Equal Length Wishbones??

[Steve-Imperial]

Just a quick query about the front suspension design of the new Imperial car:

Basically I've designed the geometry using Susprog 3-D and using a kingpin inclination of 1 degree and inboard pickups which are equal distances from the centre of the car top and bottom. This gives almost equal length wishbones.

However, I seem to have achieved the low roll centre and camber changes in roll and ride that I require. Is there a major drawback of equal length wishbones which im missing?!

The only thing I can see from experimenting with the package is that with a shorter top wishbone there's less lateral migration of the roll centre in roll. With my design it moves 200mm for a 2 degree roll, but I am using an anti roll bar to limit the roll to 1.5 degrees in a 1.5g corner and the roll centre migration to about 160mm.

Any comments would be appreciated.... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Do you think using a fairly stiff ARS on the front will cause a shedload of understeer?? Currently the front is taking 55% of the total roll couple...

[Frank]

The lateral movement goes away as you shorten the top wishbones, yes.

Too short and it starts moving laterally, the other way!

If you calculate these RC's from the centre of the contact patch, and take into account the contact patch deforming, you then require a slightly longer top wishbone, than is found with the "rigid tyre assumption"�.

I usually talk about the "ratio of widths" of the pickups on the chassis (in front view).

top width = A
bottom width = B

With no tyre deflection, a ratio of about 1.5 usually sees no lateral movement.

If you account for tyre deformation, a ratio of about 1.3 is more appropriate.

I'm not exactly sure if my explanations/methods are the most correct way to design, but I'm without better explanation ATM.

The "tyre deflections"� can, of coarse, only be estimated from tyre deflection data. Without a lateral spring constant, I assume that the lateral constant is the same as the vertical constant.

I would use the assumption of 55% front roll stiffness ratio to determine the wheel weights, with about 3 Hz FF and 1.3g cornering.

Some designs seek lateral RC migration, I fail to understand why you would want that.

Frank

[Steve-Imperial]

Cheers for the reply...

But what effect does lateral roll center migration have on handling?

So my 55% figure is good yeah?!

[RiNaZ]

i have a question frank. If you dont have the data for the tyre, how do i go about in designing the suspension geometry?

RiNaZ

[Frank]

i think the best way is to buy the tyre first, and measure the vertical spring constant

but if you don't

the vertical spring constant will be heaps bigger than your wheel rate, so it doesn't effect the spring rates you need very much

the spring rate will probably be similar to another tyre of similar make

IMO the most important thing to do is receive the tyre and check the loaded radius, especially if you don't have a lot of roll centre adjustment in your suspension

[Angry Joe]

Our 1999 car suffered from some tricky snap-oversteer due to a goof in the rear suspension points that caused the roll center to shoot out after a certain amount of roll. I don't know how severe your problem would be but you had best keep roll center migration to a minimum...



Lehigh Formula SAE Alumni
Team Captain 2002-2003

www.lehigh.edu/~insae/formula (http://www.lehigh.edu/~insae/formula)

[Steve-Imperial]

How do you measure the tyre spring rate? Is there a good and accurate way of doing this?

Im using a figure of 120kg/cm currently is that in the ball park? Anyone have any figures for Avon tyres?

I guess it depends totally on the tyre pressure etc.....