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Rotational Inertia


Perci

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I recently came across an article that help me understand the effect of the weight of tires and wheels on a car, and I thought it was worth sharing here.

http://www.superstreetonline.com/features/news/0109scc-technobabble/

 

Rotational Inertia

The rotational inertia topic was so big, it took me two months (June and August '99) to get it sorted out. Here's the deal: There is this rule of thumb among racers that adding weight to something that rotates is far more detrimental to performance than if you add it to the body of the car. This is absolutely true, and by bumbling through some physics, and after slipping and falling on a radian, I managed to get a few formulas figured out that could tell you just how much worse.

 

Any moving object has kinetic energy, as does an object sitting in place and rotating. An object that is both rotating and moving (like a rolling wheel, for example), has kinetic energy from both, meaning that accelerating or decelerating that rolling object will take more power than one that is just sliding along. How much more power is the question.

 

The answer, it turns out, depends on how the weight is distributed on the wheel. An extra pound on the tread of a rolling tire has as much kinetic energy as 2 lbs on the floor of the car. As you move toward the center of the wheel, the rotational effect drops until, at the center, a pound is just a pound. The formula I derived to determine the exact relationship between weight on a wheel and weight in the car isn't worth repeating here for one simple reason. It requires that you know the moment of inertia of the wheel, and measuring that is virtually impossible. What you need to know is that changing to tires that are 1 lb heavier will effectively add 8 lbs to the car (four tires, remember) and that adding a pound to the wheels will effectively add somewhere around 6 lbs to the car.

 

That only considers acceleration and braking; handling is dramatically affected by unsprung weight as well, but no simple formula is going to tell you how big the effect is.

 

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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  • 3 weeks later...

Another article on the effects of larger wheels and tires. One point that surprised me was the weight increase of the alloy 16" set up compared to the steel 15" of 6lbs. (Wheels and tires). A little counter intuitive, as my 'gut' would tell me alloy would be much lighter. Sigh. Closest thing I've seen for an apples to apples test.

 

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

Effects of Upsized Wheels and Tires Tested

[h=2]Plus-Size Models: Testing the performance effects of upsizing wheels and tires.[/h]

 

 

Tech Dept.

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http://media.caranddriver.com/images/media/51/car-and-driver-bars2-photo-495963-s-original.jpg From the May 2010 Issue of Car and Driver

The aesthetic appeal of larger wheels and tires is undeniable, but what about the alleged performance benefits? In general, larger wheels are heavier, and additional weight hinders performance. To examine the effects of installing larger wheels and tires, also known as “plus-sizing,” we tested five wheel-and-tire combinations—ranging from 15 to 19 inches—on a 2010 Volkswagen Golf and got a good sense of what is gained and lost in the process.

The plus-size concept basically works like this: As the wheel gets larger in diameter, the size of the tire’s sidewall must shrink to maintain the tire’s overall diameter. Keeping the overall tire diameter roughly the same is crucial to keeping the gearing the same and the speedometer accurate, as well as for making sure the wheels and tires fit within the fenders. But larger-diameter wheels and tires are often available only in enlarged widths as well. For more information on plus-sizing and details on what will and won't fit your car, call the folks at Tire Rack (800-981-3782) or visit tirerack.com.

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Our test employed nearly identical Goodyear Eagle GT ultra-high-performance all-season tires in all five sizes, inflated to manufacturer-recommended pressures. We say that the tires were nearly the same because the 15-, 16-, and 17-inchers have a lower speed rating (V, or a top speed up to 149 mph) than the 18- and 19-inch tires’ W-rating (up to 168 mph). This, however, was as close as we could get to testing the same tire in every size. We used the stock steel wheels for the 15-inch test and went to the aftermarket for the larger wheels, as most owners would do. The aftermarket wheel we chose was a cast-aluminum ASA GT1, which is available in 16- to 19-inch sizes on the Golf. It’s obvious from the test that as wheels and tires grow in size, they also grow in weight. Here’s how the test shook out:

