Meausuring suspension natural frequency with a smart phone
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- Опубликовано: 25 мар 2019
- Using a smartphone to directly measure natural frequency in car suspension. Measure bounce, roll and pitch. Cheap, accurate and fascinating!
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Hey Julian, nice Video uploads! Can you also do a video about damper histograms and application of FFT for damper setup?
I'd only do a video on that topic if I did a book on suspension. I did plan to do a book on suspension but I have put it on hold for the time being.
Brilliant video. Do the dampers change the results or should it be done without them. If so my rears are separate so that's OK but front is mcpherson. Thanks.
Dampers will make only a very small difference unless they are hugely stiff (where the suspension won't be able to be bounced!).
Hi Julian, thanks the video was very informative. I have been riding a very bouncy car and trying multiple options to fix it but have not been very successful. I need your help.
I checked the natural frequency of all four sides and here are the results.
Front right: max 3.8 and mean 2.4
Front left: max 3.8 and mean 2.5
Rear right: max 4. 8 and mean 2.5
Rear left: max 3.7 and mean 2.5
Any idea what this concludes?
Iv watched so many of your vids recently and their great. I currently have a fiesta st 2014 I'm using for track days. I have 8kg springs front and rear and got a frequency of 2.1hz on the front and 2.5 hz rear. What do you think of them figures? Thanks👍
Yes they sound about right for a road car used for track days. Now tune dampers and sway (anti-roll) bars.
@@JulianEdgar great thank you
Great videos Julian. I have a rear wheel drive vehicle with an auto box that bounces on the mounts at its natural frequency is 11 to 13 Hertz. That frequency is the same as the wheel rotation speed at 85 to 95 kph. How do I change the natural frequency to eliminate the bounce through the vehicle?
You could soften the gearbox mounts but that might lead to other issues such as driveline snatch.
@@JulianEdgar Thanks for replying Julian. The only real solution I found was road forcing the front tyres on a specialist wheel balancer which uses a rolling road to measure the tyre variation and indexing that to the rims.
The wheel variation is the trigger for the engine auto trans bounce at that frequency, which just happens to be the natural resonating frequency of the combo.
I was thinking a too stiff spring maybe not absorbing the vibration, but transfering the vibration to the resonater. Is this what they call the rattle space frequency?
I have used a phone app to measure the vibration hertz frequency, which allowed me to determine the front tyres were the problem. Great tip..it works.
Dear Julian. Thank you for introducing this. The app owner should thank you for it !
My car is on bilstein coil overs and I find it difficult to bounce because the amplitude is so small. If I took the app out for a drive, would that give me a more accurate reading ? So far I've gotten 2.4hertz at the back but the front is just so hard. I was thinking, wouldn't a slow rebound setting also affect the reading much ? Or it was meant to read that combination of spring/damper result anyway.
Yes you can do it on the road, but you'll have a much more complex waveform to work with. But the app should still be able to dig the natural frequency out of the waveform. Very, very stiff damping will alter the natural frequency, but in most cars, you can still get a clear reading. 2.4Hz is pretty stiff for a road car, but not inconceivable.
@@JulianEdgar If I compress the car slower than the natural frequency, would I still be getting the correct readings? I'm getting 2.4hertz front and back, and that worries me. But that leaves me thinking if 2.4 hertz is the speed of my compression. I have been told my Bilstein B14s are 8kg/mm fronts and 10kg/mm rears, really stiff for a 1400kg car.
Thanks for the lesson ! So it seems the impression of suspension stiffness relates completely to the speed of Z axis movement.
@@marcwang4830 It's hard to manually compress a car suspension at anything other than its natural frequency - like pushing a kid on a swing at the wrong time. Tape the phone over the front and rear axles then go for a drive.
@@marcwang4830 The natural frequency is inherent in the spring rate & proportional to the load, i.e. the mass acting on that corner of the car. You pressing out of phase won't have any effect, the "peaks" and "troughs" of maximum deflection will be the same and the frequency is still the time between them. You'd have to bounce the wheels off the floor to have an effect - that does illustrate one valid point though: you're measuring the natural frequency of the whole suspension, including the tyres.
