Overall weight vs. Rotational weight

[DubyaAreEcks]

Hi folks, gotta newb question for ya. I have done the search and couldn't find the answer to my question.

Before I ask, I would like to tell you my intentions. I own a stock 04 WRX and looking to replace the crappy RE92's. I would also like to go for a wider wheel while im @ it. I would like to wrap 17x8.5's(or maybe 8) with 245 rubber (maybe 255 if im convinced they won't rub). Since I will be upgrading the wheels to 17's I am looking for the lightest/strongest wheel that is not rediculously expensive. I have read the stock weight of the WRX wheels with tires are around 38lbs. I don't know this to be true, but I have read this here.

I have read about rotational enertia when it comes to upgrading to a larger/heavier wheel. I am confused a bit how this will effect the acceleration/handing/braking of the vehicle. From my understanding the overall weight is the main issue but ins't there another factor regarding rotational enertia?

What I am asking is: If there is an issue with rotational eneria adding weight form the increase wheel size, how much lighter than stock do I have to achieve to match the normal enertia weight exerted with the stock whees/tires.

I hope I didn't confuse anyone, maybe im thinking to much into this.

Thanks!

Dave

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[fliz]

http://forums.nasioc.com/forums/show...4&postcount=35

Those spreadsheets are great for comparing inertia (rotational and linear) of different wheel/tire combos.

[Phil Jr.]

yea in laymens terms. Get the wheels with the least mass (weight ) that is closest to the center.

SSR comps are the lightest out there and not too expensive.

also, IIRC Kuhmo Ecstas MXs are pretty light tires.

[mbtoloczko]

Originally Posted by fliz

http://forums.nasioc.com/forums/show...4&postcount=35

Those spreadsheets are great for comparing inertia (rotational and linear) of different wheel/tire combos.

I've modified the worksheets to use a more accurate estimate of the effective mass of tire/wheel combos. Fewer limitations. Works with any tire/wheel size. No need to have any vehicle mass or have an estimate of 0-60 time.

http://se30.dyndns.org:8080/SVX/rot_inertia.html

[Samurai Jack]

Personally, I think your overthinking the issue. I don't remember the weight of the stock wheels but I don't think they are 38lbs each. They just don't seem that heavy.

There are a lot of good, strong wheels out there under 25lbs each that won't cost you a fortune and your not going to notice the difference on the street.

SSR Comps vary in price from about $350 to $470 each, in the size range you are talking about. If you got that kind of cash, go for it.
http://www.tirerack.com/upgrade_gara...5&autoModClar=

You are paying for the production process, usually forged to retain strength w/lighter weight, but they are just as susceptible to damage as is a less expensive wheel that is heavier.

[EarlQHan]

The difference in weight won't be generally noticed for daily driving, although getting bigger wheels/tires will throw off your speedo a little. I've heard people say Comps aren't the best wheels for daily driving because they bend if you go over a pothole. Personally, I'm going to get Rotas for daily driving because if they bend, it's 175 to replace, but I would invest in Comps for track days.

[ripvw]

Originally Posted by DubyaAreEcks

I have done the search and couldn't find the answer to my question.

searching on "rotational" in this forum turned up these two threads among many others - of course, I knew they were there:

http://forums.nasioc.com/forums/show...al#post9293087

http://forums.nasioc.com/forums/show...ighlight=wheel

Originally Posted by DubyaAreEcks

I have read the stock weight of the WRX wheels with tires are around 38lbs.

basically correct:
16x6.5 stock WRX wheel: 16.5 lbs.
OEM 205/55x16 RE92: 21 lbs.

