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Bigger Tires

18K views 44 replies 13 participants last post by  Robotech 
#1 ·
I'll be in the market for new tires some time this season, and I would like to get a little more rubber under this car. What is the largest tire size I can fit on the stock wheel size (18 x 8, if I remember correctly) withouth rubbing? I seem to recall someone recommending 265/35R18s, but I'm a little concerned about losing rolling radius.

Has anyone run that tire size in an autocross? Do you have trouble with topping out 2nd gear on larger courses (maybe not so large courses with the shorter tire)? Does 3rd become at all useful with those tires?
 
#2 · (Edited)
5%

that is a 5% loss in diameter - according to the 1010tires.com calculator - the 265/35/18s fit wheels from 9 to 10.5" - whereas stock 245/45/18s fit from 7.5" to 9" wheels.

aslo lets you compare multiple sizes a the same time - 265/40/18s are only 1.28% smaller though still indicate 9 - 10" wheel.

255/40/18 is 2.44% smaller - fits the same wheel.

that site also has a warning that more than 3% difference can result in brake failure - doesn't explain the potential failure modes
 
#3 ·
Your brakes won't fail in any catastrophic fashion, but you may run into problems if you lose vacuum at your booster due to reduced braking force at the road with a larger diameter tire... I'm a brake engineer, so I've already thought all that through.
 
#4 ·
While we're on the subject, as I will also be in the market for new tires before NASSM, are there any wider tires we can put under these cars without rubbing or altering the wheel well? I already have my replacements picked out so more than likely I'll be getting those and sticking with the OEM size anyway, but it'd be nice to know if there's an option for a wider footprint, especially in the rear.
 
#5 ·
I know there are some with bigger tires than I, but I have 275/35/18 on 9" Rims in the back.
 
#6 · (Edited)
I have a set of Hooziers that I run autocross on. 295/30-18 A6s all 4 corners. Stock wheels. They do fit. They also lower the car about 1". Inside of the right front tire does hit the front sway bar at full left steering lock (rubs a little but not enough to really hurt if you rotate them). But they DO hold in the turns.:)
 
#7 ·
wow



interetingly those tires are larger diametr than stock - compared to the smaller than stock diameter for the tires the original post indicated.

I say wow becuase those tires are speced for 10.5 - 11.5" wheels - compared to stock tires fitting 7.5 - 9.5" wheels. While I am not saying that you cannot get a tire to mount on a wheel outside the defined range - you can end up with problems with the sidewall and load and rolling friction etc.


As for earlier posts about brakes - I suspect they are refering to changes in the centifugal forces based on changes in the mass of the wheel/tire combination - having an impact on the load under braking which could adversely affect the life of the brakes - and outside of a track environment where you might be doing a brake inspection every day (or more often) you coudl end up with a problem that could result in a failure in a time frame that you would not normally expect for a stock setup.
 

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#11 ·
Haven't gotten a flat with this car, but if it was just a nail or screw throught the tread, then the fix-a-flat and air compressor that came with the car should do the trick... for a little while. Other than that, you're going to have to call a tow truck.

Going back to the brake stuff, your brakes don't really care how much centrifigul force your wheels and tires are seeing. They do care about the inertia of the wheel/tire combo, but the change in inertia due to a large tire will be relatively small when compared to the vehicle inertia acting on that corner. As I mentioned before, the change in rolling radius (for a larger tire) will decrease the braking force at the road, meaning that you need to put more force into the pad to stop the car in the same distance you would with a stock tire. This will cause premature wear out, but we're talking a drop of probably 10 - 15%... so it's not like they will wear out in three months. Changing tire size (in either direction) can screw with your ABS/TC/ESC system, but as long as you tell the car what the new rolling radius is it shouldn't be a big problem. This, of course, assumes you are purchasing quality brake parts. Cheap pads and rotors will have a whole host of issues that will be made worse by changing tire size.

Finally, going back to the A6s, at what speed do you top out 2nd gear now? Do you find yourself in 3rd more often, or do you just let it bounce off the rev limited for longer?
 
#12 ·
kudelt07 said:
As I mentioned before, the change in rolling radius (for a larger tire) will decrease the braking force at the road.
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 :rolleyes::nono:

The things that seem to be keeping me from grasping this:

- I have seen way too many people say that the unsprung weight is a big deal to readily accept how you dismiss it as not mattering in regards to braking. I am not an engineer or an expert mechanic though, so I ask questions. :)

- Also... bigger tire (to an extent) = bigger contact patch (more on this in a second) = more friction/traction area = better braking? Unless... Are you are just saying that keeping width equal, a smaller radius tire has better braking action? (which isn't quite the same as the unsprung mass concept, even though the unsprung mass would be less in a smaller radius tire)

Ugg! :banghead: Not tracking this AT ALL. :banghead:

---

I did have a few other things to mention...