Hi @JulianEdgar . I’ve just followed the video, measuring the front natural frequency of my street car. I consistently get 3.7hz. This seems absurd if it’s true that such a frequency is usually only found on high aero race cars. Is it possible that I am measuring my TYRES frequency rather than my springs? The vehicle is usually highly damped (particularly low speed damping), although I had the adjustment wound all the way back for the test
That’s very stiff indeed. Tyre nat frequ is about 10 -12 Hz.
Does the book cover sizing the airbags? That's the bit that's always confused me, how to decide between a narrow bag under high pressure or a wide bag under low pressure.
It's only a small book, but it does cover picking the correct model of air *spring* (not just dumb airbags!) using manufacturer specs (eg from Firestone). The Firestone tech catalog allows you to make the sort of comparisons you're asking about.
@@JulianEdgar I've seen Firestone's tech info. It's very thorough, but not very clear. At least, that's my recollection of it. I'm a huge fan of air suspension thanks to twenty years in the trucking industry. But I don't trust the generic kits to provide the right spring rates for my application.
@@johnterpack3940 The correct Firestone catalog is very clear - it shows precise spring rates, natural frequencies, loads, etc for all their springs. If you buy the book, you'll see how I selected the front and rear air springs for my car, and how I changed the rate of existing air springs when I had to use a generic kit (for the front) for which no data was provided.
@@JulianEdgar I'm putting the book on my next shopping list.
Hello sir, what app is it sir? Thank you so much
www.dld-llc.com/Diffraction_Limited_Design_LLC/Vibration.html
Wouldnt a lower frequency be stiff? Stiffer components would resist the motion and allow the wheel to cycle less per second, why isnt that true? Or what am I missing? It seems counter intuitive to think of a lower frequency as soft rather than stiff. Thanks and great videos
It's the *combination* of the weight acting through the spring *and* the spring stiffness. So let's keep the springs the same and increase the weight (eg four people get into the car). The suspension sinks downwards, and then bounces more slowly up/down over each bump. (That's why the car rides better with more people in it - assuming it's not on its bump stops.) So the spring *acts as if it is softer* when there's more weight on it. So a given spring can act really stiffly (no weight acting through it) or really softly (lots of weight acting on it) ie there's no such thing as a stiff spring as such - it is soft or stiff only in the context of the weight it is supporting. And the softer the spring *behaves*, the lower will be its natural frequency. Also see ruclips.net/video/6hiy9MRW5Hc/видео.html
No, stiffer means less deflection. It's directly analogous to sound, also a wave form. Turn the pitch up on a low booming noise and it progresses to sound like a constant tone as the pitch rises.
It's conservation of energy: as the deflection decreases the frequency has to rise. Stack 10 x little 10 hertz waves on top of one another and the combined amplitude will be as high as 1 x 1 hertz wave if the same energy was put in to make it bounce
"Flat Ride" is great for rolling down the freeway, but it's bad for handling and horrible on a racetrack. It puts an artificial constraint on the springing which paints you in the corner of inadequate front stiffness, excess rear stiffness or both. Handling oriented cars *must* have a higher front ride frequency to provide the necessary chassis control for braking and a lower rear one to allow compliance and aid in acceleration traction. You just can't fix the Flat Ride understeer budget with anti-roll bars. I've been a motor racing engineer for 30 years, so I've got a fair bit of experience on this one. Low HP cars may have a front/rear ride frequency split of 0.95, while a high HP car might be more like 0.85. I don't think I've ever had a good car with the ratio over unity.
Maurice Olley was the first proponent of flat ride. He was building 1950's GM land-yachts with chewing gum for bushings and dampers that were little more than ballast. He *had* to figure out a way to make a car go down the road essentially undamped. This was a brilliant answer. We are no longer in that situation. We have very, very good dampers and, if we're willing to use them, we don't have to worry nearly as much about inducing unwanted pitch modes. The automotive industry is absolutely monkey-see/monkey-do, though, and most suspension engineers are obcessed with flat ride to the point where it destroys the handling of the car as a whole.
Olley didn't develop flat ride in the 1950s, and it was also not developed for cars with soft suspension - perhaps have a read of my book on the history of car suspension?
