Hi guys, just been giving it a quick thought hoping someone may have a bit of information.
Do any of you out there know what the spring rate of a standard rubber donut is, obviously with a coil spring it is easy to calculate but came a bit puzzuled with the trusty old donut. If any one has an exact value or any suggestions as to what it may be i would be muchly grateful, cheers mat
Rubber Donut Spring Rate
Started by
matt large
, Nov 30 2010 12:29 PM
4 replies to this topic
#1
matt large
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Posted 30 November 2010 - 12:29 PM
#2
Wil_h
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Up Into Fourth
Posted 30 November 2010 - 12:50 PM
It doesn't have one and an equivelent would be difficult to drrive. a classic spring with a singel rate would be measured in Lb/in, that is it would compress one inch with X Lbs and two inches with 2X Lbs. I know you know this, but I thought it would help explain the next bit.
With the rubber doughnut, as you compress it it gets harder and harder, so the spring rate is constantly varying, add to this the fact that as the suspention travells (especially at the front) the varying geometry also affects the spring rate. When you lower a mini you effectively reduce the spring rate (exactly the opposite of what you want).
This constantly varying rate of the doughnuts works well with the mini suspension as it works in harmony with the effective varying rate of the suspension. This is one reason why the replacement springs never really will work.
Why do you need to know?
With the rubber doughnut, as you compress it it gets harder and harder, so the spring rate is constantly varying, add to this the fact that as the suspention travells (especially at the front) the varying geometry also affects the spring rate. When you lower a mini you effectively reduce the spring rate (exactly the opposite of what you want).
This constantly varying rate of the doughnuts works well with the mini suspension as it works in harmony with the effective varying rate of the suspension. This is one reason why the replacement springs never really will work.
Why do you need to know?
#3
MRA
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Previously known as 'mra-minis.co.uk'.
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Posted 30 November 2010 - 01:00 PM
As above a fairly complex differential equation may get close, as it is pretty variable..........
#4
matt large
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Posted 30 November 2010 - 02:14 PM
Im a third yr automotive student. Just finishing a vehicle dynamics assignment and got me thinking of my autograss mini, its always nice to be able to apply what you are learning to relative real world applications.
without going to technical into it all, any ideas how well coil overs compare to the standard donuts and dampers. have never used any or been in a car with to give an accurate comparison. obvious benefit would be variety of springs available for a given application,
matt : )
without going to technical into it all, any ideas how well coil overs compare to the standard donuts and dampers. have never used any or been in a car with to give an accurate comparison. obvious benefit would be variety of springs available for a given application,
matt : )
#5
Cooperman
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Uncle Cooperman, Voted Mr TMF 2011
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Posted 30 November 2010 - 02:49 PM
In my personal opinion, coil springs as direct replacements for the rubber springs are good for very a defined and specific application which is racing. The tracks are smooth and a constant-rate spring of high 'poundage' is good, so long as you get the poundage correct for the circuit.
I think to try to replicate the variable-rate poundage of the rubber springs for other applications is not easy or worth doing as the risk of 'coil-binding' on rough roads is high and needs to be prevented at all costs.
It's been said that coils give a smoother ride on normal roads. They may well do, just as softer rubber springs with lower rates will also do, but if it then gets a bit bumpy, the limitations will soon become obvious.
I've not dealt with coil-over springs here as I don't think the damper mountings on a Mini are really up to the job, unless a lot of strengthening is done to both front top bracket and mountings and to the top of the rear turret in the boot.
Asa design engineer earlier in my life, I'm not averse to progress and evolution of products, but I do think the marketing of coil springs for anything except racing is really just another way to sell product which does not improve over the original. The Mini sales organisations seem to try to sell on the basis that if it's used in racing it must be desireable for road use - not so! Exit cynical mode!
I think to try to replicate the variable-rate poundage of the rubber springs for other applications is not easy or worth doing as the risk of 'coil-binding' on rough roads is high and needs to be prevented at all costs.
It's been said that coils give a smoother ride on normal roads. They may well do, just as softer rubber springs with lower rates will also do, but if it then gets a bit bumpy, the limitations will soon become obvious.
I've not dealt with coil-over springs here as I don't think the damper mountings on a Mini are really up to the job, unless a lot of strengthening is done to both front top bracket and mountings and to the top of the rear turret in the boot.
Asa design engineer earlier in my life, I'm not averse to progress and evolution of products, but I do think the marketing of coil springs for anything except racing is really just another way to sell product which does not improve over the original. The Mini sales organisations seem to try to sell on the basis that if it's used in racing it must be desireable for road use - not so! Exit cynical mode!
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