MikeRJ
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| posted on 6/4/10 at 05:06 PM |
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quote:
Originally posted by nitram38
Maybe there is a massive range of springs, but there will only be one set at the given diameter and length I need available.
So therefore only one spring will do.
Absolutely, my point was that you are most unlikely to be limited by "what's available". Once you have worked out what you require,
the chances are very good that it will be easily available.
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nitram38
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| posted on 6/4/10 at 05:10 PM |
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Right for 10mm of wheel travel off the ground I get the shock retracting
Front 5mm
Rear 7mm
Front before calc 300 lb fully laden
Rear before calc 450 lb fully laden
So anyone, whats your guesses?
[Edited on 6/4/2010 by nitram38]
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Ivan
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| posted on 6/4/10 at 05:50 PM |
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Don't forget that the rear springs should have a slightly higher CPM (normally 5% to 10% more) than the front that is calculated using the
wheelbase and design speed to stop the very osselations you are talking about - again Stanniforth is your friend for the maths.
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nitram38
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| posted on 6/4/10 at 08:23 PM |
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quote:
Originally posted by Ivan
Don't forget that the rear springs should have a slightly higher CPM (normally 5% to 10% more) than the front that is calculated using the
wheelbase and design speed to stop the very osselations you are talking about - again Stanniforth is your friend for the maths.
What are the maths?
That was my original question! 
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andylancaster3000
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| posted on 6/4/10 at 09:45 PM |
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Have only read through the thread quickly so apologies for any repetition. I would strongly advise getting hold of a copy of this book:
The late Staniforth gives a very easy to read and follow answer to this very problem. All the maths you need is in there.
Unfortunately there there is no one simple formula that will give you the answer. As has been mentioned the first step is determine your desired wheel
frequency, from this you can calculate your required wheel rate (this is a function of the unsprung and sprung mass at the corner). Once you have got
this you will be able to determine the spring rate by finding the motion ratio between wheel and damper through doing a bit of geometry/trig calcs (or
empirically as has also been suggested to get a rough idea).
This should give you a ball park figure of where to start.
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nitram38
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| posted on 6/4/10 at 10:07 PM |
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Guy's I'm sure Allan's book is fantastic, but I came to this forum to find help in calculating my spring rate.
I don't want to buy the book for this.
All I asked was for some help.
Instead I feel like I've just been beaten over the head with the book.
You may love this book and good on you, but if all you can suggest is buy the book, why are you on this forum?
Sorry if I've affended anyone, but I've spent hundreds of posts trying to explain in laymans terms my knowledge.
All I'm asking for is some in return.
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nitram38
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| posted on 6/4/10 at 10:12 PM |
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quote:
Originally posted by Frosty
That was quick 
I assume with a corner weight gauge you are measuring just the total corner weight and not just the sprung weight. You need just the sprung weight.
Let's assume for a moment that 220lbs is the spring weight (excludes wheel and tyre etc).
First we'll work out the suspension leverage based on your measurements.
If the front rod moves 10mm and gives 7mm at the damper, 10 / 7 = 1.43, so your front dampers have a leverage ratio of 1.43 (so force on the damper is
1.43 times greater than it is on the wheel).
Allan Staniforth recommends that a track based car should start at 130CPM and you might adjust a little from there.
The calculation to work out suspension CPM is 187.8 x sqrt( (spring rate / suspension leverage squared) / corner weight)
So your calculation based on a 200lb spring will be 187.8 x sqrt( (200 / 2.05) / 100 )
This gives a frequency of 125CPM, so looking at a 225lb spring, that will give 133CPM so I'd go with this on the front.
You need to work out how how much weight the car will be carrying when it needs to be at its best and factor this into the total weight of the car,
then to the calculations again. Best to use corner weight scales with driver (or driver and passenger) to get the total weights at each corner.
a 225lb spring will not give 133CPM when carrying two 100kg passengers you see - it will be less.
As for the rear, because of the massive leverage (2:1 ratio), you would need a 600lb spring to give you 133CPM. Again, this is with no weight in the
car, so doing these calculations again with driver and passenger would show the need for even heavier springs.
Frosty, in your calcs, where did your corner weight of 100 come from?
My front corner weights were 300 lb.
If I can understand this part of your calcs I can figure the rest thanks
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Frosty
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| posted on 6/4/10 at 11:48 PM |
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quote:
Originally posted by nitram38
quote:
Originally posted by Frosty
That was quick
I assume with a corner weight gauge you are measuring just the total corner weight and not just the sprung weight. You need just the sprung weight.
Let's assume for a moment that 220lbs is the spring weight (excludes wheel and tyre etc).
First we'll work out the suspension leverage based on your measurements.
If the front rod moves 10mm and gives 7mm at the damper, 10 / 7 = 1.43, so your front dampers have a leverage ratio of 1.43 (so force on the damper is
1.43 times greater than it is on the wheel).
Allan Staniforth recommends that a track based car should start at 130CPM and you might adjust a little from there.
The calculation to work out suspension CPM is 187.8 x sqrt( (spring rate / suspension leverage squared) / corner weight)
So your calculation based on a 200lb spring will be 187.8 x sqrt( (200 / 2.05) / 100 )
This gives a frequency of 125CPM, so looking at a 225lb spring, that will give 133CPM so I'd go with this on the front.
You need to work out how how much weight the car will be carrying when it needs to be at its best and factor this into the total weight of the car,
then to the calculations again. Best to use corner weight scales with driver (or driver and passenger) to get the total weights at each corner.
a 225lb spring will not give 133CPM when carrying two 100kg passengers you see - it will be less.
