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PostPosted: March 7, 2016, 6:31 am 
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I was looking at some F1 suspensions and wondered if anyone knew of any advantages of having super long wishbones. Im thinking with a blank slate, no particular chassis in mind. I played with vsusp for a bit and with the little i know thought i found a pretty good combination.
I am considering this to possibly create a triangular frame at the front and back of the chassis to reduce weight while keeping everything strong, stiff and supported
Let me know what you think, pros and cons, special considerations.
This seems to be good up until 3in of bump where the RC goes below the ground. Almost the same situation at 5* of roll. How important is the side to side movement? should i try to limit its migration?
AetroChimeraV1

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PostPosted: March 7, 2016, 10:14 am 
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In F1 it's all about air flow management - not an issue on our cars!

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PostPosted: March 7, 2016, 11:25 am 
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Like Kurt said, aero is the priority in F1. I don't think this is a help in our cars. What else are you going to compromise to get that shape? I think that's why you don't see this much. Do some drawings or just use masking tape on the floor and see how it all fits together...

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PostPosted: March 7, 2016, 11:43 am 
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...... and remember that you are unlikely to see roll angles beyond 1 or 2 degrees.

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PostPosted: March 7, 2016, 1:06 pm 
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Aetro wrote:
I was looking at some F1 suspensions and wondered if anyone knew of any advantages of having super long wishbones.


Long wishbones are common on formula cars of any class, formula ford is an excellent example. It's simply the inevitable result of having a wide track and a narrow chassis, rather than a goal they had in mind when they where designing the car.

The only place I can think of where long wishbones are a design goal is long travel suspensions on purpose built off road vehicles like dune buggies, Baja trucks, and the like. The benefit in that case is that a longer wishbone allows greater suspension travel for the same misalignment in the ball joints and CV joints.

On a locost, there's no real benefit.

Aetro wrote:
This seems to be good up until 3in of bump where the RC goes below the ground. Almost the same situation at 5* of roll. How important is the side to side movement? should i try to limit its migration?


In bump, don't pay any attention to RC migration with respect to the ground. What matters is the vertical distance from the RC to the Cg, which is what determines your roll moment and roll inertia. In bump, you want the RC to migrate downward. To be precise, you want it to migrate downward at the exact same rate as the Cg migrates downward. For three inches of bump you want three inches of migration. And the tales of doom regarding an RC below ground level are old wive's tales, it's really not important. If RC height is migrating from substantially positive to negative in roll, you're going to have nasty handling, but not because the ground is a sacred boundary that must never be crossed.

As for lateral (kinematic) RC migration, it's important in terms of force based roll centers. If your kinematic RC doesn't migrate vertically or laterally, then your force based RC won't migrate vertically.


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PostPosted: March 7, 2016, 2:35 pm 
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In bump, don't pay any attention to RC migration with respect to the ground. What matters is the vertical distance from the RC to the Cg, which is what determines your roll moment and roll inertia. In bump, you want the RC to migrate downward. To be precise, you want it to migrate downward at the exact same rate as the Cg migrates downward. For three inches of bump you want three inches of migration.



If you think of using the bump and droop sliders on Vsusp to represent the car pitching and rolling I don't think what you're saying is right here. Maybe other software would represent this differently. Imagine the car diving under braking or squating under acceleration. In these cases the CG might stay at the same height, for this simple example. If the car were entering the turn the front could be at 2" of bump, both wheels. In order for the RC to be at the same height it would need to rise 2".

Vsusp gives the RC in terms of the static position of the car. Maybe it should update it to the position of the "road", a line between the contact patches. It's a 2D program so doesn't understand the concept of a CG somewhere else.

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PostPosted: March 7, 2016, 7:05 pm 
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Aetro wrote:
I was looking at some F1 suspensions and wondered if anyone knew of any advantages of having super long wishbones. Im thinking with a blank slate, no particular chassis in mind. I played with vsusp for a bit and with the little i know thought i found a pretty good combination.
I am considering this to possibly create a triangular frame at the front and back of the chassis to reduce weight while keeping everything strong, stiff and supported
Let me know what you think, pros and cons, special considerations.


