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post #16 of 45 (permalink) Old 04-23-2011, 12:46 PM
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Originally Posted by gmtech16450yz View Post
I'm running 275/45/18's on the back of my Sky right now (mo powwa, mo rubba). They did have a few rubbing issues, but nothing that's not fixable. Absolutely the best traction I've had so far, they are insanely better than the stock crap rubber and size. Last time I ran the car hard, the front wheels were so hot you couldn't touch them for at least an hour after it was parked. I know what works and what doesn't.
My emphasis. I assume you first tried these tires in stock size, otherwise you would have no evidence to support that statement. Or were you making an assumption that the improvement you noted was partly due to the increase in size and not entirely due to the new tire design and compound.

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post #17 of 45 (permalink) Old 04-23-2011, 12:54 PM
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My emphasis. I assume you first tried these tires in stock size, otherwise you would have no evidence to support that statement. Or were you making an assumption that the improvement you noted was partly due to the increase in size and not entirely due to the new tire design and compound.
I have just a wee bit of experience here.
I work on an average of 10 cars a day, almost all driveability, engine, computer or electrical problems, but I diagnose every single part of the car for the guys that can change the parts but can't diagnose. Multiply that 10 by 5 days a week, then by 52 weeks a year, then by 30 years. That's about 78,000 cars I've driven. I work(ed) in a Pontiac dealer, I drove the first Solstice we got in. I got it sideways actually trying to see what it would do. I have no idea how many Solstices and Sky's I've driven since that first one, but let me tell you, it's a buttload. And when I drive a car, I DRIVE IT. (143 on my way to work the other day.)(I live in Mexico.)
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post #18 of 45 (permalink) Old 04-23-2011, 01:22 PM
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I'm well aware of who you are. I shall assume that since you didn't answer my question, you also didn't drive the new tires you bought in a stock size before making your statement about the benefit being from both size and tire construction/compound.

Thanks.

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post #19 of 45 (permalink) Old 04-23-2011, 01:30 PM
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I'm well aware of who you are. I shall assume that since you didn't answer my question, you also didn't drive the new tires you bought in a stock size before making your statement about the benefit being from both size and tire construction/compound.

Thanks.
I have two Sky's. One has stock size tires front and back. The other has stock fronts and 275/45/18's in back. Identical brands and models (Conti Extreme Contacts) Both were brand new tires at the same time. Both cars have almost identical mileage, 30K. Both cars have about 5k miles on the tires at this point. Both cars have been driven by me. The one car has at least 50-75hp more power. There is no comparison in traction between the two. Bigger, taller, wider tires (to a point obviously) equal better traction. And we're talking acceleration here obviously.

Sorry, I should have made it clearer how I came up with my conclusions I guess. I know you're a respected member Bill and didn't mean anything bad in my reply. I'm just used to dealing with people in person that are aware that I know what I'm talking about and don't just say things out my #ss or make assumptions. I have to remember that guys on the internet, especially because of the screen name I chose, assume I'm an idiot.

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post #20 of 45 (permalink) Old 04-24-2011, 11:52 AM Thread Starter
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Do you happen to have a quick explanation or a link to something that explains this? My brain is refusing to grasp this concept (granted it is late) but I don't want to derail this thread with a physics discussion
Quick physics lesson. First, recall Newton's second law, F=ma. Basically, this says that for a given mass, applied force and acceleration (or in this case, deceleration) are proportional. i.e. as you increase the force acting on the mass, you increase the magnitude of the acceleration. For this case, let's assume that mass changes (which translate to changes in inertia) are negligible. Here, the "mass" is the car, "acceleration" is the rate at which your car is slowing down, and the "force" is the force at the tire contact patch. When talking about braking, there are two radii that you need to worry about: your rolling radius (more specifically, dynamic rolling radius) and your effective radius. The effective radius is the radius from the hub center at which you can consider all of the braking force to be acting. Every company calculates this a little differently, but generally it is within a couple mm of the caliper piston center. This force generates your braking torque. The equation for torque is T=Fr, where T is your braking torque, F is the braking force at the pads, and r is either the effective radius or the rolling radius. Since the torque stays constant, when you move from the effective radius to the rolling radius (radius gets larger), the force pushing back on the car gets smaller. If you increase the rolling radius, you will decrease the force acting on the car even more.

