Rocker Ratios and Valving

[THR]

this is kinda hard to explain... but i hope you all get wot im trying to find out!

rockers on the rear suspension. they have a ratio of say 2:1 against the springs on the shocks (normal single seater type suspension setup). using a spring of say 1000lbs
the opportunity is availble on the same rockers to move the pushrods so the ratio is less say 1.5:1

the question is..

if i wanted to i could use a 750lbs spring on the 1.5:1 ratio which would give me the same effective springing, correct?

1.5:1 would mean the shock would move less, and hence the valving would be perhaps wrong.
would it require more or less rebound?

i suppose basically wot im asking is, wot effect does changing the rocker ratio have on the springs and damper settings!

[enzo]

Quickie lesson on MR's and wheel rates:

MR's can be expressed 2 ways : shock/wheel, or wheel/shock.

Your numbers are expressing it wheel/shock (I assume that the shock is moving less than the wheel)

If you use shock/wheel, the equation for finding the wheel rate is: Wheel Rate = MR^2 x Spring Rate.

If you use wheel/shock, the equaton is: Wheel Rate = Spring Rate / MR^2

Your 2/1 number means that the wheel moves twice as much as the shock. Therefore your wheel rate = .25 that of the spring - ie - a 1000# spring yields a 250# wheel rate.

Th 1.5/1 number means that your shock moves two thirds that of the wheel, for a wheel rate of 435# using a 1000# spring.

So, if you switch from the 2/1 ratio to the 1.5/1, you will need a softer spring for the same wheel rate.

Assuming that the higher ratio doesn't produce some sort of massively increasing or decreasing rate MR, I'd switch to it. Shocks need a lot of motion to act the way you would like them to and have good adjustability. Most likely you will need to soften the valving or they will be overly stiff.

Still a ****-poor MR, tho' - modern cars are generally just about 1:1, with Speedway cars set usually where the shock moves more than the wheel due to such low wheel movements.

[Jukebox]

Personally, i'd go for a 1.6:1 ratio rocker as it'll give more lift at the valve. It'll also increase the horsepower. But then it all depends on the length of the pushrod because of different angled heads, best to use adjustable pushrod.

[av8rirl]

Jukebox: You want to get off that gear. They're talking about suspension.

[THR]

so your saying the 1.5 ratio is best because it makes the shock move the most, making it more sensitive to change?
that makes sense.

the shock moves more then the car does on this car due to the ratio. like u say, as the ride height of the car probs varies 12mm the shocks need to move more and they tend to move about 20mm.
this makes the spring softer than it actually is right?

[enzo]

Your car's rockers actually move the shock more than the wheels moves? Is that correct? Highly unusual! Especially on a small-bore spec car.

If that is correct, then the spring's mechanical advantage is mulitplied - ie - at the 2/1 ratio, using a 1000# spring will give you a wheel rate of 2000#.

Double-check that you actually have the MR's correct.

[THR]

hmmm perhaps we are confusing each other!
the rocker roughly forms a 90 deg triangle.. one side is say 80mm long and one 40mm long.

the 90deg corner is the pivot, the pushrod attached to the corner of the 40mm. and the shock on the 80mm...
which makes the spring compress easier.

kinda like this picture http://www.bath.ac.uk/~en9wes/rearsusp.html

[enzo]

OK. So you have a bellcrank & pushrod system, not a rocker - rockers are what cars used prior to bellcranks and pushrods.

I am still confused as to whether or not your shock actually moves twice that of the wheel. Have you actually measured it, or are you going on the ratio of lengths of the 2 "arms" of the bellcrank?

if that is what you are doing, then the MR you think you are seeing is erroneous. The pushrod loses a lot of motion compared to the wheel because of it's reclining layout, so the actual whhel to shock movement ration is a lot lower than the ratio of the bellcrank "arm" lengths.

I find it highly doubtful (but I could be wrong) that your shock moves more than the wheel, especially on a French designed car of that vintage!

I'd suggest that you actually measure the ratio by moving the wheel in 1 mm increments and measure the shock length incremental changes.

[THR]

oh sorry yea i was confused by the names of things! bellcranks yup!!

i wasnt thinking of the effect the pushrod actually had on the movement of the actual shock. but you are right.. after drawing it, you move the hub 1mm and the pushrod moves 0.5mm pushing the shock half the distnace the car is moving up and down.

the bellcrank will then increase the movement again. so the 2:1 ratio crank will make the total movement of the system around 1:1.
with the 1.5:1 crank ratio it would make it about 1.33:1?

so wot effect does this have on the springs/valving?

[enzo]

Go back up a few posts - already answered!