General info for an uneducated AX'r, Primarily tire pressure |
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General info for an uneducated AX'r, Primarily tire pressure |
jd66921 |
Jun 6 2009, 07:30 PM
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#1
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Member Group: Members Posts: 316 Joined: 9-January 06 From: Texas Member No.: 5,394 |
Hi,
I've been doing AX's for a little more than a year. Currently I'm driving Howard's old car "Suzi". She's a pretty much stock 914 2.0l. She's been lightened a little. She has fiberglass hood and bumpers. All her heat and ventilation has been removed. Weight is 2050 lbs The engine is a stock 2 l as far as I know, and has an unmolested MPS. The transmission is stock. I do not know what kind of shocks are on her. The rears are yellow and do not seem to have any adjustments. The rear springs seem fairly soft to me? There are sway bars front and rear, I do not know what size they are. The fronts are adjustable with a number of holes in the horizontal arms. They are set to the center. I typically run about 10-12 seconds behind Corvettes, A Birkin, and a well setup Mustang Cobra. I am typically 3-4 seconds behind set up Miata's,etc. This is on courses that I would generally say favors the high powered cars. Generally, the courses are two 180's at each end, fast stuff down the middle, and most of the courses I can take flat out or with minimal lifting., except for the real corners, of course. Wheels are 5.5 inch Fuchs, with Falken Azeni RT-615's. My big questions is: I am now running 25 psi front and rear in the tires. Is that in the right ballpark? Should I go higher/lower, and why? The car feels good most of the time, with a little bit of mostly controllable oversteer. I can get some wheel spin off the line, and a little on right hand sweepers. What will changes to the front swaybar make? I've never understood them! I would prefer an answer like "increasing the front rate will increase grip" instead of "decrease over steer". Anyway, I seem to have reached the limit of my improvements without help, so I 'd like any help I can get!! Thanks, Jeff My first question is |
grantsfo |
Jun 6 2009, 08:05 PM
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#2
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Arrrrhhhh! Group: Members Posts: 4,327 Joined: 16-March 03 Member No.: 433 Region Association: None |
I had best results with Falkens on my 914 at pressures of around 35. 25 sounds a bit low.
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jd66921 |
Jun 6 2009, 08:30 PM
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#3
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Member Group: Members Posts: 316 Joined: 9-January 06 From: Texas Member No.: 5,394 |
I used to run 35, but made one spectacular slide off course (that was published
here!). My main advisor is a non racer, but seems knowledgeable. She suggested dropping tire pressure and marking the tires. Watch until they start showing signs of rolling over on the the sidewall, and that point should be best grip. I know 40 was too much, slidy, but fun Maybe I need to go up in pressure again and see how I do. Thanks for the input! Jeff I had best results with Falkens on my 914 at pressures of around 35. 25 sounds a bit low. |
Joe Ricard |
Jun 8 2009, 03:48 PM
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#4
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CUMONIWANNARACEU Group: Members Posts: 6,811 Joined: 5-January 03 From: Gautier, MS Member No.: 92 |
secret to going fast at Autocross.
"Gas is on the right, it goes to the floor"" Brake in in the middle use it hard or not at all there are gears past 2nd. try using them. 35 PSI is really where the Azenis wil be most happy. drive it harder and lean on the tire "it will stick" |
PeeGreen 914 |
Jun 8 2009, 04:02 PM
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#5
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Just when you think you're done...wait, there is more..lol Group: Members Posts: 10,219 Joined: 21-September 06 From: Seattle, WA... actually Everett Member No.: 6,884 Region Association: Pacific Northwest |
Most of these guys know their tires very well but you should understand why the tire prssure needs to be where it is as well. Do a little reading on tire pressures and contact patch and temps.
The perfect pressure is the one where your tires are heating evenly, using the correct contact, and gaining the most grip. You can tell this by looking at the tires after a run. See where the tires are getting wear. If the scrub area is too much you need more pressure. If the tires are not going far enough use less pressure. Feel if the tires are getting heat evenly across the tire or if it is in one spot. There is a lot more to this but those few tips give you a good idea what to look for. |
grantsfo |
Jun 8 2009, 09:28 PM
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#6
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Arrrrhhhh! Group: Members Posts: 4,327 Joined: 16-March 03 Member No.: 433 Region Association: None |
I agree with Joe. Trust us guys who have driven lots of AX's on these tires. Truly 35 is going to result in better times. 25 will likely result in more predictable (with falkens ...sloppy) but slower tire. Suspect your slide off course wasnt due to tire pressure.
But really you need to chuck the Falkens and get the new RE11 from Dunlop. Far better tire. Speaking of tires what are your alignment specs? This post has been edited by grantsfo: Jun 8 2009, 09:37 PM |
jd66921 |
Jun 10 2009, 07:31 PM
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#7
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Member Group: Members Posts: 316 Joined: 9-January 06 From: Texas Member No.: 5,394 |
I really appreciate all the info!
