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Fighting high speed understeer in carousel-like turns

61
44
Exp. Type
HPDE
Exp. Level
Under 3 Years
TX
You and me both. The plan was for proper coil overs this winter, but the motor refresh wiped me out. I keep doing everything backwards as I break parts. If only I had broken the stock suspension first :)

A set of Cortext/MCS coilovers are definitely on the list for this offseason if my wife doesn't end me.
Highly suggest cortex, theirs are custom made JRI shocks (they are monotube) and JRI was started by a few ex penske engineers. They will do just about anything you want except use lower gas forces / pressure for softer total damping. They do this for liability reasons, they are not comfortable making an educated guess how low to go. Gas force directly relates to available compression at a given shaft velocity. So too little and you get cavitation and major hysteresis. Too much gas force and the "Cracking force" becomes higher, meaning you can see 50+lbs of initial force needed to see shim deflection BEFORE any meaniful damping begins (sort of like an additional 50lbs/in spring force). Roehrig calls it "Force at Zero Velocity" https://www.dropbox.com/s/kg9n7ftfe7ugrey/Force_at_Zero_Velocity_Explained.pdf?dl=0 -- I linked their paper for you, I recommend checking it out, even if you end up not caring about the technical execution it can potentially help you ask informed questions when you are ready to buy.


With the info I supplied in my long term damper project thread I spoke with Cortex / JRI, then based on your recommendation 5.2 and a few others. All agree (with regards to my targeted use case) I presented them with a valid justification on my specifications. I have not had the same experience with MCS and I have not had anyone who uses them give me much in the way of useful info.


To echo my posts on M6G, I'd address the following;

-Potential aero balance // Add to front aero, or raise front ride height to shift aero balance forward (result produced by rear wing being taken out of the airstream relative to the front)
-Potential alignment // you did post pics potentially revealing an alignment issue
-Soften front springs or ARB
-Stiffen rear springs
-Address low speed damping issues (increase total low speed damping // add LS compression front // take away low speed compression rear
 
20
7
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
-Potential aero balance // Add to front aero, or raise front ride height to shift aero balance forward (result produced by rear wing being taken out of the airstream relative to the front)
-Potential alignment // you did post pics potentially revealing an alignment issue
-Soften front springs or ARB
-Stiffen rear springs
-Address low speed damping issues (increase total low speed damping // add LS compression front // take away low speed compression rear

On the aero balance side, you're backwards. You want to lower the front and/or lift the rear. This is not a stock car with a spoiler (in which case, your advice would stand), the wing is on pylons and in the airstream no matter what you do with it. Increasing chassis rake will shift CoP forward. The wing AoA difference realized through changing the chassis rake is meaningless.

The rest of the alignment would be interesting info.

While softer front springs is the correct 'book' approach, I've found front grip by *increasing* front spring rate. His car is already rolling excessively. Reducing front ride or roll rate is going to further reduce response and increase the travel window of the front end, which is already too big.

Stiffen rear springs, for sure.

The car is likely too low on compression across the board, but your changes will tend to increase understeer, not reduce it.
 
61
44
Exp. Type
HPDE
Exp. Level
Under 3 Years
TX
On the aero balance side, you're backwards. You want to lower the front and/or lift the rear. This is not a stock car with a spoiler (in which case, your advice would stand), the wing is on pylons and in the airstream no matter what you do with it. Increasing chassis rake will shift CoP forward. The wing AoA difference realized through changing the chassis rake is meaningless.

The rest of the alignment would be interesting info.

While softer front springs is the correct 'book' approach, I've found front grip by *increasing* front spring rate. His car is already rolling excessively. Reducing front ride or roll rate is going to further reduce response and increase the travel window of the front end, which is already too big.

Stiffen rear springs, for sure.

The car is likely too low on compression across the board, but your changes will tend to increase understeer, not reduce it.

If the rear wing is not in the airstream enough to cause the drag to generate downforce, how would that result in understeer? Less total rear downforce would reduce weight + grip rear. Unless I am missing something? Maybe there is not enough aero generated on his front relative to the rear for aero balance to be affected with chassis rake, hard to say either way without numbers.

Softer or stiffer springs is scenario specific, sure you can get grip with stiffer springs, or softer springs. So it is not a right or wrong answer, should he have a matched race damper / spring combo? yes, will that solve his problem the best? Yes.

In general there is many ways to skin this cat, I offered easy things to try in lieu of spending $$ on springs and dampers.

"While softer front springs is the correct 'book' approach, I've found front grip by *increasing* front spring rate. His car is already rolling excessively. Reducing front ride or roll rate is going to further reduce response and increase the travel window of the front end, which is already too big."

