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This is interesting, and i hope that there will be more opinions on this than my last tyre thread
Now, previously, i've read all sorts of things about low-profile tyres: they increase the lateral acceleration capabilities of the car, they DON"T increase teh lateral acceleration capabilities of the car, they make turn in crisper and have more consistent grip, but they don't generate as much grip, they're good 'cause you can fit bigger brakes in 'em, they're heavy and bad 'cause you can fit bigger brakes in 'em, etc. etc. etc.
keep in mind that although i know all about unsprung weight hinderences, i don't know all about "what weighs more?? more rubber, or more metal??", and obviously, what sorts of advantages and disadvantages there are to having stiffer, lower sidewalls versus taller, more flexible side walls
SO, Rich, i saw you write that they hurt performance, in what ways did you mean?? all around?? why's that?? i know you're an engineer, so you're obviously a credible source, and could give reasons why.
Grimace you subscribe to the idea that they increase performance, can you elaborate?? i don't mean to pick on you, it's just that i read your post about that just a second ago...
Alright, i don't care about "bigger rims chip and bend more easily", 'cause thats obvious, all i care about is how it affects the quickness of the car.
Thanks guys!! set your ray guns on "teach"!!
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ok, i think i can cover most of this.... if not, please feel free to correct me.
ok, the main idea we have to remember here is a little something they call "rotational mass". now, rotational mass is, basically, how much weight and material your engine moves to put horsepower to the ground. this line is (in a piston engine) the pistons, rods, crank, flywheel, clutch, input shaft, transmission gearing, output shaft, transfer case, propeller shaft, rear differential, half-shafts, brake rotors, rims, tires..... and then finally to the ground. well, the easiest way to gain horsepower isn't by increasing the ammount of combustion as is common knowledge. the really easiest way to increase horsepower is to reduce weight! when it comes down to it, every 4 lbs taken off the car transfers to 1 horsepower. nice, huh?
so now we come to rims. the lighter the rim, the less mass, the faster the acceleration. anyone seen those ugly black rims on hondas? well they weigh 8.8 lbs.... well, bigger is more mass, right? so if youy put 18's on your car, you'll be turning more mass, which will be slower, etc etc etc.... so thats why the 15' rim is probably the most popular rim for a DRAG CAR. but what about like.... a road race car? now we're coming into something a little more challenging, so let me get tires out of the way.
when you see tires on a race car, they're either really BIG or really _small_.... here's how drag car tires work....
they're called wrinkle walls. the tires are designed out of some of the softest material, because the softer the contact path, the better traction it gets. wrinkle walls do just that... the walls wrinkle under torque. this is so the inside of the tires where the bead meets the rim can turn one speed, but the outside can lag behind and speed slower, which means less burnout, which in turn means more traction, and thats what prpoels the car down the track. i love going to the local strip and watching kids in their hondas trying to drag race on stock street tires. they burn out the first 2 gears, then they chirp the rest of the gear and run a high 17.... and wonder why afterwards! "oh, why can't i be as fast as brian, we have the exact same car and he runs a 15"... well, brian knows how to drive. but when it comes to putting nearly 2000 horsepower to the ground ans just as much torque if not more, then not even the best driver in the world can control it. so the driver get help from wrinkle wall tires.
so now about road race tires. road race tires are wide, thin walled, and made of two different compunds.... the contact path is very soft, allowing maximum traction. however the sidewalls are the toughest around. why are the sidewalls so thin? because when you're in a high speed turn, it will allow less wall-roll, which is when the sidewall rolls under the rim, and can even fall off the rim.... which is bad in a race.
lemme break this down. if you want to drag race, you get small, light rims and huge tires for better traction, but you wouldn't want to make any hard turns in this. for better handling, you get bigger rims, and small tires. but there are 2 ideas i haven't mentioned yet that both styles of race setups need.... wide contact path, and a long rolling distance. wider is better, so everyone is going to use the widest tires around. but what about rolling distance? this is simply the circumference of the tires. the small the circumference, the faster the acceleration. the longer the circumference, the higher the top speed.
so thinner tires dont really hurt performance.... they're just a different kind of performance. it all depends on what you want to do with the car. drag or road.... personally, im all about handling, so im absolutely ecstatic the 8 is coming with 18's.
if this post continues for a while, i can draw up some 3D animations detailing how this all works.
Not sure if this is what you're looking for, but...
To the best of my knowledge, thereís no ďrightĒ answer to many of your questions. Iíll start off with a few cold, hard facts that canít (I donít thinkÖ) be disputed. Iíll go into a bit of detail on things you say you already understand because I think the groundwork needs to be laid before we get into how things work in a particular application. This first post is basically wheels and tires 101. Iíll try to give some of my thoughts on your question in my next post.
Fact #1: Bigger tires do NOT have an inherently larger contact patch.
