6 rear suspension travel Options

[nditiz1]

So I have posted on this before and am still confused.

How much rear suspension travel should there be on a 6?

74 chassis
2.4L engine
Springs appear to be 100 lb ones
Shocks are Bilstein C clip at lowest setting
No rear sway bar

With this setup the car sat low - cool, but there was no rear suspension travel. It was stiff back there. I push down and there is no give. Now, maybe that is how it is supposed to be?? I remember my 4 having a little bit of rebound, but that was with 200lb less back there.

So I am in the process of clicking up two spots on the Bilsteins to get some rebound back. Seems counter intuitive though. If I compress the spring more it will be more stiff. My thinking here is that there is now travel with two clicks up since before when down on the ground the spring and shock were already compressed so no suspension travel was left on the console and or the shock. Thoughts?

[Shivers]

I'm just wondering, with the extra weight, is it possible that the 100# springs are flattening out at the last setting and leaving no travel. Do you have any 140's you could try?

[mepstein]

Changing the height of the shock won’t change the stiffness of the spring. It is linear until it’s out of travel. (Unless you have progressive springs but most 914 springs are not progressive)

[nditiz1]

So let's go with the rear should have some give?

Check out my crude drawing to see if logic is correct.

[Superhawk996]

Changing the height of the shock won’t change the stiffness of the spring. It is linear until it’s out of travel. (Unless you have progressive springs but most 914 springs are not progressive)

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The notches on the rear of Bilstein's with the snap ring retainer are for adjusting ride height. Has no effect on spring rate and/or the amount of spring compression required to hold the car up at curb weight.

I'm not sure if I'm misunderstanding but want to clarify definitions:

Upward (in Z height) travel of the wheel is called compression or jounce. [Tuning terms are defined from the perspective of wheel motion]
Downward travel of the wheel is called rebound.
Sag -- this is the amount of suspension travel used to support the vehicle at curb weight.

So what you have is basically a situation where the rear damper ride height is set so low that what you have used 100% of the available compression travel for the damper and have no travel left on the damper road. Another way to think of this is that you have 100% sag. i.e. you used all available rear suspension travel just sitting at curb height.

By raising the rear spring perch, you'll gain back some of that compression travel.

I'm going to stop there or this will be come a white paper on spring selection and where the damper should be as a % of its travel at curb ride height. As a general guideline - you want more compression travel than rebound travel. If / when you start hitting the jounce bumpers, the suspension changes character quickly (in a bad way). Bottoming the suspension is also drives very high spike loads into the chassis and leads to component and/or sheet metal fatigue.

Bottom line: Make sure you have plenty of compression travel. If the rear ride height ends up being visually unappealing to you, then you'll need to step up the spring rate.

[Superhawk996]

@nditiz1

[Mark Henry]

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100lb springs and new won't have much notable "bounce" or movement at all by pushing down on a corner.
Make sure the springs are sitting correctly in the perches (zip-tie mod), set the ride height (clips) and you should be good to go.

Edit: I just sat on a teen I have here with 100lb springs and Bilsteins and it barely moves (rear corner, didn't lift my feet, so guessing 150lb).
You'll be happy once you see how they perform on the street.

[nditiz1]

Ahhhhhh, ok so the lack of compression in the rear is:

Common per Mark
and
Could also be due to %100 Sag

Now, by clicking up 2 spots on the Bilsteins I will in theory gain some compression due to Sag being at 75% which will also increase my ride height hopefully not to a "raked" look.

[brant]

The cars actually handle best with a rake
It is best for weight transfer and is the optimum stance for handling

[Cairo94507]

I have 140 pound springs in my car and it does not compress when you lean on the rear either. Yet it rides nice. (IMG:style_emoticons/default/beerchug.gif)

[Superhawk996]

Ahhhhhh, ok so the lack of compression in the rear is:

Common per Mark
and
Could also be due to %100 Sag

Now, by clicking up 2 spots on the Bilsteins I will in theory gain some compression due to Sag being at 75% which will also increase my ride height hopefully not to a "raked" look.

