Otto Mechanic

PS: The car is at my shop in Santa Cruz right now, not Paso Robles as my signature suggests. Not that it makes too much difference, but it's been raining and fogging here recently, not so much in Paso, which may have effected grip yesterday.

marc abrams

Nice car Scott. Nicer house!
Just to give you a idea I settled on 32 front and 38 rear. 40-42 rear would probably be the sweet spot for the car but not for my tush.

Otto Mechanic

Thanks Marc, I'll give that set up a try (32 front, 38 rear) and see what difference it makes. If I can get it out today I'll report back soon, otherwise we have rain in the forecast and that usually keeps me off the test track (Hwy 9 between Saratoga and Big Basin).

Regards,

V2Rocket

with that much difference in tire meat i'd think it'd be quite prone to understeer (245 vs 205 is a lot)
if you can get the rear to grip more it will understeer more/kick tail out less. softer sway bar, softer tire pressure in rear, or higher pressure in front.

Otto Mechanic

Just an outright plug, but the house and shop (not the car) just happen to be for sale, so if anyone wants to buy an equestrian B&B with great reviews and 5 years of books, that's on one of the best engineered roads in CA and also has a full shop complete with a 4 post lift and a mill/lathe for making those hard to find specialty parts, drop me a line. The RE listing for the place is at:

http://www.meganlyng.com/property/ML81650279/

Otto Mechanic

OK, now I'm conflicted. As I get it, Marc suggest higher pressure in the rear and you're suggesting higher in the front? How do I resolve this without experiment (i.e. hanging my butt over the void)?

will968

Less pressure means more contact patch and more grip, to a point. Add air and you take away from the patch and decrease your tire's grip potential.
Did you have anything special with the rear shocks as far adjustment? If so, turn down the rebound. I'm guessing setting bar to full-soft will fix your problem.

StoogeMoe

Start with the basics. You can't set the balance on your car with old rubber. I bet those tires you have are dated out.

Otto Mechanic

Will, nothing special to the rear shocks other than to replace them with Koni Sports. They're adjustable, I have both set to +3, which I believe is the middle of their range? Could that be a problem? Easy to fix if it is.

Your comment on patch strength brings back ancient memories of suspension tuning from my days running a '71 914 on this road. I'd built it to the 916 suspensions specs with 220 in front and 245 in the rear. Of course, I needed some body work to make that happen. I called it my "flexy flyer" and it ran like a bat up and down Hwy. 9 in the late 70's and early 80's until I had to move to SF, lost my shop, and sold her to a porno film maker who appreciated exotics.

It's been a very long time since I've done practical suspension tuning so I sincerely appreciate your advice.

Regards,

marc abrams

Good Point. Some times I have to wonder if too wide can hurt grip on a street car. Not to mention the semi trailing arm suspension.

I am looking for another house. That one may be a little out of my price range.

Yes that does seem true. NASCAR cup cars are always runing the lowest pressures possible to get the most grip. However I started with 32 in the rear and was still a bit loose (oversteer). I was able to tighten up (reduce oversteer) the car by increasing the rear tire pressure. Anyhow Scott a 2 psi change in tire pressure makes a very subtle change. It's easy to do and it won't make the car do anything wicked.

Otto Mechanic

Very likely. As I mentioned they were on the car when I bought it 3 years ago and they've been up on the rack the entire time until yesterday. I don't know anything about Cooper Tires or the Xeon line they sell. I've always used Goodyear Eagles on my 928 and haven't had a bad experience other than the Goodyear dealer in Cupertino putting $600 worth of tires (this was 1987, back when tires were a whole lot cheaper) on a car with a blown front A arm. I went through the entire set in 500 miles. The dealer claimed to have aligned the car, which was very obviously false. I replaced the blown A arm, had it aligned again by someone who actually knew what they were doing (and didn't sell tires for a living) and haven't had a problem since.

Live and learn I suppose.

marc abrams

Clark's Garage (RIP) has a section "suspension information and upgrades". Good reading and explains a lot.

mel_t_vin

Scott, the RS-3S have a TW rating of 300...not too bad. See if you can find their DOT label on the sidewall...usually contained in an oval area. Some tire manufacturers only put the label on one sidewall so it could be on either DS or PS. You will see 4 numbers...like 2310. That will give you the week [23] and year [2010] the tire was manufactured. As previously mentioned, these tires are probably close to their useful life span.

Regarding your suspension geometry...depending on how things progress/improve with your handling issue based on changes to tires, dampers, and sways, you may consider lowering your rear end by 10 mm.

ddombrowski

Do you know what your rear toe numbers are? Our car was really tail happy when we first got it and discovered the alignment had a lot of toe out in it.

Assuming not the toe, I agree with the others to say to soften the rear sway bar and possibly up the spring rate in the front. I've spun our race car a handful of times, always exiting from the apex under power. Everything seems fine, squeezing the power out and then.....nope.

In the back of my mind, I suspect it may be caused by a bit of wheel spin on the inside tire, but I don't have anything scientific to back that up, just a hunch and the fact that 4krpm is the sweet spot for torque.

Scott at Team Harco

Until the last post, I didn't see anything about alignment. (#1) Ride height should be set and all camber and toe settings should be to factory specs (or a good known set of numbers). No play in bushings, etc.

(#2) After that, look at the tires. Treadwear rating, age, etc. Contact patch as mentioned will be wider with the 245 vs 205 tires. All things being equal, this tire stagger should provide plenty of understeer. Most racers actually prefer tires closer to "square" to reduce understeer on cars like this (not a whole lot of power and near 50/50 weight distribution). This does not seem to be your problem.

(#3) Next it's springs and dampers. Assuming stock rear torsion bars and stock springs - shocks are next. Again, assuming all is good and consistent, move on to anti-roll bars and tire pressure.

As a general rule the following will reduce oversteer (increase understeer).

Wider tires at the rear/ narrower at front
Softer damper valving in rear/ firmer in front
Softer spring rate in rear/ stiffer spring rate in front
Softer rear anti-roll bar/ larger front bar (as mentioned - extending the lever arm of the rear bar will make it softer)
Higher tire pressure in rear tires/ lower pressure in front

It is very unusual for a car to exhibit oversteer unless something serious has been altered. Look at what isn't original and assess what it is doing for you (or against you).

Additional tuning is best performed with known good tires and a tire pyrometer. Then you can go through the whole routine all over again so you get the most of your tires.