Saturn Sky Forum banner

21 - 40 of 45 Posts

·
Registered
Joined
·
2,329 Posts
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!
 

·
Registered
Joined
·
569 Posts
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.
 

·
Registered
Joined
·
470 Posts
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.
 

·
Premium Member
Joined
·
1,640 Posts
did I say that?

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.
 

·
Registered
Joined
·
569 Posts
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.
 

·
Registered
Joined
·
2,329 Posts
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....:D

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.
 

·
Registered
Joined
·
184 Posts
Discussion Starter #27
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.
 

·
Premium Member
Joined
·
1,640 Posts
not a problem

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.
 

·
Registered
Joined
·
1,279 Posts
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. :rolleyes::banghead:
 

·
Registered
Joined
·
184 Posts
Discussion Starter #30
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.
 

·
Registered
Joined
·
1,279 Posts
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.
So, assuming there was a modification that could change ER... would a larger ER (moving the point where the calipers grab the rotor further out) increase BF?

Not that I plan to do any of this (except maybe better pads/rotors sometime in the future), lol... I am just curious, and it's somewhat worth thinking about when considering wheel/tire choices. :thumbs: :D
 

·
Registered
Joined
·
184 Posts
Discussion Starter #32
Yes, increasing your effective radius will increase the braking force at the road. That being said, I really don't think these cars need it, especially if you're not running race tires. You have to remember, you're tires can only handle so much force in a given direction before they start to slip. I haven't installed the new front brakes yet (still waiting on rotors), but my guess is that the new pads will offer more than enouigh braking torque with the current dimensions.

I did install a set of rear pads and rotors this weekend (European OE friction material) and was surprised how much of a difference it made in pedal feel. The difference with new fronts should be huge.
 

·
Registered
Joined
·
184 Posts
Discussion Starter #33
I've got a couple hundred miles on the new front and rear brakes now, and they feel great! There is WAY more grip, especially when they've warmed up, pedal feel is much more confident, and the initial bite is quite a bit better. So far no noise, but we'll see what happens after they've burnished for a little longer. I haven't had them long enough to evaluate how dusty they are.
 

·
Premium Member
Joined
·
1,640 Posts
more bite

Yes, increasing your effective radius will increase the braking force at the road. That being said, I really don't think these cars need it, especially if you're not running race tires. You have to remember, you're tires can only handle so much force in a given direction before they start to slip. I haven't installed the new front brakes yet (still waiting on rotors), but my guess is that the new pads will offer more than enouigh braking torque with the current dimensions.

I did install a set of rear pads and rotors this weekend (European OE friction material) and was surprised how much of a difference it made in pedal feel. The difference with new fronts should be huge.
Not that I have ever really had any issue with the brakes - but they do feel different than other cars I have driven and there have been a few times where I needed a tad extra braking force near the end of a stop and the pedal felt very hard - which in other cars has always felt like sorry, that's all I got - whereas in this car a little extra effort and despite the pedal feel - the extra braking force is there.

I wouldn't mind something with a different profile (of feel and effort to effect) whereby I got a more responsive feeling system that was more consistent across the range of effort applied. even if that means I end up with a system that has more total braking force than I will ever need for normal daily use.

On the other hand I don't have $4000 to spend right now on a full upgrade to something like a TCE Plus 1 for front and back - but someday.

I agree though - that upping your brakes to the point where you can more easily lock the wheels if the traction in not sufficient to transfer that force to the ground is not a good idea. And I don't think I have ever had the ABS system kick in on this car - and only maybe once in normal driving on any car - and maybe once or twice in a parking lot to get an idea of what would happen - also tried cranking on the handbrake in a snow covered parking lot - found out quickly that may help in a straight line - but don't try it under any other conditions - even on a front wheel drive with ABS - it got squirrely in a hurry - but fun when your passenger is not expecting it. And yes dear I meant to do that - because getting familiar with how your car behaves when you are not in an emergency situation helps you handle it better if and when you do get into an emergency situation.
 

