rpmagazine
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| posted on 14/7/08 at 12:27 AM |
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Designing from scratch - information from a professional
I asked the question of a very experienced and respected suspension engineer: what do we (amateur builders) need to consider when designing a vehicle
from scratch?
The answer is as below, take from it what you will -
Here's a list we compiled of the suspension-related kinematic characteristics that need to be considered when designing a vehicle from scratch.
By its nature this is a top down approach. I've tried to edit out the redundant entries, but can't claim it is 100% sorted. It obviously
is not complete.
1 WHOLE VEHICLE:
Sprung mass
Sprung CG X and Z
Moments of inertia
Maximum steady state lateral acceleration
Ground clearance
roll stiffness (degrees per g of lat acc)
ride frequency, including front to rear percentages
lateral weight transfer percentage distribution front to rear
Torsional stiffness of the body structure
Roll axis gradient and any gradient change relative to roll
roll couple distribution
2 AXLE and WHEEL:
2.1 Packaging and general
spindle length
position
track
tyre size, hence rolling radius
Jounce travel
rebound travel
wheel offset
unsprung mass
2.2 Toe
Static toe setting
Ride Steer
Roll Steer
Tractive Force Steer
Brake Force Steer
2.3 Camber
static camber
Ride Camber
Roll Camber
2.4 Castor
static castor setting
castor gain
castor trail
2.5 Roll centres
roll center height at design load
roll centre height gain
TLLTD (tire lateral load tranfer distribution)
2.6 Track
track gain in roll
semitrack gain, ie lateral displacement of contact patch per unit of jounce travel
2.7 King Pin Geometry and Steering
KPI
scrub radius
steering axis angle
hub trail
pin lead/trail
Ackerman percentage/change
steering ratios
Lateral force steer (deg/g)
Aligning torque steer (Nm/deg)
2.8 Compliances
Stiffness of the major suspension mounting points, ie on a McPherson Strut, do the strut towers move around
Aligning torque Camber compliance (small effect)
Lateral Stiffness (at ground w/o tire)
Fore-Aft Stiffness (at wheel center w/o tire)
Lateral Force Camber Compliance
2.9 Other
Anti-x
Side View Swing Arm Angle (SVSAA)
[Edited on 14/7/08 by rpmagazine]
www.racemagazine.com.au
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Alan B
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| posted on 14/7/08 at 01:28 AM |
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Yup...pretty certain Ron Champion did all of that...
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rpmagazine
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| posted on 14/7/08 at 02:13 AM |
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I'm not surprised, it is all basic stuff. The complex parts are for those that can be bothered...no doubt some will decry this as all too
complex, but then I guess it is dependant on how what you want to achieve and what you can be bothered doing.
www.racemagazine.com.au
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pbura
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| posted on 14/7/08 at 03:49 AM |
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That's a good list, thanks for posting. It includes some concepts that are mentioned occasionally and I keep meaning to learn more about, like
trail in its various flavors.
Pete
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nitram38
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| posted on 14/7/08 at 06:49 AM |
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Sometimes this stuff just scares the hell out of beginners before they start so that is why they don't!
Whatever you do to a car suspension design, it will be a compromise. One small change to one component can upset the whole setup.
There is just not one right way to do it.
Practical experience coupled with a bit of book reading will set most people on the right track.
People who bang on about one aspect of the suspension without considering other factors, will leave you confused.
My advice is Read, Read, Read first, then ask,ask, ask, until you have the basics in your head.
Limiting factors for your suspension will be the layout of your engine, passenger compartment and where you want your wheels in relation to the
chassis.
Example: Sometimes your wishbones (if you go this route) maynot be the length you want due to engine width etc.
Work stuff out with cheap paper models like making 2D wishbones with pins at the fulcrums to check camber gain/loss etc.
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ceebmoj
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| posted on 14/7/08 at 07:22 AM |
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hi all,
on the whole I don't post in threads like this because i'm not even an armchair expert. However I have seen it posted that as ammeter
builder many things are all ready set for you in the upright you choose. assuming you don't fabricate your self.
