angliamotorsport
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| posted on 1/4/12 at 08:09 AM |
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downforce, (suck/push), spring rate
I thought I would ask the question as above.
If your car produces downforce and ground effect, how do you work out what the spring rate should be given that the car will have to be travelling at
speed to achieve this.
The spring will have to be stiff enough to resist being pushed against the bump stop but able to work at lower speeds and not make the car
undriveable.
Is there a formula that would provide a good starting point or is it just a matter of track testing, suck it and see, (sorry about that).
Seems that lots of owner put diffusers, canards, wings,splitters etc. on their cars so wondered if they adjusted the spring rate to suit.
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maccmike
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| posted on 1/4/12 at 08:24 AM |
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very good question, i look forward to an answer
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kjouk
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| posted on 1/4/12 at 08:48 AM |
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The majority of aero mods are really aimed at just reducing lift and/or drag rather than creating downforce so nothing needed really. Hill climb
single seaters tend to use more complex suspension setups to allow for ride height control while maintaining good compliance. From what I have seen
most low end track cars they are less sophisticated as they can run stiff without problems.
The full maths is always quite complex but I suspect you can do fag packet calculations to get you in the ball park by looking at maximum weight
transfer + downforce in the important edge cases such as full braking, max corner G, max speed etc.
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maccmike
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| posted on 1/4/12 at 09:31 AM |
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so does that mean; you need wind tunnel to measure down force at a set speed, then add it to vehicle weight to decide on spring rate.,
I suggest you drive it and if it bottoms out at speed make alterations, if not leave it.
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angliamotorsport
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| posted on 1/4/12 at 09:41 AM |
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A bit of internet research has revealed a guy in the us runs a bike powered March based sports racer and his siolution was to use triple adjustable
penske dampers interlinked. the third cylinder?, operated a spring which increased fluid to raise or lower the ride height as downforce increased or
reduced. So spring rates did not need to increase.
I think I have got this right but please feel free to comment.
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Volvorsport
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| posted on 1/4/12 at 10:14 AM |
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you can datalog the suspension movement with various success , or you can go to a wind tunnel ....
if your car weighs 2 tons with downforce , you need the spring rate to cope with it , simples ...
but , if you look at F1 they use the spring rate to control the floor height and front wing height , so it doesnt start to porpoise , so there is
quite a few things you need to think about .
read staniforths book or mcbeaths book , its well covered , rather than me trying to explain it .
www.dbsmotorsport.co.uk
getting dirty under a bus
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balidey
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| posted on 1/4/12 at 11:09 AM |
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Wind tunnels, CFD and even basic calcs are good to a degree, but they all rely on clean air. You are hardly ever going to experience the same results
in the real world.
My opinion is, don't bother, just ignore the lift / down force contribution.
Dutch bears have terrible skin due to their clogged paws
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liam.mccaffrey
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| posted on 1/4/12 at 11:33 AM |
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active suspension...........i'll get my coat
Build Blog
Build Photo Album
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kjouk
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| posted on 1/4/12 at 11:49 AM |
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quote:
Originally posted by angliamotorsport
A bit of internet research has revealed a guy in the us runs a bike powered March based sports racer and his siolution was to use triple adjustable
penske dampers interlinked. the third cylinder?, operated a spring which increased fluid to raise or lower the ride height as downforce increased or
reduced. So spring rates did not need to increase.
I think I have got this right but please feel free to comment.
Yep, thats the kind of solution you see about. Another example is on
http://www.lee-adams.co.uk/lee_adams_gallery_1.html. A hillclimb single seater that has
to be very soft at the rear to get traction but also cope with aero loads from a large rear wing.
For seven style cars you can get good enough base data to estimate lift/drag at different speeds. As mentioned lift is not too hard to measure via
string pot anyway but drag I am finding tougher to measure and its what I am more interested in. So far I have estimated from comparing acceleration
between simulations & data logs, I guess a better option is proper coast down testing with modeling of rolling losses?
[Edited on 1/4/12 by kjouk]
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pewe
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| posted on 1/4/12 at 12:37 PM |
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I'm NO expert but surely as a base setting it has more to do with the frequency of the springs i.e how frequently the springs compress and
rebound once the car is set up?
You can measure this by enlisting the help of a couple of mates.
Have them bounce on either side at the front until they find the same harmonic in the suspension - same as bouncing it up and down when you go to buy
a car. You count the number of bounces for 15 seconds, multiply by four and divide by 60 to give so many bounces i.e. Hz/second.
Repeat for the rear.
Typically Yank motors show about 1.2/sec front - 1 rear, fast road cars about 1.8 front - 1.6 rear, anything over two is track-day/race with BTCC
going up to 5.
I don't know if mid/rear engined cars use different settings but no doubt google will be your friend for more info.
Presumably aerodynamics can only be gauged by using a wind tunnel/complex mathematical modelling or on a suck it and see basis?
I'm sure Matt (Procomp) will be along today/tomorrow to confirm or refute the above.
Cheers, Pewe10 
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MRLuke
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| posted on 5/4/12 at 08:54 PM |
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It depends how much downforce you are generating really.
The long and short of it is when the car is at its maximum downforce you need to have a bit of travel left to deal with uneven road surface.
Also not an expert but essentially you are increasing the weight of the car the faster you go, I guess the hard bit is working out how much downforce
you are generating.
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phelpsa
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| posted on 6/4/12 at 09:31 AM |
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How much downforce? Will you be doing any low speed work or is it all flowing circuits? If you have to deal with low speed corners then you may want
to think about soft springs and a 3rd bump stop.
Although in general you find that even with a few low speed corners you're better off just setting the car up really stiff, then deciding
whether there's anything to be gained by running softer at low speeds.
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angliamotorsport
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| posted on 8/4/12 at 03:48 PM |
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Some interesting comments, thanks to all.
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