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2.5" diameter rear spring rate question

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14
12
Exp. Type
Drag Strip
Exp. Level
Under 3 Years
USA
Hey Guys. I'm looking for guidance on an appropriate rear spring rate to match my 260lb front springs on a Solo course. The front spring rate is admittedly low, as I mostly street drive and am concerned that it would be too harsh if I went much higher.

I run the adjustable spring perches which use a 2.5" diameter, coil over style spring, but not coil overs.

My car weighs approximately 3900lbs w/driver and is a weekend toy that I use for casual Solo and dragstrip fun.

I currently have 10" / 200lb springs for driving and Solo.

I like the springs I have for driving, but am wondering if I should change them to something else to improve handling on the Solo course. They're quick and easy to install, so I figured I would ask.

Thanks in advance.

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179
308
Utah
On my previous car (2006 Mustang GT) I had initially mounted a 10" rear divorced spring setup (with adjusters) when I went to coilovers. I was running 275-35/18 at the time and like you found the rear to be a bit too tall.

I switched to an 8" spring of the same rate. If I remember correctly the 10" at the bottom of the adjuster compared to the 8" at the top of the adjuster resulted in a 1/4" drop. I happened to go lower than that (about 1/2" to 3/4" as I remember) so it wasn't a problem for me.

Hope that helps and that my memory is serving me (and you) well... BTW, I was on Ground Control rear adjusters, but I think most of them are similar in height. You should be able to measure your adjuster and see what the range is to get a good idea of where you will be with the 2" drop in spring height.

Nice car!
 
14
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Exp. Type
Drag Strip
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Under 3 Years
USA
Thanks Guys.

I wasn't sure if I should go down in the rear or what.

Can you tell me how high you think I would need to go in spring rates front and rear to see measurable improvement on a 43 second course? And... would it make a good thing (ride) bad on the street?

My shocks/struts are Bilstein HD's. I drive around on RT660's.

I can easily swap rear springs in/out, but the fronts, not so much. I can do it, but certainly don't want to be messing with it.

Thanks.
 
179
308
Utah
My 2006 started with the Ford Racing suspension package. I drove that suspension on track and on the street for almost 3 years before going to coil overs. I drive a road coarse and don't know much about autocross setup, but can at least tell you my experience.

I upgraded to Ground Control double adjustables, with a 450 lb front spring and 250 lb rear spring (straight rate). I would set the damping when I got to the track, then put it on full soft for street driving. Frankly it rode every bit as good or better on the than the Ford Racing setup.

It is really about the dampers being tuned to the springs. On the Ford Racing setup they were over dampened for the springs so it was generally a pretty "stiff" feeling ride where you feel every bump and contour on the road. But get into really heavy road coarse work and the spring rates eventually were too soft.

With the coil overs, the spring rates were much higher than the Ford Racing setup, but the dampers were tuned to the spring rate. So the road feel was much better actually, and the spring rates were high enough that it could handle the road course.

I went up on the front to 550 lb to balance the car on the road course, and it started to feel a little stiff for the road, but still good. All of this was with a 35mm front bar on full stiff and an 18mm rear bar off of a V6 convertible.

What springs and dampers are you running now? Are the springs straight rate or progressive? You would really want to tune the springs at this point to how the car is handling as you are pushing it. The S197 cars handle more front spring well, but I really haven't seen any except full on race cars running any higher than 250lb in the rear unless they are running really high front rates.
 
14
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Exp. Type
Drag Strip
Exp. Level
Under 3 Years
USA
Thanks Rod. It was an interesting read.

DaveW: Bilstein HD's. The fronts are BMR 260lb progressive rate. The rears are 2.5", 10" x 200lbs. I chose the rears to start with because BMR pairs a 220lb conventional rear spring (progressive) with it.

So here are my questions:

1) What is the appropriate spring rate to match my rears to front springs I have?

2a) If I were game to switch the front springs to see a handling improvement over my 260lb springs, how high would I have to go and would it be noticeably harsher?

2b) What 2.5" spring rate would pair with the spring you specify in 2a?

I did some research prior to ordering the dampers and springs and found some site pairing higher spring rates w/the Bilstein HD's. I was just hesitant on going too harsh w/spring rates when I made my purchase.

I know I'm riding a line here, just curious if more could be better without tipping the scale of making the car too harsh. I only do Solo 2-4x a year on test and tune days. Otherwise I'm just cruising on weekends, pushing for those 6/10th on back roads and the roundabouts that are everywhere around here and going to the dragstrip a few times, but that's secondary these days. Been the drag suspension route and have all of the shirts. Yuck! Ha ha.

Thanks Guys.
 

Grant 302

basic and well known psychic
Can you tell me how high you think I would need to go in spring rates front and rear to see measurable improvement on a 43 second course?
Patching in parts to ‘band-aid’ what you have now isn’t going to provide you much in the way of measurable results. Honestly, good chance you’d just make it worse from here changing spring rates. Without the ability to adjust your damping and ride height in the front, I think you’re chasing your tail. The next adjustment I would suggest if it were possible would be to raise the front…to increase your roll stiffness.
That said, without taking the step to coilovers and/or better dampers then I think working with you bar settings might help. You don’t say what settings the Whiteline bars are at (or I missed it)…so I’d start with that and look for suggestions there. Ditto for your LCA bracket settings.
might as well work with what you got first.
Going up in rates will send you down a different rabbit hole…adjustment and learning curve and budget wise. Based on you stated uses, I wouldn’t do that just yet…but that’s where I think you’d see ‘measurable improvement’ in the car.
 
14
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Exp. Type
Drag Strip
Exp. Level
Under 3 Years
USA
Grant - my main question wasn't about "patching" anything together. Good grief. Just asking if my rear spring rate was appropriate, then I went on to ask if other springs could improve things without ruining ride quality.

I'll just call a place this week.

We'll call this post: done.
 
Grant - my main question wasn't about "patching" anything together. Good grief. Just asking if my rear spring rate was appropriate, then I went on to ask if other springs could improve things without ruining ride quality.

I'll just call a place this week.

We'll call this post: done.
FWIW, you came to a racing forum and asked an open ended question with no real goal and Grant, who doesn't say things pretty, but gives damn good advice, took time to try to teach you how to even ask the correct question.


We are supposed to be nice on this forum, so I will try to help....

When you say you want to "improve" the car, what exactly is it doing wrong? If you don't know, which is an acceptable answer, then spend money on a driving school. Otherwise, we need more info? Understeer? Oversteer? Lack of forward bite?

Any good vendor is going to ask you these same questions. Most will just sell you something because you seem to want to buy something to "improve" the car....

DaveW
 
14
12
Exp. Type
Drag Strip
Exp. Level
Under 3 Years
USA
I was warned that the road race crew doesn't play well with street car or more casual guys. Guess I needed to find out for myself. So long TMO.
 

Dave_W

Cones - not just for ice cream
984
1,275
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
wondering if I should change them to something else to improve handling on the Solo course.
How does it handle now? Is there something specific you're trying to change, or just looking for a "do everything it's doing now, just faster" change? Changing rear spring rate will change the balance of the car, generally speaking rear stiffer = more towards oversteer. The inboard position of the stock spring perches don't have as much influence on roll stiffness / one-wheel bump as a "coilover" position for the same stiffness change, but affect the 2-wheel bump the same.
 

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