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      10-23-2021, 03:52 PM   #23
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Quote:
Originally Posted by tsk94 View Post
I could send you my spreadsheet, I don't use the FCM one though. Didn't find it worked well.

The stock number will be low due to the ratio of front to rear spring rate, but it doesn't take into account the bump stops, which on the stock suspension activate very early on and bump up the spring rate big time.
FCM shows separate lines for FRC with and without bumpstops on two lines.

It seems to take into consideration Track width, sway bar arm lengths, motion ratios for sway bar/damper/spring.

https://www.fatcatmotorsports.com/FC..._M3_Online.htm

I do find it hard to believe the FRC for a E46 m3 is 68% and then E9x to be in the 50s? That doesn't seem to make sense. I'm not saying my numbers are accurate either but E9xM FRC lower than E46 doesn't make sense to me.
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      10-23-2021, 04:00 PM   #24
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Quote:
Originally Posted by derbo View Post
FCM shows separate lines for FRC with and without bumpstops on two lines.

It seems to take into consideration Track width, sway bar arm lengths, motion ratios for sway bar/damper/spring.

https://www.fatcatmotorsports.com/FC..._M3_Online.htm

I do find it hard to believe the FRC for a E46 m3 is 68% and then E9x to be in the 50s? That doesn't seem to make sense. I'm not saying my numbers are accurate either but E9xM FRC lower than E46 doesn't make sense to me.
It's in the 50's when you don't take into account bumpstops. Good luck finding data on the bump stops for various cars, including the E9X M3, to get accurate comparisons across platforms. At that point the FRC will also change as you load the suspension more as the bumpstops are progressive.

Also, comparing stock FRC between cars, are even just to see what they are stock, is completely useless - this coming from someone whos spent hours upon hours of doing so.

What are you trying to accomplish finding the E9X stock FRC?
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      10-23-2021, 04:15 PM   #25
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Quote:
Originally Posted by tsk94 View Post
It's in the 50's when you don't take into account bumpstops. Good luck finding data on the bump stops for various cars, including the E9X M3, to get accurate comparisons across platforms. At that point the FRC will also change as you load the suspension more as the bumpstops are progressive.

Also, comparing stock FRC between cars, are even just to see what they are stock, is completely useless - this coming from someone whos spent hours upon hours of doing so.

What are you trying to accomplish finding the E9X stock FRC?
I'm trying to understand the math. Comparing FRC won't really matter like you said, but being a E46 without bumpstops was 68%, it seems interesting that BMW would move it to 50s for E9x. I think I'm merely looking at stock roll stiffness and ratios for a comparison to changes to our chassis.


Formulas I'm seeing:

wheel rate = spring wheel rate + ARB wheel rate

Axle Chassis roll stiff ness = (Trackwidth)^2 x (Combined Wheel rate) / (2 x (360/Pi))

Front Roll Couple = Front Chassis roll stiffness / Rear Chassis Roll Stiffness

This is omitting the bumpstop itself as well.


I'm trying to get as best of a spreadsheet to identify flat ride + a decent FRC to experiment. If my math is completely off to yours and I'm unclear which is correct, that leaves me with a very poor base to go on.
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      10-23-2021, 04:58 PM   #26
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Quote:
Originally Posted by derbo View Post
I'm trying to understand the math. Comparing FRC won't really matter like you said, but being a E46 without bumpstops was 68%, it seems interesting that BMW would move it to 50s for E9x. I think I'm merely looking at stock roll stiffness and ratios for a comparison to changes to our chassis.


Formulas I'm seeing:

wheel rate = spring wheel rate + ARB wheel rate

Axle Chassis roll stiff ness = (Trackwidth)^2 x (Combined Wheel rate) / (2 x (360/Pi))

Front Roll Couple = Front Chassis roll stiffness / Rear Chassis Roll Stiffness

This is omitting the bumpstop itself as well.


I'm trying to get as best of a spreadsheet to identify flat ride + a decent FRC to experiment. If my math is completely off to yours and I'm unclear which is correct, that leaves me with a very poor base to go on.
There's no magic FRC, largely driver, car and track dependent. I would say low 60's would be a good starting point.

The numbers look slightly different then what I'm using. I linked the spreadsheet I use above. You could punch in the same numbers to both and see how they compare. I've found this one to be accurate.
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      10-23-2021, 06:04 PM   #27
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Quote:
Originally Posted by tsk94 View Post
There's no magic FRC, largely driver, car and track dependent. I would say low 60's would be a good starting point.

The numbers look slightly different then what I'm using. I linked the spreadsheet I use above. You could punch in the same numbers to both and see how they compare. I've found this one to be accurate.
The angle between lower spring and control, the motion ratios of sway bars, all seem specific to the NC miata in that spreadsheet too. I'll play around with this tho. Thanks!
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      10-23-2021, 06:07 PM   #28
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Quote:
Originally Posted by derbo View Post
The angle between lower spring and control, the motion ratios of sway bars, all seem specific to the NC miata in that spreadsheet too.
Yes, so either go measure yours, or apply a rough angle. You'll see by playing around with the angles it makes a very small difference - almost negligible. I'd be much more concerned about getting accurate track widths, sway bar rates and weight distribution then precise spring/shock angles.

