Comparing tri5 to C4 front suspensions

[chevynut]

I thought I'd bring the discussion about spindles, a-arms, instant centers, etc. into this thread so it doesn't keep the other one off-topic. I wanted to get some actual data on the C4, tri5, and modified tri5 suspension dimensions to analyze.

I just did a quick measurement of suspension parts for both the stock tri5 suspension and both the early and late C4. The measurements were with a tape measure and they're probably accurate to about 1/8" or better.

.................................................. ..Tri5 ..................Early C4 ...................Late C4 ...............Tri5, 1" taller balljoint
Spindle height (balljoint centers) .... 9.75.................. 11.81....................... 13.31................. 10.75
a-arm pivot centers (vertically) ....... 10 ....................10.38 .........................11.13 ....................10
Upper a-arm length (pivot to bj) ..... 11 ....................8.25 ..............................8 ......................11
Lower a-arm length (pivot to bj) .....14.5 .................13.25 ...........................15 .....................14.5

Tri5:
Notice that on the stock tri5 suspension the spindle is shorter than the a-arm pivot centers by 1/4". This results in an instant center outside of the frame, about 450" outside per a quick CAD drawing I did, assuming the lower a-arm is level (balljoint center same height as a-arm shaft centerline). The spindle is taller than I thought it was due to the lower balljoint being below the spindle with the stud pointing upward. The net result is that there is positive camber gain as the suspension is compressed, because the upper balljoint moves outboard while the lower one moves inboard. Positive camber (top of tire outboard of the bottom of the tire) is bad from a performance point of view, as the tire wants to roll under the wheel.

Early C4 (84-87):
The early C4 spindle is significantly taller than the tri5 spindle (2.06"), and 1.43" taller than the a-arm pivot points. This configuration puts the instant center on the opposite side of the car centerline. My quick CAD drawing shows it to be about 48.5" from the upper a-arm pivot inboard (horizontally) to the instant center. Again, I assume the lower a-arm is level. This configuration gives negative camber gain as the suspension is compressed, since the upper balljoint instantly moves inboard and the upper a-arm is shorter than the lower one. Negative camber tilts the tire into the turn, improving handling.

Late C4 (88-96)
Again the spindle is taller than the early C4 by 1.5" and 3.56" taller than the stock tri5 spindle, but the upper a-arm pivot also moves upward .75" from the early C4. This provides even more angle to the upper a-arm, and more camber gain on suspension compression. The lower a-arm is longer, reducing the inboard movement of the lower balljoint, and the upper a-arm is slightly shorter, increasing the inboard movement of the upper balljoint. The instant center is about 34" inboard of the upper a-arm pivot, horizontally.

I didn't do any dynamic analysis, only static and assumed the suspension was at ride height with the lower a-arms level in all cases. That' means the a-arm pivots and the centers of the balljoints are at the same height.

Now, let's look at the 1" taller upper balljoint with the stock suspension. The a-arms are the same length as stock. While the taller balljoint does move the instant center to the opposite side of the car as the stock suspension, it's about 146" from the upper a-arm pivot. The net of this is that it does give some negative camber gain, but not nearly as much as even the early C4 suspension. The upper a-arm is relatively long, which increased the arc the balljoint travels, decreasing the inward movement after it goes level. Also, the lower a-arm is relatively short in comparison, being only 32% longer than the upper one. The early C4 is 61% and the late is 88% longer.

So my conclusion is that while a 1" taller balljoint helps correct the huge problem with the positive camber gain of the tri5 suspension, it isn't that much of an improvement from a performance point of view in improving negative camber gain.


EDIT- For updated numbers please see this thread

http://www.trifivechevys.com/showthr...metry-analysis

[chevynut]

Here's some interesting diagrams...

[chevynut]

And we haven't even talked about the effect of the anti-dive geometry, steering axis inclination, or scrub radius. Those parameters can't be duplicated with a stock tri5 frame and aftermarket parts.





Stock tri5 SAI is 3.5-4.5 degrees. C4 SAI is 8.744 degrees. Scrub radius depends on wheel offset and tire diameter.

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

So how does Rick's C3 solution compare?

[Rick_L]

Good summary of the suspensions.

Note that the stock 55-57 suspension has some anti-dive also. I've never measured how much, notice that you don't have a number on C4 either.

My C3 Corvette spindle conversion has similar numbers to Rocky's extended ball joint version, except that the upper control arm is shortened by 3/4" or so, and the SAI is 7 degrees instead of 3.5 degrees, improving the scrub radius. My biggest problem is the lowering from shortening the springs.

[chevynut]

The C4 has a lot of anti-dive compared to a tri5 which has some as you say. I can't find the exact numbers anywhere, but upper a-arm shaft is sitting at about 11-12 degrees as I recall, per my measurements. Here's something i found on the net:

1984 AntiSquat 51 % AntiDive 46 %
1985 AntiSquat 51 % AntiDive 46 %
1986 AntiSquat 62 % AntiDive 46 %
1987 AntiSquat 62 % AntiDive 46 %
1988 AntiSquat 62 % AntiDive 46 %
No data from later years.

I'm not sure how this all factors into handling, but we do know that the C4 suspension is a major improvement over the stock tri5 suspension. I don't know how much anti-dive a C5-6 has in comparison. Here's a pic that shows the anti-dive of the C4 suspension pretty clearly.

[Rick_L]

Stock55-57 probably has 1/3 to 1/2 that.

What's not clear is how much anti-dive is too much. Maybe I can find that somewhere.

[JT56]

Nice diagrams Cnut

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

Cnut you see mr Hinkles C4 chassis 55 nomad build on FB? Looks pretty trick so far. Similar color as yours with silver frame.

[chevynut]

Rocky, who the heck is Mr. Hinkles???? Is he related to Mr. Hanky?

Got a link to that FB page?