Cantilever suspension - Explain?

[L_O_O_M]

Hi all

Can anyone please tell me the difference between these three cantilever setups please.And like I under stand it the ratio is determent by the point where the coilover attaches to the pivot point and the point where the push rod attaches to the pivot point,so if the push rod side is 4" and your coilover side is 8" then it's a ratio of 2:1 or is 1:2? and if both ends of the pivot point is 4" then it's a 1:1 ratio https://static1.pt-content.com/images/noimg.gif (http://www.freeimagehosting.net/mkiy5)https://static1.pt-content.com/images/noimg.gif (http://www.freeimagehosting.net/brc2a)https://static1.pt-content.com/images/noimg.gif (http://www.freeimagehosting.net/pbcpk)

[Bryce]

All three systems are basically the same, just a different shaped rocker arm.

Think about how much movement the pushrod has compared to the shock.

If the pushrod has a 4" lever arm and the shock has a 8" lever arm then the shock moves twice as much as the pushrod, usually not designed that way. The coilover would have half the spring rate of a 1:1 ratio rocker arm.

[exwestracer]

THe rocker ratio is typically (not always) used to make up for an inclined pushrod. That way, the spring rate can be matched to the wheel rate. The rocker design can also be used to give a rising rate spring action, but that's a little trickier to set up.

[L_O_O_M]

Thanks for the input

So the shape doesn't necessarily matter it's the lever arm length's that determen the ratio,so If I wan't to have a 1:1 ratio I keep both side's even length but if I wan't a higher ratio,let's say I wan't the shock to move half as much as a 1:1 ratio I make the push rod side twice as long as the coilover side?Or is it the other way around?And let's say the normal spring movement is 4" and I use a Cantilever setup can I use a longer coilover that's not made for the car but has the right spring rate to mach the car?

[Ripper]

loom; you shouldn't compare ratio between the pushrod and coil over. You want to look at wheel motion compared to coil over motion. You can't just talk about the rocker ratio without looking at the ratio between wheel movement and push rod movement. Therefore it's easier to talk about wheel/coil over ratio.

I would say that the one thing you choose a push/pull rod suspension instead of a traditional is because you don't have any problem with loosing spring rate during compression. When the angle on the coil over increases compared to the road/surface, the effective coil rate will decrease. To minimize that problem and to obtain a (near) linear rate over spring compression you could use a rocker arm and design it to compensate the angular changes.
Personally, I would say that it's always eaiser to opt for a linear movement than a rising rate/exponential. Cars that behaves linear is usually easier to set up and predict (unless you need to compensate for aero). I'll use push rods in my front suspension since that was the only way I could receive a 1:1 linear (not falling/increasing rate) movement between wheel and coil over.

Sometimes you wan't higher shaft speeds on the damper to be able to dial the low speed compression easier. I'm not sure when you want that though since I don't have much real world experience from tuning push rod suspensions.

EDIT: Since I'm not native english I feel that my language slightly inhibits me from expressing me, but I'm working on it...

[RobNoLimit]

When leverage is involved, both the spring rate and the shock dampening are effected by the ratio. All of our Big-10 Classic truck chassis use a rocker arm in the rear. We set as close to linear as we can, so that the links and shocks are at 90* to the pivit points at RH. We use a 1.6 ratio so that we can have 8" of axle travel with a 5" stroke shock. With this leverage, a 250 lb. spring nets 156 lb at the axle. To us, one big benefit of a rocker arm set up is the availability to set RH seporate from shock squish / spring set. You can set the spring/shock to the center of travel, not concerned about RH, then adjust pushrod length to set the chassis level.

[19,69camaro]

You can go the complete opposite direction that Rob went. Say if you have a very low vehicle that has limited suspension travel you can run a rocker ratio that allows for more shock travel vs wheel travel. That way you will get enough shock travel to make tuning possible. It also means you can run a lighter spring so the thing won't ride like a dumptruck.

[exwestracer]

You can go the complete opposite direction that Rob went. Say if you have a very low vehicle that has limited suspension travel you can run a rocker ratio that allows for more shock travel vs wheel travel. That way you will get enough shock travel to make tuning possible. It also means you can run a lighter spring so the thing won't ride like a dumptruck.

