Topic: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Q: Does anyone have access to a spring/shock dyno in the Bay Area that I might be able to test some springs and/or struts on sometime in the next few months?

After 5 races, I'm revisiting my 95 tbird's suspension to see what's been beat to hell. Before our last race, I replaced one of my Monroe Sensatrac front struts after an offroad excursion on a track day as I knew it was blown (hydraulic juice leaking out). I did NOT however replace the front left strut which has been on my car for 5 Lemons races and 3 or 4 track days -- likely because i'm cheap. Handling was better than when the front right was blown but it didn't seem as good as it has been in the past. Hence, my desire to investigate things further. Also, the 150LB more engine up front doesn't help either.

One thing that I suspect would be useful would be to see which of my motley assortment of used struts may be matched together.
I can tell what's completely blown but I'm curious if some of my used struts may be better.

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In a related note, I've also received conflicting feedback on what happens if you cut springs (which we did years ago to lower the ride height since our tires were going to be 3" smaller than stock -- making sure not to heat up the springs by wrapping a wet towel just after the cutting point).

One camp of people have told me that if a spring has 10 coils and is rated for say 500lb/in, for example, cutting 1 coil will result in a cut spring that is 10% higher in lb/in (550lb/in in this example). The math gets more complicated with progressive springs but the idea is the same.

Another "internet" camp has told me that cutting springs (esp in the progressive type) just narrows the range of the spring resistence.
Example: if you had a 400lb/in - 600lb/in spring and cut a coil, you end up with a spring that is something like 450lb/in-600lb/in.

Both camps seem logical enough but if someone would let me buy them a sixpack of beer (or some wine) or just give some cash to test some springs/shocks, I'd love to just test my springs to know definitively which ones I have will be most appropriate for my needs.

Regards,
-g

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Typically, a spring/shock dyno is used to see if your damping factor is well matched to your spring's stiffness. In engineering speak, you're looking to see if your spring/shock combination is under damped, over damped, or critically damped and then tweak spring rate and/or valving to get your damping ratio where you want it. I'm sure some teams re-valve shocks on a dyno, but it seems a lot of work for a fairly light hearted racing series. A shock dyno would be one way of seeing if you have blown shocks though.

Cutting a spring increases your overall spring rate (ie answer one is correct - both your minimum and maximum). Take it to the extreme, if you cut your 400lb-600lb spring down only having a 1/4 coil left, it would take a lot more than 600 lbs to get it to compress an equal amount. You can also unwind the spring into a rod. If you have a long metal rod of 1/4" diameter you will be able to easily bend it at the ends. If you cut it down in length it will take more force to get it to bend the same amount. If you want some proof of the internet worthiness of this answer, you're welcome to check this spring calculator out (or google for another one).

On a Lemons level I would be much more concerned with getting the car to be neutral (ie not drastically understeering or oversteering) than getting damping just do. You can mess with this balance by swapping roll bars (or using adjustable end links). If you're understeering soften your front end and/or make your rear end stiffer. If you're oversteering, soften your rear end or make your front end stiffer.

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

I suspect much of the confusion surrounding springs has to do with folks being sloppy about their units (giggity).

I'm going to ignore progressive-rate springs, end treatments (such as closed or closed-and-ground) and a bunch of corner cases because I want to and because you either want me to or really should do.

Spring rate is expressed as pounds per inch or equivalent.  See, it's a rate.  People often refer to a 600 lb/in spring as a "600 pound" spring, which is fine for shorthand but is strictly incorrect and can be confusing.  A 600 lb/in spring will compress one inch for every 600 pounds applied to it.  Spring rate is a sort-of fundamental property of a given spring and, for a coil spring, depends upon wire diameter, coil diameter, material, and number of coils.  Note that free length, travel, and solid length don't appear in that list.

Cutting coils out of a spring will increase its spring rate in a fashion that isn't necessarily linear.  As a handy example, a notional 600 lb/in spring with 10 coils of 1/2" steel wire and an OD of 3.155" will have a rate of 686ish lb/in after cutting one coil.  Cutting a second coil takes the rate to 800 lb/in.

