I mean difference 5 pounds/wheel!!!!!!! Just trying to see whether the price difference between a light weight wheel which weighs less than 10 lbs and a regular one which weighs 15 lbs makes sense??

everything else remaining the same, how much difference it would make for a national level driver in a typical auto-x course? 1 second or 1/100th of a second? If the difference is very less , then for most of the people with less skill level, it shouldn't make any difference???


What you all think?

 

I'd be suprised if it makes any difference at all.

Probably well within the range of repeatable error.

 

No practical experience on an auto-x course but it seems like basic math to me. You exit T7 at Road Atlanta or Oak Tree at VIR and you have to rotate a 10lbs. vs 15.bs. wheel (assuming the same diameter), I got my money on the 10lbs'er every time to win that drag race.

 

Well first, it isn't 5lbs, it's 20lbs... unless you only plan on running one wheel that is. Secondly, it's unsprung weight. IIRC, the general philosophy is that 1lb of unsprung weight = 4-5lbs of sprung weight. So that would be roughly the same as 90lbs anywhere else on the car. Now, knowing that every 10lbs is worth about 1hp, that is the same as 9hp. Personally, I think that matters. That doesn't even take into account that it's rotational mass!

Light wheels = Good

 

All is well and good in theory... but I dont think it will make a difference in the bottom line (i.e. laptime) of 99% of autox'ers.

 

I am not sure it's something that can be as easily tossed aside as "theory" when you are comparing a rotating object w/ 50% more mass. Not to mention slowing it down. That's coming into play every time to hit either (brake/gas) pedal. You say it won't matter on an auto-x course, I am surprised but just have that little auto-x experience that I am willing to accept it. As for the road course part of his question, I think the math is on my side for both theory and practicality.

 

If I found a money tree, a lightweight set of rims (under 10 lbs) would be first on the list of things to get for the ita racecar. I would bet money that a 4-5 lbs reduction in wheel weight on each corner is worth at least 0.5 seconds around road atlanta. Lets setup a test!

edit: On my talon, I have 17" wheels for the street on Azenis that weigh a horrible 44 lbs per corner (wheel+tire). My autox set of 15" wheels with tires weigh 32 lbs. 12 lbs is definitely a big difference in rotational mass, but I can positively say that at the same boost level, the talon on the lighter rims would eat up its twin (if it had one) on the 17s in any acceleration test. Definitely subjective here (and the shorter gearing does play a role at certain speeds), but you can clearly feel the difference in acceleration by just how much it pushes you in the seat at any speed. I’m amazed at how much faster and brutal it accelerates every time I put on the light set of wheels. It’s like bolting on another 30 hp.


Modified by Hracer at 3:10 PM 1/28/2005

 

Spec Miata has a minimum wheel weight. There must be a reason. If wheel weight doesn't matter, then why make a minimum?

Then again, as RJ mentions, we're talking about auto-x here, not track driving.

OP - It can't hurt to have lighter wheels if you're trying to compete nationally, I'd say go lighter since every little bit helps. If you're a local only just for fun type autoxer, then the extra money may not be worth it.

 

I would say it makes just as much difference in autocrossing. Anyone who has been autocrossing for a while has inevitably lost an event by a few 100ths. Obviously driver skill could have easily made up for it, but lighter wheels could have helped too.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by r2x &raquo;</TD></TR><TR><TD CLASS="quote">There must be a reason. If wheel weight doesn't matter, then why make a minimum?</TD></TR></TABLE>

$$$

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">

$$$</TD></TR></TABLE>

I do believe $$$ is right. But that also means that people spend more $$$ for faster lap times. Therefore, lighter wheels must equal faster lap times.

 

There was a grassrootsMS article last year quantifying this. They did it by doing 1/4 miles with different weight tires. If I remember, the lightweight tires won by about 1.5 car lengths. It also had a wheel diameter comparison but I don't remember what happened. Anyone else remember this article.

Rotational mass... it crazy!!!

 

Yup, 5lbs difference in car weight alone will be unmeasurable.

