4 link bag placement

[tinydb84]

I asked the same question in my build thread on the other forum but figured I'd ask here too as there are more (and different) people on here.

I wanted to see if I could get some input from you all regarding the rear suspension (up front I have a 4inch Z and sfbd).

I was/am planning on running a parallel 4 link, bag on lower bar (Dom 2600), with the lower links on the inside of the frame and upper links on the outside of the frame. Shocks would be on the outside from top of notch to bottom of axle. Bags on the inside of the frame so I can (eventually) run really wide tires.

My main concern is handling with this set up. Are the upper links and shocks on the outside going to provide sufficient stability or should I put the bags on the outside of the frame and upper links on the inside? I don't intend on road racing (and my daily driver is a lowered 350z so I get my corner carving fix with that).

Here is a question for the engineers with too much free time.....:

If I mount the bags on the outside, how far up the arm can I move them? My tires are 29 inches and my link bars are 29 inches. Can I put the bag far enough forward to clear the tires without maxing out the bag? I don't intend on towing or haling anything (I will be raising the bed floor anyway).

I can do some drawings later if it helps. Since everyone likes pictures, here is where I am at...

[PGSigns]

Here is my .02 from building drag cars with 4 links and having a pretty good engineering back ground. Those type bars and are designed for tension and compression on the bars only and the load of the chassis being places behind the housing through a set of coil overs or bags. I see these types of kits for sale and don't agree with the design. They will take very little load on the bar with out bending it if you try and carry the weight with the bars except near the very end. If you do the math with a short arm a bag on the arm will see more load than the weight of the truck. Also with the shocks and springs(air or wound) behind the axle they travel more than the rear travels allowing the shock and spring more time and movement to control the ride and will improve it.
Jimmy

[tinydb84]

Thanks for the response PG. I have learned quite a bit from your posts. The bars at 1.5 inch dom with .25 wall. I plan on using a truss the length of the bar. I have seen a number of set ups that way but don't have a problem mounting the bag at the very end of the bar.

Any thoughts with the bags on the inside versus outside? I know wider is better but is it one of those things where it's way more impact than common sense would dictate?

[Blackpony]

I am in the process of fabbing up a parallel 4 link on my truck. My plans are to drop the front leaf spring mount down about 1.5 inches and us it as my front mounting point for the lower arm thus putting it outside the frame rail. Then installing the upper arm just inside the frame so that I will have room to install the air bag on top of the axle on the outside of the frame with the shock mounted directly behind the bag and lower arm mount. This should give good control and hopefully give me a good ride along with correct suspension geometry. I will start a thread soon with pictures.

[Ziegelsteinfaust]

Bag 1/2-2/3 way down the top bar is the best ride. Load capacity is decreased due to mentioned bending of the bar, but if your bars are thick enough not a problem. If you go further down the bar to say 3/4 it does improve load capacity, but ride suffers. Also you'll need limit straps to keep the axle from dropping to far.

Bag over axle carries the most, and typically has the roughest ride. This can be tamed by playing with bag height, bag diameter, and type or sub-type of bag used.

Bag behind axle rides better from what I have read. I just can't wrap my head onto how.

I think your bars are to short for Bag on Bar set-ups unless you run the bars inside the frame rails when you run wider tires.

[Blackpony]

Originally Posted by Ziegelsteinfaust

Bag 1/2-2/3 way down the top bar is the best ride. Load capacity is decreased due to mentioned bending of the bar, but if your bars are thick enough not a problem. If you go further down the bar to say 3/4 it does improve load capacity, but ride suffers. Also you'll need limit straps to keep the axle from dropping to far.

Bag over axle carries the most, and typically has the roughest ride. This can be tamed by playing with bag height, bag diameter, and type or sub-type of bag used.

Bag behind axle rides better from what I have read. I just can't wrap my head onto how.

I think your bars are to short for Bag on Bar set-ups unless you run the bars inside the frame rails when you run wider tires.

Good info!

I agree about not making sense of the bag behind axle giving better ride. From your statement the farthest away you place the bag from the front pivot point the harsher the ride. Which makes sense having less leverage to compress the bag on bumps. With that you would need to rjn less pressure to make the bag softer. But I would think that being behind the axle would make it even harsher then all. Unless it has something to do with the style of bag used. I have been told the tall skinny bags guve the best ride quality but intern suffer load ability. I will be using dominator 2600 bags on mine. Hopefully the ride isnt too terrible. I will also be relocating the gas tank to behind the axle. 31 gallons of fuel should add a little weight.. hehe

[Ziegelsteinfaust]

I did tons of research as I was almost going to pull the trigger on my C20 to have air ride as part of its build, but now its a 4x4.

[tinydb84]

Thanks for the input all.

The behind the rear bag set up is with sleeve bags (my brother's 74 had them and it rode great).

The bars arent my concern for strength. Putting the bag that far up in front of the tire is my concern as it is increasing the load on the bag significantly.

I wanted to get feedback from people who ran the bags on the inside with a bag on bar set up (as it is even narrower than bag over axle).

I probably have a few weeks before my steering/front end are back together to get my rear started. I'll post some pics with I get it started.

[PGSigns]

Here is why the bag behind the axle will ride the best. With the bag on the bar you loose leverage to lift the truck and the closer the bag gets to the frame mount and the more air it takes to lift the bag to a set point. So with it behind the axle it may take 40 psi to keep the bag at say 6" and the truck at ride height and with it half way down the bar it will take about twice the amount of air to keep the bag at the same height. A bag with more air pressure will be stiffer. a stiffer bag is a stiffer ride. Goal is to have the bag at ride height about the same height it is sitting on the bench with no air in it.
Jimmy

[Blackpony]

Good info jimmy! I have been searching for info as to how much air pressure guys are running for daily use and what the air bag installed height is so I know where to weld on my brackets. This helps a lot

[PGSigns]

The amount of air in the bag depends on where the bag is mounted and the weight of the back of the truck. Also the size of the bag can make a difference. My truck has the stock trailing arms and it only takes about 45 psi to lift the bags to ride height. So your bag placement and weight will determine your pressures
Jimmy