pajsh
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| posted on 16/8/05 at 04:57 PM |
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Springs & Dampers
At a recent bike track day I met a suspension Guru who said that winding in the stop plate on adjustable dampers with a C spanner doesn't
stiffen the supension but just affects the ride height as springs have a constant "rate" no matter how much they are compressed.
Maybe the rate doesn't change but there must be more energy stored in the spring which means more resistance
Sounds wrong to me but what do others think/know.
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alister667
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| posted on 16/8/05 at 05:23 PM |
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I thought that too but your guru friend is right.
Look up hookes law, for a 300lbs/inch spring it will take 300lbs to compress it the first inch, a further 300lbs to compress it another inch,a
further 300lbs to move it another inch etc ect.
If you have it compressed 3 inches by the platform position, the platform is applying 900lbs to the spring the next 300lbs will move it, you guessed
it 1 inch. The rate will always be the same.
Hope that makes sense
Ali
http://members.lycos.co.uk/alister667/
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britishtrident
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| posted on 16/8/05 at 05:26 PM |
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He is correct all you are doing is altering the preload and ride height, to alter the stiffness (and suspension frequency) you have change the spring
or move its mounting point on the lower wishbone or change the angle of the spring.
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Danozeman
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| posted on 16/8/05 at 05:36 PM |
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He is right. As britishtrident says.
Dan
Built the purple peril!! Let the modifications begin!!
http://www.eastangliankitcars.co.uk
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Nick Skidmore
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| posted on 16/8/05 at 06:19 PM |
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All you say is correct apart from the fact (in the 3" x 300lb scenario) the damper under this preload condition is being 'held open'
with a force of 900lbs which means you will have to apply a force of 900lbs to get it moving inwards and then 300lb per inch after that hence the
effect of increasing the apparent wheel rate when increasing the pre-load on the spring.
This is the way the front of modern single seater cars are set up, with loads of pre-load on a spring with no droop on the damper at ride height. They
are as stiff as buggery with very modest spring rates.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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alfasudsprint
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| posted on 16/8/05 at 11:27 PM |
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Nick,
I recommend you read Alan Staniforth's Race and Rally car Source book, its very informative on many aspects of car design relevant to what we
are doing, ie homebuilding.
Tim
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pajsh
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| posted on 17/8/05 at 09:09 PM |
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OK so if I undersatnd correctly.
Once the spring preload if overcome the spring rate (stiffness) is the same.
However if the preload is increased small bumps may not reach the threshold but bigger bumps will.
So it is not so much stiffer as harder. (Oh matron!)
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pbura
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| posted on 17/8/05 at 10:06 PM |
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quote:
Originally posted by pajsh
However if the preload is increased small bumps may not reach the threshold but bigger bumps will.
Normally, the purpose of preload is to support the sprung weight of the car, so that simply sitting there will not use up valuable shock travel.
Even the added weight of a feather will cause the spring to compress some more, though not very much! 
The case of the single-seater racer is very extreme. While it illustrates what could be done with preload, in dealing with aerodynamic downforce for
example, it doesn't describe the typical setup for a Locost.
Pete
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