Blake
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v8kid
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| posted on 14/7/08 at 08:49 AM |
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It's not just beginners it scares the hell out of it frightens me!
Very comprehensive but could it be prioritised?
For example as already pointed out most people will use a standard upright and buy available hubs brakes etc so most of the decisions re the wheels
are made for them and we have to work round it.
In my case I had to work round the tyres on the market and others will have different priorities.
Just thinking aloud I suppose but my idea of prioritising the list might involve different lists for different purposes.
Hell of a work though! Probably impractical.
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rpmagazine
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| posted on 14/7/08 at 09:24 AM |
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Martin, perhaps it is simply think of this list as a reading list - things you need to be aware of. This is the way I took it and in truth, although
it was a bit daunting it was also a relief as I had a list of areas to research - of course other areas/things have come up since.
Because we really are not designing from scratch, but are in effect recycling parts which will likely define some of these criteria, we should know a
bit about them so perhaps we can use them ourselves e.g. the castor wind off from bush compliance in braking of modern cars assists braking
performance and ability to turn under braking. If we use the bushes we can alter the compliance by changing the bush characteristics and also in using
compliant bushes we get away from the bugbear of stiction in bearings.
V8kid, as I said take from it what you will. This is not a comprehensive list from an OEM perspective. Some of it can clearly be eliminated by
choosing certain paths e.g. solid bearings (Spherical bearings) will get rid of the majority of the compliance section, but even minimal knowledge of
the area will allow greater insight into the OEM practice and might inform us as to the choices we make. Impractical...in a totality - yes probably,
though perhaps we should have some awareness of these things?
www.racemagazine.com.au
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nitram38
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| posted on 14/7/08 at 09:57 AM |
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I am not disagreeing on the awareness, just that there are so many things that builders will be frightened off.
As mentioned, the use of existing parts will dictate to the builder the way things are connected.
Sometimes it is easier to build your own uprights and start from scratch.
This is what I opted for because I didn't want the hassle of locating my rack for bump steer etc.
It wasn't your post that made me mention about "banging" on about one aspect of suspension design, I was just pre-empting those
comments (you know who you are!)
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rpmagazine
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| posted on 14/7/08 at 10:28 AM |
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I would love to be able to make my own uprights. Unfortunately local rules preclude the option, in that the level of costs due to the need to formally
engineer such things makes it pretty much impossible in the budget I have.
www.racemagazine.com.au
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CaptainJosh
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| posted on 14/7/08 at 11:05 AM |
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Seems a little OTT to me, im sure some of those variables arn't really needed in the design of a suspension setup.
I don't really have anything constructive to say, but i'll scratch my head about it and just wait to see how my car handles... then do
something about it if it dosen't 
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liam.mccaffrey
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| posted on 14/7/08 at 11:39 AM |
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I don't think you're alone CaptainJosh
Build Blog
Build Photo Album
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rpmagazine
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| posted on 14/7/08 at 11:46 AM |
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OTT...gods I haven't heard that phrase since I worked in Kent years ago!
Yep as said before parts of it we cannot alter and some of it will be less important than others...after all how many of us actually weigh components
before choosing one item...including wheels?
www.racemagazine.com.au
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kb58
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| posted on 14/7/08 at 01:30 PM |
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Posted this in the "other" thread, but it applies here as well:
Our cars exclusive strong point is their light weight. This one factor is responsible for making more of a difference than anything else, including
the RC. Face it, we aren't building F1 cars, we don't have a national runoff at stake, and we don't have sponsers to please. Plus,
our driving skills are so inconsistant, any small improvement in RC placement will be buried in the noise of our scattered lap times. (I'm sure
there are exceptions here, but you know what I mean.)
Summed up, as long as the tires are wearing evenly, that's all that counts.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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CaptainJosh
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| posted on 14/7/08 at 02:18 PM |
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quote:
Originally posted by rpmagazine
OTT...gods I haven't heard that phrase since I worked in Kent years ago!
Yep as said before parts of it we cannot alter and some of it will be less important than others...after all how many of us actually weigh components
before choosing one item...including wheels?
Errr, i do I know the difference in weight between 17, 16, 15, and 13 inch wheels and the tire choices that go with them.