OE ride rate is ~ 1.35hz front and 1.55hz rear. He has Ford Performance springs which raises front ride rate more than the rear relative to one another which IMO is the main cause to the understeer with the combination of the rear wing. If we both agree he has little to no aero in the front, then wouldn't additional mechanical grip be warranted? In this case I would raise the spring rates in total while adding more to the rear relative to the front. Targeting say 2hz front and 2.2hz rear as an example. Ultimately I do agree a spring change will have the greatest effect on his total roll, followed by damper, then ARB. He could easily be near 55% front roll couple with softer front springs (again relative to the rear), of course its mainly up to the driver how tight they want their car. So I wasn't trying to assume too much, nor cause paralysis by analysis.
 
20
7
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
I'll try to address these in order.

  1. The rear wing is on pylons and is relatively high in the airstream. It will be getting clear air regardless of the rake of the car. Stock cars can do the trick you're talking about because spoilers are relatively low in the airstream to begin. Further, the rake angle of the chassis will influence the aero forces acting on the rest of the car, particularly the nose and the body. Even though the flow will be disorganized (because it's an OEM floor, not flat bodywork), it will still be effective at providing downforce, especially with the wheel well and hood venting on the car. The front of the car will be relatively sensitive to ride height and the rear will be relatively insensitive. Lowering the front will produce more overall downforce and increase the front percentage. Lowering the rear will increase downforce slightly while shifting the balance to the rear.

    I can't say that I agree he has little aero on the front of the car. I have no idea how much aero is on the front. It appears that is a pretty healthy rear wing, so we probably have an imbalance. In the best of worlds, we can add to the front as opposed to reducing the rear. Another option is to shift weight to the rear of the car, which has the same effect. Is the battery in the back?

  2. I advised a stiffer spring package because the pictures show the chassis is really moving around. I choose springs more to address the response and feel of the car rather than to create some sort of 'perfect' roll couple distribution. A stiffer front spring will allow the driver to load the front of the car harder in the early phases of the corner. Because of this, what we calculate as an increase in understeer might actually produce the response characteristics which allow our driver to load the front in a manner which reduces understeer. A soft front end rarely feels good under heavy braking or on turn-in.

  3. The things you recommended were reasonable, but many were directionally opposite to what I would try.

  4. The Ford performance springs are quite soft for the racetrack and I find the F/R split to be wrong. For a car like this, I would tend to run the rear ride frequency about 85-90% that of the front. So, let's say we're running 2 Hz on the front, I'd shoot for 1.7-1.8 on the rear. Yes, I know this produces understeer *on paper*. On the track, it doesn't. The reason being that it allows our driver to approach the corner with much more aggression. Once we give the rear of the car the appropriate level of stability, the driver will be able to drive much more on the nose of the car. He can load the nose on the brakes and turn the wheel agressively. There's no more of this early, light, fingertip turn-ins where you get to the apex and still have 70% of the turning to do. Now you can turn in late and hard, which will allow you to arrive to the apex with a much better angle and with much less turning to do on the power side of the corner. The car generally doesn't turn that great on throttle, so we need to not ask it to do so.

  5. Roll couple is a very subjective thing. Often the best roll couple for lap time will not be the best roll couple for balance. When I'm tuning a GT car, I shoot for 2 things. A rear end that is perfectly stable on entry so the driver can brake hard and carry the brakes into the corner as far as necessary to keep weight forward and allow the car to turn. On corner exit, I want my driver to be able to plant his foot and still put power down. I don't want a slow squeeze to full throttle. O-100% throttle should happen in about 1 second. That can only happen if the driver can get the car turned early and is unwinding the steering. It's fair to mention that if I have good rear grip on both entry and exit that the car will understeer in the middle of the corner. That's fine. I'll gladly accept a lower corner minimum speed if it allows me to brake 0.1 second later and roll to throttle 0.1 second sooner. Lap time is not found in corner minimum speeds unless we're on an oval.

    Springs don't have a massive effect on roll couple, but ARB's do. Oddly, springs are more effective as shifting the handling balance. We can often make relatively large ARB changes, which you'd suspect on paper would be massive, but in reality, the change might be almost meaningless. There are many other ways of shifting handling balance. For this particular corner, just putting 25-50# of crossweight on the FR tire will have a surprisingly strong effect. It also might help the car get through the 2 longer duration right-handers in the esses as well as improving power down out of the slower corners. We don't have to do everything with springs/bars.

    ---------------------------------------------------------------------------------------------------
This should explain my position a little better.
 
61
44
Exp. Type
HPDE
Exp. Level
Under 3 Years
TX
I'll try to address these in order.

