MadaboutKitcars

Hi guys,

I am a complete beginner willing to learn and asking for advice, trying to design a suspension system to suit a chassis I have designed, It is not too late to change the chassis design if the geometry requires it. I have a few questions in the essay below that I would be very grateful for an experienced point of view / advice to be given.

So I have read plenty of books including

1. Carroll Smith’s Tune to Win
2. Frank Thiessen’s Automotive Steering, Suspension & Braking Systems
3. Milliken’s Race Car vehicle Dynamics

Brilliant set of books 1 and 3 are highly recommended for suspension geometry!

Now these books tell me everything I need to know in terms of what each aspect is and how to change them, sticking with the basics I have

1. Camber
2. Caster
3. Toe

Now I want to start the design process with the upright / hub assembly as I have components that need to fit, it is more crucial at this stage than the chassis mounting locations that can be changed.
At this stage I am designing from scratch I have access to CNC etc and would like to create my own uprights, hubs and A arms / links rather than salvage parts.

My original thoughts were to design a set of A arms in a double wishbone suspension system, have the bottom wishbone parallel with the bottom of the chassis when static and the upper wishbone arranged to control roll center etc In the back of my mind I am considering droop and wondering if parallel is a good idea?

Now I am also wondering why on pretty much all hub link fastener locations / A arm – hub joints the fixing positions are on an angle? Is this to re align the wheel dependant on the wishbone positions?

For an example see the following image I found on another website of the Cortina uprights http://www.super7thheaven.co.uk/Cortina_Uprights
Why can I not design a hub that’s straight and parallel in terms of these locations?

Can anybody tell me how to judge the amount of camber / toe I may want to design into my suspension geometry as standard? Should I set camber and toe to 0degrees and use the track rods to adjust?
I am really struggling to determine how I identify whether I need more or less camber etc and how much I should consider in the design / leave for fine tuning and adjustment?!

All / any help will be greatly appreciated

The chassis orientation is going to be Rear engine Rear wheel drive RR setup due to the components available and the type of car I want to build.

Thanks
Jason

Design Help

Jason

You are asking the classic questions car designers have been mulling over for time immemorial. I cannot answer your questions but I know a man who can, dependant on where you live that is.

I ask the question are you a closet designer or a kit builder, it sounds like you want to learn to design and then test but you are starting possibly in the wrong place if you want real results. They do degree courses in Motor Sport Car Design and perhaps you have missed out on doing this as an occupation I don't know.

Are you considering 'roll centres' (http://performancetrends.com/rc.htm) and braking effects in your design?

If I read your questions correctly you want to build in flexibility to your design to obtain the best result long term, getting the pick-up points in the right place is crucial I agree, saves a lot of hacking and welding later.

In the end its the weight that the tyre exerts on the 'road' that will determine the characteristics of your proposals for supporting it.

The other thing to remember is what you want to use it for off-road or F1, the extremes indicate the balance you are trying to achieve no one design will achieve all you desire that's why rally car designers have different designs for the same car dependant on where they are racing, gravel tracks, tarmac or grass.

You run the risk of designing something that when made and fitted does not actually do that you thought it would and that will cost time like a good'en and cash you may not want to part with.

I could understand if you had a test car to play with, on which you could modify fit and test for quicker turn round of design changes, anything else will sap your tenacity and enthusiasm in quick time.

I am not trying to put you off, just offer some insight to what other have done before you.

I think you have two options:-

Design on paper and submit for evaluation via tried and tested software evaluation tools. These tools will tell you how your design will perform under all dynamic conditions and even tell you what under-steer you will encounter on a specific corner of any race track in the UK.

The second option is to talk over your idea's with an experienced designer who can tell you what will happen based on 30 years experience in designing building and testing just exactly what you are doing yourself.

