Suspension Design and Testing

[kieran527]

Hello, as you can probably tell im a newbie
Ive had a read through the forum for ages, and im amazed at the wealth of knowledge in here, as well as being really helpful. Which is where im hoping you guys can help.

Im studying Mechanical Engineering part time at university and im in my final year, completing my final year project. The project revolves around designing suspension geometry for an open wheel kit car (caterham derivative) my mate has built and completing stress analysis of the wishbones for the front and rear suspension and uprights (which i have to design and provide geometry as well ) and then redesigning any weak links and determining a factor of safety.

I understand this is probably simple stuff for you guys, but i havent had any experience in suspension design, so any information you can provide, links to techincal articles that will help, or even people i can discuss the subject with would help massively. This will involve hand calcs for moments, forces, buckling, stress, fatigue and then comparing to software analysis for vertical, braking and cornering loads.

Im not totally wet behind the ears though, i work for a diesel engine manufacturer and have a decent technical and engineering knowledge, but no exposure to suspension unfortunatley. Ive had my motorbike and car to pieces numerous times and managed to build them back up again and im skirting round the idea of building track car. Ive followed motorsport for the last 15 years and have a very good understanding of the engineering behind it, but nothing compared to some threads on here!

So go easy on me please i hope you guys can help me, and anything i can provide back to the forum i will.

Kieran

[flynfrog]

Not sure how deep you want to go into the rabbit hole for suspension design

Tune to win by Carroll Smith is pretty good as far as basics

after that there is Milken race car vehicle dynamics.

SAE.com has a bunch of threads on people going through the same thing.

[Caito]

Yes or yes, get a copy of Tune to Win and start from there.


Let me add Tires Suspension and Handling, it's a very neat book with lots of knowledge.

And also use the search function which will probably contain a lot of information. The FSAE forum is also a recommended reading.

[Jersey Tom]

Step 1 - the most important step - figure out what you want your car to do, and how you want it to behave.

[kieran527]

Thanks for the tips, i hav ordered Tune to Win by Carrol Smith.

With regards to the vehicle it is to be used as a fast road/track vehicle, running on slicks when on the track.
The vehicle specifications have been noted and a guideline for what the suspension will need to control, with weight bias, power etc.
I am however because its purely a suspension project negating the tyre effect on the suspension, i know its a simplified suspension and not truly representative, but its the way the syllabus requires me to investigate it.

If theres anything else you guys wanna know about details or anything else you can contribute it will all help thank you.

Also, as for depth, i wanna expand my career in motorsport so nothing is too in depth, and seems as i find the subject interesting any knowledge gained will helpful.

[MuseF1]

Tune to win is a good book, i've got Carroll's engineer to win on order.

A good book to buy would be Allan Staniforth's Competition Car Suspension about £15 on Amazon which isnt a bad price at all compared to some other books

[Jersey Tom]

I am however because its purely a suspension project negating the tyre effect on the suspension

Other than controlling the body orientation for aero (seemingly not a factor here), IME the purpose of a suspension is to control the distribution of loads, and orientations of the wheels and tires.

Even if you don't have an in depth knowledge of the specific tires, doing suspension work with absolutely no concept of the impact of things is pointless. Meaningless. Would be like designing an airfoil with no concept or care of the effect of the airflow.

[kieran527]

I have already purchased Competition Car Suspension, a very good book! Thanks for the tip.

Jersey Tom, i understand what you are saying and i know this, but the project cannot use this information on tyres becuase it provides no purpose to the report. This project suspension is about forces and failure on different geometry and set ups. We are not trying to determine traction and handling from the suspension.
I hope you understand what i mean. And thank you for your input.

[Jersey Tom]

So what precisely are you still trying to figure out then? Sounds like what you're trying to achieve is pretty straightforward 2nd and 3rd year engineering classwork material... i.e. how to determine link loads, material properties and fatigue limits. Nothing specific to suspensions, just basic structures work.

[flynfrog]

do a few case loads at 2.5 vertical 5G vertical. 2.5 horizontal 5G horizontal.

Take a stab at out of operation modes. Hitting a curb @ X speed with one wheel ect. Sounds like a pretty open question.

[bettonracing]

For clarity, are you:

• [1] trying to redesign for improved component life (performance be damned), or
  [2] checking & reinforcing the existing components [where necessary] while keeping the existing suspension geometry, or
  [3] redesigning for improved vehicle performance (with safe component lifing)?

Whichever route you're going you'll need to start w/ some free body diagrams. I highly recommend writing some MATLAB code to crunch the numbers for you.

When you get to part where you input the loads you can take the engineering way (compile a matrix of the various combined load cases & crunch through them. Don't forget the + & - directions), or the typical way - all 3 max load cases at once (<-Be very careful with this method! It can lead to a false sense of security!). Keep in mind cars usually brake harder than the turn or accelerate. That braking torque applies a [significant] moment to the upright.

Also don't forget to apply appropriate safety factors based on component manufacturing methods, or failure mode analysis. E.g. if part failure results in a spinning projectile & the car becoming a cartwheel, choose (scientifically) a higher safety factor.

If this sounds overwhelming just start with the FBD's. The more you dig into it, the more you'll want to dig deeper (or you'll react like the Stig trying to play go fish). Choose your project goals from now & stick to them. Smart ppl spend 5+ years trying to learn how to learn this stuff.

Regards,
H. Kurt Betton

[Smokes]

Supension design would be somthing like this

Establish tyre performance

Establish geometry to optimise the tyre performance

One those are done you the work out the loads through the parts and do a strength vs weight vs life analysis.

This will give you the best components for your goal

You will find that the design of supension arms is very depandant on packaging it around a defined geometry.

I have had fellow students do a simalar projects but the final design is usally rubbish as it is too heavy and geometry wise dosen't optimise the tyre contact patch.

You would be better off doing a matlab simulation of the existing suspension design geometry and optimised suspension geometry to do a good final year paper.

Most of the maths can gained by reading Race car vechicle Dynamics by milliken.

Also you can use this program http://www.susprog.com/ find out if your university has a license of it kicking around.

I would strongly advise you to do a final year project for an engineering firm or comapany that needs something designing or manufacuring. This would give you more structure to do the dessertation and get feedback of established engineers and designers.

Leave the motorsports for your masters at loughbourgh or somehwhere like that.