  1. The rear wing is on pylons and is relatively high in the airstream. It will be getting clear air regardless of the rake of the car. Stock cars can do the trick you're talking about because spoilers are relatively low in the airstream to begin. Further, the rake angle of the chassis will influence the aero forces acting on the rest of the car, particularly the nose and the body. Even though the flow will be disorganized (because it's an OEM floor, not flat bodywork), it will still be effective at providing downforce, especially with the wheel well and hood venting on the car. The front of the car will be relatively sensitive to ride height and the rear will be relatively insensitive. Lowering the front will produce more overall downforce and increase the front percentage. Lowering the rear will increase downforce slightly while shifting the balance to the rear.

    I can't say that I agree he has little aero on the front of the car. I have no idea how much aero is on the front. It appears that is a pretty healthy rear wing, so we probably have an imbalance. In the best of worlds, we can add to the front as opposed to reducing the rear. Another option is to shift weight to the rear of the car, which has the same effect. Is the battery in the back?

  2. I advised a stiffer spring package because the pictures show the chassis is really moving around. I choose springs more to address the response and feel of the car rather than to create some sort of 'perfect' roll couple distribution. A stiffer front spring will allow the driver to load the front of the car harder in the early phases of the corner. Because of this, what we calculate as an increase in understeer might actually produce the response characteristics which allow our driver to load the front in a manner which reduces understeer. A soft front end rarely feels good under heavy braking or on turn-in.

  3. The things you recommended were reasonable, but many were directionally opposite to what I would try.

  4. The Ford performance springs are quite soft for the racetrack and I find the F/R split to be wrong. For a car like this, I would tend to run the rear ride frequency about 85-90% that of the front. So, let's say we're running 2 Hz on the front, I'd shoot for 1.7-1.8 on the rear. Yes, I know this produces understeer *on paper*. On the track, it doesn't. The reason being that it allows our driver to approach the corner with much more aggression. Once we give the rear of the car the appropriate level of stability, the driver will be able to drive much more on the nose of the car. He can load the nose on the brakes and turn the wheel agressively. There's no more of this early, light, fingertip turn-ins where you get to the apex and still have 70% of the turning to do. Now you can turn in late and hard, which will allow you to arrive to the apex with a much better angle and with much less turning to do on the power side of the corner. The car generally doesn't turn that great on throttle, so we need to not ask it to do so.

  5. Roll couple is a very subjective thing. Often the best roll couple for lap time will not be the best roll couple for balance. When I'm tuning a GT car, I shoot for 2 things. A rear end that is perfectly stable on entry so the driver can brake hard and carry the brakes into the corner as far as necessary to keep weight forward and allow the car to turn. On corner exit, I want my driver to be able to plant his foot and still put power down. I don't want a slow squeeze to full throttle. O-100% throttle should happen in about 1 second. That can only happen if the driver can get the car turned early and is unwinding the steering. It's fair to mention that if I have good rear grip on both entry and exit that the car will understeer in the middle of the corner. That's fine. I'll gladly accept a lower corner minimum speed if it allows me to brake 0.1 second later and roll to throttle 0.1 second sooner. Lap time is not found in corner minimum speeds unless we're on an oval.

    Springs don't have a massive effect on roll couple, but ARB's do. Oddly, springs are more effective as shifting the handling balance. We can often make relatively large ARB changes, which you'd suspect on paper would be massive, but in reality, the change might be almost meaningless. There are many other ways of shifting handling balance. For this particular corner, just putting 25-50# of crossweight on the FR tire will have a surprisingly strong effect. It also might help the car get through the 2 longer duration right-handers in the esses as well as improving power down out of the slower corners. We don't have to do everything with springs/bars.

    ---------------------------------------------------------------------------------------------------
This should explain my position a little better.

I've yet to tackle aero as much as the other aspects of the chassis. So I appreciate you taking the time. I agree with the stiffer springs, I maybe should have been more specific in my hierarchy of spring>damper>ARB. If we are talking about minimum corner speed only, I agree with you. With regard to US/OS balance I understand your approach, personally I like a little bit of flat ride and so I am biased towards that (I also have not personally run a car with any added aero). I would also agree RC is very subjective, as are ride rates since driving style can affect what those ultimately are. Of course after you figure out what the optimal setup for tyre compliance vs lap times are.

I am building and valving my own dampers, and plan on experimenting with different setup changes to validate what changes mean what for this chassis in particular. This is good info and I will add it to the list of things to experiment with.
 
20
7
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
personally I like a little bit of flat ride and so I am biased towards that

I know many people are concerned about the flat-ride characteristics when choosing springs, but, for a race car, I personally have never been able to have any success with this type of spring lash-up. Remember, this is a Maurice Olley concept. Olley was an absolute genius, but he lived in a time when dampers were almost non-existent and big, soft bushings ruled the day. It was much more important to have the car naturally bounce in a heave-only direction because he couldn't effectively control the motion any other way.
 

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