Pros/cons of torque arm vs trailing arm rear suspensions?

I like to plan builds out in my head that I may or may not ever get around to. As part of one such exercise, I noticed that a vendor sells both a torque arm and a trailing arm/2link/truck arm rear suspension for the same vehicle. This made me wonder how they might compare to each other, and if there enough differences between the two types to justify a single seller offering both as an option?

Torque Arm:

Truck arm (shown with Watts link, but can also be had with Panhard bar like the torque arm):

They both seem so similar to me. I'm not sure I understand why they'd offer both options for the same vehicle. Teach me the intricacies of solid rear axle handling setups please!

That is a truck arm suspension.  Trailing arm suspensions are IRS.

Truck arms are simpler but also have less adjustability.  The instant center of a truck arm is fixed by the chassis mounting point.  With a torque arm, the instant center is also dependent on the location of the control arms, which will also be able to cause roll steer if you want it.

Truck arms also make exhaust routing really difficult.  The need to basically have side pipes is what generally turned me off of them.

Pete. (l33t FS) said:

That is a truck arm suspension.  Trailing arm suspensions are IRS.

I think we have different terminology definitions.  Trailing arms are used on solid axles all the time. 

In reply to Curtis73 (Forum Supporter) :

Yes, but that is the difference between having trailing arms and having a trailing arm suspension 

Unless truck arm suspension is called trailing arm suspension nowadays, which I guess makes sense because they have not been used on "trucks" for fifty years... but I don't necessarily have to like it to accept it 

If we want to get pedantic, the first illustration has trailing arms on the bottom, a torque arm in the middle, and a panhard rod.  For a stick axle, it's pretty darn good, plus, the lack of upper trailing arms makes for more back seat room.  

Truck arms go around in circles very nicely, but leave you with a lot of unsprung weight.

Pete is talking of 510 Datsun/ 3 series BMW type IRS, which is better than nothing, but kinda junk. /ducks and runs/

Both of those (depending on how they're engineered) can have equal weight transfer capability for straight line by altering the height of the front mount.  Typical torque arm styles can be limited in that aspect depending on where they mount.  An F-body for instance isn't easy to change given the fact that it mounts to the transmission output.

Truck arms have the limited effect of turning the axle itself into a sway bar since the arms are rigidly attached to the axle tubes.

Both styles have the shortcoming of the main tolerance/compliance comes from the bushings.  In the torque arm design you pictured, as the axle articulates up/down the outside trailing arms move in a tighter arc than the torque arm and "pull" on the tubes at the outside.  In the truck arm design you pictured, a similar thing happens as one side articulates.  Either the truck arms are designed to twist (usually the case) or the axle tubes bear the torsion.

Extreme generalization with a million caveats.... truck arms for turning, torque arm for straight line.

I've read that torque arms put power down well but are likely to wheel hop under braking.

I've always thought truck arms were cool becuase NASCAR but I don't know jack about them.

In reply to buzzboy :

Anything with a short instant center will be prone to brake hop.  Which is where you mount the caliper to a bearing and it gets its own reaction link to the chassis, decoupling brake forces from the axle.

The binding in a torque arm is eliminated by using some type of 'slip joint' allowing the main arm to change its effective length.

The binding in truck arms is somewhat mitigated by using arms that are soft in torsion and large soft pivot bushings, but cannot be eliminated without the arms joining at a single common pivot. The further apart the pivots are, the worse it gets.

Off the top of my head, the only technical advantages the truck arm has over the torque arm is that it's cheaper and easier to build. If I had a torque arm in my catalog, I wouldn't bother adding truck arms. If I had truck arms in my catalog, I would definitely add a torque arm... And might or might not keep selling the truck arms.

The biggest attraction of truck arm suspension is that the NASCAR Cup cars used to use it.  They used it mostly due to tradition.  It's really not a great setup.  It's weirdly constrained and is very susceptible to wheel hop under decel and/or braking.

In reply to Driven5 :

One really nice thing about a truck arm is all longitudinal forces go through the center of the car.  Makes life easy if you have a chassis that is a backbone frame without much structure on the perimeter.

So, I'm reading posts saying that torque arms see wheel hop under heavy braking, and posts saying the same thing about truck arms. Some cursory reading suggests that's common with lots of anti-squat, but maybe that can be tuned a bit more with the torque arm than the truck arm? I do find the simplicity of the truck arms appealing though...and it's only going to be a street car/multi purpose cruiser rather than dedicated autocrosser, track rat, or drag car.

Is one better than the other for street driving or cruising some backroads or does that pretty much all come down to spring and shock tuning?

