Auto-X coming up

[Jordan]

You'll have to explain that again to me sometime :lol: . I took some measurements of the OMP fronts and fixed my formula. I calculated them at ~500. They are very short (7 total coils) and fairly thick between 21 and 22 (closer to 22) 32nds of an inch. I'll take a look at the rears as well but it will be difficult to measure the spring diameter while they are still in the car.

[Geoff]

Yeah, I remember the calculated values for the OMP fronts being 510 and the rears being something like 630 lb/in

I'll try to get that spreadsheet up on the forums this weekend. I need to add to it so its a little more comprehensive.

[Jordan]

Yeah if you post the values or send me the sheet in an email I can pdf it and make it available for download. Why would rear springs have a higher spring rate than the fronts?

[Luke]

Ok so I measured two springs sets I have around:

SPG: 5 active coils, .625 wire, 11.75 free height: 392 calculated
Old Sport and Rally: 6 active coils, .61 wire, 13.25 free height: 297 calculated.

I'll take my S&R springs and some rear SPG next time I go down to A1 racing and measure them too.


I also took some measurements of the rear suspension to calculate motion ratio so we can see why we have to run such stiff springs in the back.

Distance of trailing arm from front pivot to axle centerline ~ 18"
Distance from front pivot of trailing arm to center of spring seat~ 10.5"
Motion ratio = (10.5 / 18 )^2 or .34

Too put this in perspective: A 500 lbs spring on the rear = 170 at the wheel! A pretty big difference!

So to answer jordans question of why we are running stiffer springs in the rear than the front when theres no weight back there.. we arent really. Its just due to the geometry in order to obtain the rate we want out at the wheel we must use a heavy spring inboard.

It really would be nice to get some corner weights for 99 and 900's. That would help a lot in determining proper setup. I even had my car on scales once and only got the overall weight not the corner weights. Doh!

[Hans]

Regarding tires: for a 15-inch rim, the best street tire choice is a Falken Azenis. No one who's done testing runs the Hankook, and the majority of people who've been able to run Kumho MXs successfully are in very hot climates. MX wet performance is poor. For more on the Azenis, go to GRM's website and find the articles on Andy Hollis' STS2 Miata; he's done extensive testing, including datalogging.

The 205/50 works fine on a 5.5-inch rim; sidewalls are quite stiff.

Regarding corner weights: The following data is from my '86 900 notchback, with 16v turbocharged engine, no AC, with Super Aeros and 9k calipers (25+ pounds over stock set-up, and most of it rotating and all of it unsprung mass!). Approx. 10 pounds of interior stuff was missing (carpeting, knee bolster, etc.) at the time. I used an Interstate highway truck scale (four, actually) that is regularly calibrated. Multiple measurements confirmed repeatability, and measuring on multiple scales corroborated the scales' calibration. The resolution is only to 20 pounds, however, which is why the numbers don't always add up exactly.

Actual weights, without a driver, absolutely topped off fuel tank, nothing else in the car:

Total:
2740 (measured on a single scale)
2760 (calc'd from F/R weights)
2780 (calc'd from corner weights)

Front 1700
Rear 1060

Pass. Rear 500
Pass. Front 880
Driver Rear 520
Driver Front 880

Note that a hatchback Turbo will be heavier, a base model...comparable??, though prolly not for corner weights (heavier in the rear, lighter up front. (The hatch, with glass, weighs 59 pounds, incidentally.) I actually have data for my 89T but can't find it just now.

I have some *calclulated* unsprung weights for a 900, but have yet to get around to actually weighing. The part weights were taken from a WorldPac parts online reseller -- WorldPac lists part weights down to the hundredth of a pound -- or I measured them myself on a postal scale. Some weights (wheels, for example) may not be accurate, as the weights I found on the 'net were uncited.

Anyway, for a stock 88+ 900, I calculated approx 95 pounds per corner, up front. I don't have data for the rear.

And after all that bla bla bla, a car club I belong to is in the process of purchasing a set of race scales. I should have better data, and for both the notchback and my 89T hatch, in a few months...

[Hans]

Ok so I measured two springs sets I have around:

SPG: 5 active coils, .625 wire, 11.75 free height: 392 calculated
Old Sport and Rally: 6 active coils, .61 wire, 13.25 free height: 297 calculated.

Luke -- I have data for the SPG springs from a Saab manual:

They list rod diameter at 15.7 mm (.618 in) and free length at 301 mm (11.85 in). Same 5 active coils that you got.

For the rears: 9 coils, 7 1/5 active coils, rod diameter 15.8mm (0.622 in), free length 317 mm (12.48 in).

[Jordan]

Hans,

I could not find the azenis in that tire size anywhere... GRM did a recent comparison of the top auto-x tires including the MX the azenis 615 and hankook rs2, they rated the azenis as top but hankook beat out the MX. Since they are extremely cheap, I am going to give them a shot. By they time I get my car properly set up I'll probably find a 7" rim to fit some better tires on.

[Hans]

Jordan,

Try Edge Racing: http://www.edgeracing.com/tires/2055015/

Tires.com sells them, too: http://www.discounttiredirect.com/direc ... &tc=FALHH7

Have fun cone-mashing!

[Jordan]

I'm looking for 195/55/15... 205s bulge too much on the sidewalls of 5.5in tires in my opinion.

[Hans]

I'm looking for 195/55/15... 205s bulge too much on the sidewalls of 5.5in tires in my opinion.

An Azenis 205 will be fine -- they are STIFF. Yes, they do look a bit pinched. But I've gone through multiple sets on 5.5s when I ran in Stock/Street Tire class, and was regionally competitive on them.

They are, hands down, the grippiest damn tires I've ever had (they put 712s and Yok ES100s to shame). And cheap, too. Which is good, cuz they don't last very long!!

[Jordan]

I think you are probably right.. the increased stiffness off the azenis may help the wobbly feeling I got from the Kumhos... I'm sure I'll get to go through a few sets of tires to feel out which ones I like the best. Azenis are supposed to be great though...

[max]

Narrower tires are easier to turn anyway, and wider tires probably won't do you much at the speeds that auto-x is done at. So why go for a 205?

edgeracing.com is where I bought my hankooks. They were slow, but I had no other problems with them.

[Jordan]

why wouldn't you need sticky tires for auto-x? that doesn't make any sense.
And wider tires aren't neccessariy better or give you more grip. Grip is still defined by pressure and area. if you increase your tire area you decrease the pushing force of your tire on the road, grip in theory remains constant. They also tend to increase unsprung weight.

Increasing the contact patch however evens out road irregularities and helps to maintain a consitant contact patch, which is why most racers don't ride on 155 tires.

But yes, 205s on an EMS quick ratio rack isn't as fun as it sounds.

[max]

You need to go back and read my post.

I never said stickier tires weren't better. In fact, I never mentioned "stickier" anywhere in my post.

I also never mentioned that wider tires were better- in fact, I mentioned the opposite.

[Luke]

Children behave, go bicker on saabnet or something.