Zero warp stiffness!
Indeed it has.
Roll centres are quite low. Did you consider making them adjustable? .
I think he could go lower easy enough, maybe not ideal though.
Very keen to hear how it drives.
It uses an engine.
Posted 30 March 2012 - 03:26
Zero warp stiffness!
Roll centres are quite low. Did you consider making them adjustable? .
Very keen to hear how it drives.
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Posted 30 March 2012 - 06:22
I actually think the roll distribution is changed when the rocker geometry is changed.You are quite right, in principle. Controlling the roll (moment) distribution provides a method of controlling lateral balance in a turn. A roll moment distribution can be expressed as a combination of roll & warp, and it is the warp component that provides balance control. So far as I'm aware, there are three main methods of controlling roll moment bias in a turn; one is to have different (adjustable, ideally) roll centre heights, another is to control warp load directly & the last to controll the torque distribution (last added for completness).
In a conventional suspension a change in roll moment distribution is achieved by roll stiffness distribution, which yields the required warp load because the suspension has a warp stiffness. In a suspension that truely has zero warp stiffness, changing roll stiffness distribution will not (I think) change warp load, because the chassis will simply move to null the effect. What is required in a vehicle that doesn't roll, or has zero warp stiffness, is a mechanism to provide the stabilzing turn dependent warp offset.
Having different roll cente heights would be one way of achieving this. Active suspension provided a way of achieving a low warp stiffness with a directly controllable warp offset. The Lotus system, for example, embodied an algorithm that used the steering input and vehicle responses as a "trajectory" command - effectively making the vehicle neutral, at least whilst all four wheels remained in contact.
I personally think that Johan's design probably requires easily adjustable roll centre heights, or something equivalent.
Posted 30 March 2012 - 06:37
The wheels that are on now are too small, specially at the rear. It's designed for a ground clearance of around 75mm so roll centers will be 90-100mm.Roll centres are quite low. Did you consider making them adjustable? Of course your roll resistance and damping is fully adjustable. Are you confident that roll damper has identical push and pull characteristics?
Very keen to hear how it drives.
It's true I had to do some searching to find 1:1 dampers. Most people where saying that it could only be achived with trough-rod dampers (and they are probably right if you want perfect symmetry and adjustability) but Protech shocks made one (non-adjustable) from a standard body that is symmetrical enough for my purposes at a very reasonable price. Leda offered to make a non-adjustable trough-rod damper for £400.The only drawback I could see with this principle is that he'd possibly need 'non-conventional' dampers to avoid different damping characteristics in left and right turns (I think 'normal' car dampers should have difference between bump and rebound damping by some 3x)...
Posted 30 March 2012 - 06:57
Posted 30 March 2012 - 08:39
It's true I had to do some searching to find 1:1 dampers.
Most people where saying that it could only be achieved with trough-rod dampers
Edited by cheapracer, 30 March 2012 - 08:42.
Posted 30 March 2012 - 08:59
As been described to me the problem is the volume of the rod on one side of the piston.A number of bike shocks I have rebuilt could easily have the shim stack either side of the piston built up the same for 50/50 as well
I think using 2 in parallell is the easiest way of getting adjustability. But it adds weight and some complexityyou could get 2 and make one Frankenshocker out of them. In fact 2 in hand would give you the 2 equal shim stacks.
Edited by Johan Lekas, 30 March 2012 - 09:06.
Posted 30 March 2012 - 09:39
As been described to me the problem is the volume of the rod on one side of the piston.
Have also thought of MC steering dampers,
Edited by cheapracer, 30 March 2012 - 09:47.
Posted 30 March 2012 - 10:04
Edited by gruntguru, 30 March 2012 - 10:08.
Posted 30 March 2012 - 13:29
You're maybe right about the adjustability of the roll-damper, but this was the simplest way forward. And I can still adjust the roll stiffnes with the rockers (and have a lot of fun )I think for this application an adjustable damper would be very useful. You could have lots of fun optimising the spring and damper rates in roll.
What are your initial selections for the front and rear spring/damper units? There is a world of opportunity there to play with dive and squat without affecting wheel rates a great deal and almost no effect on under/oversteer.
EDIT. Have you analysed your extended damper/linkage for buckling? The L/R ratio looks large but I am guessing the peak compression loads will be low enough to avoid buckling.
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Posted 30 March 2012 - 18:52
Posted 31 March 2012 - 08:17
I think it wll still have zero warp stiffness (statically) but the sprung mass will have to roll a little, ie it will follow the axle with the greater roll stiffness allocation. Of course in a dynamic sense this will add some resistance to warp due to roll inertia of the sprung mass.I probably need to spend some more time thinking about your design, but I don't see how warp is decoupled from roll for anything but equal (50/50%) roll stiffness distribution. It seems to me that adjusting the rocker ratios front or rear will create stiffness in the warp mode. Therefore changing roll stiffness distribution would indeed change your warp load (or cross-weight) and thus your balance, but you have have a nonzero warp stiffness
Posted 31 March 2012 - 08:28
I think it wll still have zero warp stiffness
Edited by cheapracer, 31 March 2012 - 08:28.
