897na

iTrader: (3)

I've got an 89 GTUs that I will occasionally autocross in STS2. I'll be installing KYB AGX & Tanabe GF210 springs.

Does anyone know how much of the bumpstops can be cut (front and rear) before any contact is made?

I'll be using the stock 16x7 wheels. Tires at the moment are the stock 205/55/16, but I'm thinking of getting Azenis RT-615 215/45R16 which are supposedly 5% shorter and an inch wider.

I read where DaveTurnerMotorsports raised his ride height because of bumpsteer issues, so perhaps "as low as you can go" in the front is too low. I assume the tires hitting the fenderliners is what will hit first.
I plan to put at least part of some microcellular bumpstops below the stock ones to soften the blow some, because those springs should be pretty low and are not that all stiff.

Thanks,
Jim

RacerXtreme7

iTrader: (2)

few things. Those springs appear to be geared more towards looks then performance. The really low drop leaves little suspension travel and the soft rates leaves big body roll and bottoming out. to further aggrovate all the above, being that low will make your roll center farther away from your center of gravity leaving you with even more body roll and little camber gain through what little suspension movement there is left. Lower does NOT mean better handling. Theres a compromise of lowering a vehicle for handling and looks. Though some lowering helps by lowering the center of gravity, your suspension roll centers remain the same making your CG (center of gravity) FARTHER away from your RC (roll center). The way to properly have a car really low and have the suspension work like its should is to lower your suspension roll centers as well. This can quickly turn into a HUGE page of explaination, I suggest you educate yourself. To make things easy and short, don't lower your FC more then an 1". If you do, use proper coil over suspension with seperate coil preload and ride height adjustments to keep suspension travel. To maintain optimume suspension performance at such low ride height, lower your roll centers. Your going to have to look this stuff up, as at least I don't have an hour or so to spoon feed ya how suspension works. Maybe someone else feels like typing up a few pages?

205 tire is 205mm wide (or 8.07"). theres 25.4mm in an inch.

215 tire is 215mm wide (or 8.46"). Not even 1/2" wider let alone 1". mind you, all tire brands and models vary from their printed width size.

Good luck with your choices concerning your ride. It looks like your searching for correct choices in the parts you pick, and thats good!

~Mike...........

jgrewe

iTrader: (1)

I was nodding my head until this point. You the results correct with body lean but the reason is wrong. What happens is your roll center goes down farther than the center of gravity and that is what makes the car want to lean more. Think of grabbing a sledge hammer at the bottom of the handle, holding the head "up", and trying to move it(the head) back and forth.(like an upside down pendulum) Now, think about it by grabbing the handle right under the head = easier to move.

You want to lower the car without lowering the roll center or at least lower it equal to the amount the roll center drop which is hard to do without moving pivot points. As stated above pick up a book on suspension, check the sticky list for titles.

897na

iTrader: (3)

I'm no suspension guru, but I know what you're talking about. I have a 95 Miata with Ground Control coilovers with 550/375 at the moment. The Miata's stock bumpstops (and mounts in the rear) will stop you a bit short of what is ideal. I have Flyin' Miata's rear mounts which give extra travel. I have also spent time with the springs out to see what's available. I'm using cut microcellular bumpstops with spacers. This subject gets lots of time on the Miata forum. I plan to do some measuring with the shocks in and springs out when I get it apart.

Maybe the FC is already ideal in this area?

My GTUs is my backup car and truck (other cars are Miatas) and doesn't get a lot of miles, and I would like to use it as a backup autocross car. They sit a little bit high and are not all that stiffly sprung. I didn't really want to spend $400 on GC coilovers on this car. I really just wanted new shocks to get rid of the play I have in the front end. I spent more than I wanted on KYB AGX because of the deals available on EBAY. I would like it a little bit lower and a little bit stiffer. The price helped lure me to the Tanabes. You know those GTUs's sit pretty high.

So are the stock bumpstops the best bumpstops to use, without changing mounts or suspension geometry?

Thanks,
Jim

897na

iTrader: (3)

On the tires, it's the difference between what's on it now:
Potenza RE940 205/55Z16 that list a tread width of 6.9", and the
RT-615 215/45R16 that list 8.0". The 215/45s are a little shorter also.
Hankook Ventus Rs2 Z212 215/45R16 lists 7.6" for the tread width, but the same diameter as the RT-615s.

Unless I get lazy and short-cut it, I'm going to remove the spring from a front corner, then jack it up and measure wheel center to fender lip, look at tie rod angle, and see if it looks like that should be it.
Then repeat w/o the bumpstop and see how far it goes, and look at the tie rod angle.
Then decide what I'll do with the bumpstop.

Then repeat for a rear corner.