5lbs difference in rotational weight * 4 wheels is a big deal. Why do you think aluminum flywheels exist, to lower the weight of the car? No, to decrease rotational inertia. In the case of the wheels, weight hurts the car's performance everytime it's sped up or slowed down. One important note is, 5lbs out of what? 5lbs out of a 50lb wheel isn't going to matter much, but 5lbs off a 20lb wheel makes a big difference. And it's more then just the straight weight difference because it's the square of the mass. Assuming the weight difference is roughly at the same radius, a 15lb wheel versus a 20lbs wheel will actually have 44% LESS rotational inertia.

So yes, I'd say there'll be a measureable difference even in an autocross situation.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by speedracer33 &raquo;</TD></TR><TR><TD CLASS="quote">Well first, it isn't 5lbs, it's 20lbs... unless you only plan on running one wheel that is. Secondly, it's unsprung weight. IIRC, the general philosophy is that 1lb of unsprung weight = 4-5lbs of sprung weight. So that would be roughly the same as 90lbs anywhere else on the car. Now, knowing that every 10lbs is worth about 1hp, that is the same as 9hp. Personally, I think that matters. That doesn't even take into account that it's rotational mass!

Light wheels = Good</TD></TR></TABLE>

Science fiction is an amazing field of human endeavor. Newton is rotating in his grave, all the unsprung mass of him. Imagine that the unsprung weight in the front of the car like a Honda is 130 lbs (you know, tire, wheel, brake disk, caliper, shock, spring, hub, spindle, bearing, half shaft, A-arms, wheel nuts, studs, hardware, etc). If you multiply that by 5 you get 650 lbs and by two for both sides you get 1300 lbs. Now add the rear unsprung weight and you can now completely ignore the sprung weight, as the effect of your unsprung weight alone is going to be more than the total weight of the car. Maybe I won't even need springs as the car will hold itself up, kind of levitating there, Houdini like.

 

I trust you're kidding... or maybe not.

We are discussing a 5lb change of a rotating component in the unsprung assembly. Everything else you've listed is unimportant. We noted that a 5lb change counts for a lot more since it's rotating.

 

Last I checked the brake disk at least is rotating, as is the hub and studs and wheel nuts. Do you have any idea how much a brake disk, hub, wheel studs & nuts, wheel and tire weigh? As I recall, the axle is rotating too, plus all those parts bounce up and down, along with the stuff that doesn't rotate.

The tire weighs over 20 lbs, and it is far more important than the wheel which might weigh only 13 lbs, and the radius for a tire is about 12" so the inertia of the tire is mr^2 or 20*12*12 = 2880 lb-in2 while that of a 13 lb wheel is 7.5^2*13 = 731 lb-in2, assuming for simplicity that a 15" wheel has all of its mass at the rim and the tire has all of its mass at the tread. Thus the tire has an inertia of 4 times that of the wheel. If you want to really make a difference in rotational inertia, use lighter or smaller diameter tires. Change the wheel by 5 lbs up say, and its inertia is 7.5^2*18lbs = 1012 lb-in2. Thus for the 13 lb wheel and tire, the total inertia is 2880+731 = 3611 lb-in2, while for the heavier 18 lb wheel the total inertia is 2880+1012 = 3892 lb-in2. Add to each the brake disk, which might be 14 lbs for an ITR sized disk, and its moment of inertia is 0.5mr^2 (solid disk), or 0.5*5.5^2*14 lbs = 212 lb-in2 for an 11" disk. Total inertia for the 13 lb wheel combo is now 3611+212 = 3823 lb-in2, while for the 18 lb wheel it is 3892+212 = 4104 lb-in2, or an increase of 7%. The actual increase will be slightly less if we consider the other rotating stuff, but it is in the ballpark. So the torque required for the same acceleration of the heavier wheel combo will increase 7% for a wheel weight increase of 33%. Wheel weight is important, but much less than many people think. Bigger brakes will also consume power to accelerate, unless you get those ceramic ones. As for the unsprung weight, it is also important, and here a wheel increase of 5 lbs brings the total up to 135 lbs, or a increase of less than 4%. There are many old wife's tales floating around. A few simple calculations can shed some light on the effect. Anyone who says that 1 lb of wheel weight is equivalent to 5 lbs of sprung weight has been grossly mis-informed. It is just not that easy to go faster. Spend your money on increasing the power rather than buying expensive light wheels. If you already have 13 lb wheels, then the effect of going 5 lbs less to 8 lb wheels will be even less as now the tire's inertia is even more dominant, and 8 lb wheels are going to cost you a ton.

