Spring rate help

[TOPTON]

When I bought the Tiv last year I knew the front suspension had been converted (read bodged) to use coilovers, but I also knew the mod was a week point and the springs were certainly not strong enough as the car sat real low, probably on the bump stops.
I have strengthened the lower wishbone and fitted new AVO's. My problem now is spring rate. I read somewhere, mistakenly I now know, that 350lb/in springs were correct. WRONG, they squashed right down. So without doing any calculations (stupid i know now) I bought some 450lb springs. These also squashed right down. I preloaded them by 1" and the car settled 2", about 1" above bump stop.

I have 11x8 AVOs, so 3" of travel, I want it to settle 1" leaving 2" of travel. Am I right in thinking 900lb springs would give me this with about 1/2" of preload???

This is my suspension set up, the angle of the shocker is 13.14 not 13.4



The 709lb corner weight is sprung weight taken from the cars weight of 2535lb with a 56-44 distribution.

Thanks for any replies and help. I never realised that suspension geometry was a science all on its own

Edited to correct a mistake

Edited by TOPTON on Wednesday 12th January 09:50

Edited by TOPTON on Wednesday 12th January 09:52

[Mr2Mike]

The Eibach website has worksheet that describes how to calculate spring rates.

You are missing some important parameters however:

1) Unsprung weight i.e. weight of wheel, tyre, brake components, upright, half the weight of the wishbones and weight of half of spring plus moving part of damper both corrected for motion ratio (can probably be ignored unless a very heavy shock).
2) Required natural frequency - how stiff you want the suspension. Probably somewhere between 1 and 2 Hz for a road car.

Spring Rate (SR) = 900
Corner Weight (CW) = 709 lbs
Unsprung Weight (UW) = ?? Take a wild guess at 100lbs, might be miles out!!
Sprung Weight (SW) = CW - UW = 709 - 100 = 609
Motion Ratio (MR) = 6.5/12 = 0.541
Angle Correction (ACF) = cos(13.14) = 0.974 (assuming your bottom arm is horizontal)
Wheel Rate = MR^2 * SR * ACF = 0.541^2 * 900 * 0.974 = 257 lbs/in

Suspension Frequency = 187.8 * SQRT( WR / SW ) = 187.8 * SQRT( 257 / 609 ) = 122 cps
122/60 = ~2Hz which will be a very stiff road car.

Static deflection at the spring = SW / ( MR * ACF * SR) = 609 / ( 0.541 * 0.974 * 900 ) = 2.57 inch.

I suspect 900 pounds will be too stiff, but until you get a good idea of unsprung weight you won't know for sure. You could probably get a reasonable estimate by jacking the car up, removing the coil-over and then gently lowering the tyre onto some bathroom scales until suspension is at normal ride height.


E&OE - It's late and the office coffee machine is broken. Check everything!

Edited by Mr2Mike on Thursday 13th January 15:50

Edited by Mr2Mike on Thursday 13th January 15:54

[TOPTON]

Thanks for putting the time in to respond. I have not done any kind of maths, apart from every day stuff for 25 years or so, so very rusty on calculations.

http://azbusas.yuku.com/topic/1082/t/Calculating-S...

The web site above that I was using to explain things gives different answers to yours. EG, ACF was .839 on that one and .974 with you.

I tried the 'bathroom scales' and seeing as mine only went up to 266lb then they were of no use. Although they do continue to rotate round and start from 1 again, I wasn't sure if the readings were correct, besides that, the tyre squashed them in the middle and they give up.

You know the saying "a little bit of knowledge is dangerous", well that's me now. A hint I picked up as a ball park guess was, if the spring is 50% down the lower arm, in relation to the wheel/pivot, then the spring could be twice the strength of the corner weight with no sag.

