Knee Speed/ Velocity Split for damper

[OGR34]

Hey guys I'm trying to figure out the ideal knee speed or velocity split for a particular car that I'm designing. What I'm trying to do is find the knee speed (or crossover speed) of my damper that directly relates to the crossover point in the displacement transmissiblity graph (SQUARE ROOT OF 2) so that I can achieve optimum comfort in most situations. Please refer to the .png files below that I've attached. Any help would be very appreciated!

KNEE SPEED

DISPLACEMENT TRANSMISSIBILITY

 

[GregLocock]

Setting the knee velocity is done by the magic pants people, although I do know we have proprietary guidelines and methods for setting it.

How do you physically bias the slope of the low speed part so that it is different for jounce and rebound? I'd have thought that was a recipe for nasty ride.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[OGR34]

I'm not very concerned about the bias, only the knee velocity. This is for theoretical purposes only

 

[Tmoose]

Don't most passenger car, motorcycle, and even many competition dampers have pretty weak (or at least way less than rebound) compression damping?

http://i76.photobucket.com/albums/j35/Peterflux/KoniFSDDamping.jpg

 

[BrianPetersen]

All of the good-quality dampers that I've dealt with (for pavement use, both car and motorcycle) have much more rebound damping than compression damping.

The good-quality motorcycle shocks have separate oil circuits for compression and rebound damping, including the low-speed damping, with either check valves or shim stacks that act like check valves so that they act in one direction only. Slightly-less-good dampers may have one bleed orifice that affects both compression and rebound, but a check-valve circuit that bypasses it in one direction only which contains another bleed orifice that acts in parallel on compression only (which ensures that low-speed compression damping will be less than low-speed rebound damping).

As for that "knee speed" ... all I can think of is that if you work out the natural oscillation frequency, and from there work out what the maximum speed would be when going through a full-stroke oscillation, you probably want that to happen somewhere near the bounds of the low-speed-damping regime, but at how much beyond that you want to start "blowing off" fluid through the shim stack ... is a darn good question that I have no idea of the answer to. 25 mm/s sounds low, but I'm just guessing.

 

[GregLocock]

Yes that is a very small knee velocity for a road car

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[OGR34]

So I went about getting a knee velocity of about 6 in/s for compression and 2 in/s for rebound based on the maximum peak velocities. These peak velocities are in the resonant region of their respective damping ratios that are based on the low speed damping values that I've chosen. Sounds good to me. Although these are only an estimate at which to start from. What do you guys think?

 

[GregLocock]

I've looked at a few shock curves and it seems that (a) low speed rebound rate = low sped compression rate and (b) knee velocity is the same for each direction.

I'd also add there was (a long time ago) an interesting article in Vehicle Dynamics International by John Miles, in which he pointed out that for B class road cars the ratio of high speed compression to high speed rebound damping varied over a wide range yet all the cars sold reasonably well, hence presumably had 'good enough' ride.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Tmoose]

Unless I'm running over 2X4s or whacking sharp edged pot holes am I invoking/provoking high speed compression damping?
Is high speed rebound involved anytime the suspension is allowed to rebound freely?

I've never driven a modern Mini, but just yesterday I stumbled on some comments allegedly by Alex Moulton hisself about his take on the BMW Mini and its ride qualities. It appears near the bottom of the page here -

http://www.mgfcar.de/hydragas/ehydragas.htm

"Alex Moulton has some forthright views to share on the BMW Mini: “It’s enormous. The (original] Mini was the best-packaged car of all time this is an example of how not to do it. The interior space is not much bigger than the old Mini, but it’s huge on the outside and weighs the same as the Austin Maxi! The crash protection has been taken too far. I mean, what do you want... an armoured car? Princess Diana was killed in a two-tonne Mercedes: you can have a fatal accident in anything if you drive fast enough.
With the original Mini, we set out to prevent any accidents by having excellent handling, not by cushioning people from the consequences of their own folly The old Mini was the absolute apogee of this philosophy of built-in safety via the handling —people avoided accidents by driving around them. The suspension of the [BMW] Mini Cooper is set far too stiff, giving a most uncomfortable ride. To he honest, it’s an irrelevance in so far that it has no part in the Mini story.
We were also able to get a second opinion on the BMW Mini’s handling from Doug Milliken, an American suspension engineer who happened to be visiting. After his first drive in the BMW Cooper he confirmed Alex Moulton’s opinion that the suspension was too hard for comfort and that it was overdamped. His conclusion was basically that he would not want to drive it on anything other than dead-smooth tarmac."

 

[GregLocock]

Yes but that's a BMW thing not a Mini thing. If you think about the curve of what we might loosely call ride comfort vs handling then BMW have always tended toward the handling end, and get praised by journos. There's also the double whammy of their run-flat tires, which are very prone to impact noise and vibration.

So far as what part of the curve you are in, that's a really good question and hidden somewhere on this computer are data files of suspension displacements vs time on various different surfaces. Bear in mind that circuit cars see up to about 1.5 m/s, production cars much more than that.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[OGR34]

1.5 meters per second knee speed? That sounds extremely fast. Most curves I've seen are about 2-6 inches per second

 

[GregLocock]

No, maximum suspension velocity, not knee speed.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?