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SleeperBug
07-27-2004, 06:55 PM
Greetings all! This is my first post.

I live in the states and want to build a fast (light), smog exempt sleeper. This means I must use a car pre 1973.
I decided on the vw bug. I want it to be a clean euro look on the outside, but with a radical mid engine square tube frame underneath the skin. Only the body will be vw, everything else will be aftermarket or custom fab'd.

My idea:

Frame - Chrome Molly Square tube frame.

Suspension - IFS / IRS rear. Probably strut front / rear due to space.

Engine/ tranny - Acura TL v6 powered (270 hp / 240 ft-lbs) or Acura RSX-S i4 (200hp).

Plan - set the engine / tranny mid engine over rear axle, protruding into original rear firewall area.

Cooling via radiator in the front.

Custom fab floor pan and front rear firewalls.

Goals, <2000lbs curb wight, emphasis on handling. Street sleeper / smile maker.

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About me:
I have worked as an auto fabricator, also have machine shop and welding experience.
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I need help with the suspension part. I am lousy at math and don't know where to begin. I am familiar with basic terms such as Steering Axis Inclination, Scrub radius, camber effects etc.


I need help with the figuring out of where the suspension points should mount to the frame. I can weld machine what I need. I need to learn things like, should front a arm mounting points be even (level) or not (dive anti-dive); how does the angle of the strut on the lower control arm to the mounting point effect suspension geometry, etc...

Any help or ideas would be extremely appreciated. Any suggested books or links (for the math impaired) would help too.

Thx

-SleeperBug

KnifeEdge_2K1
07-27-2004, 08:44 PM
err this is off topic but is it possible to change the type of suspension on a car while keeping the original mounting points ?

like from macpherson struts to a double wishbone setup?

i know most of the theoretical stuff bout suspension design but no application

obviously you CAN do it, but what im asking is how much work would be needed to convert

SleeperBug
07-27-2004, 09:22 PM
Well the original mounting points are very week as is the frame. With the car originally having something like 45hp - then after a few years a bump up to 60 something hp I would think that any amount of traction would bend all kinds of stuff. Plus the car originally had torsion bar rear suspension, yuck!

Basicaly I want to lower a old vw beetle body onto a new rolling chassis with matching body mounting points.

I know that things like the pitch of the lower a arm can effect dive / anti-dive. I just don't understand the relationship between cause and effect that well on suspension design.

I would like to use production suspension parts, a-arms, spnidles, etc. Maybe just modify the upper mounting points of the struts. I was thinking of trying to use an MR2 or some other already in production cheap mid engine car. This way it's roughly set-up for the mid engine weight etc...

Matra et Alpine
07-28-2004, 03:11 AM
Basicaly I want to lower a old vw beetle body onto a new rolling chassis with matching body mounting points.
This has been done so often to Beetle's over the course of its long life that I'd think you'd be able to get a good basic structure from VW owners clubs etc.

I know that things like the pitch of the lower a arm can effect dive / anti-dive. I just don't understand the relationship between cause and effect that well on suspension design.
Do you want to ?
Your optoins are to take a working suspension and build chassis muntin points to take it - so getting all the benefits the original car had.
Or develop your own suspension - making it better tuned to your needs.
BUT, you will have to consider how your local vehicle licensing laws work. In the UK, if you build it all yourself, you MUST get a written engineers report that it's sound and that can be expensive for 1-off. Using donor parts reduces that effort and cost.
If you DO want to develop it yourself then it needs a LOT of study. There are many good books out there on racing car design which covers all aspects. There is VERY little on road car design - so you may not design something easy to park for example ( caster angles ) !!
As well as study, it usually involves trial and error. I know guys who've built their own grass-trackers and autotest cars and they usually went through 2 or 3 iterations of suspension and chassis before they got soemthing 'right' :(

I would like to use production suspension parts, a-arms, spnidles, etc. Maybe just modify the upper mounting points of the struts. I was thinking of trying to use an MR2 or some other already in production cheap mid engine car. This way it's roughly set-up for the mid engine weight etc...
Good choice - so you were probably already aware of the pitfalls above :)
Some still exist as taking an original suspension and chanigng it's height and distance-to-centre mountings can alter the characteristics.
Why dismiss torsion bar ? ( Especially if you have contacts for steel and machining ) It was good enough for the 911 ( and my Matra's )
The fundamental drawbacks of torsion bar come if you want large amounts of movement. In a 'dropped' car that's not an issue. The it's down to being able to source steel with the torsional compliance required. That's a specification task and maching job to determine the right lengths. The BIG advantage torsion bar brought was compactness.
However, with modern low-cost VERY good coil-overs, I'd go thate route too :)


To the design and fabrication.
There are 2 approaches ....
What I've heard called the "hill-billy" approach which is to slap some steel on in roughly the right place, weld it up and if that doesn't work, weld another bit of steel on until you get get it working ( Jesse James of bike building fame ). So don't do too much design and build it-situ to match the bits you've got.
Secondly, with the advent of cheap 3d software you can model the whole thing on a PC. A guy near me does kits for Vipers and it is an excellent piece of enginnering and construction. He did the WHOLE thing on the PC before he made a single cut, weld or lay-up. To do this you want to get all the measurements for the suspension you intend to use ( this can either be done by measurement or from manufacturers bodyshop repair documentation ). Then you can design a chassis to those measurements, see how it fits together, check clearances etc. The Viper guy reckoned it took him 3 months to get his done, but that included body parts.

