No, read the first post again carefully. Jack made the point that the 17" wheel allows you to run a lower-profile tire which is less heavy than the larger 16" one, thus putting less mass farther from the center for the 17" wheel

There are no demerits here.

-Wayne

Wayne, he was just busting me for using 'farther' instead of 'further.' A slip writers like to catch. ;) But in his example, he also implied that I had found a 17-inch tire which was heavier than a 16-inch, which isn't the case.

No. I've never talked about any of the wheels or tires on my car -- just the difference between a 16-inch Fuchs, a 17-inch 2-piece Fuchs of the same width, and two tires for each wheel which happen to have the same width and outside diameter. It's true that some people switch to wider wheels and tires when they move to 17- or 18-inch wheels, but that doesn't have anything to do with this particular discussion.

Aside from tongue in cheek writer nitpicking, my intent was to say the mass isn't distributed linearly from the center. Can you be sure the 17" wheel, while being lighter, isn't still presenting more mass farther from the axle even though it has a lighter tire ?

having trouble wrting out a continuous train of thought with Mr. Dog freaking out over the thunder and sticking his nose in my armpit...

In this case, I probably can, since the reason for the 17-inch wheels reduced weight can only be attributed to the new portion that's been welded in, which is the outside rim. It's significantly lighter than the same part of a Fuchs rim, and it's the part that's farthest ;) from the center.

Or is that furthest? Now, I'm going to have to look it up again. :cool:

Can't see where you got that one ;)

I would assume that tread and sidewall construction being the same that any equivalent width/diameter 17 would be a bit lighter than a 16, unless the longer cable in the bead canceled out the missing sidewal.

I can still hear my retired-english-teacher aunt lamenting the news anchor talking about storms moving further away...... :cool:

Jah Mon, would you speculate that the inner barrel of your wheel is smaller than the average sensible large diameter wheel. That would make that big flat band of alloy a smaller diameter than what we were getting with off the shelf wheels, aha!

ok...so more weight out at the edge is harder to get rolling...but what about once it is going? Any momentum gains?

My experience in these arguments are from the bicycle world. In general, your racing wheels wanted to be as light as possible, so light tires were all the rage. But then you'd get into arguments over wheel diameter, wind resistance, compliance over bumps, etc. But you were also working with a lot weaker engine, so little things made a big difference over a half hour crit or multi hour road race. But I don't know that the time trial guys on the flat worry so much about the rotational weight becuase once they are up to speed it isn't an issue. But for climbs, everything needs to be light as possible.

Not sure how direct a comparison though. In part becuase there is not diameter equivalence for bikes. You can't get a 700C and 650C wheel to have the same diameter by using ahigher profile tire...

Agreed - not a direct comparison, as you don't speed up and slow down a lot when you're biking (not like at a racetrack). For a car like the ones they race at Nascar, I would think that weight would possibly be an advantage, as you mentioned...

-Wayne

I had no intention of making blanket statements only attempting to clarify the inertial physics of rotating masses. I thought it important to say that the wheel weight itself is not the important factor but weight distribution across the wheel diameter.

Other folks have summed things up better in saying that in these cases the inertia of the larger wheel is not as significant as other variables.

Back to my rock I crawl.

Momentum gains yes but deceleration issues certainly. Hard to get going and hard to stop.

l had 6/7`s factory Fuch`s on my 84 and l switched to 7/9 factory 993 cups.l retained 205`s up front,went 255`s rear.The 17"wheel tire package is heavier and one thing l noticed right away is that l can`t do e-brake 180`s anymore with the bigger package.The increased inertia won`t let the tires lock up so l can`t spin the car around! l use this car for stunt driving so the advantages of quick turn-in with the 17`s is appreciated but...also during stunt sessions it`s good to have a lot of sidewall to absorb crazy tarmac.One guy l`ve seen with a work car,black 79 turbo,has super wide 15" Fuch`s,he swears by them.l have to say the 15`s on his car look very hot,the tires he`s running are huge too.All this "plus one,plus two/three" is getting old.l think it was Grassroots..maybe European car did a big track test to check performance gains from"plus one" etc.l seem to remember that l little bigger yielded slightly better laps,bigger still and lap times decreased..l`m thinking 14`s with the big gumballs , early F1 style big and littles should show off the tailights..