Originally Posted by DubyaAreEcks

I would like to wrap 17x8.5's(or maybe 8) with 245 rubber (maybe 255 if im convinced they won't rub). Since I will be upgrading the wheels to 17's I am looking for the lightest/strongest wheel that is not rediculously expensive.

you'll have to define ridiculously expensive for us in a moment

Originally Posted by DubyaAreEcks

If there is an issue with rotational eneria adding weight form the increase wheel size, how much lighter than stock do I have to achieve to match the normal enertia weight exerted with the stock whees/tires.

you can determine this by using the "new and improved" spreadsheet that mbtoloczko was kind enough to post above. I've already downloaded the spreadsheet - and noticed that mbtoloczko has added a new "worst case scenario" to it - so lets plug in some numbers:

the stock wheel and tire weights I listed above, so I plugged in sort of a "worst, worst case scenario" for you based on your desires: a Rota Slipstream 17x8.5 weighs 21.25 lbs. according to Dave @ Oakos, and the very highly regarded Yokohama Advan Neova AD07's weigh 26 lbs. in 255/40x17 according to TireRack (you would have to roll the fenders to get this combo to fit on your '04 WRX sedan, by the way). We end up with a difference of 9.8 lbs. per wheel or 39 lbs total static weight difference - in this case, it is all unsprung weight which impacts the car's ability to handle bumpy roads. In addition, we have the worst case rotational weight difference, (a theoretical wheel/tire combo that has most of it's mass on the outer edge) that gives us 39.5 lbs for a total of 78.5 lbs. difference, i.e. a very tiny and/or anorexic girlfriend is along for the ride.

of course, there will be consequences in terms of handling feel by adding this much weight - I've already mentioned the fact that it will not handle bumpy roads as well - plus it may very well feel more sluggish off the line in terms of linear acceleration - it could actually measure slower 0-60 and 1/4 times for those who live for such things.

If we adjust the numbers down by 5 lbs on the wheel and then the tires, you will see that reducing the weight of the tires makes more of a difference than reducing the weight of the wheel: 32.9 lbs. total weight if we drop the tire weight by 5 lbs, 50.4 lbs. total weight if we drop the wheel weight by 5 lbs. This makes sense since the tire is at the outer edge of our wheel/tire combo.

so what combo of 17x8.5 wheel and 255/40x17 tire would get us closest to the stock wheel/tire weight? The lightest max performance 255/40x17 that TireRack sells is the Pirelli Pzero Rosso Assimetrico @ 22lbs. The lightest 5x100 17x8.5 wheel currently sold by TireRack is the SSR Competition - but unfortunately the weight is not listed. However, since the 17x7.5 SSR Comp weighs 12.8 lbs. and the 18x8.5 weighs 15 lbs, let's say the 17x8.5 weighs 14 lbs. and check out the result: we now have a wheel/tire combo that weighs 6 lbs. less than stock, and if we factor a worst case scenario for an edge-loaded wheel/tire - we end with rotational weight adding 7.3 lbs for a total weight gain of 1.3 lbs - a small puppy instead of the anorexic girlfriend is now your traveling companion.

Slipstream 17x8.5 @ Oakos for $750 the set shipped
SSR Comps 17x8.5 @ TireRack for $409 each.
255/40x17 AD07's @ TireRack for $214 each, the Pirelli's are $188 each.

so, are we done? not by a long shot. All of this calculating ignores 2, probably 3 very important issues:
1. grip, i.e the reason we're supposed to be buying new tires in the first place
2. potential gearing advantage/disadvantage of certain tire sizes based on their outside diameter when compared to the stock tire
3. the "real world" differences on the street, i.e. can you feel it, is it worth it?

even given the worst case weight differences discussed above, there can be no doubt that the max performance 255/40x17 Yoko's on 17x8.5 Rota's will absolutely destroy on the track or auto-x any stock sized wheel/street tire combo we can come up with in terms of dry grip. a comparison with the stock RE92's would of course be laughable, but it really doesn't matter what tire you pick - hard to argue the basic physics of a wheel 2 inches wider running equivalently wide max performance rubber. which is why, of course, so many auto-x'ers are running something similar. linear acceleration may suffer, but lateral acceleration will be enhanced to a huge degree.

many have made mods to their engines and drivetrains to help deal with the linear acceleration issues, such as lightened crank pulleys - another example of rotational weight reduction at work - catless uppipes, downpipes and free-flowing catback exhausts, ECU enhancements, etc., etc. Of course, if you start with an STI instead of a WRX overcoming the inertia of a heavier wheel/tire combo is pretty easy.