Many people think that bigger tires have a bigger contact patch. This is not true. The size of the contact patch is related to the pressure in the tire, the weight that is supported, and the strength of the sidewall. Letís ignore the sidewall effect for a moment. The pressure in the tire is measured in pounds per square inch (or the metric equivalent, if you live in a civilized country). What does that pounds per square inch measurement really mean? If each tire is supporting ľ of the 3000 lb. weight of the car, thatís 750 lbs. per tire. If the pressure in the tire is 30 lbs. per square inch, thatís 25 square inches of contact space. This does not change no matter what the size of the tire! If the tire is 6 inches wide, the contact patch will be a hair over 4 inches long. If you stick 12 inch wide tires on, the patch will be about 2 inches long (Weíll get more into this later). Stiff sidewalls can hold up a bit of the weight, so the contact patch will be less than is calculated here, but that is clearly a secondary effect. If that didnít make sense, try this example. Picture an uninflated balloon. If you set it flat on a table, it will have a fairly large amount of surface area touching the table, because thereís very little pressure in the balloon. Now if you blow it up so that itís really really full (lots of pressure), only a very small part of the balloon will be in contact with the table. Now, press down with your hand on the balloon. More of the balloon touches the table. You could use this balloon as a scale, if you wanted! All youíd need is the size of the part of the balloon thatís in contact with the table and the pressure in the balloon. Neat!
Fact #2: Unsprung weight is BAAAAD. Unsprung weight is the weight that is not supported by the springs of the suspension. This means everything from the tires, brakes and wheels up to the springs themselves, which are part sprung and part unsprung. Itís much more difficult to explain the physics of why this weight is bad, but I think an example works well. Imagine youíre driving in your car, and a wheel hits a bump. The job of the springs is let the wheel and tire move over the shape of the bump without jolting the car (upsetting its balance) or its occupants (upsetting their coffee). Shocks keep the springs from, well, springing. Left to themselves, the springs would just bounce and bounce and bounce. The shocks dampen this. Ok, what does this have to do with unsprung weight? The more weight the springs have to move, the slower and less effective they are. The more weight that gets moved by the springs, the harder a time the shocks have in settling the springs down. The less weight the springs have to deal with, the more effective they are at doing their job. Now, the sprung weight (everything supported by the springs) factors into this. If youíve got a very heavy car, you can have beefy springs and shocks which can deal with the heavier unsprung weight better. This is why big wheels and tires have a huge effect on a Miata. Just an extra 7-8 lbs. in each wheel can make a Miata feel like itís driving through molasses. The RX-8 is going to be about 500-700 lbs. heavier than the Miata, so unsprung weight will be less of an issue, but itís still an issue. The ratio of unsprung weight to sprung weight is really the important thing.
An interesting aside Ė Why is it that many Miata drivers use very heavy tires for autocrossing? Simple, autocross courses are usually in a flat parking lot thatís swept before the race. In other words, they arenít worried about taxing their springs and shocks with up-and-down movements. The stickiest tires are oftentimes heavy, and stick matters more in this context than weight.
Fact #3: Bigger tire circumference has the effect of changing the gearing to hurt acceleration and lowers cruising RPMs. This isnít inherently bad, it just is. Iím not going to go into the gearing thing again (he hears a collective sigh of relief from the audience), but suffice it to say that if you keep everything the same and just change the wheels and tires, youíll get more torque to the ground with a smaller rolling circumference than you will with a larger circumference. You will gain acceleration, but your engine will rev higher for any given speed. This isnít usually a factor, since you screw up your speedometer and odometer if you go with wheels and tires that have a different sized circumference.
Ok, now letís get into the opinion part. Why did I say that big wheels hurt performance, since the bigger wheels are being put on the performance package? Why do Ferraris, Porches, and Corvettes all come with huge wheels? Well, itís not actually true that big wheels are bad all the time. 13Ē wheels on the RX-8 would be bad for performance, as would 24Ē wheels. Is there a happy medium, or are there multiple tradeoffs, and therefore different happy mediums for different people? First, Iím going to assume that weíre talking about a constant rolling radius and constant width for now. In other words, the outside of the tire is the same size, itís just the metal part in the middle thatís changing. If you picture a plain rubber disc, weíre just cutting out different size holes in the middle to fit the wheel into. We can get into different sized rolling circumferences and different widths later.
What are the benefits of big wheels?
You can fit large brakes into bigger wheels. Big brakes donít make the car stop faster, but they do help reduce brake fade. If your brakes can lock the wheels, then the factor limiting your stopping distance is the tires, not the brakes. On the track, however, fade is a big concern. No one should ever reach a point of brake fade on public roads, period. Either you donít know how to drive (in the case of using the wrong gear going down the mountains) or youíre driving dangerously. So, big wheels for big brakes can be beneficial on the track, but not on the road for braking.
