LuigiAndretti
LuigiAndretti New Reader
5/4/22 11:25 p.m.

I need the hive’s help designing a rear suspension for my current project. In a nutshell, I want to take a classic American muscle car W2W endurance racing.  The exact model is unimportant; all that matters for the sake of this discussion is that it left the factory with leaf springs in the rear, and they aren’t gonna cut it in Champ/LuckyDog/AER/etc.

I’m trying to decide between a three link setup and a torque arm setup.  The three link has the advantage of being essentially infinitely adjustable, as in I’m not afraid to cut up the floor to put the upper link and mounts wherever they need to be to optimize the setup.  The downside to the three link is that the chassis side of the upper mount and its associated bracing will take up a fair bit of real estate in the back seat area, and I was really hoping to mount the fuel cell there. 

That’s not a problem with a torque arm setup.  The entire suspension would be under the chassis, leaving the back seat area free to mount the cell. However, I’ve read that torque arms tend to be limited in antisquat due to their low instant center.  I briefly tried to calculate antisquat for my hypothetical torque arm, then discovered that you need to know the car’s center of gravity to complete that calculation.  So, does anyone have a reasonable guess about where CG would be, assuming that the engine is an aluminum LS moved back about 18” from stock?  Yeah, me either.

Either setup will be run with a Watt’s link for lateral locating of the suspension, unless someone has a compelling argument in favor of a Panhard bar.

So, what say you? Is there a distinct advantage to one style or the other?  Disadvantages?  What considerations have I failed to consider?

dean1484
dean1484 MegaDork
5/5/22 8:10 a.m.

I am not much help with suspension other than there are kits out there for most popular cars that bolt/weld in that all the engineering has been done for you. No this is not the cheep way but you will have a roller in a weekend.  

 

Fuel tank/cell in the back seat area is not something I would not do. 

frenchyd
frenchyd MegaDork
5/5/22 10:05 a.m.

In reply to LuigiAndretti :

Fuel cell has to be behind the trunk firewall.  Can't be in passenger area.  Since most newer Corvettes use a torque arm suspension that's your math solved for you. 
One point.  Depending on what track you run on you'll find an advantage to having things adjustable. Long radius corners like one setting. Tight radius need another.   Figuring out which corners make the biggest change in lap times is the result of experiance. The pro teams in NASCAR  have different cars for different tracks. While you're not building truck arm suspensions. The principle applies.  

ian sane
ian sane Dork
5/5/22 10:14 a.m.
frenchyd said:

In reply to LuigiAndretti :

Fuel cell has to be behind the trunk firewall.  Can't be in passenger area.  Since most newer Corvettes use a torque arm suspension that's your math solved for you. 
One point.  Depending on what track you run on you'll find an advantage to having things adjustable. Long radius corners like one setting. Tight radius need another.   Figuring out which corners make the biggest change in lap times is the result of experiance. The pro teams in NASCAR  have different cars for different tracks. While you're not building truck arm suspensions. The principle applies.  

When you say newer, do you mean c4? Because that's the only one I can think of that uses anything resembling a torque arm. Everything newer runs the transmission in the back.

 

 

How classic are you thinking? 3rd/4thgen camEro/firebirds use a torque arm rear, stock. I've seen some bandaids with a decoupled version. I think from Unbalanced Engineering? If they're still around.

dps214
dps214 Dork
5/5/22 10:39 a.m.
ian sane said:

When you say newer, do you mean c4? Because that's the only one I can think of that uses anything resembling a torque arm. Everything newer runs the transmission in the back.

Everything since the C2 had some form of independent rear suspension so none of them seem at all relevant here.

LuigiAndretti
LuigiAndretti New Reader
5/5/22 10:45 a.m.
frenchyd said:

Fuel cell has to be behind the trunk firewall. Can't be in passenger area.

Sorry, should have been clearer.  If the fuel cell is in the backseat there will be a bulkhead/firewall fully enclosing it and the remote fill tubing, starting right behind the main hoop.

Driven5
Driven5 UberDork
5/5/22 10:51 a.m.

I think frenchyd is confusing torque-arm and torque-tube. Two very different things.

