RKD in OKC

Wait a minute...Accelerating with a heavier rotating mass, ie heavier wheels takes more time, BUT slowing that heavier inertial mass in braking does not.??? Yet my experiences autocrossing, which is different than track driving, or street driving for that matter, are suspect. Hmmmm no wonder.

I will just go back to my observations with regards to the video, that my experiences going from 18 inch wheels and tires to 22inch wheels and tires on my Cayenne did not result in the lower performance numbers the video claims.

mark kibort

where did you get that? sure, braking and accelerating both work under the same laws. the weight on the wheels is if that weight was in your car, its that simple. (by the way, the heavier wheels under braking might be an advantage in autox, because it would be better at fighting lockup) if you see another result, look in other places. Again, max the weight would effect the car is 2x as if it was sitting in the car, period.
basic physics 101.... well, maybe 201.
your test are certainly suspect! 3 seconds a lap.....really? you think its due to this, and yet you had a test where you had 300lbs of passenger and your times didn't change? you think 8lbs of wheels x 4 = 32lbs x 2 = 64 lbs has an effect of more than 300lbs in the car. (5x)??
you can believe in what you want, but the science is the science.

you have to look at your test conditions. different day, different track conditions, car might be different, track might be different...... scientific testing means you limit or know all the variables and have some statistical significance. 3 runs one day vs 3 runs another day, doesn't amount to anything statistical, and that makes the results more anecdotal.

even tossing out your 300lb friend and seeing the same lap times, is not statistically significant...... what changed? you would need some data aqu system to see really what happened there. trust me, something happened when your friend got out of the car. you were 300lbs lighter, yet no faster? better look at your driving inputs during the run.

by your post, it seems you are trying to separate the rolling mass with the static mass... when rolling its all tied together... the effects of the rolling is very easy to compute, but make no mistake, its all one big inertia connected to the ground via gravity the tires rolling friction.
Think of it this way: if you have a spinning wheel, once it is at a speed you want, if there was no internal bearing friction or aero skin drag, the wheel would never stop.... its weight INDEPENDANT. if you had a box sliding on ice and you reached a desired speed..... and you could remove the aero drag, and the friction, it would continue on its way forever and never slow or stop. if the goal here is maximizing MPG, its not the weight of the wheels. if there is no change in velocity, there is no effect of added weight in the wheels vs in the car. If acceleration or braking is involved, then yes, the effect is about 2x of the weight as if was in the car.

its not true. the percentage change will be constant for the efficiency loss and that's due to the wider tires. The test saw 2mpg for a 14lb wheel change, comparing 15" wheels to 19s!!! AND the tires were 195 vs 235. the 235 tire is mUCH wider than the 195 as you know. the weight is NOT the issue here. its the rolling friction.... again, do you understand that the weight is still the weight, but rolling. when a car is rolling, all the inertia of the car and the tires are added together. its one big inertia.... the effect of the tires and wheels is a little higher than if it was sitting in the car, but once you have that figure ( car 1 3000lbs car 2 3040lbs) those two cars will not get any significant different MPG. we are talking about 20lbs of additional wheel weight , spinning , so X by 2 = 40lbs. does your car get 6 less MPG because you have some groceries in the back?????
the wider tire might get 1mpg less, and if you are getting 45mph, that might be 2mpg less as a %. 6 is not even remotely possible. you might want to conduct your test with a more controlled environment. the list of things that potentially could have changed, is very long.
In a somewhat controlled test by Car and Driver magazine.... they showed (I posted the results) less than 2mpg for 14lb wheel change! that was not on a 928, but a GTi.... probably a 35mph type car. who's test was more controlled?

Debrue

forget wheel weight tire size. in an suv gas mpg is all about foot weight

RKD in OKC

Please Read Posts, I talked about 2 separate cars. One added a passenger, the other heavier rims and removed weight. I will also remind you I am talking about AUTCROSSING NOT THE TRACK. More turns, ie more acceleration and braking events and slower speeds. Small changes make a bigger difference in lap times.

