Increased RWD 3link design freedom

[BillyShope]

Commonly, a 3link is considered to have 2 symmetrically positioned links, in plan view, with a third more centrally located link. Page 40 of my site provides a spreadsheet which allows much more design flexibility. (The site is essentially devoted to the presentation of foundational suspension material for the young people engaged in dragracing, but I believe Page 40 might be of interest to other automotive engineers.)

The user can pick any 3 points along the axle for the link rear pivots. Of course, there must be sufficient point spread in the Z and Y directions. Among other parameters, he inputs the distance forward to the instant center and the desired percent antisquat. He is free to use different link lengths for the 3 links. The spreadsheet output provides the 3 link angles for the desired percent antisquat AND which provides complete cancellation of the driveshaft torque effect.

http://www.racetec.cc/shope

 

[hemipanter]

Why did you design a 3 link for drag racing?
Goran

 

[BillyShope]

The traction/cornering performance of a tire pair is maximized when the tires are equally loaded. With a RWD beam axle car, the driveshaft torque tends to unload the right rear tire. Some form of suspension asymmetry is necessary to cancel that driveshaft torque effect. While an asymmetric adjustment of the competition 4link...used in dragracing...will achieve this goal, a 3link offers a more direct solution.

With a properly configured 3link, the car can have equal rear tire loading at the beginning of the run and maintain that equal rear tire loading throughout the run.

http://www.racetec.cc/shope

 

[hemipanter]

I thought you would say so, but I wanted to reconcile.
A 4 link that has not parallel bars, or worse, a very short Ic, this will lock up the motion of roll, which will not happen with a 3 link. However, a drag racing car with 4 link is equipped with a fairly stiff swaybar to prevent from roll, (but soft vertical suspension to use the "rise" to plant the tires). Most of the time you are using non-parallel 4 link, so there's really not much roll present if any, which leads me to believe that there is no advantage with a 3 link, since we do not want sideway motions.
Just a thought from me.
Goran

 

[BillyShope]

Chassis roll during launch does not occur when the driveshaft torque is cancelled. (In other words, the front lifts evenly.) Chassis roll during cornering does produce link bind when the upper and lower links are not parallel. As you've indicated, this can be minimized by placing the instant center a few hundred inches forward. The limiting condition would, then, be parallel link lines meeting...at infinity...with the desired constant percentage antisquat line. With more antisquat on the right side than on the left, driveshaft torque cancellation can be achieved. The result is then as "streetable" as a 3link.

http://www.racetec.cc/shope

 

[hemipanter]

Yes, as I mentioned, we want an shorter instant centre in order to calibrate the agressivity in the separation. What many road racing car using 4 link does, is to have very long parallell legs out to the side of the cars, (inside the body fibre glass). This is not practical for a street car.

But for a street car that sees the dragstrip now and then the 3 link would be very good. In fact many roadracing cars that have liveaxle will benefit from a 3 link.
Goran

 

[BillyShope]

Since the resultant of the dynamic forces acting through the links has a line of action which passes through the rear tire patch and the instant center (Page 38)and, in addition, since a force can be considered to act anywhere along its line of action, there is no reason for a "short" instant center. "Separation" or "hit" (dragracing terms which might not be familiar to most readers of this forum, but which correspond to antisquat percentages above 100)can just as readily be adjusted with "long" instant centers. In fact, with parallel links, this adjustment would be made with an instant center an infinite distance forward.

(Dragracers commonly keep the lower link of a 4link nearly horizontal and make adjustments only with the upper link. This keeps the instant center relatively close to the rear axle or "short." But, the same antisquat changes can be made with adjustment of both links while keeping the instant center further forward or "long.")

http://www.racetec.cc/shope

 

[hemipanter]

Okay, you think that if rods are parallel or describing shorter Ic, it will change the Force Line angle at the same raste, at a given change in ride height. I might remember wrong, but I thought it made a difference.

Then I wonder if not the attachment points of the chassis will be of less quality places, at certain anti-dive%, with parallel rods?

I can, for comparison, refer to both the Corvette and Viper that have both anti-dive and antisquat, and none of the cars have parallell A-arm attachments and fairly short Ic, around 1500mm. Any thoughts?

Goran

 

[BillyShope]

You are attributing an understanding to me which I do not possess, which indicates that you have misunderstood my posts. I believe I have covered these matters adequately at my site, but it took me far more time to develop those pages than I care to devote to forum posts. I can only suggest, then, that you search for your answers at my site.

I do not, however, cover matters of structural integrity relating to suspension attachment points.

http://www.racetec.cc/shope

 

[hemipanter]

I see that I discuss a bit at the side of your posts, but I was interested to see how you looked at 3 and 4 link on a more practical level. I think it's very good and useful "spreadsheet" on your page. Sorry if I gave the impression of something negative.
I have built some 4 Link for pure drag racing cars, and do not see that it is so easy to build very different from the normal. BUT, it is easy to be blind to, so therefore I am always looking for inputs.

Goran

 

[GregLocock]

Billy, I have discussed many of Goran's ideas with him over the years, I promise you, he is looking for answers, not criticising. You two have a very similar attitude to forces, and experiments.

Sadly I work on IRS at the moment, that takes all the fun out of it.

Cheers

Greg Locock


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