Hot Rods Generic Suspension Question

Springs hold up the car. Shocks control the wheel movement(s). (see gimpy).
Forgot to mention: if you grease twixt the springs, use screen to hold in the grease, & wrap outside w/tape or leather to help keep grease in & dirt out. Another guy mentioned leaf spring liner, which is the plastic I was referring to.
Lots of other info on springs to consider . Very tunable.
Marcus...

 

I had a rough riding car. Turned out the shocks were not too strong, I changed to the correct shocks and everything was better.

 

Here is the latest shocking facts;
1. The shocks I took off yesterday were Monroe #33033.
14 1/2 inches fully extended
11 1/2 inches fully compressed.
2. Replaced with KYB #344266 (used)
16 1/2 fully extended
10 1/2 fully compressed
3. Shocks I got today Monroe #31000 (not installed yet). got on trial, not paid yet
19 1/4 fully extended
13.00 fully compressed

All measurements center of mounting holes.
Shock mounting brackets; 13 1/2 inches top to bottom.

Any thoughts on the new Monroe #31000? I picked them out because of the extra travel?
If these sound good, will probably get them in QA1

 

I predict your latest pair with a 13" compressed length will not like your mounts that are only 13.5" apart. You do realize that will give you a half inch of travel, right?

 

If that shock mounting bracket spacing top to bottom is measured with the weight of the car on the axle, you need to put a new set of the KYB's back on it, if those are not stiff performance shocks.

The next option would be the Monroe #33033s, at least they would be able to compress 2" before bottoming out, which would probably work most of the time.

The Monroe 31000 shocks would only be able to compress a 1/2" before bottoming out, those won't work at all for you.

Lots of talk about the leaf springs, how much use have the springs on the car seen? Were they unused when installed on this car?

 

If I understand your dimensions, you have 13.5" upper shock mounting hole to bottom shock mounting hole at ride height. If the 31000 shock is fully compressed at 13", then you will have only .5" of compression left. Not going to work.
You probably need about a 5" or 6"stroke shock. With your 13.5" ride height, with a 6" stroke shock, you need a compressed length of about 9.5", and an extended length of about 15.5". This allows 2/3 of total stroke for compression and 1/3 total stroke for rebound.

 

Measure the distance between the wheel rim and wheel arch.
Then jack the car off the ground and measure this again with the wheel hanging!

I need to know how much load is on the spring
Get back to me when you've done this.

 

OK time for the maths!

Shock angle plays a role here, too. Unless your rear shocks are installed straight up, you need to solve for the angle.

1-inch of travel up and down is not 1-inch of travel on the shock.

This is an exaggerated example:


Your shocks are not on the vertical side of this triangle, they are on the hypotenuse (denoted by the 2a, according to the Pythagorean Theorem, only in this example).

The angle can be measured with an angle finder, or enough data can be gathered with a plumb bob and a tape measure. The process sounds complicated, but once you do it, you will see that it is easy.

Without the angle calculation, it is still safe to simply make sure that the you get a shock that will handle full droop, and not bottom out internally even when the bump stop is smacked by the axle.

 

Don't confuse 'em too much with maths

Laying the shocks over decreases the efficiency but increases travel.
known as the motion ratio.

Also, when the suspension compresses the angle becomes more acute [decreasing the stiffness with more travel]
a shock at 60-65 degrees is a good compromise between straight line "Jounce" and the body rolling on an arc.

here is a pretty picture

 

What is the distance between rear shock mounts
1) With the car sitting on 4 wheels, with rear shocks removed. 13.5" ?
2) With the car on jack stands, rear suspension hanging
3) with rear suspension compressed

I'd start with finding a shock that will match travel of the vehicle, preferably have the travel in the middle of where it sits at ride height. The shorty aftermarket shocks, on average, perform pretty poorly.

It may be worth moving the shock mounts to be able to use a more common factory shock. You'll have readily available options. VS high priced specialty shocks.

 

IIRC , some vehicles ,the axle had to be supported while removing/ replacing rear shocks . The shocks were the travel limiting ( droop) component . I also recall some ( British?) sports cars that had leather straps limiting the axle / shock travel ( droop) . Point is , how far the suspension may let the axle droop , may not be an accurate determining factor in choosing shock travel .

 

Just about all coil spring suspension is like that. [needing spring compressors to assemble]
Soft suspension has a higher spring load

 

WOW!!!
The shocks were measured off the car. (full extended and full compressed)
The shock mounting brackets measured sitting on the ground, with the KYB shocks installed.
Back to the garage and try to get requested measurements to get answers to all questions asked.

Thanks everyone, did not mean to open such a can of worms, for what I thought was going to be a simple question with a simple solution. WOW, was I ever more wrong?

Jack

 

Had the springs built new, maybe 10 years, 50,000 miles ago.

