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Old 01-19-2017, 06:37 AM   #1
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Tire size combo for track

On my 986 S I am doing some suspension upgrades, going to M030 ROW springs/struts with camber plates on the front. For now I will use stock S sway bars. The camber plates will get me to around 2 deg camber Front from < 1 now. Rear camber will be around 2 deg

I am looking at a couple of different tire combinations and wondering what would be the best setup. I have 2 sets of 17" wheels I will be using

Fronts
7 et 55
8 et 50

rears
8.5 et 48
9 et 52

the sets are 7 & 8.5, 8 & 9

I am currently running 225 F & 245 or 255 R
I am looking at 235 F and 245 R as a possibility or 235 F 255 R

The tires diameters are

225/45 25.0 "
235/45 25.4 "

245/45 25.7"
245/40 24.7"
255/40 25.0"

Part of my issue has do do with what class I run in and based on points if I want to change to adjustable sway bars, I can do a 235 / 245 combination. If I stay with fixed sway bars I'm fine with 235 / 255

Am I better off using a smaller or larger diameter rear tire?
I don't have PSM, just regular ABS and in any case the largest difference 25.0 / 25.7 is under 3%

Also, if the new Hankook R-S4 is a good tire choice, they wont be available in 245/45 only 245/40. Short term, I will stay with RE-71R. Later maybe the R-S4s

What is the best combination for track as well as AX?

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Last edited by JayG; 01-19-2017 at 07:19 AM.
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Old 01-20-2017, 12:47 PM   #2
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For AutoX, to be competitive, I suggest running stock 17 rims to be classed lower(stock).
If you want a little wider finding the S rims which are 9 wide rears would allow you to run 265's. The other 17s will bump you up one class and you will be less competitive.

Extra tips.

Run M030 ROW with stock front plate and stock springs, camber plates will bump you to the next class. The springs are a gray zone because they weren't official original parts, but are still available through porsche.

Don't get a seat, that bumps you to the next class. Get 6 point harness and run stock seats.

If you are not worried to compete in a higher class (STX, i think its that, basically facing against S2000s.) Use the 8.5 and 9 rims with a square tire setup. (same tire front back this allows you to run the RS4s ) Get a seat and 6 point harness, we have an incorporated roll bar which makes it easy to setup. M030 row as you planned with camber plates and maybe larger bars. I would do rear first for AutoX or Front first for track.

Have fun.
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Old 01-20-2017, 03:08 PM   #3
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Quote:
Originally Posted by WorkInProgressK View Post
For AutoX, to be competitive, I suggest running stock 17 rims to be classed lower(stock).
If you want a little wider finding the S rims which are 9 wide rears would allow you to run 265's. The other 17s will bump you up one class and you will be less competitive.

Extra tips.

Run M030 ROW with stock front plate and stock springs, camber plates will bump you to the next class. The springs are a gray zone because they weren't official original parts, but are still available through porsche.

Don't get a seat, that bumps you to the next class. Get 6 point harness and run stock seats.

If you are not worried to compete in a higher class (STX, i think its that, basically facing against S2000s.) Use the 8.5 and 9 rims with a square tire setup. (same tire front back this allows you to run the RS4s ) Get a seat and 6 point harness, we have an incorporated roll bar which makes it easy to setup. M030 row as you planned with camber plates and maybe larger bars. I would do rear first for AutoX or Front first for track.

Have fun.
Thanks for the info.

In my case, I am running PCA, not SCCA, so much of the class issues you mentioned don't apply. We have a point system for performance mods and tire width. What size wheel makes no difference

My questions pertain to the handling of the car not the class it will run in

In any case, good info. Thanks
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Old 01-20-2017, 11:54 PM   #4
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Q:

Why M030 only in the front?
Why not M030 sway bars?
What tyres do (want) you use? Semi slicks, slicks, UHP?
Is rain a topic or do you only drive in dry conditions?
Do you use the car for track AX only? Trailer to the event or drive it on street to event?
What are the track conditions?
What are typical low and max speeds for the track?

What are you trying to improve? Too much understeer?

Regards, Markus

Last edited by Smallblock454; 01-21-2017 at 12:00 AM.
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Old 01-21-2017, 08:13 AM   #5
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I have run stock sways and adjustable camber on my 987 and found it needed 0.5 or so extra negative camber in the front vs the rear for neutral handling. Based on that, I expect you will probably still have understeer with your setup. If that is the case, I suggest you choose tires widths to get more grip in the front and less in the back.

