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31 Oct 2005, 15:20 (Ref:1448456) | #51 | ||
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Goran, sorry to ask stupid questions, but what angle do you call "Sai"?
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31 Oct 2005, 15:29 (Ref:1448462) | #52 | |||
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1 - a completely fixed KINEMATIC roll centre is possible with a conventional double wishbone suspension system so this isn't an advantage of the Lawrence link. 2 - The kinematic roll centre is not a motion centre so controlling it within fractions of mm is not necessarily worth bothering with anyway. 3 - You don't need a Lawrence link system to run no ARBs - just ask Lee Stohr who doesn't run them on his D-sports runoff winning car. 4 - Who told you it was better than a Lotus 18? Non other than the guy who invented it. Did you know gullable's been removed from the latest edition of the dictionary... Ben |
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31 Oct 2005, 15:40 (Ref:1448467) | #53 | |||
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Goran Malmberg |
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31 Oct 2005, 18:40 (Ref:1448626) | #54 | |||
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I do know that the lower ball is set further into the rim than the upper one, so I guess I must have some kpi At this point I have to admit that there is the distinct possibility that I have no idea what you're talking about, and in fact my car goes round corners better than virtually anything I've raced against by complete fluke! This is born out by the fact that I seem to be the only car on track with no anti-roll bars. (Oh, except for my team mate who watched how my car corners and decided to throw his away too) |
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31 Oct 2005, 19:15 (Ref:1448657) | #55 | |||
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31 Oct 2005, 19:17 (Ref:1448660) | #56 | ||
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For those interested i put the "wheelrate" images out on my site, fare down the side. You will be amazed what it shows.
http://hem.passagen.se/hemipanter/ Goran Malmberg |
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31 Oct 2005, 22:19 (Ref:1448847) | #57 | ||
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Yeh, ok sorry about sounding a bit dense. I have actually read about kpi and at the time I understood what it did and why. It's just that as I get older it becomes harder to retain all this sort of stuff in my head at the same time. I just start to think I have a good visualisation of what's happening, then some one reminds me about a bit I've forgotten and it all falls apart again
FWIW, this is how I went about setting up my geometry: For no particular reason other than my own intuition, I reasoned that a good place to start to find good cornering was to keep all of the tyre tread flat on the ground. Sooo.... I plotted out my front lower wishbones, uprights, stub axles and tyres on a cad package. I set the ride height with the lower wishbone pivots horizontal then postioned both tyres upright. Ignoring the upper wishbones for the mo, I rolled the car about its (approximate) roll centre by about 3 degrees in each direction keeping the tyres upright. I then introduced the upper wishbones and plotted locus of the inner pivots around the top ball joints for all three positions to see if there was a mounting position which would satisfy the vertical tyre requirement. There wasn't such a position, so I experimented with different wishbone lengths to see if I could find one. It turned out that if I lengthened my existing wishbones by 40mm and moved the pivot down and in towards the centre line of the car, I had a system which would keep the tyres vertical regardless of car roll. Oh, and to keep everything simple I have all my wishbone inner pivots pretty much parallel to the centre line of the car (within my welding ability that is ). From that point I just added a few degrees of caster to help the outer tyre walls during turn in. After a bit of track testing, I also added half a degree of static negative camber to even out my tyre wear. This may all seem a bit Heath Robinson to you, but in strictly practical terms, working straight from the tyre tread to the inner wishbone pivots and simplifying everthing else as much as possible, allows me to picture what's going on without having much mathematical understanding. Guess its just as well that I'm the driver as well as the race engineer and designer. No translation problems |
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1 Nov 2005, 08:24 (Ref:1449100) | #58 | |||
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Right, tire µ for the highest total friction is the key factor. 2 Logical way of working, great. 3 Work in a siilar way myself. Even if experimenting, I use real life models in ored to involve every factor there is. And if studying a physical model one discover things that might else not have been detected. Math is a great thing when one know what to calculate, and to shorten work time. 4 If working in the way you suggest, you automatically involved yourself as a driver-factor in the prosess when judging the car. Goran Malmberg |
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1 Nov 2005, 09:51 (Ref:1449180) | #59 | ||
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Thanks for that Goran
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1 Nov 2005, 16:23 (Ref:1449517) | #60 | ||
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ben the results show deep sandersons being quicker in the wet.
You have strong opinions - maybe you should write a column for us? |
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1 Nov 2005, 20:44 (Ref:1449772) | #61 | |||
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However my take on this is that Lawrence had an obsession with not running ARBs. What's the first thing you're told to do in the wet? Disconnect your ARBs because the roll resistance requirement reduces due to the lower lateral accelerations. All of sudden you get better (i.e. less) load variation on each tyre and therefore more grip and more performance. Why did the Lotus's not do this? I couldn't answer that. A more plausable explanation I think you'll agree, and nothing to do with this ridiculously over-hyped Lawrence link system. Don't know about a column though. Would that be under my assumed name as per the letters page this month :-) Yes I guess I have some strong opinions. The thing that bugs me is the obsession with things like the Lawrence link. You need more articles about how to deal with the multiple interactions that constitute a race car rather than panaceas that don't exist. If the Lawrence link was so good why did Lawrence not design it into the Morgan Aero 8, and for that matter why was the Aero 8 so damn slow at Le Mans last year if he's any good at what he does? Ben |
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1 Nov 2005, 21:47 (Ref:1449833) | #62 | |||
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1 Nov 2005, 22:21 (Ref:1449872) | #63 | ||
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Where is the best description of the Lawrence and the X-sytem to be found?
