Do lighter rims make a car show more whp on a dyno? Reason I ask is cuz (theoretically) with the lighter weight rims, the drive shafts will spin them faster, which leads to the dyno interpreting it as more power, right?

FWIW I'm reducing from 23 lbs to 18.5 lb rims.

 

i would think that it would. Although its only a few pounds difference it IS rotational mass that you are reducing. I reduced my wheels/tire combo from 41 to 35 pounds! Your tire weight is also very important for the weight on the outer edge makes more of a difference.

 

I went from a 51.5lb wheel / tire combo to a 37.5 lb combo and did see a difference. Although it was not a direct comparison, it was impressive.

So Also pay attention to alignment.

 

Thats why they call it Wheel Horse Power

Different size and weight wheel and tire combo will effect the ability to put down power.

 

What wheel/tire combo are you using now? I'm getting rid of these g35 wheels. Ride and handling went out the window compared to the stockers.

 

http://forums.maxima.org/showthread....402&highlight=

 

track wheels=show?

 

I have MMEV5's, which are known to be heavy, those used to be my 'show' wheels. Now, I have the tracks, which are light show wheels.

Anyone who is serious about lightweight wheels know that 17" is the way to go.

 

Remember that on a dyno you're spinning up a 3000lb roller as well. a couple lb of change on the wheel weights is going to make very little, if any, measurable difference.

you'll see it more on the track in your first couple gears than you ever will on a dyno.

 

Umm, No. They call it Wheel Horsepower because it is measured at the wheels, and therefore subject to drivetrain loss, VS. being measured at the crank, like auto manufactures do to post the cars HP.

 

Wouldn't the weight of the 3000lb roller be factored into the dyno's power measuring process?

More like the last couple of gears. Heavy wheels will make more of a negative impact to the vehicle's acceleration from higher speeds than they will from lower speeds.

 

Wrong.

Wheel moment of inertia will have an identical effect on acceleration as the car's total weight. As speed increases wind resistance increases as well as rolling resistance--the torque of the engine has to overcome forces acting on the body before it is divided up amongst accelerating the wheels and the car itself, and all of a sudden inertia is no longer such a huge factor in acceleration. Given the same engine, Cd and transmission, a 1000lb car will have the same top speed as a 3000lb car, because top speed is drag limited instead of weight/inertia limited. The 1000lb car will get there much quicker, but the fact remains.

While wheel weight is not as severe in its differing effects between gears as, say, flywheel weight, the only upgrade that will be more noticeable at speed than at rest is if you manage to somehow lower the Cd of the vehicle.

That said, the affects of wheel weight should technically be more noticeable in second gear at 20mph then in first gear at 20mph, but this is negligible and not really what anyone meant.

 

Currently using a 17x8 15 pound wheel right now. And deff felt a difference when i put them on. Now considering those buddy club QF's. When i first saw their price i said absolutely not but they have stayed in the back of my mind ever since. CONSTANTLY.

 

Yeah, they are pretty ridonk.... I really wish I could get some 16x9 SSR comps or something and wrap them in 255/45 T1Rs

 

Blackbirdvq hit me up a while back about some SSR comps. You can see if he still has them, doubt it though i mean they are SSR's.

 

When I said "I wish I could get" I meant more like "I wish I had that kind of money and the will to spend it on wheels instead of a motorcycle"

 

This is only true for a load based i.e. Mustang Dyno, not a Dynojet.

 

Where did I mention a dyno? Read the thread man.

 

Your post is perfect for explaining how a Mustang dyno works, so I'm book marking it for t3h 's

 

Changing the mass of the flywheel only directly affects rotational mass on the input side of the trans and this effect shouldn't change if the output side of the trans is spinning faster or slower based on gear change (i.e., change in vehicle speed). The only operating speeds that a flywheel will encounter are idle to fuel cut. If anything, the changing the mass of the flywheel will be more noticeable in lower gears because lower gears give the input mass a chance to accelerate at a faster rate.

