Thread: tri-4 bar geometry

Tweet

On a tri-4 bar setup, let's say hypothetically you only used the two long bars (obviously this wouldn't work but just for the example). In that case, the axle housing would follow the radius of the ends of those links while travelling. So the axle wouldn't go straight up and down, it would curve along a radius equal to the length of the bars.
Now if we add in the triangulating bars, or any bars at all of a different length, doesn't that throw in a conflicting radius that the axle will want to follow. Now we have two different length bars controlling the axle, and not only that, the triangulating bars come into the axle at an angle, so now the radius' of the two different bars aren't even in the same plane.
How can it work when you use bars of different lengths, and therefor different radius'?
Also, does the angle of the coil over make a difference?
I'm asking this because I ordered a tri-4 bar setup and I want to make sure I have these theories straight in my head before I start cutting and welding.

Actually you need to go back to picturing the axle with all four bars working. Your postulation would be accurate if the bars didn't pivot at both ends (e.g. lots of ladder bar set ups). What the unequal 4 bars do is limit the rotation of the axle housing keeping the differential yoke in a relatively stable verticle plane.

I thought that was the case, because one connects on top and the other on bottom, so it makes the axle rotate with respect to the bar, but not with respect to the frame. . . or at least not much. I'm psyched to set up my 4 bar.
thanks

Richard,

Excelent analysis. Takes me back to dynamics class . . .

In a practical sense, though, I doubt if there's enough suspension travel on most cars for the swing to produce a significant problem with pinion angle. After all, ladder bars work reasonably well - of course they're generally made long enough to minimize the pinion angle change too.

Richard, in an absolute sense you are correct. That's why I used the qualifier "relatively". The change in pinion angle in a typical 4 bar setup in very slight as you know. As I'm sure you're also aware, part of what you describe in the front double A arm arrangement is not transferable to a rear application as the front arms are not typically parallel to one another in either plane so that various factors like camber, caster, anti-dive, etc can be engineered in. On a solid rear axle those factors don't apply. Some anti-squat can be factored in by either dropping the front of the upper bars about an inch, or raising the front of the lowers that amount, (race car considerations excluded here), but the rear bars usually are nearly parallel fore to aft.

I see all your points. The pinion angle would have to rotate down as the axle goes up because the bar attached on top of the axle is shorter, so the top bracket would get pulled inward as it passes over the midpoint of that curve.
BUT, the part I'm still confused about is how it works with the top bar coming into the axle at an angle. Now the plane of this radius goes through the frame at about a 30 degree angle. So if you imagine the axle travelling with ONLY one diagonal bar attached, and also imagine it blown way out of proportion, the axle would get pulled to the side along this angled plane as it travels.
So with both trianglulated bars attached, one is pulling to one side and the other is pulling to the other side, obviously with both attached it can't go either way, but doesn't this cause bind in the links as those pivots try and move along their angled plane and the axle only moves in a plane parallel to the frame?

tcodi,

Don't let the angle distract you. With the limited movement of the rear axle and a well-designed setup, there's no bind. I'm with Richard. I don't think there's a better setup for a street-driven car.

Richard,

Is it time to start talking about Watts linkage?

yeah, I realize the actual movement is tiny, but I'm just thinking about it way out of proportion to see what's really happening and it was bugging me. The thing is, mine isn't a kit or anything for my particular car, I'm just going to have to put it together and I wanted to make sure I knew all the little things.
I'll just tack it all together and move the axle housing up and down with my arms and make sure it's working right.
Thanks

The movement isn't all that tiny - it can be 6 or 8 inches, but the bars still don't bind.

Good for you for building your own. That's what rodding is all about. Just watch your geometry. The up and down angles in those bars make all the difference in the world.

I've placed my 4 links on my suspension, the last thing I have to do is place the upper connection of the coil overs.
I believe they are 14" open. I was going to put the upper mount hole 4 1/2" inward from the lower mount point.
I arrived at this number from placing them on the brackets and holding them at an angle until it looked right.
Does anyone have an opinion on what the proper angle is for coil overs, or will this work ok.

Coil-overs are set at all kinds of angles. The ones on my '34 are inset 5-1-2" at the top.

There is an effect in changing the angle. The more the shocks are slanted inward, the less the shock action . . . vector mechanics works on cars too.

I think you're fine.

By the way, my theory is that the distance from the center of the left rear tire footprint to the center of the upper shock bolt should be equal to the distance from the left tire footprint to the center of the lower shock bolt. (Of course, the other side is a mirror image) That's based on the arc the axle swings when one tire goes over a bump, and the other doesn't. Probably overkill, but engineering school does that to ya.

I figure if the shocks are 14" long unloaded, and I set it 5" inward, that will give me an angle of 21 degrees, which will transfer 92% of the downward load in the direction of the shock.
If the springs actually settle down 1 1/2" under the weight of the car, it will extend that angle to 23 degrees, decreasing the shock action to 91%.
I guess the angle will mostly determine how soft or stiff the ride is, but my shocks are adjustable so I don't think I need to worry about a few percent here or there.
I think 5" sounds good.
Thanks guys.
I'll post a picture when it's done.

Your calculations deserve a math team cheer:

Sine! Cosine! Cosine! Sine!
Three Point One Four One Five Nine

You are all right!

I used auto cad to duplicate the arcs of both upper and lower rods on my roadster, I feared a radical change in the pinion and kingpin angles.
I found that there was more wheel base change than angle change.
Out of the 5 0r 6 different lenghts I tried, the worse was 7/16" in an 8" ark, or 4" up/ 4" down. The short rods were half the length of the long ones.
All that means is your Okay