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Grip differences between 16" and 18" tires on the track

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Grip differences between 16" and 18" tires on the track

Old 07-24-09, 10:38 AM
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Grip differences between 16" and 18" tires on the track

I received my first taste of HSAX with the FD last weeked and am an addict. As Fritz predicted in one of our converstations, there is no turning back. I have built a check list of things I will need to do to continue enjoying this sport and have hit a question that I honestly have no experience with. Tire and rim size/weight and overall traction.

I currently have 18" rims with tires that will hold up for the rest of the season, but as I plan for winter projects and hitting the track running for the beginning of next season, tires and rim choice have me confused. replacing my tires on one of my two 18" rims for HSAX will be about 1200 dollars. I know what to expect on my current rims traction wise. They are about 8lbs heaver than the stock 16s but my only experience of the amount of grip and speed I got was phenominal.

I have the stock 16s with no tires currently on them. The cost for tires are cut in half if I go with Star Specs or Ecstas on 16s vs 18s but going from 225/40/18s in front and 265/35/18s in the rear to 240/50/16s all around I am sure will provide a much different feel.

I am not trying to be cheap (I have radiator, 2nd oil cooler, among other things to mod) but I want to make a good decision on whether spending the extra cash on keeping 18" rims for HSAX vs the stockers is a good idea performance and $ wise.

I appreciate the opinions of the group.
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Old 07-24-09, 12:22 PM
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HSAX? Is that autocross or track? I've never heard of it.

The main benefit of the bigger wheel and tire is they are generally available in wider sizes, and have a thinner, stiffer sidewall. They'll also clear bigger brakes of course.

If you're just starting out you're going to wear out your first set of tires before you get good enough for it to really matter, so I'd save my money and get the cheaper tires on the 16s and have fun learning. Dial the boost back if you're overwhelming the rears Going to equal sizes all around will make the car a little more over-steery or at least less under-steery than your current stagger. Besides just being cheaper, there is an additional financial benefit of running the same size all around as you can move the tires around if you chew them up or flat-spot them.
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Old 07-24-09, 02:19 PM
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HSAX Is high speed autocross on the racetrack. It is a little less crowded as there are typically 6 - 8 cars on the track at a time running timed single laps. Practice sessions have more cars obviously but there are rules on passing etc.

I still have the stock brakes so what you say makes a lot of sense. I am definitely learning so if I am gonna chew up tires quick the first year I should probably stay on the stockers for now.
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Old 07-24-09, 05:08 PM
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You can also learn more on street tires than on proper competition tires... Competition tires are too good and they'll hide your mistakes right up to the point that you overdrive them and then all hell breaks loose. Street tires give up much more gradually and give plenty of noisy feedback. You should also definitely look into doing HPDE days where you get an instructor in your passenger seat.

I see you're in Chicago, so the NASA great lakes is probably your home region, they do some good tracks.

http://www.nasagreatlakes.com/
more info about HPDE in general: http://www.nasaproracing.com/hpde/

Also the stock braking system itself is acceptable until you get pretty fast, but you will need better brake pads very soon. How much power are you making?
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Old 07-24-09, 05:16 PM
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Yes, NASA is one of the groups in the affiliation known as Midwest council. I joined the NSSCC this year and got a chance to get an instructor last week who helped me out with during a HSAX drivers school (similar to a HPDE day). All those years of GranTourismo has made it easy to pick out the line in real life, but now its more the fundementals and really understanding the the feel of how far and fast I can take the car. I have a lot to learn for sure.

Blackhawk Farms Raceway, GIngerman, and the AutoBahn are the 3 main course in my area with Road America up north in WI and a new club course being built in IN.

Maybe oneof these days I will get my FD a little more EAST to join the pros
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Old 07-25-09, 07:21 PM
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Street tires are much easier to drive well because they hold grip better as you go past the limit and they talk to you. As a beginner this is valuable. I've gotten quite comfortable autocrossing on r-comps but for track days I'm happy to have the sound of a street tire because I really don't want an off.

16" vs 18" does not affect grip. I'll repeat that - it does not affect grip. The majority of the grip you get comes from choosing the right tire and getting the best tire pressure and alignment. The stock FD wheel with a grippy tire can get you a heckuva lot of grip for not a lot of money. Plus keep in mind that the tires are much cheaper and more available on the stock size compared to many aftermarket rim sizes. I've done very well with stock FD rims and lightly used competition tires without spending much money.