Looking at the generic sizes and what those are supposed to yield is one thing. Make sure you are also checking out tread width as well. I was also looking to see if there were contact patch gains to be found in changing up tire sizes. I noticed that an increase in size does not always yield a bigger tread width. (sadly, tire rack does not always list tread width for every tire)

Here's the first example I could find (tread width is in bold):
Continental ExtremeContact DW
275/40ZR17 25lbs 9-11" 10.2" 25.7"
285/40ZR17 27lbs 9.5-11" 10.2" 26"

So, in this case you add 2lbs per tire, raise the car .15" and have esentially the SAME amount of rubber meeting the road (maybe .1" more do to the increased circumference). :banghead:

I have seen this on several lines of tires. Knowing how the tire molding process works (thank you so much "How It's Made"), I am gonna guess that some companies reuse the same tread mold for different sizes.

It's just one more thing to check when looking for tires :) :rolleyes:

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And then...

MCW Sky said:
I say wow becuase those tires are speced for 10.5 - 11.5" wheels - compared to stock tires fitting 7.5 - 9.5" wheels. While I am not saying that you cannot get a tire to mount on a wheel outside the defined range - you can end up with problems with the sidewall and load and rolling friction etc.
Those Hoosiers are just racing slicks in disguise :thumbs::D

Hoosier A6

They're stupid soft and insanely sticky with some groves cut into them so they can pretend to be "street legal" :lol:

If you look around the threads in this section you'll see people talking about the funky tire pressures and alignments to make these beasts work. Even then the life expectancy is pretty darn low with in comparison to a normal tire.

---

Ok... off to bed...
 
#13 ·
Centrifugal force

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 :rolleyes::nono:
short answer - centrifugal force - as you increase the diameter of the wheel and or width of the wheel/tire - you are generally not only increasing mass overall but also moving that mass farther out from the center of the hub - meaning there is more momentum in the spinning wheel that needs to be overcome by the braking force applied.

Or in other words - two identical vehicles traveling at the same speed - where one had 245/45/18s and the other has 285/35/19s - unless the second has magnesium wheels or some other factor that compensates - will have more momentum that spinning mass will have - meaning more braking effort required.
 
#14 ·
I'll be in the market for new tires some time this season, and I would like to get a little more rubber under this car.
Always curious why people want wider tires. A theoretical performance advantage, or a style statement? Just curious.
 
#23 ·
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.
 
#15 · (Edited)
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.
 
#16 ·
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.
 
#18 ·
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.
 
#19 · (Edited)
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.
 
#22 ·
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.
 
#25 ·
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.
 
#26 ·
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.
 
#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.
 
#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.
 
#34 ·
more bite



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.
 
#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.
 
#35 ·
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.
 
#39 ·
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...
 
#40 ·
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
 
#41 · (Edited)
Are you running the stock BC 7ks all around? If so, drop to the 6ks up front first and put your stock swaybar on up front. Gives a LOT of bite. The RE-71Rs helped a ton too. I think with the 6ks my setup would be spot on for me.

Looking at your setup I think you made the same mistake I did. The 7k Springs with Z0K (GXP) bars and that much camber doesn't work. Soften the front spring and bar and I think you'll get enough roll up front to use all that camber. Right now I bet if you turned the camber down to 1.5 or so you'd get more front bite. With the 7k springs and Z0K bar up there, you're just not going to be rolling over enough to get the most out of those front tires. the softer springs will also let you shift more weight up front under braking and thus helping with your late braking technique. LOL

Depending on where in the turn you have push says a lot too. Curing it on corner entry can be a lot different than mid corner or corner exit.

I don't know where you're at ride height wise but I want to say the GM recommended height for their racing Solstices was 4.5" or 5"...somewhere around there measured from ground to frame rail.

One thing I learned with braking (took like a novice course offered by our San Diego SCCA groups) is all I needed was a quick, hard stab of the brakes while driving straight and that would be enough to bleed off most speed. It surprised me that you didn't need a long braking period, just a short, quick, but firm braking period.

But yea, they put me in the top class too. I run against all the sports cars. ACR Vipers, Z06 Z07 Vettes, supercharged 350Zs...but the fastest guy in the group is a supercharged FR-S who works for Hotchkis Suspension (who is the title sponsor of the event) as a Suspension engineer. LOL Go figure he's the fast guy.

Me on course:
 

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#42 · (Edited)
Thanks Robotech, My FE2 bar is at the scrap yard LOL. They are cheap and available. With 6k front/7k rear springs and a FE3 bar front, where would you set your dampening to start? Full soft front and full hard rear? The same goes for a FE2 up front. If I drove within the limits of last years setup and "curbed my enthusiasm" the push would have been less of a factor. The biggest problem with my car is the driver! I'm working to improve both.
I think your right about the camber on my setup. I've read as you go wider and stickier there is a point of depreciating returns with camber. I will try try to brake a bit earlier, a sharp stab on the brakes on a straight line. Do you want the front to recover before you start your turn? I think DDM Dave's custom street alignment had a bit more rear stance (higher) than than the book states for the racing setup.
Where do you start your tire pressures front and rear with the RE-71 R's?
I do appreciate your interest in the sport and sharing with the group what has worked for you.
Craig
 
#43 ·
I am warning everyone up front, this is going to be a LONG one.

