Why do auto makers design the a-arms with a shorter arm on the top?

Karl

well i know with the race cars i work with that have excess of 24in of wheel travel the equal length will cause bumpsteer through the full cycle of the suspension with un equal length the bumpsteer is very minimal so that is why the manufaturs use them less problems down the road equal length do work just alot of work with the spindles lots and lots of geometry sharpen that pencil

hmmm thats a good question...

1. metal, if they saved 2 lbs on each arm, 4lbs for each car, multiply by 100,000 cars thats 400,000 lbs of metal saved.

2. in my eyes the upper arm just keeps the spindle straight and the lower arm takes all the punishment,

3. the lower arm has the torsion bar connected to it.

4. upper sits on the side of the frame rail, lower sits under it

the exact reason why it is shorter is unknown to me, i dont know what exact benefit that apply's. To me it seems right to make it longer, i dont know why, it just looks and sounds better to make it longer and stronger.

kevan

i know most of my answers are going to be quoted <img src="/forums/images/graemlins/scared.gif" alt="" /> <img src="/forums/images/graemlins/smile.gif" alt="" />

OK...Let me clearify the question so as to narrow down the responses.

I'm thinking purely from a geometry standpoint.

The shorter arm will have a smaller arch, thus pulling the top of the tire "in" as the arms become unparallel. Why do the designers want this to happen?

KM

ok, you need an engineering degree to answer this one, but I'll try to remember from what I was told (BTW, scott at rockstomper is really good at this too, he built his own control arms for his long travel IFS toyota).

you can get more :edit: equal (meaning less wear on the tires, keeping them flat) :/edit: travel out of unequal length control arms. <img src="/forums/images/graemlins/smile.gif" alt="" /> see there you have it. <img src="/forums/images/graemlins/confused.gif" alt="" />

hehehe, I'm not qualified to say anything past that <img src="/forums/images/graemlins/wink.gif" alt="" />

Ok, i have a degree, well will have a degree in engineering, but its still not easy to describe. I drew up a quick picture on autocad to show what happens when the suspension cycles up and down. The picture didnt seeem to come out too well, but I will try to edit it later, but just for arguements sake, here it is.

Dan
And the picture didnt work...

Here you go:

http://www.allpar.com/ed/suv-suspensions.html

"Unequal length control arm suspensions are the most complex of the designs considered, but also, potentially, the most "tunable" thanks to the isolation and adjustability of the various dynamic parameters, such as anti-dive, roll control, anti-squat, travel, etc. This very complexity entails analysis and determination of desired handling characteristics far more demanding, however. All of the manufacturers of Class 1/2/3/4/5 trucks have utilized this basic design over the years, but cost of the components has continued to be a justification stumbling block.

The term "unequal length" comes from the fact that, by design, the pivot length of the upper control arm (UCA) is shorter than the lower control arm (LCA). This is measured between the pivot axis and the ball joint, forming the triangle or "A" shape. Usually, the LCA is the longer of the 2 arms and travels through the lower measurement chord arc height. The UCA, being shorter, travels through a shorter chord arc height. (This means the chord arc height is measured horizontally.) The upper control arm moves the top of the tire a greater distance laterally in the vehicle than the lower control arm moves the GCP. This means, in a dynamic condition, the the tire wobbles at the top a lot more than the ground contact patch. This instability is what must be controlled by the geometry design to prevent a vehicle roll-over condition and directional instability."

I'm sure a mod will chime in to clarify here, Ive seen this discussed in another post. Try the search, it might be buried though.

This is what I remember. Equal length A- arms will keep the camber consistent throughout the suspension cycle, but the problem is that the wheel moves out farther from the chassis as the arms get to be more level with the ground. So, under constant cycling the wheel gets pushed in, out, in, out. This wears tires down a whole lot.

Unequal length a-arms change camber throughout the cycle, but dont move in and out, for better handling and tread wear. They do all sorts of nasty calculus to get this to work, IIRC.

This is from what I read awhile ago. I may be screwing it all up, so HTH. <img src="/forums/images/graemlins/zombie.gif" alt="" /> <img src="/forums/images/graemlins/cyclops.gif" alt="" /> <img src="/forums/images/graemlins/scared.gif" alt="" /> <img src="/forums/images/graemlins/frown.gif" alt="" />

The way I understand it, its for handling.

Two equal legnth A-arms create a paralleligram. This keeps the camber of the wheels constant. This gives you a flat foot print on the tire at all times. This is great for heavy travel suspensions (long travel) with vehicles that go in straight lines (dune buggies are a good example).

Unequal A-arms are able creat a camber efffect when turning right // or left \\. This gives better traction thru a turn and better handleing.

Well thats the way I understand it <img src="/forums/images/graemlins/scared.gif" alt="" />

Scott F.