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PostPosted: January 28, 2018, 8:37 pm 
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Joined: November 26, 2012, 2:29 pm
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Location: Sioux Falls, SD
So I'm trying to figure out my suspension geometry in the front of my car and I'm wondering how other people have tackled their suspension. Currently I have 6.49" of travel from fully extended to fully compressed. I did some math and at 5G's of upward bump force it would move the suspension 10" which of course my suspension would only take 6.49" of that force.

I'm mostly curious what everyone else's spring rates, and total suspension travel are, as well as if you had figured in the maximum amount of bump G-Force that the suspension would have to withstand.


Currently I have 200 lb/in springs at an average of 152 lb/in with the angle of the coil-over taken into account. I'm kinda curious if I should try to make the springs stiffer or if my current coilover setup would work ok.


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File comment: 2.22 is the Ride Height.
6.49 is total suspension travel.

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PostPosted: January 29, 2018, 3:30 am 
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So you inherently have "1G" worth of available droop from rest, which appears to be ~2.2 inches. While I have not tested this out for myself yet, from my research I believe that an additional 1G of bump (~4.5 inches total from full droop) before hitting the bumpstop should generally be sufficient.

Another way I've seen people look at it is that your at rest droop should be 1/3 of your total available travel, while 2/3 of your total available travel should be reserved for bump travel. This method does not include the reduction of available travel from the bumpstop in the analysis, so the ultimate result is it typically seems to end up relatively close to the first method I mentioned once the bumpstops are factored back in.

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Last edited by Driven5 on January 29, 2018, 12:31 pm, edited 2 times in total.

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PostPosted: January 29, 2018, 8:40 am 
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Location: Holly, MI
It seems like you are missing a few things in these calculations. First off, where are you accounting for the force the shock absorbs? Shock force is proportional to speed, so a 5G bump in the road will create more force in the shock than in the spring. Second, where is the motion ratio of the suspension? Without seeing your calculations, I can't tell without doing the math (and I didn't do that). The force the wheel sees is not the same as the wheel because they aren't traveling the same distance.

I think the practical answer to your question is to use some known geometries and rates and then test it on the street. Put some travel indicators on the shocks and see if you are bottoming them out.

Ken


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PostPosted: January 29, 2018, 10:07 am 
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Location: 4AGE in S.E. Michigan
I would say that 95% of all Seven style suspension systems only have about half the travel of your current design.
As already mentioned you need to look at jounce loads of 2G, along with a vectored braking of 2G, using the 1/3 to 2/3 travel ratio.
The only time you see 5G's is when you missed the corner and end up in the ditch. [Structurally design for 5Gs]
DaveW


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PostPosted: January 29, 2018, 11:17 am 
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Location: Sioux Falls, SD
BB69 wrote:
It seems like you are missing a few things in these calculations. First off, where are you accounting for the force the shock absorbs? Shock force is proportional to speed, so a 5G bump in the road will create more force in the shock than in the spring. Second, where is the motion ratio of the suspension? Without seeing your calculations, I can't tell without doing the math (and I didn't do that). The force the wheel sees is not the same as the wheel because they aren't traveling the same distance.

I think the practical answer to your question is to use some known geometries and rates and then test it on the street. Put some travel indicators on the shocks and see if you are bottoming them out.

Ken


I only took a snapshot of a portion of my spreadsheet, I do have some math for motion ratio in there but I did not account for anything as far as shock vs spring forces because I have no idea how to do that. I literally only figured (my sprung weight x 5)/max spring rate for the 5G figure.

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PostPosted: January 29, 2018, 11:19 am 
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davew wrote:
I would say that 95% of all Seven style suspension systems only have about half the travel of your current design.
As already mentioned you need to look at jounce loads of 2G, along with a vectored braking of 2G, using the 1/3 to 2/3 travel ratio.
The only time you see 5G's is when you missed the corner and end up in the ditch. [Structurally design for 5Gs]
DaveW


Anywhere where I could look to learn about computing Jounce loads? I literally have been searching online for most of my sources on equations for figure out suspension. Know of a book that I could buy that would have more info on this in it?

I'm glad though that I don't have to really figure 5Gs into normal driving, I picked that number off of a UK locost site and thought it was a bit much but w/e. I'd rather overbuild than underbuild for my own safety.

