Problem with triangulated 4-link

Five years ago, I fabricated and installed a triangulated 4-link rear suspension on my '29 Hiboy w/'32 frame that was originally built in the 80's, so this was a major upgrade. It worked great until I blew it apart for painting after shaking it out for three months. Since re-assembly, I can feel it bounce and skate through bumpy corners. It's been five years and feels worse every year. Maybe I'm just more sensitive to it.

The suspension geometry duplicates that of the Mustang II, although I raised the forward attachment point of the upper bars slightly to reduce squat on launch (which works well). I also have 180 lb. Carrera coil-overs angled in 20 degrees each at their tops, giving an effective sprint rate of 169 lb. According to an article in Rod & Custom a few years ago, that should be correct for a fiberglass-bodied fender-less car with a small block and auto trans. And it was, initially.

It feels as if it's binding somehow and causing the car to bounce around, but I can't find where, and did not feel so before re-assembly. I've checked and lubed every joint several times, and (if I do say myself) all of the parts dropped into place - nothing was tweaked to get things to line up.

Any thoughts other than binding?

It could be the pre load on one bar has changed. My suggestion is to have some one there to help you do this. If you can get underneath without jacking it up this will be more accurate. If you can't then you need to duplicate the way it sets as close as possible. Take the top left bolt out of the bracket on the housing or on the frame. adjust the length so the bolt slides in and out with you in the drivers seat. Have at least a half tank of gas. The better you duplicate the driving conditions the better this will work. If you have to jack it up make sure you have the same amount of rake you do on the ground. My 2 cents.
Ken

Yep--something is off by a tiny amount---did you by chance mix right and left bars up when you put it back together???

I must have missed the follow-up e-mail on this one or I would have responded sooner. When I put everything back together, all of the parts just dropped into place. In addition, during the fabrication phase, I set it up so that the corresponding bars on each side were the same lengths. That is, both top bars are the same length as are both bottom bars, so swapping them wouldn't have mattered. It's easy when you can set the bar lengths first, bolt them onto the frame tabs and then weld the tabs onto the frame and rear axle. Sort of like shooting a hole and then drawing the bull's eye around it.

That said, I'm going to check the preload as suggested as a possible cause for binding. If that is the case, I'll then have to figure out what changed over time to cause that. Another thing I did during fabrication was what the alignment shops call a "thrust alignment" so that the rear axle is perpendicular (within 1/32") to the frame center line (i.e., pushing the car straight ahead), and then align the front axle parallel to the rear axle. I can still let go of the steering wheel and the car drives straight as an arrow.

Setting the bar lengths equal is a good start, but the load on each bar from side to side with the car sitting on the ground can change things as Ken suggested. I've played around with this on my chassis scales with various cars, great setup tool for the street or any form of racing..... Another consideration would be the springs on the coilovers, perhaps one is weaker then the other. Also, you didn't mention, but do you have the adjustable Carrera's or the non-adjustable valving?????

I have the adjustable Carrera coil-over shocks, probably on the street setting. I recently viewed a Stevie Smith video on setting up a sprint car chassis that demonstrated how many variables there are and how changes can effect the chassis loading. When I initially set this up, I was more concerned with getting everything right, and then because the improvement was so drastic, I never refined the setup. I'm sure that it's now time to do that.

Yup, good video... Part of my collection, too.... The more you get into chassis setup, the more you want to learn about it.....It will make some very dramatic differences in how a car handles and drives.....

I agree, and compared to many other street cars, this one is very simple and adjustments are easy to make. Not that I want to reconstruct the rear suspension again, but it only took a weekend to tear the old arrangement out, fab the new parts, and install the new configuration. It only took one additional day to remove, paint, and reinstall those new parts after driving it for a few months.

By the way, I used to sweat about color matching, etc., but those days are gone. I still try to keep the car looking like I know what I'm doing, but minor details don't nag me as they used to. Until the handling got really bad, I didn't sweat that too much, either - maybe I'm just mellowing as I get older.

Just an update - I verified that there is no binding in the suspension links, so I'm now looking into my coil-overs. I weighed the rear end of the car (using a 2nd class lever and a bathroom scale) which worked out to 1050# with two adults. The QA1 tech support recommended a spring rate of 125#/in., but I bought 140# springs in case my weighing procedure is off. The springs should arrive in a few days. When I get the new springs installed and take it for a test drive, I'll post the results. I had 200# springs, so this should make a serious difference.

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Originally Posted by Dorsey

Just an update - I verified that there is no binding in the suspension links, so I'm now looking into my coil-overs. I weighed the rear end of the car (using a 2nd class lever and a bathroom scale) which worked out to 1050# with two adults. The QA1 tech support recommended a spring rate of 125#/in., but I bought 140# springs in case my weighing procedure is off. The springs should arrive in a few days. When I get the new springs installed and take it for a test drive, I'll post the results. I had 200# springs, so this should make a serious difference.

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Yup, way too much spring for that weight. When you put the new springs on, might want to make sure the shocks are adjusted the same, too.....

The springs arrived today, and 45 minutes later I was on the road. As I'd hoped, it made a big difference and the car doesn't jump all over the road when I hit a bump, which was my main goal.

However, the car now bottoms out on bumps that it didn't before, but I started with the basic unloaded settings so my next move is to adjust the springs so that the shock travel and frame clearance are what they were before with me sitting in the car.

Good deal, Dorsey... I've even done some fine tuning on the springs with a spring spacer rather then cranking the adjuster up too far... Usually not a problem, just something to use if you need it...

