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 Post subject: Spring rates rear-ADVICE
PostPosted: January 26, 2019, 9:23 am 
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Joined: September 2, 2013, 9:12 am
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Location: Michigan
I picked up another Locost recently. The car seems very tail happy and I believe it has springs that are too stiff in the rear. The original builder installed 450 lb. springs all the way around (leftovers from a FFR Cobra build). What size spring would you guys suggest to get me in the ballpark? The car has the standard front setup and the builder took the whole IRS assembly out of a Miata for the rear. No swaybars front or rear. Thanks Rich


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Last edited by spitfun on January 26, 2019, 6:29 pm, edited 1 time in total.

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PostPosted: January 26, 2019, 10:16 am 
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I recognize that car being from Huntsville, AL.
I'd swap for 300lb on the front, 250 on the rear due to the less effective mounting angle on the front. Looks like 10 inch springs. Measure installed, fully extended length without adjusting seats (full droop). Use spring closest to that length.

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PostPosted: January 26, 2019, 5:30 pm 
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Joined: January 31, 2012, 12:49 pm
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Location: Louisville KY
MV8 --

Trying to figure out my springs... Dennis Brunton used to be the go-to guy here.

Do you have good "starting points" or "Ballparks" or "rules of thumb" for, say, IRS, solid rear axle, etc etc?

Tim

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PostPosted: January 26, 2019, 6:54 pm 
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I first determine the minimum rate required. In general, set the car at ride height with at least one front and rear dampener installed. Measure the eye to eye length, seat to seat, or mark the shaft, etc to determine the dampener position at ride height.

Jack the car to where those wheels are off the ground to check the dampeners at full droop. This provides how many inches of compression you have to reach the corner weight minus the unsprung weight (wheels, brakes, hubs, half the control arms and dampener).

This is the minimum rate required and would provide the best ride and traction but would likely flop around a lot from cornering without antiroll bars, as well as braking, and accel. I’d look at 50-75 lb more per inch above the minimum to comp for lack of bars and/or adjustable dampening. Most people go much higher and it does seem sportier. Just depends on what you want.

Editing to add that Carroll Smith's technique is to:

*Start by fitting the softest rear springs that will keep the suspension from bottoming out.
*use the same size antiroll bars front and rear.
*use stiffer front springs as needed to adjust under/over steer.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


Last edited by Miatav8,MstrASE,A&P,F on January 31, 2019, 11:50 am, edited 1 time in total.

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PostPosted: January 28, 2019, 1:56 am 
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Intuitively seems a bit stiff on both ends to me, and the motion ratios look to most likely favor having a higher front spring rate than rear to balance the wheel rates out. But by how much is anybody's guess at this point. How do you like the feel of your current overall ride and roll rates? Also, have you thought about adding a sway bar instead of, or in addition to, any spring rate change?

There are too many variables for me to make any specific suggestions. The brute force solution is to simply keep incrementally softening the rear in relation to the front until you get it to handle how you want.

The analytical approach takes a bit more effort, but is the way I would be inclined to go. Since you have a complete car, pretty much everything can be measured directly, which is certainly helpful. However, this method also leads down the proverbial rabbit hole of suspension tuning madness...I mean...theory.

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