SN95 Rear lower control arm angle

[Ron McCall]

Hello,
I am curious as to what people's real life experience is with the angle of the rear lower control arm is. I have a ,new to me , SN 95 race car that has a Cortex SLA ,Cortex Watts link , MM torque arm and I run Dunlop slicks.
The front of the car is pretty amazing. The turn in is excellent , it is very predictable and never has even the slightest amount of push. The rear ,however, is a different story. It feels loose at turn in ,decent through the middle of the turn and is loose on exit. I never get that confident feeling from the rear. I have made a few adjustments ( lowered the watts, lowered the rear of the car and played with tire pressure) these changes seemed to help but only slightly . To me , the rear of the car still seems a bit higher than it should be so I had planned to lower it a bit more and see what it does. That's when I noticed that the lower arm is already lower in the front than the rear and lowering the car would only make that worse. I know , from my drag racing days that lowering the rear of the lower bar helped traction quite a bit . I was curious what effect it would have in a road race setting?
Anyone have first hand experience with this in a Fox or SN95?

Thanks,
Ron

 

[blacksheep-1]

A big plus for the front is Steeda 2x ball joints, if you don't already have them, they were the biggest plus I ever did on my new edge cars, although, because of the rules, I had to run that crappy triangulated 4 link. I know the change in trailing arm height worked for you at the drags, but keep in mind these cars are also required to brake extremely hard, and that forward bite doesn't always translate into good braking geometry. Assuming you have at least, urethane bushings in the trailing arms. It appears that your kit should have come (originally) with lower trailing arm relocation brackets, if so, are they on the car now? That right there might solve your problem, or at least give you room to experiment.

 

[Ron McCall]

A big plus for the front is Steeda 2x ball joints, if you don't already have them, they were the biggest plus I ever did on my new edge cars, although, because of the rules, I had to run that crappy triangulated 4 link. I know the change in trailing arm height worked for you at the drags, but keep in mind these cars are also required to brake extremely hard, and that forward bite doesn't always translate into good braking geometry. Assuming you have at least, urethane bushings in the trailing arms. It appears that your kit should have come (originally) with lower trailing arm relocation brackets, if so, are they on the car now? That right there might solve your problem, or at least give you room to experiment.

The front definitely doesn't need help. It has the Cortex SLA with their billet hubs. I am very happy with the front .
The lower arms are from Cortex and have solid attachments . However, they mount in the stock location and are stock length.
I'm considering a IRS swap or possibly a cambered rear housing. May do that over the winter . Looking for some help in the mean time.

 

[blacksheep-1]

The IRS units are very heavy, and unless you have a 03 or 04 terminator unit, which are premium priced, they are all weaker units. I've never destroyed any of the units, but I've spun a few halfshafts out of the earlier ones. If you are considering that route, Bruce at Full Tilt Boogie Racing is the guy to talk to. I'm not sure anyone makes a cambered unit for the SN95 body style and they are very pricey.

 

[302 Hi Pro]

That's when I noticed that the lower arm is already lower in the front than the rear and lowering the car would only make that worse.

This is your problem, (sprung v. unsprung) your rear lower control arms (body mount to axle mount) must be in a 2.0-3.0 degrees (max) downward slope to the rear axle. Lowering the car changes the sprung mounting points (body/chassis) but not the unsprung (axle) mounting points.

Look for RLCA relocation brackets to adjust the rearward downward slope of your RLCA’s

“It feels loose at turn in, decent through the middle of the turn and is loose on exit.”

Think about this in relationship to your rear axle movement between power off, coast & power on. Your rear axle dives downward on power off (turn in) neutral during coast & then drives upwards during power on, (turn out).

This unsettles the rear axle and causes (forgive me for this term if it’s wrong as I’m not an engineer), but causes rear axle tramp. I call it excessive vibration thru the rear axle that lifts the rear tire patch contact, this is why it feels loose & from turn to turn on the track and it feels so unpredictable.

I never get that confident feeling from the rear. - To me the rear of the car still seems a bit higher than it should be so I had planned to lower it a bit more and see what it does.

