la_strega_nera
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

Just to ensure we're on the same page - this is what Digressive damplers plot out like:

Image

I hope you're not suggesting you'd put something that's progressive on a car - jesus, I've ridden bikes with very basic fixed orifice dampers (gives a damping force that is proportional to velocity squared - so are "progressive") and they suck really badly - by the time you get enough damping into them to control low speed movements, the reaction to small square edged bumps is horrendous - kicks and unsettles the bike really badly. Replaced the orifice damping with a digressive pop-off type (without changing anything else in the system) and the difference was phenominal once it was set up right (had to tweak the pop off preload and the low speed orifice size).
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Re: Spring Rate to Damper Ratios???

Post by Duk »

la_strega_nera wrote:Just going over your Numbers there Duk - you haven't been squaring the motion ratio in your calcs.
The equivalent coil over rate of the torsion bar is actually 242lb/in according to that wheel rate and motion ratio.
so slinging a 225lb spring on there is only 87lb/in extra at the tire (180lb/in total wheelrate).
D'OH!!! :shock:
Cheers for telling me that. :)

la_strega_nera wrote:RSR supply springs that vary from 80 to 125kg/cm for the front (440lb/in to 688lb/in) and 40 to 70kg/cm for the rear (220 to 385lb/in) for their kits using the stock TBs, so you might want to look at a bit more spring?
So is the 93 lb/in wheel rate of the standard short torsion bars correct?
All of my various TB size wheel rate calculations are based on that figure.

Using your corrected motion ratio (and I believe Mats pointed this out to me a while ago that I hadn't squared it :oops: ) you get about 38.67% of the spring rate at the wheel.
Using that figure, the softest of the RSR springs would give a WR of 263lb/in when combined standard TB (assuming the 93lb/in figure is correct). That's slightly softer than a 30mm TB.
Even using the 688lb/in springs give a wheel rate damn near the same as a 32mm TB.

Anyway, this is great stuff. I was really struggling with some of the Bilstein damper forces when I was thinking that the spring rates(force) were so low and even the softer of the damper forces were getting close to my spring forces.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

I believe that's about right for the TB number for a stock v6 75
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Re: Spring Rate to Damper Ratios???

Post by Zagato »

la_strega_nera wrote:Just to ensure we're on the same page - this is what Digressive damplers plot out like:

Image

I hope you're not suggesting you'd put something that's progressive on a car - jesus, I've ridden bikes with very basic fixed orifice dampers (gives a damping force that is proportional to velocity squared - so are "progressive") and they suck really badly - by the time you get enough damping into them to control low speed movements, the reaction to small square edged bumps is horrendous - kicks and unsettles the bike really badly. Replaced the orifice damping with a digressive pop-off type (without changing anything else in the system) and the difference was phenominal once it was set up right (had to tweak the pop off preload and the low speed orifice size).
What you are showing here is quite normal damping curves, if quite badly executed.
I would newer accept how the curwes "wawes about" (if you understand what I mean)
on the return (underside of the diagram) it is ether bad material or badly executed or both.
If you go to the BILSTEIN- Not Tein homepage i think you can fin a cure that looks as it should! look under B16 PSS10 for example!
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Re: Spring Rate to Damper Ratios???

Post by Mats »

Zagato wrote:To be honest, I have newer seen such a thing as a linear damper, not in cars anyway. They all had curves, but maybe not digressive pistons! And I have seen hundreds in a professioal testing machine. But it's your choice.
I'm pretty sure everybody is talking damper curves, and they are all digressive (except yellow Konis which are complete sheit).

What's your references to back up your claims? Or do you just know it to be true..? 8)
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Re: Spring Rate to Damper Ratios???

Post by Duk »

Since I need to recalculate my required additional spring rate that I need to use to get a wheel rate of my chosen 233lb/in (equivalent to a 28.7mm TB) or close enough to, I'm going to need to add a 360lb/in coil over (either a 350 or 375 will be settled on).

So the combined spring rate (coilover and standard torsion bar) would be equivalent to about 600lb/in.

Numerous bits of research have hinted at 65% of spring rate as being a very good starting point for damping and having twice as much rebound damping as there is bump damping.
So I'm guessing that 65% is applied to the rebound damping, because if it's applied to the bump valving, then there would be 780lb/in worth of rebound damping.

Any comments or thoughts?

*** Apologies for persistent use of imperial measurements, despite being a metric person 99% of the time, imperial measurements just work for me better when talking suspension forces :? . ***

Bilstein Damper Valving Forces:

Valve #10 Force in Newtons 800 Force in Pounds 180

Valve #20 Force in Newtons 1200 Force in Pounds 270

Valve #30 Force in Newtons 1600 Force in Pounds 360

Valve #35 Force in Newtons 1850 Force in Pounds 416

Valve #40 Force in Newtons 2100 Force in Pounds 472

Valve #45 Force in Newtons 2350 Force in Pounds528

Valve #50 Force in Newtons 2600 Force in Pounds 585

Valve #55 Force in Newtons 2900 Force in Pounds 652

Valve #60 Force in Newtons 3200 Force in Pounds 720

Valve #70 Force in Newtons 3800 Force in Pounds 855

So if I have the equivalent of about 600lb/in spring, 65% of that is 390lb, so a bit more than a #30 valve. Half of 390 is 195, so a bit more than a #10 valve.
A reasonable interpretation of what's going on and a some sort of starting point????
Last edited by Duk on Sun Apr 24, 2011 7:52 pm, edited 2 times in total.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

I've got a spreadsheet that i've been messing with (care of another forum with some guys who really know their shit, and I've asked a question or two) and should have some usefull answers to this soon.
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Re: Spring Rate to Damper Ratios???

Post by Duk »

Cheers, I'm looking forward to getting more info and seeing what the vastly more experienced have to say on this topic :D :D :D .
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

Here's a handy spread sheet.
So the ball park targets are 65-70% of critical damping at approx 10 inches/s at the wheels... the rest is motion ratios and estimated masses. One gotcha to keep an eye out for, the motion ratios are inverted - so the front motion ratio is 1.59 or something.
Attachments
Koni damper calculations.xls
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

Additional tidbit - try to get the Body natural frequency at the rear to be about 20% greater than the front - this ties in with "Flat ride theory" where the faster response of the rearend allows the car to respond to bumps etc better.
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Re: Spring Rate to Damper Ratios???

Post by Mats »

la_strega_nera wrote:Additional tidbit - try to get the Body natural frequency at the rear to be about 20% greater than the front - this ties in with "Flat ride theory" where the faster response of the rearend allows the car to respond to bumps etc better.
You mean the other way around right?
RWD = softer in the rear, FWD = softer in the front. Universal truth.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

Yeah, that's what I always thought, but apparently flat ride theory says otherwise. Mind you, my workmate (ex V8SC) says 10% more wheel rate is the way to go.
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Re: Spring Rate to Damper Ratios???

Post by GTV27 »

OK, I'll bite, what is flat ride theory?

edit - google revealed this, seems interesting

http://www.optimumg.com/OptimumGWebSite ... hTips.html
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Re: Spring Rate to Damper Ratios???

Post by Mats »

I didn't look very hard into that but basically it seems to be a way to optimize the chassis to move parallell to the road when riding over concrete highway seams?

Not sure how that applies to my world to be honest, maybe down in the Karusell? 8)
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera »

I guess it depends on how smooth your tracks are and how often you bounce off ripple strips.
Anyway, thinking about it more, i'd probably stick with the known quantity of about 10% more wheel rate on the front.
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