Both the overall geometry and useable range of adjustability in your 3-link idea may be more limited than you realize if you're also planning on reusing the forward leaf mounts for your non-adjustable lower arm mounting points. Make sure you calculate your anti's throughout your range of motion for this design, as you're not only worried about what the static anti-squat is, and even what it does under acceleration, but also what it does when turning into anti-lift while on the brakes. Brake hop is something you'll want to avoid. This could be great, but will be a lot more time and effort to design and build.

I am not aware of the anti-squat from a typical torque arm being considered ill suited to road racing. I'd consider this the 'easy button' solution.

A good starting point for center of gravity is probably around 20 inches, maybe even lower at race height. Then move it up and down from there by an inch or two and see if the sensitivity of each design is even to worry about getting a more exact measurement.

Consider that every track will have 360 degrees more turns one direction than the other, so the asymmetric characteristics of a Panhard may not always be undesirable either.

LuigiAndretti
LuigiAndretti New Reader
5/5/22 11:00 a.m.
ian sane said:

When you say newer, do you mean c4? Because that's the only one I can think of that uses anything resembling a torque arm. Everything newer runs the transmission in the back.

How classic are you thinking? 3rd/4thgen camEro/firebirds use a torque arm rear, stock. I've seen some bandaids with a decoupled version. I think from Unbalanced Engineering? If they're still around.

C5/C6/C7 use a torque tube setup, which is theoretically and mathematically similar to a torque arm.  None of those setups will work in my case because of differences in wheelbase and overall driveline length, as well as rules restrictions.

How classic?  1968 - 1972 era.  3rd and 4th gen camarobirds did use a torque arm, which had all the strutural integrity of a wet noodle.  There are a number of aftermarket solutions for them, and adapting one to my application would probably be easier than fabbing from scratch.  However, I don't know whether they are correct length for my car. I've seen the UE decoupled torque arm; it's an interesting concept but I'd be worried about how it would react when you're on and off the throttle through a corner. 

 

MattGent
MattGent HalfDork
5/5/22 11:00 a.m.

I'd think you could get a fairly reasonable estimate of Cg from a few measurements and some FBD calcs. Even with the motor mods. 

Driven5
Driven5 UberDork
5/5/22 11:12 a.m.
LuigiAndretti said:

C5/C6/C7 use a torque tube setup, which is theoretically and mathematically similar to a torque arm.

Torque tube is neither theoretically nor methematically similar to a torque arm. The torque tube is purely an IRS design, with all of the theory and math that entails. The torque arm is a live axle design, from which the theory and math is all derived.

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
5/5/22 11:15 a.m.
MattGent said:

I'd think you could get a fairly reasonable estimate of Cg from a few measurements and some FBD calcs. Even with the motor mods. 

Yes, you can calculate it exactly on any car if you have scales. Read once flat and then read again with the front or rear raised up about a foot. 

The formulas are detailed in a book called "fast car physics" but they are probably available in other places too. I have a spreadsheet setup to just enter in the corner weights and the dimensions and it spits out the answers. 

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
5/5/22 11:18 a.m.

for live axle, I thought that not much beat the simplicity and performance of long 'truck arms', similar to what is found on 70s-80s chevy trucks (and NASCAR). 

Gearheadotaku (Forum Supporter)
Gearheadotaku (Forum Supporter) UltimaDork
5/5/22 11:40 a.m.

I would go 3rd link. Very few if any purpose built race cars use a torque arm from what I've seen. Watts link is a good choice to go along with it.

The 3rd/4th gen F body arm works well if you mount the front to the cross member instead of the transmission. Made that change to mine and its so much nicer, even in a "stock" car.

LuigiAndretti
LuigiAndretti New Reader
5/5/22 11:43 a.m.

In reply to Driven5 :

Using the forward leaf mounts for lower arm mounting points is the easy button but is not set in stone.  Even within the confines of the leaf mounts there is enough room to have multiple (vertical) mounting options to give adjustability.  Rear axle centerline height will be roughly 13.5", so there probably won't be any need to go higher than the limit of the floorpan at the leaf mount without crossing that centerline.

How much anti-squat is considered good for road racing?  In RCVD, Milliken makes a comment to the effect that antisquat values much above 30% are difficult to achieve with a torque arm.  Conversely, I've read through a number of forum posts indicating that 100% or greater antisquat is easily achievable with a torque arm, though those posts were on drag-centered fora so their applicability may be limited.  Obviously many people run torque arms quite successfully, so I'm not sure what to make of any of that information.

