MotionRatio and pushrod/spring angle

[Ratna Mitheysh]

Hi,
Motion Ratio is defined as MR=wheeltravel/springtravel.
Now I have no confusion if my pushrod/spring is perpendicular to my lower wishbone. But when it is inclined at an angle, how does it affect the motion ratio?
RCVD does not talk about inclined coil springs.
Carroll smith(Tune to Win)says it to be the wheeltravel to spring deflection ratio.
And another worksheet I found at this link:

http://performance-suspension....pensionworksheet.pdf (http://performance-suspension.eibach.com/uploads/File/upload/ERS-16_suspensionworksheet.pdf)

keeps motion ratio as the ratios of distance from the chassis pivot point of the lower wishbone, and introduces an ACF(angle correction factor) while calculating Wheel/spring rate, which RCVD and Tune to Win do not.

Is the approach followed in the worksheet right?
What I primarily wish to know is that on keeping the wheel rate fixed, will the spring rate change on inclining the spring?

Sorry if these questions seem trivial.
Thanks in Advance.

[RobbyObby]

I have never encountered the Angle Correction Factor mentioned before. As far as the formula's I've studied, Motion Ratio is simply Z-displacement of the wheel divided by linear shock travel, regardless of the orientation of the shock, or the type of suspension.
The worksheet provided looks to be summarizing the general case where the spring/damper is mounted directly to the outer ball joint, i.e. no push/pull rod or rockers.
However, I am by no means a suspension guy, so someone feel free to chime in with more insight.

[JDS]

I am not quite sure I understand the question? Are your springs direct actuated (no rocker)? In which case, are you asking how to do trigonometry?

[Ratna Mitheysh]

Originally posted by JDS:
I am not quite sure I understand the question? Are your springs direct actuated (no rocker)? In which case, are you asking how to do trigonometry?

My question is "What is the correct definition of Motion Ratio?"
Does it incorporate the inclination of the spring or only the position where it is mounted?

Say I have a direct actuated spring mounted at the hub at an angle of 60 degrees with the wishbone.
Is my Motion Ratio 1:1 or 1:sin60?
The method in the above mentioned worksheet by "eibach springs" confused me.

I am designing the suspension for the first time. Hence I want to make sure my approach is not wrong.
Any help is appreciated.

GRAM http://fsae.com/groupee_common/emoticons/icon_smile.gif

[JDS]

My question is "What is the correct definition of Motion Ratio?"
Does it incorporate the inclination of the spring or only the position where it is mounted?

Say I have a direct actuated spring mounted at the hub at an angle of 60 degrees with the wishbone.
Is my Motion Ratio 1:1 or 1:sin60?
The method in the above mentioned worksheet by "eibach springs" confused me.

I am designing the suspension for the first time. Hence I want to make sure my approach is not wrong.
Any help is appreciated.


Motion ratio isn't always defined the same way, usually it's wheel-travel / spring travel, but it is sometimes reversed, so spring travel / wheel-travel. It doesn't matter which you use, as long as you are consistent in your calculations. The spring travel number you are looking for is how much the spring compresses along the centerline axis. So yes, for a direct actuated spring you could get an approximation of travel using the appropriate trig function based on the angle of the spring.

[Adambomb]

If you look at the spreadsheet it just says that the ACF is simply defined as the angle between the shock and direction of suspension travel. So it is simple trig. And I am ashamed to admit that there was a year we forgot that...

For a direct-acting shock it is exactly as described there, while when using rockers, use the angle to the pushrod or pullrod.

[Ratna Mitheysh]

Shouldn't the ACF be squared??
My effective force on the spring changes and so does my spring compression.
Correction for both should give me an ACF squared right?

[shark.ashwa]

Originally posted by GRAM:
Shouldn't the ACF be squared??
My effective force on the spring changes and so does my spring compression.
Correction for both should give me an ACF squared right?
The spring compresses because the force acting along the spring has changed. Hence, it shouldn't be squared.

[JDS]

The force required to lift your wheel centers is directly related to spring force (not counting friction, arb's etc...) and the force required to compress the spring is directly related to spring travel. That should be all you need to know.

[typeh]

you have to take angel in consideration in case of no push/pull rod

Alexandria University Motorsports

[Ratna Mitheysh]

Yeah, Got it.
I was trying to account for both the force change and displacement independently while they were related.
Hence the ACF squaring confusion.
Feel kind of stupid now but Thank you all for your help.

GRAM http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Nbruno]

Another MR question...This time using a pushrod setup. I want to use a MR of 1 (first year team, keep it simple) and have done a bunch of calcs using that value and they come out to reasonable numbers. Now I'm starting to get into the geometry of the cranks themselves. Looking at a bunch of pictures of cranks, most of them have a shorter distance from the pushrod pickup to the pivot than the distance from the damper pickup to the pivot. The difference in this distance for some cranks is very large.
I was under the impression that the MR is determined by the ratio of these two distances. If that is true, many people are running MRs nowhere close to 1. My current thinking is that the reason for the difference is the distance between the center of the wheel and the pushrod pickup point on the LCA must be compensated for in the crank in order to get a MR of around 1. Is this correct?

[Buckingham]

My current thinking is that the reason for the difference is the distance between the center of the wheel and the pushrod pickup point on the LCA must be compensated for in the crank in order to get a MR of around 1. Is this correct?

Correct. Most choose to define their MR as "ShockTravel/WheelCenterTravel". To get MR=1.0, the bellcrank needs to compensate for the rest of the system.

To sort out your bellcrank geometry, just use any 3-d sketch tool.