 

What’s immediately apparent from the results is that as the wheel-and-tire packages get larger and heavier, acceleration and fuel economy suffer. Neither is a huge surprise, but we measured a 10-percent drop in fuel economy and a four-percent degradation in 0-to-60-mph acceleration from the 15s to the 19s, which is worth considering should you be thinking about “going big.” Increasing wheel diameter and width, in turn, requires wider tires with shorter and stiffer sidewalls, which we found will increase skidpad grip, but as our test shows, there is a limit to this assertion. The 19-inch package came with the widest tires (235/35R-19) mounted to the widest wheels (8.5 inches), but this setup had less grip around the skidpad than the narrower 225/40R-18s on 8.0-inch-wide wheels. We asked the folks at Goodyear why that might be, and they postulated that the added width may have given the outside tire more grip, which would increase body roll and could therefore decrease the load on the inside tire enough to lose 0.01 g on the skidpad.

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Surprisingly, the 225/40R-18s showed a big skidpad advantage compared with the 225/45R-17s (0.89 g versus 0.85 g). We *suspect the difference is due to the grippier compound (lower wear rating) of the W-rated 18s. Goodyear does offer a W-rated GT in the 225/45R-17 size, which is what we intended to use, but Goodyear sent us the V-rated version for our test.

Subjectively, both the 17-inch and 18-inch wheels and tires were in the sweet spot of grip, braking performance, ride comfort, and steering feel. Moving from 17-inchers to 18s barely degrades ride quality, and the additional grip is welcome. We’d guess that a W-rated Eagle GT in the 17-inch size would come close to matching the grip of the W-rated 18, but we can’t say for certain without testing. Even with V-rated rubber, the per-form*ance of the 17s felt *similar to that of the 18s. But since the 18-inch setup costs only $112 more than the 17-inch package, we’d probably opt for the 18s if we lived in a region with smooth roads and looks were a priority.

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We do know that the heavy, 19-inch setup suffered from the most impact harshness and seemed to tax the suspension the most. In contrast, the two smallest wheel-and-tire combos showed a propensity for more understeer on the skidpad but provided a more controlled and supple ride. And although it didn’t register on the dBA-meter, the 15- and 16-inch arrangements had a more pleasant sound quality than the larger tires.

The 19-inch wheels definitely look the coolest. But the 17- and 18-inch setups offer a better compromise of grip, acceleration, price, and ride harshness, so we’re not surprised VW uses 17- and 18-inch sizes on its hot Golf, the GTI. If it’s acceleration you’re after, stick with the smaller, lighter wheels and tires. And remember, unless you believe it is better to look good than to feel good, take our advice and stay away from extremely low-profile sidewalls and massively heavy wheels.

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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  • 10 months later...

And as long as we are talking tires the same webzine compared a new Continental tire to their older DW and the Michelin PSS. Of interest to me as I am replacing the Conti DW on my Boxster S, and have the Mich PSS on my 07 Cayman, which I found stellar, but pricey. I go through a lot of tires. ;)

 

http://www.motoiq.com/MagazineArticles/ID/4456/TESTED-Continental-Tire-ExtremeContact-Sport.aspx

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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agree Perci.....it is counter intuitive.

 

One only has to look at the wheels and tyres used in Formula 1 :)

 

my 928 rides far better, smoother and quieter on the stock 17s than the far larger diameter and wider 18s

Edited by malcolm

'21 Macan GTS PDK

Miami Blue/Black, 7k kms

 

'93 928 GTS 5 speed

Polar Silver/Linen, 192k kms

 

'88 928 S4 A/T

Black/Ivory, 190k kms

 

'09 Cayenne GTS Tiptronic

Nordic Gold/Black, +336k kms

 

'88 928 S4 A/T

Red/Champagne, 110k miles (US car)

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You could argue that in F1 the tires are the suspension! ;)

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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Correct - 50% of the suspension travel in an F1 car occurs in compressing the sidewall of the tire. There's been a move afoot to move to low profile tires to be more "relevant" to road cars (likely as much to do with marketing as actual manufacturer R&D). Not sure how I feel about the result:

 

25879.3.jpg

 

In terms of reducing unsprung mass, I did read an article (and will try to find the citation - MotoIQ?) that found that some of the most lightweight "performance" wheels actually hurt lap times as the spokes/rim will physically deform under cornering loads, lifting some of the tire tread off the pavement.

