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PostPosted: June 12, 2017, 4:29 am 
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In all honesty original Alfa Spiders are more Grand Touring type cars than fling it about sports cars.

Seems my original Alfa Romeo rear axle is a type of three-link design.
Works well but the original design is needlessly heavy due to use of a massive cast steel upper link.
I also note that the upper pivot point is off-center which I expect is less than ideal.
The space-frame chassis design I have calls for a four-link setup.
To make a four-link means more welding work for bracketry and probably heavier than a three-link.
Three-link will be simpler to fabricate if I can re-use the original upper pivot point.
The chassis books I've been reading claim advantages for each, but do not state that one is always better than the other.

So, front engine of around 150 HP, expecting close to even F&R weight, live axle rear, space-frame chassis, coil-over shocks, desire for neutral handling and overall design biased toward performance over comfort.
Which will be more likely to satisfy me?

Tried a search but it failed to yield useful results.

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PostPosted: June 12, 2017, 8:58 am 
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Many of the live-axle FFR Cobras use three links, apparently successfully. There are some three-link set-ups in the build logs, including one V-8 powered one and a couple of track-day/auto-cross builds. If the donor had a three-link why not do the same to utilize existing brackets, with the caution that packaging the upper link might prove problematic, depending on geometry.

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PostPosted: June 12, 2017, 9:58 am 
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Warren is correct in that many FFR Cobras use the offset 3-link setup. I believe ALL of the FFR Spec racers use it. As for the offset upper pivot, that could be done to counteract lifting of the right wheel under acceleration. This was why Jaguar used it in the D-type.

Never had any trouble with the 3-link in my dirt track stock car. Even won a few races!

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PostPosted: June 12, 2017, 11:26 am 
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The alfa uses a triangulated 3 link so the upper link is locating the axle for and aft and side to side.
The f5 cobras use a parallel 3 link with a track bar.
The biggest problem with the triangulated 3 link is the high roll center because the ball joint at the axle end of the upper arm is what sets the roll center height.
It works well for Alfas that are being used for the street but most that want too push them hard or race them generally swap in a parallel 3 link with either a track bar or watts link.
Alfahhalics (worlds best for alfa builds and parts) even has a kit for this that uses the stock upper axle mount.


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PostPosted: June 12, 2017, 1:14 pm 
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The good news is that as I am building everything from scratch I have no real limitations on what I do.
Since I have some hope of doing more of these I would like to keep it as simple as possible.
From the pics I have found so far it seems the three-link may be the way to go for less modification to the rear axle assembly.

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PostPosted: June 12, 2017, 4:34 pm 
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I've a 3link w/ panhard in mine. It's not dialed in yet, so I can't say much about how it works. First autoX is (hopefully) this Sunday so may have more info.

My main reasoning for 3-link over 4 is that 4-link MUST have the correct angles or you will get bind. I wanted a car to run hard on the street and track so wanted at least 3" of bump travel with good antisquat. It can be hard to get that from a 4link and even if you do, you are stuck with those settings (unless you want to change brackets etc). I wanted to be able to easily adjust/tune the chassis and have more options than 4link. The main reason to run 4 over 3 is if you have too much torque for the 3link to handle the loads. That being said, I've got ~280ftlbs with R-comp tires and 130% antisquat which is probably more twist than most Sevens see.

Downside of 3link can be elbow room. Especially if you have longer links. I would say if your axle is already built for 3link, it's a no-brainer to keep it.

Cheers.

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PostPosted: June 12, 2017, 6:14 pm 
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the three link will not get in a bind like a 4 link and on the race car (that I played with) had very little roll, but was still able to get some rear steer and that's a good thing too.

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PostPosted: June 12, 2017, 6:43 pm 
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Functionally, the Alfa 3-link is more of a 'fully-triangulated' 4-link than what most people probably think when discussing a 3-link. I also believe it's offset to the 'less desirable' (driver) side on the axle. Is it "ideal"? Not really. It's a compromise, just like everything else...But if it's the easiest solution to implement, it's probably more than adequate for having a good time.

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PostPosted: June 12, 2017, 9:06 pm 
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Since I am building along the lines of a sports-racer the entire body on mine is intended to be easily removable.
So if the three-link does not work out it should not be too hard to go with a four-link later.
Hoping the three-link works so that the axle will not require welding on.
Want other people to be able to build these on their own with my supplying as few parts as possible.
Looks like I should be able to create bolt-on trailing arms, upper link, and Watts link.
DOM CroMo with rod ends instead of the boat anchor casting should help a lot.

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PostPosted: August 19, 2017, 1:09 am 
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G'Day Guys,
The 4 link (trailing arm) rear won't bind if you lower the front top pivots slightly. (around 5/16") The other way is to use nylon bushes at the chassis end and QA1 rod ends at the axle end, works a treat with the reduced movement in a Clubbie. The other thing that causes bind (in roll) is placing the shock bushes along the axle centreline. They need to be parallel to the centreline of the car on the axle end of the shock. Obviously Rod End equipped shocks don't suffer from the binding that bushed ones do. Hope this is helpful.
Regards,
Mike.


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PostPosted: August 19, 2017, 5:13 am 
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You could always go Independent rear suspension. Which is lighter, completely adjustable, and generally considered better.

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PostPosted: August 19, 2017, 12:34 pm 
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IRS would require either tossing out the donor live axle or very heavy modification to use the center.
Both defeat the desire to keep these single donor.

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PostPosted: August 20, 2017, 9:57 am 
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It's hard to beat the utter simplicity of the standard Locost 4-link with parallel arms. It's NOT optimal, but it's very well behaved, generally speaking, and for a street car, a good choice. Packaging is also easy for a standard Locost chassis. If you build in adjustability to the links, it's also very "tuneable." The lateral locator is key to roll center height with the standard Locost 4-link design.

3-links are simpler (in theory), but also sensitive to the location of the top link off-center, and to it's length, as well as it's angle in space. I have no information on your Alfa Romeo, or what the engineers who designed were thinking, but it does sound like they used some of those considerations in their design. Now, will your stock 3-link set-up work as well in your Locost as it did in the Alfa? Unless you do an analysis of the Alfa design, that's pretty much unknowable because you won't know what kinds of torques and forces it was designed to produce and how that will work in your chassis, especially in combination with your front suspension design, which is likely different from the Alfa.

Probably the most modern, understandable, and compact book on 3- and 4-link suspension design is "Advanced Race Car Chassis Technology" by Bob Bolles. He covers all these issues plus a bunch you probably have not considered yet.

I hope this helps. It's a very complex subject.

Cheers,

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PostPosted: August 23, 2017, 10:52 pm 
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I would try to make the 3-link work. Not having to modify the rear axle is a big deal, not just because it reduces the amount of work during the build, but also when you have to replace it. If it's not the perfect setup, think of it as a good compromise. I also think the parallel 3-link with Panhard bar is probably the way to go.


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PostPosted: August 23, 2017, 11:15 pm 
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Just to update:

3link with panhard works nicely. Reaaaaaaaaally nicely. I have a TON of rear grip on throttle. Even when turning the car will just grab and go. I have floored it from a stop on a 90° corner and it just digs in and goes, even with all the torque. I have a lot of anti-squat which is the main ingredient, but I'm pretty certain the compliance of the 3link is what lets me get away with it. The only negative trait from it is when hard on gas over humps in the track, it gives a slight sensation of lift or twist. It's a little unnerving, but so far it's stayed planted with no drama. I'm two events in, with another this Sunday.

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