Cutting coils also makes a spring shorter, which should be obvious to one versed in the art.  It makes the spring shorter in a fashion that also reduces its travel.  The increase in spring rate and the decrease in spring travel work against each other when you look at total load capacity.  This may not matter but it is interesting. 

For our notional spring above if we start with a free height of 10" it will have a travel of 5" (because it has a solid length of 10 coils times 1/2" per coil equals 5") and go solid at 3000lb of load .  Cutting one coil yields a 686 lb/in spring with a free length of 9", a solid length of 4.5", 4.5" of travel, and 3086lb load at solid length.  Ten percent fewer coils means 10% shorter height, 10% shorter solid length, 10% less travel, 14 percent greater rate, and less than 3% greater load-carrying capacity.  We don't want to be running our springs that close to solid anyway, so it doesn't make a lot of difference in the case.

What you really care about is how much the cut will lower the car, right?  If we assume (probably incorrectly) that the car at rest compresses the spring 25% of its available travel (1.25"), that means it is carrying 750lb.  Let us further incorrectly assume the suspension has a 1:1 motion ratio (like a beam axle with the spring mounted directly to it) and that you have a giraffe-like ride height of 5" at the pinch weld.  Cut a coil and that 750lb will compress your new spring not quite 1.1", but it starts 1" lower, so your ride height drops to 4.15".

All of the above said (at excessive length): when we cut coils we take a wild guess at the number (usually 1.5), hack 'em, install 'em, set the car down and roll it, squint a little, and cut a bit more off.  We're pretty happy if they wind up the same side-to-side.  It helps to have extra springs laying around.

Fun (not really) fact: when springs "sack" they get shorter but not softer.  Conventional wisdom is that nothing on earth can change a spring's rate unless you change coil diameter, wire diameter, or the Young's Modulus of the material.  Even if you anneal the shit out of it the spring rate stays constant.  Esoteric corollary: I've seen coil springs in high-load high-cycle testing suffer a significant and repeatable reduction in spring rate.  The spring manufacturer just shrugged and said "yeah, we see that a lot".  My pet theory is that the material was micro-cracking, effectively reducing the wire diameter.  It certainly wasn't anything else we could measure.

And a final bit of knowledge for those still reading along: compression springs act like torsion bars with curvature effects.  The wire is being twisted along its length when the coils are compressed.  The curvature effects mean that round wire is stressed unevenly around its circumference (unlike a pure torsion bar).  Some dudes calculated the optimal cross-section for wire in a compression spring.  It's an egg shape that varies with spring geometry.  It would actually want to be hollow, too.  Anyway, as far as I know nobody has really gone there yet.

Scott

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Thanks for both of your inputs here; the inputs here jive with other more reputable (sounding) feedback as well.
https://www.eatondetroitspring.com/cutt … culations/

As far as ride height it concerned, I think we are OK where the car sits now but I was concerned about how the car seems to not be as neutral after the 150LB weight gain of the V8 we swapped in over the V6 the car originally came with that we ran for our first 3 races.

I did take some measurements and realized that the swaybar I have on the car now isn't the thickness I expected; going to a slightly thicker one I have from a donor car should help but i also wanted to see if I had a strong enough spring for the added wieght on the front end front end; based on this feedback, I suspect i do so now its a question of swaybars/dampening and perhaps my rear isn't stiff enough since the front end is pre-compressed more (than with the V6) b/c of the additional weight. I suspect that my front dampeners (cheap monroe sensatracs are blown).

I also know that going with too stiff a spring will cause unpredictable handling as the cars tires start lifting/bouncing all over the place.
-g

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Shocks/dampners don;t hold the car up, that is the job of the springs.
Adding 150 lbs to the front you need to increase your spring rate significantly, at least to factory V8 Springs but likely heavier again.

This handy dandy chart will let you know what your stock spring rates were and the difference to stock V8 springs and may also help you find some replacements that will give you the desired rating and size either from the junkyard or new from Rock Auto/Amazon etc....

http://classiccarsprings.com/coil-sprin … tions.html

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6 (edited by gunn 2016-10-21 09:45 AM)

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Brett85p wrote:

Shocks/dampners don;t hold the car up, that is the job of the springs.
Adding 150 lbs to the front you need to increase your spring rate significantly, at least to factory V8 Springs but likely heavier again.