Erik

 

2001 911 Turbo X50

1980 911SC - Black, 3.2 litre short stroke SOLD!

2002 BMW M3 - Carbon Black over Cinnamon

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no arguement, thats exactly what the F1/tyre engineers do.

 

a debate that often came up at the apres race party was the newer tyres offered but the weight increase. this was back in my days in m/cycle roadracing where weight was everything.......I worked out a lot back then :)

'21 Macan GTS PDK

Miami Blue/Black, 7k kms

 

'93 928 GTS 5 speed

Polar Silver/Linen, 192k kms

 

'88 928 S4 A/T

Black/Ivory, 190k kms

 

'09 Cayenne GTS Tiptronic

Nordic Gold/Black, +336k kms

 

'88 928 S4 A/T

Red/Champagne, 110k miles (US car)

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I think that's why I struggled the last time I attended Speeders!

Erik

 

2001 911 Turbo X50

1980 911SC - Black, 3.2 litre short stroke SOLD!

2002 BMW M3 - Carbon Black over Cinnamon

Link to comment
I think that's why I struggled the last time I attended Speeders!

 

Hey, that's the perfect segue into, "We should get a group together for a Speeders night".

Perhaps a Wednesday in the near future?

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100% in for that, particularly if we can get enough people to fill a run group or two.

Erik

 

2001 911 Turbo X50

1980 911SC - Black, 3.2 litre short stroke SOLD!

2002 BMW M3 - Carbon Black over Cinnamon

Link to comment

Perci,

I read the article on the tire test of the new Conti, quite the endorsement. Have you checked with suppliers in Calgary to get a read on pricing?

 

Marc

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Not in Calgary, but I ordered a set out in BC for my Boxster. 2x225+2x255= $1K

 

 

Sent from my iPad using Tapatalk

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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Perci,

SpecR Motorsports is a new advertiser with us and will appear in the next newsletter. They can get overnight delivery of the new Conti. They have a rebate program ($65) which has gone live through to the end of May. Their package deal quote for my size was excellent.

 

Marc

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Too late for me, but others may be interested.

 

 

Sent from my iPad using Tapatalk

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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Applied for the rebate today.[emoji2]Six to eight weeks to get the rebate[emoji50]

 

 

Sent from my iPad using Tapatalk

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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Coming from a past competitive cycling perspective, rotational weight was extremely important. I can't complain buying new wheels for a car when I would spend $2500 for a set of light weight hoops and $200/tire (you buy these more than once/season). If anyone wants to feel the difference of rotational weight, all you need to do is get on a bicycle with heavy wheels, then jump on the same bike with light weight wheels and feel the snap. You were always better off buying a cheaper bike, then investing the difference in a set of light wheels.

 

Here's a set you can buy for $7900 USD http://www.gokiso.jp/en/products/

87 Carrera Coupe

75 914

04 GT3

81 928

89 Vespa (former Italian Postal scooter)

My legs, they work too

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Not sure I'd ever spend that for my car, let alone my bike. But your example is an excellent case of what the article was trying to say. Reduced weight at the tire has greater effect than reduced weight at the hub. But without the complex math.

 

 

Sent from my iPad using Tapatalk

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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I think that's why I struggled the last time I attended Speeders!

 

For me it's too much 'road hugging weight' [emoji23]

 

 

Sent from my iPad using Tapatalk

Perci

 

PCA DE Instructor

 

2014 Cayman S

2000 Boxster S

2000 Carrera

2010 Panamera 4S

1989 928GT

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