Bob C
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| posted on 17/4/04 at 10:55 PM |
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rising rate suspension
I've always been led to believe that the large angle of the 7 style front spring dampers from the vertical meant that it had a reducing rate, ie
the lbs/in went down as the wheel was pushed up. But when I think about it, it actually looks like a rising rate to me (a much better thing). Am I
going soft? or have I believed someone a long time ago who I shouldn't have....?
Cheers
Bob
PS I think it was a certain Mr Staniforth!!
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Tblue
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| posted on 17/4/04 at 11:06 PM |
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As the lower wishbone rises the angle to the shocker moves nearer to a right angle, so it is a rising rate system. Probably not a great deal
though.
Look at the rear suspension linkage of a monoshock motorcross bike to see a proper rising rate system in action.
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Spyderman
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| posted on 18/4/04 at 04:17 PM |
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It is falling rate!
The deflection of the wheel is measured in a vertical direction. The fact that the wishbone and damper are getting nearer to 90 degrees is
irrelevant.
As the wheel rises, so does the lower end of the damper, increasing the angle away from vertical.
Ideally the damper movement should be in the same plane as the wheel movement, unless catered for by pivots and/or levers. The lower wishbone does not
qualify as an appropriate lever in this case due to the the difference in angles.
The only possible way of making the suspension rate rising would be to move the top mount outward so that as the suspension rose the damper became
more vertical! Other than going inboard suspension that is! 
Hope this is understandable!
Terry
Spyderman
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Matthew_1
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| posted on 18/4/04 at 06:22 PM |
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Nope, it's definitely rising. I've read various "theories" about this but decided ages ago to do the maths instead just to
see.
For argument, given a wishbone of lenth 100mm, which has a top mounting 125mm above the wishbone pickup, then for a step in wheel rise of 5mm
gives:
Rise Shock Length Change
5 141.443 --------
10 137.022 -4.421
15 132.501 -4.521
20 127.871 -4.630
25 123.121 -4.750
30 118.238 -4.883
35 113.209 -5.029
40 108.016 -5.193
45 102.639 -5.376
50 97.056 -5.583
So you can see from the change column that the rate of change is INCREASING, therefore it is a RISING rate design. Yes I've also done the
figures for an actual chassis design withing reasonable bump and droop, and it's still rising rate. Excuse the bad tabulation, this thing wont
let me put more spaces in !
[Edited on 18/4/04 by Matthew_1]
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JoelP
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| posted on 19/4/04 at 10:09 AM |
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i believe its rising rate too, but it might depend on whether you go by degree of wishbone movement or by mm of wheel movement. its probably rising
rate compared to either, and obviously you should do it to the wheel movement anyway.
Beware! Bourettes is binfectious.
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drmike54
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| posted on 19/4/04 at 05:30 PM |
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Rising Rate
I believe it is a Rising Rate. From reading on of the various Racing design books I have purchased in the last 30 Years. I have a one book that
discusses the state of the art Lotus 19. To determine if the suspension design is rising or falling rate with a conventional wishbone suspension.
If the shock/spring mount is above the plane of the top wishbone mount it is rising spring rate. If the top shock mount is below the plane if the top
shock mount you will have a falling spring rate. These rate are not much like in a purposely designed RR Suspension. But the mild RR will help keep
the car off of the bump stops.
Matthew_1 how did you do the calculations . I have been trying to determine the formula that would define the spindle movement but my old brain is to
warn out. 
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NS Dev
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| posted on 19/4/04 at 11:22 PM |
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Matthew 1 - yes, looking at the design (and obviously your calculation) it is slightly rising rate. Over the suspension travel used to be honest it is
not far from linear rate though really. Over a short travel that's no bad thing, rising rate is really more benefit over a very long travel
(like the motocross bike mentioned)
I don't think any of this is an issue with normal (ish) locost/westfield front suspension really.
Other silly point - obviously whatever the geometry, you can still have whatever you want by using progressive springs!!
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jcduroc
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| posted on 20/4/04 at 06:14 PM |
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In a "normal" double wishbone configuration such as the Locost design it is a rising rate in bump and decreasing rate in droop although,
in this case, at a lesser rate.
However, more important is the wheel rate (combining tyres, springs/shocks and anti-roll bar) as it is here that the forces are acting.
Joćo
JCM
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britishtrident
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| posted on 20/4/04 at 06:26 PM |
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The tricky bit is to work what effect lateral cornering forces have on the wheel rate, I suspect it knocks the small ammount of rising rate on the
head.
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MikeP
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| posted on 20/4/04 at 10:06 PM |
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quote:
Originally posted by drmike54
Matthew_1 how did you do the calculations . I have been trying to determine the formula that would define the spindle movement but my old brain is to
warn out.
One way to avoid too much geometry is to draw it out on 2D CAD or make a paper model ala Carroll Smith. You measure the wheel movement relative to
the spring length. Check every 1/2" or 1" or so, if the spring shortens more than the previous measurement up to the max bump it's
a rising rate.
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jcduroc
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| posted on 20/4/04 at 10:56 PM |
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quote:
Originally posted by drmike54
...Matthew_1 how did you do the calculations . I have been trying to determine the formula that would define the spindle movement but my old brain is
to warn out.
Consider the ensemble "wheel-hub-spindle-outer wishbones/upright mountings" as a BODY.
If you're going bump or droop move the ground = wheel travel (disconsider the tyre deformation); if you're going to roll, twist the ground
around the Roll Centre and you'll see what happens to the susps.
HTH
Joćo
P.S.: Either in 2D CAD or on paper, if done accuratly the result's the same.
[Edited on 20/4/04 by jcduroc]
JCM
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Matthew_1
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| posted on 27/4/04 at 06:09 PM |
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I've done the calculations in a number of places. Initially just in Excel, with a bit of trig. you can work out the results as above (I can send
the .xls if anyone wants it)
I've also modelled my whole design in both Susprog3D and Inventor. Susprog is very good for both raw angles/distance measurements etc and
working out spring rates. It takes a bit of getting into and a fair amount of background reading to understand the principles, but it's all
interesting stuff.
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drmike54
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| posted on 29/4/04 at 04:37 PM |
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Excel File
I am interested in theExcel file.
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