Although 17inch wheels look very "bling" I opted for some 13x6 superlights, a quick way to loose aload of unsprung weight!
I even made my engine choice on its PowerToWeight rather than its all out power. Not even mentioning ease of install, but im doing it right rather
than doing it easy
[Edited on 14/7/08 by CaptainJosh]
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pbura
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| posted on 14/7/08 at 02:38 PM |
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I often use 'awareness checklists' in my work as an accountant, and they are helpful for bubbling up issues that might be easily
overlooked, such as tax and regulatory requirements. The idea is to not spend a lot of time on them.
Take 'scrub radius', for example. Usually there's not much you can do about it, but knowing what you have might save some trial and
error in wheel alignment or deciding whether to buy that quick rack.
Pete
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nitram38
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| posted on 14/7/08 at 02:50 PM |
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If you make your own uprights then you should be able to get the scrub right!
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CaptainJosh
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| posted on 14/7/08 at 04:26 PM |
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quote:
Originally posted by nitram38
If you make your own uprights then you should be able to get the scrub right!
I second that! But one can only dream
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ceebmoj
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| posted on 15/7/08 at 07:37 AM |
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quote:
Originally posted by CaptainJosh
quote:
Originally posted by nitram38
If you make your own uprights then you should be able to get the scrub right!
I second that! But one can only dream
Hi all,
There have been a number of thread on fabricating your own uprights over the years. in fact I was going to start another this evening if I get sat
down this evening and finish my drawings off so that I can post them up for every one to laugh at. while ther is alot to be a where of it does not
nesasery look like it needs to be as costly as it looks i.e. along while back there was a thread where alanB (I think) was talkinga bout a sellf
jigging laset cut upright that used a bold on stub axle I cant find the thread or the out come right now but it looked interesting at the time.
blake
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rpmagazine
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| posted on 15/7/08 at 10:08 AM |
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But how much scrub is the correct amount?
www.racemagazine.com.au
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Fred W B
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| posted on 15/7/08 at 11:12 AM |
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I haven’t posted on the threads re geometry design. Problem is it seems there is a lot of noise around the theory but a distinct lack of real
practical useful info from the experts. See upright thread here for example
upright question
So we go back to making educated guesses based on what we can read / learn from those who do share. If it wasn’t for Staniforth I would not have had
the confidence to at least attempt to design and build a car.
Edit - add fabricated upright thread
update here a while ago
Cheers
Fred W B
[Edited on 15/7/08 by Fred W B]
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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rpmagazine
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| posted on 15/7/08 at 11:38 AM |
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thing with practical and useful information is it might well be wrong for you but right for me. Apart from that I am obviously not an expert so fear
to provide advice on specifics given all of the variables.
Re uprights can I suggest a couple of things:
Have a look at the bolt on FWD bearing/hub sets. The bearings are much larger than the RWD cortina style and have far less compliance...which is a
good thing. They are also compact and do away with the need for an axle.
Also try and get the steering arm and top ball joint mounted on the same arm that is attached to the upright via shims ala RALT cars. This allows you
to alter camber without changing toe!
www.racemagazine.com.au
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andygtt
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| posted on 2/2/09 at 02:27 PM |
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Its all very well saying you need to consider things like this when even the experts cant give any kind of indication of what combination of the
factors will work.
All they do do is allow you to work out when you have something wrong... which is no doubt imortant
I ended up paying an expert to design my geometry simply because after reading the books and doing calculations they were a complete waste of my time
as they gave no indication of what I needed to aim for... not even a baseline.
Andy
please redefine your limits.
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andygtt
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| posted on 2/2/09 at 02:42 PM |
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quote:
Originally posted by rpmagazine
But how much scrub is the correct amount?
bloody good question!
thats one thing i spent ages pondering with no answer... zero is ideal technically but will lack 'feel' apparently.
Andy
please redefine your limits.
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kb58
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| posted on 2/2/09 at 09:09 PM |
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All you "arm-chair" experts spouting off like you actually know something. Like headless chickens you are...
Sorry, could resist. I really miss Syd... just kidding.
[Edited on 2/2/09 by kb58]
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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