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Old 07-26-09, 01:54 PM
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Originally Posted by dfoster154 View Post
They are about 8lbs heaver than the stock 16s but my only experience of the amount of grip and speed I got was phenominal.
8# is huge in rotating, unsprung mass. You won't notice it as a function of grip, but you likely will notice it in acceleration, braking, and suspension performance. Depending on who you ask they say 1# of rotating, unsprung weight is worth 4# or even 10# on the chassis...

Also keep in mind that if you try to keep the same OD of the wheel/tire combo you'll end up with more sidewall with the 16" wheels. And sidewall is effectively a spring...
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Old 07-26-09, 06:42 PM
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Thanks for the insight, everyone. I didn't even take into consideration the unsprung weight difference. I will be buying some 16" Star Specs for the track.
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Old 07-26-09, 06:58 PM
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Really I think the larger dia wheels are only necessary when a) you can't fit brakes under the current size or b) the stiffness of the sidewall isn't high enough for your suspension tuning.

Dave
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Old 07-26-09, 11:03 PM
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Have you looked at 245/45/16s? It's a good upgrade from the stock size and is pretty much the same OD of the stock tire plus this tire size looks great on stock wheels. No bulging from the sides or anything.
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Old 07-27-09, 10:58 AM
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Originally Posted by wan View Post
Have you looked at 245/45/16s? It's a good upgrade from the stock size and is pretty much the same OD of the stock tire plus this tire size looks great on stock wheels. No bulging from the sides or anything.
245mm = 9.65in ... how does 2.65 more inches of section width than rim width not look pinched? Stock wheels are 16x7, right?

Edit: Brain fart. FD not FC. Sorry.
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Old 07-27-09, 04:10 PM
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The stock 16s came with 225/50/16s

I am sticking with that size as Dunlop Direzza Star Specs are only $115 a tire with a $60 rebate... that is basically $100 per tire for all out track fun. Now I can focus my $$ on a simple Stage one Brake Kit until I am ready to take the next step.

Dgeesaman, you bring up an interesting point on suspension setup... I have simply gone the basic route of Tokicos with Eibachs as I get started. that should still be forgiving enough to run stock tire sizes in my estimation.

I am running 5s in front and 4 in the rears for the tokicos.

Appreciate the feedback.
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Old 07-27-09, 04:19 PM
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Gene, sorry I missed your power output question. I am making 300RWHP on a Dynodynamics at 14PSI. Mods listed below:

'94 BB Rx7 PEP w/33K on the ODO - orignal engine - stock ports
SS Downpipe
High Flow Cat (could go midpipe for more power)
Racing Beat Exhaust
M2 CAI
M2 Medium IC
PFC with Stock Twins (Steve Kan tuned @ 14PSI - ECU boost controled @~13PSI)
550cc primaries, 1200cc secondaries
Cosmo Fuel pump
Aluminum AST
R1 Strut Bar
Tokicos and Eibachs

HKS Twin Power and 9's all around about 1 week away....

I am hoping to get on the road and dyno with Banzai before summer is up to clean up the tune a little further.
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Old 07-28-09, 08:21 PM
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Why do you need to do a brake upgrade? Are you experiencing brake fade?

I would imagine if you went to a set of good track pads, flushed your fluid with some high temp stuff like Motul and made sure your rotors are up to specs you will have as much brakes as you will need for a 300RWHP car running stock width tires.
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Old 07-28-09, 09:10 PM
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Stage One Brake kits that I have seen come with Brembo Slotted Rotors and Hawk Pads for the stock setup. The hightemp fluid is a great idea!

Brakes haven't faded yet, but they are still the originals so if I don't handle it soon I know I will have brake fade soon. And I will hopefully be pushing 330ish after a retune and map cleanup.
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Old 07-28-09, 09:11 PM
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^ agree. Especially on street tires.

When your driving and power level does exceed the pad/rotor/fluid upgrades, then cooling ducting will take you yet another step up in brake performance. The big brake upgrade works but IMHO I think it's only justified if you have exhausted the other approaches or you have a few thousand dollars to toss away.
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Old 07-29-09, 08:44 AM
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The grip is only similar in 16 to 18" if you're talking about the same width. Typically, when you go 18" you're talking about a 285/30/18 tire vs. at most a 245/45/16 on a stock 16" rim.

There WILL be more grip, provided of course you get the proper heat into them.
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Old 07-29-09, 09:31 AM
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from 225/40/18s in front and 265/35/18s in the rear to 240/50/16s all around
You have a point: there will be a slight grip increase in back due to the 265's being wider than the 245's stock.

But if your fronts are still the same you won't make the turn any faster if the car is tending to understeer. For me on the track that's most of the time.

David
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