I really don't know what I am doing. I've run maybe 15 autocrosses in two 914's. Suzi is my favorite! At least she has some power! I was running 35 psi, but I spun off course once and decided to reduce tire pressure. The tires do not roll over onto the sidewall, that is why I have been reducing tire pressure. I will try higher the next AX and see where I go. Sorry, right now I can't afford more expensive tires. I use the Falken's as street tires as well, so it isn't practical. I'm not sure I have enough wheels to dedicate some to race tires yet. Of course I need a LOT more experience to figure out what to do, that is why I am asking. It seems like I should be closer to the fast cars though, even with the HP gap! It's tough though, with them doing 80 and me doing 50 down the straights! Alignment specs, ?? Stock as best I know. No sure I can tell the difference! What should they be set to?? And why?? I'll be asking more later, thanks!! Jeff |
koozy |
Jun 23 2009, 03:39 AM
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#8
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Senior Member Group: Members Posts: 646 Joined: 21-July 07 Member No.: 7,931 Region Association: None |
I believe the RE-11 is a Bridgestone tire.
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koozy |
Jun 23 2009, 04:07 AM
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#9
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Senior Member Group: Members Posts: 646 Joined: 21-July 07 Member No.: 7,931 Region Association: None |
Here is my take on it.
Raise the pressure to 35 (or even higher) and see how it feels. The Faklen tires get slippery when hot so low pressures will contribute to a "greasy" feeling tire. I ran mine at 44 and did quite well. They seemed to like the 44. I didn't see much roll on the tires at any pressure since they have a reinforced sidewall. Have the car aligned. Best $70 you'll spend on AX prep. (if you don't do all that much street driving go with 1/16th toe out and have them add a bit of negative camber. This will help in the twisty stuff at AX. I have the car at 1/16 toe out and street driving isn't all that bad. it likes to find the grooves on the highway but it's not uncomfortable. Try to disconnect the rear bar and see how that feels. (just disconnect one side and use some wire to hold it up) Figure out if the front looses grip before the rear, or vise-versa and then you can make adjustments to combat the issue at hand. (only one thing at a time) Very important: Only change one thing at a time so you know what the result of the change is. More than one change and ya end up chasing your tail tryin to figure out what the heck happened. (ask me how I know...) Look out... this is the beginning of what some call the "slippery slope" and if ya get the bug it can be the most fun ya ever had changing stuff on the car every couple of months... ha ha ha Don't sell yourself short. After 15 autocross events you know what you are doing. If ya didn't, you wouldn't know ya needed anything at all. Have fun and keep us posted on your changes, results and progress. Remember, the closer ya get to the fast guys the farther away it looks. |
grantsfo |
Jun 23 2009, 11:44 AM
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#10
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Arrrrhhhh! Group: Members Posts: 4,327 Joined: 16-March 03 Member No.: 433 Region Association: None |
Biggest thing with alignment is that you want to get at least -1 degrees of camber. If its a street car you probably dont want to get any more aggressive. If you dont drive much on the street and you want to be competitive you will want at least -2 to -3 degrees of camber.
Negative camber helps put contact patch square with surface when you are in turns creating more traction. 914's are prone to understeering with stock alignment settings. Falkens are decent tires for the price. I like the new Bridgestones RE11 better. Worth a look if you are getting ready to change tires in the future. From what I have see so far Bridgestones grip better and also seem to wear better too. |
koozy |
Jun 23 2009, 11:56 AM
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#11
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Senior Member Group: Members Posts: 646 Joined: 21-July 07 Member No.: 7,931 Region Association: None |
Yea, those RE11s look very nice. I was watching a couple of guys with new RE11s at the Alameda AX and they were sticking real good.
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Richard Casto |
Jun 23 2009, 04:38 PM
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#12
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Blue Sky Motorsports, LLC Group: Members Posts: 1,465 Joined: 2-August 05 From: Durham, NC Member No.: 4,523 Region Association: South East States |
I have autocrossed with the Faklen on my Civic. I would agree on the tire pressures mentioned here (high 30's or close to 40 as a starting point). Tires will have optimal grip within a range. Go higher or lower and you will loose grip. Lower than optimal typically result in a more predictable loss of grip when it goes away. Higher than optimal may result in a quicker loss of grip. Depending upon your car setup, driving style and how good you are factors into deciding what direction you want to go with respect to purposely using too high or low pressure in a tire.
For me with the Civic I would typically run higher pressures in the front (close to 40) and lower pressure in the rear (low 30's). I used lower than optimal in the rear to both reduce rear grip relative to the front (to help rotation) and to help create a predictable loss of grip (to allow me to catch the car when it would try to snap oversteer). Your setup for the 914 will be different because it's not a front wheel drive car, but non optimal pressure should have the same general affect on a per tire basis. Again, don't use front wheel drive setup on a rear wheel drive car. I would also agree that any dramatic off course you had previously is not due to tire pressure issues. |
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