I was just reading in my RX-7 Sports Car Color History book about the Infini IV. It says: "Suspension modifications included spring rates increased 10 percent and shock absorbers 25 percent stiffer on compression and rebound. ... less compliant bushing... no DTSS... strut bar... With the stiffer springs a smaller 23mm anti-roll bar was fitted up front. Overall the suspension sets the Infini IV about 1in lower than standard ride height of U.S. models."

In the picture it looks fairly low. I would say they must mean 1 in lower than the TII.
So 1" lower and only 10% stiffer springs...
I've searched here and all I can find is that stock springs for my 89 GTUs should be 95/90.
Tanabe GF210: 168/140 1.5/1.5
I thought they were worth a shot for $169.

897na

iTrader: (3)

Here's how it looks now. The rears are pretty high.

897na

iTrader: (3)

I have to correct something about my ride height above. My car was a little lower than stock (89 GTUs) in front because a few years ago I swapped in a set of struts/springs that came (IIRC) from an earlier turbo that supposedly didn't have any play in them. I thought the spring rates were the same. After getting everything out, I compared the 4 front springs and found:
My originals have Green Id mark on LF, Grey mark on RF.
The ones I just removed (as in the pics above) have Grey LF, Pink RF.

897na

iTrader: (3)

Well, I got my AGXs and Tanabe GF210s in, and took some measurements before removal, then with springs and bumpstops removed. I'll post about the new setup later. Note that I didn't measure super carefully as I was also trying to get done - but the numbers should be fairly accurate.

I measured the ride height 1st (as in pics above):
F: 13 5/8 13 5/8
R: 15 1/4 15 3/8

Ride height measured in inches from wheel center to fender brim.

I removed the LF suspension, and put it back together without the spring and bumpstop.
I jacked up the wheel and found that the tire started rubbing the fender liner at ~ 10 1/4. Tires are very worn Potenz RE940 205/55Z16.
The tie rod looked level with the rack at ~ 14 1/2. There is a BIG angle at 10 1/4!
I think it will contact the bumpstop at ~ 12 1/2, but I didn't get a good measurement of this. Those old bumpstops are hard too - won't compress much.

I did the same on the RR.
I got the wheel up to ~ 9 5/8 and tire still didn't wasn't rubbing on anything, but that is too low.
Contacts bumpstop somewhere ~ 13 3/8. These bumpstops will compress a little.

Summary:
Front:
minimum: 10 1/4
bumpstop contact: 12 1/2?
tie rod level: 14 1/2

Rear:
minimum: let's say 10
bumpstop contact: 13 3/8

Anybody else got numbers or ideas about what to use as bumpstops?

897na

iTrader: (3)

Measurements after install of AGX & Tanabe GF210 lowering springs.

ride height (wheel center to fender brim)
LF: 13 1/4 RF: 13 1/4
LR: 13 3/8 RR: 13 3/8

Camber (measured with $40 bubble type)
LF: 0 (stock mount is min camber (will change to max camber), max castor)
RF: -0.75 (stock mount is max camber, max castor)
LR: -2.4 RR: -2.45

Anyone know the best way to adjust camber?
RBs vertical link says 60 degrees less camber by tilting the rear subframe for $110.
Then there are CAMBER ADJUSTMENT LINKS from Mazda Motorsports for $187. Anyone know how much these adjust?

Jim

dpf22

Get the links, the single camber bar yields uneven camber when you adjust it. The links are much more accurate. I have awr camber links in mine. love it.

dpf22

897na

iTrader: (3)

Thanks for the response. This thread was getting like a blog.
These springs drop the car too much in the rear. It was level and looked good when I first put them in, but then I filled up the tank and it looked like it dropped an inch! I may try to put something under them to raise the rideheight, or get different springs.


Pic of bumpstops:
Left is the OEM (~2").
Middle is 54mm progressive bumpstop available from Ground Control. These crush down to about 10mm.
Right is what I put in front. On the bottom is the very top of the OEM bumpstop, just enough to hold on to the OEM boots. Above that is the 54mm bumpstop.
As you can see I will contact the bumpstop about a half inch higher then stock, but it will be much softer. I haven't measured carefully, but there's only around 1/2" of travel before I contact them now. I just ordered wider bumpstops that fit the 22mm (7/8") strut rod. I think I'll cut them down so they have 1st contact a little lower then stock.

The OEM rear bumpstop was about 3 1/4 ~ 1/2" long. I cut the rear OEM bumpstop down to about 1 1/2" and added the 54mm bumpstop. That should have it touching the rear bumpstops right around where it did stock.
There's only about 3/4" of travel until these are contacted. I think I'll cut these down some too.

Here are the bumpstops:
http://www.ground-control-store.com/...tion.php/II=10

Here is a compression graph of 2 of GCs bumpstops, and
a rubber Miata one. The site is where I bought them
because I have a Miata and need spacers too.
http://www.fatcatmotorsports.com/iga..._63mm_NA_1.JPG
http://www.fatcatmotorsports.com/FCMstops.htm

Any opinions of people that know what may work well is welcome.