Motion Ratio's

[Lukin]

Firstly, gday all! Took me ages to find a decent FFord dedicated forum but glad I finally did. I work with a car in the state championship in Western Australia. I mostly do data and basic setup, trying to get expernience.

Now the intro's are out of the way...

Does anyone know the motion ratio's for a VD RF94 rear? Our car has been converted to dual shock front so that won't be a widely known one. But does anyone know the rear MR? Tomorrow I am going to do it the old fashioned way; moving the wheel in small increments (with 3 mm thick shims) and measuring the displacements.

From the wheel frequency data and the known values for spring and tyre stiffness, I calculated 1.50 for the front and rear. What values should I be expecting?

Cheers

[JustinDawkins]

say what?

Just drive the ****er man!

[Lukin]

Originally Posted by JustinDawkins

say what?

Just drive the ****er man!

Nice sentiments, but I dont drive!

Wanna make the car quicker. Also get some experience to move onto bigger and better (more technical) things in the future.

[JustinDawkins]

Its OK - I jest!

Seriously though welcome to the forum, unfortunately I have no idea what you are talking about, but some techie people here will so Mackmot and Mattray make yourselves known!

[StephenRae]

I,m looking forward to this

[Lukin]

Frustration!

Measuring the wheel displacement against spring displacement, I got motion ratio's of 1.2 for the front and 2.3 for the rear. With the springs, it gives me whacky wheel rates.

Im bloody confused

[nitrous]

I would recheck your rear calculations. 2.3 MR for the rear seems wrong and too soft. Make sure you start with the rocker in the correct position and move the wheel 1 full inch to get the correct displacement of the spring.

[Lukin]

Originally Posted by nitrous

I would recheck your rear calculations. 2.3 MR for the rear seems wrong and too soft. Make sure you start with the rocker in the correct position and move the wheel 1 full inch to get the correct displacement of the spring.

2.3 MR seemed wrong to me too! I took the wheel off and set the upright/disc to static ride height (the 'zero' for suspension travel was still in the ADL from the previous race). From there I added 3mm shims, and monitored the linear pots. Got upto 66 mm wheel displacement, and the pots only showed 27 mm displacement. To check repeatability I removed the shims one by one, and when they were all gone the pots showed 0.1 mm displacement, so it's not a matter of innacurracy I don't think.

From the pots, we only get 35 mm total travel in the springs in the rear, which isn't much at all. When it's jacked I can get a fair bit of wheel movement just from lifting it up and down.

I'd like to get someone else to measure it to see.

[nitrous]

Normally from static ride height we assume that you have the correct push rod length, as this determines where the bellcrank angle will be and if incorrect will give you the wrong geometry for the MR. You are also moving the rocker too much and going over center of its intended movement. From your normal ride only move it 1 inch (the wheel) and calculate your MR from there. Now you may end with the the same number but that is the correct way to check it manually without the use of a kinematics program. Let me know your results.

[Matt22]

Lukin

If your dampers only have 35mm travel then this MR is probably correct. i assume your dampers are only about 8inches long? My Mygale has 13inch dampers, so more travel, and a MR of 1.7ish. Longer travel and less MR means the dampers move further, possibly giving a smoother response/ or less static friction? Longer dampers are heavier and take up more room, of course.

Matt

[Walshy]

"Ke"?

[Mackmot]

or Que? even

[Walshy]

Hey, I don't spek Spainish,
I downt go to Spainish,
So, y wood I now how two spel Spainish.......................

[Mackmot]

now you know though :P

[Triple J Motorsport]

[JustinDawkins]

................fetch the rack!......................

[MarkG]

and there I was, thinking Formula Fords are simple.

[Walshy]

Formula Fords or the drivers is that???

[MarkG]

in my case, the driver I don't have a clue about the techy stuff being discussed.

[StephenRae]

Lukin.. I have now figured out what motion ratio is...when you have measured it what do you intend to do with the information? don't forget to throw in: rising rate, spring frequency and all of the other suspension imponderables

[Lukin]

We are getting new dampers and need the motion ratio's to work out some valving and settings. Also, when talking to someone else, it's useless to talk about what springs we have without the mottion ratio. At least if we can talk wheel centre rates we are comparing apples with apples.

I actually ignore the actual frequency of the spring in itself. It would be very high and have very little effect on the performance of the system. In the same vain, I ignore the stiffness of the suspension parts and assume it's rigid. To calculate the natural wheel frequency I only use the stiffness of the springs and the vertical stiffness of the tyre (added in series) and the corner mass.

