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Author Topic:   Lets hear it guys what do think??
posted May 30, 2002 08:01 PM
lets see what everone thinks on this. How wide should the rear springs be in a modified chassis?? Do you just put them where they fit? out from the front springs? narrower than front or the same? Then tell why you think what you do. CHAMP

posted May 31, 2002 03:15 PM
Most typically it comes down to packaging. There isn't any reason to reference the front springs in locating the rear springs. Idealy you want the springs to be as close to the axle as possible.

posted May 31, 2002 06:35 PM
Well I have my owen thoughts on this but I would like to hear more thought on this before I say what I am thinking. Come on guys let hear what you think. CHAMP

posted June 02, 2002 07:32 AM
Touche Champ00! Great question. Just goes to show you are a THINKER. I admire that.

Wfoondirt is right again, in that the front springs have little to do with the placement of the rear springs. I agree that "packaging" is is the primary reason in locating rear springs. Front spring location is pretty much pre-defined for you already. Upper and lower spring buckets are already there, so that's where the springs go.

Rear spring placement is a little diffrent and "packaging" does play an important role. The width of the rear axle housing is the limiting factor on rear spring placement. I agree with Wfo that they should be placed as far out as practical. However, that is only the beginning.

Placement of the springs relative to wheel rate. Wheel rate is determined from motion ratio. Again, the front suspension motion ratio is pre-determined by the manufacturer. Rear motion ratio is determined by the builder. Simply put, motion ratio is a leverage ratio and wheel rate is the amount of spring rate the tire actually feels.

The front suspension is an independant suspension. Front motion ratio is determined by measuring the distance from the center of the ball joint to the center line of the bushings and dividing that into the measurement of the center line of the bushings to the center line of the springs. This will be a decimal number. Wheel rate is always motion ratio squared.

The rear suspension is a "semi independant" suspension because the axle housing creates a solid link between the tires. Wheel rate is computed the same way as the front, however, motion ratio on the rear is determined by the roll center. Therefore, spring location creates a variable in the equation. Motion ratio and wheel rate will change with the placement of the rear springs in relation to the roll center and centerline of the wheel. Panhard bar length and placement also add another variable to the equation, in that roll centers can be moved with the length and height of the panhard bar.

Any good chassis builder who has done his "homework" on the rear wheel rate, places the rear springs in a location that takes the most advantage of all these variables to achieve the desired wheel rate and therefore; spring rate. He, in turn, uses each of these wheel rates to select the springs for each corner of the car.

Location of the rear springs make a significant diffrence in the balance of a race car, however you are limited to some degree by the "packaging".

posted June 02, 2002 08:28 AM
Hey Fourty8. Well stated from the technical viewpoint. But, one doesn't have to be constrained by the packaging. You have to "step away from the box" as they say in commercials. I'm restoring a 94 DW4 that was leafspring, then @#$%$%ized to a swing arm. Now, it's going to be a pure 3-link. But to make this happen, I got creative on sping/shock mounts that located "outside" the normal chassis boundaries. I think my left mounts are somewhere around 4-5" outside the rails, and my right is 2". Just have to be creative on taking into account for tortional loading on the mounts, etc. If there is a will, then there's a way........ Just my couple pennies......

posted June 03, 2002 05:37 AM
Thinking "outside the box" is what make this game, we call racing, the great sport it is. 964, I commend you.

The rear end housing is the limiting factor only on the "maximum width" that the springs may be placed. It is never a constraining factor. You are absolutely right in that the load on the upper mounts are of the utmost concern, Especially on the RR. The greatest dynamic forces are applied to that mount during body roll.

Another consideration would be the location of the spring in reference to the lateral center line of the axle. Rotational forces of the rear end need to be considered here. A spring on top of the axle is basically isolated from these forces. A spring in front of the center line is subject to "loading" under acceleration and "unloading" under deceleration. A spring mounted behind, of course, reacts opposite than one in front.

Race car setup is a compromise of all factors combined to obtain what the car and the driver "like". In my opinion, the rear suspension mounts (shocks & springs) should be relocatable to allow the most diversity. Especialy if you race on diffrent tracks. Clamped on brackets work very well for this, if done right.

posted June 03, 2002 08:25 PM
Ok this is geting good now. I maybe lost out in my owen little world again but here it goes.

First you have to think about your car as a chair again you know the four leg thing. with the seat as the body of the car and the legs as the wheel. You all with me.

Now here we go.

Lets say you front springs are 30 inch wide.

It seems to me that if we put are springs 36 inches wide at the back what happens as the car goes thru the turn the back would be more stable.and should roll over on the right front easier than before.

now put the rear springs the same as the front ? the car in turns should roll over on the right side easier than before.

Now put the rear springs narrower than before like 28 now the car should roll over on the right rear easier than before.

think about it theres no math to firgure it out so what do you think?? Before you jump out there the say its wrong think about it for a while. Seems like it might work??? CHAMP

posted June 03, 2002 09:04 PM
Good analogy to demonstrate how the placement of the spring effects the wheel rate.... moving the spring in makes it act softer....moving it out makes it act stiffer.
But the same effect can be had by changing the rate of the spring instead of the location

posted June 03, 2002 09:29 PM
Same as a swing arm concept basically. wfo hit it on the nose.


posted June 05, 2002 05:06 AM
Each of you are right on the mark. However, the swing arm uses 2 motion ratios. The one determined by the location of the swing arm at the axle tube and the other being the motion ratio of the swing arm itself. This is why the swing arm suspension requires a stiffer spring rate to obtain the same wheel rate as the suspension with the springs connected directly to the axle housing.

True, the same effect can be accomplished by spring rate as opposed to location. But, be sure to use a spring with enough height when locating it as far out as you can. In my opinion, location, spring rate and spring height, go hand in hand. The farther out the spring is mounted on the axle tube, the taller the spring needs to be. The farther out on the axle tube that the spring is placed, the more it needs to compress to achieve a given amout of body roll. The farther away from the end, the less it needs to compress to get the same result.

In short, regardless of the spring location, use enough spring rate to get the amount of movement you desire and enough spring height to eliminate the possibility of coil bind.

I commend everyone here on this discussion. Knowledge is the key to understanding. It is clear that each of you are very intelligent people. My compliments to you all.

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