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Simulation Racing Leagues => Assetto Corsa => Australian Assetto Corsa League, Tuesday nights => Topic started by: Wally on August 01, 2021, 04:54:07 PM
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This is kind of interesting... a blog post from a real world Formula Ford driver as to how he went about getting a balanced setup.
https://firmtec.wordpress.com/2015/07/12/achieving-a-handling-balance-formula-ford-1600/
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I don't fully agree with this statement:
To summarise the summary; the end with the most roll is the end with the most grip!
If it were true, race cars wouldn't have roll bars, but they do.
They were invented to give more grip.
The stiff end of the car will transfer weight to the outside tyre more than the soft end when cornering.
Again, this is not entirely true.
For corner weights it is, but a well balanced ARB transfers weight to the inside wheel, for that is its purpose.
The best part of the article was the adjusting of the roll centre height with the spacers.
Unfortunately, altering the suspension geometry is not a setup option available in AC.
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Are you sure, Bacchulum?
I reckon ARBs are to prevent roll. A flat, rigid car is less prone to roll (stability) and easier to change direction with - but I've always found the softer end to have more grip.
The weight transfer is a trickier thing; it'll depend on other things as well, probably. But what's 100% certain is that the rigid setup transfers forces ("weight") quicker to the outside; there's less delay coming from the softness of the suspension.
This is probably where the funny comes in; in a completely rigid setup (imagine a block of stone) the transfer is immediate. But we often think weight=mass, however these things are talking about forces, not mass. And springs are a sinkhole for forces; they are an energy storage device to some extent. (compress it; you have stored energy you can get back by releasing the pressure).
But on the other hand, excessive body roll will probably move the center of gravity - and that can have all sorts effects, depending how/where it moves.
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My Formula Vee has no rear roll stiffness at all! Be it from springs or so called anti roll bars. I think the preference is to call them stabilisers.
I was just thinking to myself that if the unsprung weight was (magically) zero the the Swaybar would have no effect. (Dunce)
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Everything I've read says that a stiffer ARB increases load transfer to the outside wheel. An increase in load transfer leads to reduced grip. The purpose of the ARBs is to tune which end of the car has more or less load transfer, and thus affect the balance while cornering.
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Keep in mind the motion of the suspension in roll.
The outside spring compresses while the inside spring droops.
The ARB puts a compression on the inside wheel counteracting the droop, which changes the CoG on that side.
Sure, from static, the outside wheel has an increased load in roll, but the ARB reduces the loss of load on the inside wheel compared to no ARB.
It transfers some of the load directed to the outside wheel to the inside wheel.
Think of it using the traction circle.
2 tyres, in full contact, gives a traction circle with twice the radius of 1 tyre.
As the load comes off the inside wheel that radius reduces towards the traction of one tyre, which it reaches when the inside has no load.
Therefore, keeping load on the inside wheel gives more available grip.
A stiffer ARB increases the speed the weight is transferred, so you can get to the point of breaking traction quicker which is sometimes confused with there being less traction.
Which is why I said "a well balanced ARB", the trade-off between having more traction to play with and how quickly you reach the limits of that traction.
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I just did a test in the Formula Ford on the skid pad, changing only the front ARB, with my app that shows the load on the tyres.
Front tyre loads:
Full soft ARB:
60 ----- 140
Full stiff ARB:
35 ----- 165
Same amount of load on the front axle, 200, but with the stiffer ARB, there was less load on the inside and more on the outside. If the load of 165 takes the tyre into the range of load sensitivity where a greater load means a greater dropoff in lateral force, then the stiff ARB is going to provide less lateral force than the soft ARB.
If you want to play with the app, it's attached.
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I recall it as an 8 dimensional equation.
Vertical load, delta vertical load change, lateral force, delta lateral force change, repeat for the other wheel.
More dimensions required for aero.
The summary was to find the balance point between losing grip on the inside (too soft) vs exceeding available grip on the outside (too stiff).
I don't have the Racecar Engineering magazine anymore to be more specific, but it did take 4 issues to deal with this matter.
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I just received my copy of Milliken's Race Car Vehicle Dynamics.
There are 15 equations to calculate roll, that will require more study.
The essential part, though, is table 16.5 Suggested Numbers for Simplified Method.
These numbers are:
Sedans 1.0 - 1.8 deg./g
Aero cars 0.25 - 0.5 deg./g
There is a sweet spot of optimal roll, hence, if adding stiffness gets you into that window, you will gain grip and vice-versa if you are on the other side of the window.
"Softer is more grip" is not necessarily true.
QED
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Yes, I don't doubt there's a sweet spot. Nearly everything in a setup is a compromise, and there's a lot more that contributes to lateral force than just rollbar stiffness. I've just read (most of) "Performance Vehicle Dynamics" by James Balkwill. Interesting stuff.
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From EIBACH, ARB manufacturers:
During cornering and performance driving, the CoG moves back to the ideal center of the vehicle as the Anti-Roll-Bar transfer corner forces from the outer to the inner wheel
This also reduces the load difference between the outer and inner cornering wheels
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My Formula Vee has no rear roll stiffness at all! Be it from springs or so called anti roll bars. I think the preference is to call them stabilisers.
I was just thinking to myself that if the unsprung weight was (magically) zero the the Swaybar would have no effect. (Dunce)
And the DeltaWing has no roll stiffness at the front, (similarly for three wheelers) also has semi decoupled damping at the rear so bounce and roll rates are seperate.
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From EIBACH, ARB manufacturers:
During cornering and performance driving, the CoG moves back to the ideal center of the vehicle as the Anti-Roll-Bar transfer corner forces from the outer to the inner wheel
This also reduces the load difference between the outer and inner cornering wheels
If you look at my post a few up where I measured the load in the Formula Ford on the skidpad, the opposite happens in AC (at least with that car). AC gives you a direct readout of the load on each tyre. A stiffer ARB increased the difference in the inner and outer loads.
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An AC flaw? ???
I'm just going on real world info. to check my sanity/memory.
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Nearly everything I've read says that increased roll stiffness increases the load on the outer tyre. Look at Milliken P587 (chapter 16), he states load transfer without a front ARB. Then on P590, he's fitted a front ARB and there is greater load transfer to the outer tyre. See screenshots if your page numbers are different.
The total lateral load transfer is the same (only height of CoG and wheel track can change that), but the stiffer rollbar at the front means that more of the lateral load transfer to the outside occurs at the front than the rear.
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Looks like my sanity/memory is broken.
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Well, like a lot of these topics, there is heaps of contradictory information on the web. I think that is because for a lot of these questions, the only accurate answer is "It depends".
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I'm pretty sure my info was from Racecar Engineering magazine and Eibach, a manufacturer, agreeing is confusing.
Maybe I'm thinking of lateral loads, not vertical loads?