S197 suspension motion ratios

[Alatalo]

Hello everybody, new member from Sweden here, on the hunt for information...

Has anybody measured or calculated the motion ratio (a.k.a. leverage ratio) between each tire's contact patch and the corresponding suspension element...?

Just looking without taking any measurements, the motion ratio for the front strut can't be far off 1:1...? However, it does appear to sit slightly inside the center of the tire contact patch and also on an angle, so...Also looking without taking any measurement, the motion ratio for the rear spring has to be very close to 1:1...? At least for all kinds of heave and pitch motions, but not necessarily for roll motions, as it appears to sit straight up from the axle but also quite far into the car...? And the dampers are clearly on a different motion ratio than the springs.

If anybody has information or more precise numbers they want to share, I am all ears. I used the search function, but came up short. Also on Trackmustangsonline, where motion ratios are actually discussed, but without mentioning any firm numbers. I will use the information to tinker and make some basic calculations, just to know what I have got before purchasing any aftermarket parts.

 

[WJBertrand]

Seems like you should be able to get underneath and measure this yourself. Curious what your interest in this is? It’s not something that can be easily adjusted (cutting, welding, etc.).

 

[Alatalo]

I will, but I will have no possibility to measure things for another month or so. So just checking if somebody has already got measurements or calculations they want to share.

I am not intending to change any of the motion ratios. I just want to know what they are, in order to calculate what effect a suspension adjustment has at the tire contact patch.

 

[kerrynzl]

The motion ratio of a MacPherson Strut is 1.11:1 due to "king-pin inclination" [angle of the strut]
This can also be calculated as 0.9:1 depending on you base calculation method

1" at the strut = 0.9" at the spindle
or
1" at the spindle = 1.11" at the strut

The motion ratio also changes [due to trigonometry] as the suspension increase .
The only way to reduce this change is to have the LCA perpendicular to the strut [but this lowers the roll center to infinate below the tire contact]

I will, but I will have no possibility to measure things for another month or so. So just checking if somebody has already got measurements or calculations they want to share.

I am not intending to change any of the motion ratios. I just want to know what they are, in order to calculate what effect a suspension adjustment has at the tire contact patch.

The motion ratio is needed to calculate suspension stiffness [wheel rate] but spring swaps or ARB tuning is a lot easier than motion ratio changes.
You need to know you CGH and RCH to calculate "overturning moment" [or bodyroll] for a maximum lateral acceleration. then you factor in the desired roll stiffness.

I had a race car with push rod A-arm suspension. We had a selection of "bell cranks" to dial in the wheel rate [as well as coil over springs] this was purpose built and easy to adjust.

For your S-197 I will suggest buying a tire pyrometer and learn to read temperatures across the tires [then make camber/toe adjustments to correct the footprint]
I've seen showroom stock racers clean up at the track with stock suspension because they knew how to set up the alignment.

S-197's are easy to get handling on the track but there is no "free lunch". A good handling track car is a nightmare to drive on the street.

 

[Alatalo]

Thank you for the proposals, but in this case the purpose is not to build a race car or even take it to track days. Just to understand where the OEM car is and what direction some of the popular aftermarket options will take it. If anything, this particular car will end up as a mild dragrace car rather than a corner handling car.

So far, I have only taken some basic measurements that pretty much confirm the 0.95:1 (front at ride height) and 1:1 (rear at ride height) mentioned on another forum.

 

[kerrynzl]

Thank you for the proposals, but in this case the purpose is not to build a race car or even take it to track days. Just to understand where the OEM car is and what direction some of the popular aftermarket options will take it. If anything, this particular car will end up as a mild dragrace car rather than a corner handling car.

So far, I have only taken some basic measurements that pretty much confirm the 0.95:1 (front at ride height) and 1:1 (rear at ride height) mentioned on another forum.

This was posted in the corner carver section so your intended usage was misleading.

My advice then is don't **** with it , just buy decent tyres and be done with it.
As soon as you lower an S-197 you start upsetting things and you need to start correcting induced geometry problems.

99% of the time these cars are driven as daily drivers.
If you want to occasionally drag race it, then install a nitrous set up [but leave it standard for daily use]

 

[Alatalo]

This was posted in the corner carver section so your intended usage was misleading.

My advice then is don't **** with it , just buy decent tyres and be done with it.

Dude, calm down. I am not intending to mess with any of the motion ratios. I am just intending to perform some basic calculations to know my starting point.

I posted my question under the corner carver section because I assumed that would be the place where somebody would know the answer. If that was the wrong choice or a choice that is offending somebody, can a moderator please delete the entire thread…?