[frankenstang57]

I've got a couple of questions for you guys regarding 4-link rear suspensions. I'm building a SBC powered Triumph Spitfire for the Grassroots Motorsports $2005.00 Challenge. I'm planning on back-halving the car and running a budget rate 4-link. So far I have acquired a whole bunch of used 3/4" heim joints, (not quite spanked yet, but getting there), some aluminum control arms, a set of coil-overs, and a big rear sway bar for the drags, and a narrowed 8.8". I was thinking about picking up a set of pre-drilled mounting plates from Jegs. This is the first time around building a chassis(or at least half) for me, do you guys have any pointers that may be helpful? I'm planning on using 2x3" rectangular tubing, should I be worried about any shifting/warping from heat during welding? Any rule of thumb ride height/axle clearence tips? Any help is greatly appreciated. Thanx!

[cboy]

Welcome to the board, frankenstang,

My first suggestion would be that you use the "search" function available at the top of the page. Enter words like 4-link, 4-bar, four-bar, triangulated 4-link, etc. and you will finds many many answers and tips on setting up that rear end. Also, use the "Knowledge Base" function and do the same thing - there is a whole section on suspensions and some very good info. Once you have digested all that you can ask whatever specific questions you have that have not already been answered before.

Dewey

[frankenstang57]

Thanx. I have been doing a ton of reading on here in the last couple days and I think I found what I need. The data base function really helped. Best info: http://www.rpmnet.com/techart/4link.shtml I guess the only specific question I have is wether or not the control arms will be strong enough. They are from a sprint car. They seem to be pretty beefy, what do you guys think?

[cboy]

I don't know that much about aluminum control arms but my guess is if they are threaded for 3/4" rod ends then they are stout enough. Billyshope or one of the other suspension "experts" could tell you for sure. If they are tubular, what is the exact O.D.?

Also, just so you are aware, the info in the link you cited is for suspension systems using a "birdcage". If you are NOT using a birdcage then the engineering data will not apply.

Dewey

[frankenstang57]

By definition, what exactly is the "birdcage"? Is that a reference to the bracketry on the axle tube? I was planning on building something similar to those weld-in kits from Jegs. I'm not sure about what the OD is on those control arms. They are pretty beefy. I'd guess probably about 1-1/4" or so.

[BillyShope]

Originally posted by frankenstang57
By definition, what exactly is the "birdcage"? Is that a reference to the bracketry on the axle tube?

You got it! Unless the links are parallel and of equal length, the 4link will bind during cornering UNLESS some provision is made for at least one of the link pairs to rotate on the axle housing. Birdcages are commonly used in oval track racking, but are seldom seen at the dragstrip.

So, to avoid binding in a car that must also compete in an autocross, you'll want to keep the bars parallel. But, you also don't want excessive squat on launch, so you'll need to angle the parallel set upwards, from the rear, at an angle with a tangent equal to the CG height divided by the wheelbase.

You mentioned increasing the rear roll stiffness for the drags. Good thinking! But, if you put some asymmetry into your 4link setup, you won't need to do that. In other words, you can go directly from autocross to drags without touching the rear suspension.

Your idea in increasing the rear roll stiffness was to decrease the effect of driveshaft torque on rear wheel loadings. The same thing can be achieved by having a greater angle on the right side link pair than on the left. In fact, you can achieve complete driveshaft torque cancelation and end up with equal rear tire loading under all drag conditions.

Your first reaction might be that this would cause tremendous disparity between left turn and right turn performance at the autocross. But, if you stop and think about it, this actually tends to cancel out the roll steer effects and merely push the entire axle assembly either forward or back during cornering. I'm certain you'll find such a setup quite "driveable."

(As an aside, roll oversteer should never be a big concern for the racer. In fact, it only exists because the SAE includes steering wheel angle in its under/oversteer definition. With a race car, changes in rear axle roll steer don't affect wheel loadings one whit! If a car pushes at breakaway, you can increase rear axle roll oversteer all you like and it will still push at breakaway. All you've done is rotate the car relative to the tire patches, requiring a different steering angle. There is, admittedly, a psychological effect on the driver, however, when the steering angle changes and it might take a few laps to convince him that he's capable of the same times.)

Personally, I'd try to improve my $2005 budget by selling those aluminum suspension pieces and then I'd fab pieces of steel. But, use your own judgment. When it comes to those pieces holding the wheels under me, I prefer steel and then I'll make up the weight difference with a haircut.

[frankenstang57]

Thanks Billy, that's very helpful. I think I will ditch the aluminum for steel. I don't really have too much in them, I think about $1.00 each. Biggest problem with them is I don't have 4 that are the same length. Only 2 of each. I can shorten 2 of them, but I don't feel up to welding aluminum. What size tubing should I use and where can I get lh nuts? Thanx!