Coilover Spring Rate question (IRS)

[medrick]

Ok so ive got a couple questions. I bought a set of Bilstein specific MM coilovers for the front of my Mach1 with 250lb springs and then found a set of Bilstein specific MM rear coilovers with 500lb springs.

1st question, is 500lb a good spring rate for autox/roadracing?
2nd question, what should I put on the fonts to match the rears, 500 as well or is it a ratio for fronts to back?

Thanks!

 

[Teej281]

Ok so ive got a couple questions. I bought a set of Bilstein specific MM coilovers for the front of my Mach1 with 250lb springs and then found a set of Bilstein specific MM rear coilovers with 500lb springs.

1st question, is 500lb a good spring rate for autox/roadracing?
2nd question, what should I put on the fonts to match the rears, 500 as well or is it a ratio for fronts to back?

Thanks!

Taken right from Maximum Motorsports website:

Q. How do I compare a Mustang front coil-over spring rate to a conventional Mustang spring in the stock location?

A. The spring rates must be converted into wheel rates. The wheel rate is the spring rate measured at the wheel. The conversion is done through the use of what's called the motion ratio. For a stock Fox Mustang front suspension, the wheel rate is 25% of the stock location spring's rate. For example, an 800 lb/in stock location spring has a wheel rate of 200 lb/in. For a coil-over suspension, the wheel rate is approximately 90% of the coil-over spring's rate. For example, a front coil-over spring rated at 350 lb/in would result in a wheel rate of about 315 lbs/in. Note: The exact motion ratio conversion of the wheel rate for a front coil-over suspension is determined by the angle of the strut. That angle is the result of the camber and caster settings, and by the SAI (Steering Axis Inclination).

Q. How do I compare a Mustang rear coil-over spring rate to a conventional spring in the stock location on my independent rear suspension (IRS) equipped Mustang?

A. The wheel rate for the rear of a Mustang with IRS is approximately 33% of the spring rate in the stock location. For example, a 600 lb/in stock location spring has a wheel rate of 198 lb/in. For a coil-over suspension, the wheel rate is approximately 50% of the coil-over spring rate.

I'm personally running 325lb front springs and 525lb rear springs. I'd say what you got is probably decent for road racing/auto-x. I'd be running stiffer springs but I couldnt get the dampers necessary to run the higher spring rates. Soon though. I'd stick with what you got with the stock shocks and struts though. Maybe go to the spring ratings that I have. BTW, you have to get the shocks grooved to accept the coilover kit. Not the struts though, just the shocks.

BTW, my MM coilovers aided in me getting 5th at the auto-x at Mustang Week in Myrtle Beach...in my IRS-GT with minor bolt-on's. Best of luck! :beer:

 

[medrick]

shocks are already grooved, the guy bought them all together new from MM. I read that but it still doesnt answer my question of whether the front and rear spring rates should be the same or what the ratio from front to back should be.

thanks

 

[Teej281]

There is no setup that says you have to run this setup for that kind of driving. It's all personal preference. Id run stiffer springs though. If you have the bilstein hd's, max that MM recommends is 325lb front springs and 525 rear springs. I'd usually say that you want around 200lb more spring rate in the rear. Also would help if you knew what the weight distribution and all that was. I mean you want a little more wheel rate up front than out back because these cars are nose heavy. I'd say as a rule of thumb, keep the front spring rating right around 200lb less than the rear with the IRS.

 

[ac427cobra]

It sort of depends on how much your car weighs, how competetive you want to be at your auto-x's and open track events and how much you are willing to compromise your cushy street ride you have now.

450# front and a 550# rear is going to give you a pretty competetive car at the track but the street ride is going to be compromised quite a bit. You also need shocks/struts to match those spring rates.

400-425# front and 500-525# rear will get you a better street ride but make the car less competetive at the a-x course or road course. Of course shocks and struts need to be matched to those rates.

FWIW

:thumbsup::coolman::beer:

 

[Teej281]

Bruce, can you school me on why you need all that spring in the front but not in the rear? My car with 325/525 springs feels incredibly balanced and I feel as if you add a lot more spring up front that you would make the car feel unbalanced, like the rear would be lacking at that point.

I'm always eager to learn about suspension, and if my setup would need reworking to be better, I'd like to improve upon it when I can.

 

[medrick]

It sort of depends on how much your car weighs, how competetive you want to be at your auto-x's and open track events and how much you are willing to compromise your cushy street ride you have now.

450# front and a 550# rear is going to give you a pretty competetive car at the track but the street ride is going to be compromised quite a bit. You also need shocks/struts to match those spring rates.

400-425# front and 500-525# rear will get you a better street ride but make the car less competetive at the a-x course or road course. Of course shocks and struts need to be matched to those rates.

FWIW

:thumbsup::coolman::beer:

interesting. thanks for the info.

so let me ask you this, is 250 in the front way too soft for street driving? and if not would it be safe to run that with 500 out back on the streets and when i go to the tracks swap in some 400 in the front?

 

[Andy M]

It sort of depends on how much your car weighs, how competetive you want to be at your auto-x's and open track events and how much you are willing to compromise your cushy street ride you have now.

