Torque arm suspension

[agent fabricateur]

Has anybody seen a FB using a torque arm suspension system for the rear axle? The V8 swap guru at Granny's speed shop sells one and I had always considered it more of a drag racing system. The only OEM application I'm aware of is the 1980s Camaro, which was notorious for its poor handing and nasty axle hop under braking, but it seems there are now some extremely well engineered systems on the market for American cars with live axles and they perform extremely well on road courses in conjunction with a panhard bar or watts link.

If you're not familiar, it's basically a three link with lower links being much the same as our cars, but with a very long middle that attaches directly to the differential housing and mounts at the balance point of the car, roughly. Under acceleration this system can be configured to put the entire weight of the car on the rear tires and conversely under deceleration it can significantly reduce forward weight transfer, making for a much more stable car and increasing rear brake effectiveness.



Note the multiple mounting positions for the watts link, allowing the roll centre of the rear suspension to be moved at will. Combined with ride height adjustability this makes for a car that can be tuned for very fast corner exits.

I'm leaning towards a setup like this for my project.

Connor

[TOOL]

Way more engineering there than is required imho.

If you want these cars to handle, get the arse as low as you can then fit an adjustable watts link or adjustable panhard rod to re-centre the live axle.
Then fit the widest 15 inch wheels and tyres you can. Period look but still some flexibility in the sidewalls (unlike 17s).
Decent adjustable shocks and higher spring rate and you're good to go.

Beyond this, there's not much more you can do unless you want to start bending the axle housings for a degree of camber.

The RX2 for example is prety good in the back with a 5 link set up with panhard, vs RX3 leaf spring rear. The front is notorious for finding trees on the side of roads though, so money spent in the front is the way to go.

[agent fabricateur]

I'm not sure that I entirely agree. The upper links as they are seem to be a packaging compromise and the more you lower the car the more compromised they become. The big job of the upper links in a parallel 4 link is to control rotation of the axle housing and the longer they are the more effective they can be, as well as less sensitive to ride height changes, even during a normal range of motion, which result in undesirable pinion angles and potentially binding.

Handling is also highly subjective, some people value road holding, others transitions, others compliance over uneven surfaces. While all three of these are important to me, more important is putting down power, as my first stage of engine will make more than 300whp. It's easy to say a miata handles better than a corvette, but when you try to put down 400hp in a miata at corner exit things tend to go terribly wrong. They're engineered with different purposes.

I'm already starting with a different axle that has none of the original mounting points so some engineering is in order anyway, at least for my car. I'd also like some control over my antisquat as the car will be multipurposed. I suppose I want to have my cake and eat it too, but what's the point of having cake if you can't eat it?

Connor

[Lucky]

Interesting.

The centre steel in that pic you posted looks analagous to the Power Plant Frame that Mazda introduced in the FD RX-7s (and, I think, Miatas?). I guess the obvious issue might be that wherever you mount the leading end of it to needs to be able to support the loads it's suddenly going to be subjected to. On the FD the PPF mounts to the rear of the gearbox... which is why you really need to also brace the diff on high-powered cars otherwise they tend to smash the diff on launch... the kinda IRS equivalent of axle tramp All of which kinda makes it pointless bracing the rear end independently from the chassis!

I guess the setup you're talking about would mount direct to the chassis/monocoque at the front end? My thought would be that would risk transferring a lot of NVH unpleasantness into the car, although if it's destined for competition this might be of secondary concern. Personally, I find the trailing arm setup on the FB annoying only in terms of the mounts are terrible rust traps. Having experienced a few Yank live rear axles I have to say that Mazda did an exceptional job of providing such good axle location systems that it's easy to forget it is a live rear axle... until you catch a pothole and it all bangs off the bumpstops and heads sideways. But then my car's only a road car so hardly being subjected to horrendous stresses

In short, a lot of work but probably worth it in a competition car. Although I'm tempted to agree with MR TOOL and wonder if it's too much work to still be stuck with a non-IRS rear end?

[TOOL]

Plenty of cool stuff here on a shelf:

http://www.msfracingcomponents.com.au/m ... omponents/

[agent fabricateur]

TOOL: that's a very cool link. Oh the money I could spend on this car... Unfortunately budgets are tight right now with commodities prices this low so I have to make do with FC and mustang parts.

Lucky: while it looks similar the function is very different. The link in the picture actually has a sliding pivot point at the unseen end which transfers forces to the floor pan near the middle of the car. The sliding pivot is necessary because the arc is so much greater due to the length of the middle arm. There definitely has to structure to support it, most of these kits attach to what see called subframe connectors, basically bracing that augments the frame or unibody reinforcements underneath the car. I'm thinking that I will cut out the rockers of this car as they're rusty on the inside and run some DOM tube under there between the main hoop and front leg mounting positions of the cage. The lower links would attach to a tube running perpendicular to those, under the main hoop, and the torque arm could attach at the transmission cross member area which would be tied into the rockers. Side exit exhaust?

NVH issues aren't such a concern, but the pro touring guys in the states run this kind of suspension and comfort is a concern for them, so who knows?

The car already needs a cage and a driveshaft loop so there's not that much extra work involved in making something like this.