Have been following JPs ax thread with great interest and decided to start a new thread to see what is recommended for tire pressure on my car. Its a '74 2.0 with completely stock suspension, front and rear sway bars, boge oe shocks, etc., and is shod with Yoko AVS Intermediates 205-60-15.
What is a good starting pressure (cold) for this set up. The last AX I ran, I went with 35 psi at all 4. The car was fairly neutral with a slight tendency to understeer, but overall grip seemed a little sketchy. (may be 60 series street tires, too).
Also, if someone was to invest in some upgrades, what is the general concensus on what will bring the "most bang for the buck" , a set of adjustable Koni sports all the way around, or an adjustable sway bar up front? I want to leave the bushings, springs, T-bars alone for now, stay legal in stock class and learn how to drive this car!!

Just go to an empty parking lot and bring some chalk, a tire-pressure-gauge and a pump.

Chalk up the transit where the tire's sidewall ends and the thread begins
Do this at 12, 3, 6 and 9 o-clock positions.

Have some fun on the parking lot

Check the chalk marks.
If there's still chalk on the thread, lower the pressure
If the chalk on the sidewall is gone, raise the pressure

Chalk it up again and do another run...

Repeat until satisfied. Take pressure readings (hot) and drive home.
Take pressure readings next day (cold)

This may be a little crude, but if works just fine on a streetcar

Upon AX results, you can adjust the pressure to compensate for under/oversteer

Oversteer: lower rear pressure and/or raise front pressure
Understeer: vice versa...

Have fun!

Jeroen

Thanks, Jeroen. So at the point where the chalk is no longer coming off the tire is going to be my ultimate starting point? Is there any benefit (grip wise) to going higher? I understand about adjusting preesures front and rear to compensate over and under steer characteristics, but is there a point of diminishing returns with inflation pressures overall? It seems to me that adjusting the pressures for push or loose is just like doing slight spring rate adjustments, so I guess what I'm really trying to figure out is how I can be sure I'm getting maximum grip from my tires.

By changing the tire pressure, basically what you are doing is changing the contact-patch of the tire.

If the pressure is too high, you won't be using the full thread-witdh of the tire (that's why you will be able to see a chalk mark on the thread).

Setting the pressure too low would give you a wider contact patch, but then you are using part of the sidewall of the tire as thread (and that's not quite what it's intended for )

Also a too low pressure will have the tire move (more) in relation to the wheel

All said... a lot depends on correct camber settings
If you have too much negative camber, you may not get enough or any wear on the outside of the thread. If you do, then you're probably using the inside sidewall as a contact patch.

Same goes for too little negative camber (or too much positive). You will get good use of the outside of the thread, but little use on the inside of it.

I hope it makes some sence...

cheers,

Jeroen

Yeah, you're making great sense. Currently my set up is about .5 degree of negative camber all around.. The ouside of my front tires appear to be doing more work than the inside ( they are slightly cupped?), so as to indicate a need for possibly more negative camber? Also, it now seems that that could indicate a need for more pressure to keep the tire from rolling over. I don't have a pyrometer to check tire temps to see what the effect the current camber settings are having, is there a parking lot test for that, too?

I also found that under inflation or inconsistant pressure in both front tires contribute's to steering wheel shimmy when speed is varied!

The "chalk method" is fine to get the tire working across the full thread
If you can't get that to happen with the pressures, you need to change the camber settings (more negative if the outside wears more and vice versa)

The pyrometer test could just as well be done on a parkinglot. It's more refined than the chalk method and you can do that once you get the entire thread width working to make final adjustments.
The temp-differences will show you where the tire has most contact (higher temps)

This again will have you fiddle with the pressures and cambersettings...

cheers,

Jeroen

Great, I'll try all that. How about my question regarding where best to spend a little$ - adjustable Konis or adjustable front sway bar?

Hey Jdogg,

Depends... what kinda shocks are you running now? What shape are they in?
Also how do you drive the car?

cheers,

Jeroen

Hey Drumms66,

Yep, that could happen... the tire is moving (sideways) in relation to the wheel

cheers,

Jeroen

Jeroen - the shocks are newer OE Boges, in great condition. The car sees occasional weekend use on the street, once a month autocrosses and some DE track events. Ride comfort is not a major concern.

Note that, particularly with a street tire, often going higher with the pressures will cause the tire to stick better! You say the car is neutral with about 35 PSI in all four tires. On your test day, check the pressures and go play a bit. Then let 3-4 PSI out of the front tires and play some more. See how that affects the balance of the car. If you have an air tank, try adding 3-4 PSI and seeing how that affects things.

As far as bang-for-the-buck, seat time is the way to go. Go run more events!! If you're really intent on changing the car, a stock front swaybar (or small-ish aftermarket one) would probably be more effective than new shocks on a car that doesn't have swaybars.

I personally would just try to get the car balanced (using the tire pressures and whatever other adjustments you have), and then go drive. You can easily get into a mode where you concentrate too much on what you need to do to the car, and not enough on driving it. So set the car up so that it's reasonable, and then just drive it.

--DD

DD, you speak the truth regarding seat time being the best bang for the buck, but don't forget one of the big reasons we play with these cars is so we can modify them!!! My car has stock front and rear bars, and stock oe type boge shocks, and my question was which route would provide the most immediate benefit, adjustable shocks, or a bigger adjustable sway bar?

Tough choice, cause I'd do both
So which to do first?
Maybe take DD's advice and start with a 19mm adjustable swaybar

cheers,

Jeroen

I'll just chime in on this note.

I'd do the swaybar first.
Then good tires.
All the while drive events every chance you get.

The difference that the 19mm swaybar makes is really astounding. Go with an adjustable bar to really have some fun.

Thanks for all your inputs. Any favorites regarding brand names? I know I've seen stuff here before - Tarrett, Smart Bar - Weltmeister - how about online sources for this stuff too.

BBforB? R spec tires....no doubt in my military mind.
2 sec a lap and a ton of confidence builder. Nuthin' else comes close.

Paragon Products sells both the Weltmeister and Tarett swaybars

Or you can go to Tarett directly

The Tarett is a little more expensive than the Weltmeister, but worth it IMO (once you've added adjustable droplinks to the Weltmeister, the difference with the Tarett becomes even less)
I'm not sure if Tarett has a 19mm bar though so ask!

The Smart Racing swaybar is prolly the nicest of 'em all, but quite a bit more expensive (and again, I don't know if they have a 19mm bar).
Sorry, don't have a link to the Smart Racing site...

cheers,

Jeroen

http://www.smartracingproducts.com/

Just about all the aftermarket parts places carry Weltmeister stuff. Several carry Smart Racing stuff or can get it. Pelican carries WM, and can get Smart Racing stuff. Jason at Paragon can probably get just about anything for you that is suspension-related. He's one of the Good Guys. (Not quite as Good as Pelican, of course! )

The Tarrett piece looks really very nice. Uhh, at least, I think that was the one B brought up to Portland. The Weltmeister is the least expensive of the lot. Charlie Bars from Wrightwood Racing used to be "just the best", but it sounds like Tarret and SR may have surpassed them.

--DD