WTH Spider Rear Racing Suspension!

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ST3Racer
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Last Name: McMahan
First Name: Bill

Re: WTH Spider Rear Racing Suspension!

Post by ST3Racer » Mon Aug 12, 2019 11:23 pm

Andy,
Mounted how, exactly? Inboard of the long arms? Attached how? Or concentric? I am having trouble concepting this.
Bill McMahan
Parker, Colorado
'69 124 Spider 2L ex EP racer, ex Volumex NASA ST3 Racecar - retired
'69 rebodied with '79 backdated chassis as '72 DP 1.8L Vintage Racer
'74 124 Abarth Stradale Replica
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rridge
tremila club
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First Name: richard

Re: WTH Spider Rear Racing Suspension!

Post by rridge » Fri Aug 16, 2019 5:05 pm

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/
Richard
'81 Turbo Spider
Rockville, MD
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ST3Racer
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Re: WTH Spider Rear Racing Suspension!

Post by ST3Racer » Sat Aug 17, 2019 1:41 am

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.
Bill McMahan
Parker, Colorado
'69 124 Spider 2L ex EP racer, ex Volumex NASA ST3 Racecar - retired
'69 rebodied with '79 backdated chassis as '72 DP 1.8L Vintage Racer
'74 124 Abarth Stradale Replica
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fiatfactory
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First Name: Steve

Re: WTH Spider Rear Racing Suspension!

Post by fiatfactory » Sun Aug 25, 2019 9:44 am

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
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