Choosing camber curves 1

[BKB001]

I am trying to design a clean sheet front (and rear, but one at a time) SLA suspension without a lot of constraints. I have been through this thread and learned a lot of valuable things, but I am still missing some fundamentals.

The suspension will go into a small light car (think Austin Healey Sprite) that will most likely be slightly overpowered and used mostly on the street with a few trips to local autoX and roadcourses.

My intentions were to provide a KPI of around 14 degrees and a castor angle of around 7 (I have heard that if KPI is twice castor that the camber curve will remain constant while turning). That may be too high, but please let me know.

The main question is about the camber curve. My first draft thought is to make the camber curve exactly follow the body roll with a little extra negative camber (1/2 degree or so) beyond. On a track width of ~45 inches, this means that at 3" compression, the tire should have around 8 degrees of negative camber. This seems excessive. Am I missing something? Is the goal reasonable?

Also, don't think that I forgot about RCs, but given the controversial nature of them on this forum I was going to leave them be for now.

Thanks in advance!!
Brian

 

[Supercar1]

> As far as wheel travel, if the car leans 3 degrees during a 1g turn, the suspension will have to compress at least ~2.8" for the outside tire to be on the ground.

No. If the track is 50", then you need 1.3" of compression on one wheel and 1.3" of rebound on the other wheel to roll 3 degrees. And that is assuming that your tires do not compress, which they do, so then you need less that one inch.

 

[BKB001]

AHHHHH.

Thanks for clearing up my blatant stupidity regarding that remark.

-Brian

 

[NormPeterson]

Be careful how you use that symmetrical +/-1.3" suspension travel. Roll of the sprung mass is not necessarily about the geometric "roll center" that's somewhat more useful in determining the front:rear distribution of lateral load transfer.

When this is done, don't be disappointed if an end-on photo shows up to perhaps a degree more roll than calculations based on suspension roll stiffnesses and roll center heights suggest. That's the effect of lateral load transfer and actual tire vertical (radial) spring rates. Unfortunately, it also represents a loss of negative camber / gain of positive camber.


Norm

 

[boatmotorgod]

racecar dataloging shows the inside suspension travels very little, while most of the travel is on the outside. I would figure 2-3" in compression depending on spring rate. To give a short car high speed stability I would use 6-8 degrees of camber. High amounts of KPI coupled with high amounts of caster realy adds lots of positive camber to the inside tire on tight turns around town. Are you going to build your own spindle\brake combination? or start with a fabricated spindle, or buy a compatible OEM product? The last 2 choices will only give you a range of KPI & stearing arm choices as your starting point.

 

[GregLocock]

And on what sort of car is that? I'd be very wary of generalising from one racing series to another, for instance NASCAR frequently use straps to prevent the inboard suspension from going into rebound, yet the somewhat similar Australian V8 supercars tend not to.



Cheers

Greg Locock

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