I hope that makes sense. I tried to make that as simple as I could, but I typed it up pretty quickly

To wspohn, yes, more contact patch will give you more grip... that's why you see guys with race cars dial in a bunch of negative camber. The stock Eagle F1s are garbage. The Potenza RE-01Rs that I put on there made a HUGE difference. Sadly, those tires are about at the end of their life. Since I'll be in the market for new tires soon, and no one makes a wider tire that has the same rolling radius, I figured I'd see what sort of experience guys on here have had.
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post #21 of 45 (permalink) Old 04-24-2011, 12:14 PM
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To wspohn, yes, more contact patch will give you more grip...
Of course, but wider tires don't always = bigger contact patch. It depends on how much the wider tire crowns - go too wide and you end up with less contact patch and a tire that will wear out fairly quickly. No idea where the limit is on the stock Solstice rims, but presumably somewhere north of the 275x45 that gmtech said work well.

BTW, in certain limited circumstances, a wider tire isn't always better. I've raced in classes that limit engine modification and any slight benefit you gained in cornering by going to a wider tire was more than offset by having to push that wider tire down the straight with limited power. I watched the other guys sticking on wider and wider rubber and going slightly slower and scratching their heads (they also got to the point where crowning became an issue for handling as well).

On an 18x8" rim, the normal maximum recommended tire is a 255x45, but one can usually push that a bit before crowning kicks in.

Sadly, all of my vintage high performance stuff has 15" and it is very hard to find any high speed rated tires in correct sizes for those any more!

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post #22 of 45 (permalink) Old 04-24-2011, 01:53 PM
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Totally agree with both of you guys.

Bill, I should have added that going bigger does definitely have downsides. One of our members PM'd me about putting bigger tires on a 2.4L and told him if he did it would be for looks only, and would have drawbacks. And let me also say, there is absolutely nothing wrong with someone choosing a big, or small even, tire strictly on looks alone. We all have our own ideas on what looks good, and like they say "beauty is in the eye of the beholder". I personally think the 275's on the back of my Sky make the car look BAD ASS! Especially now since I've lowered it since the pics that I put online here. (There's another tire size thread on here or the other forums somewhere that has pics of my car in it.)

I definitely lost some time on my 60-100 runs when I went to 275's in back. For me the trade off was worth it though because I don't like spinning the tires every time I try to leave a stop hard. I also didn't like having the back end try to pass up the front when the stock tires let loose and couldn't be brought back no matter what. I have to say the stock F1's were some of the worst tires I've ever driven on. Same with the Goodyear LS's that came on my '08 GMC Sierra, I couldn't wait to get rid of those things. I finally pulled them off and gave them to my cousin when I hadn't even gone a third of the way through their life. Put on some Yokohama Parada Spec X's and holy cr#p what a difference!

The tire width or contact patch discussion is an interesting one, there's so much more to it than we've even touched on. My dad was a high level managing engineer for one of the top 3 commercial airlines in the country, he was in charge of all ground equipment. One of the tests he was involved with was determining what tires and what runway, taxiway surfacing technique worked to provide the highest pulling power for the tug tractors that pull the airplanes around. They basically put a huge fish scale between two of the biggest tractors they had and tested how high the scale would go until the tires started to spin. (Tens of thousands of lbs here.) They then tried many different tire designs, and also different surfaces. One of the things they found that I thought was very interesting was if the tires had grooves running WITH the tread, and they had grooved concrete WITH the direction of the tire grooves, the tires tended to "lock" into the concrete and provided better pulling power than it they were "against" the grain of the grooves. (I hope the way I said that made sense)

Another thing they learned, that obvious physics would tell you anyway, is that obviously if you don't have the weight to support the larger size, you loose traction. In other words, it's all about lbs/square inch. The best analogy I've ever came up with is put a piece of paper flat in the palm of your hand, push on it with say, 1 lbs force. You can still slide it easily right? Now put a pencil point down in the palm of your hand. Push on it with 1 lbs force and I don't think you're gonna slide it too far. 1 lbs over 5sq.in., easy to move. 1 lbs over .005sq.in., not so easy to move.

