Recommended rear bags for light car

[KyleAnderson]

Ehhhh... with that extended height, AH1 wouldn't give much rebound travel. Looks like sleeve bags are the best solution here.

[Oh Damn, it's Sam]

So, the OP sent me a pic of the rear suspension setup, and since he PM'd me, I'm not sure whether he wants it in this thread or not, but here's what I see.

The rear bags are mounted on a pair of long trailing arms that are welded to the stock trailing arm with the bag mounted further from the pivot than the wheel hub. This is giving the rear suspension a motion ratio >1:1, and if I had to guess off the pic, it's about 1.2-1.3:1 or so. Since this is acting as a class 2 lever, the bag pressure is extra low compared to a more standard trailing arm setup with the bags in the stock spring location, where the motion ratio might be somewhere around 0.7:1. On such a long arm, small movements at the wheels are going to translate into large movements at the bag, which is going to create wildly oscillating spring rates - hence the bounciness.

What I'd like to see is those suspension arms done away with and either the existing bag located in the stock spring location inboard on the factory trailing arm (which would bring the bag pressure up into the 30-40 psi range) or better yet, a narrow sleeve-style bag placed in the stock spring location (which would likely push the pressure up into the 70-80 psi range). We've gotta get that bag pressure up and we've gotta get that motion ratio down below 1:1.

[richardd07]

Thanks sam. Here are the pics:




there wasnt a spring in the original suspension though, the trailing arms were controlled by torsion bars.

Would a sleeve type bacl mounted alongside shock fixed to same bolts improve the situation?

[Oh Damn, it's Sam]

there wasnt a spring in the original suspension though, the trailing arms were controlled by torsion bars.

Ah. Shows what I know about French cars. The last time I worked on a Peugeot was stealing the wheels off a 405 to put on an L-series Subaru. 4x140 is a bitch of a bolt pattern.

Would a sleeve type bacl mounted alongside shock fixed to same bolts improve the situation?

It seems like it ought to, though a bag pressing against an arm at an angle like the shock is doing is going to require greater pressure to lift, in your case probably roughly proportional to the inverse of the sine of the angle between the longitudinal axis of the bag and the axis of the trailing arm. Just worth considering in your calculations of where to place the bag. I'm not sure I would be dying to ask those shock bolts to do double-duty - I'd probably weld a plate to the frame instead.

At the very least, I'd like to see that bag come in as far as it can. Mounting it to the side of the trailing arm shouldn't be a problem - just reinforce it real good and... maybe consider putting a coat of paint on everything

[richardd07]

Hmmm im not sure what the best answer is now...

[Oh Damn, it's Sam]

Without having ever laid eyes on it, this super-sweet drawing is probably the direction I would go:

[richardd07]

I will get you more pics sam. I dont see where the top of a sleeve bag would mount as it would have to go through the shock to reach the car body.

Is there a particular rule for the position of the bag in reference to the hub and pivot?

[Oh Damn, it's Sam]

It's hard to tell what's frame and what's not in that photo, so more pics might help. Nevertheless, I think you're just going to have to get back under there and start mocking up how a bag might sit.

Bag placement usually boils down to "where are you stock springs", but in a situation like yours, I'd like to get that motion ratio to at most 1:1. The lower the motion ratio, the higher the bag pressure and the flatter the spring rate progression as the suspension compresses. Higher motion ratios produce lower bag pressure and land you further into the steep section of the spring rate curve as the suspension cycles. It's a balancing act, but I've seen cars with very nice-handling bag setups with motion ratios everywhere from 0.4:1 to upwards of 0.9:1. In fact, my old truck was 1:1, and it performed admirably, though I used the bags for load carrying capacity rather than getting it low.

[richardd07]

HSo thinking of mocking up sams idea here, running a sleeve mounted directly in line with elbow attached to trailing arm.



A worry I have though is that the bracket would twist, considering the bag would have to be mounted on the inside as shown and the elbow attached to the trailing arm on the outside.

As the bag compresses surely the bracket would attempt to rotate?

[Oh Damn, it's Sam]

The lower bracket's angle will change relative to the upper bracket as the suspension cycles, yes. A bag will tolerate a small amount of misalignment, and sleeve-style bags are quite commonly used on trailing arm suspensions, such as in the MKIV.

I would plot the arc that lower bracket is going to make as the suspension cycles and try to align it so that it neither rotates the lower piston of the sleeve bag at too sharp an angle, nor shifts the lower piston too far forward or back as it traverses that arc. Given how popular air ride is on VWs, I would look closely at the MKIV's rear suspension and consider copying its dimensions and angles if they seem like they'll work.

[richardd07]

what I mean is twisting may occur across the lower bracket, as the bag will be mounted on the very inside of the bracket instead of being mounted in the centre of the bracket as it is currently

[Oh Damn, it's Sam]

I don't love the idea of scabbing the bracket onto the side of that trailing arm either. If it's all built strong, it'll hold, but I agree it's not ideal.

I think you need to get under there with the wheel off and try and see if there's anywhere on the factory trailing arm that you could mount a bag bracket. Since your suspension was never designed with springs in mind, this may be a challenge.