bell crank ratios

[cparsons]

Let me reiterate that shocks are a mystery to me, and will probably remain that way forever.
I find I have hit the wall regarding a suspension theory and could really use some imput regarding bell crank ratios. Is a higher ratio better than a lower ratio? If my math is correct, I have room to go to a 4:1 ratio and really work the shocks/springs. If the pushrod moved 1 inch, the shock would compress 4 inches. Is this desirable over a 1:1 or 1:2 ratio? It would seem so to me for a varity of reasons like more finite adjustability, load/heat dispersion, etc. I know there are other factors involved, but once I make the decision on this I can deal with the next cloud of confusion.
Have I menioned that I envy those of you who "got" to do conversions instead of total builds?

[Brandon Dixon]

Are you just talking about the bellcrank ratio, or have you computed the motion ratio for the wheel (the pushrod angle has a big impact for instance)?

One thing to keep in mind is that the wheel rate is proportional to the square of the motion ratio. At 4:1 motion ratio your wheel rate is going to be 16x your spring rate, and I would asume that the friction in the shock/bellcrank etc. motion would become such a large part of the equation that it wouldn't work.

Most(?) modern cars have motion ratios near 1. I'm not encouraging you to follow like sheep, however.

Brandon

[cparsons]

thanks Brandon, I don't particularly want to be part of the flock, but the flock knows what's best. I know that exponentially the math gets ugly but can be delt with, IF it is better/desireable. Is a 1:1 better, cheaper, or just easier than 2:1, 3:1, 4:1. If I get this part wrong everything previous is scrap metal and I have a yard sale.

[jerry freeman]

I'm definately not an expert on this but Dustin Wright at Phoenix Race Works explained it to me pretty good. The guys at Phoenix are great to talk to. If you take the spring and shock out with the car on some blocks (4X4) and the wheel and tire is in its normal place according to ride height. If you pick the wheel up an inch and measure the amount of travel you get out of the hole where the shock mounts it will give you the ratio I think you are wanting. Not sure if this will help but maybe one of the engineers on the site will add to this.

Jerry

[Brian]

What normally happens is the long, very inclined pushrod has a very unfavorable, falling rate motion ratio. You couple that with a bellcrank that has an equal but opposite favorable rising rate motion ratio. When you add the two together you get a WHEEL to SHOCK motion ration somewhat near or probably under 1:1, and probably a slightly rising rate. From what I recall, the later VD's are in the .78 ish range and slightly rising in bump, so 1" of wheel bump equals .78" of shock travel.

As far as how much shock travel to wheel travel is desired, it depends on things like total wheel travel, available shock travel, spring rates that are available in the size you'll need, damping that's appropriate, etc. Makes no sense to design for a 4" 100 inch-pound spring or a 10" 1500 inch-pound spring - both are hard to find. Same with designing in 6" of shock travel when you only have 10" to fit the shock into.

If you are designing from scratch, I suggest you pick a car you admire and blatantly reverse engineer it, understand why the decisions made were made, and then design your own version. You'll probably end up with an 11 inch shock with about 2" of travel, a total MR of about .75 and wheel rates between 150 and 300, and springs between 400 and 800 lbs. That's for fronts, higher at the rear since the corner weight is higher at the rear.

Brian

[R. Pare]

While there are a lot of "ifs" in selecting a starting point, Here some criteria that will put you within the ranges of most decently designed modern cars:

1 - Motion ratio of 1:1, and higher if possible up to maybe 1.4:1 (shock:wheel). The high motion ratios allow the shock to pump more fluid for every increment of wheel movement, making it easier to tune the shock to get the control you want. Low motion ratios such as the VD MR quoted are an automatic handicap you will have a hard time working around.

2 - Try to keep the ratio close to linear if you can, and if you can't make the rear slightly more rising rate than the front.

3 - On cars of this type, Wheel Rates ( from the spring) of 2 to 3 times the corner weight will be common. The final Contact Patch Rate is a function of the tire spring rate and the wheel rate produced by the coil spring, and is a "springs in series" equation.

Good luck!