4 link swing arm for a door car

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cgarb
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4 link swing arm for a door car

Post by cgarb » Tue Aug 09, 2016 2:24 am

I see dragsters with the lower 4 link bars turned into a swing arm. Is there any reason this could not be done on a door car? I would imagine it would eliminate the need for a rear end locating device such as a diagonal link or a panhard rod. The only drawback I can think of is not being able to adjust the lower bars without taking the bolts out. But once the rear is measured out and square in the car why else would you need to change them other than making a hole adjustment or changing out rod ends? Anyone have a door car like this or have any input?

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Re: 4 link swing arm for a door car

Post by Brian P » Sun Aug 14, 2016 7:37 pm

Can you provide a photo of "... lower 4 link bars turned into a swing arm"? I don't understand your terminology.

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Re: 4 link swing arm for a door car

Post by MadBill » Mon Aug 15, 2016 10:34 pm

If the links are welded up into a single rigid structure like a very short ladder, it cannot accommodate chassis roll (theoretically infinite roll stiffness) and so would produce huge oversteer when cornering in a street/strip car. I'll leave it to the straight line guys to comment re the value on a drag car.
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Re: 4 link swing arm for a door car

Post by Brian P » Mon Aug 15, 2016 11:51 pm

That was my guess, too, but the original poster is nowhere to be found to clarify.

An axle-type suspension with a single trailing arm on each side (and nothing else locating the axle!) only works if the axle connecting them is flexible in torsion but stiff in bending (e.g. the open-section twist-beam axle found on the back of a lot of front-drive cars). If the axle is tubular then it will be too rigid to allow one-wheel bump or roll. "Truck arms" work because the forward end of the trailing arms is brought together so that the left and right sides are almost one point at the chassis end (deflection in the bushings and the arms themselves takes up what's left).

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Re: 4 link swing arm for a door car

Post by cgarb » Wed Aug 17, 2016 12:27 am


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Re: 4 link swing arm for a door car

Post by Brian P » Wed Aug 17, 2016 5:23 pm

OK, so this replaces both lower arms and the lateral location device and an antiroll bar with the structure shown.

In one-wheel bump or in roll, the whole structure has to twist - both the cross bar and the diagonals.

If someone has done their math correctly, this will serve as an antiroll bar while still allowing compliance in one-wheel bump or body roll and the stresses within the structure will hopefully remain in an acceptable range. If the math wasn't done correctly (or wasn't done at all) then one-wheel bump or body roll will essentially lock the whole suspension into moving both sides together (acting as a way-too-stiff antiroll bar), or it will break, or maybe both.

Antiroll bars normally have to be made of spring steel with carefully controlled heat treatment ... not welding.

I know that drag racers use all sorts of suspension designs that would be bad news if used outside of a straight-line, no-cornering, no-bumps application, and this is one of them.

If you want to do something like this, make the cross bar out of a bent sheet-metal U section with the open end facing down or back (not in an orientation that will trap water!), i.e. use a cross-section which is intentionally not rigid in torsion. The diagonals should also allow torsional flex.

Look at the rear suspension of many front-drive cars for inspiration. Here's an example:

http://3.bp.blogspot.com/_7IhKWjDc9wg/T ... waybar.jpg

The outer U-shape is torsionally flexible and allows the left and right sides to move separately without overstressing the metal.
The inner anti-roll bar is made of spring steel and contributes much more torsional stiffness than the twist-beam does.

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Re: 4 link swing arm for a door car

Post by cgarb » Wed Aug 17, 2016 11:59 pm

I wouldn't think the torsion of the swingarm would matter much, Its hooked to a solid rear end housing and the chassis has an antiroll bar on it to keep everything straight when it launches. Would not even consider this for a street car. Its made for launching straight. As fast as possible.

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Re: 4 link swing arm for a door car

Post by MadBill » Thu Aug 18, 2016 12:31 am

cgarb wrote:..Its made for launching straight. As fast as possible.
Unless you get out of shape, in which case you get a tank-slapper, trying to deal with terminal oversteer... https://www.youtube.com/watch?v=BTsrgb-YaKs
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Re: 4 link swing arm for a door car

Post by cgarb » Thu Aug 18, 2016 12:48 am

He's dealing with several thousand more HP than I have. To my eye, looks like he could use a good suspension guy. It headed right at the hit of the pedal. Not being a smart ass here just wondering how a go cart would deal with cornering? No suspension, wouldn't they have oversteer issues as well?

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Re: 4 link swing arm for a door car

Post by MadBill » Thu Aug 18, 2016 9:26 am

With no front suspension, a cart has high front roll stiffness too. As I understand it they adjust the balance via rear track width, seat mount stiffness, etc.
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