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8 November 2001
Just had my NSX dynoed at R&D in Gardena where my son John takes Nissan's development cars (very reputable dyno in socal). 261 RWHP, corrected to 250.8 RWHP. RM intake, DC headers and Tubi. Owner assures me that my heavy rear wheel/tire (45lb/corner) combo accounts for about 5RWHP loss. Food for thought.
Beats me. But the owner/tester said that barometric pressure is the biggest factor and today's was pretty high. Of course temperature has to do with it too. What amazes me more is the effect my heavy wheels and tires have. Does anyone care to comment on this ?
Well, power used to spin the wheels is power not available to spin the dyno drum (or propel the car.) Same thing goes with the flywheel/clutch weight. In both cases, where the weight is located (closer to the center or further to the perimeter) the more energy required to spin it up (or slow it down). Super wide tires add all their weight in the worst place. Heavy wheels are likely to have the excess distributed in various places depending on design.

The topic of rotational mass/inertia, moment of inertia, etc. is discussed here from time to time but I haven't seen anyone post the precise math for calculating the equivalent HP loss. I've heard seemingly incredible claims about "equivalent HP gains" from ultra light flywheel & clutch combos, but they defy reason unless it is somehow a percent of available HP, which also does not make sense.

For example, one claim was close to 30 HP. Since the entire driveline losses on a stock NSX are barely that, I'm not buying it. (literally or figuratively) Also, consider an old (very old) VW Beetle that had only 30 HP and just as much mass in the flywheel and clutch. In fact, under-powered cars use the flywheel in place of engine torque to get the car off the line. That's the purpose of it in the first place.

Anyone have the math?

Hey, what we need is a variable weight flywheel. Mass on the outside when taking off or during a shift (and under braking), but pulled back into the center during the rest of the acceleration. Sounds like a job for one of you engineer types. I'm sure Honda would pay millions for the right to use it on their hybrids.

[This message has been edited by sjs (edited 05 April 2002).]
What amazes me more is the effect my heavy wheels and tires have. Does anyone care to comment on this ?

Yes. 45 lbs is LIGHTER than stock: 26 lbs for the rear A022 plus 21 lbs for the seven spoke wheel (the 91-93 five spoke weighs less).

But Tino, are you still running the 3-piece AV3 wheels with 285 section tires? If so, it's probably more like a good 50 lbs.
Andreas, I should have put my 94+ stock rims for the test.Yes I had the AV3's with 275/35/18's on and I think it is more like 55lb, you're right !
I've got the Volk AV3 17/18 with 225/40/17 fronts & 275/35/18 rears I just sold my 91 and put back on my stock tire wheel set and noticed that the car felt faster than with the bigger tire wheel combo,I figured its probably due to the larger diamter of the rear tires and the wieght,but I kept them for my next car anyway I like the way they look.Doug
Originally posted by sjs:
The topic of rotational mass/inertia, moment of inertia, etc. is discussed here from time to time but I haven't seen anyone post the precise math for calculating the equivalent HP loss. [This message has been edited by sjs (edited 05 April 2002).]

I have done the math. Rotational mass and power loss is one of the most misunderstood areas. Let me try to explain the basics and give some numbers.

Unsprung mass and rotational mass are two different concepts. Rotational mass is just like it sounds, anything that rotates: flywheel, half-shafts, rotors, wheels, tires.... Reducing unsprung mass helps the suspension conform to the road (think about trying to control a bowling ball on the end of spring versus a softball). They are confused because some of the rotational mass is also unsprung.

Rotational mass takes energy and power to spin it up to any given RPM. The location of the mass effects the mass moment of inertia. Mass further from the center takes more engergy to spin it up. Recall as a kid hand pushing the marry-go-round type equipment in the playground. If a bunch of kids were standing on the outside edge, it was hard to push.

The power required to spin an object is proportional to
(Mass moment of inertia)RPM^2/time.
When you reduce the mass moment of inertia (lighter wheels, tires, flywheels) the power required is less. Notice that the power required is proportional to RPM squared and divided by time. This tells us that things that spin to high RPM and do it quickly have an even bigger effect. This is why spinning a flywheel in 1st gear takes up to 20 HP at high RPM, but that same flywheel might only take 1-2 HP to spin it in fifth gear because the rate of change of RPM^2 is much less.

This is why you can not quote (or even measure) the horsepower loss, because it changes at different speeds. Measuring the change on a chassis dyno is another impossiblity because the machine doesn't spin the wheels and tires at the same change in rate as an actual accelerating vehicle.

For the numbers. In 1st gear, the stock flywheel can require anwhere from 5 to 20 HP to spin it any different RPM's. Therefore, if the mass moment of inertia is reduced by 50%, you would save half this amount in 1st gear only. In higher gears, the flywheel requires 1-5 HP to spin it, therefore the lighter flywheel would only save a fraction of this amount.

Four tires (20,000 lbm in^2 total) require anywhere from 2.5 to 10 HP to accelerate them (the mass moment of inertia is high, but the RPM^2/time term is much smaller). Because most of the mass moment of inertia is in the tires, lighter wheels will only save a small fraction of the required power.

If you combine the effects of rotating mass and the regular (transational) force required to push a mass. 1 pound 8.5 inches from the center of rotation (for an NSX, it will vary with vehicle) is roughly similar to 1.5 pounds. Therefore saving 1 pound of wheel is like shaving 1.5 actual pounds. Because the tire is further from the center, saving 1 pound of tire (12.5" from center) is like saving 2 pounds.


[This message has been edited by 1BADNSX (edited 06 April 2002).]
Originally posted by sjs:
Thanks Bob, the basic formula certainly gets me closer to an answer. Time is the big factor I was missing. But where did you get the data to calculate the HP figures? Are you just estimating the stock weights and MOI?

SJS, yes, power always involves a rate of change (time) and this coupled with the strong RPM squared term have a bigger influence than the mass. This is why you see racers change the flywheel first even though there is only one flywheel and it weighs less than four wheels and tires. With this in mind, the lighter flywheel in the NSX is only really significant in 1st gear. I don't do too many first gear accelerations and you certainly do not use it on a road course.

For the wheel/tires, I estimated the mass moment of inertia to be 20,000 lbm in^2 total for all four wheels and tires. For the flywheel, I used the MOI posted on the Dali web page some time ago.