All right, I've made some more progress, but my documentation is lagging, so I'll try to get some pictures up in the next couple of days. In the mean time I've been getting ready to tackle the front suspension. I had read about the suspension calculators, and tried to understand all of the different terms, read through the Staniforth book. I am pretty sure I've understood almost every major section about suspension geometry at one time or another, but each new term seems to push the old one out of my head. Also I had not downloaded the actual program, so most of the discussions I read didn't make too much sense to me. The other day I finally got a windows computer working and downloaded the Wishbone software and tried to make a little sense out of it. Well today I worked on my suspension calculations all day, I read through all of the posts I had read time and time before. The information is out there, but its not set out in a step by step fashion to go from nothing to designed suspension easily. Everyone has a different level coming in, and different goals, and like they say its all a compromise even with the "best" suspension. I read several times that your calculations can have a lot more accuracy than most (me) can hold in the actual building process. I'll try to go through the steps I took, and document as well as possible, so someone else could follow all of the steps, but I'm writing this after I've spent a lot of time researching, and I'll probably skip over some stuff along the way. This is not a entirely to teach, especially since I'm not at all confident that I've done this "right". I'm hoping some people will step in and either confirm or critique what I've done to help me build some confidence, or enough knowledge to do it again hopefully better.
Extremely condensed quick and dirty solution:
1: Make both your lower mounting height on your chassis the same as your lower balljoint
2: Spread out your lower locations on the bottom of the framerail at the same distance from the centerline say about 1 foot apart
3: Make your upper arm length 2/3 of your lower arm
4: mount your upper arm locations where they fit with that arm length on the up/down rails at the front of your frame.
5: rotate your upright so that the lower ball joint is 7.5 deg in front of the wheel centerline (caster)
6: lean your tire in .5-1 deg (static camber)
With little/no calculations this should give you a decent suspension
Long version (detailed explanations, maybe not too much better thought out)
I read Allan Staniforth's
Competition Car Suspension a Practical Handbook 4th edition It was a good book and I got some of my suspension targets from it, but I think you could design your suspension without it if you don;t want to get a copy, but its a good book, and I'll be keeping my copy.
I downloaded the Wishbone analysis program by McDermott (its in a link in the post below)
viewtopic.php?f=5&t=1438
I had been paranoid about viruses, and my computers weren't running well, so I hadn't tried it before, but since so many people had used it, it is apparently not a virus, so I gave it a shot, and it was fairly painless. It doesn't want to run on Linux Ubuntu, so I had to install it on my non-internet windows machine I keep around for just such occasions, but supposedly if you are smarter/care more than me you can find linux workarounds further down the link above.
The file you downloaded is a setup file. Go ahead and click through its installation and you get the executable file.
When you open Wishbone you start with their example file and edit it. Do not type example.wsh it won't open, just type example.
People rightly complain about screen resolution, but if you hit enter get a starting set of results then hit F6 and enter a couple more times the input screen comes up full screen, and is much easer to see.
Starting in Wishbone you need the following measurements for input in your file.
Tire Height
Track Width (can be changed, but take a guess by starting with the same as the rear)
Wheelbase (can be changed, but take a guess by measuring where you think things will go)
upper and lower ball joint
(locations height from ground, distance from centerline of chassis, distance in front/behind the center of the wheel)
"ball-joint" on steering arm, same format.
measure the height of your frame off of the ground as a point of reference
-Remember when you are taking measurements you are measuring to the center of the ball-joint where the bend is really happening)
You will be adjusting the following inputs to design your suspension.
upper and lower front and back mounting locations
toe control location
static camber
static toe
Inputs that you probably won't actually know, but will just leave filled in or make a guess
CG height (maybe just assume its half way up the height of your engine?)
CG long (if you can put your car on scales take the weight ratio and multiply it by the length of your car)
Now you know what's required to input into the computer program to get your output, but what kind of output are you looking for?
Goals
Caster 5-9 deg (I shot for 7.5) This will make your steering wheel self center when you let go of it/makes your steering heavier
You control this by rotating your upright so the lower ball-joint leads the upper. (you could do it backwards, but that the wrong way, you are supposed to have the lower in front.
KPI 5-9 deg (mine has 10.xx) this is built in to your upright and is not adjustable, so although you might want to, you can't change it without changing your uprights, which would be cheating
Camber .8-1 deg of additional camber per deg of roll (your tires will have an ideal amount of camber they are designed for, but if you don't know, or don't even know what compound you will be using, you are just left with the initial rule of thumb. Initial camber will pretty much add that much throughout the range of roll +/- a little.
Roll Center you want this from 1 - 4 above the ground, maybe lower, but this is the range I was looking at for a performance not race car. The main thing you want is to see the roll center height stay in the same location as you change the roll on the car.
upper
Its at this point while I'm looking for the documentation of where I found these "rules" that I found the following link which is appropriately stickied in the suspension section. It has most of the thoughts I have above.
viewtopic.php?f=5&t=1227
These were a few goal that I found looking through the forum, and reading Staniforth's book.
To meet these parameters you can change:
Track Width (apparently Caterhams are a little narrower in the front)
Suspension inboard mounting points
Location angle of the upright
also you will need to mount steering rack which will affect bump steer, but that wil come later (since I haven't dealt with it in my design yet)
Where to start when you are plugging in to Wishbone?
Go back to my quick and dirty solution and use it to set up your initial mounting points.
Change the height and width of your upper mounting locations to control your outputs.
You don't want to start messing with the steering rack locations (toe control) till you finish locating the arm mounting points.
The steering rack location will determine bump steer, or your goal of lack of bump steer
Staniforth's rules are (I am using a forward mount steering rack)
Bump Droop Rack Adjustment
toe in toe out raise forward
lower rear
toe out toe in lower forward
lower rear
toe out toe out lengthen rack
shorten rack
toe in toe in shorten rack
lengthen rack
remember you can also add some static toe to change your outputs
I think that is the best I have gathered.
Here are some of my outputs, please let me know what you think and why so I can either add some more goals or change my process in some other way.
I'll try to keep updating this post with additional information as I change my design
Sorry it got so long
Thanks,
Sam