Getting very tired my experiences and observations being criticized here. Every time I mention an experience I get lambasted by you Mr Kibort. Told that I don't know what I am talking about, it must have been something else, or it is not possible because of physics 201. My experiences and observations were good enough that my mechanic and myself took a back of the pack 944 Turbo to be the car to beat by 3 seconds. AGAIN this was AUTOCROSS NOT TRACK. We changed ONE THING AT A TIME and fully explored what it did to handling and lap times before going to the next ONE thing.

I'm out.

marks gt

Really just messing with you. The data I have collected has been over 550,000 miles. But going from a 215-55-16 to a 225-40-18 equals about 4-6 mpg less on my tdi, that's all I said. I just wanted to see how scientific you would get to prove me wrong.

mark kibort

I don't have to go very far, do I.

how about this, why don't you tell me how that is possible? again, if its about a wider tire, than that's different, but adding that much weight to the car or wheel, will not have anywhere near that effect of gas mileage efficiency.

How can you not expect me to question your FTD or your "experience" when it comes in direct contradiction to your own posts and to common knowledge. you drop 300lbs, and run the Fast lap of the day, and then you say you save 5lbs on tire and wheel weight and you save 3 seconds. im sure the times are the times.... no doubt.. by the way, you wouldn't be the first guy in the world to change nothing and run a faster time.
you keep on mentioning autox track.... man, I get it, loud and clear it doesn't matter. I think you need to read the posts..... I agreed about the street car and questioned the results of the autoX ... I was very clear in my posts to separate my responses.
you will get lambasted when you make very outrageous claims here . you said, "small changes can make big lap time changes" well, then what does a BIG change like 300lbs make on the car.. isn't that a BIG change?? that's more than 10x the change in the wheel and tire weight change, and yet you saw no change in lap time, but still had " the fastest lap of the day".

this is about physics 101 and maybe 201 here. bottomline is this. you add weight on the tire and wheel and its like the same as if you put 2x that in the car, period! you get a wider tire, and that will have more rolling friction that can effect gas mileage..... how much, its not exactly known, but it seems like, from the credible tests, that its about 2mpg for a 15 to a 19" wheel diameter change. any more than that would be suspect by any sane person. 6mpg for a 16 to 18" wheel change is pretty unbelievalble and doesn't make a lot of sense. again, even car and driver only saw 1mpg change for that on a golf. I know all of us have gone from 16s to 17 or 18s and ive seen no difference in gas mileage, tracking over 140,000 of 928. driving. still see about 20mpg if I only go 70mph .... but that's hard to do in the 928 ... did the same test with the BMW 328i.. 16s to 17s still get the same 27mpg hyway and 20mpg mixed driving.

Dave928S

You've got way too much time on your hands Mark

Captain_Slow

I've wondered for years about the effect of increasing rotational inertia due to larger diameter wheels, with very open designs, concentrating more of the mass to the outside of the wheel (this would include the rim cylinder and the very low profile tire combined). I did some experiments with an old and much more massive stamped steel wheel/tire combo and an unmounted tire. I rolled them down a hill. The more massive solid wheel beat the less massive tire over and over. Total mass in this case wasn't the most important factor. Rotational inertia is very important.

So why does the more massive stamped steel wheel with tire BEAT the lighter hollow tire (aka: fancy 19 inch wheel with low profile tire concentrating more of the mass at the outside?)

It's like the ice skater spinning with arms and one leg out, vs. spinning with arms and legs pulled in tight. For rotating masses the total mass is not as important as the DISTRIBUTION of the mass. Concentrating the mass farther out creates EXPONENTIALLY more rotational inertia than the linear inertia due to the overall mass of the wheel. The total inertia for the wheel is the sum of linear inertia + rotational inertia.

This may explain why top fuel dragsters have big fat tires in the back and all wheels mounted on solid wheels (more evenly distributing mass...and deliberately trying to avoid concentrating the mass toward the outside of the wheel (away from the center).

