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Author Topic: Cantilever Suspension  (Read 2861 times)
Bull Durham Racing
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« on: October 02, 2008, 07:15:13 AM »

Since we had a decent finish at the WEROCK All Stock Nationals this past weekend we've decided to compete again next year. The only difference is we will compete at the regional level at Dayton, TN. This will give us the opportunity to run 4 regional events to qualify for the nationals plus 2 WEROCK Eastern Division events based on their tentative schedule.

There are a few changes I want to make to the rig before next season.

1. Reduce overall weight by removing the crusher corners, modifying the skid plate, modifying rear winch mount and going to a lighter rear axle. We estimate approximately 300 lbs savings from these mods.
2. Improve steering by going to a longer pitman arm and possibly a longer ram. I would like to gain 5 degrees of steering with these mods.
3. Reduce rear track width. This is where the cantilever suspension comes into play. Since my rear shocks are mounted on the outside of the frame rails I can't run a deeper backspaced wheel. Currently I'm 79.5" wide in front and 78.5" wide in the rear using 2.5" backspacing. By getting the shocks out of the way I can run 3.75" backspacing, remove the .5" spacers and reduce the track width by 1.75" per side. This will help with steering and hopefully miss more cones with the rear tires.

This is where you guys come in. I need help figuring out how to design the cantilever system. Here are a couple of pics to give you an idea of what I'm talking about.





I plan on running 8" ORI struts instead of the 8" coilovers in the pics above to allow for more adjustability and tuning. The goal is to get 12" of travel from an 8" strut. This will equate to a 1.5:1 ratio from the bellcrank.

Here is a pic of the bellcrank/pivot box I plan on machining once we figure out the dimensions.



Any and all suggestions are welcome.
« Last Edit: October 02, 2008, 07:54:17 AM by Bull Durham Racing » Logged

Bill Durham
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« Reply #1 on: October 02, 2008, 07:32:32 AM »

Sorry - no suggestions here, but it looks cool.  I can't quite wrap my brain around it quite yet - I'll go get soem coffee.

What rear axle are you going to run - Ford 9?

Also, do you have any photos of your front axle on the passenger side?  I'm trying to figure out how you fit a ram, a track bar, steering, and a shock in there.
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Ryan Shanteau
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« Reply #2 on: October 02, 2008, 07:38:00 AM »

What is the weight of the rear? I looks like the bellcrank should reduce the overall travel by about 33% thus increase the spring rate. It sounds like a fun project. When do we get started?  occasion14
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Bull Durham Racing
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« Reply #3 on: October 02, 2008, 07:43:39 AM »

I will be running a 35 spline full float Spider 9 which is 150 lbs lighter than the Solid 60 plus has 1" more ground clearence.

Creepy - The bellcrank will increase axle travel instead of reducing it and depending the length from the pivot mount to the end link mount will determine the ratio.

Ryan - I will get you a pic of my front setup.
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« Reply #4 on: October 02, 2008, 08:38:47 AM »

If I understand the setup from the pics, it's a rigid tube support frame supporting the two bell crank arms.  If that's the case then the flexing motion of the axle is dependent only on the following factors:

1) Position of the joints between the crank and the support frame frame

2) Length of the crank

3) Length of the links between the crank and the axle.  The version in the pic is adjustable, which seems to be a good idea.

It would seem that there is must less resistance to lateral motion than with a 4-link, so you will probably need a pretty hefty trackbar.  The relative position of the link from the crank to the struts should be less critical as long as it permits full desired travel.  Once you know the crank length, the position of the strut link will be easy to calculate.

Given the tabs welded to the support frame, I'm assuming that they are for a protective plate.
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« Reply #5 on: October 02, 2008, 09:50:35 AM »

The version in the pic above is a bolt system. The one I plan on building will not have all the extra structure i.e. X bracing. Since my rear bumper is welded to the ends of the frame I plan on welding shock tabs to the bumper and welding a piece of 1.75" DOM between the frame rails to mount the bellcrank. This will reduce weight and should be plenty strong.
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« Reply #6 on: October 02, 2008, 12:00:18 PM »

Sounds find.  In any case, the geometry is not affected by the support structure, only where the crank attaches.

The cross tube will have to be low enough to clear the top of the crank.  Depending on the clearance the tube may need to be bent.

In any case the geometry seems to boil down to solving two inter-related triangles.

The first triangle is variable and consists of the rear crank joint, the upper crank joint, and the shock atatchment point.  The distance between the last two will be fixed.  The shock forms a variable leg whose length is delimited by the shock's travel limits.  if you then assign a length to the third leg, it determines the range of angular motion of the crank.

Then you can plus that into the second triangle formed by the rear crank joint, the forward crank joint, and the axle.  The angular motion will determine the total vertical displacement of the axle.

The angles of the crank triangle itself will then depend on where in the shock's travel it needs to be at ride height, and thus what the division of the total axle flex is uptravel vs. downtravel.

So there are a lot of variables and possible configurations.  Once you know where the mounting points for the crank support and the shocks will be, then problem becomed more constrained.

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« Reply #7 on: October 02, 2008, 12:02:22 PM »

I will be running a 35 spline full float Spider 9 which is 150 lbs lighter than the Solid 60 plus has 1" more ground clearence.

Creepy - The bellcrank will increase axle travel instead of reducing it and depending the length from the pivot mount to the end link mount will determine the ratio.

Ryan - I will get you a pic of my front setup.

Thx! in for the chance pottytrain1
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« Reply #8 on: October 02, 2008, 12:22:29 PM »

spent some time under a couple of jk's like that at ejs.

The guy that built them was telling me that if the crank is designed rite you actually change the rate of the coilover as the eye moves.  Dont know really which ways helps or hurts but might be something to look into.

Mount the bump stop off the bell crank and it will look like the rear axle is just floating there drunken_smilie.

any thought to what you will use for the joint?  Use tapered bearings (dana 30 pinion size maybe) and they would probably never wear out.  Just machine the bell crank so a bearing race presses in either side. dontknow
« Last Edit: October 02, 2008, 12:25:47 PM by PorkChop » Logged

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« Reply #9 on: October 02, 2008, 02:02:43 PM »

I have thought about using a bearing instead of a poly joint on the bellcrank. Maybe a pilot bearing would work.
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Bill Durham
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« Reply #10 on: October 02, 2008, 03:37:11 PM »

Should get the job done.  Any idea what the forces on the pivot are?  I know most of the weight is transfered to the shock but does any sit on that bolt?

Looks like the needle bearing would work better.

http://www.eng-tips.com/viewthread.cfm?qid=174119&page=1
« Last Edit: October 02, 2008, 03:43:24 PM by PorkChop » Logged

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David Minton
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« Reply #11 on: October 02, 2008, 03:44:28 PM »

I don't know what the forces are on the bellcrank.
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« Reply #12 on: October 02, 2008, 04:44:06 PM »


Creepy - The bellcrank will increase axle travel instead of reducing it and depending the length from the pivot mount to the end link mount will determine the ratio.


You are right. I was thinking about the shock travel and not the axle when I first posted.  pottytrain1
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« Reply #13 on: October 02, 2008, 05:17:14 PM »

Brainstorming and thinking out loud so forgive the ramble/questions; 

How many more possible points of failure does this bring into the equation? 
Does the length of the cantilever determine the amount of flex?
If so, how long would the arms need to be to accomplish the flex you currently have and/or need?
Fulcrum point comes to mind.  How much force would be exerted on any one point of the system?
Would this cause any weak links in the strength of the system and increase the failure possibilities? 
Would the system decrease ground clearance?
Are there any other (KISS - Keep it simple stupid) options to deal with the need for better placement of shocks with less possible mechanical failure points?
Seems a lot of engineering, testing, proof of concept.......time and money will go into it.  Does the benefit out way the cost?

My train of thought is it seems very complex to solve a simple need.  Might be an over simplification though.   dontknow

Just my .05 cents worth.
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« Reply #14 on: October 02, 2008, 05:30:24 PM »

It is a simple need but in order to stay within the rules there really isn't any other way to get my shocks out of the way of the tire to narrow my track width. The only other options are to go to a leaf spring or stock type coil spring. Both of which do not perform as well as an air shock or strut.
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