Jack

 

I believe frame to axle in both positions would yield the same info....

 

OK. That I understand, and can do.
Jack

 

Yes, I do now. Thanks
Jack

 

Doing little googling found shocks that may work, but wanted your opinion.
KYB 349024 (5.55 Damper travel, 16.33 Extended, 10.78 Compression Range) ?????

 

I'm thinking a lot of leaf spring ford pickups , the shock was the down limiter ?

 

I have seen the chart KerryNZL has posted about the shock angles. I used it to angle my front shocks, and as stated, it helped take some of the harshness out of the ride. What I have a problem with is when he said it " increases travel". I don't see how it can, it would decrease the travel, that's why it decreases the efficiency. The shock has no valve that changes due to angle, so the only way to reduce efficiency is to reduce travel.

 

What Indyjps said. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The one caveat I would add is to check what your suspension is at full downward travel with the frame sitting on jack stands and no shocks on it.
Then remove whatever you need to remove and let it sit on the rubber bumper stops. That will tell you the maximum travel (up and down) of the rear ends shock mounting points.
Then get shocks that have travel that exceeds both of those dimensions.

 

The shock will compress less when it is mounted at an angle. The "increased travel" refers to the suspension being able to move more compared to the compression of the angled shock. Think of it as increased suspension travel for the same amount of shock compression.

 

Sounds like a plan.
I want to leave no stones unturned. Don't want to spend next several days going back and forth to parts store, and installing and removing shocks

 

Don't worry. It is not a can of worms.

This is just what we do. We want you, and everyone else, to have cars that they enjoy driving as much, or as little as they want.

Sometimes just a few mismatched parts can make the difference between a car that gets driven coast-to-coast, and one that rarely leaves the garage.

 

Measurements. Frame to axle
On the ground....6 inches
Off the ground, shock unhooked.....9 inches.
Shock angle, almost straight, maybe 5%.
Jack

 

Ok, a 5º angle we can probably just ignore. Yes, it will make a difference, but it will be fairly tiny.

From the numbers, you would need a shock with 6-inches of compression, and 3-inches of rebound.

Rebound is not constrained the same way as compression, where the axle will smack a bump stop, and go no farther.

Static droop, as with the car on stands, is likely not as far as the suspension could cycle. A hard jolt could push it farther than that. I generally like to take that into account.

I also do not like to have my shocks be the thing that stops either droop, or maximum travel. That is just as bad as using the guts of a shock as a bump stop.

I am looking for the Monroe shock catalog now. I have it somewhere.

 

Let's see if this works. The idea here is to reverse engineer.

Find a range of shocks that match the mounting style and desired lengths. Take those part numbers and search for them on the web. Find out what they are intended for (car, truck, heavy, light, front, rear, etc.), and find something that is a rear shock, from a vehicle with a similar weight profile.

Once you do that, you can see what types are made (regular hydraulic, gas, heavy duty, sport, adjustable, velocity sensitive valving, etc.)

 

Leaf springs are very tunable! And the spring rate determines the ride. If your springs are too “ stiff” no shock in the world is going to fix the ride! If your springs, like mentioned have too much friction in them it will never ride good.
Maybe ot and slightly different application , but in the seventies Ford 1/2 ton trucks had a five step progressive rate system in their rear leaf springs. I had a 1970 model 1/2 ton that rode like a car empty and one time I had 4400 lbs load that I took on a job 100 miles away and it rode about the same! Study that system, imitate it, if you want a good ride!
My point is make your leaves progressive , soften the ride THEN dampen it with the correct shocks!






Bones

 

@twenty8 answered this for everybody [thank you]

The motion ratio is altered so there is more travel at the wheel compared to at the shock.
Laying down shocks is a easy way to increase the suspension travel on motocross bikes.

Technically it is decreasing travel of the shock piston per inch of wheel travel.
So it is just grammatical interpretation.[lost in translation]

As with any motion ratios ,the valving needs to be altered to compensate for the motion ratio.

If a shock's angle is more perpendicular it becomes stiffer, so many designers lay the shock at an angle to the force, then use "Bell cranks" to stiffen them [straighten the angle] as the suspension compresses. This is how "Rising Rate Suspension" operates [in Grand Prix cars.]

Also that chart I posted previously is simplified for those not math fluent. Trigonometry and Motion Ratio formulas are far better [but generally not needed for average hotrod mods]


The biggest mistake in car builds is the Wrong Choice of Springs , It can make the difference between a nightmare or a pleasant car. [or performance]
This problem is really prevalent in the "wonderful world of coilovers" [everyone has a tendency to go too stiff]

 

I found a KYB 349024 that kinda matched a measurement some one here suggested?
It is a rear shock number. Tell me what you think, or if you come up with good Monroe number, that will be good. Probably better chance of getting it quickly.
Jack