In reply to STM317 :

Even a 4 link can have bad wheel hop under braking.  I ran into it a lot with an FB RX-7 when I used to play on asphalt.   Fox body Mustangs had cruddy rear brakes on purpose for this reason, they couldn't get much braking force back there without running into issues.   They didn't start to get good rear brakes until they got ABS in the SN95s.

GM had a torque arm in the contemporary Camaros, they lived with some wheel hop and then got ABS before Ford did.    They also had an extremely long torque arm and link geometry to have as long/low an instant center as possible.

If it is going to be an allrounder enjoyment car and not a dedicated times-chasing car, honestly I'd go with whatever fits the chassis best.  Or maybe just a Panhard rod on what I assume is the stock leaf spring setup.  I want to say avoid the truck arms because they cause exhaust routing woes and side exit exhaust is a pain to live with, but this is personal preference.

In this case, exhaust doesn't seem too difficult with the truck arms:

I think that's the first time I've seen exhaust under the axle while not having clearance issues.

A factory 4 link is second to nothing for garbage suspension, and I include leaf springs in that.  They make the upper arm short and squishy for interior room, then the angle them so they don't have to add a panhard bar. All bad in most every way.

Longer upper arms pointed straight ahead or a torque arm, and a panhard or watts link is roughly 200 times better

But that's just my opinion, I could be wrong.

I think the decision depends on what you start with.  I had a Fox chassis Mustang with terrible stock four link.  Went to a pan hard bar and one upper link for a major improvement, never quite got to installing the torque arm before I sold it.   If there is a developed option of a torque arm they can work well.  Mine added a lot of weight that is attached to the axle with the arm plus panhard and brackets.

I think a panhard with three link would work better and be lighter.

My primary solid axle experience is with four link with angled upper arms (Early RX-7/Mustang-style) and four links all aligned along the length of the car plus panhard (British Mk2 Escort/AE86/style) and I've been intrigued by the potential for a three link plus panhard. 
 

Never having experienced it, I'm trying to wrap my head around the torque arm setup. Where are the pivots on the chassis side? Looks like a long three link, but the center link is low instead of high, and possibly mounts to the other arms instead of the chassis? Or is part of the chassis also in the picture, and the two primary links pivot closer to the axle with a more conventional arm length, and the anti-wrap center link is the really long one?

Sorry I'm such a n00b with American car stuff, just trying to improve my understanding!

In reply to ae86andkp61 (Forum Supporter) :

The torque arm is simple.  Take a four link suspension, remove the upper links, and then install a 3-4' long slapper bar to the middle of the axle to handle the torque reactions.  It will need to be bushed in such a way that it can change in length but not in height.  (A bushing that the nose can slide in is the usual way to do it)  I have also seen people stick the end of the torque arm on a vertically placed rod ended link.

The axle is located longitudinally by the "lower" links, laterally by a Panhard or Watts or Mumford if you like making linkage, and rotationally by the torque arm.  Done properly, all of the links only load the chassis in one direction (fore-aft, side-side, up-down in that order) which can make the backyard engineer's job simpler.

Now if you want to get FANCY...   you could have the torque arm literally mounted like a slapper bar so that it is free to rotate down under braking, and then mount a 3-link style upper link that can telescope to handle brake reaction.   A lot of circle track chassis do this, seems to be strongly dependent on setting it up properly for the ride height you want and getting the telescoping parts' preloads and spring tensions correct.

STM317 said:

So, I'm reading posts saying that torque arms see wheel hop under heavy braking, and posts saying the same thing about truck arms. Some cursory reading suggests that's common with lots of anti-squat, but maybe that can be tuned a bit more with the torque arm than the truck arm? I do find the simplicity of the truck arms appealing though...and it's only going to be a street car/multi purpose cruiser rather than dedicated autocrosser, track rat, or drag car.

Is one better than the other for street driving or cruising some backroads or does that pretty much all come down to spring and shock tuning?

I think they both CAN have hop issues depending on how they're set up.  A steeper angle on either one will increase weight transfer on acceleration and decrease weight under braking.  If they're installed more level, they'll do neither... they won't add weight to the rear under accel and won't take weight off during braking.  I don't think brake hop has anything to do with the diagonal nature of one and the longitudinal nature of the other.

In reply to Curtis73 (Forum Supporter) :

You can also angle the lower arms of a torque arm setup down, which I am pretty sure GM did on the F bodies.  This reduces antisquat but also reduces the chance of brake hop, the longitudinal force from acceleration/braking partially cancels out the torque.

I am pretty sure this is why modern front drive cars with twist beam rear suspensions have the pivots so low.