S550 Mustang Suspension and Powertrain Discussion

[Norm Peterson]

I'm just a guy that welds stuff but I don't think wheel centerline has anything to do with motion ratio. Ball joint centerline sounds right to me. Norm will have the answer shortly.

Motion Ratio = (A/B) x cos(q) x (C/D)

C/D can be greater than 1 if the FVIC is on the "wrong" side like it is in many older cars like the GM Intermediates and I think the 1st gen Mustangs.

by center line I meant a line that cuts down the middle of the tread if you were to view the tire from the back of the car.

I know suspension parts make all sorts of crazy angles and hub spacing and all that, but transmissability of force would still apply. and like I said I was assuming the control arm stayed perfectly level so I could explain without writing a book.

The motion ratio math stays the same, but the FVIC moves around and changes the "C" and "D" numbers. Spring angle q also varies, and I guess if you get picky enough that the "center of the tire" would really need to be some sort of contact patch force centroid (which will also move around as camber varies).

There's aspirin in the medicine cabinet.


Norm

 

[Budwise]

So the rear springs are on the stiff side and the front are on the soft side? Am I getting that right?

 

[dontlifttoshift]

Yes, the rear springs are stiffer but the wheel rate is what matters for ride quality/handling.

 

[ArizonaGT]

Motion Ratio = (A/B) x cos(q) x (C/D)

C/D can be greater than 1 if the FVIC is on the "wrong" side like it is in many older cars like the GM Intermediates and I think the 1st gen Mustangs.



The motion ratio math stays the same, but the FVIC moves around and changes the "C" and "D" numbers. Spring angle q also varies, and I guess if you get picky enough that the "center of the tire" would really need to be some sort of contact patch force centroid (which will also move around as camber varies).

There's aspirin in the medicine cabinet.


Norm

Love the diagram--great (generalized) way to think of that all as opposed forces/moments.

 

[Mineral_'01]

Motion Ratio = (A/B) x cos(q) x (C/D)

C/D can be greater than 1 if the FVIC is on the "wrong" side like it is in many older cars like the GM Intermediates and I think the 1st gen Mustangs.



The motion ratio math stays the same, but the FVIC moves around and changes the "C" and "D" numbers. Spring angle q also varies, and I guess if you get picky enough that the "center of the tire" would really need to be some sort of contact patch force centroid (which will also move around as camber varies).

There's aspirin in the medicine cabinet.


Norm

Yes. and I will assume you were making a diagram for the S550 rear suspension and not the front. (because McPherson strut, no upper control arm) I will also assume the motion ration using that formula might change a bit depending on your static camber setting. (cos(q) with McPherson strut)

 

[BMR Tech]

The PP shocks have WAYYYYYYYYYYYYY more compression than the standard base model shocks. Holy smokes.

 

[Norm Peterson]

Yes. and I will assume you were making a diagram for the S550 rear suspension and not the front. (because McPherson strut, no upper control arm) I will also assume the motion ration using that formula might change a bit depending on your static camber setting. (cos(q) with McPherson strut)

The same diagram can be used almost directly to understand a strut suspension.

Instead of having a real UCA that is free to pivot at the upright, you can consider it to have a virtual UCA that is fixed at 90° to the upright (I think specifically from the center of where the strut rotates relative to the strut tower). You have to go back to the description of how to construct a strut suspension's geometric roll center to find that, after that it's just as easy to go from FVIC to that piece of motion ratio (C/D) as it is to get to the geo RC. I don't remember how long I struggled with strut geometry in Excel, but once I picked up on that it was like switching on the lights.

The construction lines are all virtual anyway, so they don't care if they're working from a real UCA or a virtual one.

Norm

 

[NUTCASE]

Great write ups Norm Peterson. If I could give you positive rep I would.

 

[ford20]

My head hurts. Is there a beginners guide somewhere to all of this suspension stuff?

 

[Mineral_'01]

The same diagram can be used almost directly to understand a strut suspension.

