Geometry for frame/suspension

What are the critical things I need to learn/keep in mind when designing my frame/suspension?

I need to pinch my new frame in about 4" maybe more not sure yet. I'm thinking I'll use just bags on the front & still kicking around ideas for incorperating bags in the rear along with the transversed spring by possibly using a cantilever.

I have a know it all friend who tells me I need to do this and need to do that but he's never built this type of frame. I tell him I'm gonna buy a book on building frames & suspensions & he says it's a waste of money just ask him what I need to know[S.

Things I need to learn a bit about right away is proper placement of 4 links & panhard bars & and the difference in their use (panhard to both sides or just one) Degree of rotation on my differential, degree of trans/motor angle.

Front suspension, How important is placement of bones? Rake/angle of axle? Akrym geometry? Bump steer?

Boy Charlie, that info could fill a book. But here are some basics for the suspension you are thinking about.

Any bagged suspension, either front or back, needs something to not only keep the axle from going fore and aft , but sideways too. Your wishbones or 4 bars will handle the fore and aft, but you will need a panhard to keep the frame from shifting sideways on the bags.

The one exception to the rule is the Cenpen airbag front suspension like David bought from my Son. It has two arms that mimick a traverse spring, but instead of those flexing the bags on top do that. An airbag setup takes some thought and work to make it right, a lot harder IMO than setting up a traditional traverse setup. Picking the right bags is also important, as well as all the supportive equipment, like pumps, tanks, lines, valves, etc.

With bags and an I beam axle setup ( or a standard rear end setu) all you can do is dial in the caster at the height you intend on driving at most often, becuase anytime you hit the switch and change height you are going to lessen or increase your caster and pinion angle. No way around it. So you just have to pick a normal ride height and engineer your caster and pinion angle around that height.

As for do you need one or two panhards, only one on front and one on rear. Mount them to the drivers side frame rail and going to the passenger's side of the axle. The longer the better and the more level the better, but not always possible due to space constraints.

Don

This will show you some of the stuff you will need to airbag the front and back. This is a frame my Son built some years ago to go under his Fordor sedan.

Thanks Don, I can't believe I've only gotten 1 reply on this thread[S

Charley Davidson said:

Thanks Don, I can't believe I've only gotten 1 reply on this thread[S

Click to expand...

It happens.

Sometimes I wonder if I said something wrong in my thread

But Don just about always is there with a good answer[cl

Instead of running a panhard with the bags in the rear, a triangulated four link would work on the rear as well, wouldn't look as good on the front, and probably not the space for it either.



Rest of the pictures of this frame here:
http://www.streetrodgarage.com/1949-1954chevroletchassispage.aspx

Another option to control side to side would be a wishbone three link, as the wishbone link would control the side to side movement. I know this isn't an overly popular choice but I have seen it done and for the application my buddy used it it worked amazingly. Typically found in offroad and mini truck suspension set ups but I think could be easily adapted to a rod.



This is more the style I have seen used, two ends of the wishbone attached at the frame with a centre mount on the rear axle. If using a cast pumpkin on the rear end you cant easily weld the mount to the pumpkin so a bridge could be made up and over to incorporate the mount. (in the picture below, for some reason this guy has a 5 link with four straight links and the wishbone, realistically you can make this with two straight links and the wishbone)

I'm running a setup, very similar to the one pistolpete showed, on the back of my rpu project. It comes out of the bagged minitruck field and doesn't require a panhard. The top wishbone limits side to side movement as well as fore and aft.

Don

What I think I know....

Panhard to keep your axle from moving side-to-side - needs to be as long as possible and as close to level at ride height. As it goes through it's arc, it will move your axle sideways. Not the best for an air system if you are running fenders - as you go through lift, your axle will move sideways and cause possible tire scrub on fenders or box/frame.

I'd say either a triangulated 4link or a 3link - both like what are pictured above. No side to side movement throughout your lift with those. I think the triangulated 4link will limit your lift because eventually it will bind, but shouldn't be a problem unless you put the bags on the links. Notice the one pic has the bags on the links instead of the axle - you'll get a lot more lift that way.

Many people just bag the rear and leave the front on springs because of the problems of bagging a straight axle front - geometry of a large range of motion and steering - and looks.

Isn't standard motor angle 3 degrees tail down? Your pinion at ride height would be 3 degrees up to match. One thing I learned, is if your axle is higher than your tailshaft when aired out, when you raise up, your tailshaft pushes in. So you need to give more pull out than the standard 3/4".

A couple links for air ride system parts:
http://thorbros.com/catalog/air-management/air-management-kits
http://www.airassisted.ca/us/AirBags-Slam-Specialties/c1_4/index.html

Don's son's system is probably a lot more extreme than what you want. I know you have scrounging and fabrication skills, so a lot of the system you can make or find on your own.

The way the Cen-Pen is set up is probably the best geometry to copy. Also, you'll want to go full 4 corner air system, not just front/back system, they lean bad in corners (so I've been told). As for bags, I liked Slam Specialties Bags.