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Last Post 13 Jul 2015 02:00 PM by  bullitt2655
Coil Over Conversion – How Do These Rates Sound?
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kpgubert
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13 Oct 2014 09:16 PM

    First, the car is a '94 Mustang GT convertible, 3400 lb. Have been running 275/40-17 A6s, but I would like to graduate to 315/35s sometime. I realize it's never really going to be a nationally competitive car, but I still enjoy running regional/divisional events and making it faster (driver mod required too). It's still licensed, but I rarely drive it on the street.

    Right now I have the springs in their stock (inboard) location. The fronts are 950-1050 lb/in H&Rs and the rears are 415-515 lb/in Maximum Motorsports. The front suspension workings are stock configuration with poly bushings and a Steeda bar. The rear is a MM torque arm, panhard bar, and a MM 0.120 x 1.0 in. swaybar. I have Koni single adjustable yellows on both ends.

    Even with these springs/bars the car still has a gross amount of body roll. Before next year I'm going to convert to a coil over setup (I planned on Maximum Motorsports). Based on what other similar cars are using for springs, 750 lb/in springs in the front and 450 lb/in in the rear seems like it may work well for me. Any thoughts on this combo?

     According to MM, the wheel rates are 25%/50% front/rear for the stock spring location, and 90%/110% for coil over. These are the numbers I've been referencing for calculations. 

    Any input or suggestions is/are much appreciated.

    47CP
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    14 Oct 2014 11:13 AM
    How is the balance now? Also, how low is the front of the car?

    You might be going too far. Your current wheel rates are ~250f/r and you are looking at 675 front and 495 rear. That is not only a huge change, but also quite a balance change.

    You might try to contact Dave Feighner or Marcus Merideth, they both have/had bad ass SN95 cars. I know Marcus/Jenn's car was using a 1600 or 1800 conventional front spring. I don't remember the rear rate. That car was plenty stiff and drove fine (It has a stack of National trophies, including one I won in it) Using MM's formula that is a 400-450 wheel rate in front.

    If the car is lowered too much, which almost all of them I see modified are, there isn't a spring stiff enough to fix the front. IIRC, the roll center goes 18 inches under the ground and is therefore 3 feet away from the CG and it just sucks.

    So, without knowing all the details on your car, I would supply this generic advice :

    -Front springs in the 400-450 range.
    -Add the Tall Steeda balljoints.
    -Get the ride height up.
    -Raise the rear rates the same percentage you raise the front to start, adding or subtracting 50 lbs to adjust balance if desired.

    Hope this helps. I am making a lot of assumptions that may be very far off since I haven't even seen the car. My overarching thought is that your proposed rates are too stiff for a radial tired car. I have run rates in that range before on my old CP car, but that was on the old floppy bias tires circa 2006. I don't think that will work on an A6, or even the newer design bias slicks.

    DaveW


    kpgubert
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    14 Oct 2014 07:20 PM
    Right now the rear seems to stick quite a bit more than the front. The front of the car is quite low; lower than most I've seen. It surprised me a bit, because I still have the spring isolators on the top and bottom of the springs. One of the reasons for converting to coil overs is to fix the ride height too.
    I've actually talked to Marcus a number of times, and yes, he did have 1800 lb. inboard front springs on their car. That is quite the car. I've been beat by it when Steve Munsell drove it in CP (Oscoda a couple years ago). I don't know what his rear spring rates are, and I don't know what Dave has on either end of his. I wasn't sure rear spring rates would translate to my setup due to torque arm verses Steeda 5-link. I remember when I first did the torque arm/panhard bar body roll was magnified. I wasn't sure if this would have been different with a 5-link.
    Thank you very much for mentioning ROLL CENTER. That wasn't even part of my thought process (stupid, I know). And yes, right now (iirc) my front control arms are "pointing up," so I'm sure it's driving the roll center underground. I didn't think of the Steeda ball joints either. Hopefully with those I can get better geometry without raising the CG too much.
    I still think the car pushes more than I'd like it too, but only increasing the rear rate 50 lb. over what the balance is now sounds like a reasonable starting point.
    Oh, and I didn't think about A6s verses old bias plys making a difference. Another very good point.

    Thank you very much again! I'll dig this thread up next year and report the outcome.
    47CP
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    14 Oct 2014 08:57 PM
    I think you will want a little more rear rate on a TA car versus the 5 link one, but that is not set in stone. I would think that matching the ballpark of Marcus's rear rate is a good place to start.

    As far as the push - raising the front and getting the front spring rate up should increase front grip relative to the rear. Less camber loss, better roll center, etc.

    DaveW
    kpgubert
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    16 Dec 2014 11:12 PM
    Well, my plans haven't really changed. I will be doing coil-overs and giving the X2 ball joints a try (I'm a little skeptical of how much of an effect they can have on geometry, but I will find out).
    I will be using 500 lb/in springs in front and 375 lb/in out back. Now I'm questioning if using that much higher of a rate with my Koni SAs is a good idea. I'm pretty happy with their rate with the current springs, but the planned replacements are a lot more spring. Any thoughts? Sam made a brief allusion to having Koni SAs revalved when I bought them (back in 2012), but I haven't been able to turn up anyone who does it. From what I have been finding revalving Bilsteins is common practice (as I've seen certain people do at the track!), but I can't find anyone doing this to Konis.
    47CP
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    17 Dec 2014 12:58 PM
    Call Lee Grimes at Koni, he can answer the first question and they also do the revalve.

    DaeW
    kpgubert
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    19 Dec 2014 12:45 PM
    Excellent information. I got in contact with Lee, but after the holidays I'm going to give him a call and discuss the best actions.
    I hope you have a great holiday season, and thank you very much (again)!
    bullitt2655
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    03 Jan 2015 04:09 PM

    We run a 2001 Bullitt with MM PH Bar/Trq Arm and Koni SA's.; We also use the MM front A-Arms and bump steer kit. When I talked to MM before ordering the coil over kit, they suggested 400 lb/in front and 250 lb/in rear. These seemed to work well in 2014, so I think we will stick with those rates. The car is quite well balanced for autoslalom and track. We use 275/40/17 A6's and NT01's. The A6 is the cats meow for slalom.

    kpgubert
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    07 May 2015 01:41 PM
    Well, after many more conversations, I placed the order today. 450 lb/in front springs and 300 lb/in rears. Many thanks to everyone here, Marcus Merideth, Ethan Bradbury, Lee Grimes (great resource, thanks for pointing me to him, Dave!), and Jack Hidley at Maximum Motorsports.
    I won't be having the dampers revalved at this time, but I bit the bullet and will be doing a tubular K-member. Maybe it's taking the easy way out for correcting front end geometry, but I can't wait to see the difference.
    After I can run an event with the setup I'll give another update.
    bullitt2655
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    13 Jul 2015 02:00 PM
    So how do those spring rates work for you? It seems we have similar setups and I am curious what you think.
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