Instead of having a real UCA that is free to pivot at the upright, you can consider it to have a virtual UCA that is fixed at 90° to the upright (I think specifically from the center of where the strut rotates relative to the strut tower). You have to go back to the description of how to construct a strut suspension's geometric roll center to find that, after that it's just as easy to go from FVIC to that piece of motion ratio (C/D) as it is to get to the geo RC. I don't remember how long I struggled with strut geometry in Excel, but once I picked up on that it was like switching on the lights.

The construction lines are all virtual anyway, so they don't care if they're working from a real UCA or a virtual one.

Norm

Got it. That makes sense now. Thanks for posting.

Happy Thanksgiving!

 

[Norm Peterson]

"How To Make Your Car Handle" by Fred Puhn is about the best introductory-level book as I've ever seen . . . if you can find it.


Norm

 

[ArizonaGT]

"How To Make Your Car Handle" by Fred Puhn is about the best introductory-level book as I've ever seen . . . if you can find it.


Norm

http://www.amazon.com/How-Make-Your...1&sr=8-1&keywords=How+To+Make+Your+Car+Handle

 

[Norm Peterson]

Thanks, Arizona.

I had no idea it would be that readily available - I've had my copy for at least 30 years (still has its $4.95 retail sticker on the cover, and "Printed in USA 11-76" appears on the inside). I still refer to it.

I've always felt that its introduction to both the concepts and the math was the book's strong points, rather than specific brand names or the alignment settings for a 1973 Vega.


Norm

 

[NUTCASE]

I think the hard part for a lot of people is the learning curve involved if you are not used to working with moments and forces at equilibrium.

Oh yeah, and the trig. but I guess if you have the eqns sitting right in front of you all you need is a calculator.

 

[frank s]

Thanks, Arizona.

I had no idea it would be that readily available - I've had my copy for at least 30 years (still has its $4.95 retail sticker on the cover, and "Printed in USA 11-76" appears on the inside). I still refer to it.

I've always felt that its introduction to both the concepts and the math was the book's strong points, rather than specific brand names or the alignment settings for a 1973 Vega.


Norm

For very little money you can have the second edition (1987). It has updated information and photos, including one of my 1979 TR-7 nosediving at a slalom (what we called autocross in them days) (p. 26). If everything works out, I'll see Fred at an old-racers meeting in a few weeks, and will try to remember to ask him how the (rumored) new revision is coming along.

 

[Mineral_'01]

Does Puhn's book or any edition of it touch on McPherson strut front suspension and provide formulas or diagrams for that type of suspension? I am thinking it would mostly talk about SLA/multilink given the time period but wanted to ask anyway. Thanks

 

[Norm Peterson]

Yes, it does, even the book's 1st edition does. Quite a few of the smaller cars did have MacStruts by then. Mercury's Capri (a Euro-built Pinto/Vega competitor in those days) did, for one, and I recall that Porsche and BMW also did. I'm sure that there were others, but I'm just working from memory.


Norm

 

[Mineral_'01]

Yes, it does, even the book's 1st edition does. Quite a few of the smaller cars did have MacStruts by then. Mercury's Capri (a Euro-built Pinto/Vega competitor in those days) did, for one, and I recall that Porsche and BMW also did. I'm sure that there were others, but I'm just working from memory.


Norm

Thanks Norm. The exact answer I was looking for. Heading over to amazon now to make a purchase. Much appreciated.

Edit: Done. I hope to acquire much more suspension knowledge in reading Puhn's book. Sorry to throw this thread a little off topic.

 

[lost won]

Okay, hooker - I'll bite.

The PP shocks have WAYYYYYYYYYYYYY more compression than the standard base model shocks. Holy smokes.

How much more, Kelly?? Share, please.

 

[BMR Tech]

How much more, Kelly?? Share, please.

I am working on trying to put together a graph / overlay.

Of course, I am waiting on someone else for this as well....so I would like to wait until I have all the hard data - and I will share, gladly.

On that note, we did finalize our Spring and Bar rates for the S550.