Front/Rear wheel weight and performance...
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TECH Regular
Joined: May 2005
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From: Bay Area, CA
Front/Rear wheel weight and performance... I was having a rather heated discussion with a few friends of mine on the benefits of reducing rotational mass and the old 1 pound of rotational mass is worth 10(?) pounds elsewhere on the car. What it came down to was the difference in the front wheels versus the rears in a RWD application. I argue that reducing the weight of the front wheels is just as important as reducing the weight of the rears as rotational mass is rotational mass. They argue that the front wheels are not nearly as important because they are not connected to the drivetrain and thus are effectively negligible in decreasing performance. Basically the extra weight of the wheels would be the same as the weight anywhere else on the car. As far as I'm concerned the drivetrain has to move the front wheels at the same rate as the rears and thus a heavier front wheel is just as detrimental as a heavy rear.
Input?
Input?
Location (front vs. rear) isn't nearly as important as rpms. The rule about 10 lbs of rotational = 100 lbs static weight, is based on parts that are moving at engine rpm's. Once that's scaled down through the rear axle the effect drops off. So flywheel, crank, even driveshaft (at 1:1 or overdrive ratios) is where the biggest effect is. Axle, wheel, and tire weight is still a factor, but not the same 10:1 that comes into play for parts moving at engine speeds.
That doesn't even make sense. They are connected through a common point he might know called THE GROUND. The drivetrain has to fight against the entire weight of the car AND the rotational inertias of all wheels/tires. It doesn't matter to the accerlation of the car as a whole if the wheels are not on the driveshaft it still has to push them longtitually and rotionaly. The driveshaft can't spin faster than the car is moving unless you broke traction.
The interial weight of the drivetrain is what he should be calling negectable as noted by revving the motor with the car in the air. I swear the drivetrain isn't really that damped by its own weight. I say your main enemy is static weight and not rotational energy. As noted that most static weight isn't needed in the car for structure and most of the rotational weight of the drivetrain is needed for structure. You don't make to make those smaller unless you want to break something!
The interial weight of the drivetrain is what he should be calling negectable as noted by revving the motor with the car in the air. I swear the drivetrain isn't really that damped by its own weight. I say your main enemy is static weight and not rotational energy. As noted that most static weight isn't needed in the car for structure and most of the rotational weight of the drivetrain is needed for structure. You don't make to make those smaller unless you want to break something!
I wanted to point out some other things. Most of the drivetrain componets can be thought about in 2d when considering inertias. Considered as rings therefore you calculate them as I=Mr^2.
If you are super concerned about this run the smaller diameter AND weight wheel you can get noting that the radius term is squared.
If you are super concerned about this run the smaller diameter AND weight wheel you can get noting that the radius term is squared.
Think for a second about the rotating weight is worth 10/1, it is actually a variable that could be more or less depending on what the part does. Keep in mind that different rotating parts on the car accelerate at different rates or more times than others. Whereas every part of the engines rotating assembly has to be accelerated to max speed in every gear the wheels only reach maximum speed once, at the end of the race. So a pound shaved of the crank will be worth many times more than the driveshaft, and wieght off the driveshaft worth more than the same amount of wieght off the wheels. Wheels/tires travel at twice the speed of the fenders LOL. The only time rear wheel wieght would be worth more performance than the fronts is on a RWD chassis dyno.
I built a heavy stick shift car once that came with a 40 lb. flywheel. I swapped it for a 15 lb. unit and gained performance that would have been equivalent to removing about 400 lbs off the front end. If I had swapped the wheels out for a set that weighed 25 lbs. less the difference wouldn't have been close. If I had taken 25 lbs. off the body I wouldn't have noticed.
Do you see what I'm saying? Reduce wieght where it does the most good first and work down. Aluminum flywheels or smaller torque convertors are one of the most effective weight mods as are lightwieght pulleys and crank dampeners. Thats why factory cars come with lightwieght pulleys now.
Vernon
I built a heavy stick shift car once that came with a 40 lb. flywheel. I swapped it for a 15 lb. unit and gained performance that would have been equivalent to removing about 400 lbs off the front end. If I had swapped the wheels out for a set that weighed 25 lbs. less the difference wouldn't have been close. If I had taken 25 lbs. off the body I wouldn't have noticed.
Do you see what I'm saying? Reduce wieght where it does the most good first and work down. Aluminum flywheels or smaller torque convertors are one of the most effective weight mods as are lightwieght pulleys and crank dampeners. Thats why factory cars come with lightwieght pulleys now.
Vernon
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Joined: May 2005
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From: Bay Area, CA
Originally Posted by black_knight
There is a certain wisdom to this. Consider that some cars put the front wheels up in the air. There certainly don't have rotational mass at that point.
No scientifit data here, but when I swapped back to a stock WS6 wheel (~20lbs) from a set of ROH ZS (~28lbs) It made a noticeable difference in the handling of the car at speed.
Originally Posted by OldSchoolFormula
That might be so, but at the point where the front wheels hit the ground again, it will take a bit of energy to get them spinning back up to the now very fast speed instantaneously.
(I'm not agreeing with your friends, here)
Actually, I just did run a calculation on the difference in initial angular velocity and it was 10% different for the same force on a race wheel (plus tire) compared to a stock wheel (plus tire).
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I think I agree with the original poster and disagree with his friends. Imagine a car with four wheels/tires but no driveshaft. Power it with a jet engine. The rotational mass of all four wheels is equally important in this instance, I think.
so stock rims and tires weight about 30lbs each
say some aftermarket rims and tires weigh 35lbs each
that 20lbs of extra rotational weight
so if you didnt have thoes aftermarket rims its like adding 80 lbs to the car
right?
say some aftermarket rims and tires weigh 35lbs each
that 20lbs of extra rotational weight
so if you didnt have thoes aftermarket rims its like adding 80 lbs to the car
right?
I noticed better acceleration when I took 100lbs of wheel weight off my car when I switched to a set of 15x6 Centerline Convo Pros up front, and a set of 15x8.5s Convo Pros out back 
Damn Impala SS wheels are heavy. 60 some lbs with a tire....
Damn Impala SS wheels are heavy. 60 some lbs with a tire.... 
Your friends are wrong. Rear wheel weight is only more important than front wheel weight if you're on a dyno. Lighter front wheels will help you transfer weight to the rear when you launch though.
Heavy wheels only really hurt you under periods of high acclerations. I have a math cad file where I calculated the effects of heavy wheels on dyno numbers. The accleration of the wheel on a dyno is fairly fast, but not as fast as say first gear.
Anyway, even on a 400rwhp car adding 15 lbs of weight in the worse possible senerio (outer edge of rim) only made like a 4 rwhp difference in a 4th gear pull.
Anyway, even on a 400rwhp car adding 15 lbs of weight in the worse possible senerio (outer edge of rim) only made like a 4 rwhp difference in a 4th gear pull.