Interesting stuff. Bottom line is, the right size, matched with the right compound, matched to the right vehicle weight, matched to the amount of power available, bingo! You got traction! For MY PARTICULAR CAR, that's 275/45/18 Continental Extreme Contact DW's. They hands down work better than the many others I've personally tried and have driven on. (BTW, the 275/40/18 Yokohama Spec S's I had on my car looked and fit awesome, and wear was PERFECTLY even across the whole tread, but I did not like the "drifting" traction of them. Like the stock tires, they had a tendency to let loose and I couldn't recover the drift at all, would end up spinning out. The Conti's are totally controllable in this situation. I believe it's because the Yoke's tread was a little too flat and the compound was a little too hard. The Conti's tread is a little more curved, even on a properly sized rim, but it actually helps in cornering quite a bit to not have that "squared off" edge.) And yes, I've had 3 different sets of tires on my car in the 10k miles I've owned it!

Good discussion guys, I like discussing WAY more than arguing! Not that we were here, it just ends up like that a lot of the time controversial theories or ideas are brought up like this. Tire discussions and questions are so common, and I know my findings and ideas aren't always in line with common internet knowledge. I know it's hard for people to believe some little guy like me over what they read on Tire Rack.com, but sometimes the "little" guys know a little bit! And like I mentioned previously, places like Tire Rack can't say things like "Sure, a 275 will fit fine on an 8" rim", they'd be leaving themselves up for a big liability issue. Doesn't mean it won't work. I can't count the times I've had tire shop "experts" tell me something will ABSOLUTELY NOT WORK. I always enjoy driving the car back to them after I've mounted up what they said won't work and see their reaction. It's always the same, "Damn! That looks hella good! And they fit perfectly! I would have never tried that! Want a job? Lol.
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post #23 of 45 (permalink) Old 04-24-2011, 02:40 PM
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Always curious why people want wider tires. A theoretical performance advantage, or a style statement? Just curious.
A little of column A, a little of column B... :P
They definitely give a more aggressive looking stance from the rear, but the real advantage is in the performance. Wider tire = more rubber on the road = more traction = you can punch it a lot harder without spinning out. My friend had a 2000 Mustang GT and put FAT tires on the rear (don't know what size they actually were buy they were MUCH wider than stock) and went from spinning out a lot to putting you hard into your seat without so much as a chirp from the tires. Its why drag racers have very wide tires.

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post #24 of 45 (permalink) Old 04-25-2011, 08:52 AM
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did I say that?

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Originally Posted by gmtech16450yz View Post
I really did not read all of this but I did notice the post right above me here... (Oh, not you Bill, you posted while I was typing!) And Bill, ABSOLUTELY there is a need for bigger tires other than style or looks. My car on stock tires would be dangerous. The GTO's are a perfect example. Those cars are super limited on rear tire sizes, they'd be happy to have tires as big as our stock sizes. What happens is they end up getting huge HP out of the motors, but all they do is spin the tires. The only way to make a fast GTO is to roll the fenders or tub it.

The theory of centrifugal force I think is a minor factor when you're talking about braking force. I could be wrong, and I'm not going to research it for an hour to find out for sure, but this is how I see it...

The analogy of the cars traveling at the same speed is flawed when you're talking about centrifugal force on braking. I'm not saying the results are wrong, I'm saying it's the wrong way to look at it. A valid comparison would be to take the vehicle weight out of the equation. Spin up a heavy wheel/tire combination on a fixture, or with the car on a rack, and measure how fast the brakes can stop the wheel from a certain speed. Then spin up a light wheel to the same speed and measure. Obviously you're going to stop the lighter wheel faster, I'm not arguing that. But without trying to slow down 4000lbs along with the wheel and tire weight, the difference between the two combinations is going to be minor. I've stopped plenty cars while running them on the rack, and believe me, you can stop all that centrifugal force pretty damn fast. WAY faster than stopping the actual moving car. I'm gonna really guess here but I'd say 90% of the braking force is used to slow down the actual vehicle weight and 10% is used to slow down the actual wheel/tire/hub. I'm judging this by how much faster you can stop the wheels in the air vs. on the ground. Running the car up to 100mph on the rack and you can stop it in a second or so, run it that fast on the road and it will take maybe 10 times that long.