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PostPosted: January 29, 2018, 12:01 pm 
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Location: Holly, MI
There are many books out there on the subject, but they can range from incorrect, to so technically detailed they put you to sleep. I found this guide that seems to be a nice summary:

http://www.kaztechnologies.com/wp-conte ... sprzak.pdf

I would also suggest you look up the Mark Ortiz chassis newsletter. He publishes a newsletter every month; anyone can join the mailing list. There are also a number of sites with old newsletters archived.

For books, some of the ones I like include:

How to make your car handle
Engineer to win
Vehicle Dynamics by Milliken



Ken


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PostPosted: January 29, 2018, 12:38 pm 
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Driven5 wrote:
So you inherently have "1G" worth of available droop from rest, which appears to be ~2.2 inches. While I have not tested this out for myself yet, from my research I believe that an additional 1G of bump (~4.5 inches total from full droop) before hitting the bumpstop should generally be sufficient.

Another way I've seen people look at it is that your at rest droop should be 1/3 of your total available travel, while 2/3 of your total available travel should be reserved for bump travel. This method does not include the reduction of available travel from the bumpstop in the analysis, so the ultimate result is it typically seems to end up relatively close to the first method I mentioned once the bumpstops are factored back in.


I haven't figured in bumpstops yet but I will probably do that soon. I'll have to take a look at this method though, seems pretty sufficient.

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PostPosted: January 29, 2018, 12:42 pm 
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Thanks for all the suggestions BB69!

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PostPosted: January 29, 2018, 1:13 pm 
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Well, you're just about spot on for the (1/3)/(2/3) setup. So with a reasonably sized bumpstop (about 1/3), you'll be on par with my thoughts too. Eibach makes some nice progressive MCU (micro-cellular urethane) bumpstops in 1.81 and 2.13 lengths that should fit the bill nicely.

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PostPosted: January 29, 2018, 3:46 pm 
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When it comes to springs, I would suggest just working with what you have, or buying cheap ones to start: Chances are you are going to be buying different springs once you do some testing/driving no matter how many calculations you do. It's much easier to do the math backwards and get the correct springs once you know which way you want to go (and your final weights).

I went for softer springs with a lot of travel as this is a autoX/street car. I have ~2.5" droop, ~3.5" bump and 1.5" of bumpstop. People (myself included once) tend to build cars to be track cars when they need more a rally car to deal with the bumps of the roads. Running out of bumptravel and then understeering in corners is not fun, nor fast.

Cheers.

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PostPosted: February 7, 2018, 5:03 pm 
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I went through this about 5 years ago. I ended up with springs a lot stiffer than necessary. But it turns out that when you hit a bump, the suspension doesn't soak up the full height of the bump. Not by a long shot. The chassis takes up easily half of it (i.e. you get jarred), so the suspension only sees half the compression you'd expect, or less. I have about 4" of travel in front and 2" in back. I have never hit the bump stops. Not once, not even close, in 45K miles. And I've been down some seriously bumpy roads at speed over the years. My springs are 340#/140# F/R.


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PostPosted: May 16, 2018, 4:49 pm 
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nick47 wrote:
I went through this about 5 years ago. I ended up with springs a lot stiffer than necessary. But it turns out that when you hit a bump, the suspension doesn't soak up the full height of the bump. Not by a long shot. The chassis takes up easily half of it (i.e. you get jarred), so the suspension only sees half the compression you'd expect, or less. I have about 4" of travel in front and 2" in back. I have never hit the bump stops. Not once, not even close, in 45K miles. And I've been down some seriously bumpy roads at speed over the years. My springs are 340#/140# F/R.


Have you by chance done any Aussie mods? I'm wondering because I'm building this chassis to be as stiff as possible, and I've already surpassed the book chassis stiffness by 50-75% in my model.

I would assume then that the suspension would then take the brunt of the force before the chassis.

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PostPosted: May 17, 2018, 10:43 am 
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My understanding of why you want the chassis to be stiff is so that it will force the suspension to do it's job, namely absorb forces and respond to the road in such a manner that maximum control and traction are maintained.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: May 18, 2018, 10:59 am 
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Lonnie-S wrote:
My understanding of why you want the chassis to be stiff is so that it will force the suspension to do it's job, namely absorb forces and respond to the road in such a manner that maximum control and traction are maintained.

Cheers,


That was my take on it as well. Hence the question. :D

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