True 3-link rear suspension

[mudstain1]

I didn't hit many events after the PM3L just due to other life events conflicting with autox schedule, but it was a significant improvement over the stock 4-link. The car handled better and was able to power out of the sweepers a lot better than before. My only real complaint about the PM3L is the added NVH due to the spherical joints in the upper link.
I do have a finished garage now with heat/air, but with talks of a baby in the near future I'm not sure how feasible this project is. I'll likely end up with a MM torque arm like many others here.
My tires are the weakest link at this point, 295 Federal RS-RR's, especially since the procharger addition. They were my track wheel/tire setup but with the current hp they're barely sufficient for street driving, so hopefully new wheels and either Re71's or A052's will be in this cars future.

Did you center the upper over the diff or did you offset to account for the torque steer they say happens? Do you happen to have any pics of the installed unit?

My brother just got a set of Potenze RE050A's and loves them, 295's on his c4. Said they are night and day over the same size kumhos.

 

[Fake_Snake]

Did you center the upper over the diff or did you offset to account for the torque steer they say happens? Do you happen to have any pics of the installed unit?

My brother just got a set of Potenze RE050A's and loves them, 295's on his c4. Said they are night and day over the same size kumhos.

The current setup is simply an upper spherical axle bushing with a double adjustable upper control arm mounted in the stock location on the passenger side. Here are some pics of my car on a long sweeper at the last event. Keep in mind I'm still on stock struts/shocks too, only H&R race springs. finished 4th out of 12 in Cam-T

 

[RinerAuto]

The current setup is simply an upper spherical axle bushing with a double adjustable upper control arm mounted in the stock location on the passenger side. Here are some pics of my car on a long sweeper at the last event. Keep in mind I'm still on stock struts/shocks too, only H&R race springs. finished 4th out of 12 in Cam-T

PM3L you should use the MM torque arm springs. you are getting a ton of body roll. The H&R super race front springs and/or larger sway bars may help as well. Ideally you want all 4 tires to stay on the ground and doing work.

 

[blacksheep-1]

PM3L you should use the MM torque arm springs. you are getting a ton of body roll. The H&R super race front springs and/or larger sway bars may help as well. Ideally you want all 4 tires to stay on the ground and doing work.

not always the case, without body roll the car can't handle., even on the GT4 stuff the inside tire is barely touching the pavement during hard cornering and they are IRS cars

 

[mustanghammer]

This car has a three link that goes into the interior of the car and attaches to the cage. All three links were the same length. My bracket for the third link was different than the ones I am seeing in this thread. Basically it was like an upside down Griggs torque arm mount in that it used rear end cover bolts plus the ears by the pinion. I didn't use the OE upper control arm mounts.

I tamed front end lift by installing the biggest OE rear sway bar I could find. Before that the car would lift the front end so high I couldn't see where I was going on concrete. BTW this was an evolution I saw in the CP class at Solo II Nationals. In the early 90's when the rules changed to allow 12" wide wheels extreme front end lift became an issue for all of the fast guys. Over the years they all figured out how to get this under control.

My RX7 road race car also has a three link and it three wheels on corner exit too.

 

[Fake_Snake]

This car has a three link that goes into the interior of the car and attaches to the cage. All three links were the same length. My bracket for the third link was different than the ones I am seeing in this thread. Basically it was like an upside down Griggs torque arm mount in that it used rear end cover bolts plus the ears by the pinion. I didn't use the OE upper control arm mounts.

I tamed front end lift by installing the biggest OE rear sway bar I could find. Before that the car would lift the front end so high I couldn't see where I was going on concrete. BTW this was an evolution I saw in the CP class at Solo II Nationals. In the early 90's when the rules changed to allow 12" wide wheels extreme front end lift became an issue for all of the fast guys. Over the years they all figured out how to get this under control.

You have sparked my interest again... do you have any pictures you can dig up of this setup???

 

[mustanghammer]

No unfortunately this was back before digital cameras and smart phones were not a thing so I didn't snap allot of photos.

I whipped up a paint drawing that shows a side view of how the mount looked. The grey parts are the mount. I omitted the upper control arm ears from the drawing. The mount fit between them. The top plate was 5/16" steel with 1/8" angle welded at each end. I used 1/2" bolts through thick wall tubes to attach to the flanges that run along side the pinion in the casting. The tubes are attached to the bracket using 1/8" strap that was welded to both sides of each tube. The rear of the plate used the top 4-5 cover bolts. To connect the third link to the plate I used a 1/2" shock mount.

Originally I had a bracket on a 7.5 rear end in the car but I kept breaking parts. So I went with an 8.8 with a Detroit Locker. Never had any issues with the mount coming loose or cracking. It was very sturdy.


Here is how the car looked like on the inside. I don't have any with the cover off.



Below are some photos of how I built the interior mount on my RX7. The mustang set up was similar except for the mustang it had a removable lid so I could adjust the arm from above. This wasn't as handy as it sounds so I built the RX7 to be serviceable from the bottom. The cover is big in both installs so that there is plenty of room for tools. The mount is fully connected to the cage. in both installs. This is an extremely critical thing to do.

 

[johnjamesmiller]

Hoping to pick up where Fake_Snake left off. I like how his axel side mount hung further back so you can get a slightly longer 3rd link arm.

I think I can get a 11 or 12 inch arm vs the stock 9.25 inch. I wonder if Speedway 5/8 Inch Chromoly Swedged Tube will be strong enough?

Does anybody have guestimates on the amount of force and directions of force? From what I remember 3rd link primarily wants to pull out of the body on acceleration and push into the body on braking? I assume MM panhard bar will take all of the side to side load.

For the body side mount I am going to try to tie into the two original mounts similar to evolution motor sports picture posted by black sheep.