As for the rear, because of the massive leverage (2:1 ratio), you would need a 600lb spring to give you 133CPM. Again, this is with no weight in the
car, so doing these calculations again with driver and passenger would show the need for even heavier springs.
Frosty, in your calcs, where did your corner weight of 100 come from?
My front corner weights were 300 lb.
If I can understand this part of your calcs I can figure the rest thanks
Sorry that 100 should be 220.
I wrote it in kg and not lbs, but obviously is has to be relative to the spring, so 100 should be 220, which is the corner weight you mentioned back
on page 1. This is where I got the corner weight from.
If all you want is just to know a good starting point for the springs, you can work it out from the above.
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nitram38
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| posted on 7/4/10 at 06:10 AM |
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Thank you, it was the only part of the formula that confused me!
I now understand how it works and the results might suprise everyone!
I'll redo my sums later and post them.
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nitram38
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| posted on 7/4/10 at 08:18 AM |
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Front ratio is 10/5 = 2
Rear ratio is 10/7= 1.43
Front 187.7 x sqrt (575/4/300) = 129.93
Rear 187.7 x sqrt (450/2.05/450) = 131.2
This figures do make sense considering the rear cams give the spring more leverage (spring on longer side of cam) and the fronts give the wheel more
leverage (spring on shorter side of cam).
Wierd as it seems the results are
575 lbs front 450 lbs rear
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hughpinder
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| posted on 7/4/10 at 10:54 AM |
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Your calcs look correct to me.
I might say go for slightly higher rear cpm to get to the reccommended 5 to 10% higher rate at the rear, as commented earlier
Regards
Hugh
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nitram38
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| posted on 7/4/10 at 04:52 PM |
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so 475 or 500?
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MikeRJ
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| posted on 7/4/10 at 08:54 PM |
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quote:
Originally posted by nitram38
Front ratio is 10/5 = 2
Rear ratio is 10/7= 1.43
Front 187.7 x sqrt (575/4/300) = 129.93
Rear 187.7 x sqrt (450/2.05/450) = 131.2
The motion ratio is the ratio of spring movement to wheel movement, in your case:
Front: 5/10=0.5
Rear: 7/10=0.7
The full formula is:
SF = 187.8 * SQRT( ( MR^2 * SR ) / SW )
If you want to find spring rate for a given sprung weight and frequency then re-arrange:
SR = ( SW * (SF/187.8)^2 ) / MR^2
Where
SF = Suspension frequency in cycles per minute (cpm)
MR = Motion ratio (spring motion/wheel motion)
^2 = Squared
SQRT() = Square Root
SR = Spring rate
SW = Sprung Weight i.e corner weight minus the unsprung weight.
The sprung weight part is important; for lightweight cars where the sprung weight is often a significant fraction of the corner weight ignoring this
will lead to a softer than intended spring rate.
Or you can use this online suspension rate calculator, but be aware that "Suspension
leverage" is expressed the other way, i.e. 0.5 for the above formula would be 2:1 for the web page above.
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nitram38
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| posted on 7/4/10 at 09:06 PM |
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I totally give up!
You are saying to calculate the ratio one way and Frosty another.
I don't know who the hell to believe!
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prawnabie
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| posted on 7/4/10 at 09:08 PM |
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Perhaps a conversation with procomp might yield some more information?
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prawnabie
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| posted on 7/4/10 at 09:08 PM |
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Perhaps a conversation with procomp might yield some more information?
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radom
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| posted on 7/4/10 at 09:45 PM |
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quote:
Originally posted by nitram38
I totally give up!
You are saying to calculate the ratio one way and Frosty another.
I don't know who the hell to believe!
imho these formulas will give the same result,
the only difference is that Mike is multiplying spring-to-wheel ratio instead of division by wheel-to-spring ratio
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MikeRJ
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| posted on 7/4/10 at 10:07 PM |
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quote:
Originally posted by nitram38
I totally give up!
You are saying to calculate the ratio one way and Frosty another.
I don't know who the hell to believe!
Well, the notes I have agree with both the online calculator I gave you the link to and
Eibachs suspension worksheet. Your
choice.
Taking unsprung weight into account is important though.
[Edited on 7/4/10 by MikeRJ]
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Frosty
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| posted on 7/4/10 at 10:58 PM |
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quote:
Originally posted by radom
quote:
Originally posted by nitram38
I totally give up!
You are saying to calculate the ratio one way and Frosty another.
I don't know who the hell to believe!
imho these formulas will give the same result,
the only difference is that Mike is multiplying spring-to-wheel ratio instead of division by wheel-to-spring ratio
That's correct. As you say, Mike suggests you multiply when using motion ratio, and have have suggested you divide using suspension leverage
ratio. It will give you the same answer though.
10 * 0.5 is the same as 10 / 2.
With your 10mm travel = 5mm at the damper, Mike says you should multiply by 0.5 squared, whereas I have given a formula where you divide by 2 squared
- they give exactly the same answer.
If you want me to explain why this calculation comes back the same then I will, but I fear you just want a quick answer and that is that, in which
case you should go with the springs you decided on back in page for which look right based on what you are telling us.
Just ensure that the weight you are calculating with is only sprung weight. As has already been said, unsprung weight will distort your calculations
quite a bit on a light car.
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nitram38
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| posted on 8/4/10 at 05:08 AM |
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Well my airshock spring rating tops out at 735lb and are rated from 135lb.
I run the air at around 80 psi. Max is around 100 psi. This generally gives me the impression that my spring rates will be close enough, but I reckon
I'll end up changing them!
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nitram38
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| posted on 19/5/10 at 10:23 AM |
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I've fitted the front shocks so far and after a road test seem spot on!
 
Description
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