Mix
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 posted on 11/1/05 at 03:47 PM |
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IRS camber change
Hopefully someone will be able to help me.
I'm playing around with CAD and my IRS at the moment.
I have set the length of the lower bones and am now looking at finalising the upright and upper bone design.
I'm working 'backwards' to define the length of the upper bone. I was quite surprised to see how much a small increase in camber
change affected the length of the wishbone.
What sort of camber change should I be looking to incorporate?
I plan to use the car for road use only.
Sorry if this a bit vague but it's all new to me.
Cheers Mick
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Bob C
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| posted on 11/1/05 at 04:08 PM |
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Yur basic camber change control is "swing axle length" (SAL). If you extend the lines joining the top wishbone pivots (outer & inner)
and the bottom ones, these lines will cross somewhere at the other side of the car (unless you're making a tiger avon). The distance to the
crossing point is the SAL. 7 or 8 feet seems common. This is controlled by the vertical separation of the wishbone pivots on the chassis.
You can then mess with the length of the top wishbone, and the vertical position of the chassis pivots to control the roll centre. If you want - there
are some on here who reckon it's not worth it, others (like me) who don't really know but can't see any reason not to!
I enjoyed the research & messing about on this part of my build, & there are a few guys who'll help if you get totally flummoxed!
Cheers
Bob
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Mix
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| posted on 11/1/05 at 04:20 PM |
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Good info thanks.
I presume you measure the SAL at ride height.
I've done drawings that, (unintentionally show this).
I agree I am finding this part of the build very satisfying if not frustrating at times.
Don't suppose that you have any figures do you? Ball park will do 
Cheers Mick
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pbura
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| posted on 11/1/05 at 05:32 PM |
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Mick,
If you haven't used this program before, it's terrific for checking your work:
Wishbone calculator
It seems that the newer IRS cars (the Elise and the new Caterham, for example) have rather low rates of camber change in bump, on the order of .4 to
.5 degrees per inch of bump. This gives them better ability to put the power down straight ahead. They also have substantial anti-roll bars to
minimize roll.
The Elise (dunno about the Caterham) uses tires specially designed for it, that tolerate large amounts of negative camber. This eliminates concern
about the outside wheel generating large amounts of positive camber in roll. IIRC, static camber is set at something like -3 degrees.
For my own purposes, having no fancy tires, I'm leaning toward a swing axle length of about 100". This gives about .6 degrees camber
change per inch of bump, so at 2" bump and with static camber of -1 degree, camber will be about -2.2 degrees. At 2 degrees roll, the loaded
tire is still quite vertical (less than .5 degrees positive), and the inside tire is about -2.5 degrees. I think this range is a decent compromise
for a road car.
If you have a particular tire in mind, it's good to research a bit to try to find out what its optimal grip range is. For example, from
comments I've seen online, Yoko A032Rs seem to like -1.5 degrees.
Working in CAD and Wishbone together throws a ton of light on the subject. CAD is great for determining your pivot points and checking your final
layout, and Wishbone for squidging your pivots around and testing under dynamic conditions.
All IMHO, of course!
Pete
Pete
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britishtrident
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| posted on 11/1/05 at 05:35 PM |
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For an upright of "average" height start experimenting at 2/3 the lower wishbone length. Cars such as current F1 cars which are ultra
ultra stiff can use almost equal length wishbones, but if you look at older F1 cars on which the suspension actually moved for example a Lotus 33 or
49 you will see they used closer to 2/3 the length of lower wishbone --- a Locost is quite close in suspension requirements to car of that era.
To avoid jack effects a long virtual swing axle length is a good idea but not too long or it will spoil the lateral grip -- 6 to 12 feet in the
normal range but a lot depends on the roll centre height --- avoid the "Triumph Herald tuck under effect" caused by short swing axle
length combined high roll centre.
The rule of thumb is is a high roll centre in similar (but with more side effects) to adding an anti-roll bar -- it reduces roll at the cost of
grip.
With a rwd IRS the rear roll centre is generally higher than the front -- the front end is then stiffened (hence most rwd cars have an anti-roll bar
at the front) to trim out the oversteer.
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