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New here, Angled 4 link rear end, datsun 200sx, Help

Shocks, Springs, Brakes, Frame, Body Work, etc

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Mackdan
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New here, Angled 4 link rear end, datsun 200sx, Help

Post by Mackdan »

Wow, I just found this site, while searching for an answer to why cylinder pressure is higher than calculated CR * 14.7 pis (ambient air pressure). Signed up immediately! Thanks to the team for running this.

OK. I have a 1981 Datsun (Nissan) 200sx, that I run in 24 hours of Lemons and Lucky Dog endurance racing series races on the west coast. S110 chassis. The front end is struts and lower control arms (LCAs) with tension rods (TC) from the front radiator support. I have spacers under the struts to lower the LCAs to control camber changes, since the car is lowered a bit via coil overs on the 280zx front struts (slightly bigger, vented rotor upgrade). Adjustable LCAs to move the bottom of the struts outward (camber adjust), and slight modified upper mounts to provide camber (static mount). Stock anti sway bar.

the rear of the car is a solid rear axle with straight front to rear- long lower link arms mounted to the underside of the rear seat floor. The upper links are angled out from the rear differential area, to under the rear seat area. A rear anti sway bar is hung under the rear axle and the ends are tied to the lower links. Coil springs. See link below for a picture.

The car feels like the rear end is angled out in corners, wants to walk out. The roll center on the front is low, near the ground. But the rear roll center is defined by the upper link arms, so its High, at the top of the differential. I think that is a problem. I've been trying to figure out how to fix this, thinking that the change in length of the rear links under cornering is causing the rear axle to twist vs the body center line and causes the outside rear wheel to move back, while the inside moves forward, and that is why the rear end feels like its walking outward.
I've though about adding a panhard rod to move the roll center lower. but I'm looking for ideas. Thanks

http://repairguide.autozone.com/znetrgs ... 099ba8.gif
dannobee
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Re: New here, Angled 4 link rear end, datsun 200sx, Help

Post by dannobee »

First you need to carefully measure your suspension pick up points and graph them out. Full size works best for accuracy. I used to use the shop floor and snap chalk lines on the ground after CAREFULLY measuring from the ground to the middle of the bolts at ride height. Since you're working with a 4-link type rear suspension, you'll need to measure from the side view AND the planar view (from the top looking down) to accurately find the rear roll center. With a 4-link, both the upper and lower control arms dictate the rear roll center. See the link below for how we establish the rear roll center. A higher rear roll center will cause the car to be loose, usually on corner entry and in the middle.

https://www.crawlpedia.com/4_link_suspension.htm

While I don't know what the rules are for your given class, but often in the lower division classes a stock suspension type must be used. Is it within the rules to modify the rear suspension to a 3-link with panard bar? (Keeping the 4-link and adding a panard bar will likely bind up the rear suspension). If the 4-link must be maintained, altering the pick up points is certainly a possibility. And there is nothing inherently wrong with 4-links. GM A and metric chassis cars incorporate them and are very successful in lower division classes. One thing to watch though is to make sure all of the suspension bolts and bushings are in serviceable condition. Solid bushings are the norm on the GM chassis cars. If they're loose, it's likely that the driveshaft will get twisted up like a pretzel or the driver will feel a vibration under accel/decel.

If you're dead set on using a panard bar and aren't inhibited by the rules, a 3-link with panard bar is much easier to tune. But remember, the forces on the rear axle are very high. Make double sure that any new mounts are solid, heavy, and well braced. Make the panard bar mount adjustable on the body side so that tuning the rear suspension is easy. And a longer panard bar often makes the handling more predictable, especially on longer runs.

But before all of that, have you checked the alignment? At the track we would use string to establish a perfectly square box around the car (using thin string and jack stands, all carefully measured for square, then measure from the string to the car). At the shop we were fortunate to have a 4-wheel alignment machine. Have you went through a basic setup to establish front and rear roll centers, front roll couple, and wheel rates (with spring and sway bar rates)? If not, I'd start there. Do you religiously check tire temps?
j-c-c
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Re: New here, Angled 4 link rear end, datsun 200sx, Help

Post by j-c-c »

The angled 4 link is common, cheap, fairly easy to package, and garbage. The 3 link is the ticket if rules allow. I'm wondering if the GM solid bushing comment is for drag application, not cornering? An angled 4 link MUST have built in compliance somewhere or you soon discover snap oversteer because of bind. The panhard only works if the system is overly compliant and then helps settle the rear down, if its tight and you add a stiff Panhard solution, things gets really wierd.

Ask why all the fox body guys ditch them if they can.
dannobee
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Re: New here, Angled 4 link rear end, datsun 200sx, Help

Post by dannobee »

Something solid with spherical bearings, like these.

https://www.speedwaymotors.com/1973-88- ... ,9021.html

Dunno about fox body 4-links, but there are literally thousands of circle track GM cars running 4-link rear suspensions. The problems have long since been worked out. Like I said, the only problem they had was worn/torn/slipped control arm bushings that would cause the driveshaft to bend up like a pretzel.
Protech Racing
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Re: New here, Angled 4 link rear end, datsun 200sx, Help

Post by Protech Racing »

You;ll probably go faster without the rear sway bar. That style does bind up and the dynamic spring rate goes way up as it binds.
I try to use little or no sway bar on the driven wheels for a lowered road racer.
Bailey Groutage
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Re: New here, Angled 4 link rear end, datsun 200sx, Help

Post by Bailey Groutage »

If you're using 280ZX struts and factory or chopped factory springs. It's just going to be way too soft. I'd go on ebay and look for a used pair of good coilovers with 6k to 8k rates. Almost any pair will work, evolution, Sti, corolla (fwd or rear), s13, fr-s.

Just weld the bottoms onto the strut tubes like the fancy ground control kits sell. Redrill the shock towers to suit the new camber plate if necessary.

And, raise the car so that at minimum the control arms are not inverted up front

You can buy a pair of S14 rear toe arms, and convert them into spherical front tension rods for your car. $25-30. Bolt the toe arm bracket end into your tenaion rod bushing hole (use washers etc to get rid of the rubber). Then slide the heim into the new bracket. Then you should be able to cut the original tension rod and slide it into the s14 aftermarket piece (may have to shave it down a mm or 2). Weld it on and done
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