Truck 3-link geometry review

[Big_KID]

I have read quite a bit on the topic so understand the theory of IC placement but don't have the practical application experience to know what would actually work the best for what I am trying to achieve. Currently the truck has a non-adjustable wishbone system that I am wanting to convert to an adjustable offset 3-link with panhard bar. The main goal is better acceleration traction, mostly from standstill/low speeds/corner exit. Secondary goals are less rear steer to carry speed thru sweeping curves and slaloms, along with less rear squirm on hard braking. My understanding from research is the instant center needs to be moved back closer to the center of gravity and lower.
Below is a model based on measurements made of the existing links (solid cyan) and proposed mounting locations (dotted cyan) and their corresponding IC base on position (dotted red). The IC's plotted below are the ones I think would be the most beneficial to my goal. Any of the suspension gurus want to provide feedback?
216359

Some pertinent info on the truck that could be helpful...
~700whp/tq via a positive displacement SC so lots of low end torque.
Truck weight 4440 without me (290), scaled 61/39 F/R, 49/51 cross bias with leafsprings. Has not been corner scaled with new front and rear suspension.
200tw tires (currently 305/35/20 Goodyear SC3)

[dontlifttoshift]

First, I really like the SC3 but they are the absolute worst for putting power down. Even after a tire change, with that kind of torque, you will never "hook up," all you can do is manage it.

A lot of anti squat is going to unload the rear tires under hard braking. It's a tradeoff but I don't think you are going to like the 80% A/S. Since you brought up roll steer, I would start there and make sure you are happy with those numbers first. Then use the upper arm to determine your instant center. You will have to test to get the highest AS number without wrecking the braking.

Why is this thing so heavy and can you move any of it to the rear?

I don't think you can get the roll center too low in the back, but I would certainly try. This is going to be the lowest hanging fruit in the whole project, I think. If it is the typical mini truck wishbone 3 link that is on there now, (gotta pic?) none of that is engineered for going fast and the RC is way too high for what you are trying to do and that is likely the bulk of your issues.

[Big_KID]

First, I really like the SC3 but they are the absolute worst for putting power down. Even after a tire change, with that kind of torque, you will never "hook up," all you can do is manage it.

They are not good cold but once they have some temp, they are lightyears better than the NT05 I had previously. I just found out Falken released the 660 in my size so may be going to that soon.


A lot of anti squat is going to unload the rear tires under hard braking. It's a tradeoff but I don't think you are going to like the 80% A/S. Since you brought up roll steer, I would start there and make sure you are happy with those numbers first. Then use the upper arm to determine your instant center. You will have to test to get the highest AS number without wrecking the braking.

That is why I was thinking trying the -2,-2 setting first as that drops the AS% down a bit from where it is now but adds some load to the tire with the IC being just forward of the CG. With that, the lower bar would still be at a slight incline providing a bit of rear steer for tight autox courses but may work better on long fast courses.


Why is this thing so heavy and can you move any of it to the rear?
Yea, she's a big girl and not the best platform for this sort of thing but I enjoy the challenge. It is full street trim with full interior, stereo, ect. I did recently replaced the stock buckets with some Sparcos but likely added the weight right back with the harness bar. Nearly 200# of nose weight is added with the supercharger and associated cooling added on top of an iron block. I have aspirations of going to a big cube aluminum engine but what I have works good and I love the way it puts down power (if I can get it to the ground). Immediate plans are to ditch the stock fuel tank along the driver side frame for a fuel cell in teh spare tire location. Also move the battery from right front back to the right rear hanger location. With those two moves, I calculate a 56/43 F/R, 49/51 cross balance.


I don't think you can get the roll center too low in the back, but I would certainly try. This is going to be the lowest hanging fruit in the whole project, I think. If it is the typical mini truck wishbone 3 link that is on there now, (gotta pic?) none of that is engineered for going fast and the RC is way too high for what you are trying to do and that is likely the bulk of your issues.