Doubt this can be such a big effect? Try this experiment: Take a 2-3 foot long dowel rod and stick two same size blobs of clay on each end. Hold it (making a fist) in the middle and try to twist it (like an axle trying to turn a wheel/tire). Now move the blobs to the inside, so the blobs of clay are touching each side of your fist, and twist it again. I promise you will be amazed if you aren't already familiar with this demo from a high school physics class.

The crazy thing about the rotational inertia effect is that it is much less affected by the overall mass of the rotating object, rather it is the distribution of the mass that matters most. Low mass, large diameter, open design wheels, with low profile tire concentrating mass at the very outer edge, have much higher rotational inertia than the stamped steel and more massive wheel/tire combo.

Here are the ideal formulas: For a hollow disc/cylinder (more closely resembling a 19 inch wheel of open design with low profile mass at the outside edge): Rotational inertia is more closely approximated by I=mr^2

For a solid disc (more closely resembling a stamped steel wheel/tire combo): I=0.5(mr^2)

The rotational inertia of a solid disc is half that of an open cylinder of same mass and same radius. Since rotational inertia is also directly proportional to the radius squared, it would support the statement in the video (use smallest wheel that will cover the brakes).

Low profile tires on large open rims certainly provide better cornering, but seem to work against acceleration. My gut tells me the trend is more about cosmetics. I still have my manhole covers...but no tires on them. Would be fun to try the rolling-down-the-hill test with an 18 or 19 inch wheel/tire combo against the 16 inch manholes (near as possible to same tire tread diameter).

mark kibort

Jon, and others..... I don't know why this is so hard to grasp. as far as fuel efficiency, there is no difference between spinning mass (inertia ) or that mass in the car. force is only needed to overcome the friction of the road and the wheel bearings (and Aero drag). acceleration is different, and I gave the equivalents there.....Again, if the wheel is about 16-17" and the tire diameter about 24', if the weight is on the outer portion of the wheel, the effect is about 1.5 times that if that weight was not spinning but in the car. if its at the tire, its about 2x. so, 40lbs of wheel weight difference, would have the effect of 60lbs as if it was in the car.... . you just saved or lost 6hp. 6hp is right in line to what the professional tests showed. Not 3 seconds a lap, but 1 tenth or two 0-60. but that's 10lbs per wheel. MPG should not be effected. after all, why would it. It only takes more power to drive inertia, if there is acceleration /deceleration. weight on the wheels has nothing to do with gas mileage.... weight of the car, including the wheels determines its rolling friction. and if a car gets better mileage with lighter tires, its because its contact patch is smaller, or the rubber compound/tread design is difference, or/and it has less frontal area for less aero drag.

now, back to your experiements and claims..... they are a little confusing. If you had two wheels, both with same tires, but one was hollow spoke (lighter wheel), the lighter wheel, even though more of its mass was concentrating to the outside now, would spin easier, with less force to accelerate at the same rate. this is because its total mass and reflected inertia to the shaft spinning it would be less. If you are saying if we could get a lighter tire on the outside off-set a heavier rim, I would agree... there is a tire weight savings that could of set a heavier wheel.

as far as your "rolling down the hill "tests, I don't really understand the comparison. two identically weighted wheels with one with weight concentrating on the outside vs inside, yes the lower inertia wheel and tire combo would be better... (i.e. higher percentage of its weight toward the center = lower inertia). again, this ONLY applies to acceleration.. steady state speed (constant velocity) there is no difference with the force needed to keep constant velocity, ignoring bearing friction and aero drag.
think about it... a wheel in space If rotated, will continue to rotate until its acted upon by another force, just as if it was moving forward in space.

RKD in OKC

Mark, you are the one that needs to read. I referenced different changes to two different cars.