Brake hop is a serious consideration; where do you make the tradeoff between anti-squat and anti-lift?  All of my RWD track experience is in Miatas, so this is all way outside my wheelhouse.

20" CG height sounds like a good starting point; I vaguely remember reading somewhere that it typically winds up being right around camshaft height on most domestic V8s, so 16 - 20 would be a good range to test.  How to you estimate the longitudinal location of CG?  The engine and driver will both be set back roughly 18" from stock, placing the front of the engine under the cowl and the driver right by the B pillar. 

 

LuigiAndretti
LuigiAndretti New Reader
5/5/22 11:52 a.m.

In reply to Robbie (Forum Supporter) :

Yes, scales would be the right way to do it, and ultimately will be done that way.  Unfortunately the car is still completely disassembled on the chassis jig awaiting a suspension, so measurements will have to wait.  In the interim, theory and wild guesses will have to suffice.

APEowner
APEowner SuperDork
5/5/22 12:05 p.m.

It's possible to get leaf springs to work well on a road race car.  I run leafs on my Camaro.  If you use spherical eye bushing in the front of the springs and sliders in the rear it takes the non-linearity out of the leaf setup and positions that axle side to side pretty well.  If you need more lateral constraint you can add a Watts link.  A panhard bar will bind since it tries to move the axle laterally in it's own arc and the leafs constrain it pretty well.  I don't have a bar on the Camaro.

You can set cross weight with shims under the sliders and you can adjust anti-squat and axle wrap by adjusting the stiffness of the front of the leaf pack.

 

Driven5
Driven5 UberDork
5/5/22 1:05 p.m.
Gearheadotaku (Forum Supporter) said:

I would go 3rd link. Very few if any purpose built race cars use a torque arm from what I've seen. Watts link is a good choice to go along with it.

Purpose built race cars in my mind typically mean tube frame, which have far fewer constraints as well... Speaking of which, none of the rules I've seen for these types of series' seem particularly friendly towards hacking up the floorpan. So that may be an important factor in the decision here too.

MotorsportsGordon
MotorsportsGordon Dork
5/5/22 1:30 p.m.

A 3 link would probably be your best option. It's what the fast majority of pavement late models run or probably all the road racing late model chassis use.

MotorsportsGordon
MotorsportsGordon Dork
5/5/22 1:31 p.m.
Gearheadotaku (Forum Supporter) said:

I would go 3rd link. Very few if any purpose built race cars use a torque arm from what I've seen. Watts link is a good choice to go along with it.

The 3rd/4th gen F body arm works well if you mount the front to the cross member instead of the transmission. Made that change to mine and its so much nicer, even in a "stock" car.

Dirt late models and modified use torque arms although they call them a lift bar

LuigiAndretti
LuigiAndretti New Reader
5/6/22 12:14 p.m.

In reply to Driven5 :

Respectfully disagree. All modern iterations of the torque tube are IRS but plenty of antique vehicles used a torque tube with a live axle, including millions of Model Ts.

LuigiAndretti
LuigiAndretti New Reader
5/6/22 12:22 p.m.

In reply to APEowner :

I had considered retaining the leaf springs - after all, the vintage Trans Am guys seem to get around pretty well with them. I decided not to go that route for several reasons, but mostly because they restrict the available real estate for wide tires.  As it sits now, I have confirmed that 345/30R19s will clear the lower arms for a 3 link or torque arm without having to pocket the arms inside the frame rails. May be able to fit 355s, but haven't tried yet.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/6/22 12:43 p.m.
LuigiAndretti said:

In reply to Driven5 :

Respectfully disagree. All modern iterations of the torque tube are IRS but plenty of antique vehicles used a torque tube with a live axle, including millions of Model Ts.

Those are essentially torque arms in function, not relevant at all to an IRS.

Driven5
Driven5 UberDork
5/6/22 3:57 p.m.
LuigiAndretti said:

In reply to Driven5 :

Respectfully disagree. All modern iterations of the torque tube are IRS but plenty of antique vehicles used a torque tube with a live axle, including millions of Model Ts.

Theoretically and mathematically, the Model T used a torque tube suspension while the Corvette uses a torque tube chassis. These functionalities cannot simply be applied interchangeably, as was done above.

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