I suspect you're looking at 140 lbs/in springs to get the look you want and to sill have sufficient compression travel.

You don't want to have the damper having used up 75% of it's potential travel at curb height.

On street use, it is virtually impossible to get wheels in the air (i.e. to have run out of rebound travel). This is why you want to bias toward having more compression travel than rebound travel.

If I read your comment above correctly, it sounds like you're proposing the opposite.

[Superhawk996]

The cars actually handle best with a rake
It is best for weight transfer and is the optimum stance for handling

(IMG:style_emoticons/default/agree.gif)

[nditiz1]

Ahhhhhh, ok so the lack of compression in the rear is:

Common per Mark
and
Could also be due to %100 Sag

Now, by clicking up 2 spots on the Bilsteins I will in theory gain some compression due to Sag being at 75% which will also increase my ride height hopefully not to a "raked" look.

I suspect you're looking at 140 lbs/in springs to get the look you want and to sill have sufficient compression travel.

You don't want to have the damper having used up 75% of it's potential travel at curb height.

On street use, it is virtually impossible to get wheels in the air (i.e. to have run out of rebound travel). This is why you want to bias toward having more compression travel than rebound travel.

If I read your comment above correctly, it sounds like you're proposing the opposite.

Not at all, will increasing ride height, increase shock travel? Compression?

Also, in terms of a formuala

Compression travel = Rebound travel - just realized the idiocy of this formula - if the body were to lift then there would be less rebound travel than compression

I should have said I don't want excessive rake

[Superhawk996]

For simplicity sake, let's say the motion ratio between wheel travel and shock travel are 1:1.

So for 1" of movement of the wheel you get 1" of spring compression and of course, 1" of damper rod travel.

Assume; added 200 lbs for /6 engine, lines, oil, oil tank etc. (assume it was all added the rear corners - won't be true but let's assume for simplicity)

So with 100 lbs/in springs that extra weight would result in the car sitting 1 inch lower than it would due to the 200 extra lbs has to be supported. Each spring will compress 1" adding 100 lbs of vertical force x 2 springs = 200 lbs. This means the car will be sitting 1" lower at rear.

But, you also LOST 1" of compression travel.

By pushing the lower rear spring perch up 1" you get your compression travel back and you're back to the original ride height. There is still an extra 1" of compression in each spring that is there - it was needed to support the "extra" 200 lbs of weight added to the vehicle. You've just moved the whole compressed spring upward (and the body with it).

Now let's look at 140 lb/in springs:

The added 200 lbs from the /6 engine would result in 0.71" lower ride height since you're now using stiffer springs to hold the rear of the car up.

The Bilstein rear ride height adjustment notches are about 0.5" apart.

So now, you would probably only have to push the rear spring perch up 1 notch (instead of 2) to offset the lowering caused by the extra 200 lbs. Overall, you would lose 0.21" of compression travel, the car would be at 0.21" LOWER than the /4 position and you would have less rake vs. the /4 starting position.

This is overly simplistic because the assumptions I've laid out ARE NOT true. But it gives you a general idea of why you might want to move up to 140 lb springs if you can't get the rake of the vehicle where you want it with the 100 lbs springs.

I should state the obvious. In order to prevent high speed, front end lift, you NEVER want the front of the vehicle higher than the rear.

[Mark Henry]

For a street car lowered about the max before bump steer, I look for it to be on the ground flat with no rake. With a 15" rim, on flat ground, top of rim lip to bottom center of the rear fender lip I want to see about an 1-1/2" of tire. Then adjust the front to match. If you want to be anal sandbag the drivers seat to match your weight. If you want it higher raise it up.
Once all done you need an alignment, you need an alignment anytime you raise or lower the front, so make sure you're happy with the stance first.

If as low as me you do have slight bump steer, I wouldn't go any lower without mod struts.