Suspension design priorities? Scrub, camber change, track width change....?

[jpndave]

I am designing the suspension for my project and have a model built and close to ready, I think. What I have so far is; unequal length parallel (at starting height) A-arms, 16-18" of travel, 7 degrees of kingpin angle, 11" of separation between ball joints (using uniballs), lower arms about 28" upper arm about 17". I have the scrub (track width) near zero through the entire range of travel but camber change is not ideal. Bump steer is zero as is steering scrub. What are the most important aspects of these? I want to keep zero bump steer for sure, the other things I am not positive about. What about caster? All of this seems like a big compromise. Do I hold the scrub and bump steer at zero, deal with the lack of ideal camber change in chassis roll and just try and minimize body roll in the corners?

BTW, I have and have read, Tune to Win & Engineer to Win by Carroll Smith, Chassis Engineering by Adams, Competition Car Suspension by Staniforth, Race Car Chassis Design and Construction by Aird, Race Car Engineering and Mechanics by Valkenburgh. What other books and sources do you recommend? Is there an inexpensive independent design computer software program out there?

Thanks for any input here, I really appreciate it.

Dave

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[Scott_F]

I would trade some side scrub and scrub radius for zero bump steer or a better camber curve any day. Your SAI seems too small. It IS a big compromise.

[jpndave]

Thanks for the response Scott. What is the SAI? What camber curve do you typically look for?

[Scott_F]

SAI = Steering Axis Inclination = A-arms
KPI = King Pin Inclination = I beams

Camber curve is personal preference. Most designs gain camber both in bump and droop, in the ~ range of 3-6*. Short course cars gain crazy camber like 10-15*.

[atomicjoe23]

Most dezert trucks are running anywhere from 12-14* of KPI/SAI from what I have seen and been able to glean from the guys that have and build these things. . .

[jpndave]

Ok, so angle through upper and lower ball joints/uniballs at the outer(steering) side is SAI which is also what is referred to as King Pin Inclination on the early axles, right? What do you recommed that angle to be? I can set it wherever, no problem. My number was derived from Chassis engineering, Staniforth and Carroll Smith's books. As I understand that angle, less is better as long as you can get the steering scrub at zero (or minimized). Adams recommends 7-9 degrees (page 46). Staniforth states 7 degrees and "the less the better" (page 108). I couldn't find the reference to it in Smith's books. I am no expert on this and would really rather find out where I am wrong now rather than later. What should the SAI be? What advantages/disadvantages are there for the steeper angle?

I think that is about where I am at on my Camber gain, I'll look again tonight. It does start to gain rapidly at the extemes of travel.

Thanks again for your comments.

[jpndave]

Thanks for the comment Joe.

I think often they go to the steep angles to get the steering scrub to zero and still clear brakes, deal with wheel offsets, and one thing I have considered is the need to separate the two joints more for strength. The steeper angle allows them to get the upper joint outside and above the top of the wheel. That would be an advantage on a 15" wheeled car, especially the 2WD versions where the lower arm is really high on the hub for clearance. On those applications, there really wouldn't be a choice but to take the upper joint outside the wheel and go steeper on the angle to keep steering scrub down. The 4x4 (mine is in this catagory) have the lower arm below center to clear the cv and halfshaft which compromises clearance but gives the separation on the KPI/SAI. One of the major reasons I decided on 17" wheels was to try and keep things tucked up inside the wheel. Am I following all this correctly?

The steering would "self center" more with a steeper angle as it lifts the car when steered more. What other reasons are there for the steeper angle?

[atomicjoe23]

I also started off with a KPI similar to what you are starting off with based on those books, but those books are biased toward road racing cars not off-road racing, something you have to keep in mind (by no means does this invalidate their usefulness. . .I used Heb Adams book extensively building the A-arms for our first buggy). . .

. . .with a larger KPI/SAI you also change the ratio of your UCA to LCA length. . .the shorter your UCA is compared to your LCA the more negative camber you will gain through a turn during body roll (and it appears that a lot, if not most, of the desert trucks have a LOT of body roll!!!) allowing you to have a better contact patch through the turn. . .

. . .suspension design is definitely a compromise. . .you have to decide what is most important for your application and then design it to suit your needs. . .I'm definitely not a pro at suspension design and am just trying to learn as much as I can and pass on what I have learned/been told to others as well. . .

[jpndave]

I feel the same way Joe. What KPI are you running now? I would really like to get things as close to ideal as I can the first time around. I can adjust the lengths of UCA to LCA to whatever works best. The KPI needs to be decided first or it will change everything else if adjusted later.

The theories all the books present seem to be valid. The problem comes in the extended travel and high cg heights that the off-road cars have. Trying to find a compromise that works almost seems an effort in futility. Minimizing weight and setting it as low and close to center as possible will work regardless. But, the height and long travel is unavoidable. Both of those are counter productive it seems on the handling. Minimal camber gain because of the long travel doesn't help in the handling dept. Long moment arm because of the high cg makes controlling roll difficult at best. I am looking at several ideas to control body roll without killing off road suspension travel. For now, I need a solid suspension geometry design. I will focus on the roll later.

I am going to try and minimize the body roll as much as I can. It makes sense that they have huge amounts of body roll as the "lever arm" between the roll center and cg would be quite large. They would need massive sway bars to controll all the roll. If the roll can be controlled, without killing the travel, things would be much easier to work out.

What do you think of a system like this?
http://www.fourwheeler.com/featuredv...ews/index.html
http://www.4wd1.com/didjano5.htm
http://aftermarketbusiness.search-au.../detail/504515

[B Page]

What about setting your suspension geometry to place the roll-center closer to the CG, as a way to help minimize body roll in the corners.

How much negative efect would this cause to other handeling parameters?