lifting rear tire

[kbeefy]

I'm in alaska and my car only get driven a few times a year, in the past 4 years it's only been driven on the track. I road race it, and unfortunatly we don't have many oppourtunities to 'tune' our cars on the track (it's only available 2x a year and is located about 350 miles away from home).

So, I've decided the rear suspension just doesn't handle hard cornering as well as I'd like. Accelerating out of a sharp corner it lifts/spins the inside wheel until the chassis settles and is able to grab with both tires. It has (I believe) a trac-loc in it now, and I plan on upgrading to a Torsen T2R before the next race (memorial day). What other adjustments can I do to help keep the rear flat in a hard corner?

Heres some pertinent car specs...

'87 GT hatch
302/T-5/3.73
6 point cage
Maximum Motorsports Chassis stiffeners,
front:
Maximuim Motorsports strut tower brace
Caster/Camber plates
Adjustable Tokikos
rear:
Torque Tube
Adjustable lower control arms
Panhard bar
adjustable swaybar
Tokiko adjustable shocks
I think the springs are in the 800F/350R range, whatever MM recommended when we bought the package.

 

[347HO]

You need to adjust your (4-link?) for more neutral or anti squat.

In other words, move your instant center more forward when the chassis is loaded.

In your case, what's happening is when you load the chassis in the turn and accelerating out the back you're actually pulling the rearend up into the chassis.
You can band-aid this with a larger anti roll bar, stiffer springs, quicker rebound shocks.

To make it right, you'll need to raise your upper control links 1 to 3" depending on how low your ride height is from stock, and how much your suspension travels when loaded.
You should raise your lowers up also if you move the uppers 2" to 3".

 

[kbeefy]

with the MM suspension I don't even have uppers any more, it's a 3 link with adjustable lowers, a 'torque arm' and a panhard bar. I think that system is designed to eliminate what your refering to, but I'm pretty dumb on suspension tuning. I'm also not in any position to be redesigning this rear suspension. I was hoping to avoid any 'bandaid' fixes.

 

[347HO]

At ride height, how many degrees down angle (most likely) do you have on your lower control arms?
If you have an inclinometer, just put it on top of the arm.

 

[347HO]

I'll bet if you called MM, you'd get some pretty good information from them also.

 

[kbeefy]

yeah, I should probably do that. They were really helpful when we bought this stuff.

Not sure on the LCA angle, I was looking through my pics and couldn't find a good one of that. I'll probably be at my shop today, I'll check it out.

 

[kbeefy]

At ride height, how many degrees down angle (most likely) do you have on your lower control arms?
If you have an inclinometer, just put it on top of the arm.

Well, I tried to check it out but it was hard to get a good look at without moving my subaru (thats up on jackstands right now) and taking a tire off. The MM LCA's are bent into a J to allow for the adjustable spring perches so an angle reading would be incorect, but I'd guess the difference between the height of the front and rear mounting point of the arm to be about 1" lower in the front, and by my uncalibrated eye and thumb scale I'd guess about 10* down towards the front.

Heres a bad picture that doesn't solve anything but might help illustrate my awnser.

 

[347HO]

When both upper and lower control arms are pointed down (looking from the rear), that is bad and promotes squat. When you're hard in a corner and accelerating you're actuall pulling the rear suspension up into the chassis and the chassis appears to be dropping onto the rear tires.

When the upper is more horizontal or even very slightly pointed up 1* or 2* and the lowers are also pointed up making your instant center higher and closer to your roll center, you promote a more anti-squat suspension under load and acceleration.

My explanation is very, very basic to help understand a somewhat difficult subject.

Let me know what you come up with on your measurements.
Dana

 

[347HO]

Okay... I see what you're saying.

If you take a piece of string and go from the bolt at the rear to the front attaching bolt and hang an inclinometer to it... you'll know exactly what angle you have.
1" down angle, that is why you're losing traction. You will have to find a way to decrease that down angle to either 0* or higher than 0*.

In my case I re-drilled some mount holes 2.5" higher than stock on the torque boxes and 1" lower on the rear mount for the lower control arms.
I have the 4link version and I also had to increase my uppers 2" higher than stock.

My suspension manufacturer makes relocation brackets, so I'm assuming MM does also.

 

[kbeefy]

I should probably learn myself a little more 4-link geometry.

I was thinking about it, and if it's an angle issue, it would depend upon the incidental angle between the two arms and they're length, right? Well, by installing the torque arm and removing the uppers it changes that geoetry, but I cant really remember what the original upper angle was.
This is the best sketch I could produce on the computer to represent my suspension geometry.

 

[347HO]

If this is on the car/chassis in your avatar, your rear lower control arm @ ride height was near 0* or flat level, while your uppers @ ride height had some nose down angle... enough to create your instant center ON an imaginary line drawn from your front upper ball joint to the bottom of your rear tire.
I'm thinking Ford engineers designed it this way to give you the best overall performance and comfort. It would be considered a "neutral" instant center setup.

Do some research on the subject, enough to have a some understanding and call MM up. Talk with one of their techs and maybe follow up with their suggestions.
Because you bought their equipment, it might be best to go with their advice and only use my information to help you understand your suspension.
IMO, it's never good to use multiple sources to build a race setup.
If you need more help understanding this stuff, and/or MM doesn't satisfy your requirements, drop me a PM.

Dana

btw... I like your drawing. It's always very good to have drawings or pictures with this stuff.
I'll post a link with some very good drawings in it for you.

 

[347HO]

FOUR

 

[kbeefy]

wow, that was quite an article. I must be at least a little smarter now.

I'm gonna get the rear tires off and get a better idea of angles, then give MM a call. Thanks for the tips and the link 347!