Car is unsettled under heavy high speed braking

[Earlj]

I am experiencing a loose feeling in the rear when I brake hard at high speed like when I am entering the bus stop at the Glen. It seems the rear of the car starts moving around quite a bit and if i go in too deep it wants to come around on me.

It does not seem to do at under 100 MPH entry speeds.

I have all BMR and full tilt boogy suspension upgrades in the rear, JRI coil overs, Eibach sway bars, and NTO1 305 squared.

I have zero toe at the front and rear. 2.5 camber in the front and 1.5 in the rear

The car works real well everywhere except in the high speed, heavy braking areas.

I ws wondering if anyone else is experiencing this.

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[Eritas]

I am experiencing a loose feeling in the rear when I brake hard at high speed like when I am entering the bus stop at the Glen. It seems the rear of the car starts moving around quite a bit and if i go in too deep it wants to come around on me.

It does not seem to do at under 100 MPH entry speeds.

I have all BMR and full tilt boogy suspension upgrades in the rear, JRI coil overs, Eibach sway bars, and NTO1 305 squared.

I have zero toe at the front and rear. 2.5 camber in the front and 1.5 in the rear

The car works real well everywhere except in the high speed, heavy braking areas.

I ws wondering if anyone else is experiencing this.

There's your problem. Add toe-in at the rear, and add a GT350TP spoiler or a proper wing.

 

[BmacIL]

There's your problem. Add toe-in at the rear, and add a GT350TP spoiler or a proper wing.

This.

0.10 deg per side.

 

[Norm Peterson]

Agreed.

Makes me wonder if the problem is rear bump steer or rear compliance steer though. Be nice to know which.


Norm

 

[Eritas]

Why is it necessarily a problem? I don't think any GT, Prototype or Formula car runs 0 static rear toe.

 

[BmacIL]

Agreed.

Makes me wonder if the problem is rear bump steer or rear compliance steer though. Be nice to know which.


Norm

It depends on how much he's lowered. Lowered below ~0.6" from stock in the rear, there is all sorts going on. Roll center migration, bump toe, reduced camber gain (IIRC).

 

[Earlj]

It depends on how much he's lowered. Lowered below ~0.6" from stock in the rear, there is all sorts going on. Roll center migration, bump toe, reduced camber gain (IIRC).

I kept the ride height stock for that very reason. I have raced ovals for years and delt with bump steer and roll centers a lot.

I am going to check the bump steer both front and rear.

I am running a fairly soft set up 450 springs in the front and 500 in the rear. Iam also on the softer side of my shock dampening setting.

I had Filip at Cortex engineering provide the shock and spring setup.

I am thinking that i might be experiencing more dive in the front that i should have.
When i bought the car i did the fluids and took it to the Glen, and it did it then as well but much worse. I had the complete rear suspension out to replace all the bushings and links and made sure the cradle is square and centered in the chassis.

Thank you for all the input, i appreciate it.

 

[BmacIL]

I kept the ride height stock for that very reason. I have raced ovals for years and delt with bump steer and roll centers a lot.

I am going to check the bump steer both front and rear.

I am running a fairly soft set up 450 springs in the front and 500 in the rear. Iam also on the softer side of my shock dampening setting.

I had Filip at Cortex engineering provide the shock and spring setup.

I am thinking that i might be experiencing more dive in the front that i should have.
When i bought the car i did the fluids and took it to the Glen, and it did it then as well but much worse. I had the complete rear suspension out to replace all the bushings and links and made sure the cradle is square and centered in the chassis.

Thank you for all the input, i appreciate it.

FYI 450 isn't soft for the front, quite stiff on the contrary. 500 rear springs at the rear shock position puts you at around 280 lb/in wheel rate, which is much, much lower than the front.

 

[Earlj]

FYI 450 isn't soft for the front, quite stiff on the contrary. 500 rear springs at the rear shock position puts you at around 280 lb/in wheel rate, which is much, much lower than the front.

This is the softest set up Cortex suggested for track use.

 

[BmacIL]

This is the softest set up Cortex suggested for track use.

I'm kinda perplexed by that, but ok. It's much more of an understeer balance than stock.

 

[Eritas]

Did cortex also recommend the 0 rear toe?

 

[Earlj]

Did cortex also recommend the 0 rear toe?

No, they did not give me set up info.

 

[Norm Peterson]

Why is it necessarily a problem? I don't think any GT, Prototype or Formula car runs 0 static rear toe.

Any rear instability is a problem (one possible exception being autocross). It's fine to know that x amount of static toe-in fixes it, but knowing which effect(s) the little static toe-in is compensating for would make for a better understanding. IOW, "this is the effect you're fixing and that's why you're doing it".


Norm

 

[Eritas]

Any rear instability is a problem (one possible exception being autocross). It's fine to know that x amount of static toe-in fixes it, but knowing which effect(s) the little static toe-in is compensating for would make for a better understanding. IOW, "this is the effect you're fixing and that's why you're doing it".


Norm

I think we agree that rear toe-in has a stabilizing effect, which is why the S197 Mustang is not known to be a stable car under braking due to its stick axle and zero toe.

The S550s rear toe link is ahead of the wheel centerline and at stock ride height, the arms are almost parallel with the ground and just slightly angled downward from the chassis to the hub, this means the car toes-in under droop and barely toes-out initially before toeing-in under compression. Due to the angle of the arm, I wouldn't be surprised if there was no measured initial toe-out under compression.

Depending on how much Earl lowered his car with 0 rear static toe, it's possible his car is toeing-out under droop travel under braking. Even if he didn't lower it that far, he will still be in the arc of the toe link curve to not be getting toe-in under braking anywhere near as early in the droop travel if at all.

Again, his "problem" of a lack of rear stability under braking is due to a lack of stabilizing rear toe-in.

 

[Norm Peterson]

Everybody is in agreement about the toe not being what it needs to be, but not necessarily about why it needs to be statically 'in'. Knowing why is something you can take with you to help understand other cars better.


On stick axle cars in general, rear camber and rear toe do not necessarily have to be zero even straight off the production line. You can assume that camber and toe changes are zero, though.

On the S197 in particular, compliance in the big hydro-bushing at the rear of each front control arm can result in amazing amounts of wheel movement under braking, especially after they've been in service for a while. And of course, any S197 with rear toe out is likely to be even more nervous under braking.


Norm

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