I've owned/built two VE-5 3rd gens that were/are almost identically modded when my tests were made. They both have/had the same ride height, tire width, and vehicle weight. One has a little more power, heavier brake rotors, and heavier wheels. Which one will have a high top speed and easier time getting to its top speed? I know because I went 155-160mph (on a track) in the one with less power and much less rotational mass. The other one would run out of breathe around 140... I've even raced these two cars (on a track) against each other. From 0-80, the Maxima with more rotational mass would win and between 80-100 the other one would catch up and start to pass the one with more rotational mass.

Here's a quote pertaining to inertial and mass. I found it HERE

"Mass as a Measure of the Amount of Inertia

All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity which is solely dependent upon the inertia of an object. The more inertia which an object has, the more mass it has. A more massive object has a greater tendency to resist changes in its state of motion."

FYI, I'm not referring to dyno physics, I'm merely referring to real world physics. Although, in my mind, I would think that the two would be fairly similar.

 

Absolutely

 

Flywheels are only noticed in the lower gears, yes, because more of the torque will be used to accelerate the flywheel in the lower gears. I am not sure why you thought I meant otherwise, I made it pretty clear.

Which one had more top end? Did they both have the same alignment? Correct tire pressure? Were the rolling diameters the same? Were the speedos calibrated identically? Were the drivers identical? etc

Anyway you are not listening. It is simple physics.

If we lived in a vacuum, with ideal (lossless) gearboxes, tires and bearings, the top speed of our cars would be C (speed of light). We would have no drag to overcome and all the power would go into accelerating the wheels and the body of the car. It would take awhile to get to 300 thousand kilometers a second, but it would get there given enough time. If you put the car in neutral it would never stop or even slow down.

The reason we cannot attain this top speed, besides our obvious time constraints, is because we have friction in our bearings and between our gears, but more importantly because we have to move a fawkton of air around our car very quickly the faster the car goes. This soaks up energy. If you put your car in neutral in the real world at 80mph it will slow to 70 in a matter of 10 to 20 seconds. But this funny thing happens--if you put your car in neutral at 140mph it will slow to 130 in about 5 seconds, and if you put it in neutral at 40mph it will take much longer to slow to 30. This is the force that your car has to overcome at high speeds, the force it has to obliterate before it can even think about accelerating your car and overcoming inertia. This force increases with a degree of speed CUBED. That is why a 100hp Scion xPOS can do 100mph but it takes a 640hp 'vette ZR1 to do 200mph.

Did you ever wonder why a brand new 600RR with 120hp and 380lbs (+150lb rider), a power to weight ratio of 450hp/ton can only do 150mph while a 5.5 gen with a power:weight ratio of 160hp/ton can attain the same speed? The bike will get to 150mph much quicker than the Maxima, but the only reason it's able to go faster than a 120hp civic is that it has a much smaller profile and does not have to move nearly as much air.

Wheels have a set amount of rotational inertia. Their inertia DOES NOT CHANGE WITH SPEED. For every 1mph increase in speed, it takes a set amount of energy to increase the rate of wheel rotation by a certain rpm (corresponding to 1mph increase), exactly like for every 1mph increase, it takes a set amount of energy to increase the velocity of the car by 1mph due to inertia. They have an identical effect on the acceleration as weight--the effect decreases as speeds rises (although NOT AS SEVERELY AS THE EFFECT OF A FLYWHEEL, which will DECREASE IN THE HIGHER GEARS).

If you want to get very technical about it, when speeds increase the tires will actually bubble a little which will increase the rolling diameter and set the moment of inertia of the wheel relative to its rolling diameter less, meaning that even in a vacuum, the effect of wheel weight on acceleration will decrease with speed.

Oh yeah, do not try and quote physics sites. I know what inertia is and you are just making an *** of yourself.

 

Sport Compact Car did a build-up on a CRX several years ago, and switching to lighter wheels showed a consistant 3-5 hp gain all the way through the rpm range on the dyno pull.