That grassroots motorsports test actually revealed some results that differed somewhat from Jack's. In that particular test, where they tested a lot tires, they found that wheels tended to be heavier on average than tires. Of course with variations, but it seemed that aluminum was the bigger culprit in weight gains. One must keep in mind that not all tires are equal. In that same article, as in others I have read, it mentioned that 17 inch is always the biggest one can go for ultimate gains on the average weight passenger car since there are losses when going to 18. 16 and 17 always had very close results. Another issue that cannot be discounted, because in fact it is extremely important when handling is an issue, is shock absorber workload, heat and heat dissipation. Next time you get behind a rice boy with pimp diameter sized rims on rubber rings, take the time to notice the hopping over road imperfections. I always get a laugh out of those ricers when I see their oversize wheels barely coping over bumps while the shocks are holding on to dear life. Simply put, for every inch increase in wheel diameter, there are much greater demands placed on the shock absorbers. Especially on a not so perfect surface. That is why if one cares about performance in a car that weights less than 3,000 pounds, one should not go bigger than 17 unless resorting to some very specialized$$ shocks. But I am sure there is some person in the Rice Boy Istitute of Engineering who is presenting a dissertation discounting that theory. No Weissach engineer would even dare compete with the expertise of those fellows.

While some here mentioned the 13 inch rims used in F1 having the purpose of limiting brake size, that isn't quite true. The reason being that at this moment, the brakes are the single most effective devices on the cars and the main part of the package that gets noticed the most when tested by the uninitiated. Simply put, even with the extreme levels of performance in those cars, they are actually far above the rest of the package as a whole. Amost out of balance with the cars. Hard to believe, but true. There are real advantages to those cars running 13's. Even back in the seventies there were. At that time, rim and tire sizes were free which is the reason they ran very small diameter fronts and huge rears. And during that era, not only were 15 inch rims of the same diameter widely available, but so were 19's or 15 inches in width. There was a virtual smorgasboard of slick tires sizes in that era. Today, it is impossible to find a 19 inch slick for a 935. The small diameter F1 fronts were there to reduce frontal area which is one of the reasons for the Tyrrell P34 along with the larger contact patch afforded by the increased contact patch of four front tires. But there is another reason for the 13 inch rims in Forumla 1, along with just 15's in Cart, just 18's in GT1, GT2, GT3, ETCC, BTCC, DTM and so forth. It is because in professional racing today, it is widely accepted by engineers that the target size of the rim for a particular application is mainly dictated by the overall size of the braking system. And braking systems are never, ever larger than they need to be due to the constant goal of reducing unsprung weight. Notice this is in the rarified atmosphere of high level professional racing where engineering is at its peak. There are no mistakes in sizing choices at that level. And sure enough, inertia is a big issue as well and one of the reasons for trying to keep the diameters as small as possible. Even though sometimes they are quite large. Neither do those teams want to place higher demands on the shock absorption part of the equation.

So would F1 cars run 18 inchers if they could? No, they are working quite well on the 13's as it is. Then there is the fact that they have been able to reduce to weight of a current day F1 tire to a level so low that no metal used in wheels could ever match. Pick one up and it weighs no more than a basketball. Maybe they would run 15 as in some other Formulas. But never, ever larger than that since from that point on, with the extremely low overall weight of the car, only God himself could come up with a shock that could cope with the demands.

The reason Jack's Fuchs are so light is due to the very light weight inherent in cold rolled aluminum halves. That is why some 3 piece rims become very light when used in conjunction with a forged center. Of course there is less strength to the lips, but that isn't really that big of a problem unless one gets into the 18 inch size with even lower sidewalls. One would be hard pressed to find a one piece wheel that approaches the low weight of fuchs of the same size. Unless it was one of the one piece BBS units that Roock and Konrad used to use on their 993 GT2s. They were actually made in BBS's Japan facility where the F1 BBSs are made as well. Same technology. As for SSRs, they are not the same quality forgings as fuchs and would never match their strength. That is why I always say that if ditching fuchs, never do so for a cheap cast wheel.

Something of interest in this matter is the reason for the 19 inch diameter wheels used on the 935/76, 935/77, K3, K4 and some 934 and a halfs. The reason for those was the simple fact that a 15 inch wide tire would not provide the grip required by those beats with the limited grip level in tires of the time (amazing). The solution was to increase the oveall diameter of the rear wheels, since that was free by the rules, to an outrageous number which had the effect of ingreasing the contact patch longitudinally. While giving some attention in trying to keep the sidewalls within discretion, the only solution was a 19 inch diameter rear wheel. This worked quite well. Not quite as well as something wider, but it did the job. Eventually even the BMW 320i turbos followed suit and used them as well since, after all, the tires were plentiful due to the large number of 935s. Beautiful wheels. It is a shame that one cannot get tires in those sizes today. Like I mentioned earlier, back in those days you could buy a slick in just about any size. Today in the age of demographics, if one cannot sell ten million widgets, chances are the widgets won't get produced. But I suspect that one of these days, maybe Avon will start producing them again. In fact, just a couple of years ago, Avon started production of the Tyrrell P34 front tires just for the sinle one in competition today in vintage F1. British tire company respecting a British treasure. You gotta love Avon.