Big wheels mean the tires will have shorter sidewalls (lower profile). Short sidewalls make at-the-limit performance of the tires more abrupt. High profile tires generally can let you know when youíre approaching the limit, while low profile tires can go from grip to slip with little warning. Also, the effective spring rate increases with low profile tires. This will improve responsiveness but will reduce the consistency of grip on less than ideal surfaces. Note Iím not talking about gravel roads, I mean anything that causes the suspension to work. If the suspension is tuned for this particular spring rate, this problem should be minimized, but itís still a tradeoff. The ride will be noticeably degraded with a stiffer sidewall, but the increase in responsiveness on a perfectly smooth track is worth it. One other factor is that lower profile tires mean bent wheels. If you run really short and stiff sidewalls, you will bend a wheel, but if you have a large sidewall, you wonít as often.
Bigger wheels mean sales. This is the main reason big wheels get put on cars. It often happens that the engineers want smaller wheels, but consumers see big wheels and they think ďfastĒ, or ďaggressiveĒ. Why? I donít know, but it feeds on itself. People think that way, so manufacturers put bigger and bigger wheels on their sporty cars, and it gets reinforced.
What are the downsides of big wheels? Cost and weight!
It is simply impossible to get an 18Ē wheel that weighs the same as a 16Ē wheel at the same cost with the same quality. Perhaps a $500 18Ē could equal the weight and strength of a $100 16Ē wheel, but you could always jump up to a $200 16Ē wheel and still win the game. Big really costs, and thatís just the wheels. Itís not quite as big a jump in cost for big tires, but almost. If you want to keep the unsprung weight down, it costs way more the bigger you go. If you could have the same tire compound and wheel/tire weight for the same money, larger wheels would probably be better due to the improved responsiveness of the lower profile. However, you can always get a better tire compound or lower weight by going with a smaller wheel at any given price point. This is the crux of the pro-small wheel argument.
Why do Porches, Ferraris, Corvettes, etc. have big wheels? They can afford to put lightweight wheels on as stock equipment, and they also are generally heavier cars. If youíve got an extremely capable suspension and a lot of sprung weight, you can take a few lb. hit in unsprung weight to make your car look sporty. If everything is dialed in correctly, it may actually not hurt performance as much as the benefits that you can get from having the responsiveness of a low profile tire. However, that assumes a favorable unsprung/sprung weight ratio, very lightweight wheels, a driver that can deal with the abrupt at-the-limit handling, a consumer thatís willing to pay a premium for all of these factors, and an owner that is willing to pay $$$ for replacement wheels when they get bent and $$$ for new tires. Whew!
Now, everything Iíve talked about so far assumes the same rolling circumference and width. Maybe Iíll add something on changing those later. Width adds a whole new dimension (heh ), so Iíll want to put a bit of thought into that before I write anything.
One other thing, the rotational mass thing is a bit of a strawman. If you work out the math, the amount of torque required to overcome the rotational inertia of the wheels and tires is so much less than the torque used to accelerate the car that itís negligible. Think about it this way. How hard is it to get a wheel rolling up to 10 mph just by pushing it? How hard is it to get a whole car rolling to 10 mph by pushing it? Thatís oversimplifying things, but not by too much.
You really think 18" wheels are better for the track? Rim width does play a part here more than the overall size no? What about unsprung weight? An 18" wheel and tire will definitely weigh a lot more than a 17" setup. I'm still not sure if I like having 18" for track/autox.. I might just get aftermarket 16" size.
The one plus of having larger wheels is you can fit larger brake discs/calipers for better braking performance... So maybe a 17" wheel would be a good compromise.
You're absolutely right that rim width plays a part, and I did ignore it just to make some of the other thoughts more manageable. I'll think about writing some more on that later, if no one beats me to it.
Also, I'm not sure I would want 18" wheels on the track either. I made an artificial situation by specifying that I wanted to keep the rolling diameter constant. With that constraint, for driving on a very smooth track that doesn't have lots of ups and downs I'd want minimum sidewall for maximum turnin response and max spring rate.
btw, I'm no expert on this. I could be totally wrong...
If you're debating which size to run for track/autoX, you have to consider a few things:
1) tire selection: there are only so many R-compound tires on the market, so if you're looking for a particular one, then that will be a big factor. For example, Kumho V700 Victoracers are only offered in sizes up to 17".
2) tire size: in general, you want a wider tire, but you still need to consider the overall diameter of the tire. If it's larger than stock, you'll have taller gearing and slower acceleration; smaller than stock will give you shorter gearing and quicker acceleration. For autoX, it's more important to have shorter gearing. Again, this also falls back on tire selection, since they only make so many sizes for each tire.
3) cost: Smaller wheels and tires are cheaper. Race tires and lightweight wheels aren't cheap to begin with! Over the course of a season, multiple sets of R-tires starts to add up to $$$.
In general, the only real reason to go to a larger tire/wheel is if you need the added clearance for bigger brakes. In some cases, you may have a better choice of tires, so that may be a reason to go to a larger size as well.