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Old 25 Nov 2006, 16:23 (Ref:1774785)   #1
breezeblock
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Roll centres and swing axle lenghts

what height should a roll centre be at hi low or below ground level and what is better long or short swing axle lenghts
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Old 26 Nov 2006, 03:32 (Ref:1775033)   #2
trikesrule
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U racing on dirt or bitumin? Circle track, circuit or what? What sort of car? Engine in the back or front? Rear drive or front? You've just opened a can of worms.......trikes
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Old 26 Nov 2006, 18:18 (Ref:1775367)   #3
breezeblock
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circuit based fat lotus 7 lookalike rear drive engine in front
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Old 2 Dec 2006, 11:54 (Ref:1780263)   #4
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Panard bar in the rear with wish bones up front? Is the car already built?........trikes
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Old 2 Dec 2006, 12:54 (Ref:1780295)   #5
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double wishbones front and rear
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Old 3 Dec 2006, 10:34 (Ref:1780850)   #6
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Quote:
Originally Posted by breezeblock
double wishbones front and rear
There isn't one optimum number for either paramer.

A long swing arm length will give worse roll camber compensation but better bump camber control. Neither is better or worse. You will have to find a compromise.

A low roll centre will mean you need stiffer springs/bars for a given roll angle and your load transfer rate will be lower. A high roll centre will require softer springs/bars for a given roll angle but increase the rate of load transfer.

Do you want a different load transfer rate at either end of the car?

What load transfer distribution do you want front:rear?

What ride frequency do you want?

Ben
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Old 4 Dec 2006, 08:23 (Ref:1781406)   #7
trikesrule
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I've used a suspension Programe called SusProg with good results. Contact Bevan D. Young Automotive Technical Books. It's a good program and it's pretty easy to use. I've always worked my teams 3 cars suspensions with the front roll centre lower than the rear. I wonder if there is any advantage to doing that with a circuit car? U know - drop a little performance in one direction to gain in the majority of others. What do u think guys?.........trikes
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Old 4 Dec 2006, 08:33 (Ref:1781416)   #8
trikesrule
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I've always set the front roll centre lower than the rear. U need to play with a suspension program like Susprog. I use it and it's creature friendly.....trikes
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Old 5 Dec 2006, 19:48 (Ref:1782770)   #9
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Originally Posted by trikesrule
U know - drop a little performance in one direction to gain in the majority of others.
I'm not sure I understand what you mean by this.

Ben
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Old 6 Dec 2006, 08:12 (Ref:1783087)   #10
trikesrule
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With Speedway I set the car with a higher left side weight than the right. Around 3% more on the left works ok. If your usual track has mainly left or right handers wouldn't it be beneficial to run a slightly higher weight bias on the inner side? I suspect they've been doing that very thing with the V8 Supercars for sometime. Get those inside tyres to do a bit more work.........trikes
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Old 6 Dec 2006, 19:10 (Ref:1783512)   #11
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Quote:
Originally Posted by trikesrule
With Speedway I set the car with a higher left side weight than the right. Around 3% more on the left works ok. If your usual track has mainly left or right handers wouldn't it be beneficial to run a slightly higher weight bias on the inner side? I suspect they've been doing that very thing with the V8 Supercars for sometime. Get those inside tyres to do a bit more work.........trikes
Yes, that makes sense. I thought you were talking about roll centre heights.

I'm amazed the number of circuit guys who don't consider that they do 360 degrees more cornering in one direction than the other. Further to that some of the corners in the other direction might be "non-event" corners, increasing the potential further.

Claude Rouelle pointed out when I did his seminar that if you take Long Beach it's worth setting a car up to brake hard into right handed corners because these are the only passing places and you can block everywhere else.

Asymmetry and non-linearity are frowned upon by most amatuer racers and people like Staniforth who write for them. Just because linearity and symmetry are easier to conceptualise doesn't make them optimum.

Ben
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Old 7 Dec 2006, 16:55 (Ref:1784259)   #12
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I don't know, ubrben, at one track this year the car went out equal in practice, tyre temps checked and camber on one front leg was adjust along with a pressure change on the opposite side. 1 second a lap quicker which is a 1/4 of the circuit over 15 laps. Car came in and temps checked again ... smaller adjustments and again a little gain.. now was this playing with the handling or just the driver bedding in to the circuit??? So there is a few clubbies out there adjusting.