With a FE2 or FE3 front bar and the 6K front 7K rear with a rear ZOk bar I would start the BCs at the mid point both front and rear and then tweak from there. I only say that because I have never had that setup on my car and I know with just the 5Ks up front and the FE2 bar I need to have it full soft in front and full hard in rear. So, based on that, I'd start at a mid point and speculate I'd be going a bit softer in front and a bit harder in rear but knowing it is going to come down to how the car feels with that setup first.

For braking, you need to understand what you are doing when you brake. Most people feel it is to slow down but with autocross, that's not really the point. Yes, you're going to bleed off SOME speed but the main reason for the quick stab on the brakes is for weight transfer and the reason you do it in a straight line has to do with how much traction you have available in your front tires.

Think of your available traction of your front tires as a circle and, when the car is cruising at a constant speed, the dot representing how much traction you're using is at the center of that circle. We call it the traction circle. As you accelerate, this dot moves to the bottom of the circle. As you brake, it moves to the top. If you turn right, the dot moves to right side of the circle and if you turn left, it moves to the left. The maximum traction the tire can provide is defined by the outside edge of the circle. if the dot stays in the circle, you have traction...if it moves outside the circle, the tire has lost traction and is slipping instead of gripping. Slipping = push generally.

So...the other thing to note is the more weight you put on a tire, the more traction the tire can generate. The less weight on a tire, the less traction it can generate. So more weight equates a bigger traction circle and less weight means a smaller traction circle.

Now then, if you brake super hard but don't lock up the tires, your "dot" will rest on the upper edge of the circle. If you're turning really hard to the left, the dot will be on the left edge of the circle. If you do both at the same time, the dot will be completely off the circle to the top left...and you're pushing. Again, that's bad.

So remember I said we don't brake to slow down in autocross? What we're doing by braking hard is shifting the weight of the car onto the front tires. When you brake, and you feel the nose of the car drop, that's putting weight on the front tires and thus increasing their amount of available traction.

Your question was do you want to quickly stab the brakes and then let the front recover before turning and the answer, as you have probably already figured out, is NO! You do all your braking in a straight line because, when it comes time to turn, you wan to use as much of your traction circle for turning. If you're doing any braking, you are reducing how much turning you can do. So the technique should be just before you hit the spot where you want to turn into the corner, you stab your brakes quick and firm to shift that weight onto the front tires and get that traction circle really big. Then you let off your brake as you start initiating the turn in with your steering wheel.

With your foot off the brake you are no longer asking the front tires to use any of their grip to slow you down. Yet, by doing that just before starting your turn in, you have weighted the front tires thus increasing how much traction they have to make the turn.

Ride height is doing much of the same thing. You mention DDMs settings and while I have my car set to about 0.2" lower in front than rear, I can't comment on what is "correct". What I can say is that when you're moving a couch, always pick up your side before the other guy does. LOL The reason being is that whichever side of an object is lower, that side is carrying a higher percentage of the object's weight. So if your front end is slightly lower than the back, your front end is carrying slightly more weight (and thus the front tires have a slightly larger traction circle) than the back. Doing this reduces the car's tendency to squat (and push) under acceleration.

For my RE-71Rs, they like about 33 psi of pressure. If you're not using RE-71Rs, that info won't help you as each tire will have a different "sweet spot" for pressure.

When it comes to camber, remember the goal is to use as much of the tire tread as possible when cornering and have even usage. to check your camber, go out and do a session (2-3 laps) and then immediately take a temp gun and shoot the inside portion of the tread, the middle of the tread, and the outside portion of the tread across a straight line parallel to the ground. These temperatures should be within a degree of each other. If not, the tire is telling you something is wrong.

If you get a reading like this (from inside to outside): 120 115 110, the tire has too much negative camber as the inside edge of the tire is hotter than the middle, which is hotter than the outside edge.

If you get something like: 110 115 120 then you have too much positive camber as the tire is colder on the inside edge than the middle, which again is colder than the outside.

Now, you could get something like this too: 110 115 110. This is an overinflated tire where the center is higher than the edges. Underinflated is just the opposite: 115 110 115.

Where it gets tricky is if you have an overinflated tire with too much camber. Then it can look like: 115 115 110

Or too little camber: 110 115 115

Or underinflated with too much camber: 125 115 120

Or with too little camber: 115 110 125

I'd pick up a temp gun and just see where you're at with tire temps and see if that helps give you an idea of how close you are on your camber and pressure setups.
 
#44 ·
Thanks for providing ALL that information. You made it very easy to understand. It looks like I'll be ordering more suspension parts for my car. You just can't have enough anti-sway bars LOL. To correct most of the push ( I grew up with tail happy cars ) and have it more neutral would be a big plus. When it oversteers I stop looking well ahead and become like a deer in the headlights. After last years debacle using 8 year old Hankook V12's, too stiff of a suspension and (most importantly) my lack of ability, I now have the tools to improve my times.
I thought it was going to be point and shoot but soon found out it's walking the course, looking for the correct line and in the end... having a great time with my Pontiac and fellow drivers.
Thanks again !
Craig
 
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