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PostPosted: December 26, 2017, 10:37 pm 
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Joined: November 26, 2012, 2:29 pm
Posts: 98
Location: Sioux Falls, SD
I've been trying to figure out my bellcrank design recently and I want to make sure my excel spreadsheet for figuring this out is made correctly because it seems to give me the opposite kind of results that I want. For instance with my suspension compressed it gives e a softer spring than when it is uncompressed.

Would someone mind checking it out and give me some pointers? Maybe I could get a bit of help with the design of the bellcrank itself? I'd like to shoot for something near 80-100 CPM at static, so something around 154 lb/in spring rate right between the compressed and uncompressed values.

https://docs.google.com/spreadsheets/d/ ... sp=sharing

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Bellcrank 2.PNG
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Bellcrank 3.PNG [ 170.37 KiB | Viewed 503 times ]

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PostPosted: December 27, 2017, 2:00 am 
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Mid-Engined Maniac

Joined: April 23, 2006, 8:26 pm
Posts: 5859
Location: SoCal
Just by eyeballing it, the rocker arm is backwards; turn it around and see where you are.

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PostPosted: December 27, 2017, 1:59 pm 
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Joined: July 6, 2009, 11:50 pm
Posts: 155
First you should use SolidWorks to verify your spreadsheet calculation of the motion ratio. Move the wheel up an inch in the assembly and see how much the spring compresses. I can see right away that your suspension spring rate formula is wrong. The wheel rate should be spring rate/(motion ratio^2).

As for the geometry of the bellcrank, it looks like the pushrod side is gaining advantage with compression and the coilover side is loosing it. Try lifting both connection points to make the rocker arm look more triangular. You want that 68* at the coilover side to be increasing as the suspension is compressed and the 74* at the pushrod side to be decreasing (or at least increasing at a lower rate).


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PostPosted: December 27, 2017, 8:14 pm 
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Joined: November 26, 2012, 2:29 pm
Posts: 98
Location: Sioux Falls, SD
Zac88GT wrote:
First you should use SolidWorks to verify your spreadsheet calculation of the motion ratio. Move the wheel up an inch in the assembly and see how much the spring compresses. I can see right away that your suspension spring rate formula is wrong. The wheel rate should be spring rate/(motion ratio^2).

As for the geometry of the bellcrank, it looks like the pushrod side is gaining advantage with compression and the coilover side is loosing it. Try lifting both connection points to make the rocker arm look more triangular. You want that 68* at the coilover side to be increasing as the suspension is compressed and the 74* at the pushrod side to be decreasing (or at least increasing at a lower rate).


Awesome. Thanks for the help. I should be able to figure it out now. It was too long of a christmas weekend and I couldn't think straight, too much eggnog probably haha.

I'll see if tonight or tomorrow I can get a better design made up.

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