Front Suspension Capstone Project 1

[OGR34]

Hey guys,

I have been assigned a Capstone project involving the design of a front suspension system for an Intermeccanica Speedster (shown below)

The current front setup of the vehicle consists of a classic VW transverse torsion bar suspension. My partner and I intend to replace this outdated setup with an SLA double wishbone type suspension. Links, springs, dampers etc. will all be purchased. The suspension geometry will be designed by us and the springs and dampers will be selected to achieve engineering requirements. Ultimately we would like to achieve a balance of good comfort and performance however, we're not quite sure of what engineering requirements we would need to achieve this. Currently I have very little background with suspension systems and have started reading some automotive textbooks, namely Race Car Vehicle Dynamics by Milliken. The book is very descriptive and reading it has allowed me to become familiar with most of the key words of terms of a front suspension system. However, in terms of starting a design, I have absolutely no idea where to start! The amount of parameters in suspension geometry is quite overwhelming and right now I'm trying to figure out what my starting point should be in the design process as well as how I can piece all the information I've learned to achieve my goal. Does anyone have any suggestions? Any help is greatly appreciated.

 

[BUGGAR]

Read all the books you can, of course. Then check out websites like locostusa.com that have discussions on how suspension works and even have a computer program (Vsusp) that you can plug in your suspension dimensions and watch the wheels wobble.
Your final exam will cover Olley's rule.

 

[GregLocock]

do not cross post

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[OGR34]

Sorry! I'm quite new to the thread

 

[BUGGAR]

Sorry also

 

[GregLocock]

Buggar -it wasn't aimed at you, responding to x posts is a waste of time for you, the main thread is at AE other topics.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[pwildfire]

The process is this:
1) Identify geometrical limitations of the connection points at the frame and knuckle. Also identify desired suspension travel and available spring sizes, rates, and configurations. You will most likely want to use coilovers as they are easiest to package.
2) Identify desired camber gain rates and static roll centers at ride height. In your case since you are only doing the front, a good starting point is to put the roll center at the center of the car, and half the height of the rear roll center. Camber gain depends on the width of the tire and the intended use of the vehicle. If possible you would also look at tire grip curves, but you are unlikely to get those for a DIY project.
3) Identify desired kingpin inclination, caster, and scrub radius. If you are making your own knuckle you will need to read up on this, if not you are stuck with what you have. Also identify steering rack location, travel, and angle.
4) Draw in knuckles at track width, and using the information from (3). This will determine a range of locations where it is possible to put the ends of your control arms.
5) Use results of the above to draw in links. Depending what variables you have, you will have more or less freedom. For example, if your possible LCA mounts are fixed, you can pretty much set the LCA angle and will have to use the UCA length and angle to achieve the RCH and camber gain you want.
6) Once you have front-view links sketched in, you need to consider anti-dive, which is the side-view angle of the control arms. Then you can sketch in the arms, and figure out how to package them so that they don't interfere with other components. Also do a top view and sketch in steering arms, keeping in mind desired steer angle and rack travel, etc. Front view tie rods need to be radially aligned with instant centers to avoid bump steer problems.
7) Now determine where to mount both ends of coilovers, including motion ratios.
8) Iterate 2-7 as necessary until everything works.
9) Now you can choose spring rates, damping ranges, bump stops, anti-roll bars, and package it all together.
10) Finish mechanical design of all components using some assumption from (9) for maximum loading. Don't use rod ends in bending, use ball joints and spherical bearings where possible, etc.
11) Build and test, revise as necessary.
12) Speaking of (11), leaving room for adjustability in your design is a pretty good idea.

Reading: Milliken, Fred Puhn's "How to make your car handle". I haven't read Carrol Smith's "Design to Win" yet, but I hear it's good. None of these will tell you everything you need to know, but you can piece it together.

 

[BUGGAR]

GregLocock,
Are we allowed to respond to this?

 

[GregLocock]

You can respond all you like but it would make more sense to do so in the main thread

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[dynatune4xl]

Designing a front suspension from scratch is not an easy exercise and most certainly not for a novice. I do not know what your timing horizon is but in case you are under a certain pressure to deliver a result I would look for some expert of your trust for guidance on what to do. I would also do some proper "benchmarking" by looking at some cars with a double wishbone front suspension and try to understand how it was designed and whether it could fit under your car. That would give you an indication of your package limitations around your steering rack position and so on. If you do want to do it all by yourself then I would recommend to read as many books as possible on suspension design, get yourself a suspension kinematics software and start doing analysis. One thing will lead to another and step by step you will progress. It is a long road though, very rewarding, but very long and steep uphill .

Fantastic Car by the way

Cheers,
dynatune4xl