450# front and a 550# rear is going to give you a pretty competetive car at the track but the street ride is going to be compromised quite a bit. You also need shocks/struts to match those spring rates.

400-425# front and 500-525# rear will get you a better street ride but make the car less competetive at the a-x course or road course. Of course shocks and struts need to be matched to those rates.

FWIW

:thumbsup::coolman::beer:

Agree with Bruce on this. My Terminator has the complete Maximum Motorsports coilover set up and I have 450lb springs in front and 650's in the rear. Keep in mind my car is set up for the maximum track experience as I do not drive my car much on the street and even when I do, I am ok with the degraded ride quality as my car handles like a dream on the track. I also have the full complement of delrin bushings on my car as well, which further adds to the track experience.

I would say if you want a car which handles good on track but you drive your car more on the street, I would go for the 400/500 set up, and as Bruce recommends, with the matching shocks/struts from a valving standpoint.

 

[Andy M]

FYI: in case anyone is interested... here is the suspension set up on my Terminator. Again, this setup is for the MAXIMUM level of grip and performance on track. :banana: It handles much better than my old Cobra R with stock spring suspension. Feels alot lighter and turns in much better!epper:

FRONT SUSPENSION:
BILSTEIN FRONT STRUTS RACE VALVED
MAXIMUM MOTORSPORTS FRONT COIL-OVERS
HYPERCOIL SPRINGS 450LB/IN
MAXIMUM MOTORSPORTS FRONT CONTROL ARMS WITH DELRIN BUSHINGS
MAXIMUM MOTORSPORTS SOLID STEERING SHAFT AND URETHANE RACK BUSHINGS
MAXIMUM MOTORSPORTS CASTER/CAMBER PLATES
MAXIMUM MOTORSPORTS ADJUSTABLE TIE ROD ENDS
STEEDA 35MM TUBULAR SWAY BAR

REAR SUSPENSION:
K-MEMBER AND UPPER/LOWER CONTROL ARMS PAINTED SILVER AND CLEAR COATED
MAXIMUM MOTORSPORT REAR K-MEMBER URETHANE BUSHINGS
KENNY BROWN DIFFERENTIAL BUSHINGS
BILSTEIN REAR SHOCKS WITH RACE SPEC-2 VALVING
MAXIMUM MOTORSPORTS REAR COIL-OVERS
HYPERCOIL SPRINGS 650LB/IN
KENNY BROWN IRS INNER TIE ROD STEERING KIT
MAXIMUM MOTORSPORT OUTER TIE ROD KIT
CUSTOM REAR SWAY BAR END LINKS
DELRIN UPPER AND LOWER CONTROL ARM BUSHINGS

 

[03 DSG Snake]

I would call Maximum and ask what they recommend for what you want to do.

They set me up with 375/575 and S1 Bilsteins.

 

[ac427cobra]

Bruce, can you school me on why you need all that spring in the front but not in the rear? My car with 325/525 springs feels incredibly balanced and I feel as if you add a lot more spring up front that you would make the car feel unbalanced, like the rear would be lacking at that point.

I'm always eager to learn about suspension, and if my setup would need reworking to be better, I'd like to improve upon it when I can.

The front and rear spring rates are different for a couple of reasons. Your engine is up front and with a Terminator your F/R weight distribution is 57/43. The leverage points for the front and rear are not identical that is also why the rates are not similar.

If you have any photos of your car on track I bet you'd see quite a lean in the front. That's not a bad thing unless you're a Track Ho looking for maximum handling and speed at the track.

Your car feels incredibly balanced because it's much better than it was on stock suspension and you feel a huge improvement.

interesting. thanks for the info.

so let me ask you this, is 250 in the front way too soft for street driving? and if not would it be safe to run that with 500 out back on the streets and when i go to the tracks swap in some 400 in the front?

250 in the front is fairly close to what you had in the stock confuguration. You still have a nice street ride along with adjustability. Hopefully you don't have a 500 in the rear with those 250's up front? :dw:

I would not change springs going back and forth to the track. With coil-overs, you should be corner balancing the car any time changes are made. That seems like a lot of work to me!? Get your car set-up so it's not past the breaking point (ride comfort) on the street yet still can hold its own at the track.

Agree with Bruce on this. My Terminator has the complete Maximum Motorsports coilover set up and I have 450lb springs in front and 650's in the rear.

MM has always recommended a bit higher rate spring for the rear that I have not always agreed with. I like the stiff spring in the front with a slightly lighter spring in the rear. This makes the car a little more loose, and I like a loose car. Particularly if it has an IRS. IMHO loose is fast. And I'm not the only one who says that! :-D

Everyone has a different driving style and a different comfort level as to what feels good to them. I love how the IRS in my Cobra R handles. I'm VERY comfortable in it. I can drive it right on the ragged edge and even over the edge by quite a bit with complete confidence. I wish I could say that for my new Boss. I'm not comfortable in the Boss yet. It feels tight and when it does give, it's not the same kind of give I'm used to with an IRS.