On an Escort (mk1-2) the roll centers are normally a few inches apart with the front being lowest, shows up in the 3 wheeling stance in corners with the problems of an unloaded tyre and overloaded diagonally opposite one
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Old 8 Dec 2006, 17:52 (Ref:1785270)   #13
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Quote:
Originally Posted by ubrben
A high roll centre will require softer springs/bars for a given roll angle but increase the rate of load transfer.

Ben
Ben, I'm curious about this note...assuming that the CofG is in the same place relative to a high or low roll center example, how does a higher RC increase the rate of load transfer? Is this so because we now have a shorter lever arm acting on the CofG?

I'm not questioning your assertion, I don't understand the physics involved.
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Old 9 Dec 2006, 09:14 (Ref:1785745)   #14
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Originally Posted by meb
Ben, I'm curious about this note...assuming that the CofG is in the same place relative to a high or low roll center example, how does a higher RC increase the rate of load transfer? Is this so because we now have a shorter lever arm acting on the CofG?

I'm not questioning your assertion, I don't understand the physics involved.
To transfer load into a spring you have to deflect it, this takes time. In general it will occur at the sprung mass natural frequency - i.e. 1-3Hz maybe up to 5Hz on a car with downforce.

With a roll centre above ground, you are taking some of the load transfer through the suspension linkages themselves. These (should!) be considerably stiffer than you springs and bars meaning the deflection is small. The load transfer into the links generally occurs at wheelhop frequency which can be above 20Hz.

The load transfer calculations you see in books are steady state and assume a constant lateral acceleration. They won't tell you anything about how quickly the relative sprung and unsprung load transfers occured.

This is why rear roll centres are higher to increase the rate of load transfer at the rear. This is because rear load transfer starts later due to the delay between the front and rear slip angles forming. This delay will be influenced by wheelbase, tyre stiffness and yaw inertia, amongst other things. This is why there isn't a "correct" solution for roll centre position or swingarm lengths.

Ben
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Old 11 Dec 2006, 13:51 (Ref:1788005)   #15
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Thank you. I'll need a little time to digest your response. Milliken is a bit over my head in many areas - this is not my field - but I often sense missing information when I read the term ' steady state'.

Your last sentence is wonderfull and I think spot on based upon my limited experience tinkering with suspension setups. It's kind of like pulling a thread in a plaid patterned cloth; one thing affects everything else.

Michael

Last edited by meb; 11 Dec 2006 at 13:55.
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Old 12 Dec 2006, 07:48 (Ref:1788688)   #16
ubrben
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Quote:
Originally Posted by meb
Thank you. I'll need a little time to digest your response. Milliken is a bit over my head in many areas - this is not my field - but I often sense missing information when I read the term ' steady state'.

Your last sentence is wonderfull and I think spot on based upon my limited experience tinkering with suspension setups. It's kind of like pulling a thread in a plaid patterned cloth; one thing affects everything else.

Michael
Glad I could help :-)

The big problem with "steady-state" as a phrase in vehicle dynamics is that it refers to something that's accelerating!

Simply put, steady-state means cornering at constant speed and constant lateral acceleration or straight line motion at constant speed.

All the load transfer stuff assumes a constant lateral G and calculates the roll required to resist it.

Ben
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Old 18 Dec 2006, 14:49 (Ref:1793612)   #17
ubrben
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To further clarify this issue, the following is the Glossary definition of anti-roll from Damian Harty’s (Chief Dynamics Engineer at Prodrive) book on vehicle dynamics
Quote:
Anti-Roll

Anti-roll is a geometric property of the suspension that means the reaction of roll moment is reacted entirely by compression in the mechanical suspension members (100% anti-roll), entirely by the suspension springs (0% anti-roll) or a combination of the two. Less than 0% anti-roll is described as pro-roll.

One effect of anti-roll geometry is to speed the transfer of load into the tyre for a given roll moment. By modifying the load transfer differentially from front to rear, strong changes in character can be wrought, though they rarely result in fundamental changes in vehicle behaviour. This is not to say they are of no importance; race performance hinges on ‘character’ since it leads towards or away from a confident driver)

Different anti-roll geometry from front to rear also acts to provide a yaw/roll coupling mechanism. For typical saloons coupling is such that roll out of a turn produces a yaw moment out of the turn. This is given by more anti-roll (higher roll centre) at the rear than at the front axle.

Vehicles with less anti-roll at the rear have a distinctive impression of sitting ‘down and out’ at the rear when driven aggressively; it rarely results in confidence.
My emphasis added

Ben
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