Aldin, the spring rate comment isn't strictly true... No matter what, we have to have enough spring to counteract the expected loads without bottoming the car. If we only have 2" of clearance available before something bottoms out, even if the shock is travelling 6" at the same time; the added pounds of loading against the spring have to be the same. Not only is the spring moving further, it is also moving FASTER because it compresses much more than the wheel itself. Now, using a progressive wound spring will give some of the effect you mentioned, as it can run in the "soft" portion of the spring at ride height, and get into the "stiff" portion faster with rocker ratio. A linear spring won't do that, however.

[19,69camaro]

Ray, Thanks for the explaination. I guess I assumed that if you had a rocker arm that allowed for more shock travel than wheel movement it would reduce the force.
I also heard that you loose the mechanical advantage at extreme compression, due to the force no longer being applied perpendicularly to the lever. In doing so this creates a rising spring rate to help compensate for lower spring rates.
Does this have any bearing on a pt car?

[exwestracer]

Ray, Thanks for the explaination. I guess I assumed that if you had a rocker arm that allowed for more shock travel than wheel movement it would reduce the force.
I also heard that you loose the mechanical advantage at extreme compression, due to the force no longer being applied perpendicularly to the lever. In doing so this creates a rising spring rate to help compensate for lower spring rates.
Does this have any bearing on a pt car?

One of the problems with rising rate (either from progressive windings or rocker design) is that the spring is acting differently in every part of the suspension travel. If you could always count on the suspension moving the same amount over evey bump or in every corner, rising rate would be a really good idea. Unfortunately, the suspension is doing something different almost every time we encounter a corner or bump in the road.

Not saying rising rate CAN'T work, it just makes tuning the suspension a WHOLE lot more complicated....

[overZealous1]

i thought you do push rod style just cause it looks coooool! ;)

[L_O_O_M]

Thanks for all the input it's really help full,but can anyone tell me what I should do in regards with what coilovers to use because this car's coilovers is non existent?

[overZealous1]

what car? what is your current linear spring rate? leaf spring? what point of axle are you setting push rod? rocker ratio? the coilover question is small potatoes before answering some of those questions. what have you done with the front? why the want or need to do a push rod set up?
the biggest question, are your driving skills honed enough to know the difference while in car? purpose for the car? needless to say, you will be fabbing and cutting and removing some floor pan. on a side note, you never said if it was for front or rear, so floor pan mods may or may not happen. point is, there is alot more to think about.
i am doing a push rod rear because it is easier to keep all of the suspension components associated with the frame i have built rather than transfer it to the roll cage. but, my project is well beyond a bolt on type set up. for me, a push rod is easier in some aspects. either fab an upper mount for a coilover, or, fab a mount to do a rocker/pushrod.

if you are still set on it, the answer for the correct coilover really lays upon the answers to above questions.

[L_O_O_M]

Hi overZealous1,Wow that a lot of questions but here go's

1:what car?1967 mercedes benz 250 w108
2:what is your current linear spring rate?Not sure
3:what point of axle are you setting push rod?Not sure can you please explain?
4: rocker ratio?What do you wan't to know
5:what have you done with the front?Stock for now still working on the rear
6:why the want or need to do a push rod set up?I wan't a different rear end instead of the swing axle setup so I was think about a nissan 350z rear setup so I thought I can use the push rod setup in the rear to make it easier for me to make it fit
7:the biggest question, are your driving skills honed enough to know the difference while in car?I think so
8:purpose for the car?Day to day that doesn't have to stand back for most thing on the road
9:needless to say, you will be fabbing and cutting and removing some floor pan.Don't have a problem with that(project car)

Hope this answers some of your questions.

[exwestracer]

Scott, have you seen his other thread?


https://www.pro-touring.com/showthread.php?93755-Custom-sub-frame

[overZealous1]

nope, didn't see that. looks like a cool project though! i am a fan of the 350z irs! haha.

loom, those questions were more for you to ponder. just different things to look at and make descisions on while doing it. the 350z rear is about the easiest to do a swap with. i personally am doing a 350z irs and am doing a pushrod spring set up with it. https://www.pro-touring.com/showthread.php?84491-350z-rearend-67-camaro
my next move is to build the rockers and set up the coilover mounts. then weld together my rear frame to the speedtech front. keep an eye on it and it may give you some ideas. luckily ray is never too far away and has helped answer some of my questions and keep me from doing stupid stuff! haha.

[L_O_O_M]

Wow your project looks cool,what does the 350z irs rear end go for in the US?I will definitely be keep an eye on your project.