Hi Julian, I’ve been trying to tune out porpoising from a 911 with Tractive dampers and a third party PASM controller (DSC) that allows for very fine-grained control over damper behavior. What are some good strategies for diagnosing cause of porpoising? My hunch is that front and rear damping stiffness ranges are too different. I’ve made some improvements by increasing rebound stiffness in front and increasing overall damping in the front. Would love to get some rules of thumb or approaches to trial and error when diagnosing porpoising causes. Thanks! Will try the Velocity app as well.
What do you mean by porpoising? *Pitch* is caused by incorrect ratios of front:rear spring rates, and the same may be true of stiff damping.
@@JulianEdgar it feels like low frequency pitching. How can I learn more about front:rear damping or spring rate ratios?
It sounds like I can measure my improvement with the Vibration app by measuring pitch?
@@AntonDAuria I cover it a bit in one of my books, but basically you want the natural frequency of the rear to be lower (ie softer) than the front eg (a rear natural frequency 70 per cent of the front). That gives you a pitch (front and rear going up and down at different rates) that dies away quickly after a bump has been met. Measure the natural frequencies f & b with damping initially as low as possible and see what that relationship is first.
@@AntonDAuria Yes I would think so. But get some basic f/r measurements done first (with low damping rates).
Is natural frequency directly tied to spring rate compression and rebound? Many coilovers systems available have one-way adjustability. Does this mean that the frequency changes as the rebound and compression settings are slowed down?
I think what I'm trying to get at is that my vehicle's suspension was tuned to somewhere close to 2.5Hz on a Toyota MR2 Spyder weighing 2450lbs (1111kg). The current spring rates are 5kg front and 9kg rear. The issue that I experience is on 200 treadwear tires and excessive oscillation even at slightly lower dampening settings at high rates of speed. The slower (stiffer clicks for those reading this) I set the rebound/compression, the greater the bouncing in the rear.
Would lowering the rear spring rate improve, worsen, or not change the symptoms I'm experiencing? Another RUclips video accurately describes the symptoms I experience on track and attributes those issues with too high compression. However, if the car is at it's lowest setting and rolls too much, but at a slightly higher setting the tires vibrate and osccilate up and down in a turn, what would be the issue?
Thank you kindly
What happens when you change tyre pressures?
@@JulianEdgar If I run 32psi on all corners, the car feels like it's on skates once pressure increases. I run the tires at 29 psi hot all around and it understeers up front, but allows for predictability out back with grip at the limit. However, once I am at the limit on certain technical turns, the tires vibrate within the turn, as though skipping and not really allowing me to put power down unless a) I let off gas and widen the turn radius, or b) enter the turn much slower overall. Conversely, the same turn gets harder and harder to turn at when the track is at temperature mid to later in the day. It doesn't feel like oversteer at all, in fact, I'm fighting understeer simultaneously.
The manufacturer is under the impression that I'm riding bump stops, but that makes no sense as it's only under one specific turn, which is called a Chicago turn (a hard 90 degree turn that widens out). If you'd like to see that turn, it's turn 7 at Dominion raceway in Virginia, USA.
Perhaps measure actual suspension deflection. Running out of travel does sound a good point. Does it do it at Turn 2 as well?
@@JulianEdgar I'll look into measuring deflection. It really only does it on turn 7, but throughout the rest of the track, the ride is bumpy and when it's on the softest setting, it's not pliable. There's too much rotation.
I did leave out that I'm only running a front sway bar and was recommended to omit the rear to reduce oversteer. I'm contemplating reintroducing the rear sway bar and increasing the diameter of the front, apart from increasing ride height to the adequate height.
Hi sir,
Actually, i came across a fact, which states that suspension frequency of car is related to Sleeping frequency, that why it is designed between 1-2Hz . Is it True?
If so, then driver may also feel sleepy during runtime.
No, closer to walking frequency. Frederick William Lanchester (died 1946) was the first to define suspension frequency as relevant to ride quality and he chose about 1Hz based on walking frequency. It's easy to show this if you use the Vibration app in a phone and walk around with the phone taped to a hat (so that takes into account head movement as well).
What app is that
apps.apple.com/us/app/vibration/id301097580
What is the exact app name. ' vibration' turns up a zillion hits.
apps.apple.com/us/app/vibration/id301097580