:thumbsup::coolman::beer:

 

[Andy M]

Agree 100% with Bruce on this. It really comes down to your driving style. My car understeers probably more than a guy like Bruce prefers, however, it fits my particular style and skill level perfectly. With my current set up, my car feels very neutral (at least to me) AND it is not loose out back, unless I press the happy pedal way down of course - in which case I can break the tires loose easily.

Bottom line, I would stay away from the stiff springs unless you plan on doing a fair amount of track and you don't mind giving up some ride quality.

Guys, Bruce is an expert on our IRS equipped cars, so I would definitely listen up when he makes a recommendation!:rockon:

 

[Teej281]

Now Bruce, hear me out on this. Doing some math here and you had mentioned that the weight balance is 57/43. So would you also want your wheel rate at that same percentage to give it the correct balance? If so, you would want a combo of 450/625 for the ideal balance of a stock weight cobra in the realms of spring rates good for handling. Does that make any sense? Am I completely off my block? And I have an IRS-GT so the weight balance is probably closer to 55/45 or 54/46. I need to weigh my car and have it corner-weighted and balanced as well as aligned. But I first want to get my cage put in and my new front suspension.

Edit: Just looked up the weight distribution for a GT and it was apparently 55/45 factory so my WD should be about 53.5/46.5 with an IRS in the back.

 

[ac427cobra]

Now Bruce, hear me out on this. Doing some math here and you had mentioned that the weight balance is 57/43. So would you also want your wheel rate at that same percentage to give it the correct balance? If so, you would want a combo of 450/625 for the ideal balance of a stock weight cobra in the realms of spring rates good for handling. Does that make any sense? Am I completely off my block? And I have an IRS-GT so the weight balance is probably closer to 55/45 or 54/46. I need to weigh my car and have it corner-weighted and balanced as well as aligned. But I first want to get my cage put in and my new front suspension.

Edit: Just looked up the weight distribution for a GT and it was apparently 55/45 factory so my WD should be about 53.5/46.5 with an IRS in the back.

Teej:

I think you skimmed over part of my post. I've highlighted the pertinent information:

The front and rear spring rates are different for a couple of reasons. The leverage points for the front and rear are not identical that is also why the rates are not similar. :

This means from the pivot points of the control arm to the connection point of the coil-over is not the same front to rear. This makes the rates completely differerent from front to rear.

I don't have a Terminator, I have a NA Cobra R. It does not have all of the additional weight of a Terminator with the S/C and all of the related goods that come along with it. It now sports an AL Ford GT block as well. I have not changed the spring rates. (yet) I may at some point but the car is enjoying semi-retirement at this time.

The car has been corner balanced after the AL block motor install but I don't have the numbers in front of me.

To sum this up, if you want a kick ass track car, I would not go less than 425 on the front or more than 575 on the rear.

 

[LS-WHAT]

not to steal the tread but i have no clue about the suspension stuff and im trying to learn cause i know my car drives and handles like shi...............t! but i have c/o up front 14-175 w/stock struts and h&r race springs out back that are rated 770 linear. my question is im changing my fronts to a 375+w/ diff. struts but what about the rear spring being 770 linear? what dosr this linear mean? and sould i change the spring rate in the rear

 

[Teej281]

I don't think that you understand what I'm saying. I'm going off of wheel rates based off of calculations from the pivot points and spring location. I'm sure that you know a rule of thumb that front coilovers run about 90% of the spring rate and for the rear of the IRS, you get about 50% of the spring rate. The rates you post would put the wheel rate percentages right at 57/43.

425*.9= 382.5
575*.5= 287.5
____________
670

Distribution of wheel rate puts it at 57/43. Does that make sense? Im trying to numerically evaluate this and try to wrap my head around it. Not challenging you whatsoever. Just trying to comprehend it.

Teej:

I think you skimmed over part of my post. I've highlighted the pertinent information:



This means from the pivot points of the control arm to the connection point of the coil-over is not the same front to rear. This makes the rates completely differerent from front to rear.

I don't have a Terminator, I have a NA Cobra R. It does not have all of the additional weight of a Terminator with the S/C and all of the related goods that come along with it. It now sports an AL Ford GT block as well. I have not changed the spring rates. (yet) I may at some point but the car is enjoying semi-retirement at this time.

The car has been corner balanced after the AL block motor install but I don't have the numbers in front of me.

To sum this up, if you want a kick ass track car, I would not go less than 425 on the front or more than 575 on the rear.

 

[ac427cobra]

Teej:

I never calculated the wheel rates, or even cared for that matter. Calculations worked out on paper don't always work out in the field. In my case, I was looking for maximum handling.

My car ended up 450 front and 550 rear. My rear was a tick loose and instead of softening the rear more, because it was already too soft for the dip at the apex at Gingerman in turn 11, I decided to stiffen the front sway bar instead. That was just the trick I was looking for. This was all done with my SOTP dyno and a lap timer. I do my own corner balancing, bumpsteer and alignments. A lot can be done for handling with alignments as well.

Everyone likes a different feel in their car. I had the opportunity for Joe Foster to drive my car at Road America when he was the guest speaker for the Windy City BMW instructors (at one of the events) and he liked it. So it can't be too bad! :-D

:thumbsup::coolman::beer: