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Old 04-12-2005, 01:23 PM
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Rich Lackey
 

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Scratch built IFS?

I think I have been around this forum just about long enough, and made enough friends to to ask some stupid questions and get away with it (I hope).

I'm a engineer at heart, and I like solving engineering problems. I have a few of those with my particular project. Some of these problems are legitimately created by a lack of ready to weld or bolt-in kits and parts available in South Africa, and some of them I have created myself.

The retractable hardtop is an example of a engineering problem I created all by myself, but my chassis build up has a few problems created by circumstance more than anything else.

I recently posted a question here about a 4-link (or 4-bar... someone please explain to me the difference?) setup, and the reason for that is because my rear leaf springs are shot, and I can't find replacement new ones very easily.

This also brought into my mind the possibility of cutting the frame rails just forward of the rear axle, welding a crossmember in and rewelding the rear frame rail sections on risers, and narrowed just a few inches so I can fit some bigger rubber in there. Obviously this would require a narrowed axle and tubs to match.

So far so good, I like this idea. Now onto the front. The original crossmember, control arms and spindles just aren't going to cut it for this particular car, in function or aesthetics. A normal solution would be a Mustang II with dropped spindles and big disc brakes, but I find the original Mustang II hardware ugly (I probably wouldn't be able to find it either), and the pretty polished stainless aftermarket kits aren't readily available.

So enough of the intro. What are the big problems or challenges associated with scratch building a IFS setup?

The crossmember seems simple enough, as I can even get the sheetmetal components laser cut from CAD drawings ready to weld up. I don't see much room for error fabricating a crossmember.

Control arms are another story altogether as they have to be perfect. So, my thoughts are rather than employing solid welded stainless steel control arms, use a pair of stainless steel rods with adjustable rod ends for the upper and lower arms (four in total per wheel). The fact that there is some adjustment there would allow me to dial in the precision in alignment that might not be possible in welding up solid control arms without the right jigs and so forth.

I would utilise standard Mustang II ball joints and spindles, as these are small enough I can get my parents to smuggle them in from the U.S. when they visit in July.

Instead of a normal coil over setup, I would employ some kind of a pushrod with a bellcrank and a coilover running parallel to the frame rail, or a torsion bar.

What I am envisioning would end up looking more like a F1 car front suspension than a normal Mustang II.

That's my idea, it's open for advice, opinions, suggestions and criticism, I can take it!

I want to learn, bring it on!

Rich

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Old 04-12-2005, 01:33 PM
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Rich,

I'm no expert. I hope others will chime in. This could be an interesting thread.

One thing you haven't mentioned is steering.

As I understand it, getting the geometry of the steering in relationship to the suspension is critical to avoid bumpsteering. Or, if you plan on sticking with the factory steering, you'll have to design the suspension around that.

There are several good books on suspension design out there, as many people scratch-build race cars, so its definitely a "do-able" project.
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Old 04-12-2005, 02:06 PM
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Designing a front suspension is real easy. Designing one that functions and handles reasonably well is a bit tougher...
Look here...
http://www.gmecca.com/byorc/designpu...ionsbooks.html
and here...
http://www.longacreracing.com/articles/art.asp?ARTID=13

You HAVE to have some background and understanding of the entire system before you can ask the right questions...
Do a little research, learn some of the terminology and how all of the components relate to one another then start asking your questions.
Mark
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Old 04-12-2005, 03:53 PM
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Mark,

Thanks for the links, especially the article on bumpsteer:

"In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm"

This is basically what I already understood as necessary in designing proper steering linkage. If I were to use an existing power steering rack, then I would have to design the whole front end around it in terms of keeping the above factors in check. The rack would then basically determine my upper and lower a-arm pivot points, and the corresponding length of the a-arms in order to keep the tie rod ends where they need to be in relation to the imaginary line running between the upper and lower ball joints.

However, I imagine if I used a power steering box rather than a rack I could build my own linkage keeping all the geometry in check according to my chosen a-arm lengths and pivot points rather than letting a rack determine these dimensions.

I am sure there must be a lot more involved in determining the correct a-arm lengths, pivot points, travel and so forth for a given situation so you are right. I would do well to read up on it.

Just wanted to get some ideas here first as to if I should even bother thinking along these lines.

So far, I get the impression it is doable with the right information in mind.

Rich
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Old 04-12-2005, 05:56 PM
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Quote:
Originally Posted by rlackey
I recently posted a question here about a 4-link (or 4-bar... someone please explain to me the difference?).... Rich
Google "Art Morrison" and look at his product line.

Quote:
Originally Posted by rlackey
Control arms are another story altogether as they have to be perfect.
Rich
Not really. I was adapting a Pinto spindle and ball joints to a British car. I took both sets of lower arms to the chop saw. I then mix-matched, with a little steel plate at the interface, eye-balled everything and welded them up. Everything aligned properly and the car handles and drives beautifully. I would definitely avoid the adjustment features (beyond those typically used for caster and camber) if at all possible.

As for design evaluation: You mentioned that you have the ability to use CAD software. Many of the CAD packages allow you to operate linkages as an assembly. If you have any kind of 3D capabilities (in the software), you can, if the software doesn't allow that which I've just described, build the assembly in different positions to check it out. Just keep your models as crude and simple as possible.
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Old 04-12-2005, 10:56 PM
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I say go for it. Don't set your mind on just MustangII spindles. Try to see if you can find something over there that you know will work. The entire rest of the suspension depends on the spindles! Make sure you pick some you like and also can get dropped spindles for if you want.

Start with buying some of (the late) Carroll Smith books! They are a very good read and are full of practical information.

As for bump steer and rack placement the angle of the links makes the most difference! If you are 1" lower then optimal you will have bad bumpsteer. If your rack is 1" shorter (on each side) then optimal you might not even notice it. If you go to a parts store the catalogs should have dimensions in them.

I don't think doing a pushrod suspension is such a great idea for your first suspension. Designing the bellcranks can be quite difficult! Most people also don't have a good idea of how much load goes into a bellcrank.

The sad thing (not for you...for the engineers that designed them) is that so many of the factory suspensions SUCKED SO BAD that if you put half an ounce of thought/research into your suspension it will turn out much better then factory.

Last, ask questions about things you arn't sure of (and even about some of the things you are sure of) and don't forget to post A LOT of pictures
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Old 04-13-2005, 02:25 AM
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Quote:
Originally Posted by BillyShope
As for design evaluation: You mentioned that you have the ability to use CAD software. Many of the CAD packages allow you to operate linkages as an assembly. If you have any kind of 3D capabilities (in the software), you can, if the software doesn't allow that which I've just described, build the assembly in different positions to check it out. Just keep your models as crude and simple as possible.
Billyshope,

I use AutoCAD 2004, and a 3D nurbs modelling package called Rhino3D for 3D modelling.

The retractable hardtop mechanism I am working on is being modelled and tested in 3D until it is perfected, then I'll have the individual components CNC machined directly at a local prototyping firm with CNC lathes and mills that do a lot of one-off type work like this.

The IFS would be done the same way so I can test and troubleshoot without spending any money or building anything, all it will cost is my time.

Rich
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Old 04-13-2005, 07:19 AM
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Rich,

What type of vehicle are you planning the retractable hardtop for?

I've been pondering something along those lines myself. I'd be interested in seeing what you've come up with. Are you going to retract the entire top as a single piece, or section it into two or more pieces?

(Sorry to hijack the thread off in a different direction)

- Chris
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Old 04-13-2005, 07:32 AM
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Rich,

Coincidentally, there's another new thread on retractable hardtops:

http://www.hotrodders.com/showthread.php?p=441296

You guys might be able to help each other out.

- Chris
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Old 04-13-2005, 07:52 AM
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Hi Chris,

It's a '54 Chevy Bel Air, here are the drawings of the mechanism so far, these were generated from the CAD software I am using. I am splitting the roof into three sections, and despite the weight, I am using the original steel roof but modified quite extensively in a few places.



I hope the image shows properly, if not just download the attachment and open it.

Rich
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Old 04-13-2005, 09:57 AM
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Back to front ends

Located here are some computer programs that allow one to analyze their chassis..

Look here This could be helpful in designing your own chassis and front end..

OMT
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I have tried most all of it and now do what is known to work..
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