Damper Histograms

[Angry Joe]

It seems like the bigger issue here is this: a 'dumb' suspension is a compromise for many requirements and conditions because it reacts the same way to any input. A skateboarder, like an active suspension, can adapt on the fly for any bump, ramp or whatever. Replace his brain with a command that says "bend knees at 100 lbs/inch" and you end up with the same problem.

You could set up a desert racer to perfectly absorb a particular bump without upsetting the truck at all - but it will be crap everywhere else.

Friction may be a concern, but it seems to be secondary to other issues.

[Frank]

hey racer chick,

heres a spreadsheet

http://www.uq.edu.au/fsae/frank/Suspension_Statics.xls

regards to Mr Rouelle

[Lukin]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by racer_chick:
The engineers here are working on that spreadsheet right now, and by the end of the week I should be sending it out to everyone who has attended the FSAE seminars. </div></BLOCKQUOTE>

Any chance I could get a copy too? I did Claude's non-FSAE seminar July 25-29 in Melbourne this year.

[racer_chick]

Yeah, don't worry. We'll be sending it out to all of the seminar attendees, FSAE or not. I think it is done today, just waiting for the final OK before I send it out.

Cheers!

[Pavan Dendi]

Will that include previous years? I attended the Roulle seminar for FSAE 2004 and never received a spreadsheet. I never bothered to pursue it, but I would appreciate a copy.

monqy_at_mail_dot_utexas_dot_edu

[Z]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Bucknell:
Z, ... I just can't visualize your longitudinal z-bar... could you draw a schematic? </div></BLOCKQUOTE>
John,

A sketch would explain it, but difficult to post on this forum ... I've got an idea I'll try later... Meanwhile, here are some words.

"U-bar" and "Z-bar" are shorthand names for spring elements that interconnect a pair of wheels. The central section of the bar is a torsion spring that is mounted on the chassis, and at each end it has a lever-arm, either bent in the same direction (U) or opposite directions (Z), with the ends of the lever-arms attached to the wheels.

A "U-bar", like a typical anti-roll-bar, resists differential movements of its ends (one up, the other down), but offers no resistance to similar movements (both up, or both down). A "Z-bar" resists similar movements, but allows differential movements.

A leafspring that is pivotted to the chassis at its centre, and connected to a wheel at each of its ends, is also a Z-bar (think about the central section of the Z getting shorter until it disappears).

U-bars and Z-bars can interconnect any pairs of wheels. Standard ARB's are U-bars that interconnect front-pair and rear-pair of wheels, hence stiffen up 4-wheel roll and twist modes. If the Z-bars "that link opposite corners" that you mentioned linked diagonally opposite corners, then they would stiffen bounce and twist modes - not really desirable 'cos twist should be soft.

My suggestion of side-pair Z-bars, on a FSAE car, would be something like this. A torsion bar running down each side of the car, about 1/2" diameter (maybe tubular) and as long as the wheelbase. A lever arm about 2"-3" long at each end (and cranked in opposite directions), attached via pullrods to the upper wishbones. Thus each longitudinal Z-bar resists its side of the car going down, so resists 4-wheel bounce and roll. But no resistance to pitch or twist (ie. front wheel moving up, while rear wheel moving down). A third Z-bar, perhaps as a centrally pivotted, transverse leafspring, at the more heavily laden rear wheels would provide pitch (and bounce) stiffness (could also have similar at front, but not necessary).

The Citroen 2CV, designed in late 1930's, released post WWII, had this sort of system, although somewhat different pitch control. The 2CV "Z-bars" are actually coil springs and tie rods that link leading (F) and trailing (R) arms, but same principle. BMC cars of 1960's+ had a similar hydraulically connected system ("Hydralastic" - rubber springs, and "Hydragas" - pneumatic springs). There was a Packard pre- or post-WWII (?) that had, I believe, the actual longitudinal torsion Z-bars, although I've never seen it.

I converted a VW Beetle "paddock basher" (ie. no body) to side-pair Z-bars (with 2CV style coils and a rear leafspring-Z-bar) back in the 1980's. I have no idea why everyone else in the world is not using them??? They work well!!!

There are many other ways of doing interconnected suspensions, some better than the above 'cos more separation of the modes, and hence more scope for tuning, etc,. But the above is very simple (only 3 springs instead of 4 + 2 ARB's), has good packaging (with the longitudinal torsion bars), and is a long way better than the standard approach (which has a too stiff twist mode).

There are a lot of other details, and very little readily available information (probably why so few people use interconnected suspensions), but they do work well.



Angry Joe,

I accept that you need dampers on a standard suspension, and if they are crappy dampers then the car doesn't work too well. My point was that there are a lot of ways of doing the springing better, the above being one simple way, and there are more advantages to be gained by "smarter" springs, than with better dampers.

I think using a standard springing arrangement, and then trying to make it work well with expensive, hi-tech dampers, is a dead-end, no-win situation. Just my opinion though .

Z

[romkasponka]

I would like to get a copy too
romkasponka @ gmail.com

[Lukin]

Is there an ETA on that file?

[Z]

Since I have been waffling on about interconnected U-bar and Z-bar suspensions above, I thought I'd add this here.

The recent Racecar Engineering (Oct. 2005) has an article on the "Creuat" suspension as fitted to the "Racing for Holland" Le Mans racecar (although not used in the actual LM race). The article doesn't go into much detail about how the Creuat suspension works, so I had a quick look at their patent (US Patent 6,942,230 by Josep Fontdecaba). Here's the gist of it;

In essence the system uses two "U-bars" that interconnect diagonally opposite pairs of wheels (so a bit like conventional ARB's but crossed over - LF to RR, and RF to LR). These provide all the stiffness for the car's 4-wheel pitch and roll-modes (conventional ARB's stiffen roll and twist-modes). This arrangement was shown by Mark Ortiz in his 1997 RE article "Interconnected Suspension - Part 2".

To these two U-bars Fontdecaba has added a single "Z-bar" that carries the weight of the car, ie. controls the bounce-mode. This Z-bar is connected to lever-arms at the middle of each U-bar, turning the U's into W's. Thus the bounce-mode stiffness is determined by the single Z-bar in series with the two W-bars. The actual implementation of U/W and Z-bars is done with hydraulics, pneumatics are used for the springing, and damping is via normal type damper valves in the hydraulic circuits.

This is how I understand the system from the patent. The system used on the RFH car may be different or have extra features (from the article I think it must).

Nevertheless, the main benefit claimed for the system is the soft twist-mode (called "axle-crossing" by Fontdecaba). However, in certain conditions, this is also considered a disadvantage for the very aero-sensitive Le Mans car (from what I call "tilting on the diagonal" - see the article - although I reckon this is easily fixed).

Regarding the advantage of a soft twist-mode the RFH engineer is quoted as saying "with shock absorbers [normal suspension] over the kerb you can really feel it, but with this system you don't know you are on the kerb."

Z

[John Bucknell]

Per Z's above comments, some 3,000 words (i'm just posting them as a favour, these are all from him):



Citroen 2CV and BMC, from "New Directions in Suspension Design", Colin Cambell, 1981 (with some comments by Z)




Packard, from "Chassis Design...", based on notes by Maurice Olley, pre-1960? (with more comments...)


Z-bar sketches by Z