"solved the anti-roll issue once and for all" sounds a bit strange as a statement. Really must see what they mean by that. Goran Malmberg |
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1 Nov 2005, 22:22 (Ref:1449876) | #64 | |
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It was the DAX suspension I think your on about. it wasnt good over bumps,
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1 Nov 2005, 22:29 (Ref:1449883) | #65 | ||
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Sorry TEAM78, I knew it was a Caterham look-alike but was guessing which one. I remember it having two sets of diagonal links across the front wishbones, plus some complicated linkages at each end. Do they have anything about it up on the net anywhere?
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1 Nov 2005, 22:36 (Ref:1449894) | #66 | ||
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http://locost7.info/files/suspension...CamberComp.jpg http://images.google.co.uk/imgres?im...lr%3D%26sa%3DN I found this, it was in racecar eng ages ago. some guy was using it in the Caterham championship. There doesnt seem to be much on the net. think how the system works in one wheel bump then you relise what its down fall is. |
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1 Nov 2005, 23:18 (Ref:1449931) | #67 | ||
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Aha... that's the baby! But I ain't even gonna try to get my head round it. You up to that Goran? :-)
err. ok the daxcamb3.jpg makes it fairly clear, but yes, that does make one wheel bump a bit tricky. It's late, 3/4 bottle of red wine, sorry! |
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2 Nov 2005, 09:36 (Ref:1450205) | #68 | ||
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It's kinematically similar to a beam axle. We're back to the fact that if you want to control each wheel with a single spring/damper an individual wheel will always have 1dof. Therefore the pair of wheels together have 2dof. You can have synchronous or oppositional movement (heave or roll) one wheel bump is equal amounts of both and therefore always a compromise.
Ben |
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2 Nov 2005, 09:59 (Ref:1450229) | #69 | ||
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So now lets go loony again - Creuat Suspension - as used by Racing For Holland - complex but seemingly effective RfH insist it is better and the drivers prefer it for sure. Has anyone considered using it?
Ben - you clearly dislike the LL system not quite sure why so strongly - students I think need to explore what may be blind alleys to fully get around the problem of suspension design - so they can justify why the did it the way they did. Am I right in thinking that you would like more Ortiz and McBeath and less Valkeburgh |
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2 Nov 2005, 11:31 (Ref:1450312) | #70 | |||
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I dislike the hype things like the LL create because on a kinematic basis, anything it can do a double wishbone could get close enough that it would make no difference. It's not innovative in that sense. We contemplated designing our Bikesports car from scratch with a mechanical version of the Crueat system. I corresponded with Josep from Crueat about four years ago and have been a big fan of the principles behind the system since then. We have decided not to do it because we've never designed a full blown circuit racer before and felt that we'd have enough on our plates as it was. Ben Edit: the difficulty they've had at RfH is apparently related to the relatively free warp movement allowing too much movement of the aero platform. I suspect the damping curve for the warp mode is crucial to getting it to work. |
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2 Nov 2005, 12:39 (Ref:1450359) | #71 | |
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Real swing arms?
While we're discussing unusual suspensions:
Problems (compromise) with double wishbones include the need for camber compensation in roll to compensate for the upper chassis mount moving outboard in roll. The more camber compensation you have for the roll the more camber changes in bump/dive/squat. Why does nobody run a swing arm suspension with very long and low mounted swing arms? I'm thinking along the lines of swing arms mounted on the bottom of the chassis, either on the centreline or even on the opposite side of a wide chassis (so the LH wheel has a swing arm mounted just inside the RH wheel, low down.) I think this puts the roll centre fixed at the bottom of the chassis height, so less jacking than, say, a VW beetle rear swing arm. Due to the long swing arm length you would get little camber compensation, but would need none as there is little effect on camber in roll (except that the inner pivot will move up in roll, causing the outer wheel to go positive, slightly). There;s got to be a reason why it doesnt work (other than structural and weight considerations, which can surely be overcome in teh design) otherwise people wouldnt go to the trouble of the Dax system etc. G |
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2 Nov 2005, 12:41 (Ref:1450362) | #72 | ||
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RfH seem to have overcme this and are now using it in LMES races - whereas previously they only qualified with / practised with it.
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2 Nov 2005, 12:52 (Ref:1450371) | #73 | |||
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What you can't do with a single swing arm is change the length as a function of wheel travel. Ben |
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2 Nov 2005, 16:15 (Ref:1450549) | #74 | |||
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They talk about higher rollcentres, and I dont really get the point here. Forces not rolling the car wil go straight down to the contact patch. Ben mentioned camber as a function of virtual swingarm length, or should we say the distance to instant centre. However, the shorter and more angle the upper A-arm, the more the camber compensation. Of course it follows that the instant centre is moving around. This is not the behaviour of a swing arm. Also, as I show at my site, these sort of cambercompensation mess upp the wheelrate. Goran Malmberg |
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