4 link geometry review (calcs done)

[JB5812]

Hey guys, new here and building an F100 for a while now that will be an all-purpose street truck that I'd like capable of street, track, or whatever-- making a versatile build that is a good middle-ground for whatever I may do. I'm building a custom triangulated 4 link and have had to make a few compromises due to the narrow (32" center > center) frame. The big one is in keeping a minimum 30* angle from truck centerline, I'm looking at roughly 12-13" long upper links. I know this is short and I'm trying to determine if this is TOO short, or with the right setup it will be alright. To really explore this, I've input my proposed setup and a few changes into a 4 link calc. The changes I've made, such as lower link length, have been based on keeping the shown pinion angle change to a minimum. The rest of the shown info I'm not 100% sure on which is why I'm bringing this more technical info here for some help! So any feedback and what I should be adjusting to see is much appreciated.

The first scenario is both links parallel from section view with link lengths adjusted until the pinion angle change is minimized as much as possible. To determine this, I've set the travel to 1.5", which with coilovers all around and a pretty good spring rate, I don't expect to see much travel. Here are the results:

For reference, X axis is the frame (0 is axle), Y is the axle (0 is the truck centerline), and Z is height from ground.

Scenario 1, links parallel (section, side view), lengths set to keep pinion change 0. The does result in shorter lower links though. Not sure if that's an issue

Scenario 1, 1.5" travel

Scenario 2, top link angled down from axle to frame, bottom angled slightly up from axle to frame, no travel

Scenario 2, 1.5" travel. Pinion change is present, and without shortening the lowers significantly, it sits as shown.

These were just a starting point. The uppers are unfortunately pretty set because of the narrow frame and having to keep the 30*, but I can definitely change up the slope on them. The front upper link bracket I have have 3 holes on them. As it sits, the top one would be used at parallel. Again, any help is appreciated and there are other tabs on this calc that show other data. If anyone feels like seeing something or has some suggestions, I would love to input them and see what happens. Thanks in advance!

[RobNoLimit]

Version #1, with parallel bars, will have NO weight transfer, and there fore, very limited traction. Version #2 shows an actual IC that you can work with. If you are building from scratch, why are you choosing a compromised system to begin with? (I'm not picking, but trying to help) The Big-3 automakers used these types of suspensions, and so everybody thinks they are great. Wrong. It was about $$$. Tri-4 bars meant no locator was needed. So, they could save 2 brackets, 2 bushings, 1 bar and some bolts, on every car. They used big soft rubber bushings to minimize the binding, (and 'easy to twist' formed bars) and besides, these were 300 HP, G-70-14 bias ply tire wearing, around town cruisers. NOT real performers. - even the worst corvettes never had a tri-4 bar. When it comes to suspension performance and ride quality, specialization is the key. How about a 4-bar (link) or offset 3-link designed to control axle rotational forces (and do something with them) and a lateral locator (Panhard or Watts) to control side/side motion (only). p/u's have terrible weight balance to start with, so if wou want it to work, you don't want to 'shake' the tire (shifting ft/rr or s/s due to short bar length) think about 24" to 30" lengths. You will want an A/S around 80 to 120%. Try to keep the lower bar level or running slightly uphill (to the chassis mount) at RH. Not sure the year or body of your F100, but there is room to mount bars and coil-over outboard and run a 12 tire in most. The wider suspension base keeps the trucks more stable as well. Take a look at our FatBar 4 links www.nolimit.net. Hope this helps.

[JB5812]

Version #1, with parallel bars, will have NO weight transfer, and there fore, very limited traction. Version #2 shows an actual IC that you can work with. If you are building from scratch, why are you choosing a compromised system to begin with? (I'm not picking, but trying to help) The Big-3 automakers used these types of suspensions, and so everybody thinks they are great. Wrong. It was about $$$. Tri-4 bars meant no locator was needed. So, they could save 2 brackets, 2 bushings, 1 bar and some bolts, on every car. They used big soft rubber bushings to minimize the binding, (and 'easy to twist' formed bars) and besides, these were 300 HP, G-70-14 bias ply tire wearing, around town cruisers. NOT real performers. - even the worst corvettes never had a tri-4 bar. When it comes to suspension performance and ride quality, specialization is the key. How about a 4-bar (link) or offset 3-link designed to control axle rotational forces (and do something with them) and a lateral locator (Panhard or Watts) to control side/side motion (only). p/u's have terrible weight balance to start with, so if wou want it to work, you don't want to 'shake' the tire (shifting ft/rr or s/s due to short bar length) think about 24" to 30" lengths. You will want an A/S around 80 to 120%. Try to keep the lower bar level or running slightly uphill (to the chassis mount) at RH. Not sure the year or body of your F100, but there is room to mount bars and coil-over outboard and run a 12 tire in most. The wider suspension base keeps the trucks more stable as well. Take a look at our FatBar 4 links www.nolimit.net. Hope this helps.