One car very well setup a 944 Turbo, added 300 lbs and while it affected overall top speed on the autocross lap, it did not affect matching FTD time. Because the speeds are too slow Autocrossing for Aero to help, more weight while negatively effecting acceleration and braking, positively effected cornering speed enough that on that particular course and day I matched the FTD I had made without the 300lb passenger.

The effects of the heavier wheels was on a stock 2001 Boxster S. Completely different car than the 944 Turbo. Adding the weight of the wheels, which was pretty much all in the barrels and not centers, slowed the car 3 seconds. And in an attempt to see if lightening the car would make a difference to try and get back some of the time I lost to the wheels, i removed the passenger seat and other stuff totaling 80lbs. and it only reduced my autocross lap time by 1 second. The increased 3 seconds due to heavier wheels was an average comparing my times to a Z06 that we fought back and forth for FTD before the heavier wheels. After changing to the heavier wheels I was an average of 3 seconds from his FTDs. This was comparing times over a couple of seasons of autocross events.

While the peak horsepower and total weights of both these cars was very similar, 944 Turbo 250/3000lbs and Boxster S 248/3000lbs, the 944 Turbo had a better and fuller power curve in the rpm ranges used on the typical autocross course where most of the acceleration is done towards the top of 2nd gear.

I am a VERY consistent driver. I only change or work on one thing at a time so I can tell what the difference really is. My mechanic tells me that is why I was able to make the 944 Turbo go from back of the pack to FTD to beat. I does take a lot longer to get a really fast setup that way, but it is more repeatable!

mark kibort

I know you were talking about two different cars. that was obvious. (My orig. comment was meant to say you lost 3 seconds in one car, and saw no change with 300lb change in the other)
the point is, you might be a consistent driver, but sometimes that can work against you. Ive been doing this for a long time and can safely say ive seen a lot..... studied a lot too...... the point here, and ive proved it with many students and even myself, is that if you are consistent, that might be a curse, as you might drive the same way, even though the car can drive faster. Clearly, if you lose 300lbs and run the same lap, something is wrong. increasing 300lbs does not allow you to corner better, and where did you get that idea? Let me guess, more weight puts more pressure on the road?? that's true, until you look at what happens in a turn. adding weight LOWERS your g loading capabilities. it negatively effects cornering ability. Maybe if you were driving around a circle in one direction, there could be conditions where that could be possible with positioning that weight, but not likely and certainly not likely on a road course or autox.
Now, the 3 seconds for wheel weight is not likely and I don't think anyone with any motorsports or physics knowledge would agree. sure, most of the autox is starting and stopping from slower speeds, but keep in mind, the tires are spinning slower too vs a road course. the effect is TRUELY only 1.5 x as if the weight was in the car.. this is the absolute truth as far as the wheel Barrel weight effect on acceleration. the slight intangible is unsprung weigh for handling. that can make the wheels less controllable up and down with the shocks and springs, and have a gyroscopic effect that can increase body roll on quick turn ins. however, with a well tuned car, that can be fixed unless the track is very bumpy , which most autoX's are not.

you may have changed things one at a time, thought you were controlling your test environment, but in actuality, if you saw 3 seconds a lap on a 60second lap, and you think its due to only wheel weight, im sorry , there is no real logical reason why that would hold true. you lost 80lbs of car weight and got one second back on that day. im positive, if you put those other wheels back on you wouldnt notice more than 1 second... But, you didn't do that test, so most of the test data, just doesn't have enough statistical significance, would you agree? The effect of losing 8lbs per wheel or 32lbs total rotating weight on the wheel barrel is the same as losing or gaining 32 x1.5 as if it was in the car, or...............50lbs.... your 80lbs of weight loss would cover that easily and if the wheel weight was the culprit, your 80lbs would have found the time back vs the lighter wheels in the boxster.
again, the physics is clear. 1.5x the weight in the car is what the extra wheel weight would be equivalent to. This is fact.. now, the unsprung effect on handling is something else, but I don't think it could effect times by 6% as it did you. Something else changed and the list could be long.

as a note, for years I bounced back and forth between Toyos and hoosiers. 3 lb heavier tires during qual/practice and the race. that's tire weight, so its effect is 2x vs the weight sitting in the car. I never felt any difference handlingwise, (body roll) other than the hoosier was stickier. for years, I was always 1 second faster a lap with the hoosier which was lighter. most of that was due to the tire compound, not the weight.

