SoloVR6

Like I said earlier, I am not positive of any numbers. I overheard the conversation so I am not sure if they meant the comptech prototype or what. Forgive me for posting rumors.
Again, I will not make any descisions or for that matter post any more comments until I have researched the topic further.

RacerX

As far as the calculations, Windscreen would be the person to ask about that, he's the one that did them after he looked up the formulas. He ran both Bill's original, hollow bar and the solid bar through his number crunching cerebellum.

An interesting side note - We all met a uniquely set-up white s2k at the Cendiv championships this weekend. Matt McCabe drove Chuck Behr's white s2000 to the championship using a tempered steel non-adjustable front bar (another one-off). What was interesting was that the bar was about 250% stiffer than stock :eek and they had to run tire pressures that increased oversteer, because the bar had gotten the car to understeer! Obviously, the setup worked for Matt, and it just goes to show that there is no one way to set up any car

Banannie

[QUOTE]Originally posted by etgar
[B]Bringing the other Evolution school instructor (that I know of) into the mix - do we really have any details from Joe Goeke on why he thinks the S2000 is faster than the Boxster?

Orthonormal

[QUOTE]Originally posted by etgar
[B]Was Jason running his JDM wheels?

etgar

Bill's comment to me was that the weight off the wheels is rotating and unsprung weight - and equivalent to taking the square of it's weight off of the car's sprung mass.

We're saving ~3.5 lbs per wheel * 4 wheels = 14 lbs

14^2 = 196.

Are you saying that taking 100 lbs off of unsprung rotating mass is equivalent to taking 200 lbs of sprung weight? Gut feel that seems off - but I'm not an engineer.

I'm just repeating here and don't have a 'Halliday and Resnick'.

I will say that with JDM wheels and Hoosiers the car get's up and moves - and I don't have ANY problem getting wheels spin on launches - I needed to tone down my launches to not get too much wheel spin. And no - my Hoosiers are not heat cycled to death.

If Bill is incorrect I would definitely like to know it - and know the proper 'rough' calculation. At this point I'm giving Bill the benefit of the doubt.

Etgar

Orthonormal

Who's Bill?

He probably means that heavy wheels impact you in two different ways: the acceleration/deceleration effect of the rotating mass, and the handling effect of the unsprung mass. I doubt he literally meant that you should take the square of the weight, because the units wouldn't make sense.

As far as acceleration/deceleration goes: if you assume the worst case that all the weight of the tire and the wheel is all the way out at the tread of the tire, then the speed of the "tread" around its circumference is equal to the speed of the car over the ground. Why? Because the tread touching the road is not moving relative to the road, therefore it's moving at the same speed relative to the axle as the road is.

So, you have to accelerate the car and the wheels in a straight line. At the same time, you have to accelerate the mass of the wheels in its circular paths around the axles. Thus the mass of the wheels/tires counts twice in the amount of mass you have to accelerate.

Now, if some of the mass is closer in towards the axle (quite likely, since you have spokes, and the rim of the wheel doesn't touch the ground, etc.) then that mass doesn't have to go as fast in its circular path and therefore doesn't take as much energy to accelerate.

It's also very straightforward to show the same thing using linear momentum, force, angular momentum, and torque, but it would be pretty ugly on a message board.

You know, I just remembered that someone told me this was covered in Dave Coleman's tech column in a recent issue of Sport Compact Car...so if what I said above makes no sense, try there.

As to the effect of unsprung mass on handling, it's very hard to quantify, because it primarily affects the wheel's ability to track over bumps without leaving the pavement or upsetting the car. It has very little effect on smooth pavement; more as you get more and larger bumps. I'd say it's more of an additive effect than a multiplicative effect...

etgar

Thanks for the reply Orthonormal.

Bill is Bill Gendron - he is a mechanical engineer ex test engineer for Pratt(sp?) and Whitney, 1997 National BMod champ, Designer and builder of the 1999 AMod National Champ car and as you might guess - the designer/builder of the 'Gendron Bar'. He has been involved in the road racing/autoX community for ~30 years.

I would imagine that if my post above is incorrect then it was either my misunderstanding or a lapse on Bill's part - as I'm pretty sure that he would know the right answer.

In the same conversation with Bill he did mention that the ~3.5 lbs per wheel should be enough to feel 'seat of the pants' and I can say that the combo of the JDM wheels and Hoosiers do make a VERY noticable difference - alot also due to the lighter tires of course. I would have to say that while this is very noticable to me - I can't say that I've noticed a difference at all between running a full tank vs. nearly empty tank which would be in the 13 gallons * ~7 lbs (weight of gasoline) = 91 lbs range (which would be close to what double the weight savings of both the tires and wheels would be). Others of course might have more sensitive 'seats'. 8^)

Based on what you posted I'm going to call him on it and get to the root of the matter - but it'll probably be a couple of weeks until the next AutoX.

I'll also attempt check out the magazine - should be a good read.

Thanks for taking the time Orthonormal,

Etgar

ultimate lurker

Coleman's piece isn't what I'd call particularly useful. Andrew Trevitt from Sport Rider did a much better piece several months ago. There was a discussion on this on VTEC.net in the tech forum (search for moment of inertia using a guest login). Moment of inertia is the key and its effect is dependent upon the acceleration rate of the wheel/tire. Thus, reducing rotating mass will have a more significant effect at lower speeds where the acceelration rate is higher.

You can actually make an estimate for how much torque it will take to accelerate a particular wheel/tire combo if you know the MoI and the acceleration rate. Since MoI is difficult to measure precisely, we have to estimate in most cases. We can also make some assumptions about peak acceleration (about 0.7g peak in 1st gear with a hard launch and diminishing after that).

The short synopsis is that for a typical car wheel/tire combo the tire has a MoI 3x-4x that of the wheel. Thus, losing weight in the tire can make a big difference in acceleration. Less so losing it in the wheels. As orthonormal stated, even if you assume a worst case MoI (immeasurably thin ring distribution of mass) you're only looking at a effective mass of about twice the measured mass. This can be verified by calculating the torque required to accelerate the wheel/combo and then looking at the total mass of the wheel tire combo as a percentage of total vehicle mass and then taking that percentage and multiplying it by available wheel torque in a particular gear. You'll see that the numbers are similar in magnitude.

This is one reason why Hoosier tires are attractive to me. They are noticeably lighter than other brands. If they provide similar grip, they are a worthwhile investment - moreso than wheels that are 2-3 lbs lighter. If you have to make a choice that is :-)

UL

Penforhire

UL, that acceleration estimate is dead-on. Using a G-Analyst I measured standing start acceleration peaks between 0.7 to 0.8 G in my stock stook on the street.

Orthonormal

[QUOTE]Originally posted by ultimate lurker
[B]Moment of inertia is the key and its effect is dependent upon the acceleration rate of the wheel/tire.