I adjusted the calc and have quite a few setups with the IC all over the middle of the truck to firewall area. Those were just a couple on each end of the spectrum as a starting point. A 3-5 degree upper link angle (down towards frame) and parallel lower resulted in less than .5* differential change over 3.5" bump or droop (more than I'd ever have) and a bit under 3/8" X axis shift in length from the shortened arc, which for pretty short links seems decent enough. Well to be honest, I didn't really think of it as a compromised setup at all. Granted, I have less room than ideal for the tri 4 link, but just working with what I have to make that type of 4 link work within the F100 frame (1970 BTW). Coilovers and lower link indeed going on outside of frame. I've looked extensively at all the 4 link kits out there. Absolutely nothing against them and have seen quite a few of the Fatbars being installed recently. I have nice heims for all 4 rear points and bushings up front. Not too worried about binding as I definitely did not want one of the many generic $300 tri 4 link kits that come with generic bushings all around and not much attention to link lengths or location. I also don't anticipate nearly as much travel as a factory car would see. Other than the shorter length, I'm still not quite sure what would make it compromised if there is no bind issue and the geometry is within acceptable limits. I get the bump steer and other issues, just questioning if I'd experience this with minimal travel. I'm not a chassis expert by any means and actually have much more experience with off road rigs and beefier fabrication than this, but in general have always been a fan of tri 4 links. I'm still only into everything like $400 including heims and a few long sticks of 1 3/8" DOM I'll use regardless, so I'm not married to anything. Haven't tacked a single thing, so if it doesn't work, not the end of the world!

Long story short, this is why I've asked though. I appreciate the input from others that know this stuff and am still sorting out what I actually want (and can do) for the rear setup.

So in your opinion, even with rear heims on links, a good starting ride height geometry with IC focused near center of gravity, and some anti-squat (got around 64%), this would still not perform well enough to justify going this route? Mainly due to the shorter links?

Thanks for the help!

[SSLance]

JB... First off welcome to the board. Second, when you get a chance make a signature line on your profile with your real name in it...it helps us long timers get to know you quicker.

Rob is a master at this, take his words seriously. He's probably already tried anything you can dream up with truck suspension geometry and knows what works and what doesn't.

What kind of HP will this truck have, manual or automatic, overall finished weight, and size and type of tires once complete? I think knowing these variables will help dial in Rob's suggestions with your finished chassis.

I have and am stuck with a tri 4 link in my car and have thought long and hard about alternatives over the years. Yes, there are better ways to do it especially if starting from scratch, but something to think about is those "better ways" also come with setup and tuning variables that can be a bit more daunting than a simple tri 4 link setup. Several aftermarket companies have designed, built, and sold many very stout performing tri 4 link setups for other platforms so i tcan be done...like Rob said though...just make sure you are getting the geometry you want the first go around.

[JB5812]

JB... First off welcome to the board. Second, when you get a chance make a signature line on your profile with your real name in it...it helps us long timers get to know you quicker.

Rob is a master at this, take his words seriously. He's probably already tried anything you can dream up with truck suspension geometry and knows what works and what doesn't.

What kind of HP will this truck have, manual or automatic, overall finished weight, and size and type of tires once complete? I think knowing these variables will help dial in Rob's suggestions with your finished chassis.

I have and am stuck with a tri 4 link in my car and have thought long and hard about alternatives over the years. Yes, there are better ways to do it especially if starting from scratch, but something to think about is those "better ways" also come with setup and tuning variables that can be a bit more daunting than a simple tri 4 link setup. Several aftermarket companies have designed, built, and sold many very stout performing tri 4 link setups for other platforms so i tcan be done...like Rob said though...just make sure you are getting the geometry you want the first go around.

Thanks Lance.

That's the second time I've heard that about Rob today actually. I definitely value the opinion of many of these guys that have a much better idea of whats going on than I ever will and really, will ever NEED to know. Not trying to reinvent the wheel here, so in the end, I'll try to do the practical and overall best thing since I am indeed building from scratch.

The truck will be sitting at around 700hp (custom Dart SBF), manual tranny (T56 Magnum), not sure on weight but a bit lighter than factory.. maybe 3200 or so. I have 275/40-18's now and will probably run those for a while. I'll sit out at the truck and think it over and play with some layouts. A 3 link wouldn't be much more work for me to do and I'm not limited on space due to a fuel tank or anything else. Here is the truck as it sits. Thanks for the reply. It's been fun looking through builds and different suspension setups.

[TheJDMan]

Welcome to PT Josh,
My brother lives in West Lynn not sure how close that is to you. There are a lot of talented car builders here as well as those of us still learning. Looks like you have a cool project I hope you consider starting a build thread in the project section.

[JB5812]

Welcome to PT Josh,
My brother lives in West Lynn not sure how close that is to you. There are a lot of talented car builders here as well as those of us still learning. Looks like you have a cool project I hope you consider starting a build thread in the project section.

West Lynn is very close. About 20 minutes down the 205 from me. Nice area. Yeah its been great looking through all of the sections. I have been on a few forums and groups, but most are so generic that it is harder to find performance specific info and setups. Hoping I can tap the resources here as I go through setting things up. Thanks!