Suspension geometry analysis

[chevynut]

Using all the C4 dimensions above, and with a level lower a-arm I calculated the instant centers and roll centers.

For the early C4 suspension I get an instant center of 83.80" from the center of the car and a roll center 3.149" ABOVE the ground.

For the late C4 suspension I get an instant center 66.42" from the center of the car and a roll center 3.792" ABOVE the ground.

Both of these suspension geometries move the instant center a lot closer to the center of the car than the stock tri5 geometry and the roll center is quite a bit higher (~5"!!) as a result...which is a good thing for handling. It's around 3 1/4-4" higher than using the 1/2" taller balljoint. If you start with a lower a-arm pointing DOWN 1" at the outboard end, the roll center is even higher, but the CG rises 1" too. I may look at that scenario.

Another thing you can see from these layouts is that a shorter tire lowers the CG of the car, lowering the instant center the same amount, but the roll center doesn't drop that much.

[chevynut]

18 degrees doesn't sound right at all. Does your layout let you measure scrub radius? Is it close to zero with the layout you have? The 8.75 degree SAI with the early suspension is a bunch more like I expected.

I agree Rick, but I measured The SAI on my late Nomad spindle with my digital level. It's right at 18 degrees according to my layout and my level. My layout doesn't work out if I use the 8.744 degrees of the early spindle. The scrub radius on the early suspension is 3.34" and on the late one it's basically zero using the 18 degree SAI on my layouts.

Here's the two spindles (late on the left) with the hub faces sitting parallel...the difference in SAI is readily apparent. This is news to me.

[chevynut]

I just put the late spindle and hub on my level welding table, studs down, and put a 1/2" rod through the balljoint stud holes. The SAI is indeed 18 degrees per my angle finder.

But I realized I made a mistake on the scrub radius....I used the wheel mounting surface, not the center of the tire as the contact point. The wheels have a 50mm positive offset. I'll have to re-do the roll center slightly too, but it won't make much difference. I'll edit the numbers above.

So with the 50mm offset I get 1.37" scrub radius for the early suspension, and a NEGATIVE 2.02" for the late one.

What does a negative scrub radius do?

"Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero."

[Rick_L]

A comment on stock ride height and my layouts/calculations, especially to rocky.

My layout has the stock lower control arm centerline at 10.36" above the ground. According to the 55 assembly manual, section 3, sheet 5, the lower control arm centerline is 11.0" at "curb weight" and 10.00" at "design load".

Since I looked at "stock height" and 1" and 2" lower than stock height (by my definitions), to me the 10.36" dimension is probably as accurate as some of the other measurements/assumptions. rocky, didn't you mention a 10.49" dimension?

Since we're looking at trends and trying to see what happens with certain changes, as well as comparing to other designs, I don't see any need to revise anything based on what the stock ride height is.

If someone wants to see results at some other ride height, I can do a couple more layouts if there is real interest. But they will just fill in the gaps so to speak.

[rockytopper R.I.P 5-13-2017]

A comment on stock ride height and my layouts/calculations, especially to rocky.

My layout has the stock lower control arm centerline at 10.36" above the ground. According to the 55 assembly manual, section 3, sheet 5, the lower control arm centerline is 11.0" at "curb weight" and 10.00" at "design load".

Since I looked at "stock height" and 1" and 2" lower than stock height (by my definitions), to me the 10.36" dimension is probably as accurate as some of the other measurements/assumptions. rocky, didn't you mention a 10.49" dimension?

Since we're looking at trends and trying to see what happens with certain changes, as well as comparing to other designs, I don't see any need to revise anything based on what the stock ride height is.

If someone wants to see results at some other ride height, I can do a couple more layouts if there is real interest. But they will just fill in the gaps so to speak.

Rick the 10.49 came from the diagram that is posted in the other threads as dimension A height at curb weight which is what should be used in these analysis sense cars are aligned at curb weight not design weight an assumption on my part. So if 11 is a true number then so be. It would only make the bottom arm point down more. If your model shows the tire contact patch touching the ground with cnuts tire specs and lower arm level then something is wrong with your layout. If 11 inches is the real number then my car is dropped close to 3 inches from stock. My dimension A measured at curb weight is approx 7.7 -7.8 measured. I just went out and jacked the car up up to put the lower pivot 10.38 and the lower arms are pointing down at least 5 degrees. It is looking like if I had cut less coil and stay closer to 2 inch drop my arms would have ended up almost level and not .56 high on outbd. Any how I think ride height is very inportant. When you guys correct the assumpton that the lower is level at stock ride height your analysis will change alot for the worse. I still haven't gotten a clear answer of how to determine roll center when ic is on wrong side like stock trifive is. Some show you still run the line to the tire patch center but the link I shared and we discussed about F1 says that in this case you create a bisecting line between the to arm lines and project that back to the center to determine roll center in that case. Still lots of conflicting info about this case.

[Rick_L]

You need to remember that the lower ball joint center is pretty close to the top of the control arm, while the shaft center is in the middle of the control arm. Plus the arm is curved on the bottom. This gives the illusion that the arm is going down from shaft to ball joint even at stock ride height.

As for your question about the location of the roll center when the instant center is outboard, I explained that in the other thread.

Not sure what's really agitating you as the analysis shows that the extended ball joints do what's claimed. Are you have a tough time accepting that the dropped spindles do what they do? Or do you think that the extended ball joints do more than what I said? Also the extended ball joints do move the instant center to the supposed "good" side, as does the C3 spindle, which is essentially the same thing. I can post the I.C. locations if that would help, I edited them out. They were in the unedited post but you probably couldn't match them up to the various conditions because of the presentation.

How much wheel travel do you have in bump with the 7.7" dimension? Can't be much.

[rockytopper R.I.P 5-13-2017]

Rick I believe the lower ball joint attaching bolts front and back are approx .5 below the pivot give or take. My lower arms are straight tubular no curve my level measues at least 5 degrees down at stock ride height jacked to 10.38 dimension A. My approx ball joint Measures approx 9 in or so from floor. The lower arms are clearly not level at stock ride height. This has been the bases for both of you guys argument against what I have stated in ever discussion. If they were truely level in stock form I would have never ever argued my case or called drop spindles junk. If you or C'nut cannot accept the fact that the arms are not horz at stock height then we will always have a stalemate about this topic. Once you agree I'm correct about the lower arm being down we can then all get on the same page. If the arm is level at stock ride height I'll eat every word todate.
I did not join in hear again when I said I wasn't to start more bs. We just need to get the real truth out. So no I'm not nick picking your statements.
Rocky

[rockytopper R.I.P 5-13-2017]

As for your other question measuring the rubber bumper on my tube arms about 1-1/2. Jacked to 10.38 little less than 3. How much travel per spec does stock have?

[Rick_L]

rocky, I don't think it really matters where the stock lower arms start out.

The results are going to be similar, maybe even worse. I can do a layout with the lower arms where you say and show this. Will post tomorrow.

[rockytopper R.I.P 5-13-2017]

Rick when you do the camber goes postive from the get go what I have always said. This is not good for handling. Any thing below level is going positive from the start of travel using the trifive data points you have. I'm not disputing any of cnuts measurements of the individual parts I assume all are correct except the upper pivot delta close to 5 mine at least in my case 4ish with the arms I have. Do we agree on it will go postive below level? If so that has always been my argument against drop spindles because they keep the same postive curve as stock. It does not correct for the better.