Drop me a PM if you want to know the contact details for Dave Gallop of Track Developments.
If you can visit Dave he has models that demonstrate the effects of set up and design errors, he can tell you what will work under different conditions and he can apply your design, if you choose to pay the test fees, to a software evaluation tool before you spend time and money on something that will not work.
He has supported race teams, he has designed (and is currently designing) suspension systems and knows what will and will not work. He can build a test model and can improve on your idea's if you choose to accept his experience.
This will save you time and money and get you someone to talk too who you can trust, without costing you a fortune.

Hi Eaa53

Sorry I should have started with a little more background, I am based in bristol (UK) and a Mechanical Engineer by trade, my career is in aerospace however, my love is motorsport. I have a manufacturing degree and work in a technical design environment. Unfortunately the degree course in motorsport is not an option at this stage in my career and its just a case of having a chassis I can produce easily and wanting to learn how to create a basic suspension system.

Unfortunately this means my skill set lacks stress analysis although I have full access and help with FEA models and can evaluate my chassis. I am structural designer and therefore all components I make are usually static.

It is early days as I am 23 and only recently passed out of an apprenticeship, I am looking to learn / teach myself some concepts that will allow me to build a kit car similar to the locost / Haynes roadster, track day car (tarmac) that can be driven on the road for short distances or sunny days 

I want to setup the suspension for race road holding and handling the main problem is even with the books i have read i have been unable to find information to help me determine the wheel position / path while static / moving so that i can implement some camber, caster and toe into my design. Unfortunately theres just so many inputs and not enough answers

I am considering roll centres but have not looked at braking effects yet (thanks for the link this piece of software looks really useful) I will be downloading the demo when i finish work and having a play with some of its features.

Yes flexibility is definitely something i will implement in terms of the simple adjustments that can be made, the available adjustments however, I do need to make sure the wishbone length and distance between the wishbones / angle of wishbones is suitable to my application, there doesn’t seem to be any particular recommendations 

I have done a couple of weight calculations with some sensible assumptions where necessary i can work out the centre of gravity from my model and this is really where it comes to a halt.

Thank you for the advice and i really appreciate you taking the time to reply, I understand exactly what you are saying relating to having a test chassis, this car will be designed for a purpose and I just want to ensure it is as good as i can possibly make it and i have considered as much as possible before manufacture. I can change aspects later and make adjustments to areas its just about justifying my decisions today and having reasons for making the A arms xyz and reasons for the ride hight / roll centre all theoretical obviously im sure reality is slightly different ide just like to understand exactly why I designed something the way i did. Easy for me to draw up some A arms and hubs tonight, understanding aspects of the design and doing my best to achieve something positive is another matter.


The two options you outline seem very viable,

Can you recommend an extensive tool I can use possibly within a small budget or free?  I will definitely look at the roll centre software, only ask as i know more advanced software is available, generally I am a bit of a geek and good at learning software.

If you can visit Dave he has models that demonstrate the effects of set up and design errors, he can tell you what will work under different conditions and he can apply your design, if you choose to pay the test fees, to a software evaluation tool before you spend time and money on something that will not work.

The offer of a contact at Track developments sounds brilliant, PM will be sent!

Once again thanks very much for your time

Hi Jason,
I have read most and a lot more of the books you have on your list! (costin and phipps is a good one)

I've done a few calcs and things on geo setup for my Shelsley and the next project so I kind of understand your dilema but for the moment, decide a couple of things:

Wheel type and offset
track and wheelbase
what you want to be the controlling factor i.e. roll centre change, camber control etc
engine layout (or idealised weight distribution)
suspension type
car use

then get the card and string, pinboard pins and pencil out, make a 1:2 or 1:5 scale in section through the hub and chassis and play with the positions - much quicker and more intuitive and itterative than going to cad straight away

Just be methodical about it and draw/sketch it, be realistic, e.g. don't have the lower wishbone hub mounting point 50mm from the ground and outside the wheel if you intend it to be used on the road, but you may find that a production hub is close enough to what you need if you look hard enough through the scrap yards.