I'm using one of those online suspension calculators and motion ratio is an input. The front is Miata bits, so I found that ratio easily enough, but what is the ratio for a live axle rear? Best I've found so far is to divide the distance between the spring mounts by the track width. I'm not sure about that bit. Anyone have knowledge on the topic?
My understanding is it is where the spring is in relation to the axle. Example: spring perch on top of axle tube = 100%. Spring perch in middle of lower control arm such as a foxbody mustang = 50%. Coilover mounted behind the rear axle = slightly over 100% (105% or something)
I'm sure track width might come into play somehow but that is not something I've ever seen factored in.
This might be helpful. It's talking about Mustangs but should be applicable.
DeadSkunk (Warren) said:I'm using one of those online suspension calculators and motion ratio is an input. The front is Miata bits, so I found that ratio easily enough, but what is the ratio for a live axle rear? Best I've found so far is to divide the distance between the spring mounts by the track width. I'm not sure about that bit. Anyone have knowledge on the topic?
If I'm understanding what you're saying, that should be more or less correct for the motion ratio in roll. But be aware that that's not the same as the motion ratio in straight bump/droop. The difference is in pure roll the pivot point is more or less the inside wheel, in pure bump the pivot point is the suspension mounts. For a setup with the spring somewhere along a suspension link and not particularly close to the axle, I think it gets a bit complicated and might need some empirical data to get your answer.
In single bump, it gets a little complicated since both springs will be moved, assuming a stationary chassis.
I'm trying to create an autocross car, so I'm ASSuming I need a motion ratio in roll.
In reply to DeadSkunk (Warren) :
What are you trying to figure out? Spring rate?
In reply to Patientzero :
Until I get the car constructed and weighed I'm just playing with the calculator to get a starting point for springs. Sway bars came with the car. so they're fixed for budget reasons.
My understanding of motion ratio is how much the spring is compressed for each unit of axle movement.
So, if the axle moves up 1" and compresses the spring 1" then the ratio is 1:1
If the axle moves up 1' and the spring compresses only 1/2" then its 1:2
Obviously in many leaf spring cars/trucks the motion ratio for the axle can be much different than that for the shock, because many times the shocks aren't vertical.
Many times we remove the springs then by moving the axle up and down we can get lots of readings to graph the ratio. Many times the ratio changes with the amount of movement from the static position.
I've always calculated solid axles with essentially a 1:1 motion (depending on shock placement and angle, but yeah), and it's worked fine.
I've also played with calculations as you suggest looking at shock distance vs track distance, but it didn't seem to make a noticeable difference doing so.
I like to calculate for suspension frequency for a decent ride quality, then add sway bar to deal with roll resistance.
In reply to SkinnyG (Forum Supporter) :
I'm intending to try to hit a suspension frequency of 2.2 - 2.4 and it appears that the springs I have can work if I can keep the car weight to something similar to a Miata. The sway bars came with the car, which has been recouped down to zero, so I'll use the existing bars rather than buy something else. I'm just trying to understand how close I can get with the components I already have, It's interesting to play with a calculator and see how sensitive the theoretical car is to various adjustments, like sway bar arm length.
Whatever answer is, I suspect it is a moving target since it is not "Independent" of what the other wheel is doing.
For an ideal situation where the one wheel is static and the other is swinging in an arc with the pivot being the fixed wheel, I would treat it as two separate calculations: one model where there was only the inboard spring near the pivot-point wheel and a second for the outboard spring near the wheel in motion.
If I recall, when both wheels hit the same bump at the same time, it doubles the rate and that is why you feel it so much when you hit a straight line non-level cut in the road during construction.
It's the difference between the tread with and the spring mounting point.
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