Spring Rate to Damper Ratios???

Duk
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Spring Rate to Damper Ratios???

Post by Duk » Tue Jan 25, 2011 2:56 pm

What sort of information do I need to help determine damper rates for a known spring rate?

Bilstein shocks quoted with numbers like:
40/20 210-120 Dampening or
60/20 320-120 Dampening or
30/50 160-260 Dampening.

http://www.eshocks.com/bil_ORgd.asp?Manf=All says:
Understanding Bilstein Valve Ratings
Damping forces of Bilstein valvings for Off-Road are measured in Newtons at a velocity of 0.52 meters/seconds (approximately 20 inches/second). The ratings shown correspond to those measurements; rebound force is the first number, followed by compression force (rebound / compression). Conventionally, the ratings are written as one tenth the damping force in Newtons.
EXAMPLE: Valve rating: 275 / 78
Rebound force is 2750 Newtons at 0.52 m/s
Compression force is 780 Newtons at 0.52 m/s
Higher numbers mean higher (firmer) damping forces. For example, 360/80 has more control (is firmer) that 275/78, while 170/60 has less control (is softer) than 275/78.

10 Newtons = 2.248 pounds force.

What other information do I need to start making decisions about which damper valving I need?
Zagato
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Re: Spring Rate to Damper Ratios???

Post by Zagato » Sat Feb 26, 2011 9:02 am

There is much more to it than the damping figure at 0,52m/sek, this is a figure that is beeing used only for a ruff comparision between dampers. Actually you can build very different damping curves for different purposes that ends up with the same figure at 0,52m/sek. What is your porpose with the car? Street, Clubsport, or pure racing? Will it be standard weight, or will you strip it out? If you give me some clues i can help you a bit on the way.

Regards

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Re: Spring Rate to Damper Ratios???

Post by Duk » Mon Feb 28, 2011 1:32 pm

Ahhhh, input :D

The car is intended to be purely for road use and is standard in weight.
Spring rates have been changed a fair amount.

The front is 2.5 times as stiff (standard TB + a 225lb/in spring) I calculated it as having the equivalent of a 375lb/in spring over the damper and a wheel rate of 232.5lb/in.

The rear, well I didn't know the standard rear spring rate but it has 185lb/in springs in it now. I haven't worked that out as a wheel rate or worked out the motion ratio between the spring and damper.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera » Mon Feb 28, 2011 2:45 pm

Hey Duk, give Heasmen's in Sydney a call, they're pretty much the Aussie Bilstein experts, they should be able to tell you... and post up what you find out.
Bilstein's generic circle track shocks which are what I'm guessing you're looking at, come in both linear and digressive valving - digressive is worth having although heasmen's told me that digressive can lead to wheel hop on a leaf sprung setup...

Of course my approach to it was to simply provide the spring rates etc to Heasmens and let them re-valve the billiies.
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Re: Spring Rate to Damper Ratios???

Post by Zagato » Tue Mar 01, 2011 3:38 pm

I would say that digressive is not for this purpose, more for a car that you want to get maximum wheel travel and comfort out of the suspension.
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Re: Spring Rate to Damper Ratios???

Post by Zagato » Tue Mar 01, 2011 3:43 pm

Duk wrote:Ahhhh, input :D

The car is intended to be purely for road use and is standard in weight.
Spring rates have been changed a fair amount.

The front is 2.5 times as stiff (standard TB + a 225lb/in spring) I calculated it as having the equivalent of a 375lb/in spring over the damper and a wheel rate of 232.5lb/in.

The rear, well I didn't know the standard rear spring rate but it has 185lb/in springs in it now. I haven't worked that out as a wheel rate or worked out the motion ratio between the spring and damper.
What diameter torsionbar do you have in your car, is it the long or the short ones (separate or fixed beam under the floor) app pre or after 1985?
How did you calculate the total springrate at the front?

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Re: Spring Rate to Damper Ratios???

