fj40 rear 4 link calculations
- Thread starter dude47
- Start date
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I've only designed and built one 4 link setup, but did a couple months worth of reading on the subject and asked a lot of questions to people who have built them. So keep that in mind.
Is this going to be driven on the street? What's the drivetrain? Are you planning coils, coilovers or air shocks?
The first thing that I see is that your oversteer is really high. The "general" rule of thumb is to shoot for -1 to +1 roll axis angle. But depending on the type of driving/trails you do, maybe you want more or less angle. The vehicle CG height seems pretty high... how did you come up with that number? Usually people use the top of the bellhousing for the CG height. Another "general" idea is to keep the upper links as flat as possible. The anti-squat looks like a good all-around number though.
As you know these frames are narrow and packaging constraints are many for stuffing in a link setup. Have you done any measuring on the actual frame to see how far forward of the rear axle you can put your frame end mounts? Based on your numbers I'm going to guess your frame end rear link mounts are going to be right next to your t-case, and at 32" off the deck your upper mounts are going to be very near the bottom of the tub. Plus there's a fair chance they'll be in the way of your t-case output/driveshaft. Make sure you plan for height adjustability in the frame end upper link mounts. The other 3 can be fixed heights.
38" long lowers and 34" uppers on a 98" wheelbase truck is a lofty goal. For comparison, mine is a 107" wb with 38" lowers and 34.5" uppers. The rear axle is pushed back ~12.5" from stock. My frame end lowers are directly beneath the t-case output, and the uppers are right at where the factory boxed inner frame ends (I boxed in the rear section of frame).
Just remember that when you think you have a good set of numbers from the spreadsheet, those will change once you start positioning brackets on the truck.
Is this going to be driven on the street? What's the drivetrain? Are you planning coils, coilovers or air shocks?
The first thing that I see is that your oversteer is really high. The "general" rule of thumb is to shoot for -1 to +1 roll axis angle. But depending on the type of driving/trails you do, maybe you want more or less angle. The vehicle CG height seems pretty high... how did you come up with that number? Usually people use the top of the bellhousing for the CG height. Another "general" idea is to keep the upper links as flat as possible. The anti-squat looks like a good all-around number though.
As you know these frames are narrow and packaging constraints are many for stuffing in a link setup. Have you done any measuring on the actual frame to see how far forward of the rear axle you can put your frame end mounts? Based on your numbers I'm going to guess your frame end rear link mounts are going to be right next to your t-case, and at 32" off the deck your upper mounts are going to be very near the bottom of the tub. Plus there's a fair chance they'll be in the way of your t-case output/driveshaft. Make sure you plan for height adjustability in the frame end upper link mounts. The other 3 can be fixed heights.
38" long lowers and 34" uppers on a 98" wheelbase truck is a lofty goal. For comparison, mine is a 107" wb with 38" lowers and 34.5" uppers. The rear axle is pushed back ~12.5" from stock. My frame end lowers are directly beneath the t-case output, and the uppers are right at where the factory boxed inner frame ends (I boxed in the rear section of frame).
Just remember that when you think you have a good set of numbers from the spreadsheet, those will change once you start positioning brackets on the truck.
- Thread starter
- #4
It is currently SOA right now so it is sitting higher than stock. I have all brackets clamped in place right now I pushed the axle back 6-8" uppers are as far forward as I could get them, that is why the uppers are so short. As for the angle. I angled them more so get the anti squat down. I have a 5.3L V8 and 700r4 with an Orion on FJ60 axles. Have a set of Coil Overs I am planning on using.
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- Thread starter
- #6
Plying with the calculator and It seems that I can make the lower link flatter but then anti squat will go way down. Right now I have the lower link frame mounts on the side of the frame. It I have them as low as I can go on the side of the frame. If I were to go any lower it would be mounted to the bottom of the frame(I know that is usually how it goes). But that will lower the height of the frame side of the lowers down 5" will will put the AS at -7% and the Oversteer at 4.86. It seems with the single trianglulated 4 link it is hard to get -1 to 1 roll.
- Thread starter
- #7
Think I might be using the calculator wrong. In first boxes on the link specs I was putting actual link lengths rather than as it says when you hover over it (distance FW of rear axle.)
Ok NVM there is very little difference in distance.
Ok NVM there is very little difference in distance.
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- Thread starter
- #8
Ok Mace if I shorten my lowers and put them on the bottom of the frame and I build some beefy brackets off the frame to lower my uppers on the frame side this is what I can get. What do you think? I am thinking it looks like a winner.
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Agreed