 

wow!!!! this much more than I was looking for.....I am lost now....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by schumi &raquo;</TD></TR><TR><TD CLASS="quote">wow!!!! this much more than I was looking for.....I am lost now....</TD></TR></TABLE>
welcome to algebra-tech.com

 

Hmm, this is interesting. I have actually been pondering something similar. The difference in weight between the two sizes of Kumho MX's I have been looking at is listed as 5 pounds. I have been debating on whether to go with the 15 or 16 inch wheel/tire combo for street/autox. Part of the reason is that there is a wider size available in the MX for the 16s, but the difference in tire weight is listed as 20 vs 25 lbs on Tire Rack.

I got lots of time to think about it though, since I won't be buying anything till the weather warms up.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kb58 &raquo;</TD></TR><TR><TD CLASS="quote">
Why do you think aluminum flywheels exist, to lower the weight of the car? No, to decrease rotational inertia. </TD></TR></TABLE>

A flywheel has a much bigger effect, since it's effective gear ratio is MUCH higher. It has to spin the same rpm as the engine. Whereas if the wheels spun at 7000 rpm, you'd be looking at something like 8-10 miles PER MIN!

 

with my 7.Xlb/each EVOs on either of my cars,i can tell a big difference over my 12.xlb/each Nippon knockoffs in acceleration.same size wheels,same size tires.different brand tires,so there could definitely be a weight difference there.hard to say if there's any noticeable difference in handling as the two different sets of tires are at totally different ends of the performance spectrum,so they make a huge difference that would be hard to discern from any difference due to the weight.
if this would actually make a difference in lap times?hard to say.even if it did,i'd say it'd have to be with a very good driver,driving near the limit where any advantage would be helpful.i'm sure with ME driving,a difference in wheel weight is far from the weakest link
Chris

 

Well, descartesfool sn makes sense.

http://www.grmotorsports.com/plustest.html

not weight realated, but a good read anyway

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by stormy &raquo;</TD></TR><TR><TD CLASS="quote">
welcome to algebra-tech.com</TD></TR></TABLE>

I don't need the math, my auto-x wheels due to the tires weight in at 6lbs heavier tha my street whels and tires. I had to run my auto-x wheels on the street for a short time and I noticed a huge difference in acceleration. Now the Outside diamter is alos larger which has me comptiplating a move down to 195.60.14 instead of the 205.50.14's I am running

 

I think to make it all simple without math, you just tell yourself that rotating mass is more important to the car's performance than non-rotating mass. Any increase in the weight of the tires, wheels or brake disks will slow you down in acceleration due to the rotational inertia, with the importance varying according to the radius squared. For a 15" 13 lb wheel and a 20 lb 24" dia. tire (225/50/15), the tire's effect is 4 times that of the wheel. So a 1 lb decrease in the weight of the tire is the same as a 4 lb decrease in the weight of the wheel in terms of acceleration. These factors change if the radius or relative weights change, but it gives a ballpark. If your tires are already as light as you can make them by proper selection, then go for the light weight wheels next, but don't expect miracles when it comes to lap times, as you are not changing the overall % of weight much. That is why money spent on HP is so good for lowering lap times, as you can change the HP by 5-10% fairly easily unless you are close to the limit, whereas changing weight by 5-10% requires you to take of hundreds of lbs. It is all relative.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">All is well and good in theory... but I dont think it will make a difference in the bottom line (i.e. laptime) of 99% of autox'ers.</TD></TR></TABLE>

I agree that for most autoXers the differences run 2 run are larger than a 5lb weight loss per wheel. However for those that are at the National level a 5lb difference can equal the margin of victory.

I would guess that the average low-torque Honda would see a time improvement between 0.01 and 0.1 seconds per run. A high torque car like a Z06 vette probalby musch less