This handy dandy chart will let you know what your stock spring rates were and the difference to stock V8 springs and may also help you find some replacements that will give you the desired rating and size either from the junkyard or new from Rock Auto/Amazon etc....

http://classiccarsprings.com/coil-sprin … tions.html

The tbird nerds have already documented what springs were available for tbirds:
Stock 3.8 V6 N/A Front: 223-282 lb./in
Stock 5.0 V8 N/A Front: Front: 270-330 lb./in
Stock 3.8 V6 SC Front (What i have on my car today w/ 1.5 coils cut off):: Front: 349-403 lb./in.

My concern was whether or not what i have is "enough" spring and the tbird nerds think so. The confirmation as to what happens when you cut springs reaffirms this thought.

My concern now is on dampening (hence the original request to see if anyone had a shock dyno I could do some testing with).

-g

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Kinda seems like we've jumped straight into discussion of fixing without being really clear on the problem.  It sounds like it handled ok with the V6 but you added weight on the front end of the car and now it doesn't handle as well, correct?

Shocks.  Basically a tuning device unless bad.  It would be great to dyno the shocks but for Lemons I would just yank up and down to get a ballpark idea if they operate similarly from side to side and start from there.  Or jump up and down on each corner and count how many cycles it takes for it to come to rest.  If one side operates different than the other, you might be on to something. If they are cheap to replace, then replace them.  If it immediately stops without cycling, either find more fat people to jump up and down or decide that the shocks are doing their job and call it good.

Springs.  It sounds like it handled ok with the V6 and that went away with the V8.  It also sounds like it has the same springs as when the V6 was in there.  Seems to me like you lost the balance and comparing spring rates is irrelevant. 

Swaybars.  I'm going to guess that the car pushes more now than before.  Putting a bigger bar on the front alone is gonna make it push more without doing anything else.

By my rough math, it looks like the factory increased the spring rate 15% or so when going from V6 to V8.  I'm going to guess you want to do the same.  I also think you might be overlooking the back of the car.  If you want to tune to become more neutral, I'd raise the rate of the rear swaybar.  On a relatively softly sprung car like we are playing with, I can't see it hurting to go with a bigger front bar but you'll need to go a few steps higher in the rear to keep your balance.  You may need to play with this a bit.  I'd raise the front spring rate 15%.  If you have a way to test, I'd see what that does.  I'd then put in your bigger swaybars in and see what you have.

1990 RX7 "Mazdarita"  1964 Sunbeam Imp (IOE 2013 Sears Pointless) 2002 Jaguar x-type (Winner C-Class 2021 Sears Pointless)
Gone bye-bye
1994 Jaguar XJ12 (Winner C-Class 2013 Sears Pointless)  1980 Rover SD1 (I Got Screwed 2014 Return of Lemonites)

8 (edited by gunn 2016-10-21 12:14 PM)

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

cheseroo wrote:

Kinda seems like we've jumped straight into discussion of fixing without being really clear on the problem.  It sounds like it handled ok with the V6 but you added weight on the front end of the car and now it doesn't handle as well, correct?

Shocks.  Basically a tuning device unless bad.  It would be great to dyno the shocks but for Lemons I would just yank up and down to get a ballpark idea if they operate similarly from side to side and start from there.  Or jump up and down on each corner and count how many cycles it takes for it to come to rest.  If one side operates different than the other, you might be on to something. If they are cheap to replace, then replace them.  If it immediately stops without cycling, either find more fat people to jump up and down or decide that the shocks are doing their job and call it good.

Springs.  It sounds like it handled ok with the V6 and that went away with the V8.  It also sounds like it has the same springs as when the V6 was in there.  Seems to me like you lost the balance and comparing spring rates is irrelevant. 

Swaybars.  I'm going to guess that the car pushes more now than before.  Putting a bigger bar on the front alone is gonna make it push more without doing anything else.