·
Registered
Joined
·
22 Posts
Wheel/Tire Fitment Question

Hi! I was hoping to piggy back off of this thread, which I know is several years old. I just have a fitment question, sort of related to this thread.

I've got a Redline and want to get a set of TIS 536 wheels for it, 20x8.5". The recommended tires for it are 245-35-20, but I've got an opportunity to get a fantastic deal on a set of 255-55-20. I've done a size comparison and know that these should be about .4" wider, and about .3" larger in diameter than total sizes of stock rims and wheels. (The recommended 245-35-20 size on the 20" is very very close to that of the stock, so there is almost no change there).

I just want to see if anyone can tell me whether I will have any fitment issues with the larger tires, regarding rubbing or hitting of the sidewall? Or any reason that I should stay away from this larger size?

Any help is appreciated.
 

·
Super Moderator
Joined
·
11,223 Posts
Damn tallcraig, even with a 55 offset!? Was it on the inner part of the liner or the outer? I could see that offset being to far in and causing a rub on the inside of the wheel well but it sounds like it was closer to the outside since you mention your camber played a role in it.
 

·
Registered
Joined
·
26 Posts
Light rubbing on the inside rear. I have a fairly aggressive autox alignment. Going with 18x10 +45 this season. I made them fit... Sorry kloos1502 for stepping on your thread.
(I would love to talk some Kappa Autox)
 

·
Super Moderator
Joined
·
11,223 Posts
Light rubbing on the inside rear. I have a fairly aggressive autox alignment. Going with 18x10 +45 this season. I made them fit... Sorry kloos1502 for stepping on your thread.
(I would love to talk some Kappa Autox)
Where you running autocross? I'm using 245/45/18 on RE-71Rs. Beat an ACR Viper with them on. What are you running on your AutoX Setup with those tires? I have 1.5 all four corners. Ran 2.5 up front but had bad tire wear since I daily drive it too (and didn't want to constantly change back and forth) so I went to 1.5. Have 5K springs in front, 7K out back but the front feels a bit soft. Have to turn up the front BCs to full stiff and set the rears to full soft to get the car to feel neutral. Front sway is the FE2 bar and the rear is the Redline Z0K. I think if I went with 6K front springs I could set the shocks to a setting that is closer front to back.

I wouldn't mind going with some wider rubber but I know SCCA limits tire size based on stock tire size. Hence why I wondered where you were AutoXing at...
 

·
Registered
Joined
·
26 Posts
I'm with the Thunder Bay Autosport Club. We run only three class A/B/C using SCCA street category guide lines and are lumped into a class from there. A base Solstice (BS) would end up in the A class from the get go. We also fill out classing work sheets. My car started out as a base model loaded and within a year and a half it had a SC, full DDMWorks brace bundle, GXP ZOK bars, BC coilovers, VDI doors, OZ rims...all stuff for car shows.
Last season I wanted to play with it more so I autocrossed for the first time. Thinking my car was "all that and a bottle of pop" I was humbled. Bad setup, old, hard, crap tires and a TOTAL lack of ability....but it was fun! We get maybe 25 cars in smallish parking lots around the city. It's a good group of people (excluding the Subie kids LOL)
Being in the top class I can do anything I like. I have two sets of 18x10 rims with 275/35 G-Force R1 S on one and 275/35 RE-71S on the other. I'm going to try a GXP FE3 front bar, Hawk HPS front and HP+ rear.
Alignment is Front -2 camber/ 8 caster/ 0 toe Rear -1.75 camber/ -4 caster /0 toe if recall correctly.
I'm hoping the FE3 helps with the oversteer. HATE the push. Braking sooner would also help. I should have the 5k springs in the front... I do have time...
I just yanked off the VDI door hinges. Trying to decide if I want to sell them or not. Hoping to get a new alignment and the car corner balanced. Still playing with ride height. Any suggestions would be appreciated Robotech and thank you for all you do for the Kappa community !
I came in 2nd in points last year on events attended not ability. LOL
 
21 - 40 of 45 Posts
Top