I was deducting 70 lbs for unsprung weight. But like you say, could be miles out.
The only thing I had to go on was my 450lb spring and what happened to it. When it was loaded with 3" (1350lb pressure) of squash, then it held up the car. As I only have 3" overall travel, I suspect it needs to be stiff

I couldn't figure what your final GUESTIMATE may have been, although you suspect 900 would be too stiff,

Edited by TOPTON on Thursday 13th January 14:43

Edited by TOPTON on Thursday 13th January 15:34

[LaurenceFrost]

You must also calculate the effective rate of the spring.

E.g. if the spring is at 13 deg, the rate is effectively reduced.

A 450lb spring held at 13 deg is reduced in rate by 2.5%.

450 x COS(13) = 438lb.

Not a massive difference, but it all adds up

[Mr2Mike]

TOPTON said:

I tried the 'bathroom scales' and seeing as mine only went up to 266lb then they were of no use. Although they do continue to rotate round and start from 1 again, I wasn't sure if the readings were correct, besides that, the tyre squashed them in the middle and they give up.

Surely the unsprung weight can't be more than 266 lbs (120kg)? Where you trying to use the scales to measure corner weight rather than unsprung weight i.e. did you remove the spring and damper before making the measurement?

[TOPTON]

Your Figures:

Corner Weight (CW) = 709 lbs
Unsprung Weight (UW) = ?? Take a wild guess at 100lbs, might be miles out!!
Sprung Weight (SW) = CW - UW = 709 - 100 = 609
Motion Ratio (MR) = 6.5/12 = 0.541
Angle Correction (ACF) = cos(13.14) = 0.974 (assuming your bottom arm is horizontal)
Wheel Rate = MR^2 * SR * ACF = 0.541^2 * 900 * 0.974 = 257 lbs/in

My figures from link above:

Corner weight = 709 lbs
Unsprung rate = 70 lbs
Sprung rate = 639 lbs
Motion ratio = 6.5/12 then squared = .293
Angle Correction (ACF) = .839 (calculator set in radians mode) ???
Wheel rate = 532.5
Then we have Spring rate = WR divided by MRxACF = a pure crazy figure of 2173 lbs

[TOPTON]

Mr2Mike said:

TOPTON said:

I tried the 'bathroom scales' and seeing as mine only went up to 266lb then they were of no use. Although they do continue to rotate round and start from 1 again, I wasn't sure if the readings were correct, besides that, the tyre squashed them in the middle and they give up.

Surely the unsprung weight can't be more than 266 lbs (120kg)? Where you trying to use the scales to measure corner weight rather than unsprung weight i.e. did you remove the spring and damper before making the measurement?

Sorry, ( a little bit of knowledge and all that) I got mixed up

I was innitially trying to corner weight the car

[Mr2Mike]

Right, I did screw up slightly in calculating the spring deflection as forgot to include the motion ratio and spring angle (now edited!). The results you got with 450 lb spring seem to tie up quite well, with the calculations predicting a static sag of 2.56 inches with no preload, using up most of the travel.

Working back it seems 900 lbs spring should work (depending on unsprung weight), you'd need to preload the spring by about 0.28 inches to get a static sag of 1 inch. This would give a reasonably stiff ride.

I have the above calculations in an Excel spreadsheet, you are welcome to a copy if you want.

Edited by Mr2Mike on Thursday 13th January 16:29

[TOPTON]

Hey Mike, a copy would be handy for me to learn from. Can you email it to me at [e-mail] Cheers.

Out of interest, the site I was working from left me with a figure of 2173 lbs as a spring rate. WOW!! Where did I go wrong on that or are his calcs all wrong.

I noticed 3 differences to yours, First off, when calculating WR, WR = SW divided by 0.4 x WT. 0.4 from somewhere not explained. ACF in Radians mode on calculator, where you didn't. I have never heard of that. I never new there was more than one setting on a calculator.

Working out the MR, the end figure was squared whereas yours wasn't.

I certainly like your end result more than mine using his method

[Mr2Mike]

I'll send the spreadsheet ASAP.

The Motion Ratio needs to be squared because you are looking at force multiplied by distance i.e. the wheel rate. If you where considering only force or only distance then the square would not be required.