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Strut/multi-link versus double wishbone

Old 03-27-04, 12:20 AM
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Strut/multi-link versus double wishbone

I dunno if this is the proper section, as it can easily be applied to the suspension section, but I figure there might be more interest in here.

If it gets moved, I'm not going to complain.

General consensus has always stated the double wishbone suspension is superior due to superior contact patch geometry.

Now, this is what I get for having too much idle time...

I'm going to make an argument for the strut type suspension might be better because of vehicle chassis roll.  Unless you're running ungodly thick diameter sway bars, the vehicle will always have a roll center when it turns.  So if you figure in the roll center, the strut suspension can be tuned to have superior tire contact with the road on the outside tires; there can be point made about the inside tires, but how much of the cornering is made with the inside tires?

The double wishbone suspension will actually narrow track width under compression and rebound, if we're assuming it's full horizontal at rest?  Or does the track width change not that much of a concern?


-Ted
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Old 03-27-04, 08:45 AM
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I'll come back to this when I have more time this evening, but a strut suspension has no handling advantages whatsoever over a multilink or double wishbone suspension. Struts are purely for cost and packaging.

No matter what the type of suspension the key is to make the inside tire do as much work as possible. This is done by striving for as little weight transfer across the car as possible in order to make both sides do equal work.

Roll center and CG location are keys. Roll center of the suspension can be placed wherever you like; it's purely a function of suspension geometry. You could quite easily make the roll center higher than the CG so the car would actually lean into corners like a motorcycle. Why does nobody do this? The car rolls to the inside in that case because the jacking forces on the outside are so high that the car is lifting and the tires unloading while the inside tires gain nothing in grip. In essence all of your weight transfer would become a jacking force as opposed to a loading force on the tires, so grip would actually decline at a terribly faster rate under cornering.
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Old 03-29-04, 11:03 AM
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You need to add static negative camber to the strut suspension. Think about what this is doing to your contact patch under straight driving and braking. Struts are cheap and easy to package. That's the only reason they are so widely used. A dual A-Arm suspension can be designed to offer a ver large camber gain with chassis roll if you so desire. It's just a matter of where in the arcs you have the arms start.

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Old 03-29-04, 11:41 AM
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Yeah, what those other guys said....
The problem is that struts only have 1 camber setting, meaning there is no camber gained in roll. So let's say you set the car up so that it has 5 degrees of camber to compensate for it's 5 degrees of body roll. That means that in any circumstance other than max lateral force, the tires contact patch is not being used... like braking. And when you DO execute that hard corner, only the outside tire is doing work, your inside tire is now so far out of whack, with respect to camber, that you might check the inside side walls for wear.
Like the other guys have said, struts are popular because they are cheap to design/build, and easy to package into the chassis.
a-arms = r0x0r5
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Old 03-29-04, 01:54 PM
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It's funny that all the "Touring Car" races have all these cars two-wheel with just the outside wheels, so why is that?

Does it have anything to do with the zero-droop that most of them run?

I noticed the wishbone suspensions run a lot of big rate (i.e. heavy) springs.  Wouldn't this just add to upsprung weight?


-Ted
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Old 03-29-04, 02:02 PM
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Originally posted by RETed

I noticed the wishbone suspensions run a lot of big rate (i.e. heavy) springs.  Wouldn't this just add to upsprung weight?

Spring rates of the springs on the car have absolutely nothing to do with unsprung weight. Unsprung weight is the physical mass of all the moving suspension parts hung from the chassis which the damper must control: wishbones, spindle, brake caliper, brake rotor, wheel/tire etc. The unsprung weight of a given car is the same no matter what springs are on it.

Don't confuse spring rate with wheel rate either. What actually counts on the chassis is wheel rate, this takes into account the lever arm in the way the spring is mounted to the chassis. It is completely pointless to compare spring rates unless the cars compared have identical suspensions.

As for racing cars on struts I would expect them to use very high wheel rates even compared to other racing cars. Why? Because you get hardly no camber gain from the strut suspension so you would want nearly zero roll to keep the tire footprint as flat as possible. This is still a poorly optimized system compared to a multi link or double wishbone, but given that's what you have to work with that's what you would do. Imagine go kart.

As for on two wheels I have never seen it unless they are bouncing over curbs. Any car on two wheels is going to have less grip than on four. Now if you're cutting corners and don't mind the car being a ballistic projectile until it settles back to earth that's fine; just realize you don't get to change your trajectory until landing
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Old 03-29-04, 07:08 PM
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also, some scrub is good as it helps build heat in the tires. damon is correct.
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Old 03-30-04, 10:29 AM
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Originally posted by RETed
big rate (i.e. heavy) springs
There are a few ways to design a stiffer spring. One is to use thicker wire, which will add mass, another is to reduce the number of coils, which will reduce mass. So stiffer isn't always heavier.
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Old 03-30-04, 12:46 PM
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Originally posted by DamonB
Spring rates of the springs on the car have absolutely nothing to do with unsprung weight. Unsprung weight is the physical mass of all the moving suspension parts hung from the chassis which the damper must control: wishbones, spindle, brake caliper, brake rotor, wheel/tire etc. The unsprung weight of a given car is the same no matter what springs are on it.
I always heard that unsprung weight all consisted of half the weight of the damper and spring combo in each corner?


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Old 03-30-04, 12:56 PM
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I was confused about your question earlier. I thought you were saying stiffer springs meant there was more unsprung weight.

As for the unsprung weight of the damper/spring yes, half is a good approximation.
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