Good luck!
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      10-23-2021, 06:14 PM   #29
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On FRC%: I think the point isn’t so much the number itself; it’s more about comparison, e.g. one setup vs. another, or an aftermarket setup vs. stock.

I tried several calculators, including the one tsk94 linked. The relationship I describe earlier was the same in all of them.
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      10-23-2021, 06:17 PM   #30
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Quote:
Originally Posted by tsk94 View Post
Yes, so either go measure yours, or apply a rough angle. You'll see by playing around with the angles it makes a very small difference - almost negligible. I'd be much more concerned about getting accurate track widths, sway bar rates and weight distribution then precise spring/shock angles.

Good luck!
This rabbit hole is getting bigger by the week.
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      10-23-2021, 06:19 PM   #31
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Quote:
Originally Posted by IamFODI View Post
On FRC%: I think the point isn’t so much the number itself; it’s more about comparison, e.g. one setup vs. another, or an aftermarket setup vs. stock.

I tried several calculators, including the one tsk94 linked. The relationship I describe earlier was the same in all of them.
That's how I use it now. At first I was curious in the number itself and trying to find the 'ideal'. I've found it much more useless to compare one setup to another and worry less about what the value of the FRC itself is.
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      10-23-2021, 09:35 PM   #32
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Quote:
Originally Posted by tsk94 View Post
The FCM spreadsheet isn't good.

Use this one: https://robrobinette.com/Suspension_Spreadsheet.htm

Much easier and more intuitive. The only thing you need to know is the sway bar rate. Which for this car is easy to find out.
The roll stiffness math looks broken on that spreadsheet; the sway bar rate isn't being added to the spring stiffness, but is instead mixed in with the radians to degrees math.
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      10-23-2021, 10:37 PM   #33
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Quote:
Originally Posted by mkodama View Post
The roll stiffness math looks broken on that spreadsheet; the sway bar rate isn't being added to the spring stiffness, but is instead mixed in with the radians to degrees math.
Don't know, didn't make it. Frankly I don't think it's that relevant. I punch in the same numbers to a spreadsheet Shahik gave me and it's close enough that I don't question it. The one I linked is simply more user friendly so I've used it.

Again, I wouldn't get caught up in the actual FRC number. It's not that important.
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      10-27-2021, 10:15 AM   #34
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Yeah a lot of people ignore bump stop and bump travel tuning which is the biggest problem with running "neutral" spring rates. Put 300-400lb spring up front and you will be down on the bump stops a lot even with shortened stops. There just isn't much stroke up front to work with due to packaging. Most coilovers are ~4-4.5" of stroke up front which gives you pretty much the same amount of wheel travel... leaves you with very little bump travel to play with once you put 700lb of corner weight on the spring. It's no wonder why many people feel the car drives better with 400-700lbs up front. By staying off the bump stops you may have an effectively lower front wheel rate in roll.

I would not say calculating FRC is useless. The data is all here including sway bar motion ratios. It's just a lot of work. It's useful for knowing how setup A or B will change the cars behavior given X or Y feedback from the driver. At the very least, playing with the spreadsheet helps the concepts sink in. You do need to be aware of all of the significant variables though and bump stops engagement is a huge one overlooked. Then there are the smaller things that move the motion ratio around a bit like track width and alignment. If you know your spring and sway are setup to produce the handling you want but the car is still missing something then you can now look at all the other little things that are easier to tune than springs and revalving dampers.

Last edited by bbnks2; 10-27-2021 at 10:26 AM..
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      10-27-2021, 08:32 PM   #35
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Quote:
Originally Posted by bbnks2 View Post
Yeah a lot of people ignore bump stop and bump travel tuning which is the biggest problem with running "neutral" spring rates. Put 300-400lb spring up front and you will be down on the bump stops a lot even with shortened stops. There just isn't much stroke up front to work with due to packaging. Most coilovers are ~4-4.5" of stroke up front which gives you pretty much the same amount of wheel travel... leaves you with very little bump travel to play with once you put 700lb of corner weight on the spring. It's no wonder why many people feel the car drives better with 400-700lbs up front. By staying off the bump stops you may have an effectively lower front wheel rate in roll.

I would not say calculating FRC is useless. The data is all here including sway bar motion ratios. It's just a lot of work. It's useful for knowing how setup A or B will change the cars behavior given X or Y feedback from the driver. At the very least, playing with the spreadsheet helps the concepts sink in. You do need to be aware of all of the significant variables though and bump stops engagement is a huge one overlooked. Then there are the smaller things that move the motion ratio around a bit like track width and alignment. If you know your spring and sway are setup to produce the handling you want but the car is still missing something then you can now look at all the other little things that are easier to tune than springs and revalving dampers.
It's amazing how many variables are in play when tuning suspension. I looking to understand it in theory and ensure it makes sense before making any large changes. Damper curves + bump stops too alter the system as well. It will be very fun/frustrating/entertaining to learn.
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