But since you have a WRX, there is another facet to this that hasn't received enought attention IMO - the benefit of underdriven tire sizes. If we go back to your original stated desires of wider wheels and tires and substitute a 235/40x17 instead of the 255/40x17, some cool things happen.

if we look at the outside diameter of the stock 205/55x16 RE92, we see from TireRack's specs that it measures 25.1 inches. By comparison, the 235/40x17 Neova - which still weighs 26 lbs - has an outside diameter of 24.3 inches. This much shorter tire has to rotate more often to achieve the same speed as the taller stock tire - revs per mile is 855, RE92 is 830. This will translate into a small speedometer error, but also a gearing advantage of over 3% in every gear.

What does this mean - it means you can go with the cheaper, heavier 17x8.5 Rotas and the grippy but heavy Yoko's (the best dry handling street tire that TireRack has ever tested) and still have a car that feels quicker in linear acceleration than the stock wheel/tire combo. Your skinny girlfriend and the puppy can both ride along and your car will feel quicker than stock. If you have the money for the SSR's - and don't need the max grip of the Yoko's - you can find lightweight 235/40x17 tires that let you benefit from both a lighter wheel/tire combo and the gearing advantage.

want a wider tire than 235/40x17? the 245/40x17 offers about a 1% gearing advantage is most cases - depends upon the outside diameter of the tire in question. you can use this link to plug in to see the how a generic 205/55x16 compares to a 245/40x17:

http://www.miata.net/garage/tirecalc.html

or use this formula to determine specific comparisons based upon specific outside diameter measurements:

rolling circumference = tire diameter x pi
rolling circumference of RE92 = 78.85
rolling cirumference of 235/40x17 Neova = 76.34
76.34/78.85 = 96.81% - 100% = -3.19% i.e. underdriven

unfortunately, the 255/40x17 size is virtually the same as the stock RE92 in outside diameter, so no gearing advantage there. this is also true for 225/50x16, 225/45x17 and 225/40x18 tire sizes. but the 6-speed STI does not need the gearing advantage - in fact, many have posted that you want taller tires, not shorter ones with the STI. so the 255/40x17 size is perfect for an STI in terms of gearing, and of course in terms of grip is good for both the WRX and STI.

final issue - will it matter on the street? I have no doubt that you will feel some difference if you get lighter wheels and/or lighter tires. whether you perceive that as a difference in weight is debatable. many have written that they can't feel a 5 lbs. per corner difference in wheel/tire weight - and others think they can feel a 2 lb. per corner difference. how sensitive are you to linear accleration issues? if you drive much on poorly maintained bumpy roads, the benefits of lower unsprung weight may also be a big issue for you.

if you decide to get lighter tires, understand that often translates to a tire with a less stiff sidewall - another feel issue that matters a lot to some people. and isn't it amazing that the two best (tested) dry performance street tires out there - the Falken ST615 and the Yoko Neova AD07 - happen to be stiff-sidewalled heavyweights? if you're looking for best wet performance I have better news - the Continental SportContact2 Y-rated 235/40x17 weighs 21 lbs - same as the stock RE92 and you get a gearing advantage on top of that. The 225/45x17 Y-rated Conti's performed very well in a European tire test that I posted a few weeks ago.

FWIW, here's what I think:
1. lighter wheels are always better - if you can afford them and their strength is sufficient for your needs. forged wheels are stronger per pound of weight, but also expensive - and expensive to replace if you do manage to break them.
2. lighter tires make more of a difference than lighter wheels in terms of rotational weight, but perhaps at the cost of a less stiff sidewall and ultimate grip. nothing wrong with picking the lightest tire that meets your needs, but buying tires strictly by weight is really, really dumb IMO.
3. for a WRX, the difference in linear acceleration between getting the lightest or heaviest wheels and tires can be totally overcome by the gearing advantages of a shorter diameter tire such as 235/40x17, 205/50x16, 215/45x16, 215/45x17, and 245/40x17. understand that the shorter diameter, lower profile tires will also have shorter sidewalls, making them ride less smoothly and potentially easier to damage if you hit a pothole. They will also protect your wheels less under those same circumstances. with some tire sizes, you need to pay attention to load capacity issues, and finally there is speedometer error - a non-issue for me personally but some people seem troubled by it.