I don't have a suggestion on tire diameter, but things to consider would be impact on corner balance, rake, and effective gear ratio.
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Old 01-21-2017, 08:30 AM   #6
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i can't help you w 17" or autocross, but will provide the following considerations that i've come across:

- wider is better, until you get to the point that your car can't turn the wheels any more.
- folks will try to 'balance' their car with different tire widths, but i struggle with the concept of removing traction from one end so as to balance with the other.
- look at the difference in diameter between the front and rear; over a, what, 3% or 5% difference and they say abs will start to act up.
- changes in rear tire diameter affect torque - think of them as your final final drive ratio.
- theoretical tire sizing suffers when you find out the tires you want are not available in the size you want.
- autocross tires are very different than track tires; i ran yoko ad08s that came up to temp quickly but wouldn't last 20 mins, vs nt01s which took a long time to come to temp but had ron jeremy lasting power.
- sway bars are a BIG deal, much more than shocks or springs.
- oversteer is faster than understeer, so chose front traction over rear if you have to; oversteer is also scarier than understeer.

that's all i got.
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Old 01-21-2017, 10:25 AM   #7
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Not to jump this thread, but can I go to a heavier sway bar on my 2004 986 and still remain in S01 (stock class) for PCA auto cross? Or would I get bumped to a different class? Thanks. Apologies for jumping in.
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Old 01-21-2017, 05:21 PM   #8
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Originally Posted by Blackcloud View Post
Not to jump this thread, but can I go to a heavier sway bar on my 2004 986 and still remain in S01 (stock class) for PCA auto cross? Or would I get bumped to a different class? Thanks. Apologies for jumping in.
You need to check your region rules, but most likely if you make ANY changes that are not a stock or factory option, it will boot you out of a stock class and put you in a prepared class

I know in Zone 8 where I am, it would need to be a N. American option, not a ROW option in order to stay in stock.
Of course our zone decided to change the tire rules to something that is idiotic for stock, you can run a 200TW tire if it is a N spec, or any 201 or higher tire. Nothng lower than 200 TW even if N spec , even if it was a factory delivered tire.

We run a point based system we cal CC class and there are points assessed for things like Tire TW, Tire width, springs, sways, camber plates, pretty much anything that improves (?) the performance of the car. Your total points determines what class you are running in. With this system, you can have a variety of cars in the same class, like 914, 944, old AC 911, 986, 986, 981, 996, 997, 991 928, you get the idea, could all be in the same class depending on how that are setup
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Last edited by JayG; 01-21-2017 at 05:26 PM.
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Old 01-22-2017, 09:48 PM   #9
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Quote:
Originally Posted by Smallblock454 View Post
Q:

Why M030 only in the front?
Why not M030 sway bars?
What tyres do (want) you use? Semi slicks, slicks, UHP?
Is rain a topic or do you only drive in dry conditions?
Do you use the car for track AX only? Trailer to the event or drive it on street to event?
What are the track conditions?
What are typical low and max speeds for the track?

What are you trying to improve? Too much understeer?

Regards, Markus
Geeze Markus, that's lots of questions LOL

Front and back M030
I'm not sure what sways I want to go with, so stock S for now
I am currently running RE-71R's for track and AX, Pilot Super Sports for DD
I live in San Diego, rain is not an issue
I drive to the track and AX
Track conditions, depends on tehtrack
Depending on the track, top speeds are approx 110-135

Trying to add camber and reduce understeer
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Old 01-23-2017, 09:10 AM   #10
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JayG, I've got ROW M030 and 18's on my '02 S (you've seen me at the Q occasionally), and with max front stock camber (over 1deg) and ROW alignment, it does not understeer-indeed it borders on oversteer. It's also nervous on the highway, but I'm used to it. Good luck in CC06 (or 7), where the 911 SC hotrods rule.
Well, we've only got the Q for another year or 2 anyway...
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Old 01-23-2017, 10:22 AM   #11
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Hello JayG,

for track usage i would recommend M030 with M030 sway bars. The M030 sway bars are thicker and stiffer. A stiffer Sway bar gets you less wavering, the car is more stable and you have more traction.

If you have too much understeer you could use the stock ROW swaybar in front and the M030 sway bar in the rear.

Camber:
I think you talk of -2 deg in front an rear. Maybe OK for AutoX on lower speeds, but think too much for a real raceway and DD usage. Especially on DD usage this will kill and eat your tyres. Negative camber makes your car nervous on ruts. -2 deg will it make very nervous in daily usage on straights; especially when braking and having ruts. Also it might cause the tyres to loose grip on the track if they get too warm. So it's very important to find the right tyre.

