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Re: WTH Spider Rear Racing Suspension!

Posted: Mon Aug 12, 2019 11:23 pm
by ST3Racer
Andy,
Mounted how, exactly? Inboard of the long arms? Attached how? Or concentric? I am having trouble concepting this.

Re: WTH Spider Rear Racing Suspension!

Posted: Fri Aug 16, 2019 5:05 pm
by rridge
The double trailing arm mod was popularized on the West Coast by Dave Voss and others. A second set of lower arms is mounted on the inside of the stock arms by doubling the length of the mounting bolts and cantilevering the additional arm on those longer bolts off the inside mounting brackets.

I ran the set up on my street Spider for several years to reduce the understeer in a way similar to a rear anti-sway bar. The bushings in the trailing arms absorb and resist the twist created by the body rolling side to side while the axle remains parallel to the ground. Most of that resistance is generated by the bushings. Twice the number of bushings gives twice the resistance to rolling.

I found the mod made a nice reduction in understeer for the stock suspension in my '81. After I lowered my car moderately and went to stiffer shocks the rear end became a little too loose for street work. The nice thing about the mod is that it is completely reversible. There is a picture and more info at this link: https://www.flickr.com/photos/rridge/69 ... 018311651/

Re: WTH Spider Rear Racing Suspension!

Posted: Sat Aug 17, 2019 1:41 am
by ST3Racer
NOW, I understand. Interesting. Using stiff springs, and stiff adjustment on the KONIs, gives me the balance I need on both the street car, and the racer. On sweepers, both cars are neutral, and on tight corners, it can be made to drift a bit to get the rotation needed to make the corner. Power on the street is calculated at about 150 crank, and the racer is about 200 crank. I run a 25mm front bar, and no rear bar, on both cars. I have found that any rear bar makes the handling unpredictable, so I make sure that the stiffness and balance of the springs gives the handling that is needed, for quick laps, or fast mountain driving.

Re: WTH Spider Rear Racing Suspension!

Posted: Sun Aug 25, 2019 9:44 am
by fiatfactory
I did exactly the same modification back in the early 1990's on a 124 coupe I prepped for track work, I coined the term "M Bar" for my first iteration as the lower arms and the triangulation were one piece (so shaped like a capital M)

small difference was the method of attachment to the differential housing, I made the central diff mount a screw in item, it screwed into the drain hole (fiat 132 diff, larger centre)

Eventually the welds failed at the front attachment point, so I end up making the central V separate, similar to how you have it, but the forward mounts a little more rearward, so the pivot points were all in a line across the car.

That car also had the upper inner arm attachment point moved upwards (simple box section in the floor in the region under the rear seat squab) and the lower arm attachment point on the differential extended to move the pivot point downwards, this returned the rear geometry to standard angles after the rear had been lowered... and the car hooked up amazingly well.

And yes deleted the panhard rod as the lateral location was taken care of, with no sideways deflection under vertical movement.

SteveC

Re: WTH Spider Rear Racing Suspension!

Posted: Sun Aug 25, 2019 5:50 pm
by ST3Racer
Steve,
The objective is to eliminate bind and weird motion(s) that disrupts the handling. There are many ways to do this, and I have learned some new ones.

Re: WTH Spider Rear Racing Suspension!

Posted: Sun Aug 25, 2019 10:15 pm
by atruscott
Richard explained very nicely. Only additional thing I did was to include a fender washer on the inside of the additional (inside) arm. It constrains the bushing nicely so you don’t get any “splurge”.

The key for me was that rather than a simple torsion from one side of the car to the other, with dual long arms there’s a torsion within each side of the car. You really feel the difference, and it’s such a simple modification.

A

Re: WTH Spider Rear Racing Suspension!

Posted: Tue Aug 27, 2019 3:12 am
by ace124
Heres how im doing it over here in Europe.
What you see is actually destined for a buddys 131, but 124 has similar mounting points and im in the process of replicating this set up for the 124.
On smooth tracks there is nothing wrong with the rear live axle.
My usual mods are, reinforcing, A arm or watts link conversion, although with a stiff rearend the panhard rod works ok. LSD, a proper one with clutches, not a torsen, although they work ok, clutch type works much better on trail turning into corners. Upper control arms boxed and moved inboard so as they sit parallel to the ground after lowering. Rose / Heim joints on all ends.
The key are the rear shocks, Bilstein suitably valved. or Konis but on a race track you need re valving, OEM type valving is for the road....even on koni yellows. And absolutely NO rear sway bar.
The only reason im going IRS on rear is because of dirt. A live rear axle will skip, and because of the extra suspension travel needed, IRS is better.

Re: WTH Spider Rear Racing Suspension!

Posted: Tue Aug 27, 2019 5:25 am
by atruscott
John - now why did you have to go and upset my calm with putting ideas like that in my head? :)

Re: WTH Spider Rear Racing Suspension!

Posted: Tue Aug 27, 2019 3:20 pm
by fp55scca
ace124 wrote:
Tue Aug 27, 2019 3:12 am
What you see is actually destined for a buddys 131, but 124 has similar mounting points and im in the process of replicating this set up for the 124.
-----------------------------------------------------------
--John, a couple of questions. First and foremost, what is the origin of this assembly?
--I'm also having some difficulty seeing how this setup will share any mounting points with a 124 Spider?
--Are those actually front hubs....just used for the picture? They appear to have steering arms attached?
Thanks John,

Re: WTH Spider Rear Racing Suspension!

Posted: Tue Aug 27, 2019 7:49 pm
by fiatfactory
Rear hubs look like X19 5 speed, steering drop arm is cast in, and there will be a connecting adjustable tie rod and this allows setting of rear toe.

Basically a copy of the 131 Abarth rear suspension, with historic rally being good business these days, companies like RSD in the UK have developed the IRS system and even 16 Valve heads and other parts to keep customers happy... most are customers with pretty deep pockets though.

SteveC