Is that true? Common sense makes me think it's the other way around. My truck has very large tires on 15" wheels. It goes over curbs and parking spots like they aren't even there, due to the fact that it has such large wheels.

Common sense would seem to me that the larger the wheel, the better the vehicle's ability to weather bumps in the road. As the wheel gets bigger, the road bumps would seem to have less of an affect on the suspension. An exaggerated example would be a monster truck. It doesn't really care about pot-holes because they are small in comparison to the tires. On the other hand, think about a 908 with 13" wheels. It would break the suspension if that wheel went over some of the pot-holes on my street.

Unless I'm looking at this incorrectly?

-Wayne

Wheel diameter Wayne, not overall tire diameter. Think about incresing wheel diameters on the same 24 and something inches of tire diameter. A huge truck tire on a 15 inch rim will have you feeling like you are riding on a cloud. The opposite being the closer the rim edge is to the pavement, the harsher the ride. Don't tell me you never felt a difference in ride softness when coming off a road bike and then getting on a mountain bike. Or for that matter heard people complaining about ride quality from the use of 18 inch rims with short sidewall tires. Of course, find a 30 inch diamter tire and place it on an 18 inch rim and there won't be any ride quality issues.

By the way, the solution for the inability of the shocks to adapt to sidewalls that are too stiff for the given weight of the vehicle is to put in stiffer shocks. Of course, you also create further stiffness that way. Kind of like fixing the problem by worsening the problem. Unless the vehicle is only intended for track use, or the shocks are Moton, JRZ, Penske, Ohlins or something comparable. Still a stiff ride that does not need to be that stiff. Hence the reason why most poeple will agree that for most sub 3000 pound purposes, 17 inch wheel diameter on 25 or less tire diameter, 17 should be the upper limit. That is if one is seeking perfection in the application. Most comparisons like the article on GRM ended up with those results supporting what a lot of people already felt.

While what Ed said is true, I think Wayne was focusing on the road damage handling improvement from the larger overall diameter.

I was explaining the same thing to a coworker about why her little Forester wheels on a softer modern suspension than her husbands old-tech hard-sprung Ferd truck didn't ride as well on the rutted mountain road up to their home. The big diameter tires just ride across the divots while her little subie dives into them.

This is an argument for "29"" mtn bikes (essentially a 700C wheel with a wider rim). The downside is that shorter riders can't get a good geometry and crotch clearance, but the bigger overall wheel diameter makes for a smoother/easier ride through the ruts.

And Wayne, you'd be suprised at the amount of braking and (especially) accelerating that goes on in certain types of bicycle racing.

Even though the rim/barrel is lighter, it is still further from the center (by 0.5"). So it is possible that the inertia could be greater on the 17" wheel, even if it is a lighter weight wheel/tire combo.

In general though, I've found that 17" wheels will be heavier than their 16" counterparts (Fikses vs Fuchs). I know I gained weight when I went to Fikses. My primary concern was getting more tire than a 9" rim could safely support.

I am sure there are many reasons why this could be a bad idea, so kids, DON'T TRY THIS AT HOME!:D

The 911 Fuchs have five petals with holes between them:

http://forums.pelicanparts.com/uploa...1085509630.jpg

The 914 Fuchs have four petals with holes between AND through them:


http://forums.pelicanparts.com/uploa...1085509711.jpg

Why not a 911 Fuchs with holes through the petals also?

http://forums.pelicanparts.com/uploa...1085509761.jpg

or

http://forums.pelicanparts.com/uploa...1085509779.jpg

:eek:

Reduce unsprung weight AND rotating mass.
Didn't Porsche drill lightening holes in some early wheels for racing?

Wasn't one of the reasons for hollow spoke technology to increase air volume in the wheel/tire unit for a less stiff ride due to additional compressable mass?

Air does have mass, a wheel/tire unit weighs more than an unmounted wheel and tire.

:confused:

disclaimer- I am not an engineer OR a writer. :D

"Air does have mass, a wheel/tire unit weighs more than an unmounted wheel and tire."

- That's a joke, right?

No joking, ever compare a full gas cylinder to an empty ?

Now I don't have a clue off the top of my head what the mass of the compressed air is at say 36lbs...


hmmmm, we must further our research to take this thread any farther....

(couldn't stop myself!)