Originally Posted by DubyaAreEcks

...maybe im thinking to much into this.

whether you are or not, I sure am.

note: edited data - original numbers failed to take into account tire size changes

[txl146]

Great summary. According to my butt-dyno, my car feels noticeably faster with stock setup than with much heavier 17" P1 and RE070 (20 + 27 lbs = 47 lbs) setup.

I will be replacing RE070 with Pirelli Pzero Nero (21lbs) as I do not race my car at the track.

[txl146]

Looking at the worst case scenario, it will be like 80lbs extra weight on the car. Can this be easily be fixed by the following mods on the vehicle?

1. Perrin crank pulley
2. 10lbs light battery
3. Removed spare and tools
4. Borla header

I apologize about lack of understanding in this subject, but would car equipped with above mods run similar 1/4 mile as car with stock rim/tire setup (ignore hp gain from header mod)?

[ripvw]

Originally Posted by txl146

Looking at the worst case scenario, it will be like 80lbs extra weight on the car. Can this be easily be fixed by the following mods on the vehicle?
1. Perrin crank pulley

IIRC, the lightweight crank pulleys are supposed to be worth 100 lbs of weight. not my area of expertise, but if true then the other mods you're suggesting would be icing on the cake

Originally Posted by txl146

...would car equipped with above mods run similar 1/4 mile as car with stock rim/tire setup (ignore hp gain from header mod)?

how much do you think 80lbs affects acceleration in the 1/4 mile? .1 second or so - that's my guess. you would have to be pretty good at launching the car correctly every time for this to be a big issue. but hey, if I was a drag racer I would look for every advantage - dry ice on the intercooler, lightweight pulley, etc. I would think that running 16" or even 15" wheels with those BFG drag radials is the way to go if you can afford an extra set of wheels and tires. I know that TireRack sells some 15" SSR's that clear the stock brakes of the WRX. or if shorter gearing helps with the launch and the first 60, running 16x7 SSR's and 215/45x16 tires @ over 5% underdriven would seem to be a good idea.

It's not my sport - sorry. I do love watching those videos where you guys kick the Camaro and Mustang owners' collective asses though -

[Louie]

I tried to use that spread sheet and I'm not an engineer or anything so what does it all boil down too? I've been told that for x amount of rotational mass increase its equivelant to z amount of weight.

I'm curious about this since I'm buying some rota battles that are 22lbs a piece I know thats about 5lbs more than my stock rims.

[HerculesPeanut]

Originally Posted by EarlQHan

The difference in weight won't be generally noticed for daily driving, .....

Yes, it is. My 17" wheel/tire assembly is 31lbs and the diff over stock is very noticable. Others who have gone the SSR/light tire route have agreed.

[mbtoloczko]

Originally Posted by Louie

I tried to use that spread sheet and I'm not an engineer or anything so what does it all boil down too? I've been told that for x amount of rotational mass increase its equivelant to z amount of weight.

I'm curious about this since I'm buying some rota battles that are 22lbs a piece I know thats about 5lbs more than my stock rims.

In terms of straight line acceleration, you could say that a tire/wheel combo weighing "x" lbs is roughly equivalent to "1.7x to 2x" lbs of non-rotating mass on the car. As other people have said though, wheel/tire weight is also important for handling on bumpy roads too. Light wheel/tires stick to the ground better.

[Valhakar]

Just as an example, I recently switched tires and wheels. I weighed each wheel and took an average of the 4 using a plain old digital bathroom scale.

Toyo Proxies 4, 225/50-16 on stock 16x6.5 wheels = 38 pounds

Hankook Ventus HRII XL, 225/45-17 on 5Ziegen Pro Racer GN+ 17.8.5 = 44.25 pounds

The Gtech Pro RR shows a slow loss in 0-60, No loss in 1/4 mile, and a slight gain in a 2 gear rolling start.

Using the spreadsheet listed above I have a 1.1 - 1.3 FPT loss for each wheel.