some quick calculations reveals that, in this test, .36 rwhp was lost per extra pound of unsprung weight. I guess an easier to remember "rule of thumb" could be, "For each 3 lbs extra unsprung weight, you will lose 1 rwhp."

So, you may ask, are you going to switch wheels? Right now, probably not. I'm going to look into my options, but I don't know if I want to spend the amount of money I would need to spend on a nice, light set of wheels.

-Alan

Your test had wider, stickier tires on the heavy wheels, which may have robbed some of that power through extra rolling resistance.


However, consider that the extra wheel weight also negatively affects suspension compliance, cornering performance, and braking performance, hurts gas mileage, hurts brake life and heat resistance...and well...light wheels are good =)

Thanks for doing that test though, its pretty illlustrative as to how cheap, heavy wheels can really slow you down. That is a BIG gap in power, and doesn't even take the front wheels into account which will double the effect almost.

Good post! It really shows the problem with adding rotating weight. Thank you, most folks won't go through the trouble to document this properly. :thumbs:

Yeah, honestly the "sticky" part is not an issue. The drag radials were quite cold for the test, and when cold, they're probably a tad harder than a street tire.

As far as everything else, I agree. :flag:

Yah, only cost me $75 for the dyno time. My tuner didn't charge me since I think he was just as interested (tweaked a few things while before getting the final number on the first combo).

You would have to post this right as I am thinking about getting some new wheels.... darn, darn, darn..... :willy:

Your choices are significantly limited if you want something close to stock weight.

I think this post just increased the value of the old wagon wheels for using as drag wheels only. Correct me if I’m wrong, but aren’t they about the lightest and strongest wheels in 17 and 18 inch sizes?

Thanks,

Clayton from Las Vegas :flag:

delete

Wait a minute, hold on, can't believe someone hasn't caught on to this yet..

There is a 4.2% difference in the rolling diameter between the two wheel / tire combos.

Is it a coincidence that there is a 3% difference in the power output between the two?

Everyone knows (or that understands how a Dynojet dyno operates) that the effective gear ratio (like running 4.10's) has an effect on the numbers as compared to the "standard", i.e. stock C5 w/ stock wheels and tires.

So I'm not saying that the extra weight of the combo had zero effect on your results, I am saying that the total rolling diameter delta probably had as much or more of an effect than the extra weight.

Gotta compare apples with apples, which did not appear to happen in this case. :cheers:

The gear ratio on dynos has an effect on power because of the efficiency of different sized gears meshing. That variable would be constant here.

Good point, I didnt catch that. This power robbing effect would apply only to rotating mass. Wheels/rotors. Not all unsprungs mass.

No, that is incorrect (a common misconception); the measured "power loss" on an inertial dyno with lower gears (3.90's, 4.10's) is NOT due to "increased frictional losses", it is due to the rate the drum is accelerated over time (which is precisely how these particular dynos measure torque and ultimately horse power).

This whole thread shows the true value and accuracy of a steady-state dyno (Factory brand dyno, etc.). :cheers:

could the higher gearing resulting from the taller tires have anything to do with this? There is over an inch difference in the height of these tires.

Very interesting, like many I have always wondered.

what is the weight of an actual stock rear tire combo?

nobody uses the 17" wheel/tire size quoted above for the rear of a C5.

a 295/35/18 on a 10.5" stock Z wheel would be a much more accurate and fair comparison.

Rotating Mass
 

Just to add to the confusion, the positioning of the rotatating mass can also rob horsepower. I saw a test on TV where they had replaced wheels, tires, and brake rotors that were all larger, but the weight was actually lighter than the stocks components they replaced. With only those components replaced they found 5 HP had been lost. Some genius engineer figured out that because the rotating mass had been moved further from the center point it took more HP to turn it.

Makes ya wonder how much HP the ricers loose going from a 14" wheel&tire to the 19" package with the big brake rotors. I guess the fart can makes up for it though. :)

:iagree: I lost over 12 RWHP when going from a chrome Z06 (10.5) with a very light Kuhmo V700 to a CCW (12) and Mich PS 335 30 :o Added a FAST and LS2 TB and 390s and broke even. Next time I make a pull I am going with OEM Z06 rims and some worn out Kuhmos for the lightest possible combo and the most narrow, well, I am not driving 100 mile on ET St radials (275 40 17s) just for a pull or 3. Good post!
Dave :cheers:

Yes, if a weight is further from the center it takes more force to accelerate it. I was very clear in stating that I used 17" rear wheels in my test. I never said that if you replace your OEM 18" Z06 rear wheels with 18" Z07s you will lose 12 rwhp.

I would think the "unsprung weight" in my formula would be obviously referring to wheel/tire weight; however, good catch regardless. Might as well make it as clear as possible for those who may not think things through.

My test was as accurate as it could be. Would I have preferred to use the OEM 18" Z06 wheel in the rear? Sure, but it wasn't available. Regardless, my test is still completely fair and accurate with regard to its results.