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Old 10-11-2018, 11:39 AM   #1
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Rear Spring Rate for Mk1 Foci Track only

So I am looking at some custom springs for my car. Previously I had Koni springs on there and I would like a bit lower but having a hard time finding a 9" spring. Seems like 8" ot 10" are standard lengths. And dont have coil overs... yet....

So to go with a 10" spring a 250 lb/in spring will put me 1/4" lower than the Koni Yellow Sport kit or go with a 300 lb/in and stay the same heigth. (i have worked out the math...)

I read somewhere that a few people are running as high as 450 to 550 lb/in springs on the rear.

I am trying to figure out a good rate and spring lenth that will get me a bit lower and also a good starting point to further my track car build.

Any input would be appreciated. Thanks!


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Old 10-12-2018, 10:44 PM   #2
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Have you converted over to a 2.5" i.d. rear spring kit. If not that is definitely step one, then the world is open in regards to rate and length. I have billet aluminum bottom plates I will never need again, did a cut and box on the struts so Im now running true coil overs. that would get you half the way. Adjustable tops I have from another kit.

Last question, what is your motion ratio for the rear, 250 is pretty soft. We ran 350 to 400 on the rear of our MINI for autocross and that was with a 1.1 m.r.
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Old 10-13-2018, 09:12 PM   #3
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Thanks. I was reading that some people here have gone with 450-550 spring rates for the rear. The motion ratio due to the spring being inbound I have read is 0.692.

I do not have the option to run a 2.5"ID but am converting for a 3" ID. The rear control arms I have has a inner perch that is 3" OD.


So I am planing on using no spring isolators on the lower control arm ( thinking of getting some liquid poly from the hardware store. put a small amount around the base so I have some protection.

For the top I am not sure what to do.

The pocket up in the cross member has a welded nub. and the metal center is not a 3" so need a isolator / guide to keep it square in the pocket.



So I am thinking of taking my old isolators

and cut them down and flat.

I am thinking of using this spring:
https://www.racingsprings.com/index..../category/569/

The trick is making sure I have the right height. I am a little worried 8" at that spring rate the vehicle will be to low. I have done the math and it should be about 1 -1 .25" lower than what I was at before with the Koni spring (making a huge assumption on the Koni Spring rate). I think I can take that drop looking at past pictures. Hard thing is I have the car already all apart because I was doing other things and this just happens to be the best time to also swap the springs. So I want to try to avoid the extra work install it with the old spring, recheck and then order and reinstall.

So I am taking a gamble that the springs might not work...
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Old 10-14-2018, 08:45 PM   #4
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Wish you went with 2.5 id as that’s the easy button. Shocked that massive stuff does a 3” setup. That being said what is the final ride height you are shooting for. My rules set min ride height at the rocker can be no lower than 5”.. we have the adjustable spring perches in the rear stuck with original shock mounting points and running original ford control arms. With a 5” rear spring at very high spring rates we measure at the rear rocker in front of the rear tire at 5.75”. We sit on a 15” wheel with a 225 tire, total circumference 23”
My spring rate on the 05 focus st crossing the scales at 2850 with driver is 900 rear and 700 front. Yes it is stiff and it’s a dedicated track car. Don’t know if any of this helps but total balance is what you should be shooting for. Make sure you look at front to rear balance, bar diameter and are you going to run a front bar at all.
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Old 10-14-2018, 09:59 PM   #5
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What do you you mean by ride height at the rocker?

I have 17" wheels on a 225/45 tire so not sure my diamater or circumference with the tires I have mounted.

I am running a front and rear bar still. Have the Eibach front and rear swaybars. Might change out the rear one eventually to the larger Steeda rear Sway bar.

3" ID diamater appears to provide just as many options as 2.5".
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Old 10-14-2018, 11:22 PM   #6
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When I was doing the bulk of my build, the front coil overs all used 2.5 so it just made it easier to go that way on all 4 corners, the bigger bar in the rear being 28.2 is a nice unit. Try driving the car with the front disconnected. It allows the front to work and use the rear stiff to follow along. Long time front wheel drive racers (Honda ) have been doing it for a long time. The pinch weld is the lowest point on the body, the Scca uses the rocker as the measuring point to determine the ride height of the car that was what and where I was referring to. Your tire is probably close to 25” in circumference.

Last edited by msperber; 10-14-2018 at 11:38 PM.
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