As far as all this tire size crap goes... Search the forums for my posts on what tires work on the stock rims. TIRE RACK IS NOT THE END ALL AUTHORITY ON WHAT TIRES FIT WHAT RIMS. For several reasons, most of all, liability.
275/40/18's WILL fit on the stock rims, WILL NOT rub and WILL wear perfectly evenly. I'm running 275/45/18's on the back of my Sky right now (mo powwa, mo rubba). They did have a few rubbing issues, but nothing that's not fixable. Absolutely the best traction I've had so far, they are insanely better than the stock crap rubber and size. Last time I ran the car hard, the front wheels were so hot you couldn't touch them for at least an hour after it was parked. I know what works and what doesn't.
I didn't say that the overall time it takes to stop the car would be affected did I? I was thinking more along the lines of you are changing the dynamic forces where the wheels/hub/caliper all interact - and that if you deviate too far from what the stock setup was designed for - then you could potentially have an issue.

Also - what I wrote is based on various things I have read and heard over the years - not being a mechanical engineer myself - or crunching the numbers personal - it is entirely possible that I got some bad info - or have integrated it in a way that is not complete or entirely applicable to the real world.

Could very well be that any change in the weight of a wheel and the diameter - multiplied by the centrifugal force is tiny compared to the overall factors of vehicle weight etc. and that a stock setup can handle a deviation an order of magnitude greater than anything you can easily achieve with commercially available parts.

Sometimes even a small change can have a large impact - I had 1 1967 Pontiac LeMans back in college - and it would snap the alternator bolt every 500 miles or so - I finally got a close look at a few other '67 Pontiacs and discovered that I had a bracket on the alternator for a car with air-conditioning - but I did not have air-conditioning - so I found a bracket for a no A/C car and got a new belt - made no other changes - and about 2000 miles later when I sold the car I had not snapped another bolt - and when that guy told me he sold it a few years later - it still had not snapped the bolt.

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post #25 of 45 (permalink) Old 04-25-2011, 03:01 PM
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Kinda funny we ended up focusing on brakes so much and what effect different tire sizes would have. We can all agree there would be some effects. How much? Who knows. I don't. I'm not sure I really care either.

The funny part I was just thinking about is how bad the stock brake pads themselves are and the fact that simply changing to better pads would make 10 times the difference in braking that any tire/wheel combo would. Sometimes we all get so focused on the little factors and forget there's way bigger one's out there. Every time I drive my son's Sky I notice how crappy the brakes are, and he's the one that's got stock sized tires. He's already got lines to go on it, he's saving money for better pads next.

MCW Sky, sorry about any misunderstanding, I wasn't intending to pick on you, or anyone else.
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post #26 of 45 (permalink) Old 04-25-2011, 05:06 PM
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The funny part I was just thinking about is how bad the stock brake pads themselves are and the fact that simply changing to better pads would make 10 times the difference in braking that any tire/wheel combo would. Sometimes we all get so focused on the little factors and forget there's way bigger one's out there. Every time I drive my son's Sky I notice how crappy the brakes are, and he's the one that's got stock sized tires. He's already got lines to go on it, he's saving money for better pads next.
But the crappy pads they used (I understand that they specced a better pad for the Opels in Europe) give the guys that want big brakes the 'in' to argue that they are doing it for safety....

I have to get around to changing out at least the front pads one of these days. Probably a Porterfield R4S, but I'll be sure to search here to see what the competition drivers have found to work.