Enjoy the cruizin' !!!!!

My first thought was you had to much spring rate. I had the same problem, I was over sprung. Reduced the rate (replaced coil overs too) and it changed everything. I have a 31 Ford Coupe all steel and I run QA1's with 110 lbs and 130 lbs springs. I've tested with both and feel the 130 lbs gives the same ride with a better appearance on the coil over. The 110 lbs requires to much preload. TCI suggested 300 lbs springs, this was back in 94 mind you when I didn't know what I was doing, let me tell ya ouch and bounce, bounce, bounce!

Keith...

My final step was to adjust the shocks to allow 3 inches of compression with me in the car, and it's made a huge improvement in both ride and handling.

In case anyone stumbles upon this thread, here's the process I used for setting up my rear coil-overs (many thanks to others in this forum):

1. Weigh the rear of the car with the normal load (you and a passenger). In my case, this was 1105 lb.

2. Know the spring length. For me, that's ten inches.

3. Measure the shock angle. I mounted them at 20 degrees.

4. Consult a reliable tech sheet, or contact the manufacturer to get the recommended spring rate factoring in the three variables above.

5. Mount the shocks and adjust them so that with the same load as in Step 1, the shock extension is what the manufacturer recommends. In my case, Carerra (now QA1) recommends 3 inches, or 60% of total shock travel.

By the way, if you don't have access to a vehicle scale, you can do as I did and use a bathroom scale with a 2nd class lever.

Sure, here goes. A 2nd class lever has the fulcrum/pivot at one end, the weight in the middle, and the (upward) force at the other end. I'm not good at pictures, so I'll try to paint a word picture, instead.

Using two stiff metal channels (5' fence posts), I placed one end (the fulcrum) of the posts (the lever) one foot ahead of center-line of the rear axle, with the scale supporting the other end, which was five feet away from the fulcrum. That made the math very easy: by multiplying the bathroom scale reading times 5, I had the weight of the rear end. Because most bathroom scales read up to 250 lb., I was pretty sure that I was in range. For heavier cars the same principal applies, just use a lever long enough to stay within range of the scale. For example, if you expect the car to weight 2,000 lb, you can use a 10' lever. Note that you might have to tare the scale to account for the weight of the lever, but in my case, that was negligible (less than one pound at the scale). Oh, yes, by "exactly", I mean within 1/16".

If you do use this technique, make sure that no part of the level touches the ground or it will throw off the readings. Due to bending, I had to raise the fulcrum three inches off the ground.

when weighing a vehicle for springs you need to take away an amount for the unsprung weight of the rear end and suspension

Interesting that that never appeared in any of the research I've done, and the QA1 tech didn't mention it, either.

At least he told you about the mounting angle

I placed a follow-up call to QA1, and was told that their recommendation is based on the actual weight at the wheel, NOT unsprung weight, so that's the weight to use when ordering parts. I'm not arguing that there's no difference, I'm reporting what number to use based on the shock manufacturer's recommendation and what they'll expect when you call.

My main point, however, was to not guess at the weight, but to actually weight your vehicle. For years, I had read estimates for a vehicle like mine ranging from 1250 to 1450 lb. at the rear, which is why I had 200 lb./in. springs to begin with. I learned the hard way that using someone else's numbers is a mistake, and I'm passing that along.

It's in the manufacturer's best interest to ask the right questions which lead to the right solution, and thus happy customers. They don't expect their customers to know sufficient mechanical engineering to divide the required spring rate by the sine of the mounting angle to determine which spring to order, but most of us can compute or measure the angle ourselves.

If someone would like to contribute design criteria based on unsprung weight rather than wheel weight, we'd all like to hear it. It would be especially valuable to provide a formula that derives the necessary spring rate for a given unsprung weight.

No formula or anything, but I weigh the rear end all assembled prior to putting it in the car...Deduct it's weight from the scale weight of the car with the rear end installed... I usually run to the soft side on springs by a few pounds, but I also have ballast in the car when I weigh it so the fuel and fluids, plus driver weight is all dialed in on the equasion.... My reason for going soft on the springs, seems most manufacturers' spring selections are too harsh of a ride and don't leave enough compression for the shocks to do their job correctly.... Like I said, no real formula or math----just a lot of trial and error....

Oh yeah--"your results may vary"--- Not all of us want the same rate springs, seen lots of cars that barely have room for an inch of movement on the tires before contacting something,,,,so they crank up the coilover spring height, and kill the shock action....

I haven looked at one of there catologes for a couple of years but if I remember right---QA 1 had about the best presentation in there about springs and shocks---mounting angle, rates relative to arm length, unsprung weight,etc

of course when I did my first pro stocker, had problems---but when checking the springs themselves they were all over the place ---ended up with a pair of evry type of spring---probably still have 30 or so of them------

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Originally Posted by jerry clayton

I haven looked at one of there catologes for a couple of years but if I remember right---QA 1 had about the best presentation in there about springs and shocks---mounting angle, rates relative to arm length, unsprung weight,etc

of course when I did my first pro stocker, had problems---but when checking the springs themselves they were all over the place ---ended up with a pair of evry type of spring---probably still have 30 or so of them------

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It's still a problem, Jerry.... Being an old circle burner, I still have a coil spring checker around.... I've had 120 pound springs test anywhere from 85 pounds to 150 pounds---- the difference in rate from one manufacturer to another is huge!!!! Tru-coil and AFCO seem to be about the closest to matching the advertised spec for pressure at height.....