suspension geometry

Eric · 6th February 2009, 12:47 AM

i've gotten my front a arms modeled for my custom chassis, and i'm starting to get rather concerned when looking at the models. the a arms are insanely short...

i'm trying to remember my dimensions off the top of my head right now, but i know the lower a arm mounting points are 18.75 inches from the center of the car and 5.75 inches below the wheel centerline where it passes through the chassis. the upper a arm mounts are 19.25 inches from the center and 5.75 inches above the centerline. and well, the pivot point between the lower a arm and front spindle is about 26 inches from the center, and the arm itself is a few degrees lower than horizontal. do the math (as best as you can understand my description without a drawing), and the lower arm is very short and the upper arm is even shorter.

right now, i'm planning on a 7 degree king pin angle, 5 degrees positive caster, 80mm scrub radius, the roll center about an inch below the ground with the car at ride height, and the reaction point for the a arms about 130 inches out from the center or the wheel (away from the car). part of my concern here is that i plan to use heim joints with 14mm threads (hopefully not too over the top) for adjustability, and there seems to be a lot of risk of binding.

i've read up, but am still new at suspension design, and i really want it right before making anything. should i try tweaking the chassis design for lower a arms first off? depending on my wheels, i'm wondering if also perhaps trying a design with the reaction point 130 inches from the wheel toward the car since the current setup looks like a really flat version of a dune racer suspension in my opinion.

i'm terrified to think of the torrent of information that may come my way, but it's all for the better.

Eric · 7th February 2009, 02:09 AM

oh, and i remembered another bit of into. my current wheel design for the front end is 20X10 wheels with an ET 11 offset. perhaps with a new design for the spokes, i could get more positive offset since i remember seeing some insane offsets for porsches and such.

Graeme S · 7th February 2009, 05:22 AM

Hi Eric, Follow this link it will solve your problems, this software is an excellent way to check out your maths etc, there is a demo version, and is fully functional it lasts for 10 days, it works in metric or Imperial and can link front and rear suspention roll centres and anything else you can dream of and is very accurate, well worth buying. and no i don't sell it, its just a very good tool.

3D Double A Arm Wishbone Suspension Bump Steer Software Program

Regards
Lambo

Eric · 7th February 2009, 08:40 AM

that looks like it would help. i just hope i can get it running though wine on my linux pc.

Tom · 7th February 2009, 04:22 PM

Eric,

I'm curious why you would set the role center below ground - won't that force you to use more spring rate/anti roll bars to control the side to side roll of chassis? Or did you decide that you needed high spring rates anyway to get the pitch control you want?

Thanks,

Tom

Jim Cowden · 7th February 2009, 07:11 PM

Eric

I went through the same issues with rose joint bind on my car when I first started working out the arm lengths.
I don't know what my measurements would be from the center line with out working it all out on the drawings.
The arm lengths STD from the inner mounts to center of ball joint is approx 280mm
Uppers are approx 185mm.
With rose joints or spherical bearings they bound in bump and droop due to the short arm length.
High spring rate would probably have sorted this in restricting travel but that is not the direction I wanted to go.
I ended up with arms 75mm longer than STD and have no bind, I probably did not need to go this long to fix the bind problem as I was also trying to reduce the dynamic roll center movement.
What I am getting to is if your arms are similar in length to my std lengths and you want suspension travel they will bind with r/joints.
There are high angularity joints they may give you what you need.
Good luck.

Jim

Eric · 7th February 2009, 09:47 PM

Quote:

Originally Posted by spud

Eric,

I'm curious why you would set the role center below ground - won't that force you to use more spring rate/anti roll bars to control the side to side roll of chassis? Or did you decide that you needed high spring rates anyway to get the pitch control you want?

Thanks,

Tom

i'm actually trying to remember why i did that now. if i remember, my chassis design books says to place the roll center anywhere from an inch below ground to three inches above, and that lower is better.

i think possibly narrowing the front box where the suspension mounts would greatly improve things in terms of binding, and i wouldn't have to order a custom fuel tank! i wonder how that will affect my pedal positioning since they're floating in the model right now with the clutch pedal dead center. the good thing is it's all in models and i can always make copies and modify them, assuming i can ever get the analysis software to work (wine gave me trouble).