Rims & tyres:
I personally would prefer 17" with the M30 as DD. With 18" it's too stiff for my taste as a DD. For DD i would go with 225/255 17 instead of stock 205/255 17.

I think it's important to reduce unsprung masses. So look for light rims and if you have the choice for light tyres (which are often more expensive).

Tyres:
Over here in Germany it's important that the tyres have an approval for street usage. I don't know the regulations in the US / you state.

Very important to choose the right ones. And with the right tyre gum combination you also can reduce understeer. If you use a little softer tyre with a little bit more neg camber in front and a medium tyre in rear with normal camber this will help prevent understeer. Also an older tyre (DOM) in the rear and a newer in the front can be enough to reduce understeer.

But be warned. It's a mid engine car. So understeer is also build in for safety and drivability.

Tyre idea (not shure if these tyres are available in the US, maybe named different):
225/45 R17 Federal 595 RS-R
255/40 R17 Federal 595 RS-R

Might be a little too soft for you climate, but they are cheap and they have grip. They need a little bit more steering lock angle, because of the tyre profile.

I would recommend to talk to your local tyre dealer. Best would be someone who is also in committed to racing. Or just talk to the guys you are racing with.

Think for the german market i could do better tyre recommendations, but for the US market honestly not.

Hope that helps anyway. Shure that guys from the US can give better infos.

Regards, Markus

Last edited by Smallblock454; 01-23-2017 at 10:24 AM.
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Old 01-23-2017, 10:42 AM   #12
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Originally Posted by Smallblock454 View Post
I think you talk of -2 deg in front an rear. Maybe OK for AutoX on lower speeds, but think too much for a real raceway and DD usage. Especially on DD usage this will kill and eat your tyres. Negative camber makes your car nervous on ruts. -2 deg will it make very nervous in daily usage on straights; especially when braking and having ruts. Also it might cause the tyres to loose grip on the track if they get too warm. So it's very important to find the right tyre.
i run -3 something on the front and have no issues. on street is ok (a bit twitchy, nothing you can't handle) and essential on the track (even real raceways i would suggest). the internet consensus i've read is to run the rear with 0.5 to 1 degree less negative camber than the front - how i have mine setup. do your rules allow you to notch strut towers? otherwise i don't think you will be getting the fronts to -2 with stock suspension.

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Old 01-23-2017, 11:51 AM   #13
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Hi,

camber:

problem with camber recommendations is ride height and also wheel/tyre dimensions / width and shock absorber stiffness and sway bar stiffness. And in the end hardness of the tyre edge and tyre pressure. So maybe it works on one car but not on the other. Also it depends on your driving style. A versatile racing driver adjusts his way to drive on the car and might be also fast in a car that someone else thinks has too much understeer or is undrivable. So … best way is to try different things yourself.

sway bar:

In general it's as i dsecribed. I know that the US Porsche guys tell you it's the opposite way. There are some factors like weight distribution and weight center that can affect the feeling. But just think about what the task of a spring a shock absorber and a sway bar is.

A spring is responsible for the ride height.

A shock absorber dampens shock – rebound and compression stage.

A sway bar connects the left and the right wheel. If for example (in a perfect world) the right wheel is pushed down, because you corner the car left, the sway bar pushes the left wheel up, so the chassis stays straight and both tyres have the maximum contact area.

If the sway bar is soft, the car will wank more.
If the sway bar is stiff, the car will wank less and will have more traction.

But there are a lot of factors that can affect that. Depends on how the chassis is designed, how stiff springs and dampers are …

Regards, Markus
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Old 01-23-2017, 12:17 PM   #14
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hah, i'd actually re-read and removed my bit on swaybars as i agree with you. in my experience, the stiffer the rear bar, the more the car wants to rotate.
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Old 01-23-2017, 12:29 PM   #15
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hah, i'd actually re-read and removed my bit on swaybars as i agree with you. in my experience, the stiffer the rear bar, the more the car wants to rotate.
Yes, but there are also different situations a car tend / wants to rotate with the rear or understeers.

First: you break, have no load on rear wheels - means decelerate - and than corner immediately car under- / oversteers.

Second: you pass the apex hit on the gas have full load on the rear tyres and than the car under- / oversteers.

Third: You bake in a straight line, the rear gets very light and the car tends to break out with the rear.

And these 3 behaviours are mostly caused by different settings. So - let's say it's difficult to answer things with one sentence or setup.

Regards, Markus


Last edited by Smallblock454; 01-23-2017 at 12:33 PM.
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