Post by Duk » Tue Mar 01, 2011 7:28 pm

Zagato wrote:What diameter torsionbar do you have in your car, is it the long or the short ones (separate or fixed beam under the floor) app pre or after 1985?
How did you calculate the total springrate at the front?
The torsion bar is standard diameter, short 1 in a 1990 75.
The standard torsion bar wheel rate of 93lb/in I got from the Alfa Bulletin Board forum.
The spring rate to wheel rate conversion (motion ratio) I calculate as being 0.62.
My additional coil over spring is 100% captive, there is no attempt at making it some sort of variable spring rate design.

Digressive valve dampers are the preferred approach. I want to use spring rate to control the mass of the vehicle, and I want to have good control of the suspension when driven enthusiastically on bumpy roads.
And have a decent ride quality as well.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera » Tue Mar 01, 2011 9:13 pm

Zagato wrote:I would say that digressive is not for this purpose, more for a car that you want to get maximum wheel travel and comfort out of the suspension.
Nope - Digressive is recognised as generally being better for most situations involving tarmac.
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Re: Spring Rate to Damper Ratios???

Post by Duk » Tue Mar 01, 2011 9:21 pm

la_strega_nera wrote: Nope - Digressive is recognised as generally being better for most situations involving tarmac.
That's been my interpretation too. Good damper control that provides decent initial (low speed) bump valving but 'blows open' when the encounter high speed bumps, helping to maintain both a good ride quality, excellent initial damping and safe 'all 4 wheels on the ground' control when the going gets rough.
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Re: Spring Rate to Damper Ratios???

Post by Zagato » Wed Mar 02, 2011 9:01 am

I would say that you have recieved some misinformation, i would agree if it was for a family saloon or a offroad vehicle where you would need as much wheel travel as possible combined with best possible comfort. But you will loose out in handling and steering sharpness. The more" eager" you want the car the less you use digressive.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera » Wed Mar 02, 2011 2:29 pm

Zagato wrote:I would say that you have recieved some misinformation, i would agree if it was for a family saloon or a offroad vehicle where you would need as much wheel travel as possible combined with best possible comfort. But you will loose out in handling and steering sharpness. The more" eager" you want the car the less you use digressive.
The Ex V8 Supercar engineer i'm sharing an office with says you're wrong.
The team won a number of championships and Bathurst a couple of times while he was with them, so I think he knows what he's on about.
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Re: Spring Rate to Damper Ratios???

Post by Zagato » Wed Mar 02, 2011 3:28 pm

I could not care less, i have met so many race engineers that did know very little about shockabsorbers, they mainly use them, and specialists made them! But there is many ways when you come to shocks, many more then most people understand.
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Re: Spring Rate to Damper Ratios???

Post by la_strega_nera » Wed Mar 02, 2011 3:56 pm

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).

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?

Zagato:
When pretty much every suspension guy I talk to points towards Digressive damping as the way to go, and the reasoning makes sense to me as an engineer, it says to me that maybe, just maybe, its right?

The only thing I can think of where a linear damper would be the setup of choice is if you're runnign on a perfectly smooth surface and never kiss the ripple strips. Perhaps on a slot car? Linear damping just upsets the car on big bumps. If your winding on lock fast enough for the knee point in the damping curve to be an issue, i think the type of damper is not the issue, maybe try being smoother... remember, the velocity at the knee point is reasonably high in terms of bump rate.


Random factoid: The V8 supercars are actually all over damped, and surprisingly soft in their wheelrates.
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Re: Spring Rate to Damper Ratios???

Post by MR2 Zig » Wed Mar 02, 2011 9:35 pm

I'm just a machinist here, but, maybe the question is not weather you go with a pure linear setup or a marshmellow digressive one....maybe one of degree on how close to linear do you want to go, or how marshmellow at the other extreme?









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Re: Spring Rate to Damper Ratios???

Post by Zagato » Thu Mar 03, 2011 3:40 pm

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.
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