Posted 31 March 2012 - 09:01
I am not familiar with standards for roll stiffness but I would imagine it should be expressed in Nm/degree.The MR for roll from axle to roll spring is 0.51 both front and rear, but is adjustable. The roll spring is 80 N/mm. In roll it's compressed from both ends, so it acts as two 160N/mm springs i series. So what one axle "sees" is a 160N/mm spring. with MR = 0.51 it gives a an "axle roll ratio" of 160*(0.51)^2 = 42N/mm, and a total for the car of 84N/mm
Posted 31 March 2012 - 12:08
There is a factor of two because I was unclear in my MR definition. The MR = 0.51 is for axle rotation, ie 1mm up at the left wheel and 1mm down at the right wheel gives 0.51mm at one end of the spring. This means that the 84N/mm is when measuring displacement at one wheel, so over the track it's 42N/mm. Track is 1400mmI am not familiar with standards for roll stiffness but I would imagine it should be expressed in Nm/degree.
Trying to get a feel for your roll stiffness, I assumed track = 1500mm, CGH = 400mm, RCH = 100mm, Sprung mass = 350 kg, LatAcc = 1G
Roll moment = 350 x 9.8 x (0.4-0.1) = 1029 Nm
Sprung weight transfer = 1029/1.5 = 686 N
Displacement at wheels = 686/84 = 8.2mm
Roll (deg) = tan-1(8.2/1500) = 0.3 degrees.
Sounds very small. I may have missed a factor of two somewhere by using full track width instead of half but still a very small amount of roll. Can you point out my mistake?
Posted 31 March 2012 - 22:56
Edited by gruntguru, 31 March 2012 - 22:59.
Posted 01 April 2012 - 05:35
0.7 is very low for a road car, and is around what you'll get from the tires. So it sounds about right.Thanks Johan, that was exactly what I was unsure of.
Do you know how that roll figure (0.7 degrees at 1G cornering) compares to cars with conventional suspension? Anyone else out there have figures for typical performance or race cars?
EDIT. Roll stiffness in Nm/Deg comes to 1029/0.7 = 1470 Nm/Deg
Posted 01 April 2012 - 13:14
Posted 02 April 2012 - 03:16
Posted 02 April 2012 - 07:43
Thanks for the kind words. I appreciate anyone putting time and thought on this and writing comments. It's valuable design reviewJohan, can I first say I think your project is brilliant, a radical concept that are putting your real money and time into, looking forward to its first test.
As I am not an engineer aolot of the comments are beyond me but I do have one question ( not criticism).
You quote an all up weight of 450kg and give the matching unsprung weights. These are quite high versus some norms as your sprung weight is very low.
The front ratio is about 1:3 and the rear 1:4 . In addition you will have to face the problem of gyro precession on a beam front axle which may be minimised by your small wheels but can't be eliminated.
My question is do you yet know to what extent the damper/spring rates will need to be set to control unsprung weight movement versus a very light chassis rather than controlling the heave/roll /. pitch dynamics of the sprung mass.
The fact that the unsprung control is focussed on a single spring/ damper in bounce and just two in roll or heave will put extra unsprung control loads on the dampers compared to the usual set up I would think.
Just a thought
Speed (mm/s) Damping ratio (part of critical damping) 0 0,3 20 0,6 100 0,3The total damping is divided on 1/3 compression and 2/3 on rebound
Edited by Johan Lekas, 02 April 2012 - 10:38.
Posted 02 April 2012 - 09:14
I rig tested a "quad" with a similar layout a couple of years ago. It was not fitted with dampers of any description (not required, apparently), but it was not devoid of (friction) damping. Unsurprisingly, perhaps, the hub modes were some way from being in control (damping ratio's < 10 percent). However, when the vehicle was subjected to a "kerb" input (a phased input down one side of the vehicle), roll was the only sprung mass mode requiring additional damping.What will happen? Large contact patch load oscillation?
Edited by DaveW, 02 April 2012 - 09:18.
Posted 02 April 2012 - 10:26
It has roll stiffness to centre it in rotation about the x axis and pitch stiffness for the Y. However, if the road has a "twist" in it the "wheel weights" will remain the same as for a flat road.Interestingly, that vehicle did not have zero warp stiffness (so far I could see), which leads to a question, with zero warp stiffness, what controls mean suspension position, especially when the c.g. is not on the vehicle centreline?
Posted 02 April 2012 - 12:37
Your quite right, gg. My muddled thinking....It has roll stiffness to centre it in rotation about the x axis and pitch stiffness for the Y. However, if the road has a "twist" in it the "wheel weights" will remain the same as for a flat road.
Posted 15 September 2012 - 05:26
Posted 15 September 2012 - 16:04
me tooBump. Very keen to hear of any progress Johan???
Posted 15 September 2012 - 16:07
going back to this - are std compression coil springs designed to work in tension [ experience of light springs suggest they stretch-]it appears that the horizontal 'roll' coilover has to work the spring both in compression and tension ?
Posted 15 September 2012 - 21:49
A standard coil spring is happy to work in tension, except you need to think about the ends.going back to this - are std compression coil springs designed to work in tension [ experience of light springs suggest they stretch-]
Posted 16 September 2012 - 07:01
Missed this bump somehowBump. Very keen to hear of any progress Johan???
Posted 16 September 2012 - 17:16
That makes more sense. To be honest I think that that is one of those drawing board arguments, yes you have a rationale, but who says equal is best? Most production cars are underdamped in roll when correctly damped for pitch and bounce.
Posted 21 September 2012 - 16:01