violentblue
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| posted on 8/9/04 at 08:21 AM |
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Suspension adjustability
I'm trying to design a suspension setup for my VW beetle. I'm not satisfied with the stock IRS rear and paralell trailing arm front. so
its time to do something different
I know what I'm going to do with the rear
I'm going to base the rear on these dimentions
for the fromt I'm planning on using the beetle balljoint style front spindles. my question is,
Is it better to have the adjustability on the top or bottom A arm?
Or juyst do all your calculations ahead of time and weld them solid?
Now I plan to take my initial drawings andmeasurements and run them through a suspension program to see if they check out, but what are the most
critical measurements to have correct?
what measurements can I just "wing it" with and correct later with fine tuning.
a few pics of my other projects
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pbura
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| posted on 8/9/04 at 12:01 PM |
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IMO, you should crack a couple of good books about suspension design. Personal recommendations are Alan Staniforth's 'Competition Car
Suspension' and Carroll Smith's 'Tune to Win'. Best to read at least a couple, because the authors agree on the important
stuff, but differ in areas in which you can develop your own opinions.
There are many choices to make based on your intended use of the car and your own taste. I think this is the most exciting thing about DIY
suspension, that you don't have to put up with someone else's interpretation of what you need. For example, US cars have buttloads of
understeer to keep you safe, while you can design a drifter if that's what you want.
Important dimensions to have beforehand are of your upright and tire: upper and lower pivots in relation to centerline and wheel mounting face, wheel
size and offset, tire width and rolling diameter. You'll also need to know your minimum chassis width and ride height.
The best affordable design software is SusProg3D, IMO. You can start doing 2D basic layout and geometry, and later progress to modeling weight
transfer and roll resistance, yaw, and stuff like that. There are also some functional free programs for geometry (search "suspension
software" in this forum). I've tested SusProg3D but use the freebies myself.
I don't think that rear suspension design is very good, because of the parallel A-arms. The outside tire will generate large amounts of
positive camber in roll. Also, the roll center would be very high, causing the rear to skip sideways on bumps in turns.
Will comment on adjustability later. I don't advise winging it now and adjusting later, though. It could work out that your range of
adjustments could be no good, either, without a design.
Pete
[Edited on 8/9/04 by pbura]
Pete
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pbura
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| posted on 8/9/04 at 01:05 PM |
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Adjustability
It's very normal, and highly recommended, to have the top wishbone adjustable for camber. However, I think you mean having adjustable
suspension geometry.
Really not hard to do. Just make your wishbone brackets out of channel so that you can have alternate mounting holes. For a racer, this would be
sort of cool for having alternate setups for road courses or autocross, for example.
When you change geometry, though, you will also change your bump steer, so you will also need to be able to adjust the position of your steering rack.
Changing geometry will also change the way weight transfers between the wheels in cornering, so it may be necessary to mess with your springs,
shocks, and anti-roll bars.
For a high-end racer, this can make a lot of sense, but it's extreme. If you're not happy with your car's handling, there's a
lot you can do with tires and springs before re-doing your geometry.
There's no harm in having a couple of extra holes in your brackets. However, if you start out with credible amounts of camber change, and
reasonable placement of roll centers (looking over my shoulder for Syd  ), these are probably the last things you'll probably seek to
change.
Pete
Pete
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violentblue
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| posted on 8/9/04 at 03:17 PM |
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the car is/will be a purposed built autox-x/show car
so the suspension dosn't really need a whole lot of adjustability, Just enough to get it dialed in.
I'll definatly go buy a suspension book today, hopefully Chapters has one of those two books.
as for the rear suspension design, the drawing shows paralel A arms, but the dimentions shown tell a different story, I think it is drawn that way for
simplicity and so as not to confuse anyone's math by taking into account the angles.
a few pics of my other projects
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violentblue
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| posted on 8/9/04 at 03:50 PM |
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after modeling the rear suspension design and playing with the angles a bit I see what you mean by paralell A arms, though they arent physically
paralell they do act that way (0 deg camber change, or at least very little)
a few pics of my other projects
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pbura
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| posted on 8/9/04 at 04:37 PM |
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See what you mean about the drawing; it's not even to scale. Doesn't tell where to place the wishbone brackets, either.
Amazon's good for the books if you can't find them near you. Might want to check prices before you go shopping. These titles are about
$20 apiece, I think.
Here's a cool website to get you started:
Build Your Own Race Car
Don't be too bummed if the material seems unclear, conflicting, or otherwise difficult at the beginning. A lot of guys say it takes a good
couple of reads before the light starts to dawn. This was my experience, too. 
Pete
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