Suspension Questions Etc...
 

Hello, I'm new to these forums. This past season I started working
on an E production '85 RX-7 SCCA car. I'm trying to find out how
to remedy an understeer problem that the car was plagued with this past season.
Prior to my working on this car, the rear suspension has been changed
from the OE four link to a panhard bar style suspension. The panhard bar
is adjustable for both height (both ends) and length. There is also another bar
on top that is adjustable in length (probably for pinion angle) The front has
adjustable coil over style struts and upper strut mounts.
As I metioned already the car has exhibited quite a bit of understeer as well
as some vibration on the end of the straighaway. (not described as a tire
or wheel vibration by the driver)

Now for the questions:
Does anybody here have a similar set up ? Have you experienced any
of these problems ? If so, what steps did you take to correct them ?
What settings seemed to work for you ? (I know that various tracks and
conditions will affect this, but I'm more interested in a baseline at this time.
Also, does Mazdaspeed offer any technical support in this area ?

Thanks in advance.

Rob

Well correcting understeer isn't voodoo. Stiffen the rear or soften the front. If the car's got adjustable sway bars then that's the place to start. Otherwise it's springs. Also, the solution can depend on whether the car's understeering on turn in, mid corner or track out.

Playing with tire pressure will also have an effect on understeer vs oversteer. Also, if you run more camber in the front that can help grip. These should really be tuned with a pyrometer though to make sure the tires are heating evenly.

Have you done anything as far as adding camber adjustability? I also have understeer issues, and have decided that camber plates will be the next step to correct that.

That is pretty much the style suspension I am running.

With E=Prod Goodyear slicks, Set front camber to -2 degrees
Set caster to 5 or more degrees.

To reduce understeer:
soften front springs
soften front swaybar
stiffer rear springs
stiffer rear swaybar

How many pound/inch springs do you have on the front?
How many pound/inch springs do you have on the rear?
What diameter front sway bar are you using?

what tires are you using? Bias ply's can be a bit diabolical. Radials will help with the suspension setup and overall performance.

the E-Production tire is usually the Goodyear canti-lever bias ply slick, size 23x9-15

I haven't been back here in a while.
Here's the update:
The vibration was from some very slight play in the rear u-joint.
We had made some air pressure changes which helped alot.
I also started bringing my IR thermometer to the track as well.
I've been zapping the tires as soon as the car hits impound.
So far everything looks good. Now we're just waiting for the
snow to go away and go racing again.

If you are waiting till impound for tire temps it's too late. They will look great, nice and even by that time. You need to get them as soom as possible after a hot lap with a probe type pyrometer. The outside cools quickly, below the surface will give you more useful numbers.

There are some tracks that suck for getting good temps. For example Road Atlanta has a long straight and only a couple quick turns before the pit entrance. The tires have cooled a little from their cornering temps by the time you get to the pits.

By the way raising and the panhard rod raises the rear roll center and reduces understeer. It is the easiest way to fine tune the balance. Make sure the panhard rod is level at ride height (with the driver in the car).
By the way a balanced car, with regard to oversteer-understeer, will have about the same air pressure gain in the front and rear tires.