A normal spindle has the ubj as close as practical to the hub cl and the ubj inboard enough for around 10 deg kpi, give or take. This would have a large effect on your arrangement. There is also a great deal of scrub.

Also, most of the spindles use a low mount tie rod, so a center tapped rack would be necessary, but the rack still needs adequate support through overall length, so the ends would stick out pretty far from the frame.

While it saves sprung weight, it raises unsprung weight. The savings in sprung weight is not so great when you consider aligning tubes between front and rear suspension lcaps around an engine. A variation of this would be L shaped control arms where the forward leg is perpendicular to the wheelbase and the rear leg is near parallel, as used on many rear engine, single seat race cars from the 50s/60s.

You might shrink you chart prefs to 20/20 for more resolution for the range we are concerned with.

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PostPosted: March 12, 2016, 8:53 am 
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Thanks for all the good info everyone.

I think i understand the tradeoffs for this idea, and since im putting an emphasis on minimizing unsprung weight ill stick with a more traditional configuration. With what ive got pictured in my head im going to have a hell of a time fitting the front suspension no matter what i do.

MV8, when you say "ubj as close as practical to the hub CL" do you mean perpendicular to the ground? Im not sure i understand this completely.

I read somewhere(i believe it was something pertaining to FSAE cars) that KPI should be matched to the amount of caster, and that you should use as little caster as required for an application. The plan is for this car to be fast on a road course while still being drivable on the street. My thinking was to use little to no caster with a bit more than normal scrub radius for steering return and feedback. my numbers at the moment are guestimates at best and place holders at worst.

Im still reading whatever i can find, so i might have bad info, faulty memory, or just overlooking something important. My planned chassis is going to be very different from a locost so im trying to simplify my suspension by minimizing the effect of one thing over the others. this might only give me a ill handling car but hopefully ill learn why and have that corrected by the time i start building

Thanks for the insight
Vincent

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PostPosted: March 12, 2016, 10:54 am 
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I believe MV8 meant "LBJ" instead of "UBJ."

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PostPosted: March 13, 2016, 1:45 pm 
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Like KB58 said. I meant lbj is very close to the hub. Plenty of photos online that show how much closer the lbj is to the hub than the ubj. The further away the lbj is, the heavier the spindle will be for a given strength.

As for caster, so much is normally built in to oem spindles. I don't think scrub is a good substitute. Scrub does not center the steering without caster.

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PostPosted: March 13, 2016, 3:38 pm 
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How can caster be built into the spindle? Do you mean the kingpin inclination which requires a lot of caster to keep your outside wheel in a turn from going to positive camber?

As far as suspension design goes, here's a pretty good writeup on it. At least the guy seems to make a lot of sense to me. He talks about the relationship between kingpin inclination and caster, as well mentioning that he likes to have the lower control arm as long as possible. It's a long read but there's a lot of info there.
http://www.lateral-g.net/forums/showthread.php4?t=42467

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PostPosted: March 13, 2016, 4:18 pm 
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I was talking about when the center of the hub does not fall directly on a line between the lbj and ubj centers; usually only on fwd for torque steer but it is kpi and not caster.

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PostPosted: March 14, 2016, 12:36 pm 
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Aetro wrote:
I was looking at some F1 suspensions and wondered if anyone knew of any advantages of having super long wishbones.
It's sure not because of suspension advantages. Kreb's right; it's all about airflow--they have long control arms because they have skinny noses, and like Abe Lincoln's legs being long enough to reach the ground, F1 control arms have to be long enough to reach the chassis.

Truth is, F1 cars wouldn't have suspensions at all if the rules didn't require them.

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PostPosted: March 27, 2016, 10:16 pm 
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Think about the arc the suspension travels. There will be more lateral movement of the contact patch if you have 6" long arms versus 18" long arms.

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PostPosted: March 27, 2016, 11:07 pm 
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a.moore wrote:
Think about the arc the suspension travels. There will be more lateral movement of the contact patch if you have 6" long arms versus 18" long arms.


And along those lines a question I've been pondering, we use arms that rotate around the central axis of the car so the contact patch varies in & out, but what about making the arc vary front to rear? Which is ultimately better for handling and ride?

In & out is easier to package, but is it really better? You can get longer arcs and less movement the other way.

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