From my measurements, the rising rate isn't that steep. It's there, but doesn't have a huge effect in the motion range. Still a bit puzzled though with our car, the motion ratio is tooooooooo slow.

[BrettFrancis]

Originally Posted by Lukin

2.3 MR seemed wrong to me too! I took the wheel off and set the upright/disc to static ride height (the 'zero' for suspension travel was still in the ADL from the previous race). From there I added 3mm shims, and monitored the linear pots. Got upto 66 mm wheel displacement, and the pots only showed 27 mm displacement. To check repeatability I removed the shims one by one, and when they were all gone the pots showed 0.1 mm displacement, so it's not a matter of innacurracy I don't think.

From the pots, we only get 35 mm total travel in the springs in the rear, which isn't much at all. When it's jacked I can get a fair bit of wheel movement just from lifting it up and down.

I'd like to get someone else to measure it to see.

Be careful relying on the pots as your source, unless you have checked the calibration yourself. Remember - garbage in, garbage out.

I assume you disconnected the spring/damper from the bellcrank to move the suspension through its range.

If you want a more accurate indication of what is going on, go through the procedure again, but this time remove the spring from the damper, reinstall the bare damper (if it's adjustable, set both bump and rebound to full soft) and as you install the shims measure the length of the damper. The easiest way is to measure the length of the exposed shaft at full extension, then repeatedly measure it as you build up the shims.

Note down your measurements in a spreadsheet, then you can get a picture of how the MR may change as the suspension moves.

You can then use this information to check the calibration of your suspension pots.

[nitrous]

Originally Posted by Lukin

We are getting new dampers and need the motion ratio's to work out some valving and settings. Also, when talking to someone else, it's useless to talk about what springs we have without the mottion ratio. At least if we can talk wheel centre rates we are comparing apples with apples.

I actually ignore the actual frequency of the spring in itself. It would be very high and have very little effect on the performance of the system. In the same vain, I ignore the stiffness of the suspension parts and assume it's rigid. To calculate the natural wheel frequency I only use the stiffness of the springs and the vertical stiffness of the tyre (added in series) and the corner mass.

From my measurements, the rising rate isn't that steep. It's there, but doesn't have a huge effect in the motion range. Still a bit puzzled though with our car, the motion ratio is tooooooooo slow.

Lukin

I will save you much time and effort because you are way off saying you ignore the "actual frequency" of the spring. A damper's primary job is to control the natural frequency of the spring once energized. You are going to dial yourself into neverland by overthinking something. No disrespect intended but stick to the KISS method and go with a known set of dampers with a small amount of adjustability so you keep out of trouble. In small formulas it really doesn't make too much difference what is on the car (dampers) so long as it is close as you are not dealing with high horsepower or downforce, thereby leaving it up to the driver to get the job done. More often than not you will over engineer a car and loose sight of what is really going on. Please accept this as constructive critisim.
By the way you cannot ignore the suspension components, have you heard of installation stiffness?

I also go back to original post about checking the MR. You only check it with a 1" wheel movement from your nominal static ride. I doubt very much if you have a rising rate suspension given the numbers you quoted. If you post your incremental numbers with 1" you should see it immediately whether you have any rising rate, linear or digressive characteristics.
Have fun!

[Lukin]

Cheers for the response mate.

I didn't explain that one properly sorry. The natural frequency I think your talking about it the same one I am referring to; the frequency of sprung mass. The square of spring stiffness/corner mass. The spring itself would have it's own natural frequency of around 70 Hz (estimate for the rear based on it's weight and stiffness) which is the one I ignore. The wheel frequency is around 1.5-3 Hz (ballpark) and is the one I concentrate on.

The dampers we are getting are common ones for FFord, but were used on a Toyota Atlantic car last year and need to be re-configured for us which does require knowledge of the MR's. It's more the likely they will be plug and play, but I'm doing this for my benefit as much as the car, learning this now will help in the future.

From the construction of the bellcrank, pushrod rod-ends etc it doesn't appear we have a large amount of compliance.

Someone else said about the measurement 1 inch. I had measurements at 1 inch, but actually did measurements in 3 mm increments on either side of static ride height so had 20 odd points to check rising rate. Check out: http://members.iinet.net.au/~bushfam.../Rear%20MR.jpg

[Lukin]

Edit: Looked at the graph, and it seems to be a slightly decreasing rate, not a rising rate. Though still don't completely trust it.