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PostPosted: Thu Jun 28, 2012 11:20 pm 
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Hi all, A few questions to go along with along with a couple of screen shots of #'s I've come up with, for the front and rear suspension, using the spring calculator found on the Brunton Performance Products web sight http://coilovershock.com/suspension_worksheet.htm.
I appreciate it is a matter of personal preference, just interested in knowing if what I have here is reasonable.

A bit of back ground.
My build is Miata based so A arms front and rear.
I plan on using narrow body Gaz shocks purchased from Jack @ Kinetic.
I need a min of 2 inches bump and droop to be street legal.
Car is intended for street with the occasional trip to an autocross.
For D2, The worksheet uses pivot to tire center line. Another spring calculator posted in the Spring Rates sticky gives different end points depending on the type of suspension used http://f-o-a.com/calculator.html. For A arms it suggests either end of pivot arm. Based on this I used end of the arm in the Brunton calculator as well (Ball joint center). I plugged the #s into both calculators and got the same # with in 2 lbs.

Screen shots from the Brunton calculator, of front 1st and rear 2nd.
Attachment:
File comment: Front
front snapshot.png
front snapshot.png [ 143.8 KiB | Viewed 873 times ]
Attachment:
File comment: Rear
rearsnap shot.png
rearsnap shot.png [ 145.67 KiB | Viewed 873 times ]



From these #'s-
Rear
angle is 10', so 182lbs (180lb spring) with 6.0' travel shock (4.9' travel + 1' for the bump stop)
Front
angle is 45', so 367lbs (375lb spring) with 4.5' travel shock (3.4' + 1' for the bump stop)

Questions
1st- Is there anything grossly amiss with the #s I have imputed.
2nd - Is the calculator itself reasonable?
3rd- Regarding a couple of the 'tips' included in the worksheet.
3rd.1- It suggests using 33.33% droop. What practical effect will there be if I deviate form this (as I have to affect frequency,and droop in a given travel)
3rd.2- From the worksheet- "Important- The front frequency should be less than the rear...theories vary, but 10-15% seems to be popular (I have tried to do this, again by changing droop % and travel front to rear). Is this important (why). I don't recall this being discussed in any other topics.
4th- What is the maximum shock angle one can have (using Jacks arms and I'm struggling to get 45' with the narrow upper arms and keeping my lower mount as far out as possible)
5th- What is the approximate OD of the Gaz springs (ID is 1.9’) so I can check for clearance on the upper arm (may help with ‘4th’).
6th- Better to round up or down for the springs (rear I've round down, front I've round up, base on what closest- calculated to available).
7th- Are springs for these shocks avalible in North America (much as I don't want to think about puchasing the next set before I've ordered the 1st set, switching springs down the line seems like a common practice so the question must be asked).

Thanks in advance for taking the time to read this and answering any questions.

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PostPosted: Fri Jun 29, 2012 12:37 am 
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I use that calculator a lot, and I trust its numbers, and actually your spring rates look reasonable, but I'm having a really hard time believing your motion ratios. Only 1.5" from the bottom of your shocks to the centerline of the wheel? How are doing that? My Miata doesn't even come close to that. Fronts are 3-1/4", which is still pretty amazing.

Other than that, I think anything in the 33-50% droop range is okay. The NF higher in the rear than the front, that's usually just the way it works out. Identical NFs could lead to some bad resonances, IMO, so a difference is good.

I've seen some crazy shock angles before, and 45 degrees isn't excessive, but IMO that should be just about the maximum. Think about cantilevering your upper shock mounts. And rounding spring rates from a calculator is totally personal preference. You're just estimating anyway. One thing you can do in the calculator is change the droop percent a slight amount--fractions are allowed--until you hit a spring rate that's an exact multiple of 25. In your case though I would definitely round up, since I still can't believe those MRs are accurate.


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PostPosted: Fri Jun 29, 2012 1:40 am 
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cboettch wrote:
Questions
1st- Is there anything grossly amiss with the #s I have imputed.
2nd - Is the calculator itself reasonable?
3rd- Regarding a couple of the 'tips' included in the worksheet.
3rd.1- It suggests using 33.33% droop. What practical effect will there be if I deviate form this (as I have to affect frequency,and droop in a given travel)
3rd.2- From the worksheet- "Important- The front frequency should be less than the rear...theories vary, but 10-15% seems to be popular (I have tried to do this, again by changing droop % and travel front to rear). Is this important (why). I don't recall this being discussed in any other topics.
4th- What is the maximum shock angle one can have (using Jacks arms and I'm struggling to get 45' with the narrow upper arms and keeping my lower mount as far out as possible)
5th- What is the approximate OD of the Gaz springs (ID is 1.9’) so I can check for clearance on the upper arm (may help with ‘4th’).
6th- Better to round up or down for the springs (rear I've round down, front I've round up, base on what closest- calculated to available).
7th- Are springs for these shocks avalible in North America (much as I don't want to think about puchasing the next set before I've ordered the 1st set, switching springs down the line seems like a common practice so the question must be asked).

Thanks in advance for taking the time to read this and answering any questions.
This is actually the first that I've played with this particular calculator, so these thoughts might just be worth what you've paid for them.