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post #27 of 45 (permalink) Old 04-26-2011, 12:49 PM Thread Starter
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I've got a set of Carbotech Bobcats ready to go on as soon as my rotors arrive. I'll let you know how they feel. They did use a different friction material for the European models, but you have to consider that Eurpoean customers are much more tolerant of dust and noise than us North American folks.
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post #28 of 45 (permalink) Old 04-26-2011, 01:47 PM
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not a problem

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Kinda funny we ended up focusing on brakes so much and what effect different tire sizes would have. We can all agree there would be some effects. How much? Who knows. I don't. I'm not sure I really care either.

The funny part I was just thinking about is how bad the stock brake pads themselves are and the fact that simply changing to better pads would make 10 times the difference in braking that any tire/wheel combo would. Sometimes we all get so focused on the little factors and forget there's way bigger one's out there. Every time I drive my son's Sky I notice how crappy the brakes are, and he's the one that's got stock sized tires. He's already got lines to go on it, he's saving money for better pads next.

MCW Sky, sorry about any misunderstanding, I wasn't intending to pick on you, or anyone else.
I didn't mean to derail the thread either - just trying to figure out what possible problems - aside from clearance - the 1010tires site might be referring to when they claim that problems can result from this - and yes I agree that changing pads, or rotors and pads - or calipers or even using those braided hoses and higher temp fluid could all be far bigger factors on braking than changes in wheel/tire combo.

In fact I would think that offset would be one of the major concerns - if you move the center of mass of the wheel/tire combo out an inch say from stock - then the forces acting on the hub and axel are likely to be far more serious than effects on braking.

I may have to upgrade my brakes when I get the GMPP kit installed. gonna stick with the stock wheels/tires for awhile.

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post #29 of 45 (permalink) Old 04-26-2011, 05:08 PM
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Since the torque stays constant, when you move from the effective radius to the rolling radius (radius gets larger), the force pushing back on the car gets smaller. If you increase the rolling radius, you will decrease the force acting on the car even more.
Well, for our example, shouldn't the effective radius also be constant? Wouldn't the only way to change that be to move the brake mounting points in or out?

In that case, r shouldn't be either Er or Rr because it wouldn't change anything if it was Er (you said Er = hub center to pad center, and that doesnt change with a bigger tire).

So, I am thinking it should it be something like: BF = (BT*ER)/DRR

Or maybe: BF = BT(ER-DRR)

(Braking Force, Braking Torqe, Effective Radius, Dynamic Rolling Radius)

We want overall breaking force over by itself since that's the important result of all of this discussion.

From what you've described, increasing ER = good so it should multiply or add to the Breaking Torque to enhance Breaking Force (maybe there is some validity to the big break kits, since bigger rotors would have a larger ER)?

On the other side, DRR increasing should reduce breaking force, so it's a reduction/divison/subtraction of the Breaking Torque.

I guess all I am really curious about is just how much of a difference does that make in comparison to changes in the rotating mass?

For example, if someone was to switch to a tire/wheel combo that is 7-9lbs lighter, but increases the DRR by a half inch... are they going to stop quicker? see no change? break slower?

--

Just for giggles, I thought I had a good analogy involving a big kid, a little kid and a see-saw (leverage), but my brain cant seem to wrap around the concepts enough to make it funny or to make it make sense.

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post #30 of 45 (permalink) Old 04-26-2011, 11:21 PM Thread Starter
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From what you've described, increasing ER = good so it should multiply or add to the Breaking Torque to enhance Breaking Force (maybe there is some validity to the big break kits, since bigger rotors would have a larger ER)?

On the other side, DRR increasing should reduce breaking force, so it's a reduction/divison/subtraction of the Breaking Torque.
What you said there is correct, though, the equation for braking force is BF=BT/DRR. As for your question about reducing mass but increasing rolling radius, you can figure it out using the following equations:

Let's assume that the wheel and tire make a thick walled cylinder (not really the case, but it'll work for what we're doing here). In this case, I=0.5*m*(r1^2 + r2^2), where I=rotational inertia, m=mass of the wheel and tire, r1=ID of wheel, and r2=OD of tire. Plug in the appropriate numbers to see how your inertia changes. A decrease in inertia will results in better braking/acceleration. Once again, this won't be a huge difference when you look at all the other mass the brakes have to slow down, but it is there.
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