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What change should I expect from increasing front spring rate?

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PatientZero

@restless_performance
824
866
Exp. Type
Autocross
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10-20 Years
Kansas City, Missouri
A couple weekends ago I got the chance to drive another Mustang prepped VERY similiar to mine. Same Maximum Motorsports suspension, same size wheels/tires, torque arm, same sway bars. The main difference was slightly different spring rates. I have 425F/275R. He had 450F/250R. While driving his car (on an autocross course), the most noticeable different was mid corner grip. Basically at the same speed and slip angle(think long 50mph sweeper) my car is right on the edge of grip but his car had enough grip to actually turn into the corner. I'm wondering if this is just in the spring rate difference or if it's alignment related. I ordered a set of 450lb springs to test out and see what happens but I assumed with the collective knowledge of the forum someone would be able to tell me what I could reasonably expect.

To add a little context to this. I originally had 425F/325R with a Steeda front sway bar and 1" rear sway bar. After talking to Jack Hindley at Maximum Motorsports he suggested I go down to 275 on the rear but with a 1 1/8" sway bar. He also had me switch to a stiffer Eibach bar on the front instead of the Steeda. His reasoning was to improve my roll couple percentage on the rear. These changes made a massive improvement. Would adding front spring rate now offset this change and make it worse?
 
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Dave_W

Cones - not just for ice cream
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1,326
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Connecticut
Just on the spring rates, your F:R ratio is 1.55:1 while the other car is 1.8:1. So with your 275# rear spring, you'd need a 500# front (actually 495, but close enough) to have the same ratio.

That said, did both cars have the same balance, or was yours more prone to oversteer? Especially if both had the same balance, I'd look to tire age and alignment settings. If it was a bumpy surface, shock settings as well.
 

PatientZero

@restless_performance
824
866
Exp. Type
Autocross
Exp. Level
10-20 Years
Kansas City, Missouri
Just on the spring rates, your F:R ratio is 1.55:1 while the other car is 1.8:1. So with your 275# rear spring, you'd need a 500# front (actually 495, but close enough) to have the same ratio.

That said, did both cars have the same balance, or was yours more prone to oversteer? Especially if both had the same balance, I'd look to tire age and alignment settings. If it was a bumpy surface, shock settings as well.
I felt like both cars had pretty similar balance. I was able to jump in that car and go 9/10's straight away because it felt about the same. The only noticeable difference was the mid corner front grip. I felt like I could have pushed that car much more without understeer. On mine I have to coax the back end around with the throttle or it will push wide.

Do you think it will help one way or the other? It seems like I have more body roll than I should just based on pictures. I've plugged rough numbers into a couple suspension calculator and they all seem to put me at a 450-475lb front spring. I'm not sure why Jack had me keep the 425's although he knows more about these cars than I ever will.

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Just curious, do you have the steeda 2x ball joints?
I was running limited rules and they made a massive difference in front grip.
You might try that first, if you haven't
 

Dave_W

Cones - not just for ice cream
1,016
1,326
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
All other things being equal, the car with a higher front spring rate should tend toward more understeer. Because you feel your car has more understeer, with softer front springs, I'm trying to find anywhere else there may be differences. If the sway bars were the same, were they using the same holes if adjustable? As I said above, alignment, tire age & psi, maybe shock settings, etc. There is the possibility that the stiffer front springs reduced body roll and kept the front tires more upright, for a flatter contact patch.
 

PatientZero

@restless_performance
824
866
Exp. Type
Autocross
Exp. Level
10-20 Years
Kansas City, Missouri
Just curious, do you have the steeda 2x ball joints?
I was running limited rules and they made a massive difference in front grip.
You might try that first, if you haven't
I don't. I'm running the Maximum Motorsports K-member and control arms. The control arms are in the top hole of the k-member, I'd assume that has the same effect as the ball joints. I need to confirm with MM if the Steeda ball joints will fit their control arms.
 

PatientZero

@restless_performance
824
866
Exp. Type
Autocross
Exp. Level
10-20 Years
Kansas City, Missouri
All other things being equal, the car with a higher front spring rate should tend toward more understeer. Because you feel your car has more understeer, with softer front springs, I'm trying to find anywhere else there may be differences. If the sway bars were the same, were they using the same holes if adjustable? As I said above, alignment, tire age & psi, maybe shock settings, etc. There is the possibility that the stiffer front springs reduced body roll and kept the front tires more upright, for a flatter contact patch.
Front sway bars are the same and not adjustable. Rear sway bars are the same, his was in hole 4/4, mine is in 3/4. 4 being the stiffest. I think I started out in 4 when I originally installed it but then backed it up one hole.

Just to muddy this up even more. I discovered at lease one of my front struts is dead today while changing the springs.
 
6,467
8,396
I don't. I'm running the Maximum Motorsports K-member and control arms. The control arms are in the top hole of the k-member, I'd assume that has the same effect as the ball joints. I need to confirm with MM if the Steeda ball joints will fit their control arms.
It may not be necessary then, I would think that MM would have factored that into their design, but when we had to run stock stuff, those ball joints were nagical in what they did to rhe car
 
Front sway bars are the same and not adjustable. Rear sway bars are the same, his was in hole 4/4, mine is in 3/4. 4 being the stiffest. I think I started out in 4 when I originally installed it but then backed it up one hole.

Just to muddy this up even more. I discovered at lease one of my front struts is dead today while changing the springs.
The dead strut isn't helping. My guess is that he's getting a little higher overall springrate front and rear with the higher front spring and rear bar on a stiffer setting and the overall balance is leading the car to be a little more rear biased which is letting it rotate better mid-corner.
 
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364
Front sway bars are the same and not adjustable. Rear sway bars are the same, his was in hole 4/4, mine is in 3/4. 4 being the stiffest. I think I started out in 4 when I originally installed it but then backed it up one hole.

Just to muddy this up even more. I discovered at lease one of my front struts is dead today while changing the springs.
I don't think that slow return automatically means it's blown. Monotubes are gas pressurized, twin tubes not so much so if at all.

 

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