This is one of the other main contributors for ditching the wishbone. The Panhard will both have one job of locating the axle, thus adding strength, and lowering the roll center significantly as I understand the wishbone's RC is at the front mount which is at the top of the frame. I am hoping this will have a significant impact on the handling. As I mention previously, it is a street truck so I dont want anything hanging too low. I was anticipating designing the mounts for adjustability from just above axle centerline to bottom of pumpkin. The current setup is Ridetech's wishbone for the GMT800 trucks with some slight modifications. From my experience, I feel the their focus for the late model truck kits are for better handling street trucks that package well. For that purpose, I think the kit is well designed/built, and is what 98% of the kits sold are going to do. Not many guys pushing GMT800s hard so not many off the shelf suspension options available.

https://static1.pt-content.com/images/pt/2025/02/80img_20200802_153138261_hdr_a317f8786c2-1.jpg

[Big_KID]

I did quite a bit of reading over the weekend, both in books and the internet. One takeaway is it seems to be a common thought that high anti-squat percentage contributes to rear brake issues where one publication states it actually the swing arm length. It is possible to have a long swing arm with high AS% by placing the IC high and forward, similar to what I have currently. Could the existing rear squirm under hard braking be more a result of the high rear roll center and/or rear steer? I don't get brake hop, its not violent, just the rear wants to come around with any steering input. I have not plotted it, but with the angle of the front control arms, I presume my front roll center is very low, thus creating an inclining roll axis front to rear. I can see how this may want to pivot about the front upon hard braking.

[dontlifttoshift]

.....now reconcile your high I/C and long swing arm length with the rear roll steer.


Don't forget, it is entirely possible your braking squirm is just a function of too much rear brake bias. I would address that after your suspension changes though.

[Big_KID]

Don't forget, it is entirely possible your braking squirm is just a function of too much rear brake bias. I would address that after your suspension changes though.

Interesting you mention that. Brakes have been something I have been sorting out recently. Current brakes (Wilwood Aero6 w/BP20 front / OEM single pistons w/DFC Ultimate rear) should be more front biased than the previous setup (05+ truck fronts w/Hawk LTS front / GMT400 front calipers w/partstore metallics rear). Rears were done after last event of the year so no track testing on them yet. Luckily I have ABS to tell me if there is lockup but unfortunately the rear is a single channel so one could lockup and the driveshaft would still spin.

Fabrication on the 3-link starts this weekend. I am going to focus on the 3rd link and panhard first to test the lowered roll center independently of any rear steer changes. Upper rear mount will stay at same location as current due to bed clearance. I don't see any reason for lowering that mount. Front mount will have adjustability from current height to 3' lower as depicted in the model. Thoughts are to initially set it up 1" lower, moving IC closer to CG and adding a bit of AS. I'm hoping that will at least help forward bite for standing starts and corner exit.

[Bthomas]

If you can fab it yourself, a decoupled torque arm design is the way to go. lets you have high antisquat numbers, and good braking at the same time.

[dontlifttoshift]

The only people I know that have used that somewhat successfully are the 3rd/4th gen camaro guys. They don't have room for a 3rd upper link so torque arm it is.

Do you have one?

[Bthomas]

Yes, but admittedly its in a 4th gen.

I have owned several gmt400 and 800 trucks, but the most extreme of any of them had a parallel 4link.
I agree on the rear bias, being a truck it probably already has too much rear brake

[Big_KID]

I have read up on the decoupled torque arms and I know of a few trucks running them very successfully, but I'm not entertaining one at this point in time. However, my pending design could transition to a decoupled 3-link with a pull-bar 3rd link. I am just hesitant to go that route due to unknown elastomer life considering the application. I don't want to have to buy and change new pucks every 6 months.

[Big_KID]

To follow up on this topic, I got the 3-link conversion done just before going to LSFestTX a few weeks ago along with a new set of Falken 660+ tires. The truck seemed to turn in better (more tires than suspension?) but the big improvement was in corner exit. It will take more power coming out of a turn so I can get on the throttle sooner and harder, and also seems to hold sweepers better at speed. Straight line acceleration from a stop didn't seem much different when testing with the old Goodyears considering the IC was moved quite a bit rearward and AS raised to 99%. This is where the 660+ improved instantly, even more once scrubbed in. I still felt some rear squirm on braking but didn't seem as bad. I can't wait to get more seat time to gain confidence in the new setup then start making adjustments to see what changes.

Model of new geometry...
https://static1.pt-content.com/images/pt/2025/06/50014145868920768933-1.jpg

And a few pics of the setup...
https://static1.pt-content.com/images/pt/2025/06/21617889941828120353-1.jpg

https://static1.pt-content.com/images/pt/2025/06/80449836526946869260-1.jpg

https://static1.pt-content.com/images/pt/2025/06/90934055050307994316-1.jpg

https://static1.pt-content.com/images/pt/2025/06/49637583597999632832-1.jpg

New vs old tires...
https://static1.pt-content.com/images/pt/2025/06/33967064653949589444-1.jpg

New tires installed...
https://static1.pt-content.com/images/pt/2025/06/56216313414942119386-1.jpg

[dontlifttoshift]

Nice job!

[Big_KID]

Thanks. I learned a lot thru the process and would do some things differently next time but pretty pleased with how it turned out.