Spring rate????

Stew Stau

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Jun 6, 2017
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I know this is an old thread but just in case someone non-technically minded finds it in a search, I'd like to correct a misunderstanding.

The rear spring rate on a '79-'04 Mustang is so low compared with the fronts for two reasons.

A - the rear roll centre is ridiculously high (16") because it is set by the two UCAs. The higher the rear roll centre, the lower the spring rate

B - The stock Mustang 4-link has terrible binding under articulation. This means the control arm bushings effectively act as springs and add to the overall rate.

This binding is also why the stock mustang rear end has such bleeding awful handling characteristics and was replaced by a 3-link in the S197.

If the live axle '79-'04 Mustang is fitted with a bind-free rear suspension with a lower rear roll centre, whether a Torque Arm (Griggs, MM, GM F-body) or a 3-Link (EVM, S197, NASCAR, etc), then the spring rate balance will change. A stock spring will be way too soft and the back end will hop like crazy.

For example:

Stock '87-'93: 425-530 Front, 200-300 Rear

'87-'93 with TA/3-link: 425-530 Front, 325-430 Rear (ish)

If you look at an S197 ('05-'13), you'll see spring rates that are much more balanced, similar to the TA/3-link example above (although much lower overall due to having a much higher wheel rate in general).
 

Stew Stau

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Jun 6, 2017
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I also forgot to add, the wheel rate on the front end of an SN95 is 25% of the spring rate and the wheel rate on the rear is 50% of the spring rate.

I guess that's the third reason, and 'C' for the list above.
 

Compuguy2142

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Was told this should be added to the list. Its nowhere else on the entire internet but got it today.

Vogtland Cobra w/ IRS Spring Rates

Front (variable spring rate)
Initial - 400
Final 742

Rear (variable spring rate)
Initial - 274
Final - 713
Although this is an 8 year old post, the information is still relative. I'd like to know if the progressive spring rates are for the Vogtland 03-04 Cobra part number 953061 or the 99-01 Cobra 953064? (And what is the difference between the two kits if anyone knows)

An additional question is would this set be a good fit for an 86 Foxbody with SN95 spindles and an 03/04 Cobra IRS?

The 86 had progressive rate O/O front springs of 425-525 and Progressive B/B springs of 200-300 which totally goes out the window with changed front geometry, rear IRS and lowering.

My plan for the front is the QA1 K member because it has the 2-position rack mount where that can be used to compensate for the 2002 front spindles by changing the rack location. And some sacrilege here, cutting the inner ears off of the upper k-member mount and using AJE spring perches to stay with the stock spring location instead of coilovers. QA1 is making their K-members for coilovers only.
 

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