Bill51sdr

This may not be a significant difference, but according to a tire/wheel chart, your larger rim/tire combo is actually 5mm smaller in diameter, causing a 0.5 MPH error (indicated speed is higher than actual) compared to the stock wheel/tire combo. Like I said, not sure if that is enough to be significant but it could be adding up over a tankful.

yardpro

Here are some physics to add to the conversation.

Objects in motion stay in motion unless acted on by another force.
The mass of the wheels or even the weight of the vehicle really has zero affect on mpg of a moving vehicle on a level road.

Drag, rolling friction, and wheel bearing friction are the only forces that will slow a car moving on a level road.

Mpg will however be affected by accelerating a heavier car/ wheel, or driving uphill as you are fighting the 9.8m/s squared acceleration of gravity on the object in question.
Maintaining a instant speed with no elevation changes requires the same energy whether the object weighs 1 pound or one ton.... Assuming they are the a apex the same and have identical drag and friction characteristics.

RKD in OKC

With your clear physics you are throwing a lot of things in the equation away when you claim 1.5x as the definitive and only effect possible.

AND consistently setting FTD, that is Fast Lap of the Day, would show my consistency is not a negative that adversely effects my times but instead validates my observations.

A couple of quotes from the old man that taught me car setup and how important it was to be consistent, observe, and only change one thing at a time.

Do what eveyone else does and you will be competitive. Do something different and you increase your chances of being faster than the competition.

Hang a bag of poop on your front bumper then go faster and everyone will have a bag of poop hanging on their front bumper.

mark kibort

Yep, exactly what I've been saying all along.
I don't think going uphill will cost any power with the additional inertia.
If the mass of the vehicle and wheels is not changing velocity, there is no cost over the power of the weight (mass) x elevation change x time, whether its going uphill or not and whether or not its on the wheels or chassis.

your fastest lap of the day, depends on the quality of the drivers. I'm sure that varies from event to event, further diluting that as a dependable factor.

understanding physics allows you to make the claim I made. its a very simple equation. the accuracy is +/- 10% max for any situation you possibly could change with the wheel and tire combo. so, any acceleration or deceleration effect would be tossed out as equal to that weight sitting in the car, and you have already proved that 300lbs didn't help any with the other 944turbo......... so why would the effect of 48lbs added or subrated to the car, make any difference either? (the effect of 8lbs per wheel added or subtracted) . The point is, it doesn't. I do believe your ability to use the 80lb change on the boxster to gain that second back. the problem is , changing the wheels probably didn't happen that same day, and lots could have changed. its why they call it anecdotal. just because you see and live it, doesn't it mean its true. it has to have statistical significance to be a valid empirical test. What if, just what if, you drove the car a little slower. You wouldn't be the first driver that ever drove on a track on a different day that got a slower time, even with NO changes to the car. (pro or amature)

the 8 lbs added on the wheels would not hurt your time by 3 seconds a lap. its the same as if you added 48lbs in the car. ( a small young person). It's crazy to blame that on the reason the car was slower. again, the effect would be on acceleration and deceleration. BUT, there would be a more difficult to quantify effect of handling due to the gyroscopic effects of the 8lbs on a 18" wheel. But, to accel and decel that extra weight, its exactly the same if it was in the car, with a 50% pentalty. 12lbs per wheel as if it was in the car, or 48 lbs.

want the simple equations?? ive posted them to the list many times and they are indisputable.

also, why did you think that adding weight would "help" handling? the lateral g loading capabilities always goes down with added weight. If you saw the same time with 300lbs added to the car, you were driving slower but ended up with the same time. that's the consequence of driving super consistent.