Annoying suspension geometry questions

[tjakeman]

Hi Everyone. Firstly let me apologise for asking the same questions as many others will have done before me. I'm struggling a bit with lack of experience.

I'm designing the suspension for a light weight sports car:

700kg
300 bhp
front engine
rear drive
twin wishbone front
twin wishbone rear
14" wheels

It will be used both on the road and track but never for racing. I intend to use a performance (road legal track tire) such as Toyo R888/Yokohama A048 etc..
It will be possible to use a slightly wider tire on the rear (but the wheels will have the same rim width)

I have design freedom for all inboard suspension points front and rear, and so roll centre positions etc.

I have a CAD assembly of a front and rear corner and a Suspension kinematic model in WinGeo.

Where I'm struggling is to decide on inboard suspension positions without tire data. I understand that the influence of springs bars and tires are far more influential than subtle changes in my geometry, I'm just trying to make sure I have a good starting point. For now I've just included roll and ride iterations below:

The front suspension is a work in progress but at present gives very similar results to the rear in terms of camber gain and roll centre migration (ignoring the affect of caster and steer angle for now)

Shot at 2012-08-14


Questions:
Does this look sensible?

Should I try and decrease the positive camber of the outside tyre at full roll?

How much anti squat should I aim for? (at the moment there is none, I have a SAE paper suggesting 80-90% for a passenger car... )

How much roll axis inclination is sensible, at present my static roll centres are:
front Z - 38
rear Z - 50
I could probably get the front on the ground with some more work.


Thanks all for your help. I'd love to be about to move on from this stage soon and start building, just want to avoid dropping a clanger..

tom

 

[GregLocock]

Your camber curve looks very dodgy, do you really want your outside wheel going positive?

Cheers

Greg Locock


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[tjakeman]

Hi Greg,

I really don't, but is it actually possible to prevent that on the rear without using a lot of static camber? (this is my first suspension design, still not totally clear what is achievable)

I read at some point that ball park camber gain for a performance car is 0.6-0.9 deg/inch bump, which I'm achieving.

Interestingly I just found this data for the rear of a Caterham SV

Shot at 2012-08-14

Their camber change per degree of roll is higher than mine!

 

[tjakeman]

Oops, wrong link, try this:

Shot at 2012-08-14

 

[GregLocock]

Ah hang on are you measuring camber relative to ground or vehicle?


Cheers

Greg Locock


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[tjakeman]

I'm measuring relative to the ground, I assume they did the same..

 

[tjakeman]

I misread their graph, camber change per degree of roll isn't as bad as mine, but its still potisive after ~2.5deg body roll.

 

[GregLocock]

OK, that makes more sense. I measure relative to body hence the confusion.

I don't see anything especially alarming there.

I see no point in dropping front roll centre to ground level, it should either be below ground level and stay there or above, and stay there.

Cheers

Greg Locock


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[tjakeman]

Thanks Greg.

I tweaked a little more and the rear RC is now ~60 with front ~38.

Whilst I have your attention, can you comment on anti squat? I'd like some, perhaps 15% but this has introduced significant bump steer. Is the game now to move the trackrod out of plane with the lower wishbone, and use its arc to try and cancel the bump steer?

 

[GregLocock]

I'm not, particularly, a huge fan of antisquat. If you are using stiff springs and your cg is low, you don't need it. I've never been very comfortable with the %age definition for a/s, I'd rather work back from a desired maximum pitch angle (or suspension deflection) at maximum acceleration, than a one size fits all %age.



Cheers

Greg Locock


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[FennLane]

Surely the Caterham has a DeDion rear Axle?

I would not be enthusiastic about anti-dive or anti squat on a race car.

 

[140Airpower]

FennLane, The last DeDion axle I am familiar with in first class racing cars was tried by Chaparral circa 1969, but it was an articulated DeDion with a vertical hinge in the middle. In F1, the last one was in the '50s. F1 cars are fairly uniform in having upper and lower wishbones front and rear. It's a matter of packaging as well as geometry. Look at the fronts, bad geometry, good packaging.