 

[blacksheep-1]

Be forewarned, not all sanctioning bodies will allow you to pierce the floor pan and intrude into the driver's compartment.

 

[Dave_W]

Does anybody have guestimates on the amount of force and directions of force?

As a simplification, to accelerate at 1g in a RWD car, the sum of force the rear tires are transferring forward through all the suspension links to the chassis & body is the weight of the car. If the suspension connection of the links is rearward of the chassis connection, the links are in compression on acceleration. To calculate the force on each link, you have to determine the geometry and do vector math, and it's been too long since I've done that to remember.

F=ma
Force = mass * acceleration
Pounds [force] = slugs [mass] * g [acceleration]
Pounds [weight] = slugs * 1g downward [Earth's gravity]

You should figure a 2x to 4x safety factor into mechanical designs. Also consider your tolerance for cascading "domino effect" failures if one link fails, i.e., do you want the remaining links to be immediately overloaded and also fail, or would you like them to (try to) hold together.

 

[racer47]

5/8 Inch Chromoly Swedged Tube

Virtually all full size, purpose built race cars use 3/4" rod ends for trailing arms, 3rd links, panhards, etc. That's what I would use.

 

[navseng]

Virtually all full size, purpose built race cars use 3/4" rod ends for trailing arms, 3rd links, panhards, etc. That's what I would use.

This , I am using 3/4 rod ends with 1/2 bolts on my 3 link. You might be able to get away with 5/8 x 1/2 but I tend to be a little hard on things so I try and overbuild a bit.

Surprised this thread is still going , seems like not many on here approve of anything that isn't factory components lol

 

[racer47]

Surprised this thread is still going , seems like not many on here approve of anything that isn't factory components lol

Just a vocal minority. Most are open for improvements as needed. Not one piece of suspension is stock on my car.

 

[mavisky]

This , I am using 3/4 rod ends with 1/2 bolts on my 3 link. You might be able to get away with 5/8 x 1/2 but I tend to be a little hard on things so I try and overbuild a bit.

Surprised this thread is still going , seems like not many on here approve of anything that isn't factory components lol

I always laugh at people who believe that just because an engineer designed it, that there weren't factors other than maximum performance that went into that design at an OEM.

 

[blacksheep-1]

This , I am using 3/4 rod ends with 1/2 bolts on my 3 link. You might be able to get away with 5/8 x 1/2 but I tend to be a little hard on things so I try and overbuild a bit.

Surprised this thread is still going , seems like not many on here approve of anything that isn't factory components lol

I think that is because most of us race in production classes where you are limited in what you can do. Once you start designing your own chassis you kind of go off the reservation. I designed and built a kart chassis. I hade about 4 builds before I was happy with it. I used to add tubing, brackets, etc for testing and brought along a hacksaw so I could cut them out during the test.
The other item is once you enter an open chassis class, there's no ceiling. The cover of HOT Rod this month has a road racing style firebird on the cover. The chassis is basically an old Renard Indy car. This is why most of us stay in production level classes, it's cheaper.

 

[navseng]

I think that is because most of us race in production classes where you are limited in what you can do. Once you start designing your own chassis you kind of go off the reservation. I designed and built a kart chassis. I hade about 4 builds before I was happy with it. I used to add tubing, brackets, etc for testing and brought along a hacksaw so I could cut them out during the test.
The other item is once you enter an open chassis class, there's no ceiling. The cover of HOT Rod this month has a road racing style firebird on the cover. The chassis is basically an old Renard Indy car. This is why most of us stay in production level classes, it's cheaper.

I understand the logic , didn't mean to take a jab. Or you can be dumb like me and build a 60s car with no class and try and make it fit in .. haha

 

[blacksheep-1]

I understand the logic , didn't mean to take a jab. Or you can be dumb like me and build a 60s car with no class and try and make it fit in .. haha

Nah, you're good, it's that once you go down that rabbit hole where do you stop?

 

[mustanghammer]

Hoping to pick up where Fake_Snake left off. I like how his axel side mount hung further back so you can get a slightly longer 3rd link arm.

I think I can get a 11 or 12 inch arm vs the stock 9.25 inch. I wonder if Speedway 5/8 Inch Chromoly Swedged Tube will be strong enough?

Does anybody have guestimates on the amount of force and directions of force? From what I remember 3rd link primarily wants to pull out of the body on acceleration and push into the body on braking? I assume MM panhard bar will take all of the side to side load.

For the body side mount I am going to try to tie into the two original mounts similar to evolution motor sports picture posted by black sheep.

I am running an 18" 5/8" swedged steel third link on my RX7 that I bought from Coleman Racing. It has survived a failure of the third link axle mount on a down shift and driveshaft failure. Both of these events were at 100MPH and fairly exciting but the swedged tube was fine. From experience with the Mustang and now the RX7, the critical areas are the axle and the chassis mounts. The off the shelf circle track stuff is pretty reliable.

 

[blacksheep-1]

This , I am using 3/4 rod ends with 1/2 bolts on my 3 link. You might be able to get away with 5/8 x 1/2 but I tend to be a little hard on things so I try and overbuild a bit.

Surprised this thread is still going , seems like not many on here approve of anything that isn't factory components lol

Well the factory 4 link really sucked so....

 

[blacksheep-1]

I am running an 18" 5/8" swedged steel third link on my RX7 that I bought from Coleman Racing. It has survived a failure of the third link axle mount on a down shift and driveshaft failure. Both of these events were at 100MPH and fairly exciting but the swedged tube was fine. From experience with the Mustang and now the RX7, the critical areas are the axle and the chassis mounts. The off the shelf circle track stuff is pretty reliable.

Speedway Engineering is your friend