Cantilevered shocks in trunk

[LowBuckX]

HAve we ever discused the Pros/cons of shocks mounted in trunk useing a cantilevered push-pull set up.... Looks trick helps in packaging IN my eyes.... what say you.

[Damn True]

As long as the bellcrank arrangements assure linearity and calculable motion ratio it should work fine.

[LowBuckX]

1:1 ratio of motion is what I have on paper

[MrQuick]

Check with this guy, he has a real cool 69 with a BMW IRS and pushrd set up in his trunk.

Barry Goldline

[email protected]

I've been meaning to invite him here.

[Bryce]

i have been working through this same idea with my falcon 3link. I think the pushrod set up is good. It offers a little less unsprung mass and allows the pushrods to have better mounting locations due to the smaller diameter. The down side is the complication of mounting and trying to fit more components. It does add a little more weight. But allows more adjustability with adjustable pushed length

[LowBuckX]

Found this on a gen 3 camaro

https://static1.pt-content.com/images/noimg.gif (http://img258.imageshack.us/i/pushrod1.jpg/)


https://static1.pt-content.com/images/noimg.gif (http://img27.imageshack.us/i/pushrod2.jpg/)

[LowBuckX]

Again Im gonna shoot for a 1;1 ratio so my feeble mind dont have to figure out the complicated math for reduction values. Unless someone can give me a quick tutural of figureing out ratio v spring rate.

[MrQuick]

should be a simple leverage calculation. 1:1 should be equal pivot lengths right? I'd like to do a longer pushrod pivot to utilize less spring pressure.

[LowBuckX]

Not sure what the useable suspension travel should be. That is the starting point in my mind. all is fine till you are stuck with 2inch (4 total) up down travel.

[frojoe]

Gotta be careful with those bellcranks though, if they're not oriented with proper geometry, they can create a rising rate of leverage upon the shock, making the perceived spring stiffness non-linear, making it stiffer the higher up the axle's pushrod goes. This happens when the axle pushrod starts travelling up a lot vertically, this decreases the "leverage arm" about the pivot point, which is what causes the shock's spring to appear to be getting stiffer.

The leverage ratios are like this: if they're equal lengths, it's 1:1. If the length from pivot to shock is kept the same, and the length from pivot to the pushrod gets longer, then the pushrod develops more leverage over the spring, requiring the spring rate to be higher, however this also gives you more pushrod travel and thus more axle travel. If you increase said pivot-pushrod length to create a ratio of say 2:1 compared to the pivot-shock length, this would require a spring 2X stiffer.

If you end up wanting more travel, best would be to keep the ratio the same but select a coilover with a longer stroke, so you can gain more axle travel while keeping the small-bump sensitivity/tunability of a cushier spring.

But I'm no suspension tech, just my 2centz.

[neki67]

Check with this guy, he has a real cool 69 with a BMW IRS and pushrd set up in his trunk.

Barry Goldline

[email protected]

I've been meaning to invite him here.

Please go ahead and invite him here. Very interested in what he has and how it works. Plenty of BMW's around here to pick parts from so it would/could be interesting . . . .

[LowBuckX]

I sent him an Email with invitation... Its a buisness email so He might not get it till monday..

[Damn True]

I doubt it. He's auto crossing his A-mod car this weekend.

[MrQuick]

Gotta be careful with those bellcranks though, if they're not oriented with proper geometry, they can create a rising rate of leverage upon the shock, making the perceived spring stiffness non-linear, making it stiffer the higher up the axle's pushrod goes. This happens when the axle pushrod starts travelling up a lot vertically, this decreases the "leverage arm" about the pivot point, which is what causes the shock's spring to appear to be getting stiffer.

The leverage ratios are like this: if they're equal lengths, it's 1:1. If the length from pivot to shock is kept the same, and the length from pivot to the pushrod gets longer, then the pushrod develops more leverage over the spring, requiring the spring rate to be higher, however this also gives you more pushrod travel and thus more axle travel. If you increase said pivot-pushrod length to create a ratio of say 2:1 compared to the pivot-shock length, this would require a spring 2X stiffer.

If you end up wanting more travel, best would be to keep the ratio the same but select a coilover with a longer stroke, so you can gain more axle travel while keeping the small-bump sensitivity/tunability of a cushier spring.

But I'm no suspension tech, just my 2centz.
Great points, can the actual length/angle of the pushrod affect this too? I've been doing a search on applying the set up to our applications but nothing yet. The best help shows to be here. http://www.jalopyjournal.com/forum/showthread.php?t=145167 copy to you favs so we don't get nailed for hot linking.


I doubt it. He's auto crossing his A-mod car this weekend.




thats right I forgot about the races this weekend. He'll get it eventually. He answers his emails fast.

[JRouche]

Gotta be careful with those bellcranks though, if they're not oriented with proper geometry, they can create a rising rate of leverage upon the shock, making the perceived spring stiffness non-linear, making it stiffer the higher up the axle's pushrod goes. This happens when the axle pushrod starts travelling up a lot vertically, this decreases the "leverage arm" about the pivot point, which is what causes the shock's spring to appear to be getting stiffer.

Good info. But you want a rising rate VS a decreasing rate on bump dont you? Thats the idea with spring setups. You can create a decreasing rate through bump accidentally, or purposely. But like what you are saying if the rising rate is too high it becomes non-linear in a bad way. Im not all up on the suspension setups but I thought most of them are set up with a rising rate. And I thought that was the purpose for using a suspension of this type. Taking advantage of the increased rising rate, stiff spring, compact area and still plenty of travel.. JR