GBaggott
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| posted on 9/12/09 at 01:35 PM |
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Advice please - Specifying spring rates on a 1 off build
Hi
I am building a rear engined mini with a space frame chassis.
A Vtec H22 engine will be mounted in the back, seats fuel tank and normal stuff in the front. A mini shell will then be put on top of the chassis.
I expect the car to weigh around 500 - 550kgs when complete and the balance to be about 300kg at the back axle and 200kg at the front.
I have some AVO adjustable dampers from a westfield but need to specify the correct springs for this chassis set up.
Is it a simple equation were the predicted corner weight is equal to spring rating?
Advice would be much appreciated, Gareth.
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tegwin
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| posted on 9/12/09 at 01:42 PM |
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Specing suspesnion is a black art..
The amount of leverage and type of suspension you have, the angle of the shock, the size and type of tyre you use etc...
But a close gestimate would be a start.. and then change the springs dewpending on how the car behaves on the road/track
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Would the last person who leaves the country please switch off the lights and close the door!
www.verticalhorizonsmedia.tv
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matt_gsxr
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| posted on 9/12/09 at 02:15 PM |
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I only know what I have read, but here goes.
The wheel frequency is determined from the wheel rate, and the SPRUNG weight on the corner as: (unsprung is around 20-25kg per corner)
F = (1/(2*pi))*sqrt(Wheel-rate/sprung-weight)
Obviously the units need to be correct (but see link at bottom for calculator).
If for 1inch of wheel motion you get 1inch of spring compression then the wheel rate is the same as the spring rate. If not then here is a page that
explains how to get the wheel rate with pictures:
link
Staniforth says that you want a frequency of maybe 100-120cpm (cycles per minute), although many advocate higher. I also thought that Staniforth
suggested higher values at rear to front, whereas I have also seen folk suggesting higher at front than rear. There are some "anti-dive"
issues to consider also.
Once you know all the parameters there is a calculator here.
link
You will do well to get down to that weight. An Ariel Atom with the same engine is 456kg dry and that has no bodywork.
My crude calcs based on your weights (a guess of your personal mass) and 20kg unsprung would suggest 125lb/in springs at the front and 175 at the rear
(assuming wheel-rate equals spring rate). Generally people set things up stiffer than this as many think the Staniforth recommendation is a bit soft.
Its a black art, but springs are quite cheap.
I don't know much (just a book learner) so I'd recommend reading up on it yourself too.
Matt
[Edited on 9/12/09 by matt_gsxr]
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nitram38
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| posted on 9/12/09 at 02:23 PM |
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I defy an atom to weigh 465kg.
The Motaleira weighs 420kg with an R1 engine and smaller chassis tubes.
If you go on the atom club forum you will find that no-one has a car under around 525kg......
I tried calculating but in the end I guarantee you will change them after driving for a while
[Edited on 9/12/2009 by nitram38]
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matt_gsxr
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| posted on 9/12/09 at 02:38 PM |
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Nitram, you should update the wikipedia page then.
Matt
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GBaggott
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| posted on 9/12/09 at 02:47 PM |
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Great stuff thanks guys,
I am making a rough weight estimate using the MNR as a base line.
The spaceframe mini chassis I have made is tiny, I'm using fibre glass doors and front clam shell (oh and its a mini pick up), westfield front
suspension, MNR rear independant suspension, raceleda front uprights.
I will be very happy if it weighs 500kg, probably find out in 4 months.
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kb58
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| posted on 9/12/09 at 02:49 PM |
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Since your exact motion ratio isn't listed, we can only go by wheel rate. A good starting point for a road car is wheel rate set to corner
unsprung weight. The more "sporty" the car, the value goes up from there to around twice the corner unsprung weight for a track car
(non-aero car of course.)
[edit] Note that we're talking wheel rate, NOT spring rate, which may end up with completely different values than corner weight directly.
[Edited on 12/9/09 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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kb58
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| posted on 9/12/09 at 02:51 PM |
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It's going to be heavier than that.
My scratch-built tube-frame Mini with a carbon shell and that exact same drivetrain behind the seats weighed 725kg... (The drivetrain alone weighs
216kg)
Weight distribution was 45F/55R.
[Edited on 12/9/09 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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aitch
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| posted on 9/12/09 at 04:30 PM |
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what about dampers?
thanks Matt_GSXR that calculator is fantastic and even tells you what wheel rates you want according to the use 
is there any similar calculator for dampners? i dont even know where to start??
thanks aitch
p.s. maybe i should start a new thread?
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GBaggott
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| posted on 10/12/09 at 11:44 AM |
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The calculator is cool Thanks Matt,
I have adjustable dampers so I guess when the car is finished you can adjust to suit track or road.
KB58 - 725kg sounds a lot I hope to be below that. Your build sounds really cool
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alistairolsen
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| posted on 10/12/09 at 11:57 AM |
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anything I was going to say has already been covered except to add that a lot of people massively underestimate unsprung weight. 20-25kg is just not
realistic unless you're on alloy calipers, alloy belled disks, uprights and hubs, crossply tyres and magnesium rims. 35 is far closer tot he
mark for the average 7
My Build Thread
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aitch
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| posted on 10/12/09 at 05:36 PM |
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quote:
Originally posted by alistairolsen
anything I was going to say has already been covered except to add that a lot of people massively underestimate unsprung weight. 20-25kg is just not
realistic unless you're on alloy calipers, alloy belled disks, uprights and hubs, crossply tyres and magnesium rims. 35 is far closer tot he
mark for the average 7
ill be buing springs and dampers last of all so i can weight things and get it as close to spot on as possible the first time - cant afford to be
buying twice
is there a calculator for dampers too ????????
also can i design chassis and then get the parts the length i want or am i best to design for a given length that is easily available ?
thanks
aitch
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