Motor Trend did a similar test but used 0-60 timing to measure the improvement. The lightest wheels got the car to 60 a full .4 seconds quicker.

 

505... don't wanna ruin your day, but you're talking out your ***.
Morpheus is right.

All except for the part where he starts talking about a wheel's moment of inertia decreasing with speed. It will actually increase if the tire stretches and bubbles out. You're moving mass further away from the center of mass, which will increase the polar moment of inertia.

 

You are increasing the polar moment of inertia but you are also increasing the rolling diameter, so the wheel will turn over less per every mph that the car is going, since T=I*alpha and alpha will decrease relative to acceleration of the car, less torque will be required to accelerate the wheel another mph.

What I said in the post was moment of inertia relative to rolling diameter/total radius, implying these things. Since the wheel's radius will stay the same and really only the middle section of the tire will bubble out only a portion of the tread will be further away from the axis of rotation, so the total inertia should increase less than the rolling diameter.

 

I would have replied sooner, but I've been really busy with work.

You got me Morpheus. I'll admit that I was wrong about 99% of what I was arguing. I'm not usually wrong, but oh well. I argued further because, I guess, I didn't understand your first explanation of how it all works.

As far as my comparison between my two VE's, it doesn't make any sense. Like I said everything was almost identical between those two. Alignment, tire width, tire pressure, I would even switch drivers.....

 

No worries. Really just sounds like the heavier car had more top-end though. the VE-5 and USIM VQ have the same power but the VE-5 will pull away at highway speeds because of the location of the fat part of the curve

 

They both had roughly the same weight, but one of them had more wheel and brake rotor weight and didn't have as much top-end. Beats me. Maybe there's some variable that I'm just simply over looking.

 

You're picking nits here, but you asked for it....

You also have to think about the forces pushing down on the tire. the tire is flexing more and there is more downward force on the tire the faster you go (in theory). since the tire pressure says the same, the contact patch area will increase linearly with the increase in downforce on the car. So it's safe to assume the tire itself stays the same height relative to ground and your applied torque is going to be the same for that issue.

You're also forgetting the fact that polar moment of inertia is measured by
J= pi * r^4 / 2

So as the tire size increases, there's:
1. less torque applied from the engine due to the increase in moment arm length. this goes down linearly with increase in radius.
2. increase in tire diameter is extremely small with increase in speed, so the overall RPM declination curve will slowly go to zero. This equation is something only the tire manufacturers can calculate... it's near zero at these speeds anyway
3. moment of inertia goes up by a factor of 4 with the same increase in radius.

so your power required to turn it goes up roughly by a factor of five and your curve of the tire diameter vs. rotation speed goes down by some unknown factor. That makes it impossible to quote concrete numbers or equations here, but the point of the matter is you still have more increases in required torque to accelerate than you do decreases.

 

Indeed I am.

Except that unless we are talking serious supercars, the net transverse force due to air moving over the car is lift.

I am not sure why you are concerned with J, we should be looking at I here. The wheel/tire is not going to twist. Are you referring to the increased torsion on the axle due to increased diameter/moment of inertia?

Inconsequential; that will be the same since we are only changing the wheel weight.

I never said it wasn't negligible. I said that technically, the effect would decrease with speed.

No, POLAR moment of inertia goes up by a factor of 4. Moment of inertia increases by a factor of 2 with an increase in diameter, except we aren't actually increasing the diameter of the wheel, or the sidewall. The middle of the tire will be bulging out, that is the only thing that will increase the moment of inertia.

 

sue me. grabbed the wrong equation. It's only been about 5 years since I've had to use any of those equations. It's a squared function instead of ^4

The simple fact of the matter is a significant portion of the rotating mass of the tire/wheel assembly is on the outer edge. A tire weighs about 25lb and 15lb of it is at the outer edge comprising the tread and belts. IF that stretches a significant amount, which it won't at the speeds you're looking at, it's going to cause losses, not gains.

 

Well, it depends on the weight of the wheel and sidewall compared to the weight of the tread/belts, as well as the degree of stretch...