How low can you go?
 

I was hoping someone could point me to the page(s) in the prepared rules for ride height. I have gone over and over the 2005 rules and can't find it. I'm sure I must be missing something.

Cary

Cary,
I just looked at them and couldn't find anything either ...

Why does the body can't be lower then the lowest point of the
wheel come to mind ?

Mike

That's what I was thinking too but I had a look. It's very clear in the GTCS rules but I don't see it as obvious in the SOLO II rules. It isn't a rule in modified and I'm fishing for new things to exploit :-)

Cary

I think Tom had a picture of a Prepared Miata .. can't remember for sure, that
was sitting on its bodywork after a tire failed ... But , at my age your memory
gets pretty bad :)

I don't think that there are restrictions on ride height in prepared classes. I know that aftermarket springs are allowed.

"No part of the bodywork or chassis, to the rear of the
front wheel opening, shall touch the ground when both tires on the
same side of the car are deflated." 17.2 B in the 2005 Prepared rules

All the SCCA rules are online this year. Go to the www.SCCA.com site and enter the solo area and look up cars classifications and rules.

Thanks Steve, I couldn't find that for the life of me.

Cary

Going to a lower ride height would be one advantage of a 16 inch wheel. (Keeping the same tire diameter for gearing) I don't know if that would counter your rotating mass theory with the 13 inch wheel or the advanced compounds of formula tires over sedan slicks, but it is what I've got so I am going to try them.

How Low...
 

You brought up a very interesting point for me. I just went out and measured the sidewall on my Hoosier's. They're 22.5 x 9 x 15 and have about a 3.5 inch sidewall. My car is currently sitting with about 5 inches of clearance. I'll try dropping it another inch all around for the next alignment. How much less is the sidewall on a 16 inch tire/wheel???
Assuming the same diameter - it must be an inch less???

The two 16 inch tires to fit 10 inch wheels I know of are the Hoosier 22x10-16 with an actual diameter of 22.9 and the Goodyear 23.5x10-16 with an actual diameter of 23.3 to start. Also I have read here that the diameter of a bias ply slick grows as it is used; just to confuse things more. The 16 inch wheel would give you a 1/2 inch more lowering using the tire deflating rule. And then figure in any other tire height differences from your current tire to the 16 inch tire. With a half inch greater radius from the 16 inch wheel centerline to the bottom of the wheel is why half an inch lower, not an inch compared to the 15 inch wheel.. P.S. I was a D math student (much to the chagrin of my rocket scientist father) so check with Cary or John on anything that matters.

In reading the rule book it only specifies one side deflated. So you have room to fudge a little more than if the car were sitting on just wheels and you have a little wiggle room with a defalted tire adding some in. And if you're going to go to this amount of work you might as well get every last little bit you can.

It's true that the tires grow. You need to keep track of this in your tire log to either keep them matched or to adjust ride height because of growth. But when you're talking circumference that doesn't translate into a lot for diameter. To give you a number after 6 abusive weekends and three hillclimbs I had gained 4 tenths of an inch, which is more than enough to require a ride height change. Every now and then you'll find a tire that grows a lot more than normal. Either toss it or adjust for it (this can be a pain when you rotate tires).

recommended accuracy:
Ride height: ± 0.05" with consistent car weight & tire radius
Toe: ± 0.05" per side
Camber: ± 0.1 degree
Bumpsteer: < 0.02" through normal suspension travel

Cary

OK - to confuse the issue even more...

If the rule says "with both tires on one side deflated" - my Hoosiers don't compress at all even with zero tire pressure. I guess the 1,900 lbs of the car is not enough to compress the sidewalls even after a few months of garage time!

Since I can get 24 x 11 x 15 tire on my 15 x 9.5 wheels - I'm thinking the added cost of 16 inch wheels will not be worth it to drop the car another half inch.

Or am I missing something??

I don't think you're missing anything. Just set the car as low as you can with the body almost touching when both tires on one side are deflated and call it good -- assuming you don't have control arms at weird angles. If the latter is the case you may be better at a slightly higher than optimum ride height.

You can also run the FA front tire on a 9.5 wheel. It's a 22x9x15. It's close to ten inches wide. Assuming autox and hillclimbs this too could be a good combo. It's slightly shorter and plentiful if you want to run used tires (hey I'm cheap). R35s is a good hoosier compound unless you run where it's really hot and then you may want to run a R45.

We've been running the FA tire for the past year and are fairly happy with it. (This is an Auto-X only car) But with the new SCCA rules allowing 12 inch wheels - we're concerned that we're gonna be completely out classed.
I guess we'll see. One problem with the 24 x 11 x 15 is they're only available in the R45 compound. Currently we're running the R35.

Your tires don't compress when empty? That sounds like a good way to
run a low car and get around the rules. Do you think this is because of
the FA sidewall construction or just cause they are old or....? I don't
want to break the rules but it sure would be fun to bend them! :)

Nope - the Hoosiers don't. I've run Goodyears that do compress when deflated - but the sidewalls of these Hoosiers are super stiff. And I don't think of it as bending the rules - the rules say nothing about the tire compressed - just "deflated" which they are... sort of :)

If my memory is correct, my car has about 2 1/4" of ground clearance to the frame rails with the tires properly inflated. The control arms are angled slightly downward to the outside. Without air, the car probably doesn't drop more than about .5 - 1 inch. I did run lower once many years ago and was dragging frame rails through the turns. This was after the first rebuild and I attempted to run 240 shocks on a 280 body. It didn't work so good... http://sth2.com/Z-car/rebuild5.jpg

The allowance of over 10" wheels shouldn't be a concern. I don't know of ANYONE in the national arena considering them. The extra 100 lbs that is required is too much of a deterrent.

Tom

LOL.... I am surprised you actually took the ditch witch out in public :)

Tom, with your new shock and spring package do you think that dragging is an issue
anymore ?

Mike

>do you think that dragging is an issue anymore ?

Not even close... I've had the car slightly airborn and not bottomed the chassis...

Getting way off topic here, I'm leaving for the Arkansas National Tour in the morning, where I will get to see just how fast John Thomas' newly rebuilt Z is. I'll try and get some pictures posted when I get back.

Tom

Here is the list of FP entries at that Toour Event :

FP Count:6

9 Jackson, Jonathan
Maumelle, AR 1973 Porsche 914-6 Ylw
Perfection-Plus, McPherson Client Services Other Arkansas Region
MW

23 Holden, Richard
Bartlett, TN 1971 Datsun 240 Z Blu
Holt Tire/Holden Racing Hoosier Mid South

51 Anderson, John
Kouts, IN 1972 Datsun 240Z Wht
J & M Autocrossers Hoosier South Bend

123 Thomas, John
Tampa, FL 1971 Nissan 240Z Blu
Hoosier Mississippi

151 Bartalone, Donnie
Saint Charles, IL 1972 Datsun 240 Wht
J&M autocrossers Hoosier Chicago Region

199 Holt, Tom
Round Rock, TX 1974 Datsun 280Z Wht
Road Trip Racing Team Hoosier Lone Star

Looks like all the fast Z's will be there :)

Mike