That's when I noticed that the lower arm is already lower in the front than the rear and lowering the car would only make that worse.

Yes it will only make it worse, this is why you must lower the unsprung RLCA mounting point with RLCA Relocation Brackets to maintain proper rear axle geometry. Think of it this way, the rear axle is not a fixed axle, it’s a ‘Live’ axle that rotates in an arc, (Note: Relocation Brackets will alter your Driveshaft & Pinion Angle. You must maintain a 3.0 degree downward angle from the transmission output shaft to the rear axle pinion. (AKA Pinion Angle).

Note: This is when you might have to employ an ‘adjustable’ UCA or Arm(s) in a 4 point setup, if you employ static LCA’s

“I know , from my drag racing days that lowering the rear of the lower bar helped traction quite a bit . I was curious what effect it would have in a road race setting?”

With the right Live Axle LRCA & Pinion Angle settings you will no longer feel rear axle hop while braking deep in your turn in’s & you'll blast out of turn exits as soon as you can see the next straight away.

Good Luck, Good Racing & please keep us posted.
All the best,

 

[Grant 302]

FWIW, LCA brackets aren’t typical for SN95 because of the spring perch on the control arm. Such a bracket with this arrangement would only raise the axle relative to the body, and just lower the car without changing the LCA angle.

I know , from my drag racing days that lowering the rear of the lower bar helped traction quite a bit . I was curious what effect it would have in a road race setting?
Anyone have first hand experience with this in a Fox or SN95?

I’m thinking you need to provide a few more details about your rear setup. From what I gather, you have rear coilovers and swapped to trailing arms? Do you have a rear sway bar? I see that the CorteX trailing arms ditch the goofy OEM sway bar configuration/mounts (and the spring perch).

Pictures of the existing setup and perhaps full measurements of everything would be helpful if your intent on working with what you have over the IRS swap.

 

[Ron McCall]

The car has coil overs installed. Here are a few pics .
No rear sway bar.
Thanks for the replies.




Ron

 

[Ron McCall]

Here's another view.

Ron

 

[Grant 302]

First thing I’d try is using the lower setting on the watts pivot.

I’d try raising the rear after that. Especially if you last adjustment was to two or more of these at a time.

After that, maybe consider a lower axle connection for the trailing arms.

Do you have dimensions on the torque arm? Or do you know what approximate anti-squat percentage it has?

EDIT:
If none of that seems to help, I’d consider a softer rear spring. Do you know the front and rear rates?

 

[302 Hi Pro]

This is your problem, (sprung v. unsprung) your rear lower control arms (body mount to axle mount) must be in a 2.0-3.0 degrees (max) downward slope to the rear axle. Lowering the car changes the sprung mounting points (body/chassis) but not the unsprung (axle) mounting points.

New info from @Fabman on RLCA angle: 3 to 5 degrees downward slope to the rear axle.

From what I see in the pics, it looks like you need to install RLCA Relocation Brackets at the axle end to lower the unsprung mounting point of the RLCA’s.

With that said, finalize your ride height and then go from there.

 

[67GTA]

Agree with both @302 Hi Pro and @Grant 302 on the need to lower the axle side of the trailing arm to induce more anti-squat, which is possible because of the relocated coilovers. There’s a few options, either bolt-on or weld in. Here’s an example from Kenny Brown:


Link to KB site here.

 

[Fabman]

New info from @Fabman on RLCA angle: 3 to 5 degrees downward slope to the rear axle.

From what I see in the pics, it looks like you need to install RLCA Relocation Brackets at the axle end to lower the unsprung mounting point of the RLCA’s.

With that said, finalize your ride height and then go from there.

3 to 5 degrees is per Cortex website. Your mileage may vary.
Its worth a mention, I typically measure with a full load of fuel and the drivers/passengers weight in the car so that what I adjust to in the shop is what the car sees on the track. Same with the panhard bar. I typically would set it about 3/8" high on the chassis side at full weight so that when the body rolls it brings it closer to level. The exact number depends of course on how far that particular car rolls due to springs, bars, RC height, how long the bar is, what track and so on.