1.) Generally speaking, your numbers don't look unreasonable.
2.) The calculator seems reasonable in the accuracy of its calculations.
3.1.) If you deviate to a higher percent than 33% it just means that it will be some amount easier to end up using your bumpstops. It's my impression that the 33% number is pretty conservative, especially if that's the number before even touching the bumpstops.
3.2.) The rear frequency higher than the front follows the "flat ride" philosophy where it minimizes the front to rear oscillations of the chassis as you encounter bumps. This is more important for street use than at the track. However if your spring rates are your only means of adjusting the roll couple, in other words no sway bars, then this could potentially become a moot point when trying to achieve the desired handling balance. Unfortunately this calculator does not look at the roll couple or roll rate.
4.) I could be mistaken, but I believe that the shocks should physically work at just about any angle.
5.) No idea.
6.) If the calculated rate is close to a standard rate, I would generally round to whatever is closest. You can always play with the numbers in the calculator to more exactly match the standard rates you're considering, and then compare the effects it has.
7.)No idea.

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PostPosted: Fri Jun 29, 2012 8:10 am 
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Quote:
5th- What is the approximate OD of the Gaz springs (ID is 1.9’)

OD will vary by spring rate. My GAZ coilovers measure ~2.67" OD.


Quote:
7th- Are springs for these shocks avalible in North America

Absolutely. Speedway motors JEGs and Summit all sell them, as do many online speedshop places, just not as many as 2-1/4" ID springs. Afco QA1, Hypercoil are just a few or the mainstream mfgrs. there are other here too. Instead of searching for 1.9" ID springs, use 1-7/8" coil springs usa I got over 200,000 hits.

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PostPosted: Sat Jun 30, 2012 1:04 pm 
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Guys, thanks for the great feedback. The list of questions just kept growing, and I really appreciate the thoughtful answers :cheers:
nick47 wrote:
I'm having a really hard time believing your motion ratios. Only 1.5" from the bottom of your shocks to the centerline of the wheel? How are doing that? My Miata doesn't even come close to that. Fronts are 3-1/4", which is still pretty amazing.
I'l check my #'s when I get home (could have Oops'd), but I am using pivot to pivot, not pivot to wheel center. That does change the MR some. Which should I be using? IIRC, everything I've read hear on the forum points to using pivot to pivot. However the illustration on the Brunton worksheet uses the wheel center and then to add confusion, states a couple of lines bellow, 'I have found that using half the distance between the center and inside edge of the tire works well in the real world' :BH:
Driven5 wrote:
3.2.) The rear frequency higher than the front follows the "flat ride" philosophy where it minimizes the front to rear oscillations of the chassis as you encounter bumps. This is more important for street use than at the track. However if your spring rates are your only means of adjusting the roll couple, in other words no sway bars, then this could potentially become a moot point when trying to achieve the desired handling balance. Unfortunately this calculator does not look at the roll couple or roll rate.
Got it. I do have th Miata sway bars that I planned to incorporate.
rx7locost wrote:
OD will vary by spring rate. My GAZ coilovers measure ~2.67" OD.

I could not find ODs listed any where for given spring rates. This wil give me a good baseline to check for clearance on my UCA.
rx7locost wrote:
Absolutely. Speedway motors JEGs and Summit all sell them, as do many online speedshop places, just not as many as 2-1/4" ID springs. Afco QA1, Hypercoil are just a few or the mainstream mfgrs. there are other here too. Instead of searching for 1.9" ID springs, use 1-7/8" coil springs usa I got over 200,000 hits.

Great. I was unsure if these springs would be interchangable w/ Gaz shocks.

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PostPosted: Sat Jun 30, 2012 11:31 pm 
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I'm not a suspension engineer, but it sure seems to me the motion ratio compares spring motion to wheel motion, not ball joint motion, so I think you really need to go with the contact patch. I trust Brunton's experience, and if he says use a quarter of the tire width as the effective center of the contact patch, then I'd use that. Which is great for me, because it'll improve the motion ratios on my car.

I think you can over-obsess about spring rates. I know I have. The rates you've got are reasonable. Somewhere on this site is a long thread about what people are using for spring rates, and the rates in that post varied all over the map. You've got plenty of travel, solid NFs front and rear, I think you'll be fine.


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PostPosted: Mon Jul 02, 2012 12:47 pm 
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My guess is that 2hz will end up being too soft for you,probably fine on the street provided you have enough ride height.If your not using bars than it may roll around like a stock miata does,my build targeted 2.5 hz(400lb fr and 300lb r springs) with miata bits and track only work but was WAAAY to soft.I'm currently at the stiffest 1 7/8's spring easily sourced(550's front and still playing around with the rear to balance but currently at 400lb rear)and no sway bars(no intention of ever adding any either).My ride height is less than 2" though.


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PostPosted: Mon Jul 02, 2012 2:25 pm 
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I believe that looking at the hz frequency is important for bump issues, but it is worth noting that the math used here ignores the roll centers and roll couple. The tendency for a car to roll can be predicted by looking at the length of the lever arm between the roll center height and the cg height, longer distances (high cg and low roll centers) will require considerably more spring rate via springs, bars or some combination than a car with a smaller lever arm.

So I guess the short version is that in this case, spring frequency may not be a useful metric for comparing tendency for body roll with other builds.

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PostPosted: Mon Jul 02, 2012 2:29 pm 
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From Keith Tanner's High Performance Miata book, "Performance street Miatas generally go for a frequency around 1.5 to 2.0 Hertz (Hz, or cycles per second), while track Miatas will run from 2.0 to 2.5 Hertz. Stock Miatas are in the 1.1- to 1.3-Hertz range. The bounce frequency is not affected by sway bars".

In my experience, talking with many Miata owners, "soft" and "firm" are very subjective. Natural frequency is pretty objective, and I agree that 2.0 would be good for a street Locost. Where I disagree is that a Locost with a 2.0 NF and low c.g. would roll nearly as much as a stock Miata. I'd have to experience that for myself before I'd believe it.


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