By my rough math, it looks like the factory increased the spring rate 15% or so when going from V6 to V8.  I'm going to guess you want to do the same.  I also think you might be overlooking the back of the car.  If you want to tune to become more neutral, I'd raise the rate of the rear swaybar.  On a relatively softly sprung car like we are playing with, I can't see it hurting to go with a bigger front bar but you'll need to go a few steps higher in the rear to keep your balance.  You may need to play with this a bit.  I'd raise the front spring rate 15%.  If you have a way to test, I'd see what that does.  I'd then put in your bigger swaybars in and see what you have.

Yes, you are correct on my intent/underlying concern.

To clarify, my plan was to go thicker on the rear swaybar first and see if that helps make the car handle more neutraly instead of understeering. I do have a thicker front swaybar but I intentionally haven't installed it because a) i recognized putting it on will only emphasize understeer compared to where I am today and b) it's a giant PITA to install with the engine in (will need to drop the subframe to get it in/out. Unless I can get an even thicker rear swaybar (I'm not going to buy a new aftermarket one because Lemons), I doubt the thicker front swaybar will ever go on my car.

I was hoping to find a way to test shock dampening other than the jump on the fender and count the bounces. I'm most concerned about the front left strut given how long it's been on the car. Since it looks like the bushings in my front LCAs are already beat to hell (very likely original parts), I'm probably taking off the suspension anyway to swap LCAs (I'll call it a safety feature) so I might as well disassemble/check the front left strut.

-g

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

Ok.  IIRC, it's about $30 a corner to have shocks dyno-ed.  I just looked on Rock Auto for your car.  Rear KYB's are $21 and front KYB struts are $46.  Shock dyno's are expensive precision tools that are generally owned by teams that are in the business of racing and not weekend racers.  They are also operated by someone who knows what they are doing so it may be a bit of a reach to find someone to do it for beer or whatever.  Also, every dyno I've seen is expecting the shock to have an eye on each end so I'm guessing you'll need to fab something so your strut will fit in the fixture.  Not trying to be Johnny Buzzkill but pointing out that you may have some hurdles to overcome to dyno your shocks.  if it's me, I just buy the KYB's and not mess with it.

1990 RX7 "Mazdarita"  1964 Sunbeam Imp (IOE 2013 Sears Pointless) 2002 Jaguar x-type (Winner C-Class 2021 Sears Pointless)
Gone bye-bye
1994 Jaguar XJ12 (Winner C-Class 2013 Sears Pointless)  1980 Rover SD1 (I Got Screwed 2014 Return of Lemonites)

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

gunn wrote:

Unless I can get an even thicker rear swaybar (I'm not going to buy a new aftermarket one because Lemons), I doubt the thicker front swaybar will ever go on my car.

You can do three things to stiffen out the rear of the car to try to make it more neutral: bigger rear bar, stiffer rear springs, or preload the rear bar. If you have access to a lathe, you can make an adjustable end link for your swaybar pretty cheaply. The only downside to doing this is that you will have some side to side weight jacking. You can also do the truly lemony thing and stack two smaller bars.

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

gumert wrote:
gunn wrote:

Unless I can get an even thicker rear swaybar (I'm not going to buy a new aftermarket one because Lemons), I doubt the thicker front swaybar will ever go on my car.

You can do three things to stiffen out the rear of the car to try to make it more neutral: bigger rear bar, stiffer rear springs, or preload the rear bar. If you have access to a lathe, you can make an adjustable end link for your swaybar pretty cheaply. The only downside to doing this is that you will have some side to side weight jacking. You can also do the truly lemony thing and stack two smaller bars.

Interesting idea. If my 0.94" bar I have in my spare parts pile isn't thick enough, I'll look into this.
I see some examples here:
http://forums.turbobricks.com/showthread.php?t=9587

http://i47.tinypic.com/2vbo1sj.jpg
http://i45.tinypic.com/2h72ypu.jpg

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Q: Anyone have a spring/shock dyno in the Bay Area + cutting springs

If you'd like to re-visit the topic of spring rates, cutting springs, swapping swaybars and such, Eyesore Racing's Dave wrote this a few years ago.  It's pretty comprehensive.

http://www.motoiq.com/MagazineArticles/ … cksaw.aspx

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