Can someone clear up these wheel weight statements....

I recently read in some guys post that light-weight wheels take off of top speed. I understand less rotational weight will help acceleration, but why would it reduce your top speed.

 

it wont.
If weight is the only difference.

 

What he said.

If anything, it will increase your top speed because more horsepower is making it to the ground.

 

Just out of curiosity... would wheels and tires that had a smaller overall diameter than stock reduce top speed by some (probably negligible) amount? Seems like it would since smaller diameter tires reduce effective gear ratios.

--buji

 

Smaller diameter wheels/tires = shorter overall gearing = less MPH for a given RPM.

 

I dont see how a lighter wheel could help top end. The only advantage I see is helping the car out of the hole better.

Edit: It almost seems like a heavier wheel would be to some advantage on the top end. Heavier wheel=more velocity. I dunno this **** is semi confusing to me.

Also, along the same lines, why is unsprung weight so important (weight of calipers, arms...). Is this important so the dampers wont have to work so hard due to the decrease in vertical velocity a lighter suspension creates?


[Modified by siisgood00, 8:43 PM 7/10/2002]

 

We middle-aged ex-junior high school teachers come in handy now and then, huh?

The trick is to break out the variables, since considering more than one at a time confuses your understanding and ability to set up your car. The PROBLEM with this is that it is seldom easy (or affordable) to really change only one physical attribute at a time. Keeping all other factors constant, consider...

Weight - given two wheel/tire packages of the same wheel diameter, tire make/size, air pressure and everything else, a lighter wheel will ALWAYS be better for performance. Vertical wheel motion will be easier to control and more responsive to road surface changes and bumps, and it will spin up (on acceleration) and spin down (under braking) more easily. The latter is because it will have a lower polar moment of inertia. Inertia is the tendency for things to remain stationary until a force acts upon them and for them to continue moving - rotating in this case - once they are doing so. A decreased polar moment will NOT increase theoretical top speed but WILL increase acceleration - decreasing time required to get to top speed or increasing the speed achieved in a given time. By the same token, INcreased polar moment will not INcrease top speed, although it will make it harder to slow down.

Wheel Diameter (keeping the outside diameter of the tire constant for now) - this is where things get wacky. Because polar moment is dependent on both the mass (almost the same thing as weight) and its distance from the center of rotation, it is possible for a lighter 17" wheel to have a greater polar moment than a heavier 14" wheel, because the part where most of the mass is (the actual "rim", where the bead seats) is 1.5" further from the center of the hub. Of course, larger wheels (maybe not good) allow for bigger brakes (good), so you have competing variables - race cars are full of these.

Tire Mass - similar to the (potential) impact of wheel diameter, a lighter tire (e.g., Hoosiers) will be better than a heavier one (everyone else), in terms of polar moment... This is ignoring the variations in grip, which are influenced by alignment, temperature and vertical load, blah, blah, blah. That is a lesson for a different year.

Tire Outside Diameter - understanding that you can increase wheel diameter and maintain the same tire OD, if you DO change the tire OD, you change the rolling circumference (the distance that you travel over one rotation). More importantly, you change the length of the "lever arm" with which the engine/driveline pushes against the ground - the "gearing" of the tire - or its radius (distance from hub center to the ground). A larger diameter tire decreases the mechanical advantage that the drivetrain has on the road. A smaller tire OD provides more mechanical advantage and so allows quicker acceleration. Conversely, a larger tire yields a theoretically, mathematical higher speed but most production type cars are limited by aero drag and the distance required to get to top speed, rather than by gearing.

There is more but that is enough for one evening...

Have fun!

Kirk



[Modified by Knestis, 2:36 AM 7/11/2002]

 

yeah what kirk said.....exactly what i was going to say!

 

Thanks for the detailed, technical explanation.

 

If anything, lighter wheels with the same overall diameter would INCREASE top speed due to less energy being "absorbed" by the high inertia wheels.

Equations for those with a quantitative streak..

Kinetic Energy = 1/2*Mass*Velocity^2 <translational motion(i.e. the car)

Kinetic Energy = 1/2*Inertia*W(angular velocity)^2 <rotation motion(wheels)

As you can see, assuming the engine can output a set amount of power, more of it is "free" to propel the car. Of course, this is negligible, and almost all factory cars are drag limited. So if you put some aero tweaks on your car(splitters, diffusers etc.), you will gain much more top speed than switching to a lighter weight wheel.

 

Energy and power are not the same. Power is a rate of energy output. Even if more kinetic energy is being stored in the wheels per mph, as long as it doesn't cause more frictional losses, it won't affect steady state balance of top speed.. of course, as said, it will affect acceleration, or how long it will take to get that top speed..

Top speed is essentially the balance point at which all the power of the engine is putting out is going to the various loss mechanisms (friction/drag/etc.) Angular Momentum is not a loss mechanism.. it's an energy storage mechanism.

Also, many aero tweaks will reduce ultimate top speed because they are intended to generate downforce, or at least reduce lift-- not so much to reduce drag (at the track, these tweaks might in some cases yield a higher effective top speed before the next braking zone because the better handling allows higher cornering speeds). A drag reducing aero tweak would be to put smooth plating on the entire underside of the car, and put little shields over the wheel wells, but that's not good for heat dissipation..