If you want a car to use on the road with others then safety of you AND OTHER ROAD-USERS becomes VERY important. Do you have the confidence to be able to do the structural design of a chassis to withstand the road shock and not crack ot shear ? What about impact ?? For these reasons I'd suggest contacting a VW club and ask opinion and seek professionaly designed chassis. ( Again with a little expenditure in software it's possible to do stress analysis on a chassis to 'test' for strength. Kind of doing the "hill-billy" stuff but on computer :) If you're only going to do track and closed roads stuff then some of this is less important - one of the grass-track designs I've seen the guy says he has to replace one of the cross-members on the chassis "regularly" because it bends !! That's OK if it's only YOU at risk, unacceptable if there are others.

Wow, feel like I've written a lot and only told you the difficulties and problems and how hard it is. Counter that with there is NOTHING as satisfying as getting something you deisnged and built onto the road/track.
You've lots of study ahead of you to make this work but hopefully can contact some local guys who can help. BTW, some of the 'best' fabricators are hot-rodders, so look up some of those clubs too.

Good luck and ask any question you like, You'll find folks here on UCP who've some experience in almost anything and willing to help.

EDIT: For the ultimate VW 'sleeper' do a google on "uniroyal team challenge". This is a Beetle look-alike spaceframe chassis race car here in Europe. You could get some ideas from them on the design :) [ It's not a complete Beetle shell though, it's just made to look like one ]

SleeperBug
07-30-2004, 12:20 AM
Thanks for the reply!

This would be a street driven car. I choose pre 1973 because there are NO inspections for the vehicle (there are laws but they are not checked), unless I move my residence to another state. So I can be as creative a s I want...

Since this is not a full race car were it has to have the lightest possible everything built only to last the duration of a single race. It will have a little extra "beef" in the chassis, etc. I want durable parts at the sacrifice of having a little extra weight.

I want to use stock or aftermarket a-arms. (I don't want to fab them myself). I just want to change the frame mounting locations to fit around my engine, and provide better then oem geometry.

Ride quality is not as important as actual performance. I want crisp predictable handling, so rubber bushings and the like can be left off my suspension.

I am having a hard time understanding the relationship between "roll center" (front and rear) and "a-arm angle".

Can anyone explain this in simple terms and then follow it up with a little science/math?

Thx

-Paul

Matra et Alpine
07-30-2004, 02:45 AM
I am having a hard time understanding the relationship between "roll center" (front and rear) and "a-arm angle".

Can anyone explain this in simple terms and then follow it up with a little science/math?

Thx

-Paul
Phew you don't ask for a lot do you :)

OK, all the possibilities for different suspension layouts and impact during cornering takes a few chapters in a book. So I'd advise going to local library and getting them to get in a racing car design book. There are lots of them around :) Mine are out on loan to mate who's building a track-special Locost :(

I think the point you've got a question on is with respect to the centre of gravity of the car chassis and the centreline of the suspension operation. So if the arms are parallel to the ground, if the CoG is equal height to the centreline of the suspension then the car will be exceptionally stable in cornering as there will be no forces altering the suspension gemoetry.

If the arms are angled downward, then the chassis CoG is now ABOVE the centreline of the suspension.Now on cornering the CoG of the body will exert a force ABOVE the suspension c/l and this will result in a tilt of the chassis within the compliance of the suspension and the subsequent change in angle of the wheels can be setup to IMPROVE the tyre grip ( or in a 'bad' design to REDUCE the grip )

Different susspension diesgns use different techniques to alter the camber and caster to assist in the handling of the car. Too many to go into in a medium where I can only type, where a whiteboard and discussion would be best.

So I advise again to use local library and study.

Oh and there is NO "RIGHT" answer. There are hundreds of possibilities each with pros and cons. You make your own choice what you want :)

The math on suspension is actually relatively simple as it can all be done with repitition of static vector forces. So with vector maths ( sin/cos ) and arithmetic you're sorted :) The difficulty is the variables and measuring/creating them and the options in differnet compliance of springs and shocks. For 'self-build' it will get difficult if you plan to use variable rate springs and/or dampers

Matra et Alpine
09-29-2012, 01:24 PM
Seems as easy to resurrect this VERY OLD thread with this updated info.

Came across a great tool while investigating the setup on the OMS single seater and how to improve for hillclimbs over track/sprints.

Only "simple" suspension, but a great start on one plane